Spatial patterns in the tumor immune microenvironment as prognostic and predictive biomarkers of panitumumab (Pmab), fluorouracil and folinic acid (FU/FA) or FU/FA maintenance therapy following Pmab+FOLFOX induction in RAS wildtype (wt) metastatic colorectal cancer (mCRC): PanaMa (AIO KRK0212) trial.

3539 Background: Patterns of tumor immune cell infiltration in the tumor microenvironment are prognostic biomarkers in CRC. Here, the spatially resolved tumor immune microenvironment was analyzed for prognostic and predictive impact in patients with RAS wt mCRC treated with FU/FA +/- Pmab maintenance after Pmab + FOLFOX induction (PanaMa AIO KRK0212 trial). Methods: A total of 12 immunological parameters (7 lymphocyte: CD3, CD8, CD45RO, FOXP3, CD20, Granzyme B, Perforin; 4 checkpoint: PD-1, PD-L1, IDO1, LAG3; and 1 monocyte: CD163) were quantified after immunohistochemistry using QuPath in spatially resolved (tumor invasive-margin Inv , and center Cen ) tissue-microarrays from available surgical tumor resections according to established local protocols. Parameters were analyzed according to percentile cutoffs, Immunoscore (combination of CD3 and CD8 in center and invasive-margin), and PD-L1 CPS. Median progression-free (PFS) and overall survival (OS) were estimated by Kaplan-Meier-method, log rank test and Cox regression. Results: In 194 patients with available surgical tumor resections, low expression of CD163 (HR: 0.717; 95% CI: 0.526-0.977; p = 0.036), and high expression of PD-1 (HR: 0.672; 95% CI: 0.491–0.921; p = 0.014) in the tumor center, were independent prognostic factors for prolonged PFS. The addition of Pmab to FU/FA maintenance was associated with longer PFS in patients with low expression of CD8 Inv (HR: 0.224; 95% CI: 0.085-0.594; p = 0.004), LAG3 Cen (HR: 0.489; 95% CI: 0.265-0.905; p = 0.025), CD163 Cen (HR: 0.552; 95% CI: 0.346-0.883; p = 0.015), Immunoscore (HR: 0.501; 95% CI: 0.179-0.901; p = 0.024), and high expression of CD45 Cen (HR: 0.403; 95% CI: 0.222-0.732; p = 0.003). In patients with CD45 Cen -high and LAG3 Cen -low, prolonged PFS translated also into a significant OS-prolongation. A positive Immunoactivation-score (defined as positivity in ≥ 2 predictive markers from univariate analysis) predicted significant benefit from additional Pmab as maintenance in terms of PFS (HR: 0.302; 95% CI: 0.161-0.567; p = <0.001) and OS (HR: 0.388; 95% CI: 0.195-0.772; p = 0.007). Conclusions: Several factors of the immune microenvironment were found to be prognostic factors for PFS and OS in patients with RAS wt mCRC. Of those, CD8, CD163, LAG3, CD45, Immunoscore, and an Immunoactivation-score were identified as independent predictive markers for benefit from the addition of Pmab to FU/FA maintenance therapy, suggesting that immune activation might be a crucial component of EGFR-antibody efficacy.