CFI-402257 in Combination With Paclitaxel in Patients With Advanced/Metastatic HER2-Negative Breast Cancer

Phase I: The purpose of the first phase of the study is to find the highest dose of CFI-402257 that can be tolerated without causing very severe side effects when receiving paclitaxel. This is done by starting at a dose lower than the one that is tolerated in patients when given on its own. Participants are given CFI-402257 together with paclitaxel and are watched very closely to see what side effects they have and to make sure the side effects are not severe. If the side effects are not severe, then new participants will be given a higher dose of CFI-402257. Participants joining this study later on will get higher doses of CFI-402257 than participants who join earlier. This will continue until a dose is found that causes severe but temporary side effects. Doses higher than that will not be given. Phase II: The purpose of the second phase will be to find out the effect that CFI-402257 has on breast cancer, using doses found to be safe in the first phase of the study, when given with paclitaxel.