Antibody-Mediated Endocytosis of Polysialic Acid Enables Intracellular Delivery and Cytotoxicity of a Glycan-Directed Antibody-Drug Conjugate

The specific targeting of differentially expressed glycans in malignant cells has emerged as an attractive anticancer strategy. One such target is the oncodevelopmental antigen polysialic acid (polySia), a polymer of alpha 2,8-linked sialic acid residues that is largely absent during postnatal development but is re-expressed during progression of several malignant human tumors, including small-cell and non-small cell lung carcinomas, glioma, neuroblastoma, and pancreatic carcinoma. In these cancers, expression of polySia correlates with tumor progression and poor prognosis and appears to modulate cancer cell adhesion, invasiveness, and metastasis. To evaluate the potential of PolySia as a target for anticancer therapy, we developed a chimeric human polySia-specific mAb that retained low nanomolar (nmol/L) target affinity and exhibited exquisite selectivity for polySia structures. The engineered chimeric mAb recognized several polySia-positive tumor cell lines in vitro and induced rapid endocytosis of polySia antigens. To determine whether this internalization could be exploited for delivery of conjugated cytotoxic drugs, we generated an antibody-drug conjugate (ADC) by covalently linking the chimeric human mAb to the tubulin-binding maytansinoid DM1 using a bioorthogonal chemical reaction scheme. The resulting polySia-directed ADC demonstrated potent target-dependent cytotoxicity against polySia-positive tumor cells in vitro. Collectively, these results establish polySia as a valid cell-surface, cancer-specific target for glycan-directed ADC and contribute to a growing body of evidence that the tumor glycocalyx is a promising target for synthetic immunotherapies.