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Indian J Physiol Pharmacol 1999; 43 (2): 251-254

PHAMACOKINETICS OF : UNALTERED BY ENALAPRIL AND IN RHESUS MONKEYS

D. K. BADYAL, S. K. GARG*, V. K. BHARGAVA AND S. MAJUMDARt

Departments of Pharmacology and Experimental Medicine t, Post Graduate Institute of Medical Education and Research, Chandigarh - 160 012

( Received on September 28, 1998 )

Abstract: A cross over single and multiple dose study was carried out to find out pharmacokinetic interactions between diphynylhydantoin (DPH) (35 mglkg, po) and antihypertensives enalapril (1.6 mg/kg; po) and amlodipine (0.4 mg/kg, po) in rhesus monkeys. Neither the plasma concentrations nor the pharmacokinetic parameters of DPH were altered by coadministration of enalapril or amlodipine, suggesting that enalapril and amlodipine can be safely administered to epileptic patients receiving phenytoin.

Key words: phenytoin amlodipine interaction enalapril rhesus monkey

INTRODUCTION plasma concentration ranges between 10-20 mg/L (7). and epilepsy are generally independent disorders but may sometimes Interactions between antihypertensives co-exist in the same patient (1, 2). Both and anticonvulsants may lead to increase ailments require long term treatment. in their adverse effects or loss of control over the disorder. Hence, the present study Enalapril, an converting was designed to evaluate the effect of enzyme (ACE) inhibitor is converted to its enalapril and amlodipine on the pharmac­ active moiety in the (3). Amlodipine is okinetics of phenytoin in rhesus monkeys a dihydropyridine blocker (Macaca mulatta). and metabolized in the liver by cytochrome P450 (CYP450) enzymes (4). Phenytoin, a METHODS drug of first choice for single drug therapy of partial, generalised tonic-clonic seizures Twenty four healthy adult male rhesus or both (5), has been frequently implicated monkeys (Macaca mulatta) weighing in clinically significant drug interactions between 4 to 7 kg were divided into four because of its narrow therapeutic index, groups consisting of six monkeys in each slow absorption and saturable kinetics group. The study design was cross over for (6, 7, 8, 9). The acceptable therapeutic each group.

*Corresponding Author 252 Badyal et al Indian J Physiol Pharmacol 1999; 43(2)

Group I: After overnight fast monkeys Analytical methods: Plasma was separated were given DPH (35mg/kg;po) through from blood samples and stored at -20°C until intragastric tube at 0800 h. Blood samples assayed for DPH using HPLC technique (2mL) were drawn from small saphenous (10). Recovery of DPH from plasma samples vein in heparinised tubes at 0,0.5, was >90% and sensitivity of the method was 1,2,3,6,9,12,24 and 48 h after drug 0.01 llg/mL. The intraassay and interassay administration. After a wash out period of coefficient of variation for DPH at different 10 days, DPH was administered along with concentrations were 9.51% and 8.71% enalapril (1.6 mglkg; p.o.) at 0800 h. Blood respectively. samples were drawn at" similar time intervals as described earlier. Pharmacokinetic Parameters: Single dose and steady Peak plasma concentration Group II: Six overnight fasted monkeys (Cmax) and time to reach peak plasma were treated with DPH (35mg/kg;po) at concentration (Tmax) were calculated from 0800h and after 10 days of wash out period, the actual plasma data. Absorption half-life they were treated with DPH along with (tlh a) and elimination half-life (Vh e) were amlodipine (O.4mglkg;p.o.). Blood samples calculated by the residual method and by (2mL) were drawn at both the occasions i.e. least square regression analysis method before and after amlodipine treatment at respectively. Area under the plasma concentration-time curve AUC _ was similar time intervals as that of group I O t animals. calculated by trapezoidal rule. Extension of the AUC data to infinity AUCt_~ was done by dividing the last observed concentration Group III: Six monkeys received DPH of the drug in plasma by the elimination (35mglkg/day;po) at 0800 h for consecutive rate constant (Ke!). AUCo_~ was sum of the 14 days. Blood samples (2mL) were drawn AUC _ and AUCt_~ at 0,0.5,1,2,3,6,9,12 and 24 hours on day O t 1,7and 14 after drug administration. Stastical analysis: To find out level of significance student's paired t test was Group IV: Six monkey received DPH applied and P value <0.05 was considered (35mg/kg/day;po),at 0800 h for consecutive as statistically significant. seven days. Blood samples (2mL) were • drawn at 0,0.5,1,2,3,6,9,12 and 24 hours on RESULTS day 7 after drug administration. DPH was continued for another seven days along with Group I: Table I illustrates the enalapril (1.6mglkg/day; p.o.). Blood samples pharmacokinetic parameters of DPH before were drawn as before on day 14. After a and after single dose of enalapril. No wash out period of 15 days, the monkeys significant difference was observed in any were treated with DPH along with of the parameters. The DPH levels could be amlodipine (0 .4mg/kg/day, p.o.) for detected upto 96h. consecutive seven days. Blood samples were drawn at similar time intervals on day 7 There was no significant difference in after drug administration. the plasma concentrations of DPH at any Indian J Physiol Pharmacol 1999; 43(2) Enalapril-Amlodipie-Phenytoin-Interaction 253

TABLE I: of DPH before and after between 7 days and 14 days treatment with single oral dose of enalapril in rhesus monkeys (n=6). DPH alone.

After multiple doses of oral enalapril or DPH Parameters DPH alone + Enalapril amlodipine the DPH plasma levels were. lower in monkeys treated with DPH along Trna. (h) 8.33 ±1.36 8.50± 0.92 with enalapril as compared to DPH alone, C m.,(~g/mL) 14.58 ±2.16 16.03± 1.30 but the decrease in plasma levels was not tlh a(h) 2.20 ±0.42 2.46± 0.40 statistically significant. No significant difference was observed in steady state DPH tlh e(h) 20.84 ±2.62 21.26± 6.86 plasma levels before and after multiple oral AUCo.j~g·h/mL) 468.24 ±84.25 618.35± 151.77 doses of amlodipine at any time point. Table Values represent Mean ± SEM III shows no significant difference in various pharmacokinetic parameters of DPH before TABLE II: Pharmacokinetics of DPH before and after and after multiple oral doses of enalapril or single oral dose of amlodipine in rhesus amlodipine. monkeys (n=6). TABLE III: Pharmacokinetics of DPH before and after multiple oral dose of enalpril or amlodipine DPH in rhesus monkeys. Parameters DPH alone + Amlodipine

Trn" (h) 9.50 ±0.92 11.00± 2.76 DPH + DPH + Parameters DPH alone Enalapril Amlodipine Cm.,(llg/mL) 17.82 ±2.19 19.60± 2.40 tlh a(h) 2.94 ±0.41 3.19± 0.79 T rn " (h) 4.67±1.14 3.17±0.60 3.50±0.87 Crn,,(llg/mL) 14.65±3.13 11.18±1.14 11.07±1.74 tV. e(h) 13.63 ±1.64 11.87± 1.62 tlh a(h) 1.20±0.36 1.02±0.22 1.20±0.29 AUCo.Jllg.h/mL) 600.12 ±94.13 550.58± 76.87 tlh e(h) 11.19±4.50 11.04±2.35 1l.83±3.62 Values represent Mean ± SEM AUCo--(llg.hlmL) 222.95± 70.98 158.48±29.41 155.79±44.23

Values represent Mean ± SEM time point before and after single dose of amlodipine. Table II compares the various DISCUSSION pharmacokinetic parameters of DPH before and after single oral dose of amlodipine Phenytoin. is one of the most widely revealing no significant difference in any of prescribed anticonvulsants. Drug interac­ the parameters. The DPH levels could be tions of DPH are important because of detected upto 72h. al terations in its efficacy and risk of toxicity. The DPH levels were significantly decreased in elimination phase after 7 days In the present study enalapril and and 14 days treatment as compared to single amlodipine did not influence the overall dose treatment with DPH. But no significant pharmacokinetics of DPH absorption or difference was observed in DPH levels elimination. The lack of phamacokinetic 254 Badyal et al Indian J Physiol Pharmacol 1999; 43(2) interaction between these drq.gs may b~ .due L ,J'hese r~sults suggest that enalapril and to their different metabolic pathways. While amlidipine can be safely administered to enalapril IS mainly deesterified by epileptic patients receiving DPH. However carboxyesterases in the liver (3), amlodipine the species variation may affect hepatic and phenytoin mainly undergo oxidatin expression of different CYP450 isozymes by CYP450 isozymes 3A4 and 2C9/2C19 (11), so the present findings need further respectively (4, 9). confirmation in epileptic patients.

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