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Nevirapine (NVP) is associated with severe liver and skin toxicity through sulfotransferase (SULT) bioactivation of the phase I metabolite 12‐hydroxy‐NVP [1–3]. The female sex, a well‐known risk factor for NVP‐induced toxicity, is associated with higher SULT expression [4] and lower plasma levels of 12‐hydroxy‐NVP [3]. Interestingly, apolipoprotein A1 (ApoA1) increases SULT2B1 activity and ApoA1 synthesis is increased by NVP [5, 6]. Herein, we explore the effect of ApoA1 levels on NVP metabolism and liver function. The study protocol was firstly approved by the hospitals’ Ethics Committees. All included individuals were HIV‐infected patients treated with NVP for at least one month. The plasma concentrations of NVP and its phase I metabolites were quantified by HPLC [7]. ApoA1 levels were assessed by an immunoturbidimetric assay. Forty‐nine HIV‐infected patients on NVP were included (53% men, 59% Caucasian). NVP plasma levels were correlated with HDL‐cholesterol (Spearman r=0.2631; p=0.0441) and ApoA1 (Spearman r=0.3907; p=0.0115). Women had higher ApoA1 levels than men (Student's t Test; p=0.0051). In both sexes, 12‐hydroxy‐NVP levels were negatively correlated with ApoA1 (male: Spearman r=−0.3810; p=0.0499 female: Spearman r=−0.5944; p=0.0415). In men, ApoA1 was positively correlated with aspartate aminotransferase (AST, Spearman r=0.5507; p=0.0413), while in women ApoA1 was associated (Spearman r=0.6408; p=0.0056) with alanine aminotransferase (ALT). These results show sex differences in NVP‐induced ApoA1 synthesis. The higher ApoA1 levels in women might stabilize SULT2B1 [6]. This would explain the lower levels of 12‐hydroxy‐NVP [3] and the higher hepatotoxicity found in women, due to increased sulfonation of this metabolite. These data support a role for ApoA1 in the sex dimorphic mechanism leading to NVP‐induced toxicity., Acknowledgments RECI/QEQ‐MED/0330/2012; PTDC/SAU‐TOX/111663/2009; EXPL/DTP‐FTO/0204/2012. References Antunes AM, Godinho AL, Martins IL, Justino GC, Beland FA, Marques MM. Amino acid adduct formation by the nevirapine metabolite, 12‐hydroxynevirapine – a possible factor [...]