Europ.J.clin.Pharmacol. 9, 373-379 (1976) © by Springer-Verlag 1976
Bioavailability and Pharmacokinetics of [3- Methyldigoxin after Multiple Oral and Intravenous Doses* N. Rietbrock, J. Guggenmos, J. Kuhlmann and U. Hess Dept. of Clinical Pharmacology, Klinikum Steglitz der Freien Universit~t Berlin, Federal Republic of Germany
Received: accepted:
March 12, 1975, and in revised November 10, 1975
form:
Berlin,
July 2, 1975,
summary. To obtain true half lives, glycoside elimination from six healthy subjects was studied for 14 days after multiple intravenous doses or oral administration of a daily maintenance dose of B-methyldigoxin 0.3 mg. After oral or intravenous administration of B-methyldigoxin ceased, the plasma concentrations declined from the 14th to the 16th days with a half life of 1.7 days. From the 16th to the 2Oth day a change from a shorter to a longer half life of 2.8 and 2.9 days was observed. Similar half lives were found in urine: after the last dose the initial slope from the 14th to the 16th day had a half life of 1.8 days, and the terminal slope had one of 3.2 days. The results indicate release of the glycoside from slowly equilibrating tissues. Key words: Methyldigoxin, repetitive and i.v. dose, volunteers.
doses,
The pharmacokinetics of digoxin, such as the elimination rate constant and biological half-life, should be related to physiological and clinical facts. After a single intravenous dose, measurement of systolic time interval (QS 2) and left ventricle ejection time (LVET) as indices of the physiological effect of digoxin have shown a good correlation between the biological T I/2 and the T I/2 of digoxin plasma level (Doherty, 1968; Weissler et al., 1972). However, a longer inotropic effect was observed following multiple intravenous or oral doses of digoxin or its derivative ~-methyldigoxin (K~nig and Ohly, 1970; Storz, 1970), which corresponded to an elimination rate of approximately 20 per cent per day (T I/2 = 72 hours). Accordingly, glycoside elimination has been studied in healthy subjects main-
eSupported by the Deutsche Forschungsgemeinschaft
bioavailability,
deep compartments,
oral
tained on single daily intravenous or oral doses of B-methyldigoxin in order to obtain accurate plasma and urine halflives of this glycoside. B-Methyldigoxin in solution is significantly better absorbed than digoxin (Beermann, 1972; Rietbrock and Abshagen, 1973). On intravenous infusion in guineapigs, cats and pigeons the glycoside was about equipotent with digoxin (Schaumann and Wegerle, 1971; Schaumann and Koch, 1974). After single or multiple doses in man the major part of the urinary and biliary excretion of radioactivity appeared to be attached to methyldigoxin and digoxin (Beermann, 1972; Rietbrock and Abshagen, 1973; Zilly et al., 1975). The degree of demethylation was significantly greater after oral administration than after intravenous injection (Rietbrock et al., 1975). The present study was also designed to compare plasma glycoside levels and urinary glycoside excretion rates as indices of b i o a v a i l a b i l i t y after intravenous and oral administration.
374
METHODS
AND M A T E R I A L S
Four h e a l t h y male v o l u n t e e r s (aged 24 33 years) and two w o m e n (aged 33 and 46 years) took part in the studies. All the v o l u n t e e r s gave their consent after disc u s s i n g the hazards w h i c h m i g h t reasonably be anticipated. None of the volunteers had a h i s t o r y of cardiac disease, and all had normal values of blood urea nitrogen, c r e a t i n i n e clearance, serum bilirubin, alkaline phosphatase, serum glutamic o x a l o a c e t a t e t r a n s a m i n a s e and serum glutamic pyruvic transaminase. BM e t h y l d i g o x i n (Lanitop®) 0.3 m g / d a y was given for 14 c o n s e c u t i v e days. Each patient received both the i n t r a v e n o u s and oral forms w i t h a 14-day interval betw e e n the two routes of a d m i n i s t r a t i o n to ensure c o m p l e t e e l i m i n a t i o n of drug from the p r e c e d i n g study. Blood samples from each subject were taken into h e p a r i n i z e d tubes 24 hours after a dose and 7 days after s t o p p i n g a d m i n i s t r a t i o n . Samples were i m m e d i a t e l y c e n t r i f u g e d and the plasma r e m o v e d and stored at -20°C until analyzed. Urine was c o l l e c t e d for p e r i o d s of 24 hours after g l y c o s i d e a d m i n i s t r a tion and 9 - 11 days after stopping medication. A f t e r m e a s u r e m e n t of total volume A a l i q u o t s of urine were stored at -20~C for up to one m o n t h prior to analysis. Both p l a s m a and urine c o n c e n t r a tions were m e a s u r e d by r a d i o i m m u n o a s s a y (Smith et al., 1969; L a r b i g and Kochsiek, 1971, 1972; Larbig et al., 1972; Falch, 1973). Blank values in p l a s m a and urine from each i n d i v i d u a l w e r e d e t e r m i n e d and s u b t r a c t e d from s u b s e q u e n t p l a s m a and urine data. 3H-Digoxin was o b t a i n e d from NEN (Boston, USA) w i t h a specific activity of 8 Ci/mM. The a n t i b o d y (from rabbits) was p r e p a r e d by L a r b i g (THbingen, Federal Republic of Germany) and was d i l u t e d to a final titer of I : 140,OOO. After r e p e a t e d doses of B-methyldigoxin, the c h l o r o f o r m - soluble m e t a b o l i t e s in urine c o n s i s t of d i g o x i n a p p r o x i m a t e l y 30 per cent, B - m e t h y l d i g o x i n 65 per cent, and a i g o x i g e n i n - bis - and d i g o x i g e n i n mono - d i g i t o x o s i d e 5 per cent, respectively (Zilly et al., 1975).Since B -methyld i g o x i n and d i g o x i n have the same affinity for the antiserum, the assay is entirely s a t i s f a c t o r y for b i o a v a i l a b i l i t y and k i n e t i c studies. All samples of urine and plasma were d e t e r m i n e d on one day. The standard dev i a t i o n e v a l u a t e d from repeated assays p e r f o r m e d on the same sample on the same day (O to 7.5 ng d i g o x i n per ml), in b o t h serum and urine and at low and high concentrations, a m o u n t e d to 3 - 4 per cent. A r e c o v e r y of 95 - 1OO per cent of ~m e t h y l d i g o x i n and d i g o x i n was found, when individual sample blanks were used in
the calculation. There was no d e c r e a s e in the c o n c e n t r a t i o n of d i g o x i n in pooled a n a l y s e d p l a s m a after storage for two or four weeks at -20°C. Despite the high rep r o d u c i b i l i t y of the assay, s u f f i c i e n t plasma was c o l l e c t e d from each subject prior to d o s i n g with B - m e t h y l d i g o x i n to p e r m i t p r e p a r a t i o n of a c a l i b r a t i o n curve in each subject's own plasma. A sensivity of O.15 ng per ml was found, w h i c h was d e f i n i t e l y d i f f e r e n t from zero. More than 1OO analyses of normal specimens all gave r e s u l t s in the range of ± O.11 ng/ml of zero. Log p l a s m a c o n c e n t r a t i o n versus time curves were p l o t t e d for each subject, and the slopes c a l c u l a t e d by the m e t h o d of least squares to yield the e l i m i n a t i o n rate c o n s t a n t kel and half - life based on the e q u a t i o n TI/2
=
in2
A two - way analysis of v a r i a n c e was done to d e t e r m i n e s t a t i s t i c a l significance.
RESULTS 1. Plasma Concentrations during "Steady State"
and Urinary Excretion
The m e a n p l a s m a c o n c e n t r a t i o n s in five v o l u n t e e r s p l o t t e d against time after either i n t r a v e n o u s or oral doses of 0.3 mg per day of B - m e t h y l d i g o x i n (Lanitop ®) for 14 days is shown in Fig. I. The observed "steady state" plasma g l y c o s i d e levels lay b e t w e e n 0.93 and 1.27 ng/ml after intravenous a d m i n i s t r a t i o n , and b e t w e e n O.61 and 0.89 ng/ml after oral dosing (Table I). This indicates that the a v e r a g e g l y c o s i d e levels o b t a i n e d after i n t r a v e n o u s i n j e c t i o n of B - m e t h y l d i g o x i n were 35 per cent higher than after oral a d m i n i s t r a t i o n of B-methyldigoxin. U r i n a r y g l y c o s i d e e x c r e t i o n rates during and after the steady state provide further c o r r o b o r a t i o n of the results. The daily e x c r e t i o n rates in the five volunteers m a i n t a i n e d on a fixed daily dose of B - m e t h y l d i g o x i n are shown in Fig. 2. During the 14 day period the subjects excreted m o r e g l y c o s i d e following intravenous i n j e c t i o n (38 per cent of the dose) than after i n g e s t i o n of tablets (29 per cent of the dose). Under steady state c o n d i t i o n s an a v e r a g e of 137 ~g (range 106 - 152 ~g) and 102 ~g (range 69 - 123 ug) of a m a i n t e n a n c e dose of 0.3 mg was e x c r e t e d d a i l y after i n t r a v e n o u s and oral a d m i n i s t r a t i o n of B-methyldigoxin, res p e c t i v e l y (Table 2). Therefore, the bioa v a i l i b i l i t y of the tablet r e l a t i v e to the i n t r a v e n o u s solution was 75 per cent (range 65 - 83 per cent).
375 Lanitop iv. Lanffop p,o. . . . . . . .
Bioavailability and Pharmacokinetics of [3- Methyldigoxin after Multiple Oral and Intravenous Doses* N. Rietbrock, J. Guggenmos, J. Kuhlmann and U. Hess Dept. of Clinical Pharmacology, Klinikum Steglitz der Freien Universit~t Berlin, Federal Republic of Germany
Received: accepted:
March 12, 1975, and in revised November 10, 1975
form:
Berlin,
July 2, 1975,
summary. To obtain true half lives, glycoside elimination from six healthy subjects was studied for 14 days after multiple intravenous doses or oral administration of a daily maintenance dose of B-methyldigoxin 0.3 mg. After oral or intravenous administration of B-methyldigoxin ceased, the plasma concentrations declined from the 14th to the 16th days with a half life of 1.7 days. From the 16th to the 2Oth day a change from a shorter to a longer half life of 2.8 and 2.9 days was observed. Similar half lives were found in urine: after the last dose the initial slope from the 14th to the 16th day had a half life of 1.8 days, and the terminal slope had one of 3.2 days. The results indicate release of the glycoside from slowly equilibrating tissues. Key words: Methyldigoxin, repetitive and i.v. dose, volunteers.
doses,
The pharmacokinetics of digoxin, such as the elimination rate constant and biological half-life, should be related to physiological and clinical facts. After a single intravenous dose, measurement of systolic time interval (QS 2) and left ventricle ejection time (LVET) as indices of the physiological effect of digoxin have shown a good correlation between the biological T I/2 and the T I/2 of digoxin plasma level (Doherty, 1968; Weissler et al., 1972). However, a longer inotropic effect was observed following multiple intravenous or oral doses of digoxin or its derivative ~-methyldigoxin (K~nig and Ohly, 1970; Storz, 1970), which corresponded to an elimination rate of approximately 20 per cent per day (T I/2 = 72 hours). Accordingly, glycoside elimination has been studied in healthy subjects main-
eSupported by the Deutsche Forschungsgemeinschaft
bioavailability,
deep compartments,
oral
tained on single daily intravenous or oral doses of B-methyldigoxin in order to obtain accurate plasma and urine halflives of this glycoside. B-Methyldigoxin in solution is significantly better absorbed than digoxin (Beermann, 1972; Rietbrock and Abshagen, 1973). On intravenous infusion in guineapigs, cats and pigeons the glycoside was about equipotent with digoxin (Schaumann and Wegerle, 1971; Schaumann and Koch, 1974). After single or multiple doses in man the major part of the urinary and biliary excretion of radioactivity appeared to be attached to methyldigoxin and digoxin (Beermann, 1972; Rietbrock and Abshagen, 1973; Zilly et al., 1975). The degree of demethylation was significantly greater after oral administration than after intravenous injection (Rietbrock et al., 1975). The present study was also designed to compare plasma glycoside levels and urinary glycoside excretion rates as indices of b i o a v a i l a b i l i t y after intravenous and oral administration.
374
METHODS
AND M A T E R I A L S
Four h e a l t h y male v o l u n t e e r s (aged 24 33 years) and two w o m e n (aged 33 and 46 years) took part in the studies. All the v o l u n t e e r s gave their consent after disc u s s i n g the hazards w h i c h m i g h t reasonably be anticipated. None of the volunteers had a h i s t o r y of cardiac disease, and all had normal values of blood urea nitrogen, c r e a t i n i n e clearance, serum bilirubin, alkaline phosphatase, serum glutamic o x a l o a c e t a t e t r a n s a m i n a s e and serum glutamic pyruvic transaminase. BM e t h y l d i g o x i n (Lanitop®) 0.3 m g / d a y was given for 14 c o n s e c u t i v e days. Each patient received both the i n t r a v e n o u s and oral forms w i t h a 14-day interval betw e e n the two routes of a d m i n i s t r a t i o n to ensure c o m p l e t e e l i m i n a t i o n of drug from the p r e c e d i n g study. Blood samples from each subject were taken into h e p a r i n i z e d tubes 24 hours after a dose and 7 days after s t o p p i n g a d m i n i s t r a t i o n . Samples were i m m e d i a t e l y c e n t r i f u g e d and the plasma r e m o v e d and stored at -20°C until analyzed. Urine was c o l l e c t e d for p e r i o d s of 24 hours after g l y c o s i d e a d m i n i s t r a tion and 9 - 11 days after stopping medication. A f t e r m e a s u r e m e n t of total volume A a l i q u o t s of urine were stored at -20~C for up to one m o n t h prior to analysis. Both p l a s m a and urine c o n c e n t r a tions were m e a s u r e d by r a d i o i m m u n o a s s a y (Smith et al., 1969; L a r b i g and Kochsiek, 1971, 1972; Larbig et al., 1972; Falch, 1973). Blank values in p l a s m a and urine from each i n d i v i d u a l w e r e d e t e r m i n e d and s u b t r a c t e d from s u b s e q u e n t p l a s m a and urine data. 3H-Digoxin was o b t a i n e d from NEN (Boston, USA) w i t h a specific activity of 8 Ci/mM. The a n t i b o d y (from rabbits) was p r e p a r e d by L a r b i g (THbingen, Federal Republic of Germany) and was d i l u t e d to a final titer of I : 140,OOO. After r e p e a t e d doses of B-methyldigoxin, the c h l o r o f o r m - soluble m e t a b o l i t e s in urine c o n s i s t of d i g o x i n a p p r o x i m a t e l y 30 per cent, B - m e t h y l d i g o x i n 65 per cent, and a i g o x i g e n i n - bis - and d i g o x i g e n i n mono - d i g i t o x o s i d e 5 per cent, respectively (Zilly et al., 1975).Since B -methyld i g o x i n and d i g o x i n have the same affinity for the antiserum, the assay is entirely s a t i s f a c t o r y for b i o a v a i l a b i l i t y and k i n e t i c studies. All samples of urine and plasma were d e t e r m i n e d on one day. The standard dev i a t i o n e v a l u a t e d from repeated assays p e r f o r m e d on the same sample on the same day (O to 7.5 ng d i g o x i n per ml), in b o t h serum and urine and at low and high concentrations, a m o u n t e d to 3 - 4 per cent. A r e c o v e r y of 95 - 1OO per cent of ~m e t h y l d i g o x i n and d i g o x i n was found, when individual sample blanks were used in
the calculation. There was no d e c r e a s e in the c o n c e n t r a t i o n of d i g o x i n in pooled a n a l y s e d p l a s m a after storage for two or four weeks at -20°C. Despite the high rep r o d u c i b i l i t y of the assay, s u f f i c i e n t plasma was c o l l e c t e d from each subject prior to d o s i n g with B - m e t h y l d i g o x i n to p e r m i t p r e p a r a t i o n of a c a l i b r a t i o n curve in each subject's own plasma. A sensivity of O.15 ng per ml was found, w h i c h was d e f i n i t e l y d i f f e r e n t from zero. More than 1OO analyses of normal specimens all gave r e s u l t s in the range of ± O.11 ng/ml of zero. Log p l a s m a c o n c e n t r a t i o n versus time curves were p l o t t e d for each subject, and the slopes c a l c u l a t e d by the m e t h o d of least squares to yield the e l i m i n a t i o n rate c o n s t a n t kel and half - life based on the e q u a t i o n TI/2
=
in2
A two - way analysis of v a r i a n c e was done to d e t e r m i n e s t a t i s t i c a l significance.
RESULTS 1. Plasma Concentrations during "Steady State"
and Urinary Excretion
The m e a n p l a s m a c o n c e n t r a t i o n s in five v o l u n t e e r s p l o t t e d against time after either i n t r a v e n o u s or oral doses of 0.3 mg per day of B - m e t h y l d i g o x i n (Lanitop ®) for 14 days is shown in Fig. I. The observed "steady state" plasma g l y c o s i d e levels lay b e t w e e n 0.93 and 1.27 ng/ml after intravenous a d m i n i s t r a t i o n , and b e t w e e n O.61 and 0.89 ng/ml after oral dosing (Table I). This indicates that the a v e r a g e g l y c o s i d e levels o b t a i n e d after i n t r a v e n o u s i n j e c t i o n of B - m e t h y l d i g o x i n were 35 per cent higher than after oral a d m i n i s t r a t i o n of B-methyldigoxin. U r i n a r y g l y c o s i d e e x c r e t i o n rates during and after the steady state provide further c o r r o b o r a t i o n of the results. The daily e x c r e t i o n rates in the five volunteers m a i n t a i n e d on a fixed daily dose of B - m e t h y l d i g o x i n are shown in Fig. 2. During the 14 day period the subjects excreted m o r e g l y c o s i d e following intravenous i n j e c t i o n (38 per cent of the dose) than after i n g e s t i o n of tablets (29 per cent of the dose). Under steady state c o n d i t i o n s an a v e r a g e of 137 ~g (range 106 - 152 ~g) and 102 ~g (range 69 - 123 ug) of a m a i n t e n a n c e dose of 0.3 mg was e x c r e t e d d a i l y after i n t r a v e n o u s and oral a d m i n i s t r a t i o n of B-methyldigoxin, res p e c t i v e l y (Table 2). Therefore, the bioa v a i l i b i l i t y of the tablet r e l a t i v e to the i n t r a v e n o u s solution was 75 per cent (range 65 - 83 per cent).
375 Lanitop iv. Lanffop p,o. . . . . . . .
