Differential effects of thiopurine methyltransferase (TPMT) and multidrug resistance-associated protein gene 4 (MRP4) on mercaptopurine toxicity

Mercaptopurine plays a pivotal role in treatment of acute lymphoblastic leukemia (ALL) and autoimmune diseases, and inter-individual variability in mercaptopurine tolerance can influence treatment outcome. Thiopurine methyltransferase (TPMT) and multi-drug resistant Protein 4 (MRP4) have both been associated with mercaptopurine toxicity in clinical studies, but their relative contributions remain unclear.

We studied the metabolism of and tolerance to mercaptopurine in murine knockout models of Tpmt, Mrp4, and both genes simultaneously.

Upon mercaptopurine treatment, Tpmt ^−/− Mrp4 ^−/− mice had the highest concentration of bone marrow thioguanine nucleotides (8.5 pmol/5 × 10⁶ cells, P = 7.8 × 10⁻⁴ compared with 2.7 pmol/5 × 10⁶ cells in wild-types), followed by those with Mrp4 or Tpmt deficiency alone (6.1 and 4.3 pmol/5 × 10⁶ cells, respectively). Mrp4-deficient mice accumulated higher concentrations of methylmercaptopurine metabolites compared with wild-type (76.5 vs. 23.2 pmol/5 × 10⁶ cells, P = 0.027). Mice exposed to a clinically relevant mercaptopurine dosing regimen displayed differences in toxicity and survival among the genotypes. The double knock-out of both genes experienced greater toxicity and shorter survival compared to the single knockout of either Tpmt (P = 1.7 × 10⁻⁶) or Mrp4 (P = 7.4 × 10⁻¹⁰).

We showed that both Tpmt and Mrp4 influence mercaptopurine disposition and toxicity.