A Study of Oral MBQ-167 in Participants With Advanced Breast Cancer

Key Inclusion Criteria: * The investigator will evaluate these and other criteria to determine whether a participant can be included in this study. * Histologically and/or cytologically confirmed advanced breast cancer which has progressed after treatment with approved therapies or for which there are no standard therapies available. * Participants with known brain metastases may be eligible if specific conditions are met. * Life expectancy ≥6 months, in the opinion of the investigator, after starting MBQ-167. * Are able to swallow capsules twice daily with a meal. Key Exclusion Criteria: * The investigator will evaluate these and other criteria to determine whether a participant should be excluded from this study. * Inability to take oral medication, or malabsorption syndrome or any other uncontrolled gastrointestinal condition (e.g., nausea, diarrhea, or vomiting) that might impair the bioavailability of MBQ-167. * Females who are pregnant or breastfeeding. * Participants who have received any anticancer treatment within 4 weeks or any investigational agent within 28 days prior to the first dose of trial drug or who have not recovered from any acute toxicity greater than Grade 0 or 1 related to previous anticancer treatment. * Participants who have received any anticancer treatment within 4 weeks or any investigational agent within 28 days prior to the first dose of trial drug or who have not recovered from any acute toxicity greater than Grade 0 or 1 related to previous anticancer treatment. * Active malignancies other than advanced breast cancer will be excluded from the study.