Substrate Report for: Mycophenolic-acid-acyl-glucuronide

Esterases hydrolyze compounds containing ester, amide, and thioester bonds, causing prodrug activation or detoxification. Among esterases, carboxylesterases have been studied in depth due to their ability to hydrolyze a variety of drugs. However, there are several drugs for which the involved esterase(s) is unknown. We found that flutamide, phenacetin, rifamycins (rifampicin, rifabutin, and rifapentine), and indiplon are hydrolyzed by arylacetamide deacetylase (AADAC), which is highly expressed in human liver and gastrointestinal tissues. Flutamide hydrolysis is considered associated with hepatotoxicity. Phenacetin, a prodrug of acetaminophen, was withdrawn due to side effects such as methemoglobinemia and renal failure. It was demonstrated in vitro and in vivo using mice that AADAC is responsible for phenacetin hydrolysis, which leads to methemoglobinemia. In addition, it was shown that AADAC-mediated hydrolysis attenuates the cytotoxicity of rifamycins. Thus AADAC plays critical roles in drug-induced toxicity. Another orphan esterase, alpha/beta hydrolase domain containing 10 (ABHD10), was found responsible for deglucuronidation of acyl-glucuronides including mycophenolic acid acyl-glucuronide and probenecid acyl-glucuronide. Because acyl-glucuronides appear associated with toxicity, ABHD10 would function as a detoxification enzyme. The roles of orphan esterases are becoming increasingly understood. Further studies will facilitate our knowledge of the pharmacologic and toxicological significance of orphan esterases in drug therapy.

In this review, novel aspects of the role of esterases, which contribute to the metabolism of 10% of therapeutic drugs, are described. Esterases hydrolyze the compounds that contain ester, amide, and thioester bonds, which cause prodrug activation or detoxification. Among esterases, carboxylesterases are well known to be involved in the hydrolysis of a variety of drugs. Additionally, other esterases have recently received attention for their pharmacological and toxicological roles. Arylacetamide deacetylase (AADAC) is involved in the hydrolysis of flutamide, phenacetin, and rifamycins. AADAC is associated with adverse drug reactions because the hydrolytic metabolites of flutamide and phenacetin appear to be associated with hepatotoxicity and nephrotoxicity/hematotoxicity, respectively. Paraoxonase and butyrylcholinesterase hydrolyze pirocarpine/simvastatin and succinylcholine/bambuterol, respectively. Although the esterases that hydrolyze the acyl-glucuronides of drugs have largely been unknown, we recently found that alpha/beta hydrolase domain containing 10 (ABHD10) is responsible for the hydrolysis of mycophenolic acid acyl-glucuronide in human liver. Because acyl-glucuronides are associated with toxicity, ABHD10 might function as a detoxification enzyme. Thus, various esterases, which include enzymes that have not been known to hydrolyze drugs, are involved in drug metabolism with different substrate specificity. Further esterase studies should be conducted to promote our understanding in clinical pharmacotherapy and drug development.

Mycophenolic acid (MPA), the active metabolite of the immunosuppressant mycophenolate mofetil (MMF), is primarily metabolized by glucuronidation to a phenolic glucuronide (MPAG) and an acyl glucuronide (AcMPAG). It is known that AcMPAG, which may be an immunotoxic metabolite, is deglucuronidated in human liver. However, it has been reported that recombinant beta-glucuronidase does not catalyze this reaction. AcMPAG deglucuronidation activity was detected in both human liver cytosol (HLC) and microsomes (HLM). In this study, the enzyme responsible for AcMPAG deglucuronidation was identified by purification from HLC with column chromatographic purification steps. The purified enzyme was identified as alpha/beta hydrolase domain containing 10 (ABHD10) by amino acid sequence analysis. Recombinant ABHD10 expressed in Sf9 cells efficiently deglucuronidated AcMPAG with a K(m) value of 100.7 +/- 10.2 muM, which was similar to those in HLM, HLC, and human liver homogenates (HLH). Immunoblot analysis revealed ABHD10 protein expression in both HLC and HLM. The AcMPAG deglucuronidation by recombinant ABHD10, HLC, and HLH were potently inhibited by AgNO(3), CdCl(2), CuCl(2), PMSF, bis-p-nitrophenylphosphate, and DTNB. The CL(int) value of AcMPAG formation from MPA, which was catalyzed by human UGT2B7, in HLH was increased by 1.8-fold in the presence of PMSF. Thus, human ABHD10 would affect the formation of AcMPAG, the immunotoxic metabolite.