The properties of cell surface proteins targeted by Antibody-drug conjugates (ADCs) have not been fully exploited; of particular importance are the rate of internalization and the route of intracellular trafficking. In this study we compared the trafficking of HER2, which is the target of the clinically approved ADC ado-trastuzumab emtansine (T-DM1), with that of prolactin receptor (PRLR), another potential target in breast cancer. In contrast to HER2, we found that PRLR is rapidly and constitutively internalized, and traffics efficiently to lysosomes, where it is degraded. The PRLR cytoplasmic domain is necessary to promote rapid internalization and degradation, and when transferred to HER2, enhances HER2 degradation. In accordance with these findings, low levels of cell surface PRLR (~30,000 surface receptors per cell) are sufficient to mediate effective killing by PRLR ADC, while cell killing by HER2 ADC requires higher levels of cell surface HER2 (~106 surface receptors per cell). Noncovalently cross-linking HER2 to PRLR at the cell surface, using a bispecific antibody that binds to both receptors, dramatically enhances the degradation of HER2 as well as the cell killing activity of a non-competing HER2 ADC. Furthermore, in breast cancer cells that co-express HER2 and PRLR, a HER2xPRLR bispecific ADC kills more effectively than HER2 ADC. These results emphasize that intracellular trafficking of ADC targets is a key property for their activity, and further, that coupling an ADC target to a rapidly internalizing protein may be a useful approach to enhance internalization and cell killing activity of ADCs.
- Received October 4, 2016.
- Revision received December 26, 2016.
- Accepted January 3, 2017.
- Copyright ©2017, American Association for Cancer Research.