Sr. Scientist Mersana Therapeutics, Massachusetts, United States
Key defining attributes of an antibody-drug conjugate (ADC) include the choice of targeting antibody, linker, and the drug-to-antibody ratio (DAR). The choice of DAR, within the constraints of acceptable physicochemical properties for the given platform, is a function of balancing delivery of sufficient payload to targeted cells with the ability to achieve sustained in vivo exposures. Previous reports have described lower DAR mc-VC-MMAE conjugates, DAR = 1-2, that demonstrated higher in vivo exposure and lower clearance when compared to higher DAR (e.g. 4-8) counterparts. In theory, high DAR conjugates may be especially desirable when targeting low antigen expressing tumors or when lower potency payloads are used, as each binding and internalization event results in greater payload delivery. Here we report a systematic exploration of DAR across a much wider range than has been previously reported, by combining THIOMAB® protein engineering technology with the Dolasynthen platform.
Homogeneous, site-specific ADCs spanning a discrete range of DARs – 2, 4, 6, 12, and 18 – were made by conjugation of Trastuzumab IgG1 THIOMAB® constructs with 1, 2, or 3 engineered cysteines to monomeric or trimeric Dolasynthen. The cytotoxicity of the resulting well-defined ADCs was assessed in vitro in cell lines with high or low expression of HER2 antigen. Pharmacokinetic data for all test articles in mice were generated in tumor bearing mice.
In high HER2 expressing cell lines, in vitro cytotoxicity by payload was comparable across DARs. In a lower HER2 expressing system, the higher DAR ADCs performed better. In vivo, our data demonstrated comparable pharmacokinetics for the Dolasynthen conjugates across all DARs. These results illustrate the utility of a DAR ranging platform, such as Dolasynthen when evaluating ADCs as it enables the interrogation of a range of antibody and payload dosing regimens.