Influence of nonsynonymous polymorphisms of UGT1A8 and UGT2B7 metabolizing enzymes on the formation of phenolic and acyl glucuronides of mycophenolic acid

Mycophenolic acid (MPA) is the active metabolite of mycophenolate mofetil (MMF), a standard immunosuppressive drug approved for clinical use in the prevention of acute allograft rejection after organ transplantation. This study examines the role of the genetic variants of UDP-glucuronosyltransferase (UGT) 1A8 and 2B7 enzymes involved in the formation of the primary metabolite of MPA, the inactive phenolic glucuronide (MPAG), and the reactive acyl glucuronide (AcMPAG). The first exon of UGT1A8 was first resequenced in the region encoding for the substrate binding domain in 254 Caucasians and 41 African Americans. Eight nonsynonymous changes were observed and led to the following amino acid substitutions: S⁴³L, H⁵³N, S¹²⁶G, A ¹⁴⁴V, A¹⁷³G, A²³¹T, T²⁴⁰A, and C²⁷⁷Y. Thirteen haplotypes were inferred, comprising only two previously described alleles, namely, UGT1A8*2 (A¹⁷³G) and UGT1A8*3 (C²⁷⁷Y). Upon stable expression in human embryonic kidney 293 cells, the UGT1A8*3 (C²⁷⁷Y), *5 (G ¹⁷³A²⁴⁰), *7 (A²³¹T), *8 (S ⁴³L), and *9 (N⁵³G) proteins were associated with the most profound decreases in the formation of MPAG and AcMPAG, indicating that these amino acids are critical for substrate binding and enzyme function. Altogether, the low-activity UGT1A8 enzymes are carried by 2.8 to 4.8% of the population. The variant of the UGT2B7 protein (UGT2B7*2 Y²⁶⁸), the main enzyme involved in the formation of AcMPAG, demonstrated a catalytic efficiency comparable with that of UGT2B7*1 (H²⁶⁸). In conclusion, although the common UGT2B7*2 variant is predicted to have limited impact, several UGT1A8 variants identified may potentially account for the large interindividual variance in MMF pharmacokinetics and deserve further clinical investigations.