The oral bioavailability of the antiretroviral efavirenz (EFV) undergoes high inter and intra-individual variability, this fact supporting its therapeutic drug monitoring. Previously, it was demonstrated that the encapsulation of EFV within polymeric micelles increases the oral bioavailability of the drug. The breast cancer resistant protein (BCRP, ABCG2) is known to be inhibited by EFV in vitro. Since ABCG2 is profusely expressed in the gastrointestinal tract, the aim of the present work was to thoroughly investigate whether the intestinal permeability of EFV is modulated by ABCG2. The functional role of ABCG2 in mediating the transport of EFV at the intestinal level was consistent with the following findings: (a) an ABCG2 inhibitor, fumitremorgin C (5-10μM), significantly potentiated the mucosal-to-serosal permeation of the drug in everted gut sacs; (b) a five-day oral treatment with 20mg/kg EFV promotes the over-expression of ABCG2 in about 100%, this phenomenon being accompanied by a clear decline in the intestinal permeability of the antiretroviral and (c) the normalization of the ABCG2 expression within 24h after the last administration of EFV was coincident with the recovery of the ability of the drug to permeate through the small intestine wall. Interestingly, no interactions between EFV and P-glycoprotein (ABCB1) were apparent. Since the intestinal permeability of a drug could be associated with its in vivo absorbability, we suggest that the oral absorption of EFV is affected by modifications in the ABCG2 intestinal expression contributing to the intra-individual bioavailability variations.
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