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.

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Hydrogen-exchange electrospray-ionization mass spectrometry is demonstrated to be an effective new method for probing conformational changes of proteins in solutions. The method is based on the mass spectrometric measurement of the extent of hydrogen/deuterium exchange that occurs in different protein conformers over defined periods of time. Results are presented in which hydrogen-exchange electrospray-ionization mass spectrometry is used to probe conformational changes in bovine ubiquitin induced by the addition of methanol to aqueous acidic solutions of the protein.