EurJ Clin Pharmacol (1991) 41:179-183 003169709100197S @ Springer-Verlag 1991

Dose-dependent absorption and elimination of cefadroxil in man T. M. Garrigues ~, U. Martin 2, J. E. Peris-Ribcra ~ and L. E Prescott 2 i Department of Pharmacology and Pharmaceutics, University of Valencia, Valencia, Spain and 2 University Department of Clinical Pharmacology, The Royal Infirmary, Edinburgh, Scotland Received: September 24, 1990/Accepted in revised form: February 12,1991

Summary. The pharmacokinetic behaviour of cefadroxil was dose-dependent in healthy male volunteers following the oral administration of single doses of 5, 15, and 30 mg. kg- l. As the dose of cefadroxil increased from 5 to 15 and 30 rag. kg 1, the peak plasma concentrations, normalized to 5 mg. kg -1, decreased significantly from 15.1 to 10.7 and 7.6 mg. 1 1, while the corresponding normalized areas under the plasma concentration-time curves from 0 to 2 h decreased significantly from 1258 to 946 and 801 min. mg. 1-~. When the same subjects were given 5 mg. kg 1 of cefadroxil together with 45 m g . k g 1 of cephalexin, the absorption of cefadroxil was slowed to a similar or greater extent than with the high dose of cefadroxil. Although the absorption rate decreased as the dose increased, the systemic availability of cefadroxil was essentially complete at all doses, as judged by the 24 h urinary recoveries of the antibiotic. Kinetic analysis of the plasma concentration-time curves gave the best fit with a zero-order followed by a first-order absorption process, consistent with saturable intestinal absorption of cefadroxil. The elimination rate of cefadroxil was directly related to dose and plasma concentrations, and the clearance at the dose of 5 mg. kg- 1 was significantly increased by the simultaneous administration of high-dose cephalexin. The renal clearance of cefadroxil ranged from 98 m l . m i n . l - 1 at total plasma cephalosporin (cefadroxil + cephalexin) concentrations less than 2.5 mg. 1-I to 156 mg. 1-1 at concentrations greater than 40 mg. 1-1. These findings are consistent with saturable active gastrointestinal absorption and renal tubular reabsorption of cefadroxil, with competitive inhibition of both processes by cephalexin. Key words: Cefadroxil; saturable absorption, saturable renal tubular reabsorption, cephalexin, competitive inhibition, pharmacokinetics

Beta-lactam antibiotics, such as amoxycillin, cephalexin, and cefadroxil, are actively absorbed from the rat small intestine [Tsuji et al. 1981; Kimura et al. 1983; Nakashima et al. 1984; Sfnchez-Pic6 et al. 1989], and similar mechanisms may apply in man [Sj6vall et al. 1985]. These drugs may also be subject to active renal tubular secretion [La Rosa et al. 1982] as well as saturable renal tubular reabsorption [Arvidsson et al. 1979]. Such properties could lead to non-linear oral absorption and disposition kinetics, and if this should occur within the usual dosage range, efficacy could be compromised because of reduced systemic availability or increased plasma clearance. However, if clearance is reduced, there may be accumulation with toxicity. Such effects might be exaggerated if higher than usually recommended dosages are used for the treatment of severe infections. Little information is available on this subject in man, and we have therefore studied healthy volunteers using a range of doses of cefadroxil in order to gain insight into its possible non-linear behaviour. In addition, a small dose of cefadroxil was given together with a large dose of cephalexin to investigate the possibility of competitive inhibition of the active transport of the drug. Cefadroxil was selected as the model drug because saturable absorption kinetics have been demonstrated in animals [SfnchezPic6 et al. 1989], and it is commonly used in clinical practice.

Materials and methods

Subjects Six healthy non-smoking male volunteers, aged 18 to 40 y, took part in the study after giving informed consent. Their body weights ranged from 55.0 to 89.1 (mean 75.1, SD 13.5) kg. They did not take alcohol or any drugs on a regular basis and they had no significant past or present medical history (including hypersensitivity to betalactam antibiotics). The findings on physical examination were normal, as were the results of haematological and biochemical screening and urine analysis. The study was approved by the local Ethics of Medical Research Committee.

180

T. M. Garrigues et al.: Cefadroxil absorption and elimination

Table 1. Mean plasma concentrations of cefadroxil in six healthy male volunteers after different oral doses Time

D o s e ( m g . k g 1)

(min)

5

15

10 20 30 45 60 75 90 120 180 240 360 480

1.00 (1.31) 3.03 (2.81) 7.06 (4.84) 11.98 (3.24) 14.69 (3.05) 13.23 (0.74) 13.56 (0.56) 12.31 (1.55) 7.12 (1.53) 4.51 (0.59) 2.06 (0.33) 1.12 (0.26)

0.78 3.73 14.11 26.06 31.30 33.88 33.28 31.37 23.77 15.16 6.81 3.16

(0.76) (4.32) (6.01) (3.17) (3.73) (4.73) (4.65) (6.93) (3.54) (1.91) (1.44) (0.60)

30"

5 + 45 b

3.92 (0.96) 15.95 (4.13) 31.88 (8.62) 46.62 (6.70) 50.89 (5.62) 53.06 (6.17) 53.82 (3.52) 49.29 (1.52) 43.27 (4.13) 33.06 (3.34) 16.13 (2.60) 8.47 (0.72)

0.20 1.19 2.79 5.28 6.96 8.03 8.38 8.38 7.58 5.28 2.33 1.32

(0.29) (0.87) (1.05) (1.26) (1.70) (1.96) (2.22) (1.44) (2.11) (1.61) (0.66) (0.55)

" n = 3; b Cefadroxil 5 mg-kg- ~taken with cephalexin 45 rag. kg-

Study design and blood sampling Cefadroxil (Baxan Suspension, 125 or 250 mg in 5 ml, BristolMyers) was given orally to the 6 volunteers in doses of 5 and 15 rag- kg- ~. Three of the volunteers also received 30 rag. kg- ~. The different doses were given in random order after an overnight fast, with an interval of at least 3 days between each dose. After completion of these studies the six subjects took 5 mg-kg t of cefadroxil together with 45 mg- kg- ~of cephalexin (Keflex Suspension, 250 mg in 5 ml, Eli Lilly). The appropriate volumes of suspension were made up with water to 200 ml and were taken over 2 rain. The volunteers were recumbent for the first 2 h after dosing and remained sitting for the next 6 h. Further 200 ml volumes of water were taken at 2, 4, and 6 h after dosing and lunch after 4 h. Blood was taken from a cannula inserted into a forearm vein at 0, 10, 20, 30, 45, 60, 75, 90,120,180,240,360, and 480 rain after dosing. The samples were heparinized and after centrifugation the plasma was stored in glass tubes at - 40 °C until assayed. Urine was collected before dosing and from 0-2, 2-4, 4-6, 6-8, and 8-24 h. The volume and pH were measured and 20 ml aliquots were stored at - 40 °C. All samples were analysed within 4 days.

Analytical methods Cefadroxil was assayed by high performance liquid chromatography using a Waters Model590 pump, a Model440 U V detector (X = 254 nm), and a Model 710B WISP automatic sample injector in conjunction with a Shimadzu Model C-RIB integrator. A 500 pl volume of plasma was vortexed with 100 gl of internal standard solution in a polypropylene tube and centrifuged. Approximately 80 pl of the supernatant was injected directly on to the column. The internal standard solution contained 0.75 or 50 rag- 1- t of N-propionyl-4-aminophenol (NPA) in 30% perchloric acid. Separations were carried out on a 300 x 3.9 mm stainless steel column packed with C18 silica (Microbondapak) and fitted with a precolumn of the same material. The eluant was a 95:8:5 (v/v) mixture of 0.1 ml. 1-1 acetic acid (pH 3.2), methanol, and acetonitrile. With a flow rate of 1.5 ml- min- ~the retention times of cefadroxil and NPA were 3.6 and 5.8 rain respectively. The calibration graph for cefadroxil standards was linear over the concentration ranges of 0.1 to 5 and 10 to 50 mg. 1-1, and the respective coefficients of variation were 14.6 and 2.9%. Quantification was achieved by relating the unknown peak area ratios to those of simultaneously determined standards. Urine was diluted with water if necessary and 200 gl fractions were mixed with 100 gl of an aqueous solution of 600 mg. 1- ~of NPA. Volumes of approximately 15 gl were injected directly into a Waters 100 x 8 mm Qs silica (Microbondapak) radial compression cartridge fitted with a precolumn. The mobile phase was a 95:10 mixture of 0.1 mol.1 ~ acetic acid and methanol. The retention times of cefadroxil and NPA were 6.2 and 12.1 min respectively at a flow rate of

3 ml. min- ~.When aqueous solutions of cefadroxil were run through the procedure the ass ay was linear over the range of 50 to 1000 rag. 1and the mean coefficient of variation was 5.9%.

Pharmacokinetic analysis Individual and mean plasma concentration curves were analysed using a single compartment model with first-order elimination. The input process was characterized in two ways: (a) as first-order absorption with an absorption lag time, and (b) as zero-order absorption, also with a lag time, followed by first-order absorption so as to give a rough approximation of Michaelis-Menten kinetics. With these models, the plasma drug concentration versus time profiles are described by the following equations: Model (a): C = f" D. k, (e_~.z(t_t@_e_k.(t_t@) (1) Vz (k~ - X~) where C is the concentration of the drug in plasma at any time t, f. D is the dose absorbed, ka and Z~ are first-order rate constants for apparent drug appearance in plasma and apparent drug elimination from it, Vz represents the apparent volume of distribution, and t~,gis the lag time between the administration of drug and its appearance in the plasma. Model (b): Fort)

(2)

Fort>T: k0 C = V~.~.~ (1 - e -~dT t~°g))e-k~(t-Y)qka" Aabs (e - ~At-© - e kdt- T)) (3) Vz (ka - ;2z) where T is the total time elapsed during zero-order absorption kinetics plus the lag time t~g, k0 is the zero-order rate constant expressed in mass/time, A,b~ represents the amount of cefadroxil remaining at the absorption site at time T, and t is the time since dosing. The other symbols are as described above. Pharmacokinetic variables and constants were obtained by nonlinear regression analysis using the MULTI programme described by Yamaoka et al. (1981). The data were weighted using the reciprocals of the experimental values and the relative goodness of fit to each model was judged according to the Akaike Information Criterion (AIC) [Akaike, 1976]. The overall plasma clearance for each dose group, CL, was determined from the classical expression: Ae (oo) CL = ~ (4) where Ae (oo) is the total amount of cefadroxil excreted by 24 h (virtually the total amount of cefadroxil absorbed), and A U C is the total area under the plasma concentration-time curve. Since the urinary recovery of cefadroxilwas essentially complete, the plasma clearance

Table 2. The input (ko, ka) and output (Xz) rate constants, and the A I C values for each cefadroxil dose obtained by fitting the mean observed plasma concentrations to (a) global first-order kinetics (Eq. 1) and to (b) consecutive zero- and first-order kinetics for input with first order output kinetics (Eqs. 2 and 3) Dose Equation i Equations 2 and 3 (mg.kg i) ka ~z AIC ko Ta (h -1) (h 1)values (mg-l-l.h t)(h)

ka %z AIC (h -1) (h -1) values

5 15 30b 5+45 c

2.75 1.70 0.85 1.05

1.20 0.67 1.28 0.60

0.53 17.9 0.62 34.1 0.34 11.9 0.51 7.7

30.3 45.8 86.2 6.2

0.42 0.36 0.72 0.41

0.43 0.41 0.37 0.40

- 3.3 - 18.1 0.8 - 19.9

a T = duration of zero-order kinetics, b n = 3, c Cefadroxil 5 mgkg ~taken with cephalexin 45 mg- kg - 1

T. M. Garrigues et al.: Cefadroxil absorption and elimination

181

5°f ,ok//

0

1

2

3

4 Time (h}

5

6

7

8

Fig. 1. Mean plasma concentrations of cefadroxil in healthy volunteers after the oral administration of 5 (- • -), 15 (- [] -), and 30 (- © -) Ing. kg -~ and after 5 rag. kg-~ of cefadroxil taken with 45 rag. kg-1 of cephalexin (- • -). The corresponding theoretical curves fitting the observed points were calculated according to Eqs.2 and 3, as described in the text was considered to be equal to the renal clearance. Clearances at specified time intervals were estimated by dividing the amount of cefadroxil excreted during each period by the corresponding AUC. The apparent volume of distribution Vz was calculated from the expression: CL Vz = ~ (5) The cumulative fraction of the dose absorbed at each sampling time f (t), was considered to be a suitable approximation of the actual absorption rates. It was calculated through a rearranged form of the classical Wagn er-Nelson equation [Wagner 1979]: A g C 0 - t + C (t)/;Lz

(6)

f (t) AUC As both the absorption and elimination kinetics of cefadroxil were found to be non-linear, systemic availability was calculated from the cumulative urinary excretion data rather than from the AUC values derived from the plasma concentration curves, and f. D was considered to be equivalent to Ae (oo). The results are expressed as means (standard deviations) and the statistical significances of differences between the close groups were determined by analysis of variance using the SPSS-PC program.

lexin are shown in Table 1. T h e s e data were fitted to Eq. 1 (global first-order kinetics) and Eqs. 2 and 3 (consecutive zero- and first-order kinetics for absorption with firsto r d e r elimination). T h e goodness of fit, as indicated by the respective A I C values, and the m e a n input and output rate constants at each dose are shown in Table 2. It is evident that Eqs. 2 and 3 gave the best fit for all dose groups. T h e theoretical m e a n p l a s m a concentration curves, calculated according to consecutive zero- and first-order absorption and first o r d e r elimination rates, c o r r e s p o n d e d closely to the values o b s e r v e d in the volunteers for all dose groups (Fig. 1). T h e m e a n m a x i m u m plasma concentrations (Cm=x), the time to m a x i m u m concentrations (tmax) and the lag times (qag) calculated f r o m the individual data according to Eqs. 2 and 3 are shown in Table 3. T h e r e were statistically highly significant decreases in the normalized C .... f r o m 15.1 (1.9) to 10.7 (1.6) and 7.6 (0.6) mg .1-1, as the dose of cefadroxil increased f r o m 5 to 15 and 30 rag. k g - 1 respectively (P < 0.001). Similarly, there were highly significant decreases in the values for the normalised A U C (0-2). Thus, as the cefadroxil dose increased f r o m 5 to 30 m g . k g -1 the A U C ( 0 - 2 ) fell f r o m 1260 (212) to 801 (84) m i n - m g . 1-1 (p ,~ 0.001). T h e n o r m a l i z e d A U C ( 0 24) did not differ significantly b e t w e e n the groups, but there was a t e n d e n c y for the area to decrease with dose. T h e m e a n values for 5, 15, and 3 0 m g . k g -~ and for 5 m g . kg-1 of cefadroxil c o m b i n e d with 45 mg. kg-1 of cephalexin were 2880 (229), 2640 (198), 2070 (232), and 2380 (588) min .mg. 1-1 respectively. T h e non-linear a b s o r p t i o n of cefadroxil is illustrated in Fig. 2, which represents the n o r m a l i z e d m e a n p l a s m a concentrations for all dose groups t o g e t h e r with the corresponding curves calculated according to Eqs. 2 and 3. T h e data were n o r m a l i z e d according to the respective a m o u n t s excreted in the urine in 24 h r a t h e r than the administered doses. T h e reduction in the rate of a b s o r p t i o n of cefadroxil with increasing dose is also shown by the plots of the cumulative fraction of the dose a b s o r b e d against time for the different dose groups calculated f r o m Eq. 6 (Fig. 3). This indicated that the rate of absorption was r e d u c e d by a p p r o x i m a t e l y a third within the first hour as the dose increased f r o m 5 to 30 m g . kg -~.

Results

Cefadroxilabsorption

Renal clearanceof cefadroxil

T h e m e a n plasma concentrations of cefadroxil after the administration of 5, 15, and 3 0 m g . k g -1 and after 5 m g . k g - ~ of cefadroxil given with 45 rag. k g - 1 of cepha-

T h e r e was essentially quantitative r e c o v e r y of the administered dose of cefadroxil in the urine with all doses (Table 3), and the 24 h urinary r e c o v e r y was t a k e n as an

Table 3. Pharmacokinetic variables and urinary recovery of cefadroxil after different oral doses in six healthy volunteers

Dose (rag- kg- 1)

Cmax(mg. 1-1) Actual Normalized

tm~ (min)

Lag time (min)

5 15.1 (1.9) 15.1 (1.9) 69.7 (16.4) 10.4 15 34.6 (4.8) 10.7 (1.6)*** 83.2 (7.0) 8.6 30a 54.1 (4.2) 7.6 (0.6)*** 80.0 (t5.4) 7.3 5+45 b 8.7 (1.9) 8.7 (1.9)*** 109.8 (19.8)** 10.5 a n = 3; b Cefadroxil 5 mg.kg -1 taken with cephalexin 45 mg.kg-1; * 5 rag-kg- ~dose group

(3.3) (1.3) (0.5) (8.6) P < 0.05,

AUG0_ 2 (rnin -nag. 1-1)

Actual

Urinary recovery (rag- kg-1) (212) 4.6 (0.5) ( 80)* 13.7 (1.7) ( 84)* 32.3 (1.8) (157)*** 4.7 (0.8) P < 0.001 relative to

Normalized

1258 (212) 1258 2839 (239) 946 4630 (505) 801 661 (157) 661 ** P < 0.01, and ***

Absorbed dose (f, %) 91.7 (10.7) 91.1 (11.4) 107.8 (6.1) 93.8 (16.8) values in the

T. M. Garrigues et al.: Cefadroxil absorption and elimination

182

Table 4. Plasma clearances and apparent volumes of distribution of cefadroxil in six healthy subjects after different doses, calculated according to Eqs. 2 and 3

20

Dose (mg- kg- 1)

0)

Clearance (ml. min- 1)

Volume of distribution (1)

5 120.3 (36) 16.2 (6.2) 15 130.3 (39) 19.7 (7.8) BOa 164.3 (11) 26.0 (2.6) 5 + 45 b 149.9 (30)* 21.7 (5,8) a n = 3; b Cefadroxil 5 mg. kg- 1taken with cephalexin 45 mg. kg- 1; * P < 0.05 compared with eefadroxil 5 mg. kg- 1

Q.

"~ 10

1

2

3 Time (hi

4

5

6

Administration of cefadroxil together with cephalexin

Fig.2. Mean plasma concentrations of cefadroxil normalized to the dose of 5 mg. kff~after the administration of 5, 15, and 30 mg. kg z and after 5 mg-kg-a of cefadroxil taken with 45 mg.kff~of cephalexin in healthy volunteers. The symbols are the same as in Fig. 1

0.8

When 5 mg. k g - ~of cefadroxil was given with 45 mg- k g - t of cephalexin, the normalized Cmaxand A U C (0-2) of cefadroxil were reduced, while the tmaxand renal clearance were increased compared with 5 mg. kg -1 of cefadroxil alone. The differences were statistically highly significant and were at least as great as those seen with 30 rag. kg- t of cefadroxil (Tables 2-4, Figs. 2 and 3).

0.6

Discussion

1.0

m

@ 0.4 O

L

0.2

0

I

I

I

I

I

2

3

&

Time (hl

Fig. 3. Cumulative fractions of the dose of cefadroxil absorbed after the administration of 5,15, and 30 mg. kg-1 and after 5 mg. kg 4 of cefadroxil taken with 45 rag. kg-~ of cephalexin in healthy volunteers. The symbols are the same as in Fig. 1

estimate of the systemic availability. The overall plasma clearances and apparent volumes of distribution of cefadroxil, calculated from Eqs.4 a n d 5 respectively, are summarized for each dose group in Table 4. The changes in clearance between doses were statistically significant only with the combination of 5 mg. kg -1 of cefadroxil and 45 m g . k g -1 of cephalexin (P