Alpha‐1‐acid glycoprotein directly affects the pharmacokinetics and the analgesic effect of methadone in the rat beyond protein binding

The effect of elevated α1-acid glycoprotein on the pharmacokinetics of TAK-272 (SCO-272), an orally active renin inhibitor, in rats

The pharmacokinetics of TAK-272 (SCO-272), an orally active renin inhibitor, was investigated in rats with subcutaneously injected turpentine oil, which was an inflammation animal model. Following intravenous administration of TAK-272 to the turpentine-treated rats, the systemic clearance and volume of distribution decreased with the elevated plasma α₁-acid glycoprotein (AGP) levels. The elevated plasma AGP levels were negatively correlated with the plasma unbound fraction of TAK-272 in the rats. Although the AUCs of total TAK-272 in the turpentine-treated rats were higher than those in the control rats after intravenous and oral administration, those of unbound TAK-272, which seem to directly contribute to the pharmacological effect and safety, were nearly equal between the turpentine-treated and control rats in the respective dose routes. TAK-272 has been shown to primarily bind to AGP in the human plasma. These results strongly suggested that the pharmacokinetic of TAK-272 in humans would also be affected by the variation in the plasma AGP levels and should be discussed with not only the total concentrations but also the unbound concentrations in the clinical trial for patients with elevated plasma AGP levels.

Modeling methadone pharmacokinetics in rats in presence of P-glycoprotein inhibitor valspodar

PURPOSE

To quantify the in vivo role of P-glycoprotein (P-gp) in the pharmacokinetics of methadone after intravenous and oral administration, using valspodar as a P-gp inhibitor.

MATERIALS AND METHODS

Methadone plasma concentrations after intravenous (0.35 mg/kg) and oral (6 mg/kg) administration were analyzed, in absence and presence of valspodar, using nonlinear mixed effects modeling (NONMEM V). Non-parametric bootstrap analysis and posterior predictive check were employed as model evaluation techniques.

RESULTS

The pharmacokinetics of methadone in the rat was successfully modeled using a two-compartmental model with a linear elimination from the central compartment and a first-order absorption process with lag time. Valspodar increased methadone F by 122% (95%CI: 34-269%) and decreased the V ( c ) and V ( p ) by 35% (95%CI: 16-49%) and 81% (95%CI: 63-93%), respectively. No effect of valspodar on other pharmacokinetic parameters was discernible. The non-parametric bootstrap analysis confirmed the absence of bias on the parameter estimates, and visual predictive check evidence the adequacy of the model to reproduce the observed time course of methadone plasma concentrations.

CONCLUSION

Valspodar increased methadone's bioavailability as consequence of P-gp inhibition, which resulted in an increased analgesic effect of methadone.

Changes to methadone clearance during pregnancy

OBJECTIVE

Measurement of plasma methadone concentration to investigate the rate of clearance of methadone prescribed for heroin dependence in the first, second and third trimesters of pregnancy. A secondary objective was to evaluate the outcome of pregnancy.

METHODS

Longitudinal within subject study of nine pregnant opioid dependent subjects prescribed methadone at the Leeds Addiction Unit, an outpatient community based treatment centre. Plasma concentration versus time data for methadone was collected during each trimester and post-partum for our subjects. Data was available for the first and second trimesters for 4/9 cases. All but one of the subjects provided data during the third trimester and data post-partum was collected from three respondents. Measurements of methadone levels in plasma were carried out using high performance liquid chromatography (HPLC).

RESULTS

Trough mean plasma methadone concentrations reduced as the pregnancies progressed from 0.12 mg/L (first trimester) to 0.07 mg/L (third trimester). The weight-adjusted clearance rates gradually increased from a mean of 0.17 to 0.21 L/hr/kg during pregnancy, although patterns differed substantially between the nine women. An assessment of relative clearance of methadone using two patients for whom we have had all three CL values (trimester 1-3) demonstrated notable change of CL (P = 0.056) over time. Eight of our subjects delivered (3 males), within two weeks of their due date the ninth (male) was premature (21 days). The mean length of gestation was 39.7 weeks (SD = 10 days) and none of the neonates met criterion for 'low birth weight' mean = 3094, SD = 368 g). Five neonates spent time (0.5-28 days) in a special care baby unit (SCUBU) and 4 of these displayed signs of methadone withdrawal.

CONCLUSIONS

General Practitioners and hospital doctors should recognise the significant benefits of prescribing methadone for heroin-dependent women during pregnancy. We recommend that if a pregnant opioid user complains of methadone withdrawal symptoms (i.e. that the methadone dose does not "hold" them) the prescribing clinician takes this observation seriously and considers a more detailed assessment. Further work on key factors undergoing changes during pregnancy accounting for differences in methadone metabolism in the mother, fetus and neonate are required.