An antibody-drug conjugate (ADC) composed of a monoclonal antibody directed against the tumor-associated antigen (TAA) carcinoembryonic antigen (CEA; carcinoembryonic antigen-related cell adhesion molecule 5; CEACAM5) and the immune checkpoint regulator carcinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6; CEACAM-6; CD66c) and conjugated to the microtubule-disrupting cytotoxic agent monomethyl auristatin E (MMAE), with potential antineoplastic and immune checkpoint inhibitory activities. Upon administration of anti-CEA/CEACAM6 ADC EBC-129, the monoclonal antibody moiety targets and binds to a specific glycosylation site that is conserved on both CEA and CEACAM6 expressed on tumor cells. Upon binding and internalization, MMAE is released where it targets and binds to tubulin and inhibits microtubule polymerization. This results in G2/M phase cell cycle arrest and apoptosis in CEA and/or CEACAM6-expressing tumor cells. CEA, a member of the CEA family of proteins that plays a key role in cell migration, cell invasion, and cell adhesion, is overexpressed by a variety of cancer cell types. CEACAM6, an immune checkpoint receptor, is associated with tumor-mediated immune suppression. Elevated CEACAM6 expression is associated with advanced tumor stages and poor prognosis.