Additional Study Information: The purpose of this study is to learn about the study drug BDC-1001 in patients with HER2-expressing tumors. Researchers will be studying the use of BDC-1001 alone as well as in combination with Pembrolizaumab. This study will help researchers learn if the study drug can help with HER2-expressing tumors and how people respond to the drug. The researchers of this study will be looking at the safety, efficiency, and tolerability of BDC-1001, as well as how the body absorbs, breaks down, and removes the drug in patients with advanced HER2-expressing solid tumors. This research study will be the first time BDC-1001 will be given to people. The BDC-1001 study drug is expected to stimulate the immune system and target and attack HER2-expressing tumors. Cancers include: Breast cancer, esophageal cancer, ovarian cancer, lung cancer, pancreatic cancer, and stomach cancer.