Assessment on the use of allopurinol to improve safety and efficacy of mercaptopurine in pediatric patients with Acute Lymphoblastic Leukemia and Lymphoma during maintenance therapy

Abstract Background Mercaptopurine is an important component of acute lymphoblastic leukemia (ALL) and lymphoma (LLy) maintenance therapy. The 6‐thioguanine nucleosides (6‐TGN) are believed to be the primary contributor to myelosuppression and immunosuppressive effects, while 6‐methylmercaptopurine (6‐MMPN) is believed to be responsible for several toxicities including hepatotoxicity, pancreatitis, and hypoglycemia. Previous reports suggest the addition of allopurinol may reduce these toxicities. Aims To assess the use of allopurinol to improve both safety and efficacy of mercaptopurine in pediatric patients with ALL and LLy during maintenance therapy. Secondary objectives included evaluating patient tolerability and skewed metabolism. In addition, we also analyzed mercaptopurine daily dose reduction upon allopurinol initiation. Methods and Results The primary endpoint was time within goal ANC prior to and after initiation of allopurinol. Secondary endpoints included; improvement in selective toxicities (hepatotoxicity, pancreatitis, and hypoglycemia) and 6‐MMPN to 6‐TGN ratio prior to and after allopurinol initiation. In addition, an exploratory endpoint assessing mercaptopurine daily dose reduction prior to and after allopurinol initiation was included. Sixteen patients met inclusion criteria and 15 (94%) of which were included in this study. Median percent of maintenance days within goal ANC prior to and after initiation of allopurinol was 27.8 (IQR 22.6–44.9) and 41.6 (IQR 20.2–58.2) respectively. All patients experienced selective toxicities; 15 (100%) hepatotoxicity, 1 (7%) pancreatitis, and 3 (20%) hypoglycemia. Improvement of toxicities was seen in 13/15 (87%), 1/1 (100%), and 2/3 (67%) respectively. Average 6‐MMPN:6‐TGN ratio prior to allopurinol initiation was 304:1 and after, allopurinol initiation improved to 15:1, resulting in a 95% reduction. Average mercaptopurine dose prior to and after allopurinol initiation decreased by about 56% (63 to 28 mg/m2/day). Conclusion Results suggest that the use of allopurinol in pediatric patients with ALL and LLy receiving mercaptopurine during maintenance therapy is both safe and effective.


Conclusion:
Results suggest that the use of allopurinol in pediatric patients with ALL and LLy receiving mercaptopurine during maintenance therapy is both safe and effective.

K E Y W O R D S
absolute neutrophil count, acute lymphoblastic leukemia and lymphoma, allopurinol, hepatotoxicity, mercaptopurine

| INTRODUCTION
Acute lymphoblastic leukemia (ALL) is a commonly diagnosed pediatric cancer and has a high overall 5-year survival rate of about 90%. 1 An important component of treatment is maintenance therapy. 2 Despite high success rates, metabolites of mercaptopurine may lead to toxicities that result in an interruption of maintenance therapy.While a majority of these toxicities are reversible, interruption of therapy and reduced dose intensity may result in inferior patient outcomes. 3rcaptopurine is a purine antagonist that leads to immunosuppression and produces anti-leukemic effects.The 6-thioguanine nucleosides (6-TGN) metabolites are believed to be the primary contributor to myelosuppression and immunosuppressive effects, while the 6-methylmercaptopurine (6-MMPN) metabolite is believed to be responsible for several toxicities including hepatotoxicity, pancreatitis, and hypoglycemia. 4In addition, xanthine oxidase (XO) metabolism inactivates up to 70% of mercaptopurine resulting in poor bioavailability. 5lopurinol inhibits XO, increasing the bioavailability of mercaptopurine. 5It is hypothesized that this shunts mercaptopurine metabolism toward 6-TGN metabolites.Case series have been published on the use of allopurinol in combination with mercaptopurine for treatment of irritable bowel disease and leukemia.8][9][10] Due to the bioavailability increase, mercaptopurine is empirically dose reduced by at least 50% upon allopurinol initiation.In summary, allopurinol has been added to mercaptopurine maintenance therapy regimens to abate toxicities and increase antileukemic effects measured by ANC.
The limited data published on this strategy has focused on reduction in hepatotoxicity.Here we report Penn State Health Children's Hospital's experience to decrease mercaptopurine toxicity while optimizing mercaptopurine myelosuppression.The primary objective of this retrospective review was to assess the use of allopurinol to improve both safety and efficacy of mercaptopurine in pediatric patients with ALL and LLy during maintenance therapy.Secondary objectives included evaluating patient tolerability and skewed metabolism.In addition, we also analyzed mercaptopurine daily dose reduction upon allopurinol initiation.

| METHODS
This is a single-center retrospective chart review report assessing safety and efficacy of mercaptopurine in pediatric patients with ALL and LLy during maintenance therapy at Penn State Health Children's Hospital (Hershey, PA, USA).This is a 146-bed pediatric hospital with 18 dedicated hematology/oncology beds. 11Patients treated by the pediatric hematology/oncology service with a diagnosis of ALL or LLy that utilized allopurinol during maintenance therapy between November 12, 2011 and August 31, 2022 were assessed for inclusion.
Exclusion was only achieved in the case of incomplete documentation.
The study was approved by the Penn State Institutional Review Board.
Providers had the option to add allopurinol to maintenance regimens in patients suspected to have toxicities secondary to mercaptopurine at any point during maintenance therapy.Allopurinol was started at a dose of 50 mg/m 2 per day with a max of 100 mg per day.
Once allopurinol was added to the regimen, mercaptopurine was empirically dose reduced by at least 50% and adjusted based on ANC.
ANC levels and labs were monitored by assessing each outpatient and inpatient complete blood count (CBC) beginning upon initiation of maintenance therapy through either completion of maintenance therapy or end of study period.The ANC value from the previous CBC was carried over until the next CBC was obtained to calculate time within ANC goal range of 500 to 1500 cells/microliter.Hepatotoxicity, pancreatitis, and hypoglycemia were defined with the following criteria; AST > 160 unit/L/ ALT > 165 unit/L/ total bilirubin > 1.8 mg/L (CTCAE Grade 3 or greater), lipase > 90 unit/L, and blood glucose < 55 mg/dL (CTCAE Grade 2 or greater) respectively. 12Thresholds were less aggressive then contemporary protocol recommendations for therapy interruption due to hepatotoxicity in order to proactively identify patients with a higher likelihood of progressing to clinically relevant toxicity.These lab values were compared prior to and after allopurinol initiation to determine improvement of toxicity.Of note, lipase levels and blood glucose were only evaluated in symptomatic patients.6-MMPN and 6-TGN levels were collected, as well as their ratio, however a specific goal was not defined.The daily doses of allopurinol and mercaptopurine, and weekly methotrexate administered were assessed.All adverse effects of allopurinol were recorded.
Statistical analysis was completed using Microsoft Excel.The primary endpoint was time within goal ANC prior to and after initiation of allopurinol.Secondary endpoints included; improvement in selective toxicities (hepatotoxicity, pancreatitis, and hypoglycemia) and 6-MMPN to 6-TGN ratio prior to and after allopurinol initiation.
In addition, an exploratory endpoint assessing mercaptopurine daily dose reduction prior to and after allopurinol initiation was included.
Of note, the patient with pancreatitis was experiencing intermittent abdominal pain since receiving their final dose of pegaspargase approximately 1 month prior to starting Maintenance.This was initially attributed to gastritis, but when a serum lipase was checked after the first Maintenance cycle, it remained at least mildly elevated until starting allopurinol.Average 6-MMPN:6-TGN ratio prior to allopurinol initiation was 304:1 and after, allopurinol initiation improved to 15:1, resulting in a 95% reduction.Average mercaptopurine dose prior to and after allopurinol initiation decreased by about 56% (63 to 28 mg/m 2 /day).No allopurinol adverse effects were reported.No patients relapsed during the study period.

| DISCUSSION
Results suggest that the use of allopurinol in pediatric patients with ALL and LLy receiving mercaptopurine during maintenance therapy is both safe and effective.Five patients were still receiving maintenance therapy, so we were not able to see the full effects of allopurinol addition however, our study supports the existing literature that addition of allopurinol to mercaptopurine maintenance regimens may resolve skewed metabolism and reduce hepatotoxic-

CONFLICTS OF INTEREST STATEMENT
The authors have stated explicitly that there are no conflicts of interest in connection with this article.
ity.Allopurinol was shown to increase median time within goal ANC, improve selective toxicities, reduce 6-MMPN:6-TGN ratio, and reduce daily mercaptopurine dose without adverse effects.No patients included in our study experienced relapse during the study period.Limitations of this study include its retrospective, single center design.In addition, it did not compare patients receiving this intervention to a control group limiting the ability to identify patients that may have improved without intervention.Adherence was measured by family report only.Further studies are needed to identify patients who might benefit from early initiation of allopurinol and to help predict an optimal mercaptopurine dose reduction upon initiation of allopurinol.AUTHOR CONTRIBUTIONS Tecca Barone: Conceptualization (equal); data curation (lead); formal analysis (lead); methodology (equal); visualization (lead); writingoriginal draft (lead); writingreview and editing (equal).Smita Dandekar: Conceptualization (equal); methodology (equal); supervision (equal); writingreview and editing (equal).Daniel McKeone: Conceptualization (equal); methodology (equal); supervision (equal); writingreview and editing (equal).Kevin Mulieri: Conceptualization (lead); data curation (supporting); formal analysis (equal); methodology (equal); supervision (lead); writingreview and editing (equal).ACKNOWLEDGMENTS Results were presented at the Eastern States Residency Conference Meeting in Hershey, Pennsylvania on May 10th, 2023 in digital presentation format.The Penn State Clinical & Translational Research Institute, Pennsylvania State University CTSA.

T A B L E 2
Percent of time within goal absolute neutrophil count.