Neoadjuvant (NA) chemoradiotherapy (CTRT) plus avelumab (Ave) in locally advanced rectal cancer (LARC): 4-year outcomes of the phase II AVANA trial and their association with computational pathology.

3632 Background: The AVANA trial investigated the addition of PD-L1 inhibitor Ave to NA CTRT in LARC. Here, we report long-term survival and explore its association with tumor microenvironment (TME) features assessed through computational pathology. Methods: AVANA is a multicenter, single-arm, phase II trial enrolling patients (pts) with LARC defined by at least one high-risk feature (cN+, cT4, or high-risk cT3). Pts received NA CTRT combined with six cycles of Ave, followed by surgery. Primary endpoint was pathological complete response (pCR) rate. Secondary endpoints included progression-free survival (PFS) and overall survival (OS). Multiplex immunofluorescence was performed on baseline biopsies and surgical specimens using CODEX platform (Akoya). A deep-learning pipeline using a spatially constrained convolutional neural network for cell detection and a classifier for cell labeling was applied, allowing identification of eight cell populations: cancer cells, normal epithelial cells, fibroblasts, lymphocytes (lym), neutrophils (neu), macrophages, endothelial (endo) cells and myocytes. Cell infiltrates were dichotomized as high or low using optimal cutpoints derived from maximally selected rank statistics. Cell density maps were generated to identify differences in cellular configurations between baseline and surgical specimens. Results: The primary endpoint was met, with a pCR of 21.8% (22/101 pts). At a median follow-up of 51.8 months, 26 of 101 pts had a progression event and the 48-month PFS rate was 74.7% (95% CI: 65.0–82.2). Thirteen deaths occurred, yielding a 48-month OS rate of 87.6% (95% CI: 79.1–92.7). Age (≥70 vs <70; p=0.371), sex (p=0.897), ECOG PS (0 vs 1; p=0.899), cN positivity (p=0.716), cT stage (1-2 vs 3-4; p=0.264), or mismatch repair status (dMMR vs pMMR; p=0.274) were not associated to PFS. Digital whole-slide images of diagnostic biopsies and surgical specimens were available for 87/101 pts (86.1%). In post-treatment surgical specimens, high neu (p=0.00025) and high lym infiltrate (p=0.0038) correlated with better PFS, whereas high endo cell infiltrate was associated to shorter PFS (p<0.0001). Comparisons between baseline and post CTRT+Ave tissues showed that increased endo cell density in surgical specimen relative to baseline biopsies had worse PFS (p=0.0070), while increased neu to cancer cell ratios HR 0.50 (95% CI: 0.21-1.18); p=0.108 and increased neu density HR 0.22 (95% CI: 0.03-1.65; p=0.106 showed trends toward better PFS, although not statistically significant. Conclusions: AVANA demonstrates the feasibility and activity of adding ICIs to standard CTRT in LARC. Computational pathology analyses suggest that dynamic changes in TME cellular composition could be associated with long-term outcomes, supporting its potential role for pts stratification. Clinical trial information: NCT03854799 .