Phase II Trial of Atezolizumab and Bevacizumab for Treatment of HPV-Positive Unresectable or Metastatic Squamous Cell Carcinoma of the Anal Canal.

Purpose: Anti-PD-L1 antibodies are associated with responses in <25% of patients with metastatic human papillomavirus-associated malignancies. VEGF signaling causes immune evasion and immune suppression within the tumor. We evaluated the anti-PD-L1 antibody atezolizumab and anti-VEGF antibody bevacizumab for patients with unresectable, advanced anal cancer.

Patients and methods: For this phase II study, participants with previously treated, immunotherapy-naïve anal cancer received atezolizumab (1,200 mg) and bevacizumab (15 mg/kg) intravenously every 21 days. Responses were evaluated every 9 weeks (RECIST version 1.1). The primary endpoint was the best radiographic response. Median survival was estimated by Kaplan-Meier and compared for selected biomarkers (including paired pre- and on-treatment biopsies) using a log-rank test.

Results: Among 20 participants, the overall response rate was 11% [95% confidence interval (CI): 1.2-32]. Median progression-free survival and overall survival were 4.1 months (95% CI, 2.6-not assessable) and 11.6 months (95% CI, 9.5-20), respectively. One grade 5 bevacizumab-related bowel perforation occurred. Analyses of 16 paired biopsies linked increases in IFN-γ (P = 0.03) and inflammatory response (P = 0.02) gene expression signatures with prolonged progression-free survival, as did increases in CD3+CD8+PD1+ (P = 0.02) cells and decreases in CD3+FoxP3+ cells (P = 0.04) from 10 paired biopsies with multiplex immunofluorescence. A subgroup of anal cancers characterized by the SBS31 "prior-platinum" signature demonstrated shorter median overall survival (HR, 6.3; 95% CI, 1.2-32; P = 0.01).

Conclusions: Atezolizumab and bevacizumab demonstrate activity similar to anti-PD-1 antibodies alone for unresectable anal cancer. Our translational data identify undescribed chromosomal and transcriptomic biomarkers associated with survival for metastatic anal cancer. These correlative findings warrant confirmation and further validation in larger, prospective immunotherapy trials for advanced anal cancer.