Eur J Clin Pharmacol (1990) 38: S 108-S 111

© Springer-Verlag1990

Stereoselective disposition of carvedilol in man after intravenous and oral administration of the racemic compound G. N e u g e b a u e r , W. A k p a n , B. K a u f m a n n , a n d K. R e i f f Clinical Pharmacology, Boehringer Mannheim GmbH, Mannheim, Federal Republic of Germany

Summary. T h e racemic c o m p o u n d carvedilol is a multipleaction oral antihypertensive drug that exhibits b o t h vasodilator and non-selective beta-adrenergic blocking activities. The effects of the levorotatory S-enantiomer IS ( - )-CARV] are vasodilatation and beta-blockade. The R ( + )-enantiomer [R ( + )-CARV] is a pure vasodilating agent. Quantitative determination of the enantiomers in h u m a n plasma by H P L C was carried out after formation of diastereoisomers with the chiral reagent 2,3,4,6-tetraO-acetyl-[3-D-glucopyranosyl isothiocyanate (GITC). The pharmacokinetics of the enantiomers were studied following i. v. (12.5 mg in I h) and p. o. (50 mg) administration of racemic carvedilol in ten healthy male subjects according to a randomized crossover design. The A U C s of S ( - ) - C A R V were significantly lower than those of R ( + ) - C A R V after both i.v. and p. o. administration. The systemic clearance of the two enantiomers was significantly different, whereas half-lives and apparent distribution volumes were comparable. Following p. o. administration, the absolute bioavailability (31.1% and 15.1%, respectively) and maximal plasma concentrations of R ( + ) C A R V were twice those of S ( - ) - C A R V . A similar difference was found in the half-lives. A close correlation existed between enantiomeric ratios after i.v. and after p.o. administration, demonstrating slight intraindividual variability. The preferential systemic clearance of the S ( - )-enantiomer suggests stereoselective hepatic metabolism of carvedilol, becoming especially apparent after p.o. administration. The small intrasubject variability in enantiomer ratios indicates a relatively constant relation of beta-blockade to vasodilation during chronic treatment.

Key words: carvedilol, enantiomer pharmacokinetics, bioavailability, first-pass effect Carvedilol is a multiple-action, oral antihypertensive drug that exhibits both vasodilator and non-selective [3adrenergic blocking activities. Carvedilol is a racemic

c o m p o u n d whose enantiomers have different pharmacologic actions. In contrast to the levotatory S-enantiomer, the R ( + )-enantiomer has no detectable [3-blocking activity. The ratio of pAl0 values of alphal-blockade is 0.7 for S ( - )-carvedilol over R ( + )-carvedilol (Bartsch et al., this issue). Therefore, both enantiomers are nearly equipotent vasodilator agents. Since stereoselective differences in pharmacokinetics have b e e n detected for several racemic compounds, it was of interest to investigate the disposition of both carvedilol enantiomers in man.

Materials, methods and subjects The pharmacokinetics of the enantiomers of carvedilol were studied in ten healthy male subjects with a mean age of 29.5 years (range, 2139 years), a mean height of 172.9 cm (range 160-184 cm) and a mean body weight of 73.9 kg (range, 56.5-98 kg). The subjects were free of disease according to physical examination, blood chemistry and urinalysis and took part after having given their written informed consent. The study was approved by an ethical committee. In an open, randomized, two-period, crossover study, the subjects received either 12.5 mg racemic carvedilol infused i.v. within i h (5-rag ampoules, batch 81984559D) or 50 mg p.o. (25-mg tablets, batch 81989620B) after an overnight fast. The treatments were repeated after a 1-week washout period. Serial blood samples were taken at intervals up to 24 h after the start of infusion or after oral administration. Plasma was separated, divided into two aliquots and kept frozen at - 20 °C until later analysis. Racemic carvedilol was determined in one aliquot of the samples according to the procedure published by Reiff (1987). This method is an internally standarized procedure using liquid-liquid extraction, followed by re-extraction and reversed-phase HPLC separation with fluorometric detection. A stereospecific HPLC method for the determination of the enantiomers of carvedilol was developed and applied to the analysis of plasma. A mixture of 500 gl plasma, 50 p.1 of an internal standard solution (BM 14.225) and 500 gl 0.1 ml/1 (BR-buffer (pH = 8) was prepared. Racemic carvedilol and the racemic internal standard were extracted with 5 ml diethylether. After phase separation, carvedilol and the internal standard were derivatized with stereochemically pure 2,3,4,6,-tetra-O-acetyl-[3-Dglucopyranosyl isothiocyanate (GITC) to the resultant diastereomers within 30 min at room temperature. After addition of 0.05 M sulfuric acid (pH 3.0 with NaOH) the mixture was shaken and cen-

G. Neugebauer et al.: Stereoselective disposition of carvedilol

S 109

200-

~150" c

..~100 + +

50-

_ / i v . f

0

•

i

adm. y= 1 . 0 9 . x + 0 . 1 6 r= 0.996 I

I

50 100 HPLC(rac)

I

150 200 (ng/ml)

Fig.1. Correlation between plasma concentrations of carvedilol determined by conventional HPLC and by the enantioselective method

y = 1 . 0 5 * x + O . 19 I I r--O .998I I 50 100 150 200 HPLC ( r a c ) (ng/ml)

0

(ng/ml) IO0

(ng/ml) 100

8O

8O

6O

6O

40

4O

20

~XXT

•

R (+) Corr.

20

,-

R (+) Caw.

0

0

2

4

6

8

10

12 14 (hours)

0

2

4

6

8

10 12 14

24 (hours)

Fig.2. Concentration-time course of R ( + )- and S ( - )-carvedilol (Carv.) after a 1-h infusion of 12.5 mg racemic carvedilol in ten

Fig.3. Concentration-time course of R ( + )- and S ( - )-carvedilol after 50 mg oral racemic carvedilol (Carv.) in ten healthy male sub-

healthy male subjects (medians and quartiles)

jects (medians and quartiles)

trifuged. The organic phase was separated and evaporated to dryness by a stream of nitrogen at 40 °C. The residue was dissolved in 250 btl mobile phase, and 100 gl was injected onto the column. The HPLC system consisted of a Shimadzu LC-6A solvent delivery system, a Perkin-Elmer ISS 100 sample injector, and a Perkin-Elmer LS4 fluorometer (excitation 285nm, slit 10/emission 360nm, slit 20). The column (125 x4.6 mm inside diameter) was slurrypacked with Nucleosil 120-5 C-18 (Macherey and Nagel, Dtiren, FRG). The mobile phase consisted of 56% 0.02 mol/1 phosphate buffer (pH 3) and 44% acetonitrile. The flow rate was 2.6 ml/min under isocratic conditions. Total analysis time was 26.5 min. Calibration graphs were prepared by assaying plasma samples to which known amounts of racemic carvedilot or different concentrations of the enantiomers of carvedilol had been added. Peak height ratios of each enantiomer relative to internal standard were plotted against the known enantiomer concentrations. The resulting correlations were linear in each case, the coefficient of correlation being between 0.9900 and 0.9999. The lower limit of quantitation is about 1 ng/ml for both S( - )- and racemic carvedilol and about 2 ng/ml for R ( + )-carvedilol. An excellent correlation existed between the racemic carvedilol concentrations and the sum of the enantiomer concentrations over the whole range of plasma concentrations (Fig. 1). The correlation coefficient was 0.99.

Non-compartmental kinetic data were calculated by conventional methods. The Wilcoxon test for paired data was used for statistical comparisons, with a significance level of alpha = 0.05.

Results T h e c o n c e n t r a t i o n - t i m e curves o f R ( + )- a n d S ( - )-carv e d i l o l a f t e r i.v. infusion are s h o w n in Fig. 2. T h e e n a n t i o m e r s e x h i b i t e d m o r e o r less p a r a l l e l i n c r e a s e s a n d d e clines, t h e S ( - ) - e n a n t i o m e r always d e m o n s t r a t i n g l o w e r c o n c e n t r a t i o n s . P h a r m a c o k i n e t i c p a r a m e t e r s are l i s t e d in T a b l e 1. Since in f o u r s u b j e c t s t h e S ( - ) - e n a n t i o m e r r e a c h e d the q u a n t i f i c a t i o n limit t o o early, n o t e n o u g h d a t a p o i n t s e x i s t e d for a valid c a l c u l a t i o n o f half-life or, c o n s e q u e n t l y , o f c l e a r a n c e a n d a p p a r e n t d i s t r i b u t i o n volu m e . H o w e v e r , t h e r e w e r e significant d i f f e r e n c e s in t h e A U C s ( e n a n t i o m e r i c R / S r a t i o , 1.3; r a n g e , 1.04-1.9), m a x i m a l c o n c e n t r a t i o n s ( e n a n t i o m e r i c ratio, 1.2), a n d s y s t e m i c c l e a r a n c e , w h i c h was 9.4% h i g h e r for t h e Sform.

S 110

G. Neugebauer et al.: Stereoselective disposition of carvedilol

Table 1. Pharmacokinetic parameters of R ( + )- and S ( - )-carvedilol following i. v. administration of 12.5 mg and p. o. administration of 50 mg racemic carvedilol and enantiomeric ratios [R ( + )IS ( - )] in 10 subjects (median values with ranges in parentheses) Route of administration

Enantiomer

A U C 0-24 (ng. h. ml- 1)

ha (h)

Cm~x (ng. ml - 1)

C1/P (ml. min - 1)

Vz (1)

f (% )

i.v.

R(+

172" (139-325) 129

3.5 (2.0-7.1) 3.2 [6]b

85* (65-107) 70

605* (310-747) 662 [6]

100

s(-)

(76-178)

(2.1-3.8)

(51-85)

(582-858)

170 (103-251) 18816] (105-284)

R(+)

214" (138-605) 78 (32-163)

9.6* (4.1-19.4) 22.1 [7] (4.7-72.5)

74* (46-138) 29 (11-48)

1791 (648-2867) 2439 [7] (1601-6256)

-

31.1 (20.2-46.5) 15.1 (10.4-24.4)

1.3 (1.0-1.9) 2.7 (1.6-4.4)

1.1 (0.6-3.5) 0.4 (0.2-1.6)

1.2 (1.1-1.4) 2.6 (1.7-4.3)

0.9 (0.5-1.0) 0.7 (0.3-1.7)

p.o.

s(-)

i.v.

R(+)/S(-)

p.o.

R(+)/S(-)

-

0.9 (0.7-1.8) -

100

2.0 (1.5-2.4)

* P < 0.05 [Wilcoxon test for paired data, R ( + ) vs S ( - )] After i.v. administration f ---1 b Numbers of evaluable subjects are shown in square brackets

The difference in AUCs was even wider after p. o. administration (Fig. 3, Table 1), with a ratio of 2.7 (range, 1.6-4.4), whereas the difference in clearance did not reach statistical significance. The maximal concentration of the R-form was 2.6-fold that of the S-form and, in contrast to that obtained with i.v. administration, a considerably and significantly longer terminal elimination half-life was found for the S-form. The absolute bioavailability of R ( + )-carvedilol reached twice the value for S( - )-carvedilol, with 31.1% (range 20.2%-36.5%) and 15.1% (range, 10.4%-24.4%), respectively. Taking the AUCs of the respective enantiomers as representative for the average concentrations, the variability was considerably wider after oral administration. In relation to the median value, the average concentrations ranged from 65% to 283% for R ( + ) and from 41% to 210% for S ( - ). After i.v. administration, the corresponding ranges were 81%-189% and 59%-138%. Thus, the S-enantiomer represented 18.6%-38.2% of the total plasma concentration after p.o. administration and 34.6%-49.1% after i.v. administration. A good correlation existed between the enantiomeric ratios after both routes of administration (Fig. 4). 4.5-

4.0-

--

maiI

~3.5 o

°3. ru

'

0

2.5 2,0

0 5

•

~

r

*x-1

/

1.5

•

Y 1.0

.32

= 0.927

I

I

I

1.5

2.0

2.5

R/S-ratio i.v.

Fig.4. Correlation between the A U C enantiomer ratios after i.v. and oral administration of carvedilol in ten subjects

Discussion

Carvedilol is a compound that is extensively metabolized, mainly by conjugation with glucuronic acid and sulfuric acid but also by oxidation at different sites of the molecule (Neugebauer et al. 1987). Metabolism also occurs during the first passage through the liver after p.o. administration, leading to an absolute bioavailability of about 25% (yon M611endorff et al. 1987). The present investigation reveals that this composite value of bioavailability is the result of a stereoselective difference in first-pass metabolism, where the R ( + )-enantiomer exhibitis twice the absolute bioavailability (31.1%) of the S ( - )-enantiomer (15.1%). Interestingly, for other [3-blockers with a stereoselective first-pass effect, e.g., propranolol, a higher oral clearance for the dextrorotatory R-enantiomer has been reported (Silber et al. 1982; Walle et al. 1988). The conjugation reaction with glucuronic acid did not seem to be stereoselective; since the enantiomer ratio of carvedilol glucuronides reflected that of the parent compound (Fujimaki et al. 1989), other metabolic pathways are presumably the cause of selectivity. For propranolol this mainly involves ring oxidation (Walle et al. 1984), which is only a minor pathway in carvedilol metabolism. Therefore, it is tempting to attribute the stereoselective metabolism of carvedilol to side-chain oxidation. A stereoselective difference was also demonstrable for systemic clearance, but at 9.4% this was relatively small. Therefore, the enantiomeric R/S ratio was only 1.3. Similar small differences were found for propranolol, which were nevertheless predictive for a pronounced stereoselective first-pass clearance (Olanoff et al. 1984). The degree of stereoselectivity after oral administration seems to vary among individuals by a factor of 2.8, whereas within the same individual there is only small variability, as demonstrated by the clear correlation between i.v. and p.o. enantiomeric ratios. Only the S ( - ) - e n a n tiomer of carvedilol has R-blocking activity. Therefore, within the range of total plasma concentrations of carvedilol in one subject receiving continuous oral treatment, the extent of R-blockade should be relatively stable. A dif-

G. Neugebauer et al.: Stereoselective disposition of carvedilol ference probably exists, however, between i.v. and p.o. treatment, i.v. administration being connected with a higher relative proportion of the S ( - )-enantiomer. Conventional [3-blockers are relatively safe, at least in patients with hypertension and angina pectoris with no other complicating diseases, over a wide range of doses up to supramaximal f3-blocking effects. Therefore, it is difficult to imagine that the small differences between i.v. and oral carvedilol treatment would have any clinically relevant implications. It remains to be seen whether or not the higher systemic clearance of S ( - )-carvedilol with an unchanged apparent volume of distribution is combined with a shorter elimination half-life, since not all our subjects were evaluable in this respect. In contrast to the situation with i.v. infusion, after p.o. administration the elimination half-life of the S ( - )-enantiomer was considerably longer than that of the R ( + )-enantiomer, although, again, not all subjects could be evaluated. In general, as previously reported (von M611endorff et al. 1987), the terminal halflives after oral administration are determined by absorption rather than by elimination. The difference between the two enantiomers is difficult to explain however. On the one hand, it could be related to limitations of the analytical assay, which mean that concentrations have to be determined near the quantitation limit. On the other hand, it may be related to selective enterohepatic recirculation of S ( - )-carvedilol, although this is conceivable only for the unchanged and conjugated compound. Glucuronidation does not seem to be responsible for this difference, because of the lower concentrations of the S ( - )form (Fujimaki et al. 1989). In any case, since the terminal phase starts at rather low concentrations, it can only be of minor importance. It is concluded that stereoselectivity in the systemic clearance of carvedilol explains the substantial preferential oral clearance of the S ( - )-enantiomer. The enanti-

S 111 omer ratio within any one individual is less variable than that between subjects. Depending on the route of administration, there may be small differences in the relative extent of beta-blockade. At the m o m e n t it is rather speculative as to whether a preferential enterohepatic circulation of the S ( - )-enantiomer is present.

References Fujimaki M, Murakoshi J, Hakusui H (1989) Stereoselective disposition and glucuronidation of carvedilol in healthy subjects. Eur J Clin Pharmaco136 [Suppl]: A 179 M611endorffE yon, Reiff K, Neugebauer G (1987) Pharmacokinetics and bioavailability of carvedilol, a vasodilating betablocker. Eur J Clin Pharmaco133:511-513 Neugebauer G, Akpan W, M611endorffE von, Neubert R Reiff K (1987) Pharmacokinetics and disposition of carvedilol in humans. J Cardiovasc Pharmacol 10 [Suppl 11]:85-88 Olanoff LS, Walle % Walle UK, Cowart TD, Gaffney TE (1984) Stereoselective clearance and distribution of intravenous propranolol. Clin Pharmacol Ther 35:755-761 Reiff K (1987) High-performance liquid chromatographic method for the determination of carvedilol and its desmethyl metabolite in body fluids. J Chromatogr 413:355-362 Silber B, Holford NHG, Riegelman S (2982) Stereoselective disposition and glucuronidation of propranolol in humans. J Pharm Sci 71:699-703 Walle T, Walle UK, Wilson MJ, Fagan TC, Gaffney TE (1984) Stereoselective ring oxidation of propranolol in man. Br J Clin Pharmaco118: 741-747 Walle T, Webb JG, BagwellEE, Walle UK, Daniell HB, Gaffhey TE (1988) Stereoselective delivery and action of beta receptor antagonists. Biochem Pharmaco137:115-124 Dr. G. Neugebauer Clinical Pharmacology Boehringer Mannheim GmbH Sandhofer Strasse 116 D-6800 Mannheim 31 FRG