• The evaluation of the metabolism of N-methylspiperone and its effect on kinetic and receptor binding parameter estimation

      Miller, Ann Kay; Young, David G. (1992)
      N-methylspiperone (NMSP) is a butyrophenone derivative that binds preferentially to dopamine D2 receptors in the brain of animals and man. This dissertation describes the development of a specific and sensitive HPLC assay for quantitation of NMSP in guinea pig plasma and brain. The metabolism of NMSP is studied in guinea pigs and rats after multiple intraperitoneal injections of NMSP. Three important metabolites found in guinea pig plasma and brain were identified as reduced NMSP, spiperone, and reduced spiperone. Only spiperone was found in rats. Furthermore, reduced NMSP was found to be a radiolabeled metabolite after single or multiple doses of {dollar}\sp3{dollar}H-NMSP and after a single dose of {dollar}\sp{lcub}11{rcub}{dollar}C-NMSP in guinea pigs. The plasma disposition of NMSP and {dollar}\sp3{dollar}H-NMSP in guinea pigs after a single intravenous dose was best described by a biexponential decline. The half-lives of NMSP distribution and elimination were 11.7 and 289 minutes, respectively. Reduced NMSP was found to follow a mono- or biexponential decline in plasma after a single intravenous dose. The mean elimination half-life was 54.7 minutes. The "donor-recipient" parameter estimation method was evaluated for usefulness in NMSP receptor binding studies. This method includes estimation of transfer rates of formation of a radiolabeled metabolite in the body and transfer of the metabolite into and out of the brain. Data was simulated for a radiolabeled parent compound and its radiolabeled metabolite in the plasma and in the two types of brain regions--those with and without specific binding of parent to the receptors. All parameters except the rate constant of metabolite elimination from the body were estimated within 1% of the true value. Parameters estimated by non-linear least squares regression analysis were found to be similar to those from the "donor-recipient" method. In evaluating the "donor-recipient" method when specific binding is assumed to be reversible, only K23 was poorly estimated. The "donor-recipient" method does provide accurate estimation of all parameters of interest in the receptor binding models determined in this analysis. However, K23, the rate of sequestration into the brain compartment, is only accurately estimated when it is truly represented by a first order process and K23 is zero. (Abstract shortened with permission of author.)
    • Factors that are important in determining the statistical significance of covariates during population pharmacokinetic analyses

      Knebel, William; Young, David G. (2000)
      Population pharmacokinetic analysis has become an integral part of the drug development process. Patient demographic and pathphysiologic characteristics, expressed as either dichotomous (yes/no) or continuous (weight, age, creatinine clearance) covariates, aid in population pharmacokinetic model development by helping to explain some of the variability that exists in the study population. Their inclusion is often used to determine groups in the population that may be more susceptible to adverse drug effects and to determine the proper dose and dosing regimen. There are number of graphical and statistical methods to aid in the detection of dichotomous and continuous covariates during population pharmacokinetic modeling. However, the effect of the range of a continuous covariate and the percentage of patients in a population analysis who are positive for a dichotomous covariate on the ability to detect the covariate during the population analysis have not been determined. This research was designed to investigate these issues. For dichotomous covariates, it was clear based upon the results of this research that a dichotomous covariate can be present in as little as 5% of the population and still be statistically significant in a population pharmacokinetic analysis as long as the effect of the covariate on the pharmacokinetic parameter is 20% or more and the interindividual variability was 10% or less. In addition, the ability to detect the covariate was highly dependent on the interindividual variability and the effect of the covariate on the pharmacokinetic parameter. In order to increase the ability to detect a dichotomous covariate, either the interindividual variability of the pharmacokinetic parameter being affected by the covariate must decrease or the number of subjects with the covariate must increase. For continuous covariates, the results of this research indicated that the magnitude of effect of a continuous covariate in population pharmacokinetic modeling should be 23% or more and the interindividual variability on the pharmacokinetic parameter affected by the covariate should be 10% or less for the covariate to be statistically significant. The ability to detect the continuous covariate can be increased by increasing the range of the covariate or by combining studies to increase the number of subjects in a population pharmacokinetic analysis. However, the latter approach of increasing the number of subjects may prove to be more beneficial with respect to increasing the ability to detect the continuous covariate. In general, the results of this research should be used to plan additional population pharmacokinetic studies in the event of the failure of the initial study or to investigate why a covariate was not detected in a population pharmacokinetic analysis. Guidelines of how to use these results for these purposes are given.
    • 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

      Fossler, Michael James, Jr.; Young, David G. (1994)
      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.