Disposition of digitoxin in renal failure The disposition of digitoxin was studied for a period of 8 days in 6 uremic patients given a single oral dose of i mg 3 H-digitoxin. in plasma, the time-course of radioactivity indicated a diminished absorption velocity of tritium compared to that of control subjects already reported 20 and, after reaching of a pseudostate-equilibrium at 24 hr, an exponential decline with a mean half-life of 8.0 days. in urine, smaller amounts of tritiated compounds were eliminated in uremic patients (8.7% of the dose) than in controls (22.5%). The average fecal excretion of digitoxin and its metabolites was not significantly increased. Chloroform extraction and thin-layer chromatography in plasma, urine, and feces suggested no qualitative alteration in the metabolism of digitoxin. Calculations of the total body tritium content (body stores) after each 24-hr interval and its pharmacokinetic behavior showed that the elimination of digitoxin is determined by the transfer constant from tissue to plasma. The differences in elimination kinetics of digitoxin and its metabolites of uremic patients and healthy subjects were not significant.
H. F. Vohringer, N. Rietbrock, P. Spurny, J. Kuhlmann, H. Hampl, and R. Baethke Berlin, Germany Department of Clinical Pharmacology, Klinikum Steglitz, Dialysis Center of Klinikum Westend, Free University of Berlin
It is often suggested that the pharmacokinetics of digitoxin in man is not altered in patients with impaired renal function. 1, 6, 9, 12, 13, 15, 17 Only lelliffe 5 and his associates have postulated that a decrease in creatinine clearance would increase the digitoxin plasma halflife leading to greater accumulation. On the other hand, Storstein's investigations 18, 19 gave evidence for enhanced elimination of digitoxin during uremia. The unaltered digitoxin elimination during Supported by the Deutche Forschungsgemeinschaft. Received for publication June 12, 1975. Accepted for publication Oct. 6, 1975. Reprint requests to: Dr. N. Rietbrock, Institut fiir Klinische Pharmakologie, Klinikum Steglitz der FU Berlin, 1000 Berlin, 45, Hindenburgdamm 30 Germany.
uremia may be due to compensatory biliary excretion of digitoxin or its metabolites. 15, 18, 19 The experiments to be reported here were performed in patients with severe renal failure to determine whether the increased metabolic rate or the increased rate of fecal excretion of unchanged digitoxin and its metabolites is responsible for the unaltered elimination of digitoxin during uremia. Materials and methods
The subjects were 6 patients (2 females and 4 males) with severe renal insufficiency. They were studied under closely controlled conditions in our dialysis center. The diagnosis and the respective clinical data of each patient at the time of the investigation are listed in
387
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388
Clinical Pharmacology and Therapeutics
Table I. Clinical data of patients with severe renal insufficiency Plasma-
Sex
Age (yr)
Body weight (kg)
F
57
67.5
2
F
39
60.5
3
M
54
79.0
4
M
32
67.8
5 6
M M
48 61
101.7
Patient
Mean
48.5
69.3
Diagnosis
Renal cystic disease Chronic pyelonephritis Chronic glomerulonephritis Chronic glomerulonephritis Primary gout Nephrolithiasis
74.3
Special abbreviations used dgt digitoxin dgt-bis digitoxigenin-bis-digitoxoside dgt-mono digitoxigenin-mono-digitoxoside dgt-g digitoxigenin dgt-epi 3 epi-digitoxigenin dg digoxin dg-bis digoxigenin-bis-digitoxoside dg-mono digoxigenin-mono-digitoxoside dg-g digoxigenin dg-epi 3 epi-digoxigenin TLC thin-layer chromatography
Table I. None of the patients had previously participated in a hemodialysis program or had received digitalis glycosides. After a standard breakfast, a single dose of 1 mg 21/22 3H_ digitoxin (spec. activity 68 /LCi/mg, radiochemical purity above 95%; Fa. Boehringer, Mannheim) dissolved in 2 ml 98% (v/v) ethanol was administered per os together with 100 ml tea. Venous blood, 10 to 15 ml, was drawn 30 min, 1 hr, 2 hr, 4 hr, 8 hr, and 12 hr after administration and on every morning thereafter for a period of 8 days. Urine samples were collected after 12 and 24 hr; subsequent 24-hr urine and feces samples were obtained during the remaining observation period. Extraction procedure, TLC and determination of radioactivity
The samples of blood, urine, and feces were extracted, purified, and subjected to thin-layer
BUN (mg%)
Serum creatinine (mg%)
Creatinine clearance (mllmin)
Protein conc. (gm%)
Albumin conc. (gm%)
76
8.7
5.4
6.9
3.7
76
15.7
2.4
6.4
3.2
119
9.5
4.8
6.9
3.5
132
24.0
1.4
6.8
3.7
85 85
8.5 12.8
7.2 6.0
6.3 8.8
3.8 4.2
95.5
13.2
4.5
7.0
3.7
chromatography as described previously. 20 An improved procedure of the tritium extraction from the body fluids lead to a mean recovery in plasma of 90.1 ± 0.87%, in urine of 95.0 ± 0.6%, and in feces of 9l.0 ± 0.8%, respectively, from the primary radioactivity count. These results were obtained in comparison to those previously reported,20 from lyophilized plasma samples extracted 4 times with 10 ml dichlormethan/methanol 1: 1 (instead of 3 times). The evaporated CHCI3-s01uble compounds of urine that were eluted from trimagnesium silicate into a flask were redissolved sequentially 3 times (instead of once) in 5 ml of distilled water. The samples of feces were homogenized 5 times (instead of 3) with three volumes of acetone. For the examination of the recovery in feces, a portion of feces was lyophilized and approximately 0.5 gm of the pulverized sample was combusted in a Packard Tri-Carb Sample Oxidizer, Model 306. The increased values of recovery did not alter the fractional distribution of tritium in the TLC. It has been confirmed that the losses caused by an incomplete recovery of tritium as has been described 20 have proportionally affected all of the compounds applied to the starting line of the chromatogram. The TLC was performed on silica gel plates (Fa. Merck, Darmstadt), 0.25 mm layer, flow distance, 15 cm. An aliquot of the CHCL3-s01uble residues was applied to TLC plates. In order to assure that an aliquot of the CHC13-
Digitoxin in renal Jailure
Volume 19 Number 4
. ;;:::. .. ~
E
~
Q.
389
- - mean of uremic patients
------ mean of controls [n tt 6]
5.0 40 3.0
~
.,
2.0
'0
to
0 ."
£
"#.
.5
..,:I:
.8 .6 .5 0 36912
24
48
72
120
96
144
168
192
[hours]
Fig. 1. Tritium concentrations in plasma after an oral dose of 1 mg 3H-digitoxin to 6 uremic patients. The plot is semilogarithmic. (Values of control subjects are reported in CUN. PHARMACOL. THER. 16:796-806, 1974.)
Table II. Pharmacokinetic parameters in uremic patients calculated Jrom plasma concentrations and renal excreted amounts oj tritium per 24 hr after oral dose oj 1 mg 3 H-digitoxin * lmax
*
cmaxt
Half-life in plasma (days)
Half-life in body stores (days)
ke (plasma) (day-l)
ke(body stores) (day-l)
Total clearance (mllmin)
Patient
(min)
1
60 240 120 120 480 30
3.3 2.4 2.2 1.8 3.7 3.1
9.7 5.1 (>10) 10.0 5.3 9.9
15.1 10.9 25.5 23.0 6.9 36.2
0.071 0.136 (0.004) 0.069 0.131 0.070
0.0458 0.0635 0.0272 0.0301 0.1002 0.0192
2.2 3.8 (0.2) 2.7 2.3 3.0
175
2.7
8.0
19.6
0.096
0.0477
2.8
2 3 4 5 6 Mean of uremic group
-------------------- NS ----------------------
Student's t test Mean of control group
40
4.1
6.8 ± 0.47
14.5 ± 2.1
0.104 ± 0.007 0.051 ± 0.007
2.9 ± 0.14
'Values from control group are reported in CUN. PHARMACOL. THER. 16:796·806, 1974. tc max : maximum of the plasma concentration in % of the dose/L plasma: t max : time at whieh e max is achieved.
soluble residues does not produce a disproportionate pattern of the metabolites, several aliquots of the residues were applied to TLC. The fractional distribution of tritium was without exception the same in all chromatograms of the same sample of plasma, urine, and feces. Solvent system I: chloroform/acetone (1 : 1), 4 times; solvent system II: diisopropylether / methanol (9: 1), 5 times. 21 No tritium was found at the origin and the solvent front of the chromatogram. The solvent systems I and II showed the same fractional distribution of tritium among compounds. If digitoxin and the metabolites dgt-mono and dg/dg-bis developed in solvent system I were isolated from the plate and sequentially chromatographed in solvent system II, the same
result was obtained as in solvent system I. The metabolite dgt-bis was rechromatographed in pyridine/chloroform (1 :6), double development (solvent system III), for separation from dg-mono and dg-g. No tritium-labeled compound other than dgt-bis could be detected in solvent system III. In all three solvent systems, digitoxin and the metabolites found in plasma, urine, and feces exhibited the same chromatographic behavior as the authentic compounds. The radioactive bands of samples of urine and feces were located with a radiochromatogram scanner (thin-layer scanner II, Berthold). Quantitative analysis was performed by automatic integration of the activity peaks (Integrator Berthold LB 2437). The tritium between the identified peaks in the chromato-
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390
Clinical Pharmacology and Therapeutics
Uremic patients
Controls
30
. 0
1
2
3
4
5
1
2
3
4
5
6
25 20
."
~ '0
'"
15 10 5
URINE
0
40 35
30
. "
25
~
~
20
'0
15
>It
10 5 0
Fig. 2. Cumulative excretion of tritium in urine and feces in [92 hr following an oral dose of 1 mg 3H-digitoxin to 6 uremic patients. (Values of control subjects are reported in CUN. PHARMACOL. THER. 16:796-806, 1974.)
grams was regarded as blank values. In all chromatograms, the blank value consisted of no more than 6% to 8% of the radioactivity detected in the whole chromatogram. A proportional adjustment of the activity peaks from the blank value was calculated in order to attain a 100% recovery of the CHCla-soluble metabolites. For the evaluation of the total percentage distribution of digitoxin and its metabolites in urine and feces, the CHCl3 -s01uble and CHCl3 -insoluble fractions were related to the total radioactivity, and hence the fractions of tritium in urine and feces are presented as equaling 100% in each subject. For the plasma samples, each sheet of the plate was divided into O.S-cm bands in order to scratch out and to count the radioactivity separately. Nonlabeled reference compounds were crystallized and purified: dgt, dgt-bis, dgt-mono, dgt-g, dgt-epi, dg, dg-bis, dg-mono, dg-g, and dg-epi.
Radioactivity of all samples was assayed in a Packard Tri-Carb liquid scintillation spectrometer, Model 3380. The counting efficiency was determined by the channel ratio method employing an external standard; 10 ml of scintillation fluid (10% naphthalene, 0.98% diphenyloxazole, 0.02% 2,2-p-phenylene-bis-[Sphenyloxazole] in dioxane) was added to each sample. Calculations and statistics
Statistical analysis of the data was performed with Student's t test for unpaired samples; all values are reported as the arithmetical means and standard errors of the means (SEM), unless indicated otherwise. The pharmacokinetic parameters (half-life, elimination rate constant, total clearance) were calculated with a program (Olivetti Programma) based on the method of least squares regression analysis. Results
Kinetics of the total radioactivity in plasma, urine, and feces. After oral administration of I mg 3H-digitoxin in 6 uremic patients, tritium peak levels in plasma were reached between 30 min and 8 hr(mean 2.9) (Fig. 1 and Table II). The mean maximum was 2.7% of the dose per liter plasma (range, 1.8 to 3.7). The maximum level of the plasma concentration was followed in 5 out of 6 patients by a rapid decline and then an increase between 12 and 24 hr after administration. Thereafter the tritium plasma levels decreased slowly with a half-life ranging from S. I to > 10 days. This time-course of tritium concentrations is compared with values from subjects with normal renal function in Fig. 1 and Table II. It is evident that the mean plasma levels of radioactivity in the two groups differ considerably in the first 6 hr after administration of the glycoside. The mean maximum of plasma concentration in uremic patients was reached 2 hr later and remained about 34% lower, indicating slower absorption velocity than in controls. In the later distribution and postdistribution phases, the mean concentration and the average decline of radioactivity (Patient 3 with a halflife of > 10 days is omitted in these calculations) are no longer statistically different. The cumulative excretion of total radio-
Digitoxin in renal failure
Volume 19 Number 4
391
Uremic - Group
Control-Group
PLASMA
dgtdgt bis
dgt dgt 1H5
URINE
do'
tMs
dO'
J~~ do
bis
dg
I
do' ~ do btl
FECES
\
~.~~~ mono bis
dgt bis
bis
I
[
~
dg. ~. meno bis
~
dot do
bis
dg
Fig. 3. TLC-separation of CHCla-soluble fraction in plasma, urine, and feces. (Radiochromatograms from control group are reported in CUN. PHARMACaL. THER. 16:796-806, \974.) Solvent systems: chloroform/acetone (I: I) and diisopropylether/methanol (9: I).
activity in urine and feces is shown in Fig. 2. In patients with severe renal failure there is a decrease of tritium elimination. Within 192 hr, 22% of the given dose is excreted in urine of controls and only 8.7% in that of uremic patients. The fecal excretion is increased in Patients 2 and 5, while in the others it is the same as that in controls. The average amount eliminated accounts for 22.2% of the dose in uremic patients and 13. I % in controls.
Subtracting the total radioactivity excreted in urine and feces from the dose (100%), the total body tritium content (body stores) can be evaluated after each 24-hr interval. In this way the rate of disappearance of radioactivity from the body can be determined. In Table II the half-lives of total radioactivity calculated from log plasma concentration and log body stores vs time are shown for the patients. Only in Patient 3 was plasma half-life time sub stan-
392
Vohringer et al.
Clinical Pharmacology and Therapeutics
Table HI. Chloroform-insoluble fraction of the plasma in percent of the total radioactivity during different collection periods after oral dose of 1 mg 3H-digitoxin* Patient 2 3 4 5 6
Mean of uremic group
48 hr
1 hr
4 hr
12 hr
24 hr
6 4 4 15 12
32 7 9 49 16
33 10 12 65 21
33 15 12 63 18
69 20
22.5
28.6
28.2
29.7
8.1
27 14 18
Student's t test
------------------------------------------ NS -------
Mean of control group
10.7 ± 0.7
* Data from
control group are reported in CUN.
10.6 ± 0.8 PHARMACOL. THER.
tially prolonged. Nevertheless, the half-life in his case of body stores is within the range of values of the other 5 patients. These patients have an average plasma half-life of 8.0 days and a mean rate constant of elimination of 0.096 day-l, from which a total clearance of 2.8 ml/min is calculated. The half-lives in body stores amount to 19.6 days, indicating a lower rate constant of elimination (0.0477 day-I) than that in plasma. The differences between the uremic patients and healthy subjects are not statistically significant. Metabolism of digitoxin in plasma, urine, and feces. When specimens of plasma, urine, and feces are extracted with CHCl3 and the CHCla-soluble extracts are analyzed chromatographically, in all specimens unchanged dgt accounted for most of it (Fig. 3). The other small radioactive areas in plasma correspond to dgt-bis and in urine correspond to hydroxylated products dg and dg-bis and to dgt-bis. In feces dgt-mono is found as a second main label of radioactivity. The comparison of the chromatographic analysis with that of control subjects does not indicate a qualitative alteration in the metabolic degradation of digitoxin in uremia. In the CHCla-insoluble metabolites in plasma of uremic patients (Table III), a wide range (4% to 49% of the total radioactivity) was found in the initial phase after administration. From 12 hr to 192 hr the relationship between CHCla-soluble and CHCla-insoluble fraction was almost constant. An analysis of the data
16.1 ± 1.8
16.8 ± 1.5
22.9 ± 1.6
16:796-806. 1974.
shows that the plasma of Patients 2 and 5 contained a remarkably larger amount of CHClainsoluble metabolites than found in the other patients. The relative composition of tritium-labeled products excreted in urine and feces during the 192-hr experimental period is summarized in Table IV. The major degradation reaction is the conjugation to polar metabolites. Dgt-bis is found in urine as well as in feces, while dgt-mono is excreted only in feces, indicating a bacterial cleavage of the polar compounds in the lower bowel prior to fecal excretion. The hydroxylated metabolites dg and dg-bis are present in urine and feces in very low concentrations. The findings indicate that hydroxylation does not playa significant role in the metabolism of digitoxin in healthy subjects or in uremic patients. Four patients (1, 3, 4, and 6) (Table IV) exhibited a low rate of digitoxin metabolism that can be attributed to a longer tritium half-life in plasma. On the other hand, Patients 2 and 5 had the shortest half-lifes of total radioactivity in body stores as well as in plasma. In both patients dgt was extensively metabolized as evidenced by the relatively larger amounts of polar metabolites in plasma and urine and the relatively lower amounts of unchanged dgt in feces. ' Discussion
The disposition of digitoxin in healthy humans has been characterized by a slow rate of
Digitoxin in renal failure
Volume 19 Number 4
96 hr
26 22 15
144 hr
192 hr
23
22 30 22 67 20
30 32 27 66 20
29.9
32.2
34.9
26.5 ± 0.9
25.6 ± 1.3
27.0 ± 3.2
64
degradation and slow renal and fecal elimination of digitoxin and its metabolites. Within 8 days, 22% of an oral dose is excreted in urine and 13% in feces. 20 Our data on the elimination kinetics of digitoxin in 6 uremic patients whose clearance of creatinine was almost totally abolished (on average 96%) show that after an oral dose of I mg 3H-dgt the mean concentrations of total radioactivity in plasma do not differ significantly from those in normal subjects. The renal elimination of tritiated compounds was reduced to about 60% of that in controls, whereas the average fecal excretion of dgt and its metabolites was not significantly increased. The thin-layer chromatography of samples of plasma, urine, and feces did not indicate differences in metabolism of digitoxin in uremic patients and in normal subjects. Since the calculated half-lives of total radioactivity in plasma and body stores are not significantly different between the two groups, it may be concluded that severly reduced renal function does not lead to a substantial change in the disposition of digitoxin in man. Apparently factors other than renal or fecal elimination determine the disposition of the glycoside in uremic patients as well as in control subjects. Following peak plasma level of radioactivity, 3H-digitoxin was rapidly removed from the plasma and distributed in the tissue. After the establishment of a pseudo-equilibrium state the tritium plasma levels decline monoexponentially. On the assumption that the body behaves as a single compartment, this time-
393
course of radioactivity can be extrapolated to time t = 0 and an initial tritium concentration of 2.2% of the administered dose per liter of plasma will be obtained. Digitoxin is known to be 93% bound to plasma proteins in healthy subjects, while plasma proteins of uremic patients bound 88% (averaged from data in References 4, 7, and 16). After a dose of I mg digitoxin the fictive initial concentration of free, unaltered digitoxin in plasma is calculated to be 2.6 /Lg/L plasma. It is assumed, first, that 58% of the body weight is represented by body water 3 and, second, that the in plasma unbound digitoxin is freely diffusible within the body water. On this basis the following conclusion can be drawn with respect to uremic patients: with an initial concentration of free digitoxin in plasma of 2.6 /Lg/L, the total amount of free digitoxin in the whole body is 113 /Lg or II % of the dose and therefore 89% of the dose is bound to plasma proteins or to tissue-binding sites. Using these calculations for the control subjects, a percentage of 93% is evaluated for the binding of digitoxin to plasma proteins and tissue. After 24 hours, when the total radioactivity in body stores is related to the tritium content in plasma, a ratio of 13.6 (±0.35) is developed in controls and of 16.1 (± 1. 9) in uremic patients. This relationship increases at the end of the experimental period to 19.1 (±0.6) in the controls and to 21.0 (±2.7) in the uremic patients. The differences between both groups are not significant. It can therefore be assumed that most of the digitoxin and its metabolites is contained in the tissue. In uremia the distribution of the glycoside between plasma and tissue resembles that of healthy subjects. Similar conclusions have been reached with respect to the disposition of digoxin in normal subjects and in patients with renal disease. 2 The enterohepatic recirculation of digitoxin and its metabolites is reported to play an outstanding role in the retention of the glycoside. 14 In the present paper the elimination kinetics of 3H-digitoxin is calcualted for the whole organism in which the enterohepatic circulation is included. The half-life time of total radioactivity amounts to 19.6 ± 4.4 days in body stores of uremic patients and 14.5 ± 2.1 days
Vohringer et al.
394
Clinical Pharmacology and Therapeutics
Table IV. Distribution in percent of digitoxin and its metabolites in urine and feces 192 hr after an oral dose of 1 mg 3H-digitoxin* Patient
dg/dg-bis
CHCI 3insoluble
4.0 4.5 2.5 4.0 3.4 3.2
5.6 8.8 7.9 10.1 10.4 7.5
12.9 20.5 8.8 5.4 55.6 16.3
68.1
3.6
8.4
19.9
(NS)
(NS)
dgt
dgt-bis
77.5 66.3 80.7 80.5 30.7 73.0
Mean of uremic group Student's t test
dgt-mono
Urine I 2 3 4 5 6
Mean of control group Feces I 2 3 4 5 6
59.4 ± 7.6
3.1 ± 0.09
50.6 13.0 66.6 28.4 10.6 19.8
3.2 7.3 3.9 4.4 4.8 3.3
Mean of uremic group
31.5
4.5
Student's t test Mean of control group
P < 0.Q25
(NS)
13.0 ± 1.8
24.5 ± 6.3
12.5 38.0 11.8 29.4 16.5 17.9
8.1 7.8 9.5 7.9 10.6 6.4
25.6 32.2 8.2 29.9 53.6 52.7
21.0
8.4
33.7
8.5 ± 1.6
25.2 ± 2.6
NS 43.0 ± 4.2
'Data from control group are reported in CUN.
3.1 ± 0.6 PHARMACOL. THER.
in normal subjects, In plasma, the tritium levels decline with an apparent half-life of 8.0.± 1.1 and 6.8 ± 0.47 days, respectively, If the published data of Lukas 10, 11 on rate of excretion of digitoxin in urine and stool after 1.0 mg of digitoxin are extrapolated to the amount of digitoxin remaining in the body vs time and if biotransformation of the drug is disregarded, a similar difference between the half-lives in plasma and body stores can be evaluated, In our study, the metabolic pattern of plasma samples shows an almost constant relationship between (mainly) the unchanged digitoxin and the CHC13-insoluble metabolites from 12 until 192 hr. These findings do not imply that the fractional distribution of digitoxin and metabolites in tissue resembles that of plasma. From the half-lives of radioactivity in plasma and body stores it is concluded that in uremic patients as well as in normals the excretion velocity of the tritiated compounds is by
20.1 ± 3.5 16:796·806, 1974.
far greater from plasma into urine/feces than from tissue to plasma. These findings suggest the existence of two different rates, one for the transfer of digitoxin and its metabolites from the tissue compartment to the plasma and the other for the transfer of the glycoside from plasma into urine and bile, The rate of transfer from tissue to plasma is suggested to be much lower than the rate of elimination. This is consistent with reported pharmacokinetic calculations of digoxin in man in which the rate constant of elimination klO was estimated 2 to 4 times as great as the rate constant k21 from peripheral to central compartment. 8 On this assumption it follows that the ratelimiting step for the elimination of digitoxin and its metabolites is the transfer constant from tissue to plasma, The low concentrations of tritium in plasma are the result of the low transfer constant, whereas the higher elimination
Volume 19 Number 4
rate constant seems to playa minor role. This is evidenced by the fact that the plasma concentrations of 3H-digitoxin are not significantly increased even when in uremic patients the renal elimination is reduced to about 60% of normal. In uremia a higher rate of fecal excretion can compensate for the diminished excretion of digitoxin and its metabolites in urine, as was observed in 2 out of 6 patients. A higher degradation rate of digitoxin, especially the formation of polar metabolites, is thought to have influenced the increased excretion of total radioactivity compared to that of the other four patients. However, since the elimination kinetics of total radioactivity even of these four patients is not significantly different from the controls, it is concluded that in uremic patients as well as in healthy subjects the disposition of digitoxin is determined by the slow transfer constant from the peripheral to the central compartment of the organism. The authors wish to express their thanks to Mrs. Gabriele Minarek for her excellent technical work.
References 1. Beller, G. A., Smith, T. W., Abelmann, W. H., Haber, E., and Hood, W. B.: Digitalis intoxication. A prospective clinical study with serum level correlations, N. Engl. 1. Med. 284:989997, 1971. 2. Bloom, Ph. M., and Nelp, W. B.: Relationship of the excretion of tritiated digoxin to renal function, Am. 1. Med. Sci. 251: 133-144, 1966. 3. Goldstein, A., Aranow, L., and Kalman, S. M.: Principles of drug action: The basis of pharmacology, New York, 1974, John Wiley & Sons, Inc. 4. Hawlina, A., Rodewig, K., and Rahn, K. H.: Studies on the binding of cardiac glycosides by plasma of patients with renal failure, Naunyn Schmiedebergs Arch. Pharmacol. 285:R 29, 1974. 5. Jelliffe, R. W.: A mathematical analysis of digitalis kinetics in patients with normal and reduced renal function, Math. Biosci. 1:305325, 1967. 6. Kramer, P., Horenkamp, J., Wilms, B., and Scheler, F.: Das Kumulationsverhalten verschiedener Herzglykoside bei Anurie, Dtsch. Med. Wochenschr. 95:444-453, 1970. 7. Kramer, P., Kothe, E., Saul, J., and Scheler, F.: Uraemic and normal plasma protein binding of various cardiac glycosides under "in vivo" conditions, Eur. J. Clin. Invest. 4:53-58, 1974.
Digitoxin in renal failure
395
8. Kramer, W. G., Lewis, R. P., Cobb, T. C., Forester, W. F., Visconti, J. A., Wanke, L. A., Boxenbaum, H. G., and Reuning, R. H.: Pharmacokinetics of digoxin: Comparison of a twoand a three-compartment model in man, J. Pharmacokinet. Biopharm. 2:299-312, 1974. 9. Lahrtz, H. G., Reinhold, H. M., and van Zwieten, P. A.: Serum-konzentration und Ausscheidung von 3H-digitoxin beim Menschen unter normalen und pathologischen Bedingungen, Klin. Wochenschr. 47:695-700, 1969. 10. Lukas, D. S.: Some aspects of the distribution and disposition of digitoxin in man, Ann. N. Y. Acad. Sci. 179:338-361, 1971. II. Lukas, D. S.: The pharmacokinetics and metabolism of digitoxin in man, in Storstein, 0., editor: Symposium on digitalis, Oslo, 1973, Gyldendal Norsk Forlag, pp. 84-102. 12. Lukas, D. S., and Peterson, R. E.: Double isotope dilution derivative assay of digitoxin in plasma, urine, and stool of patients maintained on the drug, J. Clin. Invest. 45:782-795, 1966. 13. Morrison, J., and Killip, T.: Radioimmunassay of digitoxin, Clin. Res. 18:668, 1970. 14. Okita, G. T.: Species difference in duration of action of cardiac glycosides, Fed. Proc. 26: 1125-1130,1967. 15. Rasmussen, K., Jervell, J., Storstein, L., and Gjerdrum, K.: Digitoxin kinetics in patients with impaired renal function, CUN. PHARMACOL. THER. 13:6-14, 1972. 16. Shoeman, D. W., and Azarnoff, D. L.: The alteration of plasma proteins in uremia as reflected in their ability to bind digitoxin and diphenylhydantoin, Pharmacology 7: 169-177, 1972. 17. Smith, T. W.: Radioimmunassay for serum digitoxin concentration: Methodology and clinical experience, J. Pharmacol. Exp. Ther. 175: 352-360, 1970. 18. Storstein, L.: The influence of renal function on the pharmacokinetics of digitoxin, in Storstein, 0., editor: Symposium on digitalis, Oslo, 1973, Gyldendal Norsk Forlag, pp. 158-168. 19. Storstein, L.: Studies on digitalis. II. The influence of impaired renal function on the renal excretion of digitoxin and its cardioactive metabolites, CUN. PHARMACOL. THER. 16:25-34, 1974. 20. Vohringer, H. F., and Rietbrock, N.: Metabolism and excretion of digitoxin in man, CUN. PHARMACOL. THER. 16:796-806, 1974. 21. Watson, E., Tramell, P., and Kalman, S. M.: Identification of submicrogram amounts of digoxin, digitoxin, and their metabolic products. Isolation by chromatography and preparation of derivates for assay by electron capture detector, J. Chromatogr. 69:157-163,1972.
H. F. Vohringer, N. Rietbrock, P. Spurny, J. Kuhlmann, H. Hampl, and R. Baethke Berlin, Germany Department of Clinical Pharmacology, Klinikum Steglitz, Dialysis Center of Klinikum Westend, Free University of Berlin
It is often suggested that the pharmacokinetics of digitoxin in man is not altered in patients with impaired renal function. 1, 6, 9, 12, 13, 15, 17 Only lelliffe 5 and his associates have postulated that a decrease in creatinine clearance would increase the digitoxin plasma halflife leading to greater accumulation. On the other hand, Storstein's investigations 18, 19 gave evidence for enhanced elimination of digitoxin during uremia. The unaltered digitoxin elimination during Supported by the Deutche Forschungsgemeinschaft. Received for publication June 12, 1975. Accepted for publication Oct. 6, 1975. Reprint requests to: Dr. N. Rietbrock, Institut fiir Klinische Pharmakologie, Klinikum Steglitz der FU Berlin, 1000 Berlin, 45, Hindenburgdamm 30 Germany.
uremia may be due to compensatory biliary excretion of digitoxin or its metabolites. 15, 18, 19 The experiments to be reported here were performed in patients with severe renal failure to determine whether the increased metabolic rate or the increased rate of fecal excretion of unchanged digitoxin and its metabolites is responsible for the unaltered elimination of digitoxin during uremia. Materials and methods
The subjects were 6 patients (2 females and 4 males) with severe renal insufficiency. They were studied under closely controlled conditions in our dialysis center. The diagnosis and the respective clinical data of each patient at the time of the investigation are listed in
387
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388
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Table I. Clinical data of patients with severe renal insufficiency Plasma-
Sex
Age (yr)
Body weight (kg)
F
57
67.5
2
F
39
60.5
3
M
54
79.0
4
M
32
67.8
5 6
M M
48 61
101.7
Patient
Mean
48.5
69.3
Diagnosis
Renal cystic disease Chronic pyelonephritis Chronic glomerulonephritis Chronic glomerulonephritis Primary gout Nephrolithiasis
74.3
Special abbreviations used dgt digitoxin dgt-bis digitoxigenin-bis-digitoxoside dgt-mono digitoxigenin-mono-digitoxoside dgt-g digitoxigenin dgt-epi 3 epi-digitoxigenin dg digoxin dg-bis digoxigenin-bis-digitoxoside dg-mono digoxigenin-mono-digitoxoside dg-g digoxigenin dg-epi 3 epi-digoxigenin TLC thin-layer chromatography
Table I. None of the patients had previously participated in a hemodialysis program or had received digitalis glycosides. After a standard breakfast, a single dose of 1 mg 21/22 3H_ digitoxin (spec. activity 68 /LCi/mg, radiochemical purity above 95%; Fa. Boehringer, Mannheim) dissolved in 2 ml 98% (v/v) ethanol was administered per os together with 100 ml tea. Venous blood, 10 to 15 ml, was drawn 30 min, 1 hr, 2 hr, 4 hr, 8 hr, and 12 hr after administration and on every morning thereafter for a period of 8 days. Urine samples were collected after 12 and 24 hr; subsequent 24-hr urine and feces samples were obtained during the remaining observation period. Extraction procedure, TLC and determination of radioactivity
The samples of blood, urine, and feces were extracted, purified, and subjected to thin-layer
BUN (mg%)
Serum creatinine (mg%)
Creatinine clearance (mllmin)
Protein conc. (gm%)
Albumin conc. (gm%)
76
8.7
5.4
6.9
3.7
76
15.7
2.4
6.4
3.2
119
9.5
4.8
6.9
3.5
132
24.0
1.4
6.8
3.7
85 85
8.5 12.8
7.2 6.0
6.3 8.8
3.8 4.2
95.5
13.2
4.5
7.0
3.7
chromatography as described previously. 20 An improved procedure of the tritium extraction from the body fluids lead to a mean recovery in plasma of 90.1 ± 0.87%, in urine of 95.0 ± 0.6%, and in feces of 9l.0 ± 0.8%, respectively, from the primary radioactivity count. These results were obtained in comparison to those previously reported,20 from lyophilized plasma samples extracted 4 times with 10 ml dichlormethan/methanol 1: 1 (instead of 3 times). The evaporated CHCI3-s01uble compounds of urine that were eluted from trimagnesium silicate into a flask were redissolved sequentially 3 times (instead of once) in 5 ml of distilled water. The samples of feces were homogenized 5 times (instead of 3) with three volumes of acetone. For the examination of the recovery in feces, a portion of feces was lyophilized and approximately 0.5 gm of the pulverized sample was combusted in a Packard Tri-Carb Sample Oxidizer, Model 306. The increased values of recovery did not alter the fractional distribution of tritium in the TLC. It has been confirmed that the losses caused by an incomplete recovery of tritium as has been described 20 have proportionally affected all of the compounds applied to the starting line of the chromatogram. The TLC was performed on silica gel plates (Fa. Merck, Darmstadt), 0.25 mm layer, flow distance, 15 cm. An aliquot of the CHCL3-s01uble residues was applied to TLC plates. In order to assure that an aliquot of the CHC13-
Digitoxin in renal Jailure
Volume 19 Number 4
. ;;:::. .. ~
E
~
Q.
389
- - mean of uremic patients
------ mean of controls [n tt 6]
5.0 40 3.0
~
.,
2.0
'0
to
0 ."
£
"#.
.5
..,:I:
.8 .6 .5 0 36912
24
48
72
120
96
144
168
192
[hours]
Fig. 1. Tritium concentrations in plasma after an oral dose of 1 mg 3H-digitoxin to 6 uremic patients. The plot is semilogarithmic. (Values of control subjects are reported in CUN. PHARMACOL. THER. 16:796-806, 1974.)
Table II. Pharmacokinetic parameters in uremic patients calculated Jrom plasma concentrations and renal excreted amounts oj tritium per 24 hr after oral dose oj 1 mg 3 H-digitoxin * lmax
*
cmaxt
Half-life in plasma (days)
Half-life in body stores (days)
ke (plasma) (day-l)
ke(body stores) (day-l)
Total clearance (mllmin)
Patient
(min)
1
60 240 120 120 480 30
3.3 2.4 2.2 1.8 3.7 3.1
9.7 5.1 (>10) 10.0 5.3 9.9
15.1 10.9 25.5 23.0 6.9 36.2
0.071 0.136 (0.004) 0.069 0.131 0.070
0.0458 0.0635 0.0272 0.0301 0.1002 0.0192
2.2 3.8 (0.2) 2.7 2.3 3.0
175
2.7
8.0
19.6
0.096
0.0477
2.8
2 3 4 5 6 Mean of uremic group
-------------------- NS ----------------------
Student's t test Mean of control group
40
4.1
6.8 ± 0.47
14.5 ± 2.1
0.104 ± 0.007 0.051 ± 0.007
2.9 ± 0.14
'Values from control group are reported in CUN. PHARMACOL. THER. 16:796·806, 1974. tc max : maximum of the plasma concentration in % of the dose/L plasma: t max : time at whieh e max is achieved.
soluble residues does not produce a disproportionate pattern of the metabolites, several aliquots of the residues were applied to TLC. The fractional distribution of tritium was without exception the same in all chromatograms of the same sample of plasma, urine, and feces. Solvent system I: chloroform/acetone (1 : 1), 4 times; solvent system II: diisopropylether / methanol (9: 1), 5 times. 21 No tritium was found at the origin and the solvent front of the chromatogram. The solvent systems I and II showed the same fractional distribution of tritium among compounds. If digitoxin and the metabolites dgt-mono and dg/dg-bis developed in solvent system I were isolated from the plate and sequentially chromatographed in solvent system II, the same
result was obtained as in solvent system I. The metabolite dgt-bis was rechromatographed in pyridine/chloroform (1 :6), double development (solvent system III), for separation from dg-mono and dg-g. No tritium-labeled compound other than dgt-bis could be detected in solvent system III. In all three solvent systems, digitoxin and the metabolites found in plasma, urine, and feces exhibited the same chromatographic behavior as the authentic compounds. The radioactive bands of samples of urine and feces were located with a radiochromatogram scanner (thin-layer scanner II, Berthold). Quantitative analysis was performed by automatic integration of the activity peaks (Integrator Berthold LB 2437). The tritium between the identified peaks in the chromato-
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390
Clinical Pharmacology and Therapeutics
Uremic patients
Controls
30
. 0
1
2
3
4
5
1
2
3
4
5
6
25 20
."
~ '0
'"
15 10 5
URINE
0
40 35
30
. "
25
~
~
20
'0
15
>It
10 5 0
Fig. 2. Cumulative excretion of tritium in urine and feces in [92 hr following an oral dose of 1 mg 3H-digitoxin to 6 uremic patients. (Values of control subjects are reported in CUN. PHARMACOL. THER. 16:796-806, 1974.)
grams was regarded as blank values. In all chromatograms, the blank value consisted of no more than 6% to 8% of the radioactivity detected in the whole chromatogram. A proportional adjustment of the activity peaks from the blank value was calculated in order to attain a 100% recovery of the CHCla-soluble metabolites. For the evaluation of the total percentage distribution of digitoxin and its metabolites in urine and feces, the CHCl3 -s01uble and CHCl3 -insoluble fractions were related to the total radioactivity, and hence the fractions of tritium in urine and feces are presented as equaling 100% in each subject. For the plasma samples, each sheet of the plate was divided into O.S-cm bands in order to scratch out and to count the radioactivity separately. Nonlabeled reference compounds were crystallized and purified: dgt, dgt-bis, dgt-mono, dgt-g, dgt-epi, dg, dg-bis, dg-mono, dg-g, and dg-epi.
Radioactivity of all samples was assayed in a Packard Tri-Carb liquid scintillation spectrometer, Model 3380. The counting efficiency was determined by the channel ratio method employing an external standard; 10 ml of scintillation fluid (10% naphthalene, 0.98% diphenyloxazole, 0.02% 2,2-p-phenylene-bis-[Sphenyloxazole] in dioxane) was added to each sample. Calculations and statistics
Statistical analysis of the data was performed with Student's t test for unpaired samples; all values are reported as the arithmetical means and standard errors of the means (SEM), unless indicated otherwise. The pharmacokinetic parameters (half-life, elimination rate constant, total clearance) were calculated with a program (Olivetti Programma) based on the method of least squares regression analysis. Results
Kinetics of the total radioactivity in plasma, urine, and feces. After oral administration of I mg 3H-digitoxin in 6 uremic patients, tritium peak levels in plasma were reached between 30 min and 8 hr(mean 2.9) (Fig. 1 and Table II). The mean maximum was 2.7% of the dose per liter plasma (range, 1.8 to 3.7). The maximum level of the plasma concentration was followed in 5 out of 6 patients by a rapid decline and then an increase between 12 and 24 hr after administration. Thereafter the tritium plasma levels decreased slowly with a half-life ranging from S. I to > 10 days. This time-course of tritium concentrations is compared with values from subjects with normal renal function in Fig. 1 and Table II. It is evident that the mean plasma levels of radioactivity in the two groups differ considerably in the first 6 hr after administration of the glycoside. The mean maximum of plasma concentration in uremic patients was reached 2 hr later and remained about 34% lower, indicating slower absorption velocity than in controls. In the later distribution and postdistribution phases, the mean concentration and the average decline of radioactivity (Patient 3 with a halflife of > 10 days is omitted in these calculations) are no longer statistically different. The cumulative excretion of total radio-
Digitoxin in renal failure
Volume 19 Number 4
391
Uremic - Group
Control-Group
PLASMA
dgtdgt bis
dgt dgt 1H5
URINE
do'
tMs
dO'
J~~ do
bis
dg
I
do' ~ do btl
FECES
\
~.~~~ mono bis
dgt bis
bis
I
[
~
dg. ~. meno bis
~
dot do
bis
dg
Fig. 3. TLC-separation of CHCla-soluble fraction in plasma, urine, and feces. (Radiochromatograms from control group are reported in CUN. PHARMACaL. THER. 16:796-806, \974.) Solvent systems: chloroform/acetone (I: I) and diisopropylether/methanol (9: I).
activity in urine and feces is shown in Fig. 2. In patients with severe renal failure there is a decrease of tritium elimination. Within 192 hr, 22% of the given dose is excreted in urine of controls and only 8.7% in that of uremic patients. The fecal excretion is increased in Patients 2 and 5, while in the others it is the same as that in controls. The average amount eliminated accounts for 22.2% of the dose in uremic patients and 13. I % in controls.
Subtracting the total radioactivity excreted in urine and feces from the dose (100%), the total body tritium content (body stores) can be evaluated after each 24-hr interval. In this way the rate of disappearance of radioactivity from the body can be determined. In Table II the half-lives of total radioactivity calculated from log plasma concentration and log body stores vs time are shown for the patients. Only in Patient 3 was plasma half-life time sub stan-
392
Vohringer et al.
Clinical Pharmacology and Therapeutics
Table HI. Chloroform-insoluble fraction of the plasma in percent of the total radioactivity during different collection periods after oral dose of 1 mg 3H-digitoxin* Patient 2 3 4 5 6
Mean of uremic group
48 hr
1 hr
4 hr
12 hr
24 hr
6 4 4 15 12
32 7 9 49 16
33 10 12 65 21
33 15 12 63 18
69 20
22.5
28.6
28.2
29.7
8.1
27 14 18
Student's t test
------------------------------------------ NS -------
Mean of control group
10.7 ± 0.7
* Data from
control group are reported in CUN.
10.6 ± 0.8 PHARMACOL. THER.
tially prolonged. Nevertheless, the half-life in his case of body stores is within the range of values of the other 5 patients. These patients have an average plasma half-life of 8.0 days and a mean rate constant of elimination of 0.096 day-l, from which a total clearance of 2.8 ml/min is calculated. The half-lives in body stores amount to 19.6 days, indicating a lower rate constant of elimination (0.0477 day-I) than that in plasma. The differences between the uremic patients and healthy subjects are not statistically significant. Metabolism of digitoxin in plasma, urine, and feces. When specimens of plasma, urine, and feces are extracted with CHCl3 and the CHCla-soluble extracts are analyzed chromatographically, in all specimens unchanged dgt accounted for most of it (Fig. 3). The other small radioactive areas in plasma correspond to dgt-bis and in urine correspond to hydroxylated products dg and dg-bis and to dgt-bis. In feces dgt-mono is found as a second main label of radioactivity. The comparison of the chromatographic analysis with that of control subjects does not indicate a qualitative alteration in the metabolic degradation of digitoxin in uremia. In the CHCla-insoluble metabolites in plasma of uremic patients (Table III), a wide range (4% to 49% of the total radioactivity) was found in the initial phase after administration. From 12 hr to 192 hr the relationship between CHCla-soluble and CHCla-insoluble fraction was almost constant. An analysis of the data
16.1 ± 1.8
16.8 ± 1.5
22.9 ± 1.6
16:796-806. 1974.
shows that the plasma of Patients 2 and 5 contained a remarkably larger amount of CHClainsoluble metabolites than found in the other patients. The relative composition of tritium-labeled products excreted in urine and feces during the 192-hr experimental period is summarized in Table IV. The major degradation reaction is the conjugation to polar metabolites. Dgt-bis is found in urine as well as in feces, while dgt-mono is excreted only in feces, indicating a bacterial cleavage of the polar compounds in the lower bowel prior to fecal excretion. The hydroxylated metabolites dg and dg-bis are present in urine and feces in very low concentrations. The findings indicate that hydroxylation does not playa significant role in the metabolism of digitoxin in healthy subjects or in uremic patients. Four patients (1, 3, 4, and 6) (Table IV) exhibited a low rate of digitoxin metabolism that can be attributed to a longer tritium half-life in plasma. On the other hand, Patients 2 and 5 had the shortest half-lifes of total radioactivity in body stores as well as in plasma. In both patients dgt was extensively metabolized as evidenced by the relatively larger amounts of polar metabolites in plasma and urine and the relatively lower amounts of unchanged dgt in feces. ' Discussion
The disposition of digitoxin in healthy humans has been characterized by a slow rate of
Digitoxin in renal failure
Volume 19 Number 4
96 hr
26 22 15
144 hr
192 hr
23
22 30 22 67 20
30 32 27 66 20
29.9
32.2
34.9
26.5 ± 0.9
25.6 ± 1.3
27.0 ± 3.2
64
degradation and slow renal and fecal elimination of digitoxin and its metabolites. Within 8 days, 22% of an oral dose is excreted in urine and 13% in feces. 20 Our data on the elimination kinetics of digitoxin in 6 uremic patients whose clearance of creatinine was almost totally abolished (on average 96%) show that after an oral dose of I mg 3H-dgt the mean concentrations of total radioactivity in plasma do not differ significantly from those in normal subjects. The renal elimination of tritiated compounds was reduced to about 60% of that in controls, whereas the average fecal excretion of dgt and its metabolites was not significantly increased. The thin-layer chromatography of samples of plasma, urine, and feces did not indicate differences in metabolism of digitoxin in uremic patients and in normal subjects. Since the calculated half-lives of total radioactivity in plasma and body stores are not significantly different between the two groups, it may be concluded that severly reduced renal function does not lead to a substantial change in the disposition of digitoxin in man. Apparently factors other than renal or fecal elimination determine the disposition of the glycoside in uremic patients as well as in control subjects. Following peak plasma level of radioactivity, 3H-digitoxin was rapidly removed from the plasma and distributed in the tissue. After the establishment of a pseudo-equilibrium state the tritium plasma levels decline monoexponentially. On the assumption that the body behaves as a single compartment, this time-
393
course of radioactivity can be extrapolated to time t = 0 and an initial tritium concentration of 2.2% of the administered dose per liter of plasma will be obtained. Digitoxin is known to be 93% bound to plasma proteins in healthy subjects, while plasma proteins of uremic patients bound 88% (averaged from data in References 4, 7, and 16). After a dose of I mg digitoxin the fictive initial concentration of free, unaltered digitoxin in plasma is calculated to be 2.6 /Lg/L plasma. It is assumed, first, that 58% of the body weight is represented by body water 3 and, second, that the in plasma unbound digitoxin is freely diffusible within the body water. On this basis the following conclusion can be drawn with respect to uremic patients: with an initial concentration of free digitoxin in plasma of 2.6 /Lg/L, the total amount of free digitoxin in the whole body is 113 /Lg or II % of the dose and therefore 89% of the dose is bound to plasma proteins or to tissue-binding sites. Using these calculations for the control subjects, a percentage of 93% is evaluated for the binding of digitoxin to plasma proteins and tissue. After 24 hours, when the total radioactivity in body stores is related to the tritium content in plasma, a ratio of 13.6 (±0.35) is developed in controls and of 16.1 (± 1. 9) in uremic patients. This relationship increases at the end of the experimental period to 19.1 (±0.6) in the controls and to 21.0 (±2.7) in the uremic patients. The differences between both groups are not significant. It can therefore be assumed that most of the digitoxin and its metabolites is contained in the tissue. In uremia the distribution of the glycoside between plasma and tissue resembles that of healthy subjects. Similar conclusions have been reached with respect to the disposition of digoxin in normal subjects and in patients with renal disease. 2 The enterohepatic recirculation of digitoxin and its metabolites is reported to play an outstanding role in the retention of the glycoside. 14 In the present paper the elimination kinetics of 3H-digitoxin is calcualted for the whole organism in which the enterohepatic circulation is included. The half-life time of total radioactivity amounts to 19.6 ± 4.4 days in body stores of uremic patients and 14.5 ± 2.1 days
Vohringer et al.
394
Clinical Pharmacology and Therapeutics
Table IV. Distribution in percent of digitoxin and its metabolites in urine and feces 192 hr after an oral dose of 1 mg 3H-digitoxin* Patient
dg/dg-bis
CHCI 3insoluble
4.0 4.5 2.5 4.0 3.4 3.2
5.6 8.8 7.9 10.1 10.4 7.5
12.9 20.5 8.8 5.4 55.6 16.3
68.1
3.6
8.4
19.9
(NS)
(NS)
dgt
dgt-bis
77.5 66.3 80.7 80.5 30.7 73.0
Mean of uremic group Student's t test
dgt-mono
Urine I 2 3 4 5 6
Mean of control group Feces I 2 3 4 5 6
59.4 ± 7.6
3.1 ± 0.09
50.6 13.0 66.6 28.4 10.6 19.8
3.2 7.3 3.9 4.4 4.8 3.3
Mean of uremic group
31.5
4.5
Student's t test Mean of control group
P < 0.Q25
(NS)
13.0 ± 1.8
24.5 ± 6.3
12.5 38.0 11.8 29.4 16.5 17.9
8.1 7.8 9.5 7.9 10.6 6.4
25.6 32.2 8.2 29.9 53.6 52.7
21.0
8.4
33.7
8.5 ± 1.6
25.2 ± 2.6
NS 43.0 ± 4.2
'Data from control group are reported in CUN.
3.1 ± 0.6 PHARMACOL. THER.
in normal subjects, In plasma, the tritium levels decline with an apparent half-life of 8.0.± 1.1 and 6.8 ± 0.47 days, respectively, If the published data of Lukas 10, 11 on rate of excretion of digitoxin in urine and stool after 1.0 mg of digitoxin are extrapolated to the amount of digitoxin remaining in the body vs time and if biotransformation of the drug is disregarded, a similar difference between the half-lives in plasma and body stores can be evaluated, In our study, the metabolic pattern of plasma samples shows an almost constant relationship between (mainly) the unchanged digitoxin and the CHC13-insoluble metabolites from 12 until 192 hr. These findings do not imply that the fractional distribution of digitoxin and metabolites in tissue resembles that of plasma. From the half-lives of radioactivity in plasma and body stores it is concluded that in uremic patients as well as in normals the excretion velocity of the tritiated compounds is by
20.1 ± 3.5 16:796·806, 1974.
far greater from plasma into urine/feces than from tissue to plasma. These findings suggest the existence of two different rates, one for the transfer of digitoxin and its metabolites from the tissue compartment to the plasma and the other for the transfer of the glycoside from plasma into urine and bile, The rate of transfer from tissue to plasma is suggested to be much lower than the rate of elimination. This is consistent with reported pharmacokinetic calculations of digoxin in man in which the rate constant of elimination klO was estimated 2 to 4 times as great as the rate constant k21 from peripheral to central compartment. 8 On this assumption it follows that the ratelimiting step for the elimination of digitoxin and its metabolites is the transfer constant from tissue to plasma, The low concentrations of tritium in plasma are the result of the low transfer constant, whereas the higher elimination
Volume 19 Number 4
rate constant seems to playa minor role. This is evidenced by the fact that the plasma concentrations of 3H-digitoxin are not significantly increased even when in uremic patients the renal elimination is reduced to about 60% of normal. In uremia a higher rate of fecal excretion can compensate for the diminished excretion of digitoxin and its metabolites in urine, as was observed in 2 out of 6 patients. A higher degradation rate of digitoxin, especially the formation of polar metabolites, is thought to have influenced the increased excretion of total radioactivity compared to that of the other four patients. However, since the elimination kinetics of total radioactivity even of these four patients is not significantly different from the controls, it is concluded that in uremic patients as well as in healthy subjects the disposition of digitoxin is determined by the slow transfer constant from the peripheral to the central compartment of the organism. The authors wish to express their thanks to Mrs. Gabriele Minarek for her excellent technical work.
References 1. Beller, G. A., Smith, T. W., Abelmann, W. H., Haber, E., and Hood, W. B.: Digitalis intoxication. A prospective clinical study with serum level correlations, N. Engl. 1. Med. 284:989997, 1971. 2. Bloom, Ph. M., and Nelp, W. B.: Relationship of the excretion of tritiated digoxin to renal function, Am. 1. Med. Sci. 251: 133-144, 1966. 3. Goldstein, A., Aranow, L., and Kalman, S. M.: Principles of drug action: The basis of pharmacology, New York, 1974, John Wiley & Sons, Inc. 4. Hawlina, A., Rodewig, K., and Rahn, K. H.: Studies on the binding of cardiac glycosides by plasma of patients with renal failure, Naunyn Schmiedebergs Arch. Pharmacol. 285:R 29, 1974. 5. Jelliffe, R. W.: A mathematical analysis of digitalis kinetics in patients with normal and reduced renal function, Math. Biosci. 1:305325, 1967. 6. Kramer, P., Horenkamp, J., Wilms, B., and Scheler, F.: Das Kumulationsverhalten verschiedener Herzglykoside bei Anurie, Dtsch. Med. Wochenschr. 95:444-453, 1970. 7. Kramer, P., Kothe, E., Saul, J., and Scheler, F.: Uraemic and normal plasma protein binding of various cardiac glycosides under "in vivo" conditions, Eur. J. Clin. Invest. 4:53-58, 1974.
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