Stereoselective metabolism of cibenzoline, an antiarrhythmic drug, by human and rat liver microsomes: possible involvement of CYP2D and CYP3A.

Stereoselective metabolism of cibenzoline succinate, an oral antiarrhythmic drug, was investigated on hepatic microsomes from humans and rats and microsomes from cells expressing human cytochrome P450s (CYPs). Four main metabolites, M1 (p-hydroxycibenzoline), M2 (4,5-dehydrocibenzoline), and unknown metabolites M3 and M4, were formed by human and rat liver microsomes. The intrinsic clearance (CL(int)) of the M1 formation from R(+)-cibenzoline was 23-fold greater than that of S(-)-cibenzoline in human liver microsomes, whereas the R(+)/S(-)-enantiomer ratio of CL(int) for M2, M3, and M4 formation was 0.39 to 0.83. The total CL(int) for the formation of the four main metabolites from S(-)- and R(+)-cibenzoline was 1.47 and 1.64 microl/min/mg, respectively, suggesting that the total CL(int) in R(+)-enantiomer was slightly greater than that in S(-)-enantiomer in human liver microsomes. The M1 formation from R(+)-cibenzoline was highly correlated with bufuralol 1'-hydroxylation and CYP2D6 content and was inhibited by quinidine, a potent inhibitor of CYP2D6. Additionally, only microsomes containing recombinant CYP2D6 were capable of M1 formation. These results suggest that the M1 formation from R(+)-cibenzoline was catalyzed by CYP2D6. The formation of M2, M3, and M4 from S(-)- and R(+)-cibenzoline was highly correlated with testosterone 6beta-hydroxylation and CYP3A4 content. Ketoconazole, which is a potent inhibitor of CYP3A4/5, had a strong inhibitory effect on their formation, and the M4 formation from R(+)-cibenzoline was inhibited by quinidine by 45%. The formation of M2 was also inhibited by quinidine by 46 to 52% at lower cibenzoline enantiomers (5 microM), whereas the inhibition by quinidine was not observed at a higher substrate concentration (100 microM). In male rat liver microsomes, ketoconazole and quinidine inhibited the formation of the main metabolites, M1 and M3, >74% and 44 to 59%, respectively. These results provide evidence that CYP3A and CYP2D play a major role in the stereoselective metabolism of cibenzoline in humans and male rats.

Comparative pharmacokinetic/pharmacodynamic analysis of proton pump inhibitors omeprazole, lansoprazole and pantoprazole, in humans.

The relationship between plasma concentrations and inhibitory effects on gastric acid secretion by proton pump inhibitors (PPIs) omeprazole (OPZ), lansoprazole (LPZ) and pantoprazole (PPZ), was analyzed using a pharmacokinetic/pharmacodynamic (PK/PD) model in humans. The estimated values of apparent reaction rate constant of PPI and H+,K+-ATPase (K) were 1.34 +/- 0.17 (microM(-1) x h(-1)), 0.339 +/- 0.002 and 0.134 +/- 0.006 for OPZ, LPZ and PPZ, respectively. The estimated values of apparent turn-over rate constant of H+,K+-ATPase (k) were 0.0252 +/- 0.0019 (h(-1)), 0.0537 +/- 0.0006 and 0.0151 +/- 0.0002 for OPZ, LPZ and PPZ, respectively. The apparent dissociation constants of the H+,K+-ATPase-PPI complex (k/K x fp) corrected with plasma free fraction (fp) were about 1 nM for OPZ and LPZ and 2.3 nM for PPZ. Therefore, the potency of the inhibitory effect of PPZ on acid secretion may be slightly weaker than that of OPZ or LPZ. The apparent half lives (ln2/k) of the inhibitory effect on acid secretion were 12.9 h for LPZ, < 27.5 h for OPZ, and < 45.9 h for PPZ, the recovery rate of the inhibitory effect of PPZ on acid secretion was slowest among these PPIs. In conclusion, the relationship between plasma concentrations and inhibitory effects of PPIs on gastric acid secretion could be analyzed by the PK/PD model.

Pharmacokinetics and pharmacodynamics of FK143, a nonsteroidal inhibitor of steroid 5 alpha-reductase, in healthy volunteers.

The pharmacokinetics and pharmacodynamics of FK143, a new nonsteroidal inhibitor of steroid 5 alpha-reductase, were investigated in healthy volunteers, with use of plasma FK143 concentrations and serum dihydrotestosterone levels as an index for pharmacologic effects. The area under the plasma concentration-time curve from zero to infinity [AUC(0-infinity)] and maximum plasma concentration [Cmax] were increased dose proportionally after oral administration (100 to 500 mg) while subjects were in the fed state. The AUC(0-infinity) and Cmax after 500 mg oral administration during fed conditions were significantly larger than those during the fasted state, suggesting an increase of the absorption of FK143. Dihydrotestosterone concentrations after a single administration of FK143 (100 to 500 mg) during fed conditions decreased to about 65% of predose values and thereafter slowly recovered to the same levels as predose values at 168 hours. A combined pharmacokinetic-pharmacodynamic model was constructed with use of changes in dihydrotestosterone concentrations. The pharmacokinetic-pharmacodynamic profiles of FK143 after repeated administration were predictable with use of the pharmacokinetic-pharmacodynamic parameters obtained after a single administration of FK143.

Pharmacokinetic and pharmacodynamic study of a new nonsteroidal 5 alpha-reductase inhibitor, 4-[3-[3-[Bis(4-isobutylphenyl)methylamino]benzoyl]-1H-indol-1-yl]-butyr ic acid, in rats.

The pharmacokinetic and pharmacodynamic behaviors of 4-[3-[3-[Bis(4-isobutylphenyl) methylamino] benzoyl]-1H-indol-1-yl]-butyric acid (FK143), a new nonsteroidal steroid 5 alpha-reductase inhibitor, in the ventral prostate were investigated after i.v. administration to rats. The relationship between blood concentrations at 24 hr and doses was linear in the range of 0.1 to 20 mg/kg. However, the levels of FK143 in the prostate were saturated over the dose of over 5 mg/kg. The dissociation constant (Kd) and maximum amount of binding substances (Bmax), calculated according to nonlinear kinetic analysis including a specific binding pool, was 0.0553 +/- 0.0117 microgram/ml (92 nM, estimated value +/- S.D.) and 0.908 +/- 0.092 microgram/g tissue, respectively. A combined pharmacokinetic/pharmacodynamic (PK/PD) model was constructed using change in dihydrotestosterone (DHT) levels in the prostate after i.v. administration of FK143 as an index for its pharmacological effect and blood concentration as an input function. The apparent reaction rate constant of drug and enzyme (K) was 39.7 +/- 25.1 g tissue/microgram/hr (estimated value +/- S.D.), the apparent turn-over rate constant of enzyme (k) was 0.140 +/- 0.107 hr-1, the elimination rate constant of DHT (kel, DHT) was 1.13 +/- 0.94 hr-1 and the fraction of FK143-insensitive DHT synthesis (F) was 0.461 +/- 0.037. The PK/PD analysis suggested that the duration of the effect of FK143 was related to its accumulation in the binding pool of the prostate. After i.v. administration of FK143 in the range of 0.1 to 20 mg/kg, the DHT levels in the prostate decreased to about 40% of control value, after which despite the rapid decline of blood FK143 concentration, slowly recovered according to the elimination rate of FK143 in the prostate. Moreover, the PK/PD profiles of FK143 after repeated i.v. administration were predictable by using the PK/PD parameters obtained after single administration of FK143.

Tissue distribution kinetics of a new nonsteroidal 5 alpha-reductase [correction of 5 A-reductase] inhibitor, 4-[3-[3-[bis(4-isobutylphenyl)methylamino]benzoyl]-1H-indol-1-YL ]-butyric acid, in rats.

The disposition of a new nonsteroidal 5alpha-reductase inhibitor, 4-[3-[3-[bis(4-isobutylphenyl)methylamino]benzoyl]-1H-indol-1-yl]-butyric acid (FK143), was investigated in rats. After intravenous administration of FK143 at 1 and 5 mg/kg, total body clearance, elimination half-life, and volume of distribution at steady-state were, respectively, 6.96 and 8.76 ml/min/kg, 10.31 and 9.83 hr, and 4.11 and 3.33 liters/kg. There were no essential differences between the two doses in any parameters. The serum protein binding in vitro was very high (>99%). The unidirectional uptake clearance (CL1) to 13 tissues was determined by integration plot until 10 min after intravenous administration of 1 mg/kg. CL1 values were much smaller than blood flow rate in all tissues, including the prostate, the target organ, indicating that FK143 was transported from blood to tissues by a membrane-limited process. Since the elimination rates of FK143 from the liver, kidney, lung, epididymis, seminal vesicle, and prostate were slower than from the blood, the efflux rate constant (k2) and rate constants at a binding compartment (k3 and k4) were assumed in the pharmacokinetic model. A correlation was found between the binding potential of binding compartment (k3/k4) and V(max) of steroid 5alpha-reductase, the target enzyme, suggesting that the levels of 5alpha-reductase activity or that of associated substances are a primary determinant of the specific binding of FK143 in these tissues.

A species comparison of 14C-labeled 7-ethoxycoumarin metabolism in precision-cut liver slices from guinea pig and dog using a phosphor imaging system.

The metabolism of 14C-labeled 7-ethoxycoumarin (7EC) has been investigated in precision-cut liver slices from guinea pigs and dogs. 7EC was incubated with slices in 12-well plates (4 slices/well; n = 3) for up to 8 hr. In addition, a new simple method was established for analyzing 7EC and its metabolites simultaneously by using thin-layer chromatography-radioluminography (TLC-RLG). In both species, 7EC was taken up rapidly into the slices and metabolized extensively under the conditions used (no serum fraction supplemented), showing both phase I and phase II metabolism. In guinea pig medium samples, 4-ethoxy-2-hydroxyphenylacetic acid (EHPA) and 7-hydroxycoumarin (7HC) glucuronide were major metabolites. In dogs, conjugated 7HCs (with D-glucuronic acid and sulfate) were major products but EHPA was formed only to a small extent. These results suggest that deethylation in dogs occurs to a much greater extent than in guinea pigs. These results demonstrate the advantages of precision-cut liver slices as a powerful tool to investigate the species specific metabolism of xenobiotics, since the conditions employed enabled both phase I and phase II reactions in vitro.

Stereoselective pharmacokinetics of dihydropyridine calcium antagonists.

Many dihydropyridine calcium antagonists are widely used for the treatment of angina and hypertension, and many more are under development. Most of these drugs have one or more chiral centre, and the pharmacological activity between the enantiomers for these drugs is known to be markedly different. First, the stereospecific assay methods for these drugs in plasma or serum are reviewed with emphasis on chiral stationary phase high-performance liquid chromatography for their determination. Next, the stereoselective pharmacokinetics of these drugs (nilvadipine, nitrendipine, felodipine, nimodipine, manidipine, benidipine and nisoldipine) in animals, healthy subjects and patients with hepatic disease is reviewed. Enantiomer-enantiomer interaction, enantiomeric inversion and the stereochemical aspects of pharmacokinetic drug interactions in these drugs are also described.

Further metabolism of FK506 (tacrolimus). Identification and biological activities of the metabolites oxidized at multiple sites of FK506.

To characterize the metabolic pathway of FK506 (tacrolimus), FK506 or its 31-O-desmethyl metabolite was incubated with liver microsomes prepared from dexamethasone-treated rats in the presence of a NADPH-generating system under aerobic conditions. Besides the four oxidized metabolites already reported, four new metabolites were isolated and identified by HPLC, mass spectrometry, and NMR spectroscopy, and their biological activities were examined. The di-demethylated metabolites at the 15- and 31-, 13- and 31-, and 13- and 15-methoxy groups of FK506, were designated respectively as M-V, M-VI, and M-VII. The fourth, M-VIII, was the metabolite produced after O-demethylation at the 31-methoxy group and formation of a fused 10-membered ring structure through the 19- to 22-carbon of the macrolide ring after oxidation of the 19-methyl group, and of the 36- and 37-vinyl group of FK506. The immunosuppressive activity of the isolated metabolites was estimated in a mouse mixed lymphocyte reaction system and the IC50 values for M-V, M-VI, M-VII, M-VIII, and FK506 were > 1000, 8.78, > 1000, 15.27, and 0.11 ng/ml, respectively. Reactivity of the metabolites with mouse anti-FK506 monoclonal antibody was studied and immunocrossreactivity of M-V was 92.3% of FK506, but no reactivity was observed for M-VI, M-VII, and M-VIII. FK506 thus was metabolized at multiple sites by rat hepatic microsomes and the metabolites formed (M-V) - (M-VIII) exhibited weak or negligible immunosuppressive activity.

Sex-dependent and independent renal excretion of nilvadipine metabolites in rat: evidence for a sex-dependent active secretion in kidney.

1. To clarify the mechanism of sex-dependent and independent kidney secretion of major nilvadipine metabolites (3, 7) in rat, renal clearance corrected for protein binding and glomerular filtration rate (GFR) was measured in both sexes. The effect of probenecid, an inhibitor of organic anion transport, on these measurements was also investigated. 2. Clear sex-dependent active secretion was observed in the renal excretion of 3 (3-carboxylic acid pyridine derivative). In the female rat, 3, clearance was approximately 32-fold greater than GFR and was markedly decreased by probenecid. Conversely, in the male rat, renal clearance of 3 was only a fraction of GFR and was unaffected by probenecid. 3. Sex-independent active secretion was observed in the renal excretion of 7 (5-carboxylic acid pyridine derivative). In both sexes of rat 7 clearance was about 22-fold greater than GFR and was markedly reduced by probenecid. 4. A clear presence of sex-dependent and independent active secretion mechanisms in the kidney has been demonstrated in rat. The female rat is able to eliminate 3 and 7 in urine by an active secretion mechanism that is inhibited by probenecid. In the male rat, a transport mechanism for 7 is present, but either lacks or is apparently inactive for 3.

Interaction of renal excretion between nilvadipine metabolites, M3 and M7 in rats: characterization of sex-dependent and sex-independent active secretion in the kidney.

The interaction of renal clearance between nilvadipine metabolites, M3 and M7, in male and female rats including protein binding and renal excretion was investigated to clarify the mechanisms involved. In male rats, active renal secretion of M7 (the 5-carboxylic acid pyridine derivative) was reduced in inverse proportion to the molar ratio of the plasma concentration M3/M7 after an i.v. dose of M3 (the 3-carboxylic acid pyridine derivative), and the dosed M3 was excreted only by glomerular filtration. In female rats, the active renal secretion of M7 was unaffected after an i.v. dose of M3, and the dosed M3 was excreted by active secretion. These results indicate an interference of the active secretion of M7 in male rats by M3 on competitive interaction at the renal tubular secretion, even though M3 was excreted only via a filtration process. Female rats may have two distinct and separate active renal secretion mechanisms for M7 and M3, even though these carboxylic acid compounds were eliminated by active transport in the kidney.

Age- and sex-related changes of sulfotransferase activities in the rat.

Temporal and gender-related changes of sulfotransferase activities in the rat were studied using alcoholic, amine and phenolic compounds as substrates. Alcohol and amine sulfotransferase activities in the rat exhibited different temporal and sex-related changes of those for phenol. Formation of sulfoconjugates in vivo correlated well with in vitro sulfotransferase activities in the rat liver.

Effects of twenty-three drugs on the metabolism of FK506 by human liver microsomes.

We investigated the effects of 23 drugs on the metabolism of FK506 by human liver microsomes. Acyclovir, amphotericin B, cefixime, cefotaxime, ciprofloxacin, cyclosporin A, diltiazem, enoxacin, erythromycin, ethinyl estradiol, fluconazole, fosfomycin, kanamycin, lincomycin, loxoprofen, minocyclin, nifedipine, nilvadipine, norethindrone, ofloxacin, phenobarbital, prednisolone, or rifampicin was added to the reaction media at equimolar or at ten times an excess molar ratio of the substrate concentration; their effects on FK506 metabolism were examined. Drugs known to be the substrate of cytochrome P-450 3A inhibited the metabolism of FK506, and among the drugs tested, the inhibition by cyclosporin A and nifedipine was the strongest.

Nilvadipine as a neuroprotective calcium entry blocker in a rat model of global cerebral ischemia. A comparative study with nicardipine hydrochloride.

The effects of two dihydropyridine type calcium entry blockers, nilvadipine and nicardipine hydrochloride (nicardipine), on the liberation of free fatty acids (FFAs) were investigated using an experimental model of global cerebral ischemia in rats, and were compared with their pharmacokinetic properties. Nilvadipine, but not nicardipine, at a dose of 100 micrograms/kg i.v., significantly attenuated the liberation of FFAs, particularly docosahexaenoic and arachidonic acid. Furthermore, the brain concentration of nilvadipine was higher than that of nicardipine after equivalent dosing. The results of the present study demonstrate that pharmacokinetic differences between these two calcium entry blockers might explain the difference in their pharmacological efficacy.

Sex differences in the metabolism and excretion of nilvadipine, a new dihydropyridine calcium antagonist, in rats.

1. The metabolic profiles of nilvadipine in the urine and bile of male and female rats were studied after i.v. dosing with 1 mg/kg of the 14C-labelled compound. 2. Excretion rates of the dosed radioactivity in male and female rats, respectively, in the first 48 h were 84.1% and 59.1% in bile, 12.0% and 36.9% in urine, and 2.5% and 3.6% in faeces. 3. Comparison of biliary and urinary excretion for each radioactive metabolite after dosing with 14C-nilvadipine, showed marked sex-related differences in the excretion routes of several metabolites. In male rats, metabolite M3, having a free 3-carboxyl group on the pyridine ring, was not excreted in urine, but in female rats urinary excretion of M3 accounted for 4.7% of the dose. One reason for the lower urinary excretion of radioactivity by males than by females was that the main metabolite, M3, was not excreted in the urine of the male rats. 4. To clarify the sex difference in the route of excretion of M3, this metabolite (M3) was given i.v. to rats. No excretion of the metabolite was observed in urine of male rats within 24 h but, in marked contrast, 41.5% of the dose was excreted in urine of females in the same period.

Stereoselective disposition of nilvadipine, a new dihydropyridine calcium antagonist, in the rat and dog.

The stereoselective disposition of nilvadipine (NV), a new 1,4-dihydropyridine calcium antagonist, was determined in male and female rats, and male dogs. After oral dosing of racemic NV to male rats, the maximum plasma concentration (Cmax) and area under the plasma concentration-time curve (AUC) of the pharmacologically more potent (+)-NV were 0.59-0.60 times those of (-)-NV. The apparent oral clearance (CLo) ratio of (+)- to (-)-NV was 1.69. The plasma elimination of the enantiomers were similar. In female rats, the plasma concentrations and pharmacokinetic parameters of the enantiomers did not significantly differ. In orally-dosed dogs, the Cmax and AUC of (+)-NV were 3.13 and 3.83 respectively times greater than those of (-)-NV. The enantiomeric ratio of CLo was 0.27, and the half-lives of the enantiomers were similar. After intravenous dosing to dogs, the plasma concentrations of (+)- and (-)-NV declined biexponentially with similar t1/2 beta values. The AUC of (+)-NV was 1.56 times more than that of (-)-NV. The enantiomeric ratios of systemic clearance and volume of distribution at steady state were 0.64 and 0.81, respectively. Thus, the stereoselective disposition of NV was species-dependent and sex-related in rats, and was dosing route-dependent in dogs. The free fraction value for protein binding of (+)-NV in dog plasma was only 0.50-0.51 times that of (-)-NV. The enantiomeric ratios of those values in male and female rat plasma were 1.14 and 0.98, respectively.

Stereoselective oxidation of nilvadipine, a new dihydropyridine calcium antagonist, in rat and dog liver.

The stereoselective oxidation of nilvadipine (NV), a new 1,4-dihydropyridine calcium antagonist, to the corresponding pyridine analog was studied after incubation of (+)- and (-)-NV with rat and dog liver microsomes. The rates of formation of the pyridine analog and disappearance of NV were similar for each species, indicating that aromatization of NV is the primary metabolic step. Formation of the corresponding pyridine required the presence of an NADPH-generating system and was significantly inhibited by carbon monoxide and metyrapone, indicating the participation of cytochrome P-450. In male rat liver microsomes, the apparent Km values for the formation of the pyridine from (+)- and (-)-NV were 11.2 and 8.1 microM, and the Vmax values were 7.48 and 3.37 nmol/mg of protein/min, respectively. Therefore, the Vmax/Km value, which is equivalent to the intrinsic clearance of the drug, for the oxidation of (+)-NV was 1.59-fold greater than that for the oxidation of the (-)-enantiomer. In female rats, (-)-NV oxidation exhibited two distinct apparent Km values, whereas the that of the (+)-enantiomer did not. The (+)/(-) ratio of Vmax/Km was 1.23. On the other hand, in male dog microsomes the Km values for (+)- and (-)-NV were 21.9 and 12.2 microM, and Vmax values were 3.02 and 2.45 nmol/mg of protein/min, respectively; the (+)/(-) ratio of Vmax/Km was 0.69. These results indicate that the stereo-selective oxidation of NV is species dependent and is sex related in rat liver.

Stereoselective oxidation and plasma protein binding of nilvadipine, a new dihydropyridine calcium antagonist, in man.

The stereoselectivity in the plasma protein binding and oxidative metabolism of nilvadipine, a new dihydropyridine calcium antagonist, in man was studied. The free fraction values (fp) for the plasma protein binding of (+)- and (-)-nilvadipine determined by equilibrium dialysis were 1.00 and 0.90%, respectively; the fp of the (+)-nilvadipine was a little higher than that of the (-)-enantiomer. Marked differences between enantiomers were not observed in the blood to plasma ratio. On the other hand, Vmax/Km value, which is equivalent to the intrinsic clearance of the drug, for the oxidation of (+)-nilvadipine to the corresponding pyridine analogue by human liver microsomes was 0.43-0.54 times less than that for the oxidation of the (-)-enantiomer.

Multiple-dose pharmacokinetics of nilvadipine in healthy volunteers.

Nilvadipine, a new antihypertensive and antianginal drug, was studied in six healthy male volunteers to evaluate its steady-state pharmacokinetics after oral dosing. The subjects were given a single dose of 4 mg, followed by 4 mg every 12 hours for six days after a washout period of more than 3 days. The pharmacokinetics of nilvadipine were well described by a linear model of triexponential equation with zero-order absorption. The steady state was reached by the fourth day of multiple dosing, with a twofold accumulation of trough plasma concentration and no accumulation of peak concentration. The mean plasma concentration at steady state was 1.0 ng/mL. The optical enantiomers of nilvadipine were also determined in the plasma. The plasma concentration of (+)-nilvadipine was about two and a half times higher than that of (-)-nilvadipine, and this ratio was unaffected by multiple dosing.

Oxidation of nilvadipine, a new dihydropyridine calcium antagonist, to the corresponding pyridine by rat liver microsomes.

1. The oxidation of nilvadipine, a 1,4-dihydropyridine derivative, by rat liver microsomes and sex-related differences in this activity were investigated. 2. The kinetic data (Km and Vmax) for the formation of the corresponding pyridine analogue by liver microsomes from adult male rat (7 weeks old) were similar to those for the disappearance of nilvadipine, indicating that the aromatization of nilvadipine is the primary metabolic step. 3. The formation of the pyridine analogue required the presence of NADPH-generating system and was significantly inhibited by cytochrome c, metyrapone, and 7,8-benzoflavone, indicating the participation of cytochrome P-450. Phenobarbital pretreatment of rats caused an increase in the metabolism of nilvadipine. 4. Nilvadipine oxidase activity in adult male rat was about 10 times higher than that in adult female rat. In contrast, marked sex-related differences were not seen in immature rat (21 days old), and the activity was approximately twice as high as that in adult female rat. No activity was detected in the postmitochondrial fractions of lung, kidney, brain, heart, pancreas or small intestine of adult male rat.

Pharmacokinetics of nilvadipine, a new dihydropyridine calcium antagonist, in mice, rats, rabbits and dogs.

1. The pharmacokinetics of nilvadipine in male and female rats, and in male mice, rabbits and dogs were studied after i.v. and oral dosing. 2. After i.v. dosing (0.1 mg/kg), the plasma concentrations of nilvadipine declined two- or three-exponential with terminal half-lives of 0.73 h in mice, 1.2 h in male and female rats, 3.7 h in rabbits and 5.0 h in dogs. Sex difference in pharmacokinetics after i.v. dosing in rats was not found. The systemic plasma clearance was in the order of mice greater than rats greater than rabbits greater than dogs, and nearly equalled the hepatic blood flow in each species. The volume of distribution at steady-state was high (greater than 4 L/kg) in all species. 3. After oral dosing, plasma concentrations of nilvadipine peaked within 1 h in all species except for middle and higher doses (4 and 16 mg/kg) in dogs. The area under the plasma concentration-time curves in male rats (3.2-100 mg/kg) and dogs (1-16 mg/kg) increased in proportion to the dose. Bioavailability was low in male rats (3-4%) and rabbits (2%), but in other species was 29-44%. The oral clearance in male rats was about 8 times higher than in female rats. 4. The free fraction of nilvadipine in plasma was 1.94% in mice, 1.89% in rabbits and 0.85% in dogs, with no dependence on plasma concentration over a range of 10-100 ng/ml.