Contribution of UGT2B17 to the Pharmacokinetics of Diclofenac

Overview

The purpose of this pilot study is to gather preliminary data on the (1) contribution of the understudied drug metabolizing enzyme, UDP-glucuronosyltransferase (UGT) 2B17, to the metabolism of a widely used medication, diclofenac, and (2) impact of the UGT2B17 inhibitor and natural product, curcumin, on diclofenac pharmacokinetics. Results will inform future studies aimed to assess the effects of UGT2B17 genetic polymorphisms and co-consumed xenobiotics on the pharmacokinetics and toxicity risk of diclofenac and other UGT2B17 drug substrates.

Diclofenac, a widely used non-steroidal anti-inflammatory drug, has been linked to severe adverse effects such as gastrointestinal ulcers and bleeding and cardiotoxicity. Based on cardiotoxicity reports, US and European regulatory agencies withdrew over-the-counter (OTC) diclofenac, requiring the drug to be a prescription-only medication. However, diclofenac remains OTC in many countries, including Australia, China, and India, among others. Reported metabolic pathways of diclofenac in the liver are mediated by the prominent drug metabolizing enzyme, cytochrome P450 (CYP) 2C9, and the conjugative enzyme, UDP-glucuronosyltransferase (UGT) 2B7. However, recent in vitro and in silico data indicated that diclofenac is metabolized almost exclusively by a less-studied UGT, UGT2B17, in the intestine. UGT2B17 is among the most genetically polymorphic enzymes, with highly prevalent copy number variations (CNVs). Individuals homozygous for the null allele, UGT2B17\*2 (CNV=0), are considered poor metabolizers (PMs), whereas those homozygous for the reference allele, UGT2B17\*1/\*1 (CNV=2), are considered extensive metabolizers (EMs). Deletion of this gene may lead to large interindividual variability in the pharmacokinetics - and toxicity risk - for patients taking diclofenac and other UGT2B17 substrates, including vorinostat, MK7426, tamoxifen, exemestane, and testosterone. Collectively, considering UGT2B17 CNVs on the metabolism of these drugs is critical to ensure consistent and optimum safety, efficacy, and patient outcomes. Recent preliminary data showed that curcumin, a principal curcuminoid of the natural product turmeric, is a potent inhibitor of UGT2B17. Turmeric is used worldwide and was the 2nd top-selling herbal supplement in the US in 2021, with nearly $100 million in total sales. Considering both turmeric/curcumin and diclofenac are used for arthritis and other inflammatory conditions, there is a high likelihood of patients co-consuming curcumin and diclofenac, raising concerns for variable diclofenac pharmacokinetics and toxicity risk. The purpose of this pilot study is to gather preliminary data on the (1) contribution of UGT2B17 to diclofenac metabolism and (2) impact of curcumin co-administration on diclofenac pharmacokinetics. Results will inform future studies aimed to evaluate the effects of UGT2B17 genetic polymorphisms and co-consumed xenobiotics on the pharmacokinetics and toxicity risk of diclofenac and other UGT2B17 drug substrates.