# Preclinical Assessment of Dactinomycin in KMT2A-Rearranged Infant Acute Lymphoblastic Leukemia

## TL;DR

This study tested dactinomycin for treating infant leukemia and found it has limited effectiveness despite showing promise in lab tests.

## Contribution

The study evaluates dactinomycin's potential for repurposing in KMT2A-rearranged infant ALL using in vitro and in vivo models.

## Key findings

- Dactinomycin showed significant in vitro cytotoxicity against infant ALL cell lines at nanomolar concentrations.
- In vivo treatment with dactinomycin provided only a limited survival benefit in xenograft models.
- Combination with cytarabine showed mild additive effects in vitro but did not translate to strong clinical benefit.

## Abstract

Infant acute lymphoblastic leukemia (iALL) is an aggressive form of leukemia with a poor prognosis and few new treatment options. We used patient-derived iALL cell lines to perform an in vitro drug screen with existing anti-cancer agents with the aim to repurpose them for clinical use; we found dactinomycin to have significant activity at nanomolar concentrations. In vitro combination testing of dactinomycin with conventional chemotherapeutic agents used for the treatment of iALL identified a mild additive effect with cytarabine. However, in vivo treatment of dactinomycin in iALL patient-derived xenograft models led to a limited improvement in survival. These results indicate that dactinomycin is unlikely to provide clinical benefit for the treatment of iALL.

Background/Objectives: Infants with KMT2A-rearranged B-cell acute lymphoblastic leukemia (ALL) have high rates of relapse and poor survival compared with children. Few new therapies have been identified over the past twenty years. The aim of this study was to identify existing anti-cancer agents that have the potential to be repurposed for the treatment of infant ALL. Methods: Eight extensively characterized infant ALL cell lines were treated with 62 anti-neoplastic drugs in vitro to identify agents that exhibit significant cytotoxicity. From this screen, we selected the most effective and clinically translatable agent for further in vitro and in vivo assessment to determine the potential for use in the clinical setting. Results: Our anti-cancer drug screen revealed significant activity of dactinomycin across all infant ALL cell lines. Further in vitro testing identified low half-maximal inhibitory concentrations (IC50) across our infant ALL cell lines in the nanomolar range. Combination testing with the conventional chemotherapeutic agents currently used to treat infants with ALL demonstrated additivity with cytarabine. In vivo assessment of dactinomycin identified 36 μg/kg as the maximum tolerated dose, with unacceptable toxicities at higher dose treatment. Treatment using doses of 18 μg/kg administered either once or twice a week derived a small but significant survival benefit in patient-derived xenografts. Conclusions: Dactinomycin is extensively used for the treatment of solid tumors in children and has an acceptable safety profile when used to treat infants in this context. However, despite being readily translational and exhibiting promising in vitro cytotoxicity, dactinomycin showed limited efficacy in vivo and therefore does not represent a priority candidate for integrating into therapy for infants with ALL.