New pill shows promise in shrinking Hard-to-Treat breast tumors

Copied word for word from the study's registry entry, so the wording is the study team's rather than ours.

Inclusion Criteria: * Phase 1a (Dose Escalation): Participants with histologically or cytologically confirmed locally advanced or metastatic solid tumors associated with CDK4 dependency, including HR+/HER2- breast cancer. Participants must have received prior standard-of-care therapies for their disease, unless the therapy is not available or not tolerated, or is determined not appropriate based on the investigator's judgment. * Phase 1b (Dose Expansion): Participants with selected solid tumors including locally advanced or metastatic HR+/HER2- breast cancer. * Female participants with metastatic HR+/HER2- breast cancer will be required to have ovarian function suppression using gonadotropin-releasing hormone (GnRH) agonists such as goserelin or be postmenopausal. * Male participants with HR+/HER2- breast cancer will be required to have gonadal suppression using GnRH agonists when being treated with letrozole or fulvestrant. * Patients must have ≥1 measurable lesion per RECIST v1.1. * Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 1. * Adequate organ function without symptomatic visceral disease. Exclusion Criteria: * Prior therapy selectively targeting CDK4 (prior CDK4/6 inhibitor therapy is permitted). * Known leptomeningeal disease or uncontrolled untreated brain metastasis. * Any malignancy ≤ 3 years before the first dose of study drug(s) except for the specific cancer under investigation in this study and any locally recurring cancer that has been treated with curative intent (eg, resected basal or squamous cell skin cancer, superficial bladder cancer, or carcinoma in situ of the cervix or breast). * Uncontrolled diabetes. * Infection requiring systemic antibacterial, antifungal, or antiviral therapy ≤ 28 days before the first dose of study drug(s), or symptomatic COVID-19 infection. Patients receiving prophylactic antibiotics (eg, for prevention of urinary tract infection, chronic obstructive pulmonary disease, or for dental extraction) are eligible. Patients who have recovered from symptomatic COVID-19 infection can be rescreened for this study. * Untreated chronic hepatitis B or active hepatitis C infection. Note: Other protocol defined Inclusion/Exclusion criteria may apply.