Species-specific biotransformation of moclobemide: a comparative study in rats and humans Schoerlin M-P, Da Prada M. Species-specific biotransformation of moclobemide: a comparative study in rats and humans. Acta Psychiatr Scand 1990: Suppl 360: 108-1 10. This study measured plasma concentration of moclobemide and 2 of its active metabolites in the rat after oral doses of 30 mg/kg moclobemide. The secondary amine metabolite Ro 16-3177 was found in rat plasma at all times investigated (up to 3 weeks); the peak concentration of 200 ng/ ml was reached 15 min after administration of moclobemide. The primary amine 16-6491 was found after only 30 min at about 40 ng/ml, and remained at about one half to one third the level of the secondary amine. Unchanged moclobemide appeared in much higher concentrations than the metabolites initially, but declined rapidly to about the same level as Ro 16-3177 by 3 h. In the humans, peak concentrations of moclobemide were reached more slowly than in the rats, and neither of the amine metabolites was found in human plasma at any time. Metabolism of moclobemide, as has been shown for other morpholine compounds, is quantitatively different in rats and humans. Since the concentration of Ro 16-6491 in human plasma remains below the limit of detection, only a very weak inhibition of MAO-B is produced in human platelets, and moclobemide can thus be considered a selective MAO-A inhibitor in humans.
Moclobemide (Ro 11-1163, p-chloro-N-[2-morpholinoethyl]benzamide,Aurorixm, chemical structure in Fig. I) belongs to a new generation of shortlasting monoamine oxidase (MAO) inhibitors with preferential effect on MA0 type A in the rat brain. In extracerebral organs of the rat, such as in liver and kidney, moclobemide is devoid of selectivity and markedly inhibits both MAO-A and MAO-B activity, and maximum MAO-A inhibition occurs shortly after moclobemide administration (about 15 min after 10 mg/kg orally). Marked MAO-B inhibition in liver and kidney is attained only after a delay of 30-60 min (1). Since moclobemide is virtually devoid of MAO-B inhibitory activity in vitro (2, 3), it has to be postulated that it is converted into metabolites active towards MAO-B, at least in the rat. Previous in vitro and ex vivo experiments using 14 identified or putative moclobemide metabolites clearly indicated that primary amine Ro 16-6491(chemical structure in Fig. 1) was the only compound to potently and selectively inhibit MAO-B activity in vitro as well as ex vivo (1). The secondary amine Ro 16-3177 (chemical structure in Fig. 1) was markedly more potent in vivo than in vitro (l), and this can be explained 108
M.-P. SChR8rli11,M. Da Prada Departments of Pharmacokinetic Research and Pharmaceutical Research, F. Hoff mann-La Roche, Basle, Switzerland
Key words: antidepressant; monoamine oxidase (MAO) inhibitor; moclobemide; metabolism; rat; human M.-P. Schoerlin, Department of Pharmacokinetic Research, F. Hoffmann-La Roche, CH-4002 Basle, Switzerland
by its dealkylation to the primary amine Ro 16649 1. The aim of this study was to measure the concentration of moclobemide, Ro 16-3177 and Ro 166491 in plasma of rats administered moclobemide. To assess species-specific differences in the metab-
C I ~ C O - N modobcmids H-CH:,-CH~-N~O
0
CI~CO-NH-CH~-C Ro 16-3177
I:] 1
C I ~ C O - NRo H -16-6491 CH~-CH~-NH~
(1633)
( 1:s)
(624)
1l
I I
(139)
T
0.25
2.5 3.0 2.0 1.5 I .o 0.5 hours after administration of moclobemide (30 mg/kg orally)
Fig. 1. Concentrations o f Ro 16-3177 and Ro 16-6491 in rat plasma at different times after moclobemide (means SD, n = 7 rats for each time). M e a n p l a s m a concentrations of moclobemide a r e given in parentheses.
olism of moclobemide between rats and humans, plasma from healthy subjects administered moclobemide was also submitted to the same analytical procedure.
Material and methods Male albino rats (Wistar origin, body weight 190-205 g), separated into 8 groups of 7 rats each were used. Seven groups of rats received oral moclobemide (30 mg/kg) and one group solvent. Blood was collected from the abdominal aorta by total exsanguination by means of Vacutainerm (anticoagulant, ammonium and potassium oxalate) and under Vetanarcola anaesthesia. Plasma was obtained by centrifugation and frozen at -20°C until analysis. Six healthy nonsmoking male subjects (19-29 years old, 65-86 kg body weight) participated in the study. No drug was allowed for 2 weeks prior to or during the study. On study day 1 each subject was given a single oral 100 mg moclobemide tablet after overnight fasting. After 1 washout week the same subjects were treated for 15 days with 150 mg moclobemide 3 times daily. Human blood (5 ml) was collected on days 1 and 15 just before (blank) and 0.5, 1, 2 and 3 h after the last morning dose from the cubital vein. Plasma samples were prepared as described above. The plasma concentrations of moclobemide, Ro 16-3177 and Ro 166491 were measured by a specific and sensitive (detection limit for each compound about 30 ng/ ml plasma) reverse-phase high-performance liquid chromatography (HPLC) method with ultraviolet detection (4).
Results Ro 16-3177 was detectable in rat plasma at all times investigated and the highest concentration (about 200 ng/ml plasma) was already attained 15 rnin after administration of 30 mg/kg moclobemide (Fig. 1). At this time Ro 16-6491 was not detectable with the analytical method used. Ro 16-6491 was present in plasma in measurable amounts (about 40 ng/ml) only 30 rnin after drug administration. The concentrations of Ro 16-3177 were 2-3 times higher than those of Ro 16-6491 at all times investigated (from 30 rnin to 3 h) (Fig. 1). The concentrations of moclobemide were much higher than those of its metabolites soon after dosing and decreased rapidly; at 3 h the concentrations of moclobemide and Ro 16-3177 were virtually the same. After oral administration of moclobemide, peak concentrations were reached more rapidly in the rat (15 min post-dosing, 1633 536 ng/ml plasma) than in humans, either after single (60 min
post-dosing, 785 k 391 ng/ml plasma) or repeated administration (60 min post-dosing, 1768& 330 ng/ ml plasma). Although moclobemide was found in relatively high amounts in the plasma of subjects ingesting multiple doses even at 3 h post-dosing (1174 2 14 ng/ml plasma), in no human plasma samples investigated could Ro 16-3177 or Ro 166491 be detected by our HPLC analytical procedure. Discussion
The main finding of this study is that extensive biotransformation of the MAO-A inhibitor moclobemide leading to opening of the morpholine ring is much more pronounced in rats than in humans. Marked species differences in the metabolism of morpholine derivatives have been already observed with several xenobiotics (5). Moreover, as shown here for moclobemide, in vivo formation of ethanolamine derivatives resulting from the opening of the morpholine ring and dealkylation of the nitrogen atom of the morpholine ring to a primary amine have already been observed for other morpholine derivatives (5, 6). The pharmacodynamic effects, i.e., M A0 inhibition observed after oral administration of moclobemide in the rat, closely parallel metabolite concentration-time profiles. Thus, shortly after moclobemide administration, i.e., when no Ro 16-6491 is measured in plasma, only MAO-A activity is inhibited. In contrast, MAO-B activity is markedly inhibited in the rat extracerebral organs at later times when Ro 166491 was clearly detectable in plasma. In humans, relatively high plasma concentrations ( > 1000 ng/ ml) of parent compound were found after multiple administrations of 150 mg of moclobemide. In contrast to the rat, however, no detectable plasma concentrations of Ro 16-6491 were seen. Since only a weak inhibition of MAO-B activity can be measured in human platelets after single or repeated administration of moclobemide in healthy volunteers (I), it can be speculated that moclobemide’s selectivity in humans for MAO-A inhibition is a result of its poor biotransformation into, or different kinetic behaviour of, Ro 16-6491. In conclusion, this study supports the view that, similar to other morpholine derivatives, the metabolism of moclobemide is quantitatively different in rats than in humans. Since even after repated moclobemide administration (1 50 mg 3 times daily for 15 days) the concentration of Ro 16-6491 in human plasma remains below the detection limit of the method used ( < 30 ng/ml plasma) and therefore only a weak MAO-B inhibition is produced in human platelets, moclobemide can be considered as a selective MAO-A inhibitor in humans. 109
Acknowledgements The authors thank Dr T. Guentert and Dr W. E. Haefely for helpful criticism of the manuscript.
References 1. DA PRADAM, KETTLERT. KELLERHH, BURKARDWP,
MUGGLI-MANIGLIO D, HAEFELY WE. Neurochemical profile of moclobemide, a short-acting and reversible inhibitor of monoamine oxidase type A. J Pharmacol Exp Ther 1989: 248: 400-414. 2. DA PRADAM, KELLER HH, KETTLER R et al. Ro 1 1 -1 163, a specific and short acting M A 0 inhibitor with antidepressant properties. In: KAMIJO K, USDINE, NAGATSU T, eds. Monoamine oxidase. Basic and clinical frontiers. Amsterdam: Excerpta Medica. 1981: 183-196.
110
3. DA PRADAM, KETTLERR, BURKARD WP, HAEFELY WE. Moclobemide, an antidepressant with short-lasting MAOA inhibition: brain catecholamines and tyramine pressor effects in rats. In: TIPTONKF, DOSTERT P, STROLIN BENEDETTI M, eds. Monoamine oxidase and disease. London: Academic Press, 1984: 137-1 54. 4. GESCHKE R, KORNERJ, EWERSH. Determination of the new monoamine oxidase inhibitor moclobemide and three of its metabolites in biological fluids by high-performance liquid chromatography. J Chromatogr 1987: 420: 111-120. 5. STROLINBENEDETTI M, DOSTERTP. Rtactions de phase I et de phase I1 dans le mttabolisme des xtnobiotiques comparaison entre espkces. Actualites de Chimie Therapeutique, 96me Serie. Pans: Technique et Documentation Louvoisier, 1982: 287-3 14. 6. TATSUMI K, KITAMURA S, YOSHIMURA H. The metabolism of phenyl-O-(2-N-morpholinoethoxy)-phenyl ether hydrochloride in the rabbit and rat. Xenobiotica 1975: 5: 377-388.
Moclobemide (Ro 11-1163, p-chloro-N-[2-morpholinoethyl]benzamide,Aurorixm, chemical structure in Fig. I) belongs to a new generation of shortlasting monoamine oxidase (MAO) inhibitors with preferential effect on MA0 type A in the rat brain. In extracerebral organs of the rat, such as in liver and kidney, moclobemide is devoid of selectivity and markedly inhibits both MAO-A and MAO-B activity, and maximum MAO-A inhibition occurs shortly after moclobemide administration (about 15 min after 10 mg/kg orally). Marked MAO-B inhibition in liver and kidney is attained only after a delay of 30-60 min (1). Since moclobemide is virtually devoid of MAO-B inhibitory activity in vitro (2, 3), it has to be postulated that it is converted into metabolites active towards MAO-B, at least in the rat. Previous in vitro and ex vivo experiments using 14 identified or putative moclobemide metabolites clearly indicated that primary amine Ro 16-6491(chemical structure in Fig. 1) was the only compound to potently and selectively inhibit MAO-B activity in vitro as well as ex vivo (1). The secondary amine Ro 16-3177 (chemical structure in Fig. 1) was markedly more potent in vivo than in vitro (l), and this can be explained 108
M.-P. SChR8rli11,M. Da Prada Departments of Pharmacokinetic Research and Pharmaceutical Research, F. Hoff mann-La Roche, Basle, Switzerland
Key words: antidepressant; monoamine oxidase (MAO) inhibitor; moclobemide; metabolism; rat; human M.-P. Schoerlin, Department of Pharmacokinetic Research, F. Hoffmann-La Roche, CH-4002 Basle, Switzerland
by its dealkylation to the primary amine Ro 16649 1. The aim of this study was to measure the concentration of moclobemide, Ro 16-3177 and Ro 166491 in plasma of rats administered moclobemide. To assess species-specific differences in the metab-
C I ~ C O - N modobcmids H-CH:,-CH~-N~O
0
CI~CO-NH-CH~-C Ro 16-3177
I:] 1
C I ~ C O - NRo H -16-6491 CH~-CH~-NH~
(1633)
( 1:s)
(624)
1l
I I
(139)
T
0.25
2.5 3.0 2.0 1.5 I .o 0.5 hours after administration of moclobemide (30 mg/kg orally)
Fig. 1. Concentrations o f Ro 16-3177 and Ro 16-6491 in rat plasma at different times after moclobemide (means SD, n = 7 rats for each time). M e a n p l a s m a concentrations of moclobemide a r e given in parentheses.
olism of moclobemide between rats and humans, plasma from healthy subjects administered moclobemide was also submitted to the same analytical procedure.
Material and methods Male albino rats (Wistar origin, body weight 190-205 g), separated into 8 groups of 7 rats each were used. Seven groups of rats received oral moclobemide (30 mg/kg) and one group solvent. Blood was collected from the abdominal aorta by total exsanguination by means of Vacutainerm (anticoagulant, ammonium and potassium oxalate) and under Vetanarcola anaesthesia. Plasma was obtained by centrifugation and frozen at -20°C until analysis. Six healthy nonsmoking male subjects (19-29 years old, 65-86 kg body weight) participated in the study. No drug was allowed for 2 weeks prior to or during the study. On study day 1 each subject was given a single oral 100 mg moclobemide tablet after overnight fasting. After 1 washout week the same subjects were treated for 15 days with 150 mg moclobemide 3 times daily. Human blood (5 ml) was collected on days 1 and 15 just before (blank) and 0.5, 1, 2 and 3 h after the last morning dose from the cubital vein. Plasma samples were prepared as described above. The plasma concentrations of moclobemide, Ro 16-3177 and Ro 166491 were measured by a specific and sensitive (detection limit for each compound about 30 ng/ ml plasma) reverse-phase high-performance liquid chromatography (HPLC) method with ultraviolet detection (4).
Results Ro 16-3177 was detectable in rat plasma at all times investigated and the highest concentration (about 200 ng/ml plasma) was already attained 15 rnin after administration of 30 mg/kg moclobemide (Fig. 1). At this time Ro 16-6491 was not detectable with the analytical method used. Ro 16-6491 was present in plasma in measurable amounts (about 40 ng/ml) only 30 rnin after drug administration. The concentrations of Ro 16-3177 were 2-3 times higher than those of Ro 16-6491 at all times investigated (from 30 rnin to 3 h) (Fig. 1). The concentrations of moclobemide were much higher than those of its metabolites soon after dosing and decreased rapidly; at 3 h the concentrations of moclobemide and Ro 16-3177 were virtually the same. After oral administration of moclobemide, peak concentrations were reached more rapidly in the rat (15 min post-dosing, 1633 536 ng/ml plasma) than in humans, either after single (60 min
post-dosing, 785 k 391 ng/ml plasma) or repeated administration (60 min post-dosing, 1768& 330 ng/ ml plasma). Although moclobemide was found in relatively high amounts in the plasma of subjects ingesting multiple doses even at 3 h post-dosing (1174 2 14 ng/ml plasma), in no human plasma samples investigated could Ro 16-3177 or Ro 166491 be detected by our HPLC analytical procedure. Discussion
The main finding of this study is that extensive biotransformation of the MAO-A inhibitor moclobemide leading to opening of the morpholine ring is much more pronounced in rats than in humans. Marked species differences in the metabolism of morpholine derivatives have been already observed with several xenobiotics (5). Moreover, as shown here for moclobemide, in vivo formation of ethanolamine derivatives resulting from the opening of the morpholine ring and dealkylation of the nitrogen atom of the morpholine ring to a primary amine have already been observed for other morpholine derivatives (5, 6). The pharmacodynamic effects, i.e., M A0 inhibition observed after oral administration of moclobemide in the rat, closely parallel metabolite concentration-time profiles. Thus, shortly after moclobemide administration, i.e., when no Ro 16-6491 is measured in plasma, only MAO-A activity is inhibited. In contrast, MAO-B activity is markedly inhibited in the rat extracerebral organs at later times when Ro 166491 was clearly detectable in plasma. In humans, relatively high plasma concentrations ( > 1000 ng/ ml) of parent compound were found after multiple administrations of 150 mg of moclobemide. In contrast to the rat, however, no detectable plasma concentrations of Ro 16-6491 were seen. Since only a weak inhibition of MAO-B activity can be measured in human platelets after single or repeated administration of moclobemide in healthy volunteers (I), it can be speculated that moclobemide’s selectivity in humans for MAO-A inhibition is a result of its poor biotransformation into, or different kinetic behaviour of, Ro 16-6491. In conclusion, this study supports the view that, similar to other morpholine derivatives, the metabolism of moclobemide is quantitatively different in rats than in humans. Since even after repated moclobemide administration (1 50 mg 3 times daily for 15 days) the concentration of Ro 16-6491 in human plasma remains below the detection limit of the method used ( < 30 ng/ml plasma) and therefore only a weak MAO-B inhibition is produced in human platelets, moclobemide can be considered as a selective MAO-A inhibitor in humans. 109
Acknowledgements The authors thank Dr T. Guentert and Dr W. E. Haefely for helpful criticism of the manuscript.
References 1. DA PRADAM, KETTLERT. KELLERHH, BURKARDWP,
MUGGLI-MANIGLIO D, HAEFELY WE. Neurochemical profile of moclobemide, a short-acting and reversible inhibitor of monoamine oxidase type A. J Pharmacol Exp Ther 1989: 248: 400-414. 2. DA PRADAM, KELLER HH, KETTLER R et al. Ro 1 1 -1 163, a specific and short acting M A 0 inhibitor with antidepressant properties. In: KAMIJO K, USDINE, NAGATSU T, eds. Monoamine oxidase. Basic and clinical frontiers. Amsterdam: Excerpta Medica. 1981: 183-196.
110
3. DA PRADAM, KETTLERR, BURKARD WP, HAEFELY WE. Moclobemide, an antidepressant with short-lasting MAOA inhibition: brain catecholamines and tyramine pressor effects in rats. In: TIPTONKF, DOSTERT P, STROLIN BENEDETTI M, eds. Monoamine oxidase and disease. London: Academic Press, 1984: 137-1 54. 4. GESCHKE R, KORNERJ, EWERSH. Determination of the new monoamine oxidase inhibitor moclobemide and three of its metabolites in biological fluids by high-performance liquid chromatography. J Chromatogr 1987: 420: 111-120. 5. STROLINBENEDETTI M, DOSTERTP. Rtactions de phase I et de phase I1 dans le mttabolisme des xtnobiotiques comparaison entre espkces. Actualites de Chimie Therapeutique, 96me Serie. Pans: Technique et Documentation Louvoisier, 1982: 287-3 14. 6. TATSUMI K, KITAMURA S, YOSHIMURA H. The metabolism of phenyl-O-(2-N-morpholinoethoxy)-phenyl ether hydrochloride in the rabbit and rat. Xenobiotica 1975: 5: 377-388.
