Combining Clofarabine and Fludarabine with Exposure Targeted Busulfan for Pediatric Leukemia: An Effective, Low Toxicity TBI-Free Conditioning Regimen

The impact of total body irradiation-based regimens on outcomes in children and young adults with acute lymphoblastic leukemia undergoing allogeneic hematopoietic stem cell transplantation

INTRODUCTION

Total body irradiation (TBI)-based conditioning is the standard of care in the treatment of acute lymphoblastic leukemia (ALL) that requires allogeneic hematopoietic stem cell transplantation (HSCT). However, TBI is known to be associated with an increased risk of late effects, and therefore, non-TBI regimens have also been utilized successfully. A recent study showed that patients that were next-generation sequencing-minimal residual disease (NGS-MRD) negative prior to allogeneic HSCT had a very low risk of relapse, and perhaps could avoid exposure to TBI without compromising disease control. We examined outcomes at our institution in patients that received a TBI or non-TBI regimen, as well as explored the impact of NGS-MRD status in predicting risk of relapse post transplant.

PROCEDURES

This retrospective analysis included 57 children and young adults with ALL that received their first myeloablative allogeneic HSCT from 2012 to 2017 at the University of California San Francisco. Our primary endpoint was the cumulative incidence of relapse at 3 years post transplant.

RESULTS

We demonstrated similar cumulative incidence of relapse for patients treated with either a TBI or non-TBI conditioning regimen, while NGS-MRD positivity prior to transplant was highly predictive of relapse. The presence of acute graft-versus-host disease was associated with decreased relapse rates, particularly among patients that received a TBI conditioning regimen and patients that were NGS-MRD positive prior to HSCT.

CONCLUSIONS

Our data suggest that the decision to use either a TBI or non-TBI regimens in ALL should depend on NGS-MRD status, with conditioning regimens based on TBI reserved for patients that cannot achieve NGS-MRD negativity prior to allogeneic HSCT.

Population Pharmacokinetics of Clofarabine as Part of Pretransplantation Conditioning in Pediatric Subjects before Hematopoietic Cell Transplantation

The primary objective of this work was to characterize the pharmacokinetics (PK) of systemic clofarabine (clo-fara) in pediatric allogeneic hematopoietic cell transplantation (HCT) recipients receiving either nucleoside monotherapy or a dual nucleoside analog preparative regimen. Fifty-one children (median age, 4.9 years; range, .25 to 14.9 years) undergoing allogeneic HCT for a variety of malignant and nonmalignant disorders underwent PK assessment. Plasma samples were collected over the 4 to 5 days of clo-fara treatment and quantified for clo-fara, using a validated liquid chromatography/tandem mass spectrometry assay. Nonlinear mixed-effects modeling was used to develop the population PK model, including identification of covariates that influenced drug disposition. In agreement with previously published models, a 2-compartment PK model with first-order elimination best described the PK of clo-fara. Final parameter estimates for clo-fara were consistent with previous reports and were as follows: clearance (CL), 23 L/h/15 kg; volume of the central compartment, 42 L/15 kg; volume of peripheral compartment, 47 L/15 kg; and intercompartmental CL, 9.8 L/h/15 kg. Unexplained variability was acceptable at 33%, and the additive residual error (reflective of the assay) was estimated to be 0.36 ng/mL. Patient-specific factors significantly impacting clo-fara CL included actual body weight and age. The covariate model was able to estimate clo-fara CL with good precision in children spanning a wide age range from infancy to early adulthood and demonstrates the need for variable dosing in children of different ages. For example, the dose required for a 6-month and 1-year old was approximately 43% and 17% lower, respectively, than the typical 40 mg/m²dose to achieve the median AUC_0-24of 1.04 mg·h/L in the study population. Despite the known renal elimination of clo-fara, no significant clinical parameters for renal function were retained in the final model (P> .05). Coadministration of fludarabine with clo-fara did not alter the CL of clo-fara (P> .05). These results will help inform individualized dosing strategies for clo-fara to improve clinical outcomes and limit drug-related adverse events in children undergoing HCT.