18TiP - Phase 1/2a study of double immune suppression blockade by combining a CSF1R inhibitor (pexidartinib/PLX3397) with an anti PD-1 antibody (pembrolizum...

Date 20 November 2015
Event ESMO Symposium on Immuno-Oncology 2015
Session Welcome reception and general Poster viewing
Topics Immunotherapy
Skin Cancers
Presenter Siwen Hu-Lieskovan
Citation Annals of Oncology (2015) 26 (suppl_8): 5-14. 10.1093/annonc/mdv514
Authors S. Hu-Lieskovan1, A. Patnaik2, P. Eisenberg3, J. Sachdev4, A. Weise5, D.R. Kaufman6, I. Aromin7, B.L. West7, S. Tong7, A. Ribas1
  • 1Division Of Hematology-oncology, UCLA - School of Medicine, 90045 - Los Angeles/US
  • 2Hematology Oncology, South Texas Accelerated Research Therapeutics (START), San Antonio/US
  • 3Hematology Oncology, Marin Cancer Care, Greenbrae/US
  • 4Hematology Oncology, Honor Health, Scottsdale/US
  • 5Hematology Oncology, Karmanos Cancer Institute, Detroit/US
  • 6Clinical Research, Merck & Co., Inc., Kenilworth/US
  • 7Clinical Research, Plexxikon Inc., Berkeley/US



Tumor-associated macrophages (TAMs) support tumor growth and cause tumor resistance to chemotherapy and radiation therapy, and myeloid-derived suppressor cells (MDSCs) suppress anti-tumor immunity and cause resistance to PD-1 inhibition. TAMs and MDSCs are both regulated in part by colony stimulating factor 1 (CSF1), which signals via its receptor (CSF1R). Pexidartinib is an orally administered, small molecule inhibitor of the CSF1R. The normal function of PD-1, expressed on the cell surface of activated T-cells, is to suppress excessive immune responses such as autoimmune reactions. Tumors can utilize PD-1 signaling to downregulate immune-mediated elimination. Pembrolizumab is a potent and highly selective humanized monoclonal antibody that blocks the interaction between PD-1 and its ligands, PD-L1 and PD-L2. We present the study design for a Phase 1/2a clinical trial assessing the safety, efficacy, pharmacokinetics, and pharmacodynamics of the combination of pexidartinib and pembrolizumab in advanced solid tumors.

Trial design

In Part 1 of this open-label, uncontrolled study, patients with advanced solid tumors will be treated with pembrolizumab (200 mg intravenously every 3 weeks) and escalating daily oral doses of pexidartinib to establish a recommended phase 2 dose (RP2D) combination regimen, and the safety and tolerability of the combination. In Part 2, the combination RP2D will be studied in an expanded panel of solid tumor cohorts in up to 475 patients to further determine safety and preliminary efficacy. Overall response rate (ORR) and progression free survival (PFS) will be evaluated using RECISTv1.1 criteria. Exploratory objectives include identifying novel biomarkers of clinical activity and drug mechanisms of action. A truncated sequential probability ratio test will be employed in each tumor cohort to allow early decision-making for futility or success. The results will support further development of the pexidartinib/pembrolizumab combination in the solid tumors that respond to treatment in this study.

Clinical trial identification



S. Hu-Lieskovan: Is an investigator in the clinical trial, which is sponsored by Plexxikon Inc. Consulting with Honoraria paid to UCLA: Amgen, Novartis, Emergent BioSolutions, Vaccinex. Other Research Funding: Pfizer, Genentech. A. Patnaik, P. Eisenberg, J. Sachdev, A. Weise and A. Ribas: Are investigators in the clinical trial, which is sponsored by Plexxikon Inc. D.R. Kaufman: Is an employee of Merck & Co., Inc., which is providing funding for the clinical trial. I. Aromin, B.L. West and S. Tong: Are employees of Plexxikon Inc., the sponsor of the clinical trial.