This is a Phase 1, open-label study to evaluate the safety and tolerability of ATV-1601 administered orally in adults with AKT1 E17K-mutant, advanced solid tumors and also in HR+/HER2- advanced and metastatic breast cancer, with or without fulvestrant.

This is a first-in-human, open-label, multicenter, Phase 1a/1b dose escalation dose finding, and dose expansion study to evaluate safety, tolerability, pharmacokinetics (PK), and preliminary antitumor activity of ATV-1601 as monotherapy in participants with advanced or metastatic solid tumors with the AKT1 E17K mutation, and in combination with fulvestrant in participants with breast cancer that has the AKT1 E17K mutation. This study has a dose escalation and expansion phase with ATV-1601, and an escalation and expansion phase in combination with Fulvestrant.

Inclusion Criteria:

1. Histologically or cytologically confirmed metastatic or advanced-stage solid malignant tumor or HR+/HER2- breast cancer.
2. Have progressed on, were intolerant to, or experienced disease recurrence after standard therapy and have no available effective or tolerable treatment options to derive clinically meaningful benefit.
3. Tumor must have documented specific mutation profile as outlined below based on local laboratory testing.
4. Participants with solid tumors or HR+/HER2- breast cancer with AKT1 E17K mutations.
5. Measurable disease according to RECIST v1.1 criteria.
6. Formalin-fixed paraffin-embedded tumor specimen available for submission.
7. Eastern Cooperative Oncology Group performance status of 0 or 1.

Exclusion Criteria:

1. Previously documented activating mutations in KRAS, NRAS, HRAS, or BRAF.
2. Inadequate bone marrow reserve or organ function.
3. Clinically significant abnormalities of glucose metabolism.
4. Participants who are symptomatic or have uncontrolled brain metastases.
5. Requires treatment with certain medications.

Participants must meet other inclusion/exclusion criteria.