LBA 30 - Digital Pathology in Relation to Immune Activation Following Aglatimagene Besadenovec Immunotherapy in Localized Prostate Cancer

Purpose/Objective(s): Despite treatment with curative intent external beam radiation therapy (EBRT), approximately 30% of patients with localized, intermediate- to high-risk prostate cancer (PCa) experience recurrence. Aglatimagene besadenovec (aglatimagene) is a replication-defective adenoviral vector encoding the herpes simplex virus thymidine kinase (HSV-tk) gene. Preclinical and clinical data show that aglatimagene, in combination with valacyclovir, induces local and systemic antitumoral activity that is synergistic with radiation therapy. A pivotal placebo-controlled phase 3 clinical trial of evaluating aglatimagene + valacyclovir in combination with EBRT in 745 patients with intermediate- to high-risk, localized PCa showed significantly improvement in disease-free survival after aglatimagene treatment, independent of radiotherapy regimen (conventional vs moderate hypofractionated EBRT) and short-term use of androgen deprivation therapy. We present histologic analyses of prostate biopsies assessing the biological effect of aglatimagene plus EBRT compared to EBRT alone¹.

Materials/Methods: We used digitized hematoxylin and eosin-stained prostate biopsy samples collected at baseline and after treatment with either aglatimagene or placebo (n=647). We applied machine learning models, trained on balanced pre- and post-radiation datasets, annotated by expert pathologists to identify and quantify immune infiltration in prostate biopsy samples. We then performed spatial analysis of paired pre- and post-treatment biopsies to quantify lymphocytes and their proximity to tumor regions.

Results: Across all collected biopsies, overall lymphocytic fraction increased in post-treatment samples in both arms (p<0.001), with a significantly greater increase in the aglatimagene arm versus EBRT alone (p=0.048). To define treatment-associated changes in the tumor microenvironment (TME) among patients with detectable disease, we analyzed paired pre- and post-treatment biopsies from patients with available tumor-positive post-treatment biopsies (62 aglatimagene, 46 EBRT alone). In this subset, aglatimagene significantly increased intratumoral lymphocyte fraction (p<0.001) and maintained lymphocyte/tumor interaction relative to EBRT alone, as demonstrated by enrichment in lymphocyte-tumor mixing score and lymphocyte/tumor neighborhood (p<0.001 for both).

Conclusion: These findings show that adding aglatimagene to standard-of-care EBRT increases intratumoral lymphocyte infiltration and remodels the TME beyond EBRT alone. This immune signature, detected even in tumor-positive biopsies, supports the hypothesis that aglatimagene contributes to local tumor control through sustained immune surveillance and continued lymphocyte engagement with tumor cells.