Abstract

Striking stereoselective disposition of the antidepressant and smoking cessation aid bupropion and its active metabolites observed clinically influence patients' response to BUP therapy and its clinically important drug-drug interactions (DDI) with CYP2D6 substrates. However, understanding of the biochemical mechanisms responsible is incomplete. This study comprehensively examined hepatic and extrahepatic stereoselective metabolism of BUP in vitro. Racemic-, R- and S-BUP was incubated separately with pooled cellular fractions of human liver (microsomes, HLMs; S9 fractions, HLS9 fractions; and cytosols, HLCs) and intestinal (microsomes, HIMs; S9 fractions, HIS9 fractions; and cytosols, HICs) and cofactors. Formation of diastereomers of 4-hydroxyBUP (OHBUP), threohydroBUP (THBUP) and erythrohydroBUP (EHBUP) were quantified using a novel chiral UHPLC/MS/MS method. Racemic BUP (but not R- or S-BUP) was found suitable to determine stereoselective metabolism of BUP; both enantiomers showed complete racemization. Compared to that of RR-THBUP, the in vitro intrinsic clearance (Cl_int) for the formation of SS-THBUP was 42-, 19-, and 8.3-fold higher in HLMs, HLS9 fractions and HLCs, respectively; Cl_int for the formation of SS-OHBUP and RS-EHBUP were also higher (2.7- to 3.9-fold) than their R-derived counterparts. In cellular fractions of human intestine, {greater than or equal to}95% of total reduction was accounted by the formation of RR-THBUP. Ours is the first to demonstrate marked stereoselective reduction of BUP in HLCs, HIMs, HIS9 fractions and HICs, providing the first evidence for tissue- and cellular fraction-dependent stereoselective metabolism of BUP. These data may serve as the first critical step towards understanding factors dictating BUP's stereoselective disposition, effects and DDI risks.

Significance Statement This work provides a deeper insight into bupropion stereoselective oxidation and reduction to active metabolites in cellular fractions of human liver and intestine tissue. The results demonstrate tissue- and cellular fraction-dependent stereospecific metabolism of bupropion. These data may improve prediction of bupropion stereoselective disposition and understanding of bupropion’s effects and CYP2D6- dependent drug-drug interaction in vivo.