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

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 by herpes simplex type I & II and varicella-zoster viruses. Given the high prevalence of people with conditions for which chronic use of valproic acid is indicated, and the notion that valproic acid increases the antiviral activity of acyclovir, it is not uncommon for the two drugs to be used concomitantly. As such, recent reports on the interaction between valproic acid and acyclovir with break through convulsions were a cause for concern. Since understanding the mechanism of this interaction is vital to the establishment of concrete guidelines on the use of the two drugs in patients, the aim of this study was to investigate the possible pharmacokinetic interaction between acyclovir and valproic acid. First, a high performance liquid chromatography (HPlC) method for analysis of acyclovir in plasma was developed. It involved simple protein precipitation of 200 IJl of plasma with perchloric acid, followed by centrifugation after which 20 IJl of the supernatant was injected in the HPlC. The sample was eluted with acetonitrile: 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) 51Janalytical column. 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 range of 10 - 100 IJg/ml, linearity was demonstrated by a linear regression equation of y = 0.03196 - 3.207x with a regression coefficient r² = 0.995, and accuracy by a percentage coefficient of variation (CV%) of less than 15%. The method was successfully used to analyze acyclovir in a rabbit treated with acyclovir single dose. Thereafter, the possibility of a direct interaction between acyclovir and valproic acid in vitro was investigated by monitoring the concentrations of valproic acid and acyclovir at different pH (pH 7.4 or pH 3 or pH 10) and temperatures (25°C and 37°C) when mixed in a 1:1 molar ratio or prepared separately in phosphate buffer. The samples were incubated at 25°C for 2 hours and a further 1 hour at 370C, and aliquots were drawn at 10 min., 2 and 3 hours to measure the concentration of valproic acid and acyclovir (n=3). The average concentrations of valproic acid and acyclovir from the samples containing the single drug were not different (P > 0.05) from those in the mixture of both drugs at the different temperatures and pH. However, when the temperature and pH were evaluated separately, there was a trend whereby, at high temperature (37°C), the concentrations of acyclovir (percentage detected) tended to be higher in the mixture (87%) than when it was alone (84%), while those of valproic acid tended to be lower in the mixture (89%) than when it was alone (92%). This same trend was observed at acid or alkaline pH. In conclusion, although temperature and pH did not induce significant effects on the concentrations of both acyclovir and valproic acid, increased concentrations of acyclovir were associated with reduced concentration of valproic acid when the two drugs were mixed under constrained conditions. These observations suggested a possible direct interaction between the two drugs This final part of the study was undertaken to investigate the effect of coadministration of valproic acid and acyclovir on the pharmacokinetic parameters of each other in a rabbit model. Fifteen white New Zealand rabbits were divided into 3 groups A, Band C whereby group A received acyclovir only, group B received valproic acid only, and group C received a combination of acyclovir and valproic 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 were drawn over a 10 hr period and the pharmacokinetic parameters were derived from the concentrations. After intravenous administration, the area under the plasma concentration time curve (AUC) and plasma concentrations of acyciovir in group C were higher than in group A, while the volume of distribution (Vd) and plasma clearance (CLp) of acyciovir in group C were only 12.8% and 10.36% of those of group A, respectively. A similar trend was observed after oral administration. However, the bioavailability (F) of acyclovir was 8.4% in group A versus 1.5% in group C. Of note, the concentrations and kinetic parameters of valproic acid between the two groups after oral and intravenous administration were not different. In conclusion, co-administration of single doses of acyclovir and valproic acid led to reduced oral bioavailability of acyclovir, but increased concentrations of acyclovir due to reduced volume of distribution and clearance and this was most probably due to inhibition of the membrane transport proteins for acyclovir by valproic acid. Overall, a simple and accurate HPLC method for analysis of acyclovir in plasma was successfully developed, and a possibility of direct interaction between the two drugs was observed both in vitro and in vivo. These observations call for a cautious approach to the concomitant use of the two drugs until human studies are done.