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BackgroundPatients with advanced renal cell carcinoma with sarcomatoid features (sRCC) have a poor prognosis and limited therapeutic options. First-line nivolumab plus ipilimumab (NIVO+IPI) provided efficacy benefits over sunitinib (SUN) in patients with intermediate/poor-risk sRCC at 42 months minimum follow-up in the phase 3 CheckMate 214 trial. In this exploratory post hoc analysis, we report clinical efficacy of NIVO+IPI in sRCC after a minimum follow-up of 5 years.MethodsIn CheckMate 214, patients with clear cell advanced RCC were randomized to NIVO 3 mg/kg plus IPI 1 mg/kg every 3 weeks (four doses), then NIVO 3 mg/kg every 2 weeks versus SUN 50 mg once daily (4 weeks; 6-week cycles). Randomized patients with sRCC were identified via independent central pathology review of archival tumor tissue or histological classification per local pathology report. Overall survival (OS), as well as progression-free survival (PFS) and objective response rate (ORR) per independent radiology review using Response Evaluation Criteria in Solid Tumors V.1.1, were evaluated in all International Metastatic Renal Cell Carcinoma Database Consortium intermediate/poor-risk sRCC patients and by baseline tumor PD-L1 expression level (≥1% vs ResultsIn total, 139 patients with intermediate/poor-risk sRCC were identified (NIVO+IPI, n=74; SUN, n=65). At 5 years minimum follow-up, more patients remained on treatment with NIVO+IPI versus SUN (12% vs zero). Efficacy benefits with NIVO+IPI versus SUN were maintained with median OS of 48.6 vs 14.2 months (HR 0.46), median PFS of 26.5 vs 5.5 months (HR 0.50), and ORR 60.8% vs 23.1%. In addition, median duration of response was longer (not reached vs 25.1 months), and more patients had complete responses (23.0% vs 6.2%) with NIVO+IPI versus SUN, respectively. Efficacy was better with NIVO+IPI versus SUN regardless of tumor PD-L1 expression, but the magnitude of OS, PFS, and ORR benefits with NIVO+IPI was greater for sRCC patients with tumor PD-L1 ≥1%. No new safety signals emerged in either arm with longer follow-up.ConclusionsAmong patients with intermediate/poor-risk sRCC, NIVO+IPI maintained long-term survival benefits and demonstrated durable and deep responses over SUN at minimum follow-up of 5 years, supporting NIVO+IPI as a preferred first-line therapy in this population.Trial registration numberNCT02231749.

BackgroundXL092 is a novel oral multi-targeted inhibitor of receptor tyrosine kinases. In preclinical studies, oral dosing with XL092 resulted in pharmacodynamic inhibition of MET, TAM kinases (AXL, MER), and VEGFR2 phosphorylation and was associated with significant anti-tumor activity in xenograft tumor models. Drugs targeting TAM kinases may promote an immune-permissive environment, which may enhance response to immune checkpoint inhibitors (ICIs). The combination of XL092 and anti–PD-1 ICI exhibited greater anti-tumor activity in a syngeneic tumor model than either agent alone.[1] Preliminary XL092 pharmacokinetic data indicate a median terminal half-life of 21 hours, suitable for daily dosing.MethodsThis multi-center, phase 1, open-label study aims to enroll approximately 800 patients total in dose-escalation and cohort-expansion stages (NCT03845166). The dose-escalation stage will enroll patients with inoperable locally advanced or metastatic solid tumors in a 3+3 design (monotherapy cohorts) or rolling 6 design (combination cohorts). Patients will receive escalating doses of XL092 alone or in combination with ICIs (atezolizumab 1200mg Q3W or avelumab 800mg Q2W). Following identification of the maximum tolerated dose (MTD) and/or recommended dose (RD) of XL092 monotherapy, the expansion stage will enroll XL092 monotherapy cohorts in 3 tumor-specific indications (renal cell carcinoma [RCC], metastatic castration-resistant prostate carcinoma [mCRPC], and hormone-receptor positive breast carcinoma [HR+ BC]) using a Simon’s optimal 2-stage design. Once the MTDs/RDs are identified for XL092 in combination with each ICI, the expansion stage will also enroll patients in tumor-specific cohorts for XL092+atezolizumab (RCC, mCRPC, HR+ BC, and colorectal carcinoma) and XL092+avelumab (urothelial carcinoma) using Simon’s optimal 2-stage design or a Precision-Based design. A limited number of patients will be enrolled in dedicated biomarker cohorts, with tumor and blood samples collected, to evaluate the pharmacodynamic effects of XL092 alone and in combination with ICIs. Key eligibility criteria include unresectable or metastatic solid tumors; measurable disease per RECIST 1.1 (expansion cohorts only); available tumor tissue (archival or fresh biopsy); and ECOG 0-1. The primary objective of the dose-escalation stage is to determine MTDs/RDs of XL092 when administered alone or in combination with ICIs, and the primary objective of the expansion stage is to evaluate objective response rate and progression-free survival rate by investigator per RECIST 1.1 of XL092 as monotherapy or in combination with ICIs. Secondary objectives include evaluation of the safety of XL092 alone and in combination with ICIs and evaluation of plasma pharmacokinetics of XL092 and its potential metabolites. The study is open for enrollment.AcknowledgementsMedical writing support provided by Griselda Zuccarino-Catania, PhD (Exelixis, Inc.)Trial RegistrationNCT03845166ReferencesHsu J, Chong C, Goon L, et al. XL092, a Multi-targeted Inhibitor of MET, VEGFR2, AXL and MER with an Optimized Pharmacokinetic Profile. European Journal of Cancer [abstract 33] 2020;138 Suppl 2:S16.Ethics ApprovalStudy approved by institutional Review Boards at each investigational site.

BackgroundCombining an immune checkpoint inhibitor with a tumor vaccine may modulate the immune system to leverage complementary mechanisms of action that lead to sustained T-cell activation and a potent prolonged immunotherapeutic response in metastatic castration resistant prostate cancer (mCRPC).MethodsSubjects with asymptomatic or minimally symptomatic mCRPC were randomly assigned in a 1:1 ratio to receive either atezolizumab followed by sipuleucel-T (Arm 1) or sipuleucel-T followed by atezolizumab (Arm 2). The primary endpoint was safety, while secondary endpoints included preliminary clinical activity such as objective tumor response and systemic immune responses that could identify key molecular and immunological changes associated with sequential administration of atezolizumab and sipuleucel-T.ResultsA total of 37 subjects were enrolled. The median age was 75.0 years, median prostate specific antigen (PSA) was 21.9 ng/mL, and subjects had a median number of three prior treatments. Most subjects (83.8%) had at least one treatment-related adverse event. There were no grade 4 or 5 toxicities attributed to either study drug. Immune-related adverse events and infusion reactions occurred in 13.5% of subjects, and all of which were grade 1 or 2. Of 23 subjects with Response Evaluation Criteria in Solid Tumors measurable disease, only one subject in Arm 2 had a partial response (PR) and four subjects overall had stable disease (SD) at 6 months reflecting an objective response rate of 4.3% and a disease control rate of 21.7%. T-cell receptor diversity was higher in subjects with a response, including SD. Immune response to three novel putative antigens (SIK3, KDM1A/LSD1, and PIK3R6) appeared to increase with treatment.ConclusionsOverall, regardless of the order in which they were administered, the combination of atezolizumab with sipuleucel-T appears to be safe and well tolerated with a comparable safety profile to each agent administered as monotherapy. Correlative immune studies may suggest the combination to be beneficial; however, further studies are needed.Trial registration numberNCT03024216.

BackgroundCirculating cytokines and angiogenic factors have been associated with clinical outcomes in patients with metastatic renal cell carcinoma (RCC) receiving systemic therapy. However, none have yet examined cytokine concentrations in parallel cohorts receiving either immunotherapy or targeted therapy.MethodsIn this prospective correlative study, we enrolled 56 patients who were planned for treatment with either a vascular endothelial growth factor-tyrosine kinase inhibitor (VEGF-TKI) or immune checkpoint inhibitor (ICI). Eligibility requirements permitted any RCC histologic subtype, International Metastatic Renal Cell Carcinoma risk classification, and line of therapy. Immunologic profile was assessed at baseline and after 1 month on treatment using a Human Cytokine 30-plex protein assay (Invitrogen). Clinical benefit was defined as complete response, partial response, or stable disease ≥6 months per RECIST (Response Evaluation Criteria in Solid Tumors) V.1.1 criteria.ResultsClinical benefit was similar between VEGF-TKI and ICI arms (65% vs 54%). Patients with clinical benefit from VEGF-TKIs had lower pretreatment levels of interleukin-6 (IL-6) (p=0.02), IL-1RA (p=0.03), and granulocyte colony-stimulating factor (CSF) (p=0.02). At 1 month, patients with clinical benefit from ICIs had higher levels of interferon-γ (IFN-γ) (p=0.04) and IL-12 (p=0.03). Among patients on VEGF-TKIs, those with clinical benefit had lower 1 month IL-13 (p=0.02) and granulocyte macrophage CSF (p=0.01) as well as higher 1 month VEGF (p=0.04) compared with patients with no clinical benefit.ConclusionFor patients receiving VEGF-TKI or ICI therapy, distinct plasma cytokines were associated with clinical benefit. Our findings support additional investigation into plasma cytokines as biomarkers in metastatic RCC.

Background Immune Checkpoint Inhibitor (ICI) therapies have significantly improved overall survival in numerous cancers, but colitis has emerged as the most frequent dose-limiting toxicity associated with these therapies. Patients experiencing colitis side effects have to discontinue their cancer therapy to treat the colitis, which repositions life-threatening cancer. An ideal therapeutic would offer a colon-restricted approach to treating colitis side effects, while allowing patients to stay on their ICI therapy, an approach unavailable with currently approved therapies. Methods To test this localized approach for the treatment of ICI-mediated colitis, we have developed a new chemical entity as a next-generation candidate therapeutic designed for oral administration. Permeability was tested in predictive epithelial monolayers and confirmed in rodent pharmacokinetic studies. We tested the drug in the preclinical adoptive transfer model for colitis. In this model, immunodeficient mice are hosts for adoptive transfer of naive CD4+ T cells. In the absence of regulatory T cells, the transferred cells drive systemic inflammation and migrate to the colon, causing disease. Without treatment, these mice develop signs of colitis including weight loss, altered crypt architecture and infiltration of the lamina propria by week four. Furthermore, we developed a live biopsy culture system to test drug effects on ICI-colitis patient biopsies obtained via colonoscopy. Results In-vitro studies demonstrate that the drug has minimal toxicity and that it potently suppresses T cell proliferation and cytokine secretion. Permeability studies show a limited ability to cross the colonic mucosa restricting anti-inflammatory effects to sites of ulceration and active colitis. When colitis mice were given drug by oral gavage after colitis had developed, treated mice showed a significant increase in weight over controls and improved histological scores. Importantly, markers of systemic inflammation remained unchained, and the cancer-killing ability of the primary ICI therapy was preserved. Results of the live biopsy culture studies will be presented. Conclusions These preliminary studies demonstrate that the candidate therapeutic has potential to become a novel next-generation oral therapy for ICI-colitis because it effectively limits leukocyte function in-vitro and in-vivo with minimal systemic absorption and minimal expected side effects. Given the favorable drug profile and the rapid growth of ICI therapies, our colon-restricted colitis therapeutic has the potential to improve outcomes in a large number of cancer patients. We anticipate commencing first-in-human studies in Q4 of 2021. Acknowledgements None Ethics Approval The study was approved by City of Hope Cancer Center Institutional Review Board with the approval number of 19304.

BackgroundThe clinical significance of tumor-specific genomic alterations in metastatic renal cell carcinoma (mRCC) is emerging, with several studies suggesting an association between PBRM1 mutations and response with immunotherapy (IO). We sought to determine genomic predictors of differential response to vascular endothelial growth factor–tyrosine kinase inhibitors (VEGF-TKIs) and IO.MethodsConsecutive patients who underwent genomic profiling were identified; patients receiving either VEGF-TKIs or IO were included. Clinical tumor-normal whole exome sequencing and tumor whole transcriptome sequencing test were performed using a Clinical Laboratory Improvement Amendments (CLIA)-certified assay (Ashion Analytics; Phoenix, Arizona, USA). Genomic findings were compared between patients with clinical benefit (CB; complete/partial response or stable disease for >6 months) and no clinical benefit (NCB) in VEGF-TKI-treated patient cohort and IO-treated patient cohort.Results91 patients received genomic profiling and 58 patients received VEGF-TKI and/or IO therapy. 17 received sequenced treatment involving both VEGF-TKI and IO, resulting in 32 patients in the IO cohort and 43 patients in the VEGF-TKI cohort. The most commonly used IO and VEGF-TKIs were nivolumab (66%) and sunitinib (40%). The most frequently detected alterations in the overall cohort were in VHL (64%), PBRM1 (38%), SETD2 (24%), KDM5C (17%) and TERT (12%). TERT promoter mutations were associated with NCB in the IO cohort (p=0.038); transcriptomic analysis revealed multiple differentially regulated pathways downstream of TERT. TERT promoter mutations and PBRM1 mutations were found to be mutually exclusive. While PBRM1 mutations were more prevalent in patients with CB with IO and VEGF-TKIs, no statistically significant association was found.ConclusionsOur analysis found that TERT promoter mutations may be a negative predictor of outcome with IO and are mutually exclusive with PBRM1 loss-of-function mutations.

BackgroundAtezolizumab is a treatment for locally advanced/metastatic urothelial carcinoma (mUC). However, its use in patients with renal insufficiency or UC with mixed variant histology (MVH) is not well characterized.ObjectiveTo report efficacy and safety of atezolizumab in these special subpopulations from an expanded access program (EAP).Design, setting, and participantsA total of 218 patients were enrolled at 36 US study sites (November 2015–August 2016), and the trial ended following the approval of atezolizumab by the US Food and Drug Administration. This post hoc analysis investigated outcomes in specific study subgroups.InterventionAtezolizumab 1200 mg was administered intravenously every 3 weeks until loss of clinical benefit, unacceptable toxicity, death, consent withdrawal, decision to discontinue, commercial availability, or study closure.Outcome measurements and statistical analysisResponse Evaluation Criteria in Solid Tumors V.1.1 responses and safety were evaluated by baseline renal function and histology.Results and limitationsObjective responses occurred in 0/6 (0%), 4/19 (21%), 1/27 (3.7%), and 12/62 (19%) of evaluable patients with creatinine clearance (CrCl) ConclusionsIn this EAP mUC subgroup analysis, clinical benefit of atezolizumab occurred in patients with compromised renal function or MVH UC tumors. Safety was comparable across subgroups.Patient summaryWe examined the efficacy and safety of atezolizumab for UC in certain patients participating in an EAP. We found that responses to atezolizumab occurred, and safety was similar, in most patient subgroups with varying levels of kidney functioning or less common types of tumor tissue histology.

BackgroundThe TIVO-3 trial demonstrated improved progression-free survival (PFS) with TIVO when compared to sorafenib (SOR; 5.6 mo. vs 3.9 mo., respectively; HR 0.73) and better tolerability with reduced need for dose interruptions (p = 0.0164), dose reductions (p = 0.0147), and discontinuations< sup >1. As the majority of patients diagnosed with mRCC in the US are >65 years, with the largest recent increase in incidence among those ≥75, and front-line treatment now standardly includes immunotherapy (IO), tolerability of new therapies for relapsed or refractory (R/R) mRCC must be acceptable in the elderly and/or IO pretreated populationMethodsData was analyzed to identify relationships between tolerability and advanced age or IO pretreatment. In addition to measures of drug exposure, any grade ≥3 treatment related adverse events (TRAEs) and VEFGR TKI class effect grade ≥3 TRAEs are reported by age (ResultsOf the 343 patients treated on study, 120 (35%) were between age 65 and 75 and 34 (10%) were over 75. Patients received 1.5-2x more cycles of TIVO compared to SOR and fewer overall grade ≥3 TRAEs in all age groups and irrespective of prior IO. Differences in VEGFR TKI class effect TRAEs seen in the total population were retained across most subgroups (table 1). Among patients 75 and over, there were almost half the rate of the dose reductions or discontinuations with TIVO compared to SOR. Prior IO was associated with less asthenia overall, more HTN with TIVO, and more rash but less diarrhea with SORAbstract 349 Table 1Drug exposure, dose modifications, and TRAEs in TIVO-3 by age and prior IOConclusionsTolerability benefits with TIVO compared to SOR in mRCC are retained in elderly patients and those previously treated with IO. This finding, paired with consistently improved PFS in these subpopulations (age >65: HR 0.59, prior IO: HR 0.55), suggests TIVO is a safe and effective option in the context of the current R/R mRCC treatment paradigmTrial RegistrationClinicalTrialsgov Identifier: NCT02627963ReferenceRini B, Pal S, Escudier B, Atkins M, Hutson T, et al. Tivozanib versus sorafenib in patients with advanced renal cell carcinoma (TIVO-3): a phase 3, multicentre, randomised, controlled, open-label study. Lancet Oncol 2020;21:95–104Ethics ApprovalThis trial was approved by the institutional review board or ethics committee at every centre and complied with Good Clinical Practice guidelines, the Declaration of Helsinki, and local laws. All patients provided written informed consent before any trial procedure. The trial protocol including the relevant centres is provided in the appendix of the reference 1

The approval of immunotherapeutic agents and immunotherapy-based combination strategies in recent years has revolutionized the treatment of patients with advanced renal cell carcinoma (aRCC). Nivolumab, a programmed death 1 (PD-1) immune checkpoint inhibitor monoclonal antibody, was approved as monotherapy in 2015 for aRCC after treatment with a VEGF-targeting agent. In April 2018, the combination of nivolumab and ipilimumab, a CTLA-4 inhibitor, was approved for intermediate- and poor-risk, previously untreated patients with aRCC. Then, in 2019, combinations therapies consisting of pembrolizumab (anti-PD-1) or avelumab (anti-PD-ligand (L) 1) with axitinib (a VEGF receptor tyrosine kinase inhibitor) were also approved to treat aRCC and are likely to produce dramatic shifts in the therapeutic landscape. To address the rapid advances in immunotherapy options for patients with aRCC, the Society for Immunotherapy of Cancer (SITC) reconvened its Cancer Immunotherapy Guidelines (CIG) Renal Cell Carcinoma Subcommittee and tasked it with generating updated consensus recommendations for the treatment of patients with this disease.

Background Despite the advent of checkpoint inhibitors (CPIs) for advanced genitourinary (GU) cancers, existing studies suggest that durable complete responses are observed in fewer than 10% of patients. This study sought to evaluate the association between expectations of cure reported by patients with advanced GU cancers initiating immunotherapy and quality of life (QOL), anxiety and depression. Patient and methods A single-institution, cross-sectional survey study was conducted with patients preparing to receive CPIs for treatment of metastatic renal cell carcinoma (RCC), urothelial cancer (UC) and prostate cancer (PC). Patients were assessed prior to initiation of immunotherapy for expectations of cure (divided into four quartiles), quality of life (QOL; Functional Assessment of Chronic Illness Therapy-General [FACT-G]), and symptoms of anxiety and depression (PROMIS). Results Sixty patients were enrolled, with metastatic RCC, UC and PC comprising 63, 28 and 8% of the study population, respectively. Median age of the cohort was 65 (range, 31–91), and 68% were male; 33% received CPI in the first-line setting. Despite extensive counseling from oncologists regarding potential clinical outcomes with immunotherapy, a substantial proportion of patients (23%) harbored inaccurate expectations regarding the potential benefit of immunotherapy. Importantly, patients with accurate expectations of cure reported lower anxiety scores using the PROMIS-Anxiety inventory. No significant differences were found between expectations of cure and quality of life or depression, using the FACT-G and PROMIS-Depression inventories, respectively. Conclusion The current study found that a considerable proportion of patients with advanced GU cancers harbor inaccurate expectations concerning the potential benefit of immunotherapy. These results suggest that more effective counselling may mitigate patient anxiety, and potentially promote treatment satisfaction and adherence.

Background Nivolumab is approved for patients with metastatic renal cell carcinoma (mRCC) refractory to prior antiangiogenic therapy. The clinical activity of nivolumab in patients with non-clear cell RCC subtypes remains unknown as these patients were excluded from the original nivolumab trials. Methods Patients from 6 centers in the United States who received at least one dose of nivolumab for non-clear cell mRCC between 12/2015 and 06/2017 were identified. A retrospective analysis including patient characteristics, objective response rate according to RECIST v1.1 and treatment-related adverse events (TRAEs) was undertaken. Results Forty-one patients were identified. Median age was 58 years (33–82), 71% were male, and majority had ECOG PS 0 (40%) or 1 (47%). Histology included 16 papillary, 14 unclassified, 5 chromophobe, 4 collecting duct, 1 Xp11 translocation and 1 MTSCC (mucinous tubular and spindle cell carcinoma). Among 35 patients who were evaluable for best response, 7 (20%) had PR and 10 (29%) had SD. Responses were observed in unclassified, papillary and collecting duct subtypes. In the entire cohort, median follow-up was 8.5 months and median treatment duration was 3.0 months. Median PFS was 3.5 months and median OS was not reached. Among responders, median time to best response was 5.1 months, and median duration of response was not reached as only 2 out of 7 responders had disease progression during follow-up. TRAEs of any grade were noted in 37% and most commonly included fatigue (12%), fever (10%) and rash (10%). Nivolumab treatments were postponed in 34% and discontinued in 15% of patients due to intolerance. No treatment-related deaths were observed. Conclusions Nivolumab monotherapy demonstrated objective responses and was well tolerated in a heterogeneous population of patients with non-clear cell mRCC. In the absence of other data in this treatment setting, this study lends support to the use of nivolumab for patients with metastatic non-clear cell renal cell carcinoma.

Immune checkpoint inhibitors targeting the PD-1 pathway have greatly changed clinical management of metastatic urothelial carcinoma and metastatic renal cell carcinoma. However, response rates are low, and biomarkers are needed to predict for treatment response. Immunohistochemical quantification of PD-L1 was developed as a promising biomarker in early clinical trials, but many shortcomings of the four different assays (different antibodies, disparate cellular populations, and different thresholds of positivity) have limited its clinical utility. Further limitations include the use of archival specimens to measure this dynamic biomarker. Indeed, until PD-L1 testing is standardized and can consistently predict treatment outcome, the currently available PD-L1 assays are not clinically useful in urothelial and renal cell carcinoma. Other more promising biomarkers include tumor mutational burden, profiles of tumor infiltrating lymphocytes, molecular subtypes, and PD-L2. Potentially, a composite biomarker may be best but will need prospective testing to validate such a biomarker.

Background AGS-003 is an autologous immunotherapy prepared from fully matured and optimized monocyte-derived dendritic cells, which are co-electroporated with amplified tumor RNA plus synthetic CD40L RNA. AGS-003 was evaluated in combination with sunitinib in an open label phase 2 study in intermediate and poor risk, treatment naïve patients with metastatic clear cell renal cell carcinoma (mRCC). Methods Twenty-one intermediate and poor risk patients were treated continuously with sunitinib (4 weeks on, 2 weeks off per 6 week cycle). After completion of the first cycle of sunitinib, patients were treated with AGS-003 every 3 weeks for 5 doses, then every 12 weeks until progression or end of study. The primary endpoint was to determine the complete response rate. Secondary endpoints included clinical benefit, safety, progression free survival (PFS) and overall survival (OS). Immunologic response was also monitored. Results Thirteen patients (62%) experienced clinical benefit (9 partial responses, 4 with stable disease); however there were no complete responses in this group of intermediate and poor risk mRCC patients and enrollment was terminated early. Median PFS from registration was 11.2 months (95% CI 6.0, 19.4) and the median OS from registration was 30.2 months (95% CI 9.4, 57.1) for all patients. Seven (33%) patients survived for at least 4.5 years, while five (24%) survived for more than 5 years, including 2 patients who remain progression-free with durable responses for more than 5 years at the time of this report. AGS-003 was well tolerated with only mild injection-site reactions. The most common adverse events were related to expected toxicity from sunitinib therapy. In patients who had sequential samples available for immune monitoring, the magnitude of the increase in the absolute number of CD8+ CD28+ CD45RA− effector/memory T cells (CTLs) after 5 doses of AGS-003 relative to baseline, correlated with overall survival. Conclusions AGS-003 in combination with sunitinib was well tolerated and yielded supportive immunologic responses coupled with extension of median and long-term survival in an unselected, intermediate and poor risk prognosis mRCC population. Clinical Trial Registry #NCT00678119

Recent clinical advances underscored that elimination of tumor-induced immunosuppression is prerequisite for successful therapy of advanced human cancers. The myeloid-derived suppressor cells (MDSCs) are one of the major populations limiting T cell-mediated antitumor immune responses in late-stage solid tumors, such as castration-resistant prostate cancers (CRPCs). Targeting MDSCs proved challenging due to their heterogeneity and phenotypic similarities with other myeloid cell lineages. Here, we identify a population of granulocytic MDSCs, which accumulate in patients' blood during progression of prostate cancer from localized to metastatic disease. The CRPC-associated MDSCs potently inhibit proliferation, IFN-γ and Granzyme-B production in autologous CD8+ T cells. Importantly, the MDSCs in blood and in lymph nodes of CRPC patients show high levels of activated STAT3 transcription factor, which is a crucial immune checkpoint regulator. We previously generated an original method to silence genes specifically in TLR9+ myeloid cells using CpG-siRNA conjugates [1,2]. Now, we demonstrate that the CpG-STAT3siRNA is also effectively internalized by human MDSCs and induces STAT3 silencing without the need for transfection reagents. Our further studies demonstrated that two-pronged TLR9-activation/STAT3-blocking by CpG-STAT3siRNA inhibits immunosuppressive effects of CRPC-MDSCs on CD8+ T cells, while both TLR9 triggering or STAT3 inhibition alone have only minimal effects. The reversal of MDSC immunosuppression by CpG-STAT3 siRNA results from reduced expression and enzymatic activity of Arginase I, a downstream STAT3 target gene. Overall, our study demonstrates the feasibility of overcoming immunosuppression in CRPC patients using CpG-STAT3siRNA strategy to alleviate effects of MDSCs. We anticipate that our strategy to reverse MDSC-mediated immunosuppression without depleting myeloid cell populations will generate safer and more effective immunotherapy for late-stage, metastatic prostate cancers and potentially other solid tumors. This project described was supported by the National Cancer Institute of the National Institutes of Health under award number R01CA155367 and by the Department of Defense Prostate Cancer Research Program under award number W81XWH-12-1-0132 to M.K.