Abstract 4178: PAK4 inhibition improves PD1 blockade immunotherapy in prostate cancer

Abstract Prostate cancers have low immune infiltration and do not respond to PD1 blockade immunotherapy. The objective of this study was to examine the effect of PAK4 inhibition on anti-tumor immune response in preclinical prostate cancer models. C57BL/6J mice (male, 5-8 weeks old) were injected with wild type (WT) or PAK4 knockdown (KD) syngeneic Rm1 tumor cells (2X105 cell/mouse) and treated with or without αPD1 blockade (200μg, BioXCell). Tumor growth was monitored and analyzed using ANOVA of repeated measures (n=6). In a separate experiment, 1 cm RM1 tumors were collected for RNA sequencing (RNASeq) and immunohistochemistry (IHC). RNASeq data were normalized using the variance stabilizing transformation (VST) function as implemented within the DESeq2 package. IHC images were captured with Keyence BZ-X800 and positive staining was calculated using ImageJ. CD8+ lymphocytes depletion was achieved by CD8 antibody injection (200ug, BioXCell). PAK4 inhibitors PF-3758309 (PF) and KPT-9274 (KPT) were evaluated in murine models. PAK4 KD tumors treated with αPD1 antibody (αPD1) showed a significant reduction of tumor growth when compared to growth in WT (P<.05), WT αPD1 (P<.01), and KD (p<.05). Analysis of bulk tumor RNAseq showed that the combination of PAK4 KD with αPD1 was associated with increased CD8a, IFNγ, and PD-L1 expression. IHC with αCD8 verified that PAK4 KD increased CD8+ cell infiltration in mouse tumors. Further, RNAseq showed that PAK4 KD increased the expression of cytokines, chemokines, and genes related to the IFNγ response. These results suggest that PAK4 KD primes the tumor for immune infiltration and improves αPD1 treatment response. The anti-tumor effect of PAK4 KD with αPD1 in mice was CD8-dependent and could be eliminated by CD8+ cell depletion (p<.05). We tested two different PAK4 inhibitors in mice with WT Rm1 tumors. PF is a kinase inhibitor that blocks the function of PAK4. A combination of PF and αPD1 significantly reduced tumor growth when compared to WT (P<.05), WT αPD1 (P<.01), and PF (p<.05). KPT is a dual inhibitor of PAK4 and NAMPT, which reduces the cytosolic protein concentration of PAK4. A combination of KPT and αPD1 significantly reduced tumor growth when compared to WT (P<.01) and WT αPD1 (P<.01). In conclusion, PAK4 inhibition increased immune infiltration and improved αPD1 treatment response in preclinical mouse prostate cancer models. Citation Format: Shengchen Su, Sungyong You, Yanping Wang, Patrick Tamukong, Catherine S. Grasso, Hyung L. Kim. PAK4 inhibition improves PD1 blockade immunotherapy in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 4178.