Site-specific conjugation of a cytotoxic drug to an antibody improves the therapeutic index

Systemic toxicity associated with heterogeneity in the stoichiometries and sites of drug attachment is a major hurdle to developing antibody-drug conjugates (ADCs) for cancer therapy. Junutula et al . engineer cysteine residues in constant domains to produce near-homogenous ADCs that are better tolerated than conventional ADCs, without any loss of antitumor activity. Antibody-drug conjugates enhance the antitumor effects of antibodies and reduce adverse systemic effects of potent cytotoxic drugs. However, conventional drug conjugation strategies yield heterogenous conjugates with relatively narrow therapeutic index (maximum tolerated dose/curative dose). Using leads from our previously described phage display–based method to predict suitable conjugation sites, we engineered cysteine substitutions at positions on light and heavy chains that provide reactive thiol groups and do not perturb immunoglobulin folding and assembly, or alter antigen binding. When conjugated to monomethyl auristatin E, an antibody against the ovarian cancer antigen MUC16 is as efficacious as a conventional conjugate in mouse xenograft models. Moreover, it is tolerated at higher doses in rats and cynomolgus monkeys than the same conjugate prepared by conventional approaches. The favorable in vivo properties of the near-homogenous composition of this conjugate suggest that our strategy offers a general approach to retaining the antitumor efficacy of antibody-drug conjugates, while minimizing their systemic toxicity.