Combination of paclitaxel and LAG3-Ig (IMP321), a novel MHC class II agonist, as a first-line chemoimmunotherapy in patients with metastatic breast carcinoma (MBC): Interim results from the run-in phase of a placebo controlled randomized phase II

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Background: IMP321 is a recombinant soluble LAG3-Ig fusion protein that binds to MHC class II molecules and mediates antigen-presenting cell (APC) activation followed by CD8 T-cell activation. The activation of the dendritic cell network with IMP321 the day after a low dose injection of a single agent chemotherapy may lead to stronger anti-tumor CD8 T-cell responses. We report initial results of the safety run-in of a randomized, placebo-controlled phase IIb trial in patients (pts) with hormone receptor positive (HR+) MBC receiving first-line weekly paclitaxel. Methods: In the safety run-in phase 15 pts with MBC received weekly paclitaxel (80 mg/m2; D1, D8, D15) in a four week cycle in conjunction with either 6 mg (n = 6; cohort 1) or 30 mg (n = 9, cohort 2) IMP321 injections s.c. (D2 and D16) for 6 cycles. Patients without progressive disease could continue with a maintenance phase of 12 additional injections of IMP321 every 4 weeks. Blood samples for pharmacokinetics and immuno-monitoring were taken in cycle 1 and 4 just before and after IMP321 injection. Results: In total 15 pts (median age 53 years) were enrolled between Jan 2016 and Oct 2016. No dose limiting toxicities have been reported. Cytokine release syndrome grade 1 was the only serious adverse event (SAE) related to IMP321 and occurred twice in the same patient. Grade 1 and 2 injection site reactions were the most common related AEs and occurred in 14 pts (93 %). A dose-dependent increase in serum IMP321 concentration was observed among the two dose levels with a Cmaxbetween 4 and 24 hours. Increased number of circulating monocytes, dendritic cells and increased activation were observed at both dose levels of IMP321, supporting the working hypothesis. Conclusions: The 30 mg s.c. IMP321 given every two weeks in combination with weekly paclitaxel is the recommended phase 2 dose which is used in the ongoing randomized placebo controlled phase II part of the study. Clinical trial information: NCT02614833.