Trastuzumab Deruxtecan With Nivolumab in Advanced Breast and Urothelial Cancer

This is a study of trastuzumab deruxtecan for the treatment of HER2-positive unresectable or metastatic breast cancer following two or more prior anti-HER2 based regimens. Participants will receive this study drug along with a cancer drug, an immune checkpoint inhibitor, anti-PD1, called nivolumab. The study will be done in two parts: * Part 1 is to identify the recommended dose to use for treatment. * Part 2 is to find out how well the combination works, and how safe and tolerable it is.

The purpose of this phase 1b (Part 1, Part 2) study is to assess the combination of a test drug (trastuzumab deruxtecan) with nivolumab in participants with HER2-expressing breast and urothelial cancer who had disease progression during or after prior therapies, did not respond to standard therapies, or for whom no standard therapy is available. The study will be performed in 2 parts. * Part 1 is to test different doses of trastuzumab deruxtecan when given along with a fixed dose of nivolumab, and establish the maximum tolerated dose/recommended dose for expansion, when used in combination with nivolumab * Part 2 is to assess the efficacy and safety of this dose combination.