Combinatorial Treatment Options for Highly Resistant Compound Mutations in the Kinase Domain of the BCR::ABL1 Fusion Gene

The introduction of tyrosine kinase inhibitor (TKI)-based therapies has dramatically changed the outcome in patients with chronic myeloid leukemia (CML) and Ph-positive acute lymphoblastic leukemia (Ph+ALL). However, the occurrence of point mutations in the kinase domain (KD) of the BCR::ABL1 fusion gene can impair or inhibit the efficacy of individual TKIs. In advanced phases of CML and in Ph ⁺ALL, patients frequently develop compound mutations (CMs), defined as the presence of more than one mutation on the same BCR::ABL1 molecule in a leukemic cell, which are often highly or even completely resistant to any individual TKI. Some CMs respond to high doses of the currently most potent available TKI, ponatinib, but the dose-related toxicity is an important impediment. The most frequently occurring CMs, which include the gatekeeper mutation T315I, are often resistant to any currently available TKI.

We performed in vitro analyses using a common cell line model based on Ba/F3 cells expressing a panel of 12 different CMs (V299L/G250E, V299L/E255V, V299L/F359V, F317L/Y253H, F317L/E255V, F317L/F359V, F359V/Y253H, T315I/M244V, T315I/G250E, T315I/E255K, T315I/E255V, T315I/F359V) in BCR::ABL1 constructs introduced into the cells by a transposon-mediated approach. The presence of single insertion copies of the mutant constructs was verified by FACS analysis to preclude skewed test results potentially caused by multiple copies of the mutant construct. The cells pre-tested in this way were exposed in vitro to different concentrations of various drugs including ponatinib combined with a number of other agents such as asciminib, hydroxyurea, palbociclib, venetoclax, ibrutinib, vodobatinib, and crizotinib. The drug concentrations tested corresponded to the spectrum of the respective dosing regimens used in the clinical setting. The cell viability assays performed indicated that CMs responding only to elevated concentrations of ponatinib could be suppressed by combinatorial treatment with lower concentrations of ponatinib and different other agents tested. Moreover, CMs displaying high levels of resistance to TKIs applied as single agents mostly revealed adequate in vitro responses to individual drug combinations with ponatinib, most commonly including asciminib, hydroxyurea or crizotinib. Only the most resistant CM constellation tested, T315I/F359V, failed to respond to the compounds investigated and showed a limited response to the combination of high concentrations of ponatinib with hydroxyurea. A good response of this CM in vitro was only achieved by a combination of ponatinib with a very high concentration of crizotinib indicating the need for further testing of additional candidate drugs for this constellation.

Overall, combinatorial treatment of BCR::ABL1 CMs indicated that relatively low concentrations of the drugs used are capable of effectively inhibiting in vitro survival of mutant cells in most instances, which is important with regard to the expected toxicity of combinatorial treatment approaches. Since our earlier data indicated a good correlation between in vitro test results and clinical responses, our observations may serve as a basis for novel treatment options in patients with Ph ⁺leukemias displaying challenging BCR::ABL1 KD-mutations.

[This project was supported by an educational grant from Incyte Biosciences International Sàrl]