(M-060) Impact of Drug Interactions and Genetic Variability on High-Dose Methotrexate in Children with Acute Lymphoblastic Leukemia

Keito Hoshitsuki, Pharm.D., Ph.D. – Postdoctoral Research Associate, St. Jude Children's Research Hospital; Carl Panetta, Ph.D. – Lead-Biomedical Pharmacometrician, St. Jude Children's Research Hospital; Wenjian Yang, Ph.D. – Sr Bioinformatics Research Scientist, St. Jude Children's Research Hospital; Seth Karol, M.D. – Pediatric Oncologist, St. Jude Children's Research Hospital; Yinmei Zhou, Ph.D. – Principal Biostatistician, St. Jude Children's Research Hospital; Kristine Crews, Pharm.D. – Dir-Research Operations-Lic, St. Jude Children's Research Hospital; Cheng Cheng, Ph.D. – Member, St. Jude Faculty, St. Jude Children's Research Hospital; William Evans, Ph.D. – Emeritus Faculty Member, St. Jude Children's Research Hospital; Ching-Hon Pui, M.D. – Oncology Faculty, St. Jude Children's Research Hospital; Hiroto Inaba, Ph.D. – Oncology Faculty, St. Jude Children's Research Hospital; Jun Yang, Ph.D. – Business Title Member, St. Jude Faculty, St. Jude Children's Research Hospital; Hope Swanson, Pharm.D. – Pharmacist-Clinical Specialist, St. Jude Children's Research Hospital

Objectives: High-dose methotrexate (HDMTX) and asparaginase (ASP) play crucial roles in the multi-agent chemotherapy used to cure children with acute lymphoblastic leukemia (ALL). Individualized dosing of HDMTX based on its pharmacokinetics has the potential to improve efficacy and reduce toxicity. While a few predictors of HDMTX pharmacokinetics, such as SLCO1B1, are recognized, the impact of concomitant chemotherapy agents, such as ASP, and other genetic variations are not well-established.

Methods: A cohort of 302 standard/high-risk (SR/HR) patients, aged 1 to 18 years old, with newly diagnosed ALL participated in the Total XVII trial at St. Jude Children’s Research Hospital (NCT03117751). During the consolidation phase, SR/HR patients received HDMTX every 2 weeks for 4 courses with a first dose of 5g/m2 and subsequent doses pharmacokinetically guided to target a steady-state plasma concentration of 65 µM. Prior to December 2019, they also received 1000 units/m2 of PEG-asparaginase (PEG-ASP) on day 3 of each HDMTX cycle pending clearance of methotrexate (MTX). After December 2019, PEG-ASP was removed from the consolidation phase.
Serial blood samples for HDMTX pharmacokinetics monitoring were collected pre-infusion and at 6, 23, and 42-hours after the start of infusion. MTX population pharmacokinetics were estimated using nonlinear mixed-effects models implemented in Monolix using the Stochastic Approximation Expectation-Maximization method. A linear two-compartment model was used to fit the data. Parameters estimated were clearance, volume, intercompartmental clearance, and volume of the peripheral compartment. Whole-genome sequencing was also available.

Results: There were 302 SR/HR patients with 1088 evaluable cycles of HDMTX, of which 220 were given when the patient was exposed to PEG-ASP. HDMTX clearance was 14% (p=9.1x10-12) lower when given with PEG-ASP, with a post-hoc median clearance of 76.9 vs 98.2 mL/min/m2. Due to the pharmacokinetic targeting of HDMTX, the median HDMTX dose given during consolidation cycles 2-4 was significantly lower in those who received concomitant PEG-ASP compared to those who did not (3.56 g/m2 vs 4.02 g/m2; p=0.047).
HDMTX clearance was 8.9% (p=6.1x10-3) lower in those with SLCO1B1 decreased or poor function compared to those with normal or increased function (post-hoc median clearance: 83.3 vs 94.7 mL/min/m2). There was no significant interaction between SLCO1B1 phenotype and PEG-ASP exposure on HDMTX clearance (p=0.38).

Conclusions: Our results indicate that PEG-ASP administered with HDMTX alters the pharmacokinetics of HDMTX by reducing its clearance. Due to this clinically significant interaction, protocols should be designed to avoid concurrent administration of HDMTX with PEG-ASP to avoid excessive toxicity. Further investigation is needed to determine the mechanism of this drug interaction and the potential effects of variants in genes other than SLCO1B1.

Citations: N/A