Development of a highly efficacious, first-in-class antibody-drug conjugate against a novel MUC1-C oncoprotein for metastatic colorectal cancer.

e15600 Background: Poor prognosis and mortality due to metastatic colorectal cancer (mCRC) remains a stiff challenge and the management of mCRC continues to be a significant unmet medical need. An aberrantly glycosylated Mucin 1 (MUC1) is overexpressed in > 80% of CRC and development of novel approaches for targeting of MUC1 are critically needed. MUC1 is a heterodimeric oncoprotein expressed on the cell surface and consists of two (MUC1-N and MUC1-C) terminal subunits. MUC1-N subunit is shed in the blood from cancer cells and MUC1-C subunit retains at the cell surface. Multiple attempts at targeting the MUC1-N terminal shed protein have been failed earlier due to all the MUC1-N binding antibodies bind to the shed domain in blood that act as a sink. Methods: We have generated a panel of novel, first-in-class antibodies against the extra cellular domain (ECD) of MUC1-C that remains intact on the cell surface. The development of antibody-drug conjugates (ADCs) has been a major advancement in the field of oncology as ADCs are the emerging class of anti-cancer drugs consisting of tumor targeting MAbs with highly cytotoxic payloads attached through cleavable chemical linkers. We present here making of MUC1-C (3D1)-MMAE as well as generation of MUC1-C (3D1 + 7B8)-biparatopic-MMAE with preclinical characterization. Results: We demonstrate that two MAbs, 3D1 and 7B8, bind to different epitopes (3D1 binding to a3 helix; 7B8 binding to a4 helix) in the MUC1-C-ECD with high affinity and more importantly, do not cross compete with each other. Reactivity of both MAbs is specific for MUC1 expressing human cancer cells as demonstrated by both knock-in and knock-out cell line models. Both antibodies have substantial internalization by 3 hours at 37 o C. Both 3D1 and 7B8 were separately conjugated to monomethyl auristatin E (MMAE) with vc-PAB linkers. The results demonstrate that treatment of diverse mCRC cell lines, SW620, SK-CO-1 and HCT116/MUC1 with 3D1-MMAE or 7B8-MMAE is associated with substantial killing with low nM IC 50 . To further access in vivo efficacy of 3D1-MMAE, nude mice with established HCT-116/MUC1 tumor xenografts were treated with 7.5 mg/kg once a week x 3 weeks. Significant tumor growth inhibition was observed with 3D1-MMAE without any weight loss. A bi-paratopic antibody can bind to the same antigen either on same cell or at two different cells and thereby will increase internalization substantially. Since MAbs 7B8 and 3D1 do not cross compete with each other for binding to MUC1-C-ECD, we are developing 3D1-7B8 bi-paratopic-MMAE for increasing the efficacy further. Studies are underway to test 3D1-7B8 bi-paratopic-MMAE for activity in vitro and in vivo. Conclusions: MUC1-C-ADC has demonstrated a favorable toxicity profile and potent anti-tumor activities in the mCRC. Together, these results suggest advancing the anti-MUC1-C-MMAE ADCs as a novel therapeutic for mCRC.