This is a Phase 1, two-part, open-label, nonrandomized, dose-escalation and signal-seeking study of CGT6297, evaluating the safety, tolerability, PK, pharmacodynamic (what the drug does to the body), and antitumor activity of CGT6297 in adult participants with advanced solid tumors harboring PIK3CA mutations

Inclusion Criteria:

1. Histologically confirmed advanced solid tumor harboring oncogenic PIK3CA mutations in blood and/or tumor:

   1. Phase 1b Cohort 1, participants must have PIK3CA endometrial cancer
   2. Phase 1b Cohort 2, participants must have HR-positive/HER2-negative or HER2-low breast cancer (immunohistochemistry [IHC] and in-situ hybridization results must meet ASCO-College of American Pathology guidelines for breast cancer or criteria)
   3. Phase 1b Cohort 3 will allow all solid tumors that do not meet criteria for Phase 1b Cohorts 1 or 2, including head and neck cancers, other gynecological cancers, colorectal cancers harboring PIK3CA mutations

2. Meet prior treatment requirement of:

   1. Phase 1a: previously treated with and refractory to or intolerant of existing therapy(ies) known to provide clinical benefit for their condition.
   2. Phase 1b: previously treated with or considered not appropriate for SOC first-line treatment for their condition

3. Have at least one measurable lesion according to RECIST v1.1.

4. Eastern Cooperative Oncology Group (ECOG) Performance Status 0 to 1

5. Have clinically acceptable local laboratory screening results (clinical chemistry and hematology) within certain limits

6. Resolution of acute toxicities from prior anticancer therapy to ≤Grade 1 (or baseline), including resolution of clinically significant laboratory abnormalities (other than parameters specified in screening testing as outlined below), as determined by the National Cancer Institute-Common Terminology Criteria for Adverse Events (NCICTCAE) v5.0.

7. Have an ejection fraction ≥50%

Exclusion Criteria:

1. Received small molecule chemotherapy or anticancer therapies or radiotherapy within certain timeframes before first dose of study drug.
2. Major surgeries (eg, abdominal laparotomy) within 4 weeks of the first dose of study drug
3. Treatment with radiotherapy ≤2 weeks before the first dose of study drug.
4. Clinically significant cardiac disease
5. Ongoing or planned long-term (≥4 consecutive weeks) treatment with glucocorticoid steroids at greater than physiologic dosing (defined as equivalent to >20 mg/day prednisone)
6. Diagnosis of diabetes mellitus type 1 or uncontrolled diabetes mellitus type 2 (defined as fasting glucose ≥140 mg/dL and HbA1c ≥7.0%; antihyperglycemic medical management permitted with the exception of insulin)
7. Previous molecular testing (NGS or PCR) showed tumor with the following mutations: mutations/deletions in PTEN or activating mutations in AKT, HRAS/KRAS/NRAS, EGFR, and BRAF