Discovery of HMPL-A580, a first-in-class antibody-targeted therapy conjugate (ATTC) of a novel PI3K/PIKK inhibitor payload linked to an anti-EGFR antibody

Background: EGFR is highly expressed in multiple types of solid tumors and well recognized as a driving force in tumorigenesis and disease progression. Modulation of the PI3K/AKT/mTOR (PAM) pathway is required for EGFR-mediated tumorigenesis or conferred resistance to EGFR-targeted therapy. Importantly, PAM pathway inhibition synergizes anti-EGFR therapy to enhance anti-tumor activity. Consequently, an Antibody-Targeted Therapy Conjugate (“ATTC”) with a PI3K/PIKK small molecule inhibitor conjugated to an EGFR antibody is developed and expected to enhance anti-tumor efficacy of anti-EGFR therapy while reducing off-tumor toxicities of PAM inhibitors. HMPL-A580 is a first-in-class ATTC comprising of a highly selective and potent PI3K/PIKK inhibitor payload linked to an anti-EGFR IgG1 antibody, via a cleavable linker. Methods: EGFR binding and internalization were evaluated by FACS. Confocal microscopic imaging was used to monitor endocytosis. Cell viability was measured by CCK-8 or luminescence in 3D cell viability assays. Multiple human xenograft tumors were applied in immune-deficient mice to investigate the anti-tumor activity of HMPL-A580. Results: The payload of HMPL-A580 potently inhibited PI3K and PIKK family kinases, with IC 50 ranging around 1 to 10 nM. Eurofins profiling across 418 kinases revealed the payload has excellent selectivity. By conjugating this potent payload with an anti-EGFR antibody via a cleavable linker, the ATTC compound HMPL-A580 demonstrated robust anti-tumor effect. Upon binding to EGFR-expression cancer cell line, HMPL-A580 underwent rapid internalization, lysosomal trafficking, payload release, and PAM and PIKK signaling inhibition to induce tumor cell apoptosis. In a 38-human solid tumor cell line panel, HMPL-A580 potently inhibited EGFR-expression tumor cell proliferation. The tumor cells harboring EGFR high expression, EGFR mut or PAM alterations were more sensitive to HMPL-A580. HMPL-A580 showed a strong bystander effect when EGFR-negative cells co-cultured with EGFR-expression cells. In human tumor xenograft models in mice, HMPL-A580, administered intravenously at 1~10 mg/kg once weekly for two weeks, demonstrated a dose / exposure-dependent anti-tumor activity in multiple EGFR-expression models, which is associated with much stronger target inhibition and suppression of downstream functions than antibody and payload alone treatment. The preliminary results demonstrated that HMPL-A580 was stable in human, monkey, rat and mouse plasma, and showed favorable PK property in cynomolgus monkeys. Conclusion: HMPL-A580 demonstrates strong anti-tumor activity in preclinical models and good PK characteristics, supporting further clinical evaluation.