Plexxikon will partner with Merck & Co. on a clinical trial that will assess the cancer immunotherapy combination of PLX3397, Plexxikon’s investigational CSF-1R inhibitor, and Merck’s already-marketed Keytruda® (pembrolizumab).

The Phase I/II trial will enroll patients with advanced melanoma and multiple other solid tumors, with the goal of determining the safety and tolerability of the combination therapy.

The trial is expected to begin enrollment by mid-year, the companies said.

“Cancer cells use multiple tactics to evade host immune responses, and the combination of these two agents is being investigated to re-activate anti-tumor immunity using distinct and complementary mechanisms,” Plexxikon CEO Gideon Bollag, Ph.D, said in a statement.

He added that PLX3397 and Keytruda have potential application in multiple types of cancer. Merck aims to learn which types of cancer through its ongoing clinical program for Keytruda, which has more than 85 clinical trials across more than 30 tumor types and over 14,000 patients. The program is assessing Keytruda both as a monotherapy and in combination with other therapies.

Plexxikon is separately evaluating PLX3397 in a Phase III study for tenosynovial giant cell tumor (TGCT)—historically called pigmented villonodular synovitis (PVNS) or giant cell tumor of the tendon sheath (GCT-TS). That study is recruiting patients, according to, as are four other studies assessing PLX3397:

  • In combination with radiation therapy and temozolomide in patients with newly diagnosed glioblastoma (Phase I/II).
  • In children and young adults with refractory leukemias and refractory solid tumors Including neurofibromatosis type 1 associated plexiform neurofibromas (Phase I/II).
  • In combination with another Plexxikon drug candidate, PLX9486, in patients with types of advanced solid tumors including gastrointestinal stromal tumor (Phase I/II).
  • In combination with eribulin (Halaven™) in patients with metastatic breast cancer (Phase Ib/II).

Plexxikon is owned by Daiichi Sankyo Group, which acquired the company in 2011.

Keytruda is a humanized monoclonal antibody that works by blocking interaction between PD-1 and its receptor ligands, PD-L1 and PD-L2—thus increasing the immune system’s ability to fight cancer in cells that produce the pigment responsible for color in the skin.

Keytruda—which won FDA approval in September 2014—is indicated for patients with unresectable or metastatic melanoma and disease progression following treatment with ipilimumab and, for patients who are BRAF V600 mutation positive, a BRAF inhibitor. Keytruda has been approved for intravenous infusion over 30 minutes every three weeks at a dose of 2 mg/kg.

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