Altered disposition and antinociception of [D-penicillamine^2,5] enkephalin in mdr1a-gene-deficient mice

This study was undertaken to test the hypothesis that P-glycoprotein (P- gp) modulates opioid peptide pharmacodynamics. [D- Penicillamine^2,5]enkephalin (DPDPE) (10 mg/kg i.v.) was administered to mdr1a(-/-) and wild-type mice to assess systemic disposition and antinociception. A subsequent dose-response experiment examined the impact of P-gp on DPDPE antinociception. In addition, the time course of antinociception was determined after a 0.9-mg/kg [mdr1a(-/-) mice] or 24- mg/kg (FVB mice) i.v. dose. Data were fit with a series of pharmacokinetic- pharmacodynamic models to compare the disposition and action of DPDPE in the two mouse strains. A 10-mg/kg dose produced >80% maximum possible response at all time points in mdr1a(-/-) mice; peak antinociception was <20% maximum possible response in FVB mice. DPDPE systemic disposition did not differ between the two mouse strains. Although brain tissue concentrations were 2- to 4-fold higher in mdr1a(-/-) compared to FVB mice, the dose required to elicit comparable antinociception was nearly 30-fold lower in mdr1a(-/-) mice; brain tissue EC₅₀ differed by an order of magnitude in the two mouse strains. Pharmacokinetic-pharmacodynamic modeling indicated that the difference in antinociception between mdr1a(-/-) and FVB mice was a function of DPDPE distribution within brain, as well as between blood and brain, and not due to differences in intrinsic response. The results of this study suggest that DPDPE is a substrate of P-gp, and that P-gp is responsible, in part, for the low penetration of DPDPE into brain. The substantial difference in brain tissue EC₅₀ in the absence vs. presence of P-gp suggests that P- gp modulates DPDPE-associated antinociception at sites other than the blood- brain interface.