This study is being conducted to examine the safety and tolerability of SGN-LIV1A in patients with metastatic breast cancer, and to find the highest dose of the drug that can be given without unacceptable side effects. Other goals of the trial are to find out if SGN-LIV1A has any antitumor effects, to learn about the pharmacokinetics of SGN-LIV1A, and to explore the relationship between antigen expression and SGN-LIV1A effects.