Sex-related pharmacokinetic differences and mechanisms of metapristone (RU486 metabolite)

Abstract

Metapristone is the primary metabolite of the abortifacient mifepristone (RU486), and is being developed as a safe and effective cancer metastatic chemopreventive agent for both sexes. Here, we systematically investigated the sex-related pharmacokinetics of metapristone in both rats and dogs, and explored the related mechanisms of actions. Administration of metapristone to rats and dogs showed that plasma concentrations of metapristone (AUC, C _max ) were significantly higher in female dogs and rats than in males. The sex-related differences in pharmacokinetics become more significant after ten consecutive days of oral administration. Female liver microsomes metabolized metapristone significantly slower than the male ones. The results from P450 reaction phenotyping using recombinant cDNA-expressed human CYPs in conjunction with specific CYP inhibitors suggested that CYP1A2 and CYP3A4 are the predominant CYPs involved in the metapristone metabolism, which were further confirmed by the enhanced protein levels of CYP1A2 and CYP3A4 induced by 1-week oral administration of metapristone to rats. The highest tissue concentration of metapristone was found in the liver. The study demonstrates, for the first time, the sex-related pharmacokinetics of metapristone, and reveals that activities of liver microsomal CYP1A2 and CYP3A4 as well as the renal clearance are primarily responsible for the sex-related pharmacokinetics.

Figure 1

Plasma concentration-time courses of metapristone released after p.o. or i.v. administrations to male and female rats (left) and beagle dogs (right). Each point represents the mean ± s.d. of six rats or three dogs. Note, significant differences between sexes existed in areas under plasma concentrations of metapristone based on the ANOVA analysis.

Figure 2

The three-compartmental model of metapristone released once absorbed. The model is composed of the central compartment, shallow chamber, and poorly-filled chamber. Metapristone, after absorbed into the circulation system via i.v. or p.o. administrations, is distributed from the central compartment into either shallow chamber with the fast constant rate (K₁₂), or poorly-filled chamber with the slow constant rate (K₁₃). Blood metapristone travels back to the central compartment with the constant rates K₂₁, or K₃₁.

Figure 3

Plasma concentrations and tissue levels of metapristone released. (A) Accumulated metapristone in blood at 6 h following 10 consecutive days of oral administration of metapristone to rats. Plasma metapristone in females was significantly higher than in males. (B) Female rat tissue concentrations of metapristone at 6 and 24 h after single oral administration. Each bar represents the mean ± s.d. of 4 rats receiving 45 mg/kg metapristone.

Figure 4

Metabolism rate of metapristone when incubated with male or female rat liver microsomes. Metapristone was added to the liver microsomal mixtures of male or female rats at the final concentrations of 6, 9 and 13.5 µM at 37 °C, followed by quantitative analysis of the remaining metapristone.

Figure 5

Changes of metapristone metabolism rate induced by male and female liver microsomes in the presence and absence of CYP inhibitors. (A) No significant differences in metabolism rate of metapristone (9 µM) between controls and CYP450 inhibitor sulfaphenazole (CYP2C9), quinidine (CYP2D6), or ticlopidine (CYP2C19); whereas CYP450 inhibitor ketoconazole (CYP1A2) and naphthoflavone (CYP3A4) significantly prevented metapristone from degradation in comparison with the controls during the 120 min incubation. The data also show statistically significant differences in metapristone metabolism between female and male liver microsomes in the presence of the five inhibitors. (B) α-naphthoflavone produced concentration-dependent prevention against metapristone metabolism induced by human CYP3A4; (C) ketoconazole produced concentration-dependent prevention against metapristone metabolism induced by human CYP1A2. (D) up-regulated activities of rat liver CYP1A2 and CYP3A4 after 7-day administration of metapristone. Data represent the mean ± s.d. (n = 3); *P < 0.05; **P < 0.01 of statistically significant difference (Student t test).

Figure 6

Conclusive schematic of the sex-related pharmacokinetics of metapristone and the related mechanisms. Rat blood samples were obtained after metapristone administration, and analyzed by using the validated LC/MS/MS method. The analysis revealed that metapristone metabolized by liver microsomes was slower in females than in males probably because of lower activities of CYP1A2 and CYP3A4 in females (left panel). The corrected clearance (CL/F, representing the renal clearance) of metapristone (p.o. and i.v.) from the body was significantly slower in females than in males (right panel; the data were extracted from Table 1). As a result, plasma concentrations of metapristone in female rats were higher in males.