Mereo completed a Phase 1a dose escalation clinical trial with Etigilimab in patients with advanced solid tumors and enrolled patients in a Phase 1b study in combination with nivolumab in selected tumor types.
23 patients were treated in the Phase 1a dose escalation study with doses up to 20 mg/kg Q2W. Tumor types included colorectal cancer, endometrial cancer, pancreatic cancer and other tumors. No dose limiting toxicities were observed. In the Phase 1b combination study, a total of ten patients, nine of whom had progressed on prior anti-PD-1/PD-L1 therapies were enrolled at doses of 3, 10, and 20 mg/kg. Eight patients were evaluable for tumor growth assessment, and all of these patients had progressed on PD-1/PD-L1 therapies with best responses including one patient with a partial response another with stable disease. These patients remained on study for up to 224 days. No dose limiting toxicities (DLTs) were observed and the most common related adverse events included fatigue, rash, and pruritis.
ACTIVATE, a Phase 1b/2 basket combination study of etigilimab in combination with nivolumab in select recurrent advanced / metastatic solid tumors was initiated in March 2021. The multicenter study is designed to evaluate the efficacy, safety, tolerability, PK, and pharmacodynamics of etigilimab, in combination with nivolumab, with dosing every two weeks.
As of September 12, 2022, there were 63 efficacy-evaluable checkpoint inhibitor-naïve (CPI-naive) subjects with a minimum of 1 staging scan at 8 (+/-1) weeks and RECIST 1.1 response assessment or documented clinical progression. Key updates:
Cervical cancer: 3/7 PD-L1 combined positive score CPS>1%, CPI-naïve cervical cancer subjects with confirmed complete responses (cCRs) ORR of 43%, 2 ongoing at >284 days and >142 days, and 1 withdrew consent with ongoing cCR at 163 days. Two additional patients had stable disease (SD) for a DCR rate of 71%. Of interest, biomarker analysis showed that the 2 cervical subjects with complete responses exhibited higher levels of PVR, TIGIT as well as high CD226+CD8+ co-expression. No evaluable tissue for biomarker analysis was available for the third patent with a complete response.
Uveal melanoma: 1/6 evaluable subjects, with confirmed partial response (cPR) at >347 days, ORR 17% and 2 patients were SD, for 175 and 294 days DCR 50%. The subject with cPR was noted to have high CD226+CD8+ co-expression, and was PVR positive and PD-L1 negative (CPS <1%).
Biomarker data presented at ESMO 2022 evaluated tumors for baseline expression of PVR, TIGIT, PD-L1 and CD226 by multiple modalities including validated IHC assays. Consistent with Mereo’s previously reported data (Mereo BioPharma webcast, November 30, 2021), high PVR expression was observed in subjects with decreases in target lesions (TL) from baseline and RECIST 1.1 responses. Additional noteworthy responses observed in subjects with cancer types not typically responsive to CPI monotherapy, with tumors that had high PVR expression and were either PD-L1 negative (CPS <1%), or PD-L1 low (CPS≤3%) include, (i) an ovarian PR (-80% target lesion (TL) decrease, ongoing >255 days, PD-L1 negative), (ii) an endometrial PR (-69% TL decrease, ongoing >150 days, MMR proficient, PD-L1 CPS=3%), (iii) a dedifferentiated liposarcoma PR (-80% TL decrease, ongoing cPR >267 days, high TIGIT, PD-L1 CPS=1%), and (iv) a recurrent/metastatic testicular GCT patient with stable, near normalization of elevated tumor marker alpha feto protein (doing well clinically on study >127 days), PD-L1 negative. The presentation at ESMO 2022 demonstrated robust target engagement in patients as evidenced by significant decreases in peripheral T regulatory cells while maintaining circulation levels of CD8 cells. Increases in proliferating T-cells subsets (CD8, CD4), proliferating NK cells, and intracellular cytokines (IFNg, IL2, and TNFa) were observed and sustained longitudinally. Additionally, etigilimab plus nivolumab reduced TPEX cells (CD8+CCR7+PD1+TIGIT+), progenitor cells believed to be committed to an exhausted-like fate. Further data was reported showing reductions in circulating tumor DNA (ctDNA) measured at ~5-6 weeks post-treatment correlated with clinical benefit.
These data support the role of dual checkpoint inhibition of the TIGIT/PVR and PD-(L)-1 pathways and further evaluation of these biomarkers, including PVR and CD226, as a potential enrichment strategy for the treatment of etigilimab plus anti-PD1.
Enrolment in the Phase 1b/2 ACTIVATE trial has been paused; 30 patients previously enrolled currently remain on study. The totality of emerging data from ACTIVATE and other trials targeting the TIGIT axis will direct next steps for the etigilimab program.