Allopurinol Add-on Treatment - Promising Approach to Opitimize Maintenance Therapy for Acute Lymphoblastic Leukemia in Children

Introduction Allopurinol can be used in maintenance therapy (MT) for acute lymphoblastic leukemia (ALL) to mitigate hepatic toxicity in patients with skewed 6-mercaptopurine (6MP) metabolism. These patients have high erythrocyte levels of methylated mercaptopurine metabolites (e-MeMP) associated with liver toxicity, including transaminitis and hypoglycemia, and low erythrocyte levels of thioguanine nucleotides (e-TGN), the key intermediate metabolites mediating the antileukemic effect. Retrospective studies and case reports show that the unfavorable metabolite ratio (high e-MeMP/TGN) can be modified by adding allopurinol leading to lower MeMP and higher TGN. In a recent publication https://doi.org/10.3324/haematol.2023.284390 we showed that allopurinol leads to a similar metabolic shift also in unselected pediatric ALL-patients, without previous severe liver toxicity or other signs of skewed 6MP metabolism. We have now analysed the levels of DNA incorporated TGN (DNA-TG) in our cohort since more recent studies, e.g. https://doi.org/10.1016/s1470-2045(17)30154-7, suggests that relapse-free survival is better correlated to DNA-TG than to e-TGN. Methods Pediatric ALL patients on NOPHO ALL-2008 non-high risk protocols with thiopurine methyltransferase wild-type were studied in a prospective before-after trial, NCT03022747. 6MP metabolites were measured, in total 9 blood samples per patient, during 12 weeks of standard MT, followed by 12 weeks of MT with addition of allopurinol 50 mg/m2 and finally 4 weeks of MT without allopurinol. Mean DNA-TG for each patient was calculated for all study phases separately. DNA-TG/e-TGN ratio was calculated as there was a concern that a decrease in e-MeMP might reduce the DNA incorporation of TGN, since e-MeMP is known to inhibit de novo purine synthesis. The 6MP dose was reduced by 50% when allopurinol was initiated to prevent excessive myelosuppression. DNA-TG was quantified using the same method as in the study cited above, with 1−2 µg DNA purified from whole blood and thioguanine measured with ultra-performance liquid chromatography tandem mass spectrometry. Results 51 patients from Sweden and Finland, age 0-15 (median 4) years, were included, of whom 48 completed the study. In paired analysis DNA-TG was 393 fmol/mg DNA higher (1248 vs 855) (p<0.001), e-TGN 193 nmol/mmol Hb higher (457 vs 264) (p<0.001) and e-MeMP 6324 nmol/mmol Hb lower (2614 vs 8938) (p<0.001) when allopurinol was added, Mean DNA-TG/e-TGN ratio did not change significantly, 3.15 on allopurinol compared to 3.51 before, (p=0.10). Four weeks after allopurinol was discontinued, mean DNA-TG decreased 564 fmol/mg DNA (p<0.001), e-TGN decreased 271 nmol/mmol Hb (p<0.001) whereas e-MeMP showed a non-significant increase of 777 nmol/mmol Hb (p=0.17). Alanine aminotransferase decreased by 39% a few weeks after the e-MeMP-levels decreased. Mean absolute neutrophil count (ANC) was lower on allopurinol, 1.35 x 109/L, compared to 1.72 before (p<0.001). Allopurinol did not increase the number of severe adverse events (SAE) nor cause any life-threatening episodes. In accordance with study protocol the mean given dose 6MP was halved during the weeks on allopurinol, 187 mg/m2/week compared to 379 (p<0.001) for the weeks before allopurinol. Conclusions Addition of allopurinol to standard MT with 6MP and methotrexate leads to substantially increased DNA-TG levels. DNA-TG/e-TGN ratio was unchanged indicating that DNA incorporation of TGN was not significantly affected during allopurinol treatment. The 46 % rise in DNA-TG, together with our previously published data with higher proportion of ANC levels within target, lower ALT and no increase in SAE, indicate that addition of allopurinol could be an effective strategy to optimize ALL MT.