Open-Label Study of BBO-10203 in Subjects With Advanced Solid Tumors

Inclusion Criteria:

• Locally advanced and unresectable or metastatic HER2-positive advanced breast cancer (aBC), HR-positive/HER2-negative advanced breast cancer, KRAS mutant advancedcolorectal cancer (aCRC), or KRAS mutant advanced non-small cell lung cancer (aNSCLC)

• Measurable disease by RECIST v1.1 (except for HR-positive HER2-negative aBC whereevaluable bone-only disease is permitted)

• Eastern Cooperative Oncology Group (ECOG) performance status (PS) 0-1

• Adequate LVEF assessed by ECHO or MUGA (BBO-10203 + Trastuzumab cohorts only)

• Stable brain metastases

• Patients with HER2-positive aBC: Must have had at least 2 prior lines ofanti-HER2-directed therapy. Only 1 prior line is acceptable where there is no otherregionally available standard of care (SoC)

• Monotherapy Cohort patients with HR-positive, HER2-negative aBC, KRAS mutant aCRC oraNSCLC: Must have progression on, or disease recurrence after at least one line ofSOC treatment or in the opinion of the investigator, would be unlikely to tolerateor derive clinically meaningful benefit from SoC therapy

• BBO-10203 + Fulvestrant combination cohort patients with HR-positive, HER2-negativeaBC: confirmed PIK3CA mutation, must have been treated with a CDK4/6i

• BBO-10203 + Fulvestrant + ribociclib combination cohort patients with HR-positive,HER2-negative aBC: confirmed PIK3CA mutation, no prior systemic therapy in the aBCsetting permitted

• BBO-10203 + FOLFOX + Bevacizumab combination cohort patients with KRAS mutant aCRC:One prior line of irinotecan-containing therapy for locally advanced or metastaticCRC is allowed but not required

Exclusion Criteria:

• Patients with KRAS mutant aCRC who have KRAS G12R mutation, BRAFV600E mutation,HER2amp, or dMMR/MSI-H tumors

• Patients with KRAS mutant aNSCLC who have KRAS G12R mutation, or tumors with othertargetable driver mutations (eg, EGFR, anaplastic lymphoma kinase,ROS1/BRAF/RET/MET/EGFR exon20 insertion/NTRK/HER2)

• Patients with untreated and/or non-stable brain metastases

Other inclusion/exclusion criteria are specified in the protocol