ABBV-901

Ovarian cancer (OC) is a lethal disease. The purpose of this study is to assess the safety, pharmacokinetics and efficacy of ABBV901, alone or in combination with bevacizumab, in participants with ovarian cancer. ABBV901 is an investigational drug for the treatment of ovarian cancer. This study has 4 Parts (Arms) where participants will receive ABBV-901, alone or in combination with the standard available therapy, bevacizumab. Around 219 participants will be enrolled in the study at approximately 75 sites around the world. In part 1, participants will receive escalating doses of intravenous (IV) ABBV-901 alone. In part 2, participants will receive 1 of 3 doses of IV ABBV-901, alone to determine the optimized dose. In part 3, participants will receive escalating doses of IV ABBV-901, combination with IV bevacizumab. In part 4, participants will receive recommended doses for expansion of IV ABBV-901, combination with IV bevacizumab. The total study duration will be approximately 3 years. There may be higher treatment burden for participants in this trial compared to their standard of care. Participants will attend regular visits during the study at a hospital or clinic and may require frequent medical assessments, blood tests, and scans.