The pharmacokinetic interactions between valproic acid and acyclovir assessed in vitro and in a rabbit model

[摘要] English: Valproic acid is an antiepileptic drug that is widely used for treatment of epilepsy,while acyclovir is an antiviral drug indicated for treatment of infections caused byherpes simplex type I & II and varicella-zoster viruses. Given the high prevalenceof people with conditions for which chronic use of valproic acid is indicated, andthe notion that valproic acid increases the antiviral activity of acyclovir, it is notuncommon for the two drugs to be used concomitantly. As such, recent reportson the interaction between valproic acid and acyclovir with break throughconvulsions were a cause for concern. Since understanding the mechanism ofthis interaction is vital to the establishment of concrete guidelines on the use ofthe two drugs in patients, the aim of this study was to investigate the possiblepharmacokinetic interaction between acyclovir and valproic acid. First, a high performance liquid chromatography (HPlC) method for analysis ofacyclovir in plasma was developed. It involved simple protein precipitation of 200IJl of plasma with perchloric acid, followed by centrifugation after which 20 IJl ofthe supernatant was injected in the HPlC. The sample was eluted withacetonitrile: octanesulfonic acid: ammonium acetate-citrate (vol.lvol.;5%:11.88%:83.12%) at 1.5 ml/min over a luna C18 (4.60 x 150 mm) 51Janalyticalcolumn. Gancyclovir was used as the internal standard. Under these conditions,gancyclovir eluted at 3.4 min and acyclovir at 4.5 min. Over the calibration rangeof 10 - 100 IJg/ml, linearity was demonstrated by a linear regression equation ofy = 0.03196 - 3.207x with a regression coefficient r² = 0.995, and accuracy by apercentage coefficient of variation (CV%) of less than 15%. The method wassuccessfully used to analyze acyclovir in a rabbit treated with acyclovir singledose. Thereafter, the possibility of a direct interaction between acyclovir and valproicacid in vitro was investigated by monitoring the concentrations of valproic acidand acyclovir at different pH (pH 7.4 or pH 3 or pH 10) and temperatures (25°Cand 37°C) when mixed in a 1:1 molar ratio or prepared separately in phosphatebuffer. The samples were incubated at 25°C for 2 hours and a further 1 hour at370C, and aliquots were drawn at 10 min., 2 and 3 hours to measure theconcentration of valproic acid and acyclovir (n=3). The average concentrations ofvalproic acid and acyclovir from the samples containing the single drug were notdifferent (P > 0.05) from those in the mixture of both drugs at the differenttemperatures and pH. However, when the temperature and pH were evaluatedseparately, there was a trend whereby, at high temperature (37°C), theconcentrations of acyclovir (percentage detected) tended to be higher in themixture (87%) than when it was alone (84%), while those of valproic acid tendedto be lower in the mixture (89%) than when it was alone (92%). This same trendwas observed at acid or alkaline pH. In conclusion, although temperature and pHdid not induce significant effects on the concentrations of both acyclovir andvalproic acid, increased concentrations of acyclovir were associated with reducedconcentration of valproic acid when the two drugs were mixed under constrainedconditions. These observations suggested a possible direct interaction betweenthe two drugs This final part of the study was undertaken to investigate the effect of coadministrationof valproic acid and acyclovir on the pharmacokinetic parametersof each other in a rabbit model. Fifteen white New Zealand rabbits were dividedinto 3 groups A, Band C whereby group A received acyclovir only, group Breceived valproic acid only, and group C received a combination of acyclovir andvalproic acid. In a cross-over design, the intravenous route was studied first,followed by the oral route after a two-week wash out period. Blood samples weredrawn over a 10 hr period and the pharmacokinetic parameters were derivedfrom the concentrations. After intravenous administration, the area under the plasma concentration time curve (AUC) and plasma concentrations of acyciovirin group C were higher than in group A, while the volume of distribution (Vd) andplasma clearance (CLp) of acyciovir in group C were only 12.8% and 10.36% ofthose of group A, respectively. A similar trend was observed after oraladministration. However, the bioavailability (F) of acyclovir was 8.4% in group Aversus 1.5% in group C. Of note, the concentrations and kinetic parameters ofvalproic acid between the two groups after oral and intravenous administrationwere not different. In conclusion, co-administration of single doses of acyclovirand valproic acid led to reduced oral bioavailability of acyclovir, but increasedconcentrations of acyclovir due to reduced volume of distribution and clearanceand this was most probably due to inhibition of the membrane transport proteinsfor acyclovir by valproic acid.Overall, a simple and accurate HPLC method for analysis of acyclovir in plasmawas successfully developed, and a possibility of direct interaction between thetwo drugs was observed both in vitro and in vivo. These observations call for acautious approach to the concomitant use of the two drugs until human studiesare done.

[发布机构] University of the Free State