Pharmacokinetics of didanosine in patients with acquired immunodeficiency syndrome or acquired immunodeficiency syndrome related complex The pharmacokinetics of didanosine (2',3'-dideoxyinosine) after intravenous and oral administration were evaluated in an open, escalating-dose phase I study in patients with acquired immunodeficiency syndrome (AIDS) or severe AIDS-related complex. Didanosine was administered twice a day for 2 weeks as an intravenous infusion of 60 minutes duration at doses ranging from 0.4 to 16.5 mg/kg, followed by 4 weeks of oral treatment at twice the intravenous dose. Serial blood and urine samples were obtained on the first and final day of intravenous administration and after the first oral dose, as well as at steady state. Didanosine demonstrated linear pharmacokinetic behavior over the dose ranges of 0.4 to 16.5 mg/kg intravenously and 0.8 to 10.2 mg/kg orally. There was no indication of significant changes in pharmacokinetic parameters with repeated administration. The apparent elimination half-life after oral administration was approximately 1.4 hour. Renal clearance values exceeded the glomerular filtration rate, indicating that active tubular secretion of didanosine occurs. Bioavailability of didanosine when administered as a solution with an antacid was approximately 43% for doses from 0.8 to 10.2 mg/kg in patients with AIDS and advanced AIDS-related complex. Bioavailability of didanosine from the citratephosphatebuffered solution, the formulation currently used in phase II and expanded access studies, was comparable to the formulation used in the phase I trials. (CLIN PHARMACOL THER 1991;49:523-35.)
Catherine A. Knupp, DVM, MS, Wen Chyi Shyu, PhD, Raphael Dolin, MD, Fred T. Valentine, MD, Colin McLaren, PhD, Russell R. Martin, MD, Kenneth A. Pittman, PhD, and Rashmi H. Barbhaiya, PhD Syracuse, Rochester,
and New York, N.Y., and Wallingford, Conn.
From the Department of Metabolism and Pharmacokinetics, BristolMyers Squibb Pharmaceutical Research Institute, Bristol-Myers Squibb Company, Syracuse; the Department of Clinical Research, Infectious Diseases, Bristol-Myers Squibb Pharmaceutical Research Institute, Bristol-Myers Squibb Company, Wallingford; the Infectious Diseases Unit, Department of Medicine, University of Rochester School of Medicine and Dentistry, Rochester; and the Department of Medicine, New York University School of Medicine, New York. Supported in part by the AIDS Clinical Trial Group of the National Institute of Allergy and Infectious Diseases (research project cooperative agreements 5U01 AI27658 and 5U01 AI27665) and Bristol-Myers Squibb Pharmaceutical Research Institute, Wallingford, Conn. Received for publication Oct. 25, 1990; accepted Jan. 2, 1991. Reprint requests: Rashmi H. Barbhaiya, PhD, Department of Metabolism and Pharmacokinetics, Pharmaceutical Research Institute, Bristol-Myers Squibb Company, P.O. Box 4755, Syracuse, NY 13221-4755. 13/1/27704
Didanosine (2',3'-dideoxyinosine, ddI) is a purine nucleoside analog with in vitro activity against human immunodeficiency virus.1'2 Through a series of intracellular conversions, didanosine generates the triphosphate form of dideoxyadenosine (ddATP).3 Incorporation of ddATP into viral deoxyribonucleic acid terminates chain elongation; alternatively, ddATP may directly inhibit viral reverse transcriptase activity.4 Didanosine possesses a higher in vitro therapeutic ratio than does zidovudine, thereby warranting clinical evaluation.2 The purpose of this phase I study was to evaluate safety and tolerance, to characterize pharmacokinetics, and to obtain preliminary indications of efficacy of didanosine in patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex (ARC). The results of safety, tolerance, and prelimi-
523
CLIN PHARMAC01. THER
524 Knupp et al. nary efficacy assessments are already reported.5-7 This article describes the pharmacokinetic data collected during the study.
METHODS Study design. This was an open, escalating-dose study of parallel design entailing intravenous and oral administration of didanosine to patients with AIDS or ARC. The protocol was approved by the institutional review boards of the participating clinical facilities (University of Rochester School of Medicine and Dentistry and New York University School of Medicine). All patients signed informed consent documents before enrollment in the study. Each patient received didanosine twice a day at approximately 12-hour intervals for 2 weeks as a constant-rate, 60-minutes, intravenous infusion followed by 4 weeks of oral treatment at twice the intravenous dose. Nine treatment groups, differentiated on the basis of didanosine dose, were used in the study. Each treatment group was to contain six patients, with four participating in the pharmacokinetic evaluation. Serial blood samples and the total urinary output, collected as discrete intervals, were obtained on study days 1, 14, 16, and 43 for the pharmacokinetic assessments. In addition, a single blood sample for determining the trough concentration (Cm,n) of didanosine was obtained before the first dose of the day on days 4, 7, 10, 23, 30, and 37. In addition to participating in the escalating-dose study, a group of eight patients participated in a pilot bioequivalence study designed to compare the bioavailability of didanosine when administered as a citrate-phosphatebuffered solution, relative to that from a solution given with an antacid (Maalox, Rorer Pharmaceutical Corp., Fort Washington, Pa.). The study was designed as a two-way crossover study. Beginning on study day 1, patients received 375 mg didanosine, twice a day, in the form of a solution with Maalox. On study day 4, a single 375 mg dose was given, as either a citrate-phosphatebuffered powder blend or a solution with Maalox. On study days 5 and 6 patients received the solution with Maalox again, followed by a single dose of the appropriate alternate treatment on study day 7. The doses on days 4 and 7 were preceded by approximately a 10-hour fast, and patients were not permitted to eat for 4 hours after drug administration. At the completion of this bioequivalence assessment, patients returned to their usual dosing regimen. Patients. Thirty-three patients participated in some aspect of the pharmacokinetic studies conducted during the trial. Patients were diagnosed as having AIDS
MAY 1991
or ARC according to guidelines of the Centers for Disease Control. Key criteria for entry into the study included absence of life-threatening opportunistic infection on enrollment, a CD4 lymphocyte count of 400/mm3 or less, platelet count greater than 80,000/mm3, bilirubin level less than 1.5 mg/d1, liver enzyme levels less than three times the upper limit of normal, and a creatinine clearance of greater than 50 ml/min. Patients were excluded from the study if they had been treated with any other antiretroviral drug within 1 month or cytotoxic agents within 3 months of the study, if they had any history of seizure disorders or cardiac disease, or if they had intractable diarrhea. Patients were permitted to receive medications for concomitant disease conditions, including aerosolized pentamidine. The patients had a mean (-±SD) age of 38 ± 8.4 years, a mean body weight of 70 ± 8.7 kg, and a mean height of 173 ± 8.0 cm. The majority of the patients (79%) were white men at risk for AIDS because of homosexual or bisexual behavior. Four women participated in the pharmacokinetic studies. Greater than 80% of the patients were intolerant to zidovudine because of previous therapy with this agent. The diagnosis of AIDS or ARC occurred with approximately equal frequency. Drug formulation and administration. The formulation for intravenous administration was supplied in 20 ml parenteral vials containing 250 mg didanosine as a sterile, lyophilized form. Each 250 mg parenteral vial was constituted with 16.5 ml 0.9% sodium chloride injection United States Pharmacopeia (USP) to obtain 15 mg/ml didanosine solution. The drug was administered as a constant-rate 1-hour infusion through an infusion pump twice daily for 2 weeks. The formulation for oral use initially consisted of a two-bottle system. One bottle contained a blend of powdered citrate and phosphate buffers; the other contained 250, 500, 2000, or 4000 mg didanosine. One bottle each of buffered powder and didanosine powder was constituted with 50 ml purified water. The appropriate dosing volume of didanosine solution was withdrawn from the didanosine bottle with a syringe after the solution was shaken thoroughly and then added to the bottle containing the buffer solution. The final pH of the oral preparation was 7 to 8. The buffered didanosine solution was shaken for 1 to 2 minutes and then swallowed. This formulation was given to patients who received a dose equivalent to 3.5 mg/kg or below. For administration of doses equal to or greater than 6.0 mg/kg, the vials of lyophilized didanosine were reconstituted with a 16.5 ml solution of 0.9% sodium chloride to obtain 15 mg/ml didanosine. Patients
VOLUME 49 NUMBER 5
were instructed to mix the appropriate volume of didanosine solution with apple juice and ingest the dose 2 minutes after taking 30 ml of an oral antacid (Maalox or aluminum hydroxide [Amphojel, WyethAyerst Laboratories, Philadelphia, Pal). The drug was administered on an empty stomach. The citrate-phosphate buffer formulation used in the pilot bioequivalence study contained 375 mg didanosine, 2.942 gm sodium citrate USP dihydrate, 2.13 gm dibasic sodium phosphate USP anhydrous, and 0.12 gm citric acid, as well as approximately 14.5 gm sucrose to counteract the salty taste of the buffering ingredients. The total weight of a unit dose was 20 gm. The dose was packaged into bottles and reconstituted with 120 ml water, yielding a solution with a pH of 7 to 8. Blood and urine sampling. Serial blood samples were collected after the first and last intravenous and oral doses on days 1 and 14 or 16 and 43, respectively. Approximately 3 ml heparinized blood, except for the predose sample, which was 7 ml, was collected before administration and 0.33, 0.66, 1, 1.05, 1.15, 1.25, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 6.0, 8.0, 10.0, and 12.0 hours after the initiation of intravenous administration. Sampling times after oral administration were before dosing and 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 6.0, 8.0, 10.0, and 12.0 hours after administration. In addition, a single blood sample was collected before the first daily dose on days 4, 7, 10, 23, 30, and 37 to ensure that steady state had been reached. Plasma samples were obtained by centrifuging at 1000g for 10 minutes (0° to 4° C). The samples were then stored at 20° C with quality-control samples prepared in control human plasma before sample collection, pending analysis for didanosine. The total urine output, collected as discrete intervals, was obtained on days 1, 14, 16, and 43 before dosing and 0 to 4, 4 to 8, and 8 to 12 hours after drug administration. An additional 12 to 24 hour sample was collected on days 14 and 43 only. The collection bottles were kept in a refrigerator except during voiding. At the end of each interval, the total volume of the urine was measured and recorded. The urine sample was shaken thoroughly and a 2 ml portion of the sample was transferred to a 15 ml polypropylene tube containing 4 ml 0.20 mol/L potassium phosphate buffer, pH 8.0. The buffered urine sample was stored at 20° C with quality-control samples pending analysis for didanosine. During the pilot bioequivalence study, blood and urine samples were collected according to a schedule similar to that described for the evaluation of the phar-
Didanosine in HIV patients
525
macokinetics of didanosine after oral administration in the phase I trial. The last blood sample was collected at 8 hours, although the collection of urine was extended to 24 hours after drug administration. Drug analysis. Plasma and urine samples were analyzed for didanosine by HPLC methods with ultraviolet detection.8 Quality-control samples were included in each analytic sequence to verify the stability of study samples during storage and accuracy and precision of didanosine analysis. Pharmacokinetic analysis. Plasma didanosine concentration (C) versus time (t) data were evaluated by noncompartmental methods .9' I° The highest observed plasma concentration and the corresponding sampling time were defined as Cmax and tmax, respectively. The best log-linear fit to the terminal portion of the data was determined by least-squares linear regression analysis to determine half-life (t112). The area under the plasma concentrationtime curve (AUC) and the area under the first moment of plasma concentration time curve (AUMC) were estimated by trapezoidal and log-trapezoidal methods. The values for AUC and AUMC were extrapolated to infinity by use of the standard methods .9 For single-dose administration, mean residence time (MRT) in the body was estimated as follows: MRT = AUMC(0_,)/AUC,0,
For multiple dosing, MRT at steady state was estimated 1112 as follows: MRT =
[AUMC,,
+ T[AUC(_,
AUC,,]/AUC(0
in which AUC(,) and AUMCw_;) correspond to AUC and AUMC for one dosing interval, T (12 hours).
The MRT equivalent for bolus intravenous administration was calculated according to the equation: MRT(IV) = MRT (T/2) in which T is the infusion time (1 hour). Total body clearance (CL) was calculated as follows: CL = Dose/AUC. The steady-state volume of distribution (Vss) was obtained by use of the equation: Vss = MRT(IV) (CL).13 The amount of intact didanosine excreted during each collection interval was calculated as the product of the concentration in the corresponding buffered urine sample and the total volume of urine voided in that interval. The total urinary recovery (UR) was calculated as the cumulative amount excreted within the 12- or 24-hour collection period and expressed as a percentage of the administered dose. Renal clearance (CLR) of didanosine was calculated as follows:
CL, = Amount excreted((_,)/AUC(o,)
PHARMACOI. THER
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Knupp et al.
MAY 1991
The absolute bioavailability (F) of oral didanosine was estimated as follows:
F=
AUC,0
X
Dose,,
AUCw x Dosepo
in which AUCiv is AUC(0_,) for the last intravenous dose administered on day 14 and AUCpo is AUC(3) after the first oral dose on day 16, and AUC(,) after the steady-state oral dose on day 43. The accumulation ratios (R) after multiple administration by both the intravenous and oral routes were calculated by the following different methods.I4 The equations employed in these calculations are as follows: R, =
1/(1 -°`)
= AUC,,,,,ss/AUC(,),)I
Ri = in which AUC(0)I corresponds to the first dose, AUC(o_T)SS corresponds to the last intravenous or oral
dose (days 14 or 43, respectively), Cmjn1 corresponds to the plasma concentration immediately before administration of the second dose, and Csmsm corresponds to the steady-state plasma concentration immediately before administration of any dose at steady state. Statistical analyses. Repeated-measures analysis of variance (ANOVA) was used to determine whether the pharmacokinetic parameters AUC, Cm, t112, MRT, CLR, and UR measured after the first intravenous or oral dose were significantly different from those measured at steady state. For the intravenous route, the parameters CL and Vss were also evaluated. Analyses were conducted separately for each route of administration. Because the phase I study was carried out at two clinical facilities, data for patients from both study sites were combined. Sample sizes were too small within each site for a formal evaluation of their ability to be combined to be performed, but scattergrams of the AUC and Cmax data provided no suggestion that the sites should be analyzed separately. Only patients with both a first-dose and a steady-state observation for a route of administration were included in the analyses. As a result, data from 31 patients after intravenous administration and 25 patients after oral administration were evaluated statistically. Heterogeneity of variance among dose groups and between study days was evident for all parameters. The rank transformationI5 was somewhat successful in stabilizing the variances; however, unless the analysis
of the rank data provided different conclusions, only the unweighted ANOVA findings are reported. If the findings for the two analyses differed, the sign test was used as a further test for the difference between the first dose and steady-state observations. Weighted regression analysis was used to determine whether Cmax and AUC(3_,) at steady state were dose linear or dose proportional. Weights used were the reciprocal variances for each dose group. Analyses were conducted separately for each route of administration. Tests for dose linearity were based on the t test for a linear trend across doses. Tests for nonlinearity were performed with the lack-of-fit F statistic. Dose proportionality was evaluated after dose linearity was observed. This test consisted of a t statistic for the significance of the intercept. If the intercept was not significantly different from zero, a final no-intercept model was fit to assess the decrease in r2 by forcing the line through zero. ANOVA procedures, appropriate for a two-period crossover design, were performed to determine if there were significant mean differences in didanosine pharmacokinetic parameters between the solution with Maalox and citrate-phosphate buffer solution formulations. The bioequivalence assessment of the citratephosphate buffer powder relative to the solution with Maalox was made on the basis of the two one-sided tests procedure. I6 The two formulations were considered to be equivalent if the 90% confidence limits around the mean difference, expressed relative to the reference formulation, fell within the interval (80% to 120%). Confidence intervals were constructed for AUC(o_ ), Cmax, MRT, tu2, and CLR with the mean square error term from the ANOVA model with 6 error degrees of freedom, for eight patients, with the solution with Maalox used as the reference product.
RESULTS Didanosine assays. Plasma samples were analyzed at two different laboratories. Combining data from both sites, the predicted concentrations of the plasma quality-control samples were within 10% of the nominal values, and the percentage of coefficient of variation for interassay (between-day) precision averaged 9%. Urine quality-control samples were within 18% of the nominal values, with interassay precision averaging 10% coefficient of variation. Didanosine was found to be stable in plasma and buffered urine samples for up to 3 hours when heated at 56° C or for a minimum of 12 months at 20° C. These data are indicative of accurate and precise analyses and adequate stability of didanosine in the stored samples.
VOLUME 49 NUMBER 5
Didanosine in HIV patients 3.0 DAY
DAY 14
1
2.5-
mg/kg 0.4 1.0
1.75
3.0
0.5-
0 2
4
6
8
II10 fl12
0
2
4
6
10
112
TIME (HR) AFTER DOSING Fig. 1. Mean didanosine plasma concentrationtime profiles in patients with AIDS after the administration of single and multiple intravenous doses over the range of 0.4 to 3.0 mg/kg.
2
4
6
8
10 12 0 2 4 TIME (HR) AFTER DOSING
10
12
Fig. 2. Mean didanosine plasma concentrationtime profiles in patients with AIDS after the administration of single and multiple intravenous doses over the range of 5.1 to 16.5 mg/kg.
527
CL1N PHARMACOL THER MAY 1991
Knupp etal.
528
Table I. Pharmacokinetic data for patients receiving intravenous infusions of didanosine (nglml)
Dose (mg/kg)
n
0.4
3 3 3 3
1.0 1.75
3.0 5.1
Day
1
14
3
1
3
14
6 6
1751
2818
1
3041
14
5
1
5
14
4 4
14
11.4
3 3
14
4 4
14
1
1
1
5140 5151
8918 8405 13247 11407 17974 18019
MRT (hr)
(hr) Mean
SD
Mean
SD
13 10
0.88 0.93
0.94 0.94
0.25 0.35
249 94 261 718 635 477 1082 767 2882 1682 1257 2328 3341 3131
1.23 1.34 1.45 1.36 1.17 1.19 1.36 1.28 1.72 1.47 1.56 1.36 1.48 1.49
0.30 0.36 0.14 0.38 0.38 0.23 0.27 0.36 0.45 0.32 0.80 0.27 0.16 0.28 0.23 0.17
1.30 1.42 1.09 1.23
0.30 0.34 0.29 0.16 0.10 0.16 0.18
SD
305 291 1089 870 1866
1
14
7.6
16.5
Mean
0.99 0.96 1.14 1.06 1.29 1.29 1.27 1.36 1.26 1.48
0.21 0.67 0.12 0.18 0.28 0.18 0.23
Peak plasma concentration; t1, half-life; MRT, mean residence time; AUC, area under the plasma concentration-time curve; CL, total body clearance; renal clearance; Vs, steady-state volume of distribution; UR; urinary recovery. day-I4 values represent AUC,,,. *Day-I values represent AUC,,,
CL,
Table II. Pharmacokinetic data for patients receiving oral didanosine (mg/kg)
0.8 2.0 3.5
6.0 10.2
15.2
Formulation Buffer§
Buffer§ Buffer§11
Solution with antacid Solution with antacid Solution with antacid
Mean
Day 2
16
2
43
244 343
3
16
721
3
43
799
2 2
16
43
1065 690 1494
6
16
6
43
4 4
43
3
16
3
43
Solution with antacid
3
16
3
43
33.0
Solution with antacid
2 2
43
16
t,
No SD is reported for observations less than 3.
Mean
SD
250 212
923 1112 2285 2051 2307 2960 1574 2998
0.50 0.83 0.83 0.75 0.63 0.67 0.67 0.56 0.69 0.83 0.92 0.75 0.75 1.12
0.88
Mean
SD
0.76
0.63
2257 5650 4915 2339 2381 5671 7032 5787 6041
16
22.8
SD
t2 (hr)
(hr)
(nglml)
C,,
Dose
0.14 0.14
0.44 0.20 0.13 0.24 0.14 0.14 0.25 0.00
1.10 1.62 1.61
1.60 1.15 1.12 1.24 1.37 1.33 1.87 2.74 1.50 1.63 1.64 1.90
0.84 0.72
0.29 0.36 0.16 0.31
0.84 1.34 0.26 0.25
t.
time to reach peak concentration; C,x, Peak plasma concentration; half-life; MRT, mean residence time; AUC, area under the plasma concentrationtime curve; CLR, renal clearance; F, absolute bioavailability; UR, urinary recovery. *ALIC,,,, on day 16 and AUC,.,, on day 43. tMean and SD values for the 6.0 and 15.2 mg/kg dose groups are based on data from (n 1) patients as a result of incomplete UR data in one patient in each of these dose groups. *Mean and SD values for the 10.2 mg/kg dose group are based on data from (n + 1) patients. §Citrate phosphate, pH 7 to 8. 'Patients received solution with antacid on day 43.
-
Pharmacokinetics. Mean didanosine plasma concentration-time curves for the intravenous and oral routes of administration are shown in Figs. 1, 2, 3, and 4, respectively. The mean pharmacokinetic pa-
rameters for didanosine after intravenous and oral administration are shown in Table I and II, respectively. After administration of the first intravenous dose, observed Cmax and AUC(0_,) increased in a linear
VOLUME 49 NUMBER 5
Didanosine in HIV patients
AUC (hr
6992
SD
1.30 1.51
1.96 1.76 1.73 1.60 1.76
0.66 0.72 0.41
0.29 0.17 0.25
2.58 3.39
1.09
2.01
0.48 0.45
2.30 2.56 2.35
SD
Mean
34 35
1074 1074
130 223
132 281
131
768 828
156 67 153 164 301 125 122 183
652 762 432 428 455
843 945 821 781
837 668 668 627
1.68
217 176 40
1295 6051
691
18
730
10324
661
157 187
AUC (hr
MRT (hr)
2.16 2.18 2.14
Mean
801
Mean
461
416 467 499 410 354 451
334 328 355 278
SD
Mean
2203
688 464 360 349 373
1661
341
351 421 1281
1477
349 242
2374 3377 6819 6866 4272 4429 9542
3116 2784 2647 4609 3048
13171
1983
12730 14179
1500 1419
47 140 126 281 175
208
SD
Mean
SD
59.1
8.0
60.3
58.0 61.4 70.9 50.8 62.7 57.0 47.6
17.1
69.1 58.3 52.3 56.1
5.5 13.4 16.8 19.7
80 55
54.1 53.5 48.1 50.9 47.7 56.5 54.1
118
58.6
195
275 159 112 191
492 387 330 362
257 257 301
206
fashion as the dose increased (Fig. 5). On day 1, mean Cma at the end of the 60-minute intravenous infusion ranged from 305 to 17,974 ng/ml for doses over the range of 0.4 to 16.5 mg/kg. Similar values values were found on day 14, with mean C ranging from 291 to 18,019 ng/ml for the same dose range. Mean AUC(0) values after the first intravenous dose ranged from 413 to 28,257 hr ng/ml over the same dose range. Like Cnm, day 14 AUC(,) values were also similar to those obtained after
SD
25.2 19.1
4.3 16.4
22.9 12.8 13.6 18.5 17.5 9.7 5.8 13.2
SD
Mean
52 45
130
65 60 243 347 318 142 10
48 40 32 29 42 54 54
26.1 8.8 3.7
30.6 10.6
5.8 10.4
SD
28.2 41.5 5 3
17 18
25 21
16 17
10 19
24 24
6 6
18 21
6.0 25.9 24.4 22.2 20.2
UR (%)74
42 51 41
68.1
18.6
7.0
Mean
53.7 49.6 56.0 63.3 47.8 51.7 53.5 47.4 49.2 40.8
F (%)
CLR (mIlmin)l
ng1m1)*
UR (%)
V
SD
320 720 652 768 507 1445 1329 3862 3243 2745
13040 12737 21486 19387 28257 32264
CLR (ml/mm)
Mean
SD
413 418 1662 1544 2677 2541 3726 4118 7443
Mean
CL (ml/mm)
ng1m1)*
Mean
529
32.2
3.5 5.7
19.0 10.3 16.6
10.1
26.2
14.3
18.8 25.1
17.7
18.0
5.4 13.1
16.5
8.2
3.5
13.1
3.6
9.6 6.9
the first intravenous dose and ranged from 418 to 32,264 hr ng/ml. In the weighted regression analysis, conducted on parameters calculated at steady state, both Cnia,, and AUC(0_,) were found to be dose linear but not dose proportional. The estimated x intercepts of the regression lines (or the threshold doses) were 0.12 mg/kg for Cma and 0.16 mg/kg for AUC(0). They are unlikely to be of any clinical significance. The AUC," value for 0.4 mg/kg dose group (the lowest dose) was
CLIN PHARMACOL THER
530 Knupp et al.
MAY 1991
6
mg/kg 0.8
DAY 43
DAY 16
5
2.0
0 3.5 4
6.0
o
10.2
3
2
1
0 12
012
4
6
8
12
TIME (HR) AFTER DOSING Fig. 3. Mean didanosine plasma concentrationtime profiles in patients with AIDS after the administration of single and multiple oral doses over the range of 0.8 to 10.2 mg/kg.
8.5
5.0-
0
012
4
6
12
012
12
TIME (HR) AFTER DOSING Fig. 4. Mean didanosine plasma concentrationtime profiles in patients with AIDS after the administration of single and multiple oral doses over the range of 15.2 to 33.0 mg/kg.
VOLUME 49 NUMBER 5
Didanosine in HIV patients
0
0
3
6
9
12
15
531
17
DOSE (mg/kg)
FIRST DOSE
E
+ STEADY
STATE
21
4-
0
7
0 12
18
17
DOSE (mg/kg)
FIRST DOSE
± STEADY
STATE
Fig. 5. Regression plot of peak plasma concentration (Cma) and area under the plasma concentrationtime curve (AUC) versus dose after intravenous administration of didanosine.
much lower than that of the other dose groups after normalizing the data for dose. The value for AUC(0_,.) may have been underestimated in this dose group because of limited sensitivity of the analytic method. Exclusion of the 0.4 mg/kg dose group AUC(0_7) data from the regression analysis resulted in a conclusion of dose proportionality over the range of 1.0 to 16.5 mg/kg. The ANOVA values for Cmax, AUC, MRT, t112, CL, CLR, and Vss indicated no statistically significant differences between the observations after the first intravenous dose on day 1 and the steady-state dose on
day 14. There was also no statistically significant evidence of dose dependency in t112, MRT, CL, CLR, or Vss. The overall CL and CLR were 760 ml/min and 406 ml/min, respectively, excluding the 0.4 mg/kg dose group for the previously cited reasons. The overall t172 was about 1.36 hour and mean values ranged from 0.88 to 1.72 hour. The overall Vss was 54.0 L, with a mean range of 47.7 to 61.4 L. There were no statistically significant differences in UR values after the first intravenous dose compared with steady state. Average UR ranged from 49.2% to
(AIN PlIARMACOL TI1ER
532
Knupp et al.
MAY 1991
15.0 12.5 -
x 7_,
10.0 -
E
x
5.0 -
O
2.512 15 18 21 DOSE (mg/kg) -
FIRST DOSE
24 27 30 33
STEADY STATE
20 620 16
-
12 co L.
8 -
_c
15 18 21 24 27 30 33 DOSE (mg/kg) STEADY STATE -1FIRST DOSE
6 -
9
12
Fig. 6. Regression plot of peak plasma concentration (Cmax) and area under the plasma concentrationtime curve (AUC) versus dose after oral administration of didanosine.
63.3% of dose recovered within 12 hours of didanosine administration on day 1 to 40.8% to 69.1% of dose during the 24-hour collection period after administration on day 14. The majority of the didanosine recovered in the urine was excreted within the first 8 hours after drug administration. On day 16 (first day of oral administration) the administration of single oral doses ranging from 0.8 to 33 mg/kg resulted in mean Cmax values of 244 to 5787 ng/ml, respectively. After 4 weeks of twice-a-day repeated dosing, the mean Cm values remained similar
to those observed after the first dose, ranging from 343 to 7032 ng/ml. There was no statistically significant difference between first oral dose and steady-state
oral Cniax values. After oral administration of didanosine, Cnia values increased in a dose-linear fashion as the dose increased (Fig. 6), although dose proportionality was not observed for the Cmax values. If the three highest dose groups were excluded from the weighted regression analyses, dose proportionality was observed in the range of 0.8 to 10.2 mg/kg. Mean AUC(o_s) values after the first oral dose
VOLUME 49 NUMBER 5
ranged from 351 to 12,730 hr ng/ml over the dose range of 0.8 to 33.0 mg/kg. There were no significant differences between AUC(0_) after the first oral dose and AUC(0_,) at steady state. Results of the weighted regression analysis of the steady-state data indicated that AUC(0_,) was dose proportional (Fig. 6). Mean tmax ranged from 0.50 to 1.12 hour for all dose groups regardless of the number of doses. The mean t112 ranged from 0.76 to 1.87 hour after the first oral dose and the range remained essentially the same (1.10 to 2.74 hour) after 4 weeks of oral administration. The results of the unweighted ANOVA indicated that there were no statistically significant changes in t112 during the 4-week administration period. The overall mean apparent elimination t1,2 was approximately 1.43 hour, which was slightly longer than that observed in the groups given drug intravenously (i.e., 1.36 hour). The comparison between first-dose CLR and UR values and steady-state observations, with unweighted ANOVA, indicated no significant difference in CLR and a significant difference in UR. Average CLR values ranged from 257 to 688 ml/min after the first oral dose and 206 to 464 ml/min after 4 weeks of oral therapy. UR was variable, averaging 8.2% to 28% after the first oral dose and 6.9% to 41.5% after multiple dosing. Of the 24 patients included in the analysis, 17 had steady-state UR values that were greater than those observed after the first oral dose and 7 patients had values that were less than the first dose observations. Although no statistically significant differences were observed, UR tended to decrease at doses equal to or greater than 15.2 mg/kg, supporting the observation that bioavailability decreased at doses equal to or greater than this level. The absolute bioavailability for both days 16 and 43 was estimated for each patient using individual intravenous data for day 14 (Table II). Because oral doses were administered after 2 weeks of intravenous administration, steady-state intravenous data were used as a reference for the calculation of bioavailability. Overall bioavailability for patients receiving 0.8 to 10.2 mg/kg doses of didanosine was approximately 43% (40% for day 16 and 45% for day 43). When the patients received 15.2 mg/kg or more didanosine, F decreased to approximately 24% (23% for day 16 and 25% for day 43). The estimation of the accumulation factor was essentially the same on the basis of three different methods. When didanosine was given at 12-hour intervals, there was no evidence of accumulation (accumulation factor = 1) after intravenous administration. After
Didanosine in HIV patients
533
oral administration, the accumulation factor ranged from 1.0 to 1.30. Blood samples collected to assess the trough concentration of didanosine generally contained less than quantifiable concentrations of didanosine. Bioequivalence assessment. Mean didanosine pharmacokinetic parameter values after administration of a solution with Maalox or a citrate-phosphatebuffered solution are presented in Table III. The results of the ANOVAs indicated that there were no significant sequence or period effects in the evaluation of Cm,, MRT, tu2, AUC, or CLR. The confidence interval assessment for MRT, t1,2, and CLR indicated that the two formulations were equivalent with respect to these parameters. The mean values for were 2145 and 2088 ng/ml for the citrate-phosphate buffer and the solution with Maalox, respectively. The mean estimate of the Cmax ratio, with 90% confidence limits, was 103% (88% to 118%). The respective mean values of AUC(0) were 3530 and 3231 hr ng/ml for the citrate-phosphate buffer and the solution. The mean estimate for the AUC(0_ ) ratio was 109% (96% to 122%). These data indicate that these two didanosine formulations possess very comparable bioavailability.
C.
DISCUSSION The findings of the didanosine pharmacokinetic analyses yield parameter values that generally agree well with reported values. 17 The apparent lack of dose dependence in CL and Vss indicates linear pharmacokinetic behavior for didanosine over the intravenous dose range of 0.4 to 16.5 mg/kg. Values for t2, AUC, CL, CLR, and obtained after singledose administration, do not appear to differ significantly from values calculated after 2 weeks of intravenous therapy, suggesting that the disposition of didanosine remains invariant with repeated administration. The data for C AUC, and elimination rate constant values suggest that there is no accumulation with repeated administration. CLR values exceed the average glomerular filtration rate in humans,I8 indicating that active secretion of didanosine by the renal tubules occurs after both intravenous and oral administration. In vitro experiments have shown that purine nucleosides are transported across cell membranes by facilitated diffusion mechanisms that do not appear to be structure specific. 19 In contrast to this inactive process, the active secretion of 2'-deoxyadenosine and active reabsorption of adenosine have been demonstrated in both a mouse model and humans.202I In a patient with an adenosine deam-
V.
C,
(TIN PHARMACOI, THER
534
Knupp et al.
MAY 1991
Table III. Pharmacokinetic data for patients receiving a 375 mg oral dose of didanosine, formulated as a solution with Maalox or a citrate-phosphatebuffered solution (nglml)
Dose
Formulation
(mg)
Citrate phosphate Maalox solution
375 375
,
Peak plasma concentration;
t,
8 8
Mean
SD
2145 2088
556 532
time to reach peak concentration;
1
t,
(hr)*
0.75 0.50
112
MRT (hr)
(hr)
Mean
SD
Mean
SD
1.59 1.73
0.21
2.07 2.21
0.26 0.32
0.32
t,2, half-life; MRT, mean residence time;
AUC,,
area under the plasma
concentrationtime curve; CLR, renal clearance; UR, urinary recovery. *Median is reported.
inase deficiency, the CL, of 2'-deoxyadenosine was fourfold greater than the measured creatinine clearance. It is conceivable that didanosine is secreted by the same nonselective process responsible for the secretion of 2'-deoxyadenosine and other deoxyribonucleosides structurally related to adenosine.22 CLR of didanosine accounts for approximately 50% of CL, suggesting that nonrenal pathways of elimination, such as metabolism or biliary secretion, play a significant role in the disposition of didanosine. Didanosine is rapidly degraded under the conditions of pH that exist in the human stomach.23 To help neutralize gastric acidity, didanosine was administered as a buffered solution or after the ingestion of an antacid. Although didanosine is absorbed rapidly, as demonstrated by the short tmax, individual patient differences in bioavailability may be a result of variable degradation of didanosine in the gastric environment or concurrent disease conditions that affect gastrointestinal motility and transit time. Bioavailability calculated in this study, approximately 43%, agrees well with the value of 38% reported by investigators from the National Cancer Institute.17 Although there appears to be a trend toward decreased bioavailability in the highest dose groups, dose proportionality was demonstrated at dose levels between 0.8 and 10.2 mg/kg. The apparent decrease in bioavailability of didanosine at the highest dose levels (15.2 mg/kg and above) may be attributable to incomplete absorption or saturation of an absorption process. The didanosine formulation currently used in the phase II trials and expanded access program contains powdered citrate and phosphate buffers, packaged as a sachet. Evaluation of bioavailability of didanosine from the citrate-phosphate buffer formulation relative to that from a solution with Maalox, as used in the phase I trials, demonstrated very comparable bioavailability for the two preparations. Although the two formulations appeared to be bioinequivalent with respect to AUC, based on the 90% confidence limit approach, the absolute difference in AUC(0_00) was only 9%. This slight difference in AUC between the two formula-
tions probably is not clinically relevant because of the wide safety margin of didanosine. Values for several didanosine phannacokinetic parameters were compared with those reported for zidovudine.24 After intravenous administration, the elimination t112 for zidovudine is 1.1 hour, a value similar to the 1.4 hour t112 calculated for didanosine. CL for zidovudine is considerably higher, averaging 1540 ml/min in a 70 kg adult, compared with 800 ml/min for didanosine. Zidovudine distributes more readily into body tissues, as evidenced by a Vss value of approximately 98 L. The apparent bioavailability of zidovudine is 63%, a value greater than that observed for didanosine (43%) in this study. Similar to didanosine, linear pharmacokinetic behavior has been reported for zidovudine over its therapeutic dose range. Although in vitro tests predict that the efficacious concentration for zidovudine is much lower than that for didanosine (1 versus 10 limol/L), the intracellular t1,2 for the active moiety of didanosine, ddATP, is 12 hours or longer.25 In comparison, the intracellular half-life of the analogous compound, zidovudine triphosphate, is similar to the plasma half-life of the parent compound. As a result of the prolonged intracellular half-life of ddATP, didanosine may be administered twice a day (every 12 hours), in contrast to four to six times a day with zidovudine. Less frequent administration coupled with the apparent high therapeutic index for didanosine may offer significant advantages over current therapeutic regimens with zidovudine. Phase II trials designed to address these issues are currently in progress. We thank Elizabeth A. Morgenthien for statistical analysis of the data and Frank A. Stancato, Eugene A. Papp, Gene D. Morse, and Charlene Taylor for excellent technical
assistance.
References 1.
Cooney DA, Ahluwalia G, Mitsuya H, et al. Initial studies on the cellular pharmacology of 2',3'-dideoxy-
VOLUME 49 NUMBERS
Didanosine in HIV patients
(hr
nglml)
CL, (ml/mm)
UR (%)
Mean
SD
Mean
SD
Mean
SD
3530
527 599
432 436
91 131
23.4 20.9
4.0 6.0
3231
adenosine, an inhibitor of HTLV-III infectivity. Biochem Pharmacol 1987;36:1765-8. Mitsuya H, Broder S. Inhibition of the in vitro infectivity and cytopathic effect of human T-Iymphotrophic virus type III/lymphadenopathy-associated virus (HTLVIII/LAV) by 2',3'-dideoxynucleosides. Proc Nat! Acad Sci USA 1986;83:1911-15. Johnson MA, Ahluwalia G, Connelly MC, et al. Metabolic pathways for the activation of the antiretroviral agent 2',3'-dideoxyadenosine in human lymphoid cells. J Biol Chem 1988;263:15354-7. Ahluwalia G, Cooney DA, Mitsuya H, et al. Initial studies on the cellular pharmacology of 2',3'-dideoxyinosine, an inhibitor of HIV infectivity. Biochem Pharmacol 1987;36:3797-800. Lambert JS, Seidlin M, Reichman RC, et al. 2',3'Dideoxyinosine (ddI) in patients with the acquired immunodeficiency syndrome or AIDS-related complex. N Engl J Med 1990;322:1333-40. Cooley TP, Kanches LM, Saunders CA, et al. Oncedaily administration of 2',3'-dideoxyinosine (ddI) in patients with the acquired immunodeficiency syndrome or AIDS-related complex. N Engl J Med 1990; 322:1340-5. Rozencweig M, McLaren C, Beltangady M, et al. Overview of phase I trials of 2',3'-dideoxyinosine (ddI) conducted on adult patients. Rev Infect Dis 1990;12:5570-5. Knupp CA, Stancato FA, Papp EA, Barbhaiya RH. Quantitation of didanosine in human plasma and urine by high-performance liquid chromatography. J Chromatogr 1990;533:282-90. Gibaldi M, Perrier D. Pharmacokinetics. 2nd ed. New York: Marcel Dekker, 1982:409-17. Riegelman S, Collier P. The application of statistical moment theory to the evaluation of in vivo dissolution time and absorption time. J Pharmacokinet Biopharm 1980;8:509-34. Pfeffer M. Estimation of mean residence time from data
535
obtained when multiple-dosing steady-state has been reached. J Pharm Sci 1984;73:854-6. Baner LA, Gibaldi M. Computation of model-independent pharmacokinetic parameters during multiple dosing. J Pharm Sci 1983;72:978-9. Benet LZ, Galeazzi RL. Noncompartmental determination of the steady-state volume of distribution. J Pharm Sci 1979;68:1071-4. Colburn WA. Estimating the accumulation of drugs. J Pharm Sci 1983;72:833-4. Conover WJ, 'man RL. Rank transformation as a bridge between parametric and nonparametric statistics. Am Stat 1981;35:124-33. Schuirmann DJ. A comparison of the two one-sided test procedures and the power approach for assessing the equivalence of average bioavailability. J Pharmacokinet Biopharm 1987;15:657-80. Hartman MR, Yarchoan R, Pluda JM, et al. Pharmacokinetics of 2',3'-dideoxyadenosine and 2',3'-dideoxyinosine in patients with severe human immunodeficiency virus infection. CON PHARMACOL THER 1990; 47:647-54. Altman PL, Dittmer DS. Biology data handbook, vol 3. 2nd ed. Bethesda, Maryland: Federation of American Societies for Experimental Biology, 1974:1992-9. Plagemann GWP, Wohlhueter M. Permeation of nucleosides, nucleic acid bases and nucleotides in animal cells. Curr Top Membr Transplant 1980;14:225-30. Kuttesch JF, Nelson JA. Renal handling of 2'-deoxyadenosine and adenosine in humans and mice. Cancer Chemother Pharmacol 1982;8:221-9. Nelson JA, Vidale E, Enigbokan M. Renal transepithelial transport of nucleosides. Drug Metab Dispos 1988;16:789-92. Nelson JA, Kuttesch IF, Herbert BH. Renal secretion of purine nucleosides and their analogs in mice. Biochem Pharmacol 1983;32:2323-7. Anderson BD, Wygant MB, Xiang TX, et al. Preformulation solubility and kinetic studies of 2',3'-dideoxynucleosides: potential anti-AIDS agents. Int J Pharm 1988;45:27-37. Klecker RW, Collins JM, Yarchoan R, et al. Plasma and cerebrospinal fluid pharmacokinetics of 3'-azido-3'deoxythymidine: a novel pyrimidine analog with potential application for the treatment of patients with AIDS and related diseases. CLIN PHARMACOL THER 1987; 41:407-12. Ahluwalia G, Johnson MA, Fridland A, Cooney DA, Broder S, Johns DG. Cellular pharmacology on the anti-HIV agent 2',3'-dideoxyadenosine. Proc Am Acad Cancer Res 1988;29:349P.
Catherine A. Knupp, DVM, MS, Wen Chyi Shyu, PhD, Raphael Dolin, MD, Fred T. Valentine, MD, Colin McLaren, PhD, Russell R. Martin, MD, Kenneth A. Pittman, PhD, and Rashmi H. Barbhaiya, PhD Syracuse, Rochester,
and New York, N.Y., and Wallingford, Conn.
From the Department of Metabolism and Pharmacokinetics, BristolMyers Squibb Pharmaceutical Research Institute, Bristol-Myers Squibb Company, Syracuse; the Department of Clinical Research, Infectious Diseases, Bristol-Myers Squibb Pharmaceutical Research Institute, Bristol-Myers Squibb Company, Wallingford; the Infectious Diseases Unit, Department of Medicine, University of Rochester School of Medicine and Dentistry, Rochester; and the Department of Medicine, New York University School of Medicine, New York. Supported in part by the AIDS Clinical Trial Group of the National Institute of Allergy and Infectious Diseases (research project cooperative agreements 5U01 AI27658 and 5U01 AI27665) and Bristol-Myers Squibb Pharmaceutical Research Institute, Wallingford, Conn. Received for publication Oct. 25, 1990; accepted Jan. 2, 1991. Reprint requests: Rashmi H. Barbhaiya, PhD, Department of Metabolism and Pharmacokinetics, Pharmaceutical Research Institute, Bristol-Myers Squibb Company, P.O. Box 4755, Syracuse, NY 13221-4755. 13/1/27704
Didanosine (2',3'-dideoxyinosine, ddI) is a purine nucleoside analog with in vitro activity against human immunodeficiency virus.1'2 Through a series of intracellular conversions, didanosine generates the triphosphate form of dideoxyadenosine (ddATP).3 Incorporation of ddATP into viral deoxyribonucleic acid terminates chain elongation; alternatively, ddATP may directly inhibit viral reverse transcriptase activity.4 Didanosine possesses a higher in vitro therapeutic ratio than does zidovudine, thereby warranting clinical evaluation.2 The purpose of this phase I study was to evaluate safety and tolerance, to characterize pharmacokinetics, and to obtain preliminary indications of efficacy of didanosine in patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex (ARC). The results of safety, tolerance, and prelimi-
523
CLIN PHARMAC01. THER
524 Knupp et al. nary efficacy assessments are already reported.5-7 This article describes the pharmacokinetic data collected during the study.
METHODS Study design. This was an open, escalating-dose study of parallel design entailing intravenous and oral administration of didanosine to patients with AIDS or ARC. The protocol was approved by the institutional review boards of the participating clinical facilities (University of Rochester School of Medicine and Dentistry and New York University School of Medicine). All patients signed informed consent documents before enrollment in the study. Each patient received didanosine twice a day at approximately 12-hour intervals for 2 weeks as a constant-rate, 60-minutes, intravenous infusion followed by 4 weeks of oral treatment at twice the intravenous dose. Nine treatment groups, differentiated on the basis of didanosine dose, were used in the study. Each treatment group was to contain six patients, with four participating in the pharmacokinetic evaluation. Serial blood samples and the total urinary output, collected as discrete intervals, were obtained on study days 1, 14, 16, and 43 for the pharmacokinetic assessments. In addition, a single blood sample for determining the trough concentration (Cm,n) of didanosine was obtained before the first dose of the day on days 4, 7, 10, 23, 30, and 37. In addition to participating in the escalating-dose study, a group of eight patients participated in a pilot bioequivalence study designed to compare the bioavailability of didanosine when administered as a citrate-phosphatebuffered solution, relative to that from a solution given with an antacid (Maalox, Rorer Pharmaceutical Corp., Fort Washington, Pa.). The study was designed as a two-way crossover study. Beginning on study day 1, patients received 375 mg didanosine, twice a day, in the form of a solution with Maalox. On study day 4, a single 375 mg dose was given, as either a citrate-phosphatebuffered powder blend or a solution with Maalox. On study days 5 and 6 patients received the solution with Maalox again, followed by a single dose of the appropriate alternate treatment on study day 7. The doses on days 4 and 7 were preceded by approximately a 10-hour fast, and patients were not permitted to eat for 4 hours after drug administration. At the completion of this bioequivalence assessment, patients returned to their usual dosing regimen. Patients. Thirty-three patients participated in some aspect of the pharmacokinetic studies conducted during the trial. Patients were diagnosed as having AIDS
MAY 1991
or ARC according to guidelines of the Centers for Disease Control. Key criteria for entry into the study included absence of life-threatening opportunistic infection on enrollment, a CD4 lymphocyte count of 400/mm3 or less, platelet count greater than 80,000/mm3, bilirubin level less than 1.5 mg/d1, liver enzyme levels less than three times the upper limit of normal, and a creatinine clearance of greater than 50 ml/min. Patients were excluded from the study if they had been treated with any other antiretroviral drug within 1 month or cytotoxic agents within 3 months of the study, if they had any history of seizure disorders or cardiac disease, or if they had intractable diarrhea. Patients were permitted to receive medications for concomitant disease conditions, including aerosolized pentamidine. The patients had a mean (-±SD) age of 38 ± 8.4 years, a mean body weight of 70 ± 8.7 kg, and a mean height of 173 ± 8.0 cm. The majority of the patients (79%) were white men at risk for AIDS because of homosexual or bisexual behavior. Four women participated in the pharmacokinetic studies. Greater than 80% of the patients were intolerant to zidovudine because of previous therapy with this agent. The diagnosis of AIDS or ARC occurred with approximately equal frequency. Drug formulation and administration. The formulation for intravenous administration was supplied in 20 ml parenteral vials containing 250 mg didanosine as a sterile, lyophilized form. Each 250 mg parenteral vial was constituted with 16.5 ml 0.9% sodium chloride injection United States Pharmacopeia (USP) to obtain 15 mg/ml didanosine solution. The drug was administered as a constant-rate 1-hour infusion through an infusion pump twice daily for 2 weeks. The formulation for oral use initially consisted of a two-bottle system. One bottle contained a blend of powdered citrate and phosphate buffers; the other contained 250, 500, 2000, or 4000 mg didanosine. One bottle each of buffered powder and didanosine powder was constituted with 50 ml purified water. The appropriate dosing volume of didanosine solution was withdrawn from the didanosine bottle with a syringe after the solution was shaken thoroughly and then added to the bottle containing the buffer solution. The final pH of the oral preparation was 7 to 8. The buffered didanosine solution was shaken for 1 to 2 minutes and then swallowed. This formulation was given to patients who received a dose equivalent to 3.5 mg/kg or below. For administration of doses equal to or greater than 6.0 mg/kg, the vials of lyophilized didanosine were reconstituted with a 16.5 ml solution of 0.9% sodium chloride to obtain 15 mg/ml didanosine. Patients
VOLUME 49 NUMBER 5
were instructed to mix the appropriate volume of didanosine solution with apple juice and ingest the dose 2 minutes after taking 30 ml of an oral antacid (Maalox or aluminum hydroxide [Amphojel, WyethAyerst Laboratories, Philadelphia, Pal). The drug was administered on an empty stomach. The citrate-phosphate buffer formulation used in the pilot bioequivalence study contained 375 mg didanosine, 2.942 gm sodium citrate USP dihydrate, 2.13 gm dibasic sodium phosphate USP anhydrous, and 0.12 gm citric acid, as well as approximately 14.5 gm sucrose to counteract the salty taste of the buffering ingredients. The total weight of a unit dose was 20 gm. The dose was packaged into bottles and reconstituted with 120 ml water, yielding a solution with a pH of 7 to 8. Blood and urine sampling. Serial blood samples were collected after the first and last intravenous and oral doses on days 1 and 14 or 16 and 43, respectively. Approximately 3 ml heparinized blood, except for the predose sample, which was 7 ml, was collected before administration and 0.33, 0.66, 1, 1.05, 1.15, 1.25, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 6.0, 8.0, 10.0, and 12.0 hours after the initiation of intravenous administration. Sampling times after oral administration were before dosing and 0.25, 0.5, 0.75, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 6.0, 8.0, 10.0, and 12.0 hours after administration. In addition, a single blood sample was collected before the first daily dose on days 4, 7, 10, 23, 30, and 37 to ensure that steady state had been reached. Plasma samples were obtained by centrifuging at 1000g for 10 minutes (0° to 4° C). The samples were then stored at 20° C with quality-control samples prepared in control human plasma before sample collection, pending analysis for didanosine. The total urine output, collected as discrete intervals, was obtained on days 1, 14, 16, and 43 before dosing and 0 to 4, 4 to 8, and 8 to 12 hours after drug administration. An additional 12 to 24 hour sample was collected on days 14 and 43 only. The collection bottles were kept in a refrigerator except during voiding. At the end of each interval, the total volume of the urine was measured and recorded. The urine sample was shaken thoroughly and a 2 ml portion of the sample was transferred to a 15 ml polypropylene tube containing 4 ml 0.20 mol/L potassium phosphate buffer, pH 8.0. The buffered urine sample was stored at 20° C with quality-control samples pending analysis for didanosine. During the pilot bioequivalence study, blood and urine samples were collected according to a schedule similar to that described for the evaluation of the phar-
Didanosine in HIV patients
525
macokinetics of didanosine after oral administration in the phase I trial. The last blood sample was collected at 8 hours, although the collection of urine was extended to 24 hours after drug administration. Drug analysis. Plasma and urine samples were analyzed for didanosine by HPLC methods with ultraviolet detection.8 Quality-control samples were included in each analytic sequence to verify the stability of study samples during storage and accuracy and precision of didanosine analysis. Pharmacokinetic analysis. Plasma didanosine concentration (C) versus time (t) data were evaluated by noncompartmental methods .9' I° The highest observed plasma concentration and the corresponding sampling time were defined as Cmax and tmax, respectively. The best log-linear fit to the terminal portion of the data was determined by least-squares linear regression analysis to determine half-life (t112). The area under the plasma concentrationtime curve (AUC) and the area under the first moment of plasma concentration time curve (AUMC) were estimated by trapezoidal and log-trapezoidal methods. The values for AUC and AUMC were extrapolated to infinity by use of the standard methods .9 For single-dose administration, mean residence time (MRT) in the body was estimated as follows: MRT = AUMC(0_,)/AUC,0,
For multiple dosing, MRT at steady state was estimated 1112 as follows: MRT =
[AUMC,,
+ T[AUC(_,
AUC,,]/AUC(0
in which AUC(,) and AUMCw_;) correspond to AUC and AUMC for one dosing interval, T (12 hours).
The MRT equivalent for bolus intravenous administration was calculated according to the equation: MRT(IV) = MRT (T/2) in which T is the infusion time (1 hour). Total body clearance (CL) was calculated as follows: CL = Dose/AUC. The steady-state volume of distribution (Vss) was obtained by use of the equation: Vss = MRT(IV) (CL).13 The amount of intact didanosine excreted during each collection interval was calculated as the product of the concentration in the corresponding buffered urine sample and the total volume of urine voided in that interval. The total urinary recovery (UR) was calculated as the cumulative amount excreted within the 12- or 24-hour collection period and expressed as a percentage of the administered dose. Renal clearance (CLR) of didanosine was calculated as follows:
CL, = Amount excreted((_,)/AUC(o,)
PHARMACOI. THER
526
Knupp et al.
MAY 1991
The absolute bioavailability (F) of oral didanosine was estimated as follows:
F=
AUC,0
X
Dose,,
AUCw x Dosepo
in which AUCiv is AUC(0_,) for the last intravenous dose administered on day 14 and AUCpo is AUC(3) after the first oral dose on day 16, and AUC(,) after the steady-state oral dose on day 43. The accumulation ratios (R) after multiple administration by both the intravenous and oral routes were calculated by the following different methods.I4 The equations employed in these calculations are as follows: R, =
1/(1 -°`)
= AUC,,,,,ss/AUC(,),)I
Ri = in which AUC(0)I corresponds to the first dose, AUC(o_T)SS corresponds to the last intravenous or oral
dose (days 14 or 43, respectively), Cmjn1 corresponds to the plasma concentration immediately before administration of the second dose, and Csmsm corresponds to the steady-state plasma concentration immediately before administration of any dose at steady state. Statistical analyses. Repeated-measures analysis of variance (ANOVA) was used to determine whether the pharmacokinetic parameters AUC, Cm, t112, MRT, CLR, and UR measured after the first intravenous or oral dose were significantly different from those measured at steady state. For the intravenous route, the parameters CL and Vss were also evaluated. Analyses were conducted separately for each route of administration. Because the phase I study was carried out at two clinical facilities, data for patients from both study sites were combined. Sample sizes were too small within each site for a formal evaluation of their ability to be combined to be performed, but scattergrams of the AUC and Cmax data provided no suggestion that the sites should be analyzed separately. Only patients with both a first-dose and a steady-state observation for a route of administration were included in the analyses. As a result, data from 31 patients after intravenous administration and 25 patients after oral administration were evaluated statistically. Heterogeneity of variance among dose groups and between study days was evident for all parameters. The rank transformationI5 was somewhat successful in stabilizing the variances; however, unless the analysis
of the rank data provided different conclusions, only the unweighted ANOVA findings are reported. If the findings for the two analyses differed, the sign test was used as a further test for the difference between the first dose and steady-state observations. Weighted regression analysis was used to determine whether Cmax and AUC(3_,) at steady state were dose linear or dose proportional. Weights used were the reciprocal variances for each dose group. Analyses were conducted separately for each route of administration. Tests for dose linearity were based on the t test for a linear trend across doses. Tests for nonlinearity were performed with the lack-of-fit F statistic. Dose proportionality was evaluated after dose linearity was observed. This test consisted of a t statistic for the significance of the intercept. If the intercept was not significantly different from zero, a final no-intercept model was fit to assess the decrease in r2 by forcing the line through zero. ANOVA procedures, appropriate for a two-period crossover design, were performed to determine if there were significant mean differences in didanosine pharmacokinetic parameters between the solution with Maalox and citrate-phosphate buffer solution formulations. The bioequivalence assessment of the citratephosphate buffer powder relative to the solution with Maalox was made on the basis of the two one-sided tests procedure. I6 The two formulations were considered to be equivalent if the 90% confidence limits around the mean difference, expressed relative to the reference formulation, fell within the interval (80% to 120%). Confidence intervals were constructed for AUC(o_ ), Cmax, MRT, tu2, and CLR with the mean square error term from the ANOVA model with 6 error degrees of freedom, for eight patients, with the solution with Maalox used as the reference product.
RESULTS Didanosine assays. Plasma samples were analyzed at two different laboratories. Combining data from both sites, the predicted concentrations of the plasma quality-control samples were within 10% of the nominal values, and the percentage of coefficient of variation for interassay (between-day) precision averaged 9%. Urine quality-control samples were within 18% of the nominal values, with interassay precision averaging 10% coefficient of variation. Didanosine was found to be stable in plasma and buffered urine samples for up to 3 hours when heated at 56° C or for a minimum of 12 months at 20° C. These data are indicative of accurate and precise analyses and adequate stability of didanosine in the stored samples.
VOLUME 49 NUMBER 5
Didanosine in HIV patients 3.0 DAY
DAY 14
1
2.5-
mg/kg 0.4 1.0
1.75
3.0
0.5-
0 2
4
6
8
II10 fl12
0
2
4
6
10
112
TIME (HR) AFTER DOSING Fig. 1. Mean didanosine plasma concentrationtime profiles in patients with AIDS after the administration of single and multiple intravenous doses over the range of 0.4 to 3.0 mg/kg.
2
4
6
8
10 12 0 2 4 TIME (HR) AFTER DOSING
10
12
Fig. 2. Mean didanosine plasma concentrationtime profiles in patients with AIDS after the administration of single and multiple intravenous doses over the range of 5.1 to 16.5 mg/kg.
527
CL1N PHARMACOL THER MAY 1991
Knupp etal.
528
Table I. Pharmacokinetic data for patients receiving intravenous infusions of didanosine (nglml)
Dose (mg/kg)
n
0.4
3 3 3 3
1.0 1.75
3.0 5.1
Day
1
14
3
1
3
14
6 6
1751
2818
1
3041
14
5
1
5
14
4 4
14
11.4
3 3
14
4 4
14
1
1
1
5140 5151
8918 8405 13247 11407 17974 18019
MRT (hr)
(hr) Mean
SD
Mean
SD
13 10
0.88 0.93
0.94 0.94
0.25 0.35
249 94 261 718 635 477 1082 767 2882 1682 1257 2328 3341 3131
1.23 1.34 1.45 1.36 1.17 1.19 1.36 1.28 1.72 1.47 1.56 1.36 1.48 1.49
0.30 0.36 0.14 0.38 0.38 0.23 0.27 0.36 0.45 0.32 0.80 0.27 0.16 0.28 0.23 0.17
1.30 1.42 1.09 1.23
0.30 0.34 0.29 0.16 0.10 0.16 0.18
SD
305 291 1089 870 1866
1
14
7.6
16.5
Mean
0.99 0.96 1.14 1.06 1.29 1.29 1.27 1.36 1.26 1.48
0.21 0.67 0.12 0.18 0.28 0.18 0.23
Peak plasma concentration; t1, half-life; MRT, mean residence time; AUC, area under the plasma concentration-time curve; CL, total body clearance; renal clearance; Vs, steady-state volume of distribution; UR; urinary recovery. day-I4 values represent AUC,,,. *Day-I values represent AUC,,,
CL,
Table II. Pharmacokinetic data for patients receiving oral didanosine (mg/kg)
0.8 2.0 3.5
6.0 10.2
15.2
Formulation Buffer§
Buffer§ Buffer§11
Solution with antacid Solution with antacid Solution with antacid
Mean
Day 2
16
2
43
244 343
3
16
721
3
43
799
2 2
16
43
1065 690 1494
6
16
6
43
4 4
43
3
16
3
43
Solution with antacid
3
16
3
43
33.0
Solution with antacid
2 2
43
16
t,
No SD is reported for observations less than 3.
Mean
SD
250 212
923 1112 2285 2051 2307 2960 1574 2998
0.50 0.83 0.83 0.75 0.63 0.67 0.67 0.56 0.69 0.83 0.92 0.75 0.75 1.12
0.88
Mean
SD
0.76
0.63
2257 5650 4915 2339 2381 5671 7032 5787 6041
16
22.8
SD
t2 (hr)
(hr)
(nglml)
C,,
Dose
0.14 0.14
0.44 0.20 0.13 0.24 0.14 0.14 0.25 0.00
1.10 1.62 1.61
1.60 1.15 1.12 1.24 1.37 1.33 1.87 2.74 1.50 1.63 1.64 1.90
0.84 0.72
0.29 0.36 0.16 0.31
0.84 1.34 0.26 0.25
t.
time to reach peak concentration; C,x, Peak plasma concentration; half-life; MRT, mean residence time; AUC, area under the plasma concentrationtime curve; CLR, renal clearance; F, absolute bioavailability; UR, urinary recovery. *ALIC,,,, on day 16 and AUC,.,, on day 43. tMean and SD values for the 6.0 and 15.2 mg/kg dose groups are based on data from (n 1) patients as a result of incomplete UR data in one patient in each of these dose groups. *Mean and SD values for the 10.2 mg/kg dose group are based on data from (n + 1) patients. §Citrate phosphate, pH 7 to 8. 'Patients received solution with antacid on day 43.
-
Pharmacokinetics. Mean didanosine plasma concentration-time curves for the intravenous and oral routes of administration are shown in Figs. 1, 2, 3, and 4, respectively. The mean pharmacokinetic pa-
rameters for didanosine after intravenous and oral administration are shown in Table I and II, respectively. After administration of the first intravenous dose, observed Cmax and AUC(0_,) increased in a linear
VOLUME 49 NUMBER 5
Didanosine in HIV patients
AUC (hr
6992
SD
1.30 1.51
1.96 1.76 1.73 1.60 1.76
0.66 0.72 0.41
0.29 0.17 0.25
2.58 3.39
1.09
2.01
0.48 0.45
2.30 2.56 2.35
SD
Mean
34 35
1074 1074
130 223
132 281
131
768 828
156 67 153 164 301 125 122 183
652 762 432 428 455
843 945 821 781
837 668 668 627
1.68
217 176 40
1295 6051
691
18
730
10324
661
157 187
AUC (hr
MRT (hr)
2.16 2.18 2.14
Mean
801
Mean
461
416 467 499 410 354 451
334 328 355 278
SD
Mean
2203
688 464 360 349 373
1661
341
351 421 1281
1477
349 242
2374 3377 6819 6866 4272 4429 9542
3116 2784 2647 4609 3048
13171
1983
12730 14179
1500 1419
47 140 126 281 175
208
SD
Mean
SD
59.1
8.0
60.3
58.0 61.4 70.9 50.8 62.7 57.0 47.6
17.1
69.1 58.3 52.3 56.1
5.5 13.4 16.8 19.7
80 55
54.1 53.5 48.1 50.9 47.7 56.5 54.1
118
58.6
195
275 159 112 191
492 387 330 362
257 257 301
206
fashion as the dose increased (Fig. 5). On day 1, mean Cma at the end of the 60-minute intravenous infusion ranged from 305 to 17,974 ng/ml for doses over the range of 0.4 to 16.5 mg/kg. Similar values values were found on day 14, with mean C ranging from 291 to 18,019 ng/ml for the same dose range. Mean AUC(0) values after the first intravenous dose ranged from 413 to 28,257 hr ng/ml over the same dose range. Like Cnm, day 14 AUC(,) values were also similar to those obtained after
SD
25.2 19.1
4.3 16.4
22.9 12.8 13.6 18.5 17.5 9.7 5.8 13.2
SD
Mean
52 45
130
65 60 243 347 318 142 10
48 40 32 29 42 54 54
26.1 8.8 3.7
30.6 10.6
5.8 10.4
SD
28.2 41.5 5 3
17 18
25 21
16 17
10 19
24 24
6 6
18 21
6.0 25.9 24.4 22.2 20.2
UR (%)74
42 51 41
68.1
18.6
7.0
Mean
53.7 49.6 56.0 63.3 47.8 51.7 53.5 47.4 49.2 40.8
F (%)
CLR (mIlmin)l
ng1m1)*
UR (%)
V
SD
320 720 652 768 507 1445 1329 3862 3243 2745
13040 12737 21486 19387 28257 32264
CLR (ml/mm)
Mean
SD
413 418 1662 1544 2677 2541 3726 4118 7443
Mean
CL (ml/mm)
ng1m1)*
Mean
529
32.2
3.5 5.7
19.0 10.3 16.6
10.1
26.2
14.3
18.8 25.1
17.7
18.0
5.4 13.1
16.5
8.2
3.5
13.1
3.6
9.6 6.9
the first intravenous dose and ranged from 418 to 32,264 hr ng/ml. In the weighted regression analysis, conducted on parameters calculated at steady state, both Cnia,, and AUC(0_,) were found to be dose linear but not dose proportional. The estimated x intercepts of the regression lines (or the threshold doses) were 0.12 mg/kg for Cma and 0.16 mg/kg for AUC(0). They are unlikely to be of any clinical significance. The AUC," value for 0.4 mg/kg dose group (the lowest dose) was
CLIN PHARMACOL THER
530 Knupp et al.
MAY 1991
6
mg/kg 0.8
DAY 43
DAY 16
5
2.0
0 3.5 4
6.0
o
10.2
3
2
1
0 12
012
4
6
8
12
TIME (HR) AFTER DOSING Fig. 3. Mean didanosine plasma concentrationtime profiles in patients with AIDS after the administration of single and multiple oral doses over the range of 0.8 to 10.2 mg/kg.
8.5
5.0-
0
012
4
6
12
012
12
TIME (HR) AFTER DOSING Fig. 4. Mean didanosine plasma concentrationtime profiles in patients with AIDS after the administration of single and multiple oral doses over the range of 15.2 to 33.0 mg/kg.
VOLUME 49 NUMBER 5
Didanosine in HIV patients
0
0
3
6
9
12
15
531
17
DOSE (mg/kg)
FIRST DOSE
E
+ STEADY
STATE
21
4-
0
7
0 12
18
17
DOSE (mg/kg)
FIRST DOSE
± STEADY
STATE
Fig. 5. Regression plot of peak plasma concentration (Cma) and area under the plasma concentrationtime curve (AUC) versus dose after intravenous administration of didanosine.
much lower than that of the other dose groups after normalizing the data for dose. The value for AUC(0_,.) may have been underestimated in this dose group because of limited sensitivity of the analytic method. Exclusion of the 0.4 mg/kg dose group AUC(0_7) data from the regression analysis resulted in a conclusion of dose proportionality over the range of 1.0 to 16.5 mg/kg. The ANOVA values for Cmax, AUC, MRT, t112, CL, CLR, and Vss indicated no statistically significant differences between the observations after the first intravenous dose on day 1 and the steady-state dose on
day 14. There was also no statistically significant evidence of dose dependency in t112, MRT, CL, CLR, or Vss. The overall CL and CLR were 760 ml/min and 406 ml/min, respectively, excluding the 0.4 mg/kg dose group for the previously cited reasons. The overall t172 was about 1.36 hour and mean values ranged from 0.88 to 1.72 hour. The overall Vss was 54.0 L, with a mean range of 47.7 to 61.4 L. There were no statistically significant differences in UR values after the first intravenous dose compared with steady state. Average UR ranged from 49.2% to
(AIN PlIARMACOL TI1ER
532
Knupp et al.
MAY 1991
15.0 12.5 -
x 7_,
10.0 -
E
x
5.0 -
O
2.512 15 18 21 DOSE (mg/kg) -
FIRST DOSE
24 27 30 33
STEADY STATE
20 620 16
-
12 co L.
8 -
_c
15 18 21 24 27 30 33 DOSE (mg/kg) STEADY STATE -1FIRST DOSE
6 -
9
12
Fig. 6. Regression plot of peak plasma concentration (Cmax) and area under the plasma concentrationtime curve (AUC) versus dose after oral administration of didanosine.
63.3% of dose recovered within 12 hours of didanosine administration on day 1 to 40.8% to 69.1% of dose during the 24-hour collection period after administration on day 14. The majority of the didanosine recovered in the urine was excreted within the first 8 hours after drug administration. On day 16 (first day of oral administration) the administration of single oral doses ranging from 0.8 to 33 mg/kg resulted in mean Cmax values of 244 to 5787 ng/ml, respectively. After 4 weeks of twice-a-day repeated dosing, the mean Cm values remained similar
to those observed after the first dose, ranging from 343 to 7032 ng/ml. There was no statistically significant difference between first oral dose and steady-state
oral Cniax values. After oral administration of didanosine, Cnia values increased in a dose-linear fashion as the dose increased (Fig. 6), although dose proportionality was not observed for the Cmax values. If the three highest dose groups were excluded from the weighted regression analyses, dose proportionality was observed in the range of 0.8 to 10.2 mg/kg. Mean AUC(o_s) values after the first oral dose
VOLUME 49 NUMBER 5
ranged from 351 to 12,730 hr ng/ml over the dose range of 0.8 to 33.0 mg/kg. There were no significant differences between AUC(0_) after the first oral dose and AUC(0_,) at steady state. Results of the weighted regression analysis of the steady-state data indicated that AUC(0_,) was dose proportional (Fig. 6). Mean tmax ranged from 0.50 to 1.12 hour for all dose groups regardless of the number of doses. The mean t112 ranged from 0.76 to 1.87 hour after the first oral dose and the range remained essentially the same (1.10 to 2.74 hour) after 4 weeks of oral administration. The results of the unweighted ANOVA indicated that there were no statistically significant changes in t112 during the 4-week administration period. The overall mean apparent elimination t1,2 was approximately 1.43 hour, which was slightly longer than that observed in the groups given drug intravenously (i.e., 1.36 hour). The comparison between first-dose CLR and UR values and steady-state observations, with unweighted ANOVA, indicated no significant difference in CLR and a significant difference in UR. Average CLR values ranged from 257 to 688 ml/min after the first oral dose and 206 to 464 ml/min after 4 weeks of oral therapy. UR was variable, averaging 8.2% to 28% after the first oral dose and 6.9% to 41.5% after multiple dosing. Of the 24 patients included in the analysis, 17 had steady-state UR values that were greater than those observed after the first oral dose and 7 patients had values that were less than the first dose observations. Although no statistically significant differences were observed, UR tended to decrease at doses equal to or greater than 15.2 mg/kg, supporting the observation that bioavailability decreased at doses equal to or greater than this level. The absolute bioavailability for both days 16 and 43 was estimated for each patient using individual intravenous data for day 14 (Table II). Because oral doses were administered after 2 weeks of intravenous administration, steady-state intravenous data were used as a reference for the calculation of bioavailability. Overall bioavailability for patients receiving 0.8 to 10.2 mg/kg doses of didanosine was approximately 43% (40% for day 16 and 45% for day 43). When the patients received 15.2 mg/kg or more didanosine, F decreased to approximately 24% (23% for day 16 and 25% for day 43). The estimation of the accumulation factor was essentially the same on the basis of three different methods. When didanosine was given at 12-hour intervals, there was no evidence of accumulation (accumulation factor = 1) after intravenous administration. After
Didanosine in HIV patients
533
oral administration, the accumulation factor ranged from 1.0 to 1.30. Blood samples collected to assess the trough concentration of didanosine generally contained less than quantifiable concentrations of didanosine. Bioequivalence assessment. Mean didanosine pharmacokinetic parameter values after administration of a solution with Maalox or a citrate-phosphatebuffered solution are presented in Table III. The results of the ANOVAs indicated that there were no significant sequence or period effects in the evaluation of Cm,, MRT, tu2, AUC, or CLR. The confidence interval assessment for MRT, t1,2, and CLR indicated that the two formulations were equivalent with respect to these parameters. The mean values for were 2145 and 2088 ng/ml for the citrate-phosphate buffer and the solution with Maalox, respectively. The mean estimate of the Cmax ratio, with 90% confidence limits, was 103% (88% to 118%). The respective mean values of AUC(0) were 3530 and 3231 hr ng/ml for the citrate-phosphate buffer and the solution. The mean estimate for the AUC(0_ ) ratio was 109% (96% to 122%). These data indicate that these two didanosine formulations possess very comparable bioavailability.
C.
DISCUSSION The findings of the didanosine pharmacokinetic analyses yield parameter values that generally agree well with reported values. 17 The apparent lack of dose dependence in CL and Vss indicates linear pharmacokinetic behavior for didanosine over the intravenous dose range of 0.4 to 16.5 mg/kg. Values for t2, AUC, CL, CLR, and obtained after singledose administration, do not appear to differ significantly from values calculated after 2 weeks of intravenous therapy, suggesting that the disposition of didanosine remains invariant with repeated administration. The data for C AUC, and elimination rate constant values suggest that there is no accumulation with repeated administration. CLR values exceed the average glomerular filtration rate in humans,I8 indicating that active secretion of didanosine by the renal tubules occurs after both intravenous and oral administration. In vitro experiments have shown that purine nucleosides are transported across cell membranes by facilitated diffusion mechanisms that do not appear to be structure specific. 19 In contrast to this inactive process, the active secretion of 2'-deoxyadenosine and active reabsorption of adenosine have been demonstrated in both a mouse model and humans.202I In a patient with an adenosine deam-
V.
C,
(TIN PHARMACOI, THER
534
Knupp et al.
MAY 1991
Table III. Pharmacokinetic data for patients receiving a 375 mg oral dose of didanosine, formulated as a solution with Maalox or a citrate-phosphatebuffered solution (nglml)
Dose
Formulation
(mg)
Citrate phosphate Maalox solution
375 375
,
Peak plasma concentration;
t,
8 8
Mean
SD
2145 2088
556 532
time to reach peak concentration;
1
t,
(hr)*
0.75 0.50
112
MRT (hr)
(hr)
Mean
SD
Mean
SD
1.59 1.73
0.21
2.07 2.21
0.26 0.32
0.32
t,2, half-life; MRT, mean residence time;
AUC,,
area under the plasma
concentrationtime curve; CLR, renal clearance; UR, urinary recovery. *Median is reported.
inase deficiency, the CL, of 2'-deoxyadenosine was fourfold greater than the measured creatinine clearance. It is conceivable that didanosine is secreted by the same nonselective process responsible for the secretion of 2'-deoxyadenosine and other deoxyribonucleosides structurally related to adenosine.22 CLR of didanosine accounts for approximately 50% of CL, suggesting that nonrenal pathways of elimination, such as metabolism or biliary secretion, play a significant role in the disposition of didanosine. Didanosine is rapidly degraded under the conditions of pH that exist in the human stomach.23 To help neutralize gastric acidity, didanosine was administered as a buffered solution or after the ingestion of an antacid. Although didanosine is absorbed rapidly, as demonstrated by the short tmax, individual patient differences in bioavailability may be a result of variable degradation of didanosine in the gastric environment or concurrent disease conditions that affect gastrointestinal motility and transit time. Bioavailability calculated in this study, approximately 43%, agrees well with the value of 38% reported by investigators from the National Cancer Institute.17 Although there appears to be a trend toward decreased bioavailability in the highest dose groups, dose proportionality was demonstrated at dose levels between 0.8 and 10.2 mg/kg. The apparent decrease in bioavailability of didanosine at the highest dose levels (15.2 mg/kg and above) may be attributable to incomplete absorption or saturation of an absorption process. The didanosine formulation currently used in the phase II trials and expanded access program contains powdered citrate and phosphate buffers, packaged as a sachet. Evaluation of bioavailability of didanosine from the citrate-phosphate buffer formulation relative to that from a solution with Maalox, as used in the phase I trials, demonstrated very comparable bioavailability for the two preparations. Although the two formulations appeared to be bioinequivalent with respect to AUC, based on the 90% confidence limit approach, the absolute difference in AUC(0_00) was only 9%. This slight difference in AUC between the two formula-
tions probably is not clinically relevant because of the wide safety margin of didanosine. Values for several didanosine phannacokinetic parameters were compared with those reported for zidovudine.24 After intravenous administration, the elimination t112 for zidovudine is 1.1 hour, a value similar to the 1.4 hour t112 calculated for didanosine. CL for zidovudine is considerably higher, averaging 1540 ml/min in a 70 kg adult, compared with 800 ml/min for didanosine. Zidovudine distributes more readily into body tissues, as evidenced by a Vss value of approximately 98 L. The apparent bioavailability of zidovudine is 63%, a value greater than that observed for didanosine (43%) in this study. Similar to didanosine, linear pharmacokinetic behavior has been reported for zidovudine over its therapeutic dose range. Although in vitro tests predict that the efficacious concentration for zidovudine is much lower than that for didanosine (1 versus 10 limol/L), the intracellular t1,2 for the active moiety of didanosine, ddATP, is 12 hours or longer.25 In comparison, the intracellular half-life of the analogous compound, zidovudine triphosphate, is similar to the plasma half-life of the parent compound. As a result of the prolonged intracellular half-life of ddATP, didanosine may be administered twice a day (every 12 hours), in contrast to four to six times a day with zidovudine. Less frequent administration coupled with the apparent high therapeutic index for didanosine may offer significant advantages over current therapeutic regimens with zidovudine. Phase II trials designed to address these issues are currently in progress. We thank Elizabeth A. Morgenthien for statistical analysis of the data and Frank A. Stancato, Eugene A. Papp, Gene D. Morse, and Charlene Taylor for excellent technical
assistance.
References 1.
Cooney DA, Ahluwalia G, Mitsuya H, et al. Initial studies on the cellular pharmacology of 2',3'-dideoxy-
VOLUME 49 NUMBERS
Didanosine in HIV patients
(hr
nglml)
CL, (ml/mm)
UR (%)
Mean
SD
Mean
SD
Mean
SD
3530
527 599
432 436
91 131
23.4 20.9
4.0 6.0
3231
adenosine, an inhibitor of HTLV-III infectivity. Biochem Pharmacol 1987;36:1765-8. Mitsuya H, Broder S. Inhibition of the in vitro infectivity and cytopathic effect of human T-Iymphotrophic virus type III/lymphadenopathy-associated virus (HTLVIII/LAV) by 2',3'-dideoxynucleosides. Proc Nat! Acad Sci USA 1986;83:1911-15. Johnson MA, Ahluwalia G, Connelly MC, et al. Metabolic pathways for the activation of the antiretroviral agent 2',3'-dideoxyadenosine in human lymphoid cells. J Biol Chem 1988;263:15354-7. Ahluwalia G, Cooney DA, Mitsuya H, et al. Initial studies on the cellular pharmacology of 2',3'-dideoxyinosine, an inhibitor of HIV infectivity. Biochem Pharmacol 1987;36:3797-800. Lambert JS, Seidlin M, Reichman RC, et al. 2',3'Dideoxyinosine (ddI) in patients with the acquired immunodeficiency syndrome or AIDS-related complex. N Engl J Med 1990;322:1333-40. Cooley TP, Kanches LM, Saunders CA, et al. Oncedaily administration of 2',3'-dideoxyinosine (ddI) in patients with the acquired immunodeficiency syndrome or AIDS-related complex. N Engl J Med 1990; 322:1340-5. Rozencweig M, McLaren C, Beltangady M, et al. Overview of phase I trials of 2',3'-dideoxyinosine (ddI) conducted on adult patients. Rev Infect Dis 1990;12:5570-5. Knupp CA, Stancato FA, Papp EA, Barbhaiya RH. Quantitation of didanosine in human plasma and urine by high-performance liquid chromatography. J Chromatogr 1990;533:282-90. Gibaldi M, Perrier D. Pharmacokinetics. 2nd ed. New York: Marcel Dekker, 1982:409-17. Riegelman S, Collier P. The application of statistical moment theory to the evaluation of in vivo dissolution time and absorption time. J Pharmacokinet Biopharm 1980;8:509-34. Pfeffer M. Estimation of mean residence time from data
535
obtained when multiple-dosing steady-state has been reached. J Pharm Sci 1984;73:854-6. Baner LA, Gibaldi M. Computation of model-independent pharmacokinetic parameters during multiple dosing. J Pharm Sci 1983;72:978-9. Benet LZ, Galeazzi RL. Noncompartmental determination of the steady-state volume of distribution. J Pharm Sci 1979;68:1071-4. Colburn WA. Estimating the accumulation of drugs. J Pharm Sci 1983;72:833-4. Conover WJ, 'man RL. Rank transformation as a bridge between parametric and nonparametric statistics. Am Stat 1981;35:124-33. Schuirmann DJ. A comparison of the two one-sided test procedures and the power approach for assessing the equivalence of average bioavailability. J Pharmacokinet Biopharm 1987;15:657-80. Hartman MR, Yarchoan R, Pluda JM, et al. Pharmacokinetics of 2',3'-dideoxyadenosine and 2',3'-dideoxyinosine in patients with severe human immunodeficiency virus infection. CON PHARMACOL THER 1990; 47:647-54. Altman PL, Dittmer DS. Biology data handbook, vol 3. 2nd ed. Bethesda, Maryland: Federation of American Societies for Experimental Biology, 1974:1992-9. Plagemann GWP, Wohlhueter M. Permeation of nucleosides, nucleic acid bases and nucleotides in animal cells. Curr Top Membr Transplant 1980;14:225-30. Kuttesch JF, Nelson JA. Renal handling of 2'-deoxyadenosine and adenosine in humans and mice. Cancer Chemother Pharmacol 1982;8:221-9. Nelson JA, Vidale E, Enigbokan M. Renal transepithelial transport of nucleosides. Drug Metab Dispos 1988;16:789-92. Nelson JA, Kuttesch IF, Herbert BH. Renal secretion of purine nucleosides and their analogs in mice. Biochem Pharmacol 1983;32:2323-7. Anderson BD, Wygant MB, Xiang TX, et al. Preformulation solubility and kinetic studies of 2',3'-dideoxynucleosides: potential anti-AIDS agents. Int J Pharm 1988;45:27-37. Klecker RW, Collins JM, Yarchoan R, et al. Plasma and cerebrospinal fluid pharmacokinetics of 3'-azido-3'deoxythymidine: a novel pyrimidine analog with potential application for the treatment of patients with AIDS and related diseases. CLIN PHARMACOL THER 1987; 41:407-12. Ahluwalia G, Johnson MA, Fridland A, Cooney DA, Broder S, Johns DG. Cellular pharmacology on the anti-HIV agent 2',3'-dideoxyadenosine. Proc Am Acad Cancer Res 1988;29:349P.
