Targeted Maytansinoid Conjugate Improves Therapeutic Index for Metastatic Breast Cancer Cells

This study was undertaken to target cell surface receptors other than the ones typically associated with breast cancer {estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2)}. It was also launched to use small molecules other than those most widely used for active targeting in general (e.g. folate and carbonic anhydrase IX ligands). Specifically, the focus of this study was on unique small molecules that bind the TrkC receptor, which is overexpressed in metastatic breast cancer. A conjugate (1) of a TrkC-targeting small molecule and the highly cytotoxic warhead, DM4 (a maytansinoid), was prepared. Cellular studies featuring TrkC⁺ and TrkC^- human breast cells indicated this conjugate might have a better therapeutic effect than DM4 alone. It emerged that the conjugate 1 was very efficacious in vivo, completely ablating orthotopic 4T1 breast tumor in one case and dramatically reducing the tumor size in four other mice. Throughout, no significant weight loss or obvious neurotoxic effects were observed in the animals tested.