B7-H3, also referred to as CD276, is a member of the B7 family of immune regulatory proteins. B7-H3 is overexpressed on many solid cancers, including prostate cancer, renal cell carcinoma, melanoma, squamous cell carcinoma of the head and neck, non-small cell lung cancer and breast cancer. Over-expression of B7-H3 is associated with disease severity, risk of recurrence and reduced survival. In this article, we report the preclinical development of MGC018, an antibodydrug conjugate targeted against B7-H3. MGC018 is comprised of the cleavable linkerduocarmycin payload, valine-citrulline-seco duocarmycin hydroxybenzamide azaindole (vcseco-DUBA), conjugated to an anti-B7-H3 humanized IgG1/kappa monoclonal antibody through reduced interchain disulfides, with an average drug-to-antibody ratio of ~2.7. MGC018 exhibited cytotoxicity toward B7-H3-positive human tumor cell lines, and exhibited bystander killing of target-negative tumor cells when co-cultured with B7-H3-positive tumor cells. MGC018 displayed potent antitumor activity in preclinical tumor models of breast, ovarian and lung cancer, as well as melanoma. Additionally, antitumor activity was observed toward patientderived xenograft models of breast, prostate and head and neck cancer displaying heterogeneous expression of B7-H3. Importantly, MGC018 exhibited a favorable pharmacokinetic and safety profile in cynomolgus monkeys following repeat-dose administration. The antitumor activity observed preclinically with MGC018, together with the positive safety profile, provides evidence of a potentially favorable therapeutic index and supports the continued development of MGC018 for the treatment of solid cancers.

Introduction: B7-H3, a member of the B7 family of immunomodulatory molecules, is overexpressed in a wide range of solid tumors; tumor overexpression has been correlated with disease severity and poor outcome in several cancer types. MGC018 is an antibodydrug conjugate (ADC) targeted against B7-H3 and comprised of the cleavable linker-duocarmycin payload, valine-citrulline-seco DUocarmycin hydroxyBenzamide Azaindole (vc-seco-DUBA), conjugated to an anti-B7-H3 humanized IgG1 kappa monoclonal antibody through reduced interchain disulfides, with an average drug-to-antibody ratio of ~2.7. Previous studies indicated MGC018 exhibited a favorable preclinical profile, with strong reactivity toward tumor cells and tumor-associated vasculature, limited normal tissue reactivity, potent cytotoxicity in vitro and antitumor activity in vivo toward a range of B7-H3-expressing tumor cell lines representing several cancer types. Based on these preliminary results, expanded preclinical development of MGC018 was undertaken to support clinical development. Methods: vc-seco-DUBA conjugation to obtain MGC018 ADC was performed by Synthon Biopharmaceuticals B.V. Single-and repeatdose in vivo efficacy studies were conducted in CD-1 nude mice with human tumor xenografts that express B7-H3 to explore the relationship between C max , exposure and antitumor activity, and to define the minimal efficacious dose in these models. A GLP toxicology study was conducted in cynomolgus monkeys in which MGC018 was administered at dose levels of 1, 3, 6, and 10 mg/kg every 3 weeks for a total of 3 doses.

<p>5000 Hs700T B7-H3 KO/RFP cells alone in the presence of 6.7 nM MGC018</p>

<div>Abstract<p>B7-H3, also referred to as CD276, is a member of the B7 family of immune regulatory proteins. B7-H3 is overexpressed on many solid cancers, including prostate cancer, renal cell carcinoma, melanoma, squamous cell carcinoma of the head and neck, non–small cell lung cancer, and breast cancer. Overexpression of B7-H3 is associated with disease severity, risk of recurrence and reduced survival. In this article, we report the preclinical development of MGC018, an antibody–drug conjugate targeted against B7-H3. MGC018 is comprised of the cleavable linker-duocarmycin payload, valine-citrulline-<i>seco</i> duocarmycin hydroxybenzamide azaindole (vc-<i>seco</i>-DUBA), conjugated to an anti-B7-H3 humanized IgG1/kappa mAb through reduced interchain disulfides, with an average drug-to-antibody ratio of approximately 2.7. MGC018 exhibited cytotoxicity toward B7-H3–positive human tumor cell lines, and exhibited bystander killing of target-negative tumor cells when cocultured with B7-H3–positive tumor cells. MGC018 displayed potent antitumor activity in preclinical tumor models of breast, ovarian, and lung cancer, as well as melanoma. In addition, antitumor activity was observed toward patient-derived xenograft models of breast, prostate, and head and neck cancer displaying heterogeneous expression of B7-H3. Importantly, MGC018 exhibited a favorable pharmacokinetic and safety profile in cynomolgus monkeys following repeat-dose administration. The antitumor activity observed preclinically with MGC018, together with the positive safety profile, provides evidence of a potentially favorable therapeutic index and supports the continued development of MGC018 for the treatment of solid cancers.</p></div>

<p>Supplementary Data</p>