MAUI, HAWAII – There’s a new kid on the block to worry about when it comes to thiopurine pharmacogenetics: Genetic variants in the thiopurine-metabolizing enzyme nudix hydrolase 15 have been linked to a markedly increased risk of thiopurine myelosuppression among inflammatory bowel disease patients.
The Food and Drug Administration and others already recommend screening for genetic variants in thiopurine methyltransferase (TPMT), another enzyme that metabolizes thiopurine. Polymorphisms lead to TMPT dysfunction, accumulation of cytotoxic metabolites, and increased risk of thiopurine-induced myelosuppression (TIM). Carriers are advised to use reduced doses with careful drug monitoring, or to skip thiopurines altogether.
A similar picture is emerging for nudix hydrolase 15 (NUDT15). It’s been known for several years that genetic variants are not uncommon among East Asian people and lead to TIM, but their prevalence and impact among people of European decent wasn’t clearly understood until now.
Investigators led by Gareth Walker, MBBS, of the Royal Devon and Exeter Hospital in Exeter, England, compared rates of problematic TMPT and NUDT15 variants among European inflammatory bowel disease patients who had developed TIM and those who had not, about 1,000 patients in all. The majority were on azathioprine and had Crohn’s disease. Finnish people were excluded because “their unique genetic background ... has led to the enrichment of some disease-causing gene variants and losses of others,” according to the study, which was published in JAMA.
Carriage of any of three coding NUDT15 variants greatly increased the risk of TIM (odds ratio, 27.3; 95% confidence interval, 9.3-116.7), independent of TPMT genotype and thiopurine dose. A particular variant – an in-frame deletion in NUDT15 – increased the risk 38-fold (95% CI, 5.1-286.1), and was carried by 5.8% of TIM patients.
The analysis also confirmed the importance of TPMT variants, which were found in 30.5% of TIM patients (95/311) versus 16.4% (100/608) of patients who did not develop TIM.
“Patients with variants of either NUDT15 or TPMT, or among those with variants of both genes, had a faster onset of TIM, more severe TIM, and had a greater need for granulocyte colony–stimulating factor rescue therapy. ... Our data suggest that pretreatment sequencing of the NUDT15 gene ... may be considered prior to initiation of thiopurine therapy,” the team concluded.
The prevalence of problematic NUDT15 variants among non-Finnish Europeans is about 1.6%, 6.9% among people from Finland, and almost 30% among East Asians. The team estimated NUDT15 would have to be genotyped in 95 non-Finnish Europeans to prevent one case of TIM; the number is 123 for TPMT. “Given the widespread use of thiopurines” – primarily in rheumatology and transplant medicine, in addition to gastroenterology – “these findings may have ramifications beyond the management of IBD,” the investigators wrote.
M. Alexander Otto/MDedge News
Dr. Edward Loftus
“I do think it’s worthwhile” to screen for NUDT15, said Edward Loftus, MD, a professor and consultant in the division of gastroenterology and hepatology at the Mayo Clinic in Rochester, Minn., who reviewed the study at the Gastroenterology Updates, IBD, Liver Disease Conference. “If you are a homozygote for this, your chance of getting profound leukopenia is very high, so I would probably not use a thiopurine.”
“If you are going to start low dose on everyone” with careful blood monitoring, “I suppose you could just do that, but I would say if you can get” the test and “are reassured the patient is not” carrying problematic NUDT15 or TMPT variants, “then I think you just go ahead and do full dose,” he said.
Testing for the relevant variants is available through the Mayo Clinic and several commercial labs.
“Even though the thiopurine dose is adjusted based on genotype, patients still need to be monitored closely for the development of thiopurine-induced myelosuppression as” TMPT and NUDT15 variants “do not explain all cases of” TIM, Marieke Coenen, PhD, an associate professor of pharmacogenetics at Radboud University Medical Center in Nijmegen, the Netherlands, cautioned in an editorial (Transl Gastroenterol Hepatol. 2019 Dec 12;4:81).
The prevalence of problematic NUDT15 variants is 0.7% among African and 20.7% among Hispanic people.
The work was funded by the National Institutes of Health, Crohn’s & Colitis UK, the Wellcome Trust, and others. Dr. Walker and other investigators reported numerous industry ties. Dr. Loftus is a consultant and/or has research funding from Abbott, Pfizer, and other companies. Dr. Coenen had no disclosures.
SOURCE: Walker GJ et al. JAMA. 2019 Feb 26;321(8):773-85.