Trial in Progress: A Phase I Trial of BTX-A51 in Patients with Relapsed or Refractory AML or High-Risk MDS
Authors: Ball Brian J. , Stein Anthony S. , Borthakur Gautam , Murray Crystal , Kook Karin , Chan Kyle WH , Stein Eytan M. 
Published: November 2020

Background:
For patients with relapsed or refractory (R/R) acute myeloid leukemia (AML) or high-risk myelodysplastic syndrome (MDS), low response rates and poor overall survival remain unmet clinical needs. AML cells evade apoptosis through overexpression of antiapoptotic genes and inactivation of p53. The antiapoptotic gene Mcl1 is overexpressed in AML cell lines resistant to venetoclax. Similarly, the sensitivity of AML patients' samples to venetoclax inversely correlates with the presence of a TP53 mutation or low expression of p53. In AML, p53 inactivation more commonly results from overexpression of its negative regulators, Mdmx and Mdm2. BTX-A51 is a novel, oral, direct inhibitor of Casein kinase 1α (CK1α), cyclin dependent kinase 7 (CDK7), and CDK9. CK1α phosphorylates Mdmx and Mdm2 leading to enhanced binding and degradation of p53. CDK7 and CDK9 phosphorylate RNA polymerase II (Pol II) to enable transcriptional initiation and elongation, particularly at large clusters of transcriptional enhancers termed super-enhancers (SE). Preclinical studies have demonstrated that BTX-A51 robustly increased p53 protein levels via CK1a inhibition and Mdm2 downregulation while preferentially decreasing SE transcription of key oncogenes such as Myc andMcl1, enabling selective apoptosis of leukemia cells. The combination of CK1α, CDK7, and CDK9 inhibition was synergistic and prolonged survival in multiple genetic and patient-derived xenograft AML models.
Study Design and Methods:
This is an open-label, multi-center, first-in-human Phase 1 study evaluating the safety of BTX-A51 in patients with R/R AML or high-risk MDS. The trial will be performed in two phases, a dose escalation (phase 1a) and dose expansion (phase 1b). Phase 1a utilizes a hybrid accelerated titration with single patient cohorts and a Bayesian optimal interval (BOIN) design to assess 8 potential dosing cohorts. The maximum tolerated dose (MTD) will be the dose for which the isotonic estimate of the toxicity rate is closest to the target toxicity rate of 0.3. Up to a maximum of 35 patients will be enrolled in the dose escalation phase of the study at Memorial Sloan Kettering Cancer Center, MD Anderson Cancer Center, and City of Hope Cancer Center. Following determination of the MTD, 15 patients will be enrolled in the dose expansion phase for further evaluation of dose-limiting toxicities (DLTs) and for preliminary evidence of efficacy. BTX-A51 will be dosed 3 weeks on drug, followed by 1 week off drug over a 28-day cycle. For the first cycle, patients will receive tumor lysis syndrome prophylaxis with allopurinol and intravenous fluids and be closely monitored. Key inclusion criteria are age ³ 18 years, R/R AML or R/R high-risk MDS, Eastern Cooperative Oncology Group (ECOG) £ 2 and life expectancy of ³ 6 weeks, and adequate kidney and liver function. Key exclusion criteria are receipt of cancer chemotherapy (other than hydroxyurea) within 2 weeks prior to the start of study drug, transplantation within 3 months prior to screening, active graft-versus-host disease requiring systemic immunosuppressive medications, and a white blood cell count &gt; 20 × 109/L. The primary objective for the Phase 1 study is to determine the MTD and recommended Phase 2 dose (RP2D) of BTX-A51. Secondary objectives include evaluating overall response (complete remission, complete remission with incomplete blood count recovery, and partial remission) according to the European LeukemiaNet (ELN) 2017 criteria (Döhner et al. Blood. 2017), survival (overall survival and event-free survival) and pharmacokinetics. Correlative objectives include determining the changes in SEs and SE-driven expression of antiapoptotic genes by chromatin immunoprecipitation and RNA-sequencing. Recruitment is ongoing and this trial is registered on clinicaltrials.gov: NCT04243785

Disclosures
Stein: Amgen: Consultancy, Speakers Bureau; Stemline: Consultancy, Speakers Bureau. Borthakur:BioLine Rx: Consultancy; Argenx: Consultancy; PTC Therapeutics: Consultancy; FTC Therapeutics: Consultancy; BioTherix: Consultancy; Curio Science LLC: Consultancy; Xbiotech USA: Research Funding; Oncoceutics: Research Funding; Polaris: Research Funding; PTC Therapeutics: Research Funding; Cyclacel: Research Funding; GSK: Research Funding; Jannsen: Research Funding; Abbvie: Research Funding; Novartis: Research Funding; Incyte: Research Funding; BioLine Rx: Research Funding; BMS: Research Funding; AstraZeneca: Research Funding; Nkarta Therapeutics: Consultancy; Treadwell Therapeutics: Consultancy. Murray:Salamandra: Current Employment. Kook:Salamandra: Current Employment. Chan:BioTheryx: Current Employment. Stein:Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Syros: Membership on an entity's Board of Directors or advisory committees; Syndax: Consultancy, Research Funding; Genentech: Consultancy, Membership on an entity's Board of Directors or advisory committees; Bayer: Research Funding; Biotheryx: Consultancy; Agios Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Abbvie: Consultancy; PTC Therapeutics: Membership on an entity's Board of Directors or advisory committees; Seattle Genetics: Consultancy; Amgen: Consultancy; Celgene Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Astellas Pharmaceuticals: Consultancy, Membership on an entity's Board of Directors or advisory committees; Daiichi-Sankyo: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding.