Results from a First-in-Human, Proof-of-Concept Phase 1 Trial in Pretreated B-Cell Malignancies for Loxo-305, a Next-Generation, Highly Selective, Non-Covalent BTK Inhibitor
Authors: Mato Anthony R. , Flinn Ian W. , Pagel John M. , Brown Jennifer R. , Cheah Chan Y. , Coombs Catherine C. , Patel Manish R. , Rothenberg S. Michael , Tsai Donald E. , Ku Nora C. , Wang Michael L. 
Published: February 2020

Introduction: Bruton Tyrosine Kinase inhibitors (BTKis) have transformed the treatment of patients with chronic lymphocytic leukemia (CLL) and other B-cell malignancies by inducing durable responses, improving quality of life and prolonging overall survival. Prolonged use of BTKi in the real-world setting is limited by toxicity and acquired resistance. Discontinuation rates for BTKis may be as high as 40% in relapsed/refractory CLL, with BTK C481-mediated resistance evident in many progressing patients. Alternative therapies such as venetoclax are associated with on-target (BCL2) acquired resistance. We hypothesized that a selective, non-covalent BTKi would benefit patients with B-cell malignancies in the setting of acquired resistance and/ or unacceptable toxicities following an irreversible BTKi.
LOXO-305 is a next-generation, highly selective, oral, non-covalent BTKi that inhibits wild-type and C481-mutated BTK preclinically. Here, we report results from a first-in-human, proof-of-concept phase 1 trial in patients with B-cell malignancies.
Methods: This multicenter phase 1/2 trial (NCT 03740529) enrolled patients with advanced B-cell malignancies who had failed or were intolerant to &gt; 2 prior therapies. LOXO-305 was dosed orally in 28-day cycles, using a standard 3+3 dose-escalation design with a primary endpoint of MTD/RP2D identification.
Results: As of 26 July 2019, 13 patients (9 CLL and 4 MCL) were enrolled to 3 dose levels: 25mg (n=5), 50mg (n=5) and 100mg (n=3) QD. Median age was 65 (range 51-79) years and the median number of prior therapies was 3 (range 2-6). 12 patients (8 CLL, 4 MCL) received prior chemotherapy + anti-CD20 antibody; 2 MCL patients underwent prior autologous stem cell transplantation; 5 CLL patients received prior umbralisib; 10 patients (7 CLL, 3 MCL) received prior ibrutinib (5 intolerant, 5 relapsed), including 1 who had also received venetoclax. 6 CLL patients displayed high-risk genetic features, including unmutated IGHV (4), complex karyotype (4) and del17p (3). Molecular characterization was available in 7 patients (6 CLL, 1 MCL) and revealed: BTK C481S mutations (in 2 CLL patients post-ibrutinib), a BCL2 G101V mutation (in a CLL patient post-venetoclax), and a TP53 mutation (in an MCL patient post-ibrutinib). At doses ≥50 mg QD, LOXO-305 exposure exceeded the calculated IC90 for wild-type and C481S mutated BTK. No DLTs were reported and all TEAEs are grade 1-2. Clinical activity was noted within the first cycle of therapy and at the first dose level of 25mg QD. The first eight patients were evaluable for initial response and 7 tumor responses (87.5%) were observed (by disease-defined criteria): 5/5 CLL patients (1 PR and 4 PR-L including one with BTK C481S mutation after ibrutinib and one with BCL2 G101V mutation after venetoclax) and 2/3 MCL patients (2 PR and 1 PD with a preexisting TP53 mutation). 2 additional CLL patients were awaiting initial radiologic assessment but had already demonstrated treatment-induced lymphocytosis. 12/13 patients remain on therapy, the longest 5+ months.
Conclusion: Phase 1 data with LOXO-305 demonstrate a favorable safety profile and provide proof-of-concept evidence of efficacy in heavily pretreated CLL and MCL patients, including patients with acquired resistance to available BTKis and venetoclax.

Disclosures
Mato: DTRM Biopharma: Research Funding; Genentech: Consultancy, Research Funding; Pharmacyclics: Consultancy, Research Funding; Gilead: Research Funding; Acerta: Consultancy; Janssen: Consultancy; TG Therapeutics: Consultancy, Other: DSMB member , Research Funding; Celgene: Consultancy; Sunesis: Consultancy, Research Funding; AstraZeneca: Consultancy, Research Funding; AbbVie: Consultancy, Research Funding; LOXO: Consultancy, Research Funding; Johnson & Johnson: Consultancy, Research Funding. Flinn:F. Hoffmann-La Roche Ltd: Research Funding; Acerta Pharma, Agios, Calithera Biosciences, Celgene, Constellation Pharmaceuticals, Genentech, Gilead Sciences, Incyte, Infinity Pharmaceuticals, Janssen, Karyopharm Therapeutics, Kite Pharma, Novartis, Pharmacyclics, Portola Pharmaceuticals: Research Funding; TG Therapeutics, Trillum Therapeutics, Abbvie, ArQule, BeiGene, Curis, FORMA Therapeutics, Forty Seven, Merck, Pfizer, Takeda, Teva, Verastem, Gilead Sciences, Astra Zeneca (AZ), Juno Therapeutics, UnumTherapeutics, MorphoSys, AG: Research Funding; TG Therapeutics, Trillum Therapeutics, Abbvie, ArQule, BeiGene, Curis, FORMA Therapeutics, Forty Seven, Merck, Pfizer, Takeda, Teva, Verastem, Gilead Sciences, Astra Zeneca (AZ), Juno Therapeutics, UnumTherapeutics, MorphoSys, AG: Research Funding; AbbVie, Seattle Genetics, TG Therapeutics, Verastem: Consultancy. Pagel:AstraZeneca: Consultancy; Gilead Sciences: Consultancy; Pharmacyclics: Consultancy. Brown:Teva: Honoraria; Janssen: Honoraria; Sunesis: Consultancy; Juno/Celgene: Consultancy; Gilead: Consultancy, Research Funding; Dynamo Therapeutics: Consultancy; Genentech/Roche: Consultancy; Pharmacyclics: Consultancy; Pfizer: Consultancy; Novartis: Consultancy; Loxo: Consultancy, Research Funding; Kite, a Gilead Company: Consultancy, Research Funding; Sun Pharmaceuticals: Research Funding; Verastem: Consultancy, Research Funding; TG Therapeutics: Consultancy; Octapharma: Consultancy; AstraZeneca: Consultancy; BeiGene: Consultancy; Catapult Therapeutics: Consultancy; Acerta Pharma: Consultancy; Invectys: Other: Data safety monitoring board; Morphosys: Other: Data safety monitoring board; AbbVie: Consultancy. Cheah:Roche, Janssen, MSD, Gilead, Loxo Oncology, Acerta, BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene, Roche, Abbvie: Research Funding; Roche: Other: Travel expenses. Coombs:Medscape: Honoraria; Covance: Consultancy; Cowen & Co.: Consultancy; H3 Biomedicine: Honoraria; Dedham Group: Consultancy; Loxo: Honoraria; Abbvie: Consultancy; Octopharma: Honoraria; Pharmacyclics: Honoraria. Rothenberg:LOXO Oncology Inc.: Employment. Tsai:Eli Lilly and Company: Employment. Ku:Eli Lilly and Company: Employment. Wang:BioInvent: Consultancy, Research Funding; VelosBio: Research Funding; Loxo Oncology: Research Funding; Guidepoint Global: Consultancy; Kite Pharma: Consultancy, Research Funding; Pharmacyclics: Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding, Speakers Bureau; Acerta Pharma: Consultancy, Research Funding; MoreHealth: Consultancy, Equity Ownership; AstraZeneca: Consultancy, Honoraria, Research Funding, Speakers Bureau; Celgene: Honoraria, Research Funding; Juno Therapeutics: Research Funding; Dava Oncology: Honoraria; Aviara: Research Funding.