3402 - Relationship of PD-L1 Status and Outcomes in a Diverse Patient Population with Anal Squamous Cell Carcinoma

Purpose/Objective(s): PD-L1 targeted therapy has a growing role in treating metastatic anal squamous cell carcinoma (ASCC). Here we assess the survival implications of PD-L1 expression for patients treated with definitive chemoradiation (CRT).

Materials/Methods: We identified patients treated with CRT for ASCC from 2021-2024 with PD-L1 expression reported using combined positive score (CPS). We used 5% PD-L1 expression as a threshold to distinguish low from high PD-L1 expression. Cox proportional hazards modeling was used to explore predictors of progression-free survival (PFS), with known predictors included in a multivariable model. Kaplan-Meier analysis with log-rank was used to compare PFS between groups.

Results: Sixty-one ASCC patients were reviewed. Median age was 62 and just over half were female (52.5%). One third were Black (32.8%). The remainder were white (59.0%), Asian (1.6% ), and other (6.5%). One quarter were Hispanic (26.2%). Thirty-four patients (55.7%) were immunosuppressed; 24 were HIV+ (39.3%), 3 had organ transplants (4.9%), 2 each had inflammatory bowel disease, prior malignancy, and systemic lupus erythematosus (3.3%), and 1 had sarcoidosis (1.6%). Of the HIV+ patients, 19 (79.2%) were men. Half of patients were past or current smokers and 78.7% were p16 positive. Twelve patients were T1 (19.7%), 21 were T2 (34.4%), 18 were T3 (29.5%), and 10 were T4 (16.4%); 38 (62.3%) patients had N1 disease.

Median follow-up was 17.2 months. Forty-five patients (73.8%) had a clinical complete response (CR), 6 (9.8%) had a partial response (PR), and 10 (16.4%) had insufficient follow-up to assess response. There were 6 failures: 3 locoregional (LRF), 2 distant (DF), and 1 simultaneous LRF/DF. Median time to failure was 6.2 months from end of treatment.

There were no significant differences between high and low PD-L1 groups. By univariate analysis, high PD-L1 expression, CR, female sex, and lower N- and T-stage were associated with improved PFS. In a multivariable model, only PD-L1 and female sex were associated with improved PFS (Table).

Conclusion: In this racially and ethnically diverse ASCC patient population with over one-third HIV+, PD-L1 status appears to be prognostic for survival in the setting of CRT. Extended follow-up and wider PD-L1 testing are necessary to validate the 5% cutoff as a prognostic marker that could be used for better tailoring chemoradiation for ASCC.

Abstract 3402 - Table 1