Investigating Resistance Mechanisms to Non-covalent Bruton's Tyrosine Kinase Inhibitors and Therapeutic Approaches to Overcome Resistance for Patients with B-Cell Malignancies

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PROJECT SUMMARY B-cell malignancies such as chronic lymphocytic leukemia (CLL) and other forms of Non-Hodgkin lymphomas (NHL) are common hematologic malignancies typically occurring in adults. These malignancies arise from dysfunctional, mature B-cells. Irregular signaling of the B-Cell receptor (BCR) pathway can lead to proliferation and survival of B-cell malignancies making Bruton’s tyrosine (BTK), a kinase early in the BCR pathway, an attractive target for cancer therapy. While available treatments can lead to disease remission, nearly all patients relapse, leading to the consensus that CLL remains to be considered an incurable disease. Covalent (irreversible) small molecule inhibitors of BTK, such as ibrutinib, have transformed the management of CLL, mantle cell lymphoma (MCL), and Waldenström’s Macroglobulinemia (WM). Covalent BTK inhibitors are efficacious in multiple B-cell malignancies, however due to resistance and intolerance, many patients discontinue these agents. To overcome these problems, highly selective and reversible non-covalent BTK inhibitors have been developed. They are currently being tested in clinical trials showing safety and promising efficacy in multiple B-cell neoplasms, including heavily pre-treated CLL, MCL, WM, and follicular lymphoma, demonstrating that non-covalent BTK inhibitors might address a growing unmet need for alternative therapies for these patients. Despite the beneficial features of pirtobrutinib and a high overall response rate seen in the Phase 1 studies, some patients with previously treated CLL and B-cell malignancies did not respond to treatment or relapsed after initial response to monotherapy. Based on next-generation mutational analysis of BTK, PLCG2, and additional genes recurrently mutated in CLL prior to non-covalent BTK inhibitor therapy and at the time of on treatment progression, we have now identified novel BTK and PLCG2 variants that were only detectable post-treatment. Given these preliminary findings, I hypothesize that on-target mutations within BTK or the B-cell receptor signaling pathway (such as PLCG2 mutations) result in resistance to non-covalent BTK inhibition and that combining BTK inhibitors with other CLL targeting drugs will circumvent said resistance. Currently, there are no reports of resistance mechanisms to non-covalent BTK inhibitors in patients. This proposal will identify the mechanisms by which cells develop resistance to non-covalent BTK inhibition and challenge that resistance with additional targeted therapies in the following aims: Aim 1: Identify the mechanism underlying novel BTK mutations observed in acquired resistance to non-covalent BTK inhibition. Aim 2: Test combination therapies to overcome resistance to BTK inhibitors in B-cell lymphoma. There is an exponential need for therapeutic alternatives due to an increased incidence of patients with acquired resistance. The results of this study will have a major impact for patients with a variety of B-cell malignancies. Additionally, these findings will inform the development of rational combination therapies with non-covalent BTK inhibitors.