Pharmacokinetics and pharmacodynamics of 4-aminopyridine: Evaluating the pharmacokinetics of 4-aminopyridine in Sprague-Dawley rats and modeling its effect on the EEG using Fourier analysis

Abstract

The compound 4-aminopyridine (4AP) is a potassium channel blocker under investigation as a potential treatment for a number of neurological conditions. The data resulting from a recent clinical study suggested that the pharmacokinetics of the compound may differ between men and women, with women having significantly higher peak levels than men. The data also suggested that the CNS side effects seen with this compound (mental status changes, convulsions) tended to occur at plasma levels >= 100 ng/mL. The following animal studies were performed in an effort to (1) determine whether there is a gender difference in the pharmacokinetics of 4AP in rats, and (2) determine the relationship between the plasma concentration of 4AP and its CNS toxicity using fast Fourier transform analysis of the EEG and pharmacokinetic/pharmacodynamic (PK/PD) modeling techniques. For the first study, male or female rats were given 4AP either intravenously or orally, and blood samples taken up to 3 hours post-dose. The resulting plasma samples were analyzed for 4AP by a reverse-phase ion-pairing HPLC method developed in preparation for this work. The results of this study indicated that female rats have a significantly decreased mean clearance than male rats (12.0 vs 14 9 mL/min, p < 0.05). This difference is most likely due to differences in body weight. When 4AP was given orally, it was found that female rats had a significantly longer half-life of elimination, and a significantly decreased maximum concentration (Cmax) as compared with male rats. The difference in Cmax persisted when corrected for AUC, indicating a difference in GI absorption between male and female rats. The precise mechanism for this difference is not known, but may be related to female sex hormones. For the second study male rats were implanted with cortical screws for EEG collection. The EEGs of rats dosed with 4AP showed significant increases in high frequencies as compared to rats dosed with saline. The symmetrized percent increase in power from 40-50 Hz was selected as a surrogate marker of CNS toxicity. Using this measurement of drug effect, a PK/PD model of the CNS toxicity of 4AP was developed. Rats were implanted with cortical screws, given either 1 or 0.5 mg 4AP and both EEG data and blood samples were collected over 3 hours. The resulting data were modeled using the Hill equation. The model was validated using Monte Carlo simulation techniques. The model suggests that the CNS toxicity of 4AP is reduced at plasma concentrations below 100 ng/mL, which is similar to what was seen in humans, suggesting that the EEG may be a good cross-species indicator of neurotoxicity.