Mereo BioPharma Group plc announced that Celgene Corporation (Celgene) has notified OncoMed Pharmaceuticals, Inc. (OncoMed, a subsidiary of Mereo), pursuant to the Master Research and Collaboration Agreement, dated December 2, 2013, by and among OncoMed, Celgene and Celgene Alpine Investment Company II, LLC of Celgene's decision, for strategic product portfolio considerations, not to exercise its option to license etigilimab, the anti-TIGIT antibody, one of two product candidates Mereo acquired through its April 2019 merger with OncoMed. Mereo and Celgene are working to finalize the termination of the Collaboration Agreement with respect to etigilimab, upon which Mereo expects to retain the worldwide rights to etigilimab and to initiate discussions with other potential partners for this program. Etigilimab successfully completed the Phase 1a dose-escalation portion of a clinical study in patients with a variety of late stage metastatic cancers and is currently being evaluated in a fully-enrolled Phase 1b combination portion with nivolumab in patients with select tumor types.

As a consequence of the forthcoming termination of the Collaboration Agreement, and in accordance with the terms and conditions of the Contingent Value Rights Agreement, dated April 23, 2019, by and among Mereo and Computershare Inc., as rights agent, (the "CVR Agreement"), it is not expected that holders of contingent value rights pursuant to the CVR Agreement will be entitled to receive the TIGIT Milestone Payment (as defined in the CVR Agreement). In addition, it is not expected that holders of contingent value rights pursuant to the Contingent Value Rights Agreement, dated March 14, 2019, by and among OncoMed and Computershare, Inc. as rights agent will be entitled to receive any TIGIT Payment Amounts (as defined in the OncoMed CVR Agreement). TIGIT (T-cell immunoreceptor with Ig and ITIM domains) is a next generation checkpoint receptor shown to block T-cell activation and the body's natural anti-cancer immune response.

Etigilimab is an IgG1 monoclonal antibody which binds to the human TIGIT receptor on T-cells with a goal of improving the activation and effectiveness of T-cell and NK cell anti-tumor activity. The Phase 1a/b clinical trial with etigilimab enrolled patients with advanced solid tumors into either a Phase 1a single-agent portion (dose escalation in all patients and expansion in selected tumor types) or Phase 1b combination portion with nivolumab in selected tumor types.18 patients were treated in the Phase 1a dose escalation study with doses up to 20mg/kg Q2W. Tumor types included colorectal cancer (6), endometrial cancer (2), pancreatic cancer (1) and 8 other tumor types.

No dose limiting toxicities were observed with the recommended Phase 2 dose of 20mg/kg Q2W.