An open-label, dose-escalation study designed to evaluate the safety, pharmacokinetics, and preliminary efficacy of ABBV-399 in participants with advanced solid tumors.

Inclusion Criteria:

• Participant with histologically confirmed advanced solid tumor.
• Participant must have advanced solid tumor that is not amenable to surgical resection or other approved therapeutic options that have demonstrated clinical benefit.
• Participant has an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0 to 2.
• Participant must have measurable disease per Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1.
• Participant has archived diagnostic formalin-fixed paraffin embedded (FFPE) tumor tissue available for analysis.
• Participant has adequate bone marrow, renal, and hepatic function.

Exclusion Criteria:

• Participant has received anticancer therapy including chemotherapy, immunotherapy, radiation therapy, immunotherapy, biologic, or any investigational therapy within a period of 21 days, or herbal therapy within 7 days prior to the first dose of ABBV-399.
• Participant has known uncontrolled metastases to the central nervous system. Participants with brain metastases are eligible after definitive therapy provided they are asymptomatic off systemic steroids and anticonvulsants for at least 2 weeks prior to first dose of ABBV-399.
• Participant has unresolved clinically significant adverse events >= Grade 2 from prior anticancer therapy except for alopecia or anemia.
• Participant has had major surgery within 21 days prior to the first dose of ABBV-399.