Cancer is a condition where cells in a specific part of body grow and reproduce uncontrollably. The purpose of this study is to assess adverse events and change in disease activity when ABBV-400 is given to adult participants to treat advanced solid tumors. ABBV-400 is an investigational drug being developed for the treatment of advanced solid tumors. Study doctors put the participants in groups called cohorts. Cohorts 1-8 receive ABBV-400 alone (monotherapy) followed by a safety follow-up period. Cohort 9 receives ABBV-400 in combination with a strong CYP3A3 inhibitor (ITZ) followed by a safety follow-up period. Approximately 285 adult participants with hepatocellular carcinoma (HCC), pancreatic ductal adenocarcinoma (PDAC), biliary tract cancers (BTC), esophageal squamous cell carcinoma (ESCC), triple negative breast cancer (TNBC), hormone receptor+/human epidermal growth factor receptor 2 negative (HER2-) breast cancer (hormone receptor-positive \[HR+\]/HER2-breast cancer \[BC\]), head and neck squamous-cell-carcinoma (HNSCC), Platinum Resistant High Grade Epithelial Ovarian Cancer (PROC)/primary peritoneal/fallopian tube cancer, or advanced solid tumors, will be enrolled in the study in approximately 54 sites worldwide. In cohorts 1-8, participants with the following advanced solid tumor indications: HCC, PDAC, BTC, ESCC, TNBC, HR+/HER2-BC, HNSCC, and PROC/primary peritoneal/fallopian tube cancer will receive intravenous (IV) ABBV-400 monotherapy and in cohort 9 participants will receive intravenous (IV) ABBV-400 and an oral solution of ITZ, for up to 3 years during and up to the treatment period with an additional safety follow-up period of up to 2 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 an approved institution (hospital or clinic). The effect of the treatment will be frequently checked by medical assessments, blood tests, questionnaires and side effects.