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Outcomes for men with localized prostate cancer vary widely, with some men effectively managed without treatment on active surveillance, while other men rapidly progress to metastatic disease despite curative‐intent therapies. One of the strongest prognostic indicators of outcome is grade groups based on the Gleason grading system. Gleason grade 4 prostate cancer with cribriform morphology is associated with adverse outcomes and can be utilized clinically to improve risk stratification. The underpinnings of disease aggressiveness associated with cribriform architecture are not fully understood. Most studies have focused on genetic and molecular alterations in cribriform tumor cells; however, less is known about the tumor microenvironment in cribriform prostate cancer. Cancer‐associated fibroblasts (CAFs) are a heterogeneous population of fibroblasts in the tumor microenvironment that impact cancer aggressiveness. The overall goal of this study was to determine if cribriform prostate cancers are associated with a unique repertoire of CAFs. Radical prostatectomy whole‐tissue sections were analyzed for the expression of fibroblast markers (ASPN in combination with FAP, THY1, ENG, NT5E, TNC, and PDGFRβ) in stroma adjacent to benign glands and in Gleason grade 3, Gleason grade 4 cribriform, and Gleason grade 4 noncribriform prostate cancer by RNAscope®. Halo® Software was used to quantify percent positive stromal cells and expression per positive cell. The fibroblast subtypes enriched in prostate cancer were highly heterogeneous. Both overlapping and distinct populations of low abundant fibroblast subtypes in benign prostate stroma were enriched in Gleason grade 4 prostate cancer with cribriform morphology compared to Gleason grade 4 prostate cancer with noncribriform morphology and Gleason grade 3 prostate cancer. In addition, gene expression was distinctly altered in CAF subtypes adjacent to cribriform prostate cancer. Overall, these studies suggest that cribriform prostate cancer has a unique tumor microenvironment that may distinguish it from other Gleason grade 4 morphologies and lower Gleason grades.

A lower baseline neutrophil-to-eosinophil ratio (NER) has been associated with improved responses to immune checkpoint inhibitors (ICI)-treated metastatic renal cell carcinoma (mRCC). This study investigated the decrease in NER at week 6 after ipilimumab/nivolumab (ipi/nivo) initiation and treatment responses in mRCC. A retrospective study of ipi/nivo-treated mRCC at two US academic cancer centers was conducted. A landmark analysis at week 6 was performed to assess the association between the change in NER and clinical responses (progression-free survival (PFS)/overall survival (OS)). Week 6 NER was modeled as a continuous variable, after log transformation (Ln NER), and a categorical variable by percent change. There were 150 mRCC patients included: 78% had clear cell histology, and 78% were IMDC intermediate/poor risk. In multivariable regression analysis, every decrease of 1 unit of Ln NER at week 6 was associated with improved PFS (adjusted hazard ratio (AHR): 0.78, p-value:0.005) and OS (AHR: 0.67, p-value: 0.002). When NER was modeled by percent change, decreased NER > 50% was associated with improved PFS (AHR: 0.55, p-value: 0.03) and OS (AHR: 0.37, p-value: 0.02). The decrease in week 6 NER was associated with improved PFS/OS in ipi/nivo-treated mRCC. Prospective studies are warranted to validate NER change as a biomarker to predict ICI responses.

341 Background: Neutrophilia is known to be associated with worse prognosis in metastatic renal cell carcinoma (mRCC); however, less is known about the role of eosinophils in the response to immunotherapy (IO). We investigated the association of the baseline neutrophil to eosinophil ratio (NER) with outcomes to IO-based combination treatment in mRCC. Methods: Patients with mRCC treated with ipilimumab plus nivolumab, pembrolizumab plus axitinib, or avelumab plus axitinib at the Vanderbilt-Ingram Cancer Center were retrospectively identified. Patients on >10mg prednisone and patients with prior IO were excluded. Baseline NER (at time of first IO) and association with progression free survival (PFS), overall survival (OS), and objective response rate (ORR) were investigated. Data cutoff was 9/1/2020. Analysis for PFS and OS was performed using the log-rank test and Mantel-Haenszel method, and analysis of the odds ratio for ORR was performed using Fischer’s exact test. Results: Sixty-one patients were identified: 89% clear cell histology, 74% prior nephrectomy, 69% IMDC intermediate risk, and 72% treatment-naïve. Patients with baseline NER < median (N=31) had improved clinical outcomes compared to patients with baseline NER > median (N=30) (Table). Improvement in PFS by NER was maintained when stratified by anti-PD-1/CTLA-4 and anti-PD(L)-1/VEGF (p= 0.0062 and p= 0.049); however, differences in OS and ORR were no longer significant. The median baseline NER among patients with partial response (PR) was significantly lower at 22.7 (95% CI 18.9-31.1) vs. 51.6 (95% CI 39.5-93.1) among those with progressive disease (PD) (p= 0.0054). For comparison, the median neutrophil to lymphocyte ratio was not significantly different between PR (2.60) and PD (3.84, p= 0.056). Conclusions: Patients with a low baseline NER treated with IO-based combinations had improved clinical outcomes compared to patients with a high baseline NER. Additional investigation of this parameter in larger cohorts is warranted. [Table: see text]