Arsenic Trioxide Suppresses Growth of BCR-ABL1 Positive Cells with "Gatekeeper" or Compound Mutation
Authors: Mian Afsar Ali , Raees Hadiqa , Ahmad Sujjawal , Ottmann Oliver , Lalani El-Nasir M A 
Published: November 2021

Abstract
Introduction:
Chronic myeloid leukemia (CML) and 30% of adult acute lymphatic leukemia (ALL) are characterized by the Philadelphia chromosome (Ph +), having a (9;22) chromosomal translocation. The BCR-ABL1 fusion protein is the hallmark of Ph + leukemia. BCR-ABL1 is characterized by constitutively activated ABL1 tyrosine kinase activity that determines its transformation potential. Tyrosine kinase inhibitors (TKI) have greatly improved the overall prognosis of these diseases. However, unsatisfactory responses in advanced disease stages, resistance and long-term tolerability of BCR-ABL1 inhibitors represent major clinical problems. The most important resistance mechanism against TKIs is the acquisition of point mutations within the BCR-ABL1 kinase domain that impair drug binding, restoring the oncoprotein's constitutively active tyrosine kinase activity. The selection of leukemic clones driven by BCR-ABL1 harboring point mutations, such as the E255K, Y253F/H (P-loop), H396R (activation loop) or the T315I (gatekeeper).
Second- and third generation TKIs such as nilotinib, dasatinib, and ponatinib effectively overcome point mutation-mediated resistance. Ponatinib is the only U.S. Food and Drug Administration approved TKI with activity against all known BCR-ABL1 point mutations, including BCR-ABL1-T315I. However, the emergence of compound mutations (two mutations within the same BCR-ABL1 allele) has been linked to resistance to all approved TKIs, including ponatinib, posing a clinical challenge with limited treatment options.
The anti-cancer agent arsenic trioxide (ATO) has been used to treat patients with acute promyelocytic leukemia (APL). APL patients respond very well to ATO therapy and achieve complete remission, possibly through induction of apoptosis and differentiation. In addition, it has been demonstrated that combined treatment of ATO with interferon or nilotinib significantly suppressed cell proliferation. However, the potential effects of ATO on BCR-ABL1 mutations and especially on compound mutation is not apparent. This study aimed to investigate the role of ATO in BCR-ABL1 resistant mutations, including compound mutation in Ph + leukemias.
Methods:
We undertook preclinical evaluation of ATO and compared it with approved TKIs e.g. imatinib, nilotinib, dasatinib, ponatinib and ABL inhibitor asciminib, in vitro models of CML and primary patient-derived long term cultures (PD-LTC) of Ph + ALL patients with or without mutation. The effects on mutational resistance were investigated in Ba/F3 cells expressing BCR-ABL1 with T315I mutation and T315I-E255K mutation. For non-mutational resistance, we used PD-LTCs from Ph + ALL patients with different levels of non-mutational drug resistance. Cell proliferation was assessed by XTT.
Results:
ATO efficiently inhibited the growth of all PD-LTCs in cellular assays at dosages of 200-500nM. It also suppressed the growth of Ph + PD-LTC with non- mutational resistance (BV) and the BCR-ABL1-T315I positive PD-LTC (KO) in this dosage range. In all modelsWe treated Ba/F3 cells expressing native BCR-ABL1, BCR-ABL1-T315I mutation and BCR-ABL1-T315I-E255K (compound mutation) with increasing concentrations of imatinib (250, 500 and 1000nM), nilotinib (100, 200 and 400nM), dasatinib (10, 25 and 50nM), ponatinib (10, 50 and 100nM), asciminib) (ABL allosteric inhibitor) (5, 10 and 20nM) and ATO (0.5, 1.0 and 2.0 µM). We found that all the inhibitors significantly inhibited the proliferation of Ba/F3 cells expressing wild type BCR-ABL1 in a dose-dependent manner. In contrast, the growth of Ba/F3 cells expressing BCR-ABL1-T315I was inhibited by increasing concentration of ponatinib, asciminib and ATO. ATO potently inhibited the most challenging mutation (T315I-E255K) with a clinically relevant concentration (IC50 250nM). All approved ABL kinase inhibitors (AKIs) and allosteric inhibitors like asciminib could not inhibit the growth of Ba/F3 cells expressing BCR-ABL1 compound mutation.
Conclusions:
Our findings indicate that ATO significantly suppressed the proliferation of cells expressing non-mutated BCR-ABL1, single and compound mutated BCR-ABL1. These results support including ATO in treating patients with Ph + leukemias having BCR-ABL1 resistant single or compound mutati

Disclosures
Ottmann: Novartis: Honoraria; Amgen: Honoraria, Research Funding; Celgene/BMS: Honoraria, Research Funding; Fusion: Honoraria; Incyte: Honoraria, Research Funding.