First-in-Human Study of STX-478, a Mutant-Selective PI3Kα Inhibitor as Monotherapy and in Combination with Other Antineoplastic Agents in Participants with Advanced Solid Tumor

• STX-478 is a PI3Kα small molecule inhibitor.

In this study, the sponsor and investigators want to learn:

• How much of the study drug can be given with an acceptable level of side effects
• The effects of the study drug (good and bad)
• much of the study drug is absorbed into the blood and how fast it is removed

This is a dose escalation/expansion study. Dose Escalation means that the amount of the study drug given will be increased in each group of research participants, to find the most appropriate dose for further study.

In the dose expansion part of the study, a larger number of people receive the study drug dose determined to be appropriate in the dose escalation part of the study. The study will be conducted in three Parts.

Part 1 participants will be receiving the study drug alone.

Part 2 participants will be receiving the study drug in combination with Fulvestrant.

Part 3 participants will be receiving the study drug in combination with Fulvestrant and either ribociclib or palbociclib.

The dose of the study drug you receive and in which Part you will participate will be determined by when you enroll into the study. The study regimen will be given in cycles.  Each cycle is 28 days.

You will receive the study drug once daily by mouth with or without food. If you are in Part 2 or 3, you will also receive Fulvestrant as an injection on Day 1 and Day 15 of Cycle 1, and Day 1 of every Cycle thereafter. If you are in Part 3, you will also receive either ribociclib or palbociclib as a tablet to be taken daily by mouth with or without food.