A Study to Evaluate ATX-898 in Participants With Advanced Solid Tumors

Inclusion Criteria:

• Has histologically or cytologically confirmed advanced solid tumor malignancy that is metastatic or locally advanced and unresectable
• Is ≥18 years of age at the time of signing the informed consent form
• ECOG performance status score of 0 or 1
• Adequate organ function
• Must consent to a pre-treatment tumor biopsy (collected during screening or have available archival tumor tissue in the past 5 years.

For Part 1a (monotherapy dose escalation)

• Has received one or more standard systemic therapies and additional standard therapies are either not available, or participant ineligible for available standard therapies For Part 1b (monotherapy dose expansion)
• b1: Has platinum-resistant or refractory metastatic ovarian cancer
• b2: Has recurrent or metastatic gynecologic cancer excluding the cancers eligible for b1
• b3: Has recurrent advanced or metastatic solid tumor excluding the cancers eligible for b1 or b2
• Has at least 1 measurable lesion per RECIST 1.1

For Part 2a (combination dose escalation)

• Has confirmed HR+ HER2 - (including HER2 low and ultra low) status
• Has received prior therapy in the metastatic For Part 2b (combination dose expansion)
• Has confirmed HR+ HER2 - (including HER2 low and ultra low) status
• Treatment-naïve or has received prior therapy in the metastatic setting

Exclusion Criteria:

• Has history (within ≤2 years before screening) of a solid tumor or hematological malignancy that is histologically distinct from the cancer being studied
• Has symptomatic CNS metastases at screening or asymptomatic CNS metastases requiring corticosteroids to control symptoms within 28 days prior to the first dose of ATX-898.
• Has toxicities from previous anticancer therapies that have not resolved to baseline levels or to CTCAE Grade ≤1, with the exception of alopecia any grade and Grade ≤2 peripheral neuropathy
• Has any condition for which, in the opinion of the Investigator, participation would not be in the best interest of the participant or would prevent, limit, or confound the protocol-specified assessments

Cohort Specific:

• For abemaciclib cohorts only: has a history of any events of cardiac etiology that would contraindicate dosing with abemaciclib
• For ribocicilib only: has history of pneumonitis or interstitial lung disease