Phase Ib/II Study of PLX 3397 and Eribulin in Patients With Metastatic Breast Cancer

This is a nonrandomized, open label phase Ib/II study evaluating the safety and efficacy of eribulin in combination with PLX3397, a novel CSF1 inhibitor, in patients with metastatic breast cancer. The phase II portion of this trial will be limited to patients with triple negative disease. The phase I portion of this trial is a dose escalation of PLX3397 to determine the maximum tolerated dose (MTD) of PLX3397 when given in combination with standard dose eribulin. Patients will be enrolled in cohorts of three, using the dose levels and plan outlined in the statistical section, with 6 patients enrolled at the MTD. All patients with accessible tumor will be required to have a tumor biopsy at study start before starting therapy. Pharmacokinetics of PLX3397 and eribulin, and blood levels of CSF1 will be obtained as outlined in section 14. To allow rapid accrual to phase Ib, and an earlier start to the phase II trial, patients will be enrolled in phase I with both hormone receptor positive and negative disease, and at any line of therapy assuming eligibility criteria are otherwise met. Dose limiting toxicity (DLT) will be defined as any treatment-related toxicity meeting the criteria below and occurring within the first 21 days of combination therapy. Patients must receive at least 14 days of PLX3397 and 2 doses of eribulin during the first cycle in order to be considered evaluable for DLT (unless the missed doses are due to a DLT). Patients in each cohort will be followed for at least 3 weeks (one full cycle) before opening accrual to the next dose level. If one patient in any cohort develops a DLT, an additional 3 patients will be enrolled at that level. If no additional toxicities occur in the six patients, then this particular dose would be used for the phase II trial, and the next higher dose would be considered the MTD. A minimum of 12 and maximum of 24 patients will be enrolled in the phase I study. The phase II trial will not open until the last patient in the phase I study has been followed for at least 3 weeks. The phase II portion of this trial will evaluate progression free survival (PFS) in patients with Triple negative breast cancer (TNBC) treated with PLX3397 and eribulin, using the dose of PLX3397 determined in the phase Ib study in a two-step design. Please see the statistical section for details regarding enrollment and statistical design. Treatment is preceded by a 5 to 7 day lead-in phase, in which patients will take PLX3397 alone daily. Patients with accessible tumor will undergo a core biopsy of tumor before the start of PLX3397 treatment, and then a fine needle aspiration or core biopsy will be performed on the day of or the day before the start of eribulin (day -1 to day 0).