Should Indian patients be screened for azathioprine pharmacogenomic variants (TPMT and NUDT15) before initiating therapy?

Should Indian Patients Be Screened for Azathioprine Pharmacogenomic Variants?

Yes, Indian patients should be screened for both TPMT and NUDT15 variants before initiating azathioprine therapy, with NUDT15 testing being particularly critical in this population.

Rationale Based on FDA Labeling and Guidelines

The FDA drug label explicitly recommends considering TPMT and NUDT15 testing in patients who experience severe bone marrow toxicities, and states that homozygous deficiency in either gene warrants consideration of alternative therapies due to increased toxicity risk 1. For heterozygous deficiency in either gene, dosage reduction is recommended, with patients heterozygous for both requiring more substantial reductions 1.

The ACMG 2022 technical standard reinforces this by stating that FDA-approved labeling and CPIC guidelines recommend dose adjustment of thiopurines for patients with reduced TPMT or NUDT15 activity, or using alternative nonthiopurine immunosuppressants for nonmalignant conditions 2. The guideline specifically notes that TPMT2, TPMT3A, and TPMT3C account for about 95% of individuals with reduced TPMT activity, while NUDT153 is the most commonly observed nonfunctional allele 2.

Why This Matters Specifically for Indian Patients

The genetic landscape in Indian populations differs dramatically from European populations, making NUDT15 testing especially important:

• NUDT15*3 variant frequency is 10.7% in Indians 3, significantly higher than in European populations 4
• TPMT variants occur in only 3.5% of Indian patients, while NUDT15 variants occur in 19.4% 3
• In one Indian study, all 10 patients who developed leukopenia harbored the NUDT15 variant (p<0.0001) 3
• Among Indian patients heterozygous for NUDT15*3, 92.86% exhibited thiopurine-mediated hematological toxicity 4

This contrasts sharply with the traditional focus on TPMT testing, which was developed primarily in European populations where TPMT variants are more common.

Clinical Implementation Algorithm

Pre-treatment screening approach:

1. Test both TPMT and NUDT15 before initiating azathioprine

   • TPMT: Test for *3A, *3B, *3C variants (phenotyping or genotyping)
   • NUDT15: Test for c.415C>T (p.R139C) variant, also known as NUDT15*3

2. Dosing based on results:

   • Homozygous deficient (either gene): Consider alternative therapy; if azathioprine used, start at ≤10% of standard dose 1
   • Heterozygous deficient (single gene): Reduce dose to 30-80% of standard dose 1
   • Heterozygous for both genes: Require more substantial reductions; consider starting at 10-30% of standard dose 1
   • Wild-type for both: Start at standard dose (1.0 mg/kg for rheumatoid arthritis, 3-5 mg/kg for transplant) 1

3. Monitoring remains mandatory regardless of genotype:

   • Weekly CBC for first month
   • Twice monthly for months 2-3
   • Monthly thereafter 1
   • Genetic testing does not substitute for CBC monitoring 1

Critical Pitfalls to Avoid

Do not rely solely on TPMT testing in Indian patients. The evidence shows NUDT15 is clinically more relevant than TPMT in this population in terms of sensitivity, specificity, and statistical significance for predicting toxicity 3. Testing only TPMT would miss the majority of at-risk Indian patients.

Do not assume normal genotype eliminates risk. Non-genetic factors including drug-drug interactions (particularly with allopurinol, which requires dose reduction to 25-33% of usual dose 1), renal dysfunction, and concurrent medications affecting myelopoiesis can still cause toxicity 2, 1.

Do not skip monitoring based on favorable genotype. The FDA label explicitly states that genetic testing cannot substitute for complete blood count monitoring 1.

Integrated Approach for Optimal Safety

Recent Indian case series demonstrate that combining TPMT phenotyping, NUDT15 genotyping, and therapeutic drug monitoring of 6-thioguanine nucleotide (6-TGN) metabolites represents the most comprehensive approach to optimize therapy and prevent myelosuppression 5. This triple approach allows for:

• Pre-emptive dose adjustment based on genetics
• Real-time assessment of drug exposure through metabolite levels
• Individualized dose titration to balance efficacy and toxicity

The evidence strongly supports that preemptive NUDT15 genotyping can prevent fatal thiopurine-induced toxicity in Indian patients 3, making it a critical component of safe azathioprine prescribing in this population.