Time dependency for HPV ctDNA detection as a prognostic biomarker for anal cancer.

3513 Background: Prior HPV infection is associated with > 90% of anal cancers, a malignancy with rising incidence in the United States. Standard treatment for localized anal cancer is concurrent chemoradiation (cRT). While detection of circulating tumor DNA (ctDNA) within weeks after surgery is linked to recurrence in other solid tumors, the optimal time point for ctDNA detection as a prognostic biomarker following cRT for HPV-associated anogenital cancers is not well characterized. We utilized a novel, highly sensitive HPV ctDNA assay to evaluate clinical outcomes according to HPV ctDNA status among patients with localized anal cancer treated with cRT. Methods: ctDNA was isolated from patients with stages I-III anal cancer treated at our institution with cRT prior to treatment, at the end of treatment (week 5), and at months 3, 6, 9, and 12 after treatment under an IRB-approved protocol. A droplet digital HPV ctDNA PCR assay evaluating HPV E6 and E7 oncogenes for 13 oncogenic HPV types was utilized for quantification of HPV ctDNA. A limit of quantification at 16 HPV copies/mL plasma was set for “HPV ctDNA detection.” Median recurrence-free survival (RFS) according to HPV ctDNA status was estimated via Kaplan-Meier and compared using a log-rank test. Associations between selected clinical factors and recurrence were evaluated with a Chi-square test, with a one-sided p .25 for all). Conclusions: With a novel, highly sensitive assay, detection of HPV ctDNA at 3 months after cRT was associated with RFS. Further, an HPV ctDNA-positive status outperformed baseline clinical features for prognosticating anal cancer recurrence after cRT in this retrospective series. Future clinical trials should incorporate the 3-month post-treatment time point for identification of patients with HPV-positive anal cancer at elevated risk for recurrence according to HPV ctDNA status. A clinical trial evaluating anti-PD-L1 + anti-TIGIT immunotherapy for patients with HPV-associated cancers with detected HPV ctDNA after cRT is forthcoming at our institution.