Metabolic reprogramming of tumor-associated macrophages using glutamine antagonist JHU083 drives tumor immunity in myeloid-rich prostate and bladder cancer tumors.

Glutamine metabolism in tumor microenvironments critically regulates anti-tumor immunity. Using glutamine-antagonist prodrug JHU083, we report potent tumor growth inhibition in urologic tumors by JHU083-reprogrammed tumor-associated macrophages (TAMs) and tumor-infiltrating monocytes (TIMs). We show JHU083-mediated glutamine antagonism in tumor microenvironments induces TNF, pro-inflammatory, and mTORC1 signaling in intratumoral TAM clusters. JHU083-reprogrammed TAMs also exhibit increased tumor cell phagocytosis and diminished pro-angiogenic capacities. In vivo inhibition of TAM glutamine consumption resulted in increased glycolysis, a broken TCA cycle, and purine metabolism disruption. Although the anti-tumor effect of glutamine antagonism on tumor-infiltrating T cells was moderate, JHU083 promoted a stem cell-like phenotype in CD8+ T cells and decreased Treg abundance. Finally, JHU083 caused a ubiquitous shutdown in glutamine utilizing metabolic pathways in tumor cells, leading to reduced HIF-1alpha, c-MYC phosphorylation, and induction of tumor cell apoptosis, all key anti-tumor features.

Association of PARP inhibitor treatment on the prevalence and progression of clonal hematopoiesis in patients with advanced prostate cancer.

BACKGROUND: Poly ADP-ribose polymerase (PARP) inhibitors are approved for the treatment of some men with advanced prostate cancer. Rare but serious side effects include myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). The impact of PARP inhibitors on clonal hematopoiesis (CH), a potential precursor lesion associated with MDS and AML, is incompletely understood in prostate cancer. We hypothesized that PARP inhibitors would increase CH prevalence and abundance. METHODS: We prospectively enrolled participants with advanced prostate cancer treated with PARP inhibitors. The presence of CH was assessed from leukocytes using an ultra-deep error-corrected dual unique molecular identifiers sequencing method targeting 49 genes most commonly mutated in CH and myeloid malignancies. Variant allele frequencies (VAF) of ≥0.5% were considered clinically significant. Blood samples were collected before and after PARP inhibitor treatment. RESULTS: Ten men were enrolled; mean age of 67 years. Six patients had Gleason 7 disease, and four had Gleason ≥8 disease at diagnosis. Nine had localized disease at diagnosis, and eight had prior treatment with radiation. The mean time between pre- and post-treatment blood samples was 11 months (range 2.6-31 months). Six patients (60%) had CH identified prior to PARP inhibitor treatment, three with multiple clones. Of 11 CH clones identified in follow-up, 5 (45%) appeared or increased after treatment. DNMT3A, TET2, and PPM1D were the most common CH alterations observed. The largest post-treatment increase involved the PPM1D gene. CONCLUSION: CH alterations are frequently found after treatment with PARP inhibitors in patients with prostate cancer and this may be one mechanism by which PARP inhibitors lead to increased risk of MDS/AML.

Bipolar androgen therapy plus nivolumab for patients with metastatic castration-resistant prostate cancer: the COMBAT phase II trial.

Cyclic high-dose testosterone administration, known as bipolar androgen therapy (BAT), is a treatment strategy for patients with metastatic castration-resistant prostate cancer (mCRPC). Here, we report the results of a multicenter, single arm Phase 2 study (NCT03554317) enrolling 45 patients with heavily pretreated mCRPC who received BAT (testosterone cypionate, 400 mg intramuscularly every 28 days) with the addition of nivolumab (480 mg intravenously every 28 days) following three cycles of BAT monotherapy. The primary endpoint of a confirmed PSA50 response rate was met and estimated at 40% (N = 18/45, 95% CI: 25.7-55.7%, P = 0.02 one-sided against the 25% null hypothesis). Sixteen of the PSA50 responses were achieved before the addition of nivolumab. Secondary endpoints included objective response rate (ORR), median PSA progression-free survival, radiographic progression-free survival (rPFS), overall survival (OS), and safety/tolerability. The ORR was 24% (N = 10/42). Three of the objective responses occurred following the addition of nivolumab. After a median follow-up of 17.9 months, the median rPFS was 5.6 (95% CI: 5.4-6.8) months, and median OS was 24.4 (95% CI: 17.6-31.1) months. BAT/nivolumab was well tolerated, resulting in only five (11%) drug related, grade-3 adverse events. In a predefined exploratory analysis, clinical response rates correlated with increased baseline levels of intratumoral PD-1 + T cells. In paired metastatic tumor biopsies, BAT induced pro-inflammatory gene expression changes that were restricted to patients achieving a clinical response. These data suggest that BAT may augment antitumor immune responses that are further potentiated by immune checkpoint blockade.

Divergent immune microenvironments in two tumor nodules from a patient with mismatch repair-deficient prostate cancer.

Patients with prostate cancer (PC) generally do not respond favorably to immune checkpoint inhibitors, which may be due to a low abundance of tumor-infiltrating lymphocytes even when mutational load is high. Here, we identified a patient who presented with high-grade primary prostate cancer with two adjacent tumor nodules. While both nodules were mismatch repair-deficient (MMRd), exhibited pathogenic MSH2 and MSH6 alterations, had a high tumor mutational burden (TMB), and demonstrated high microsatellite instability (MSI), they had markedly distinct immune phenotypes. The first displayed a dense infiltrate of lymphocytes ("hot nodule"), while the second displayed significantly fewer infiltrating lymphocytes ("cold nodule"). Whole-exome DNA analysis found that both nodules shared many identical mutations, indicating that they were derived from a single clone. However, the cold nodule appeared to be sub-clonal relative to the hot nodule, suggesting divergent evolution of the cold nodule from the hot nodule. Whole-transcriptome RNA analysis found that the cold nodule demonstrated lower expression of genes related to antigen presentation (HLA) and, paradoxically, classical tumor immune tolerance markers such as PD-L1 (CD274) and CTLA-4. Immune cell deconvolution suggested that the hot nodule was enriched not only in CD8+ and CD4 + T lymphocytes, but also in M1 macrophages, activated NK cells, and γδ T cells compared to the cold nodule. This case highlights that MMRd/TMB-high PC can evolve to minimize an anti-tumor immune response, and nominates downregulation of antigen presentation machinery (HLA loss) as a potential mechanism of adaptive immune evasion in PC.

MLH1 Loss in Primary Prostate Cancer.

PURPOSE: Among mismatch repair-deficient (MMRd) prostate cancers, loss of MLH1 is relatively uncommon and few cases have been reported in detail. METHODS: Here, we describe the molecular features of two cases of primary prostate cancer with MLH1 loss detected by immunohistochemistry, and in one case, confirmed via transcriptomic profiling. RESULTS: Both cases were microsatellite stable on standard polymerase chain reaction (PCR)-based microsatellite instability (MSI) testing, but showed evidence of MSI on a newer PCR-based long mononucleotide repeat (LMR) assay and by next-generation sequencing. Germline testing was negative for Lynch syndrome-associated mutations in both cases. Targeted or whole-exome tumor sequencing using multiple commercial/academic platforms (Foundation, Tempus, JHU, and UW-OncoPlex) showed modestly elevated, though variable, tumor mutation burden estimates (2.3-10 mutations/Mb) consistent with MMRd, but without identifiable pathogenic single-nucleotide or indel mutations in MLH1. Copy-number analysis confirmed biallelic MLH1 loss in one case and monoallelic MLH1 loss in the second case, without evidence of MLH1 promoter hypermethylation in either. The second patient was treated with single-agent pembrolizumab and demonstrated a short-lived prostate-specific antigen response. CONCLUSION: These cases highlight the challenges in identifying MLH1-deficient prostate cancers using standard MSI testing and commercial sequencing panels, and support the utility of immunohistochemical assays and LMR- or sequencing-based MSI testing for detection of MMRd prostate cancers.

Bipolar Androgen Therapy Followed by Androgen Receptor Inhibition as Sequential Therapy for Prostate Cancer.

Inhibition of androgen receptor (AR) signaling has been the mainstay of treatment of advanced prostate cancer (PCa) for the past 80 years. Combination and sequential AR-inhibiting therapies are highly effective palliative therapy, but they are not curative. All patients eventually develop resistance to primary castrating therapy [ie, castration-resistant PCa (CRPC)]. At this point, they are treated with subsequent lines of secondary AR inhibitory therapies. However, resistance to these agents also develops and patients progress to a state we have termed complete androgen inhibition-resistant PCa. This phase of the disease is associated with poor prognosis. At this point, treatment shifts to non-hormonal cytotoxic therapies (eg, chemotherapy and radiopharmaceuticals). However, the majority of PCas remain addicted to signaling through AR throughout the course of the disease. Resistant PCa cells adaptively upregulate AR activity, despite castration and AR inhibitors, via mechanisms such as AR overexpression, gene amplification, mutation, and expression of ligand-independent variants to permit sustained liganded and non-liganded AR signaling. Studies dating back nearly 30 years indicate that high expression of AR induced by prolonged castration becomes a vulnerability of CRPC cells in vitro and in mouse xenografts to supraphysiologic androgen (SPA), which induces cell death and growth arrest in this context. Based on these studies, we developed a counterintuitive treatment called bipolar androgen therapy (BAT) for patients with CRPC, in which SPA is administered intermittently to result in cycling of serum testosterone from the polar extremes of supraphysiologic to near-castrate levels. This rapid cycling is intended to disrupt the adaptive of AR regulation associated with chronic exposure to high or low levels of testosterone, while simultaneously targeting the spectrum of AR expression present in heterogeneous CRPC tumors. We have now tested BAT in >250 patients with CRPC. Here we present a review of these clinical studies, which have demonstrated collectively that BAT can be safely given to men with CRPC, improves quality of life, and produces therapeutic responses in ~30% of patients. As expected, resistance to BAT is associated with adaptive downregulation of AR expression. Intriguingly, this downregulation is associated with restoration of sensitivity to subsequent AR inhibitor therapies.

The testosterone paradox of advanced prostate cancer: mechanistic insights and clinical implications.

The discovery of the benefits of castration for prostate cancer treatment in 1941 led to androgen deprivation therapy, which remains a mainstay of the treatment of men with advanced prostate cancer. However, as early as this original publication, the inevitable development of castration-resistant prostate cancer was recognized. Resistance first manifests as a sustained rise in the androgen-responsive gene, PSA, consistent with reactivation of the androgen receptor axis. Evaluation of clinical specimens demonstrates that castration-resistant prostate cancer cells remain addicted to androgen signalling and adapt to chronic low-testosterone states. Paradoxically, results of several studies have suggested that treatment with supraphysiological levels of testosterone can retard prostate cancer growth. Insights from these studies have been used to investigate administration of supraphysiological testosterone to patients with prostate cancer for clinical benefits, a strategy that is termed bipolar androgen therapy (BAT). BAT involves rapid cycling from supraphysiological back to near-castration testosterone levels over a 4-week cycle. Understanding how BAT works at the molecular and cellular levels might help to rationalize combining BAT with other agents to achieve increased efficacy and tumour responses.

Androgen receptor activity in prostate cancer dictates efficacy of bipolar androgen therapy through MYC.

Testosterone is the canonical growth factor of prostate cancer but can paradoxically suppress its growth when present at supraphysiological levels. We have previously demonstrated that the cyclical administration of supraphysiological androgen (SPA), termed bipolar androgen therapy (BAT), can result in tumor regression and clinical benefit for patients with castration-resistant prostate cancer. However, predictors and mechanisms of response and resistance have been ill defined. Here, we show that growth inhibition of prostate cancer models by SPA required high androgen receptor (AR) activity and were driven in part by downregulation of MYC. Using matched sequential patient biopsies, we show that high pretreatment AR activity predicted downregulation of MYC, improved clinical response, and prolonged progression-free and overall survival for patients on BAT. BAT induced strong downregulation of AR in all patients, which is shown to be a primary mechanism of acquired resistance to SPA. Acquired resistance was overcome by alternating SPA with the AR inhibitor enzalutamide, which induced adaptive upregulation of AR and resensitized prostate cancer to SPA. This work identifies high AR activity as a predictive biomarker of response to BAT and supports a treatment paradigm for prostate cancer involving alternating between AR inhibition and activation.

Trainee-Led Quality Improvement Project to Improve Fertility Preservation Counseling for Patients With Cancer.

PURPOSE: Oncofertility counseling regarding the reproductive risks associated with cancer therapy is essential for quality cancer care. We aimed to increase the rate of oncofertility counseling for patients of reproductive age (18-40 years) with cancer who were initiating systemic therapy at the Johns Hopkins Cancer Center from a baseline rate of 37% (25 of 68, June 2019-January 2020) to 70% by February 2021. METHODS: We formed an interprofessional, multidisciplinary team as part of the ASCO Quality Training Program. We obtained data from the electronic medical record and verified data with patients by phone. We surveyed patients, oncologists, and fertility specialists to identify barriers. After considering a prioritization matrix, we implemented Plan-Do-Study-Act (PDSA) cycles. RESULTS: We identified the following improvement opportunities: (1) oncologist self-reported lack of knowledge about counseling and local fertility preservation options and (2) lack of a standardized referral mechanism to fertility services. During the first PDSA cycle (February 2020-August 2020, disrupted by COVID-19), we introduced the initiative to increase oncofertility counseling at faculty meetings. From September 2020 to November 2020, we implemented a second PDSA cycle: (1) educating and presenting the initiative at Oncology Grand Rounds, (2) distributing informative pamphlets to oncologists and patients, and (3) implementing an electronic medical record order set. In the third PDSA cycle (December 2020-February 2021), we redesigned the order set to add information (eg, contact information for fertility coordinator) to the patient after-visit summary. Postimplementation (September 2020-February 2021), counseling rates increased from 37% to 81% (38 of 47). CONCLUSION: We demonstrate how a trainee-led, patient-centered initiative improved oncofertility care. Ongoing work focuses on ensuring sustainability and assessing the quality of counseling.

Molecular and Clinical Characterization of Patients With Metastatic Castration Resistant Prostate Cancer Achieving Deep Responses to Bipolar Androgen Therapy.

BACKGROUND: Bipolar androgen therapy (BAT) is an emerging treatment strategy for men with metastatic castration resistant prostate cancer (mCRPC) whereby serum testosterone is cycled from supraphysiologic to near-castrate levels each month. BAT has been shown to induce clinical responses in a significant proportion of patients, some of which are extreme. We explored the clinical and molecular characteristics of patients with mCRPC who achieved deep responses to BAT. METHODS: We conducted a retrospective analysis of patients with mCRPC treated with BAT at Johns Hopkins. We identified 22 of 114 (19%) patients with mCRPC who achieved ≥70% PSA reductions upon treatment with BAT. All patients were treated using 400 mg testosterone cypionate intramuscularly every 28 days, together with continuous LHRH agonist therapy. Clinical-grade DNA sequencing was obtained using commercially available assays. RESULTS: Somatic next-generation sequencing was obtained for 15 of 22 (68%) patients. Of these 15 extreme PSA responders with sequencing data available, All 15 of 15 (100%) harbored a pathogenic mutation in TP53 and/or a homologous recombination DNA repair (HRD) gene. Among the subset of patients with measureable disease (N = 15), 10 patients (67%) achieved an objective response including one patient with a complete response. The median radiographic progression-free survival of these deep PSA responders was 11.3 months (95% CI, 7.9-25.0 months). CONCLUSIONS: We observed an enrichment of TP53 and HRD mutations in mCRPC patients with extreme PSA responses to BAT, with durability lasting about a year. These data support the hypothesis that BAT may most benefit patients with DNA repair-deficient mCRPC. Further studies of BAT in biomarker-selected mCRPC populations (ie, TP53/HRD-mutated) are warranted.

Forming the Hematology-Oncology Collaborative Videoconferencing (CO-VID) Learning Initiative: Experiential Lessons Learned From a Novel Trainee-Led Multidisciplinary Virtual Learning Platform.

PURPOSE: COVID-19 challenged medical practice and graduate medical education. Building on previous initiatives, we describe and reflect on the formative process and goals of the Hematology-Oncology Collaborative Videoconferencing Learning Initiative, a trainee-led multi-institutional virtual COVID-19 learning model. METHODS: Clinical fellows and faculty from 13 US training institutions developed consensus needs, goals, and objectives, recruited presenters, and generated a multidisciplinary COVID-19 curriculum. Weekly Zoom conferences consisted of two trainee-led instructional segments and a trainee-moderated faculty Q&A panel. Hematology-oncology training program faculty and trainees were the targeted audience. Leadership evaluations consisted of anonymized baseline and concluding mixed methods surveys. Presenter evaluations consisted of session debriefs and two structured focus groups. Conference evaluations consisted of attendance, demographics, and pre- or postmultiple-choice questions on topic learning objectives. RESULTS: In 6 weeks, the initiative produced five conferences: antivirals, anticoagulation, pulmonology, provider resilience, and resource scarcity ethics. The average attendance was 100 (range 57-185). Among attendees providing both pre- and postconference data, group-level knowledge appeared to increase: antiviral (n = 46) pre-/postcorrect 82.6%/97.8% and incorrect 10.9%/2.2%, anticoagulation (n = 60) pre-/postcorrect 75%/93.3% and incorrect 15%/6.7%, and pulmonary (n = 21) pre-/postcorrect 66.7%/95.2% and incorrect 33.3%/4.8%. Although pulmonary management comfort appeared to increase, comfort managing of antivirals and anticoagulation was unchanged. At the conclusion of the pilot, leadership trainees reported improved self-confidence organizing multi-institutional collaborations, median (interquartile range) 58.5 (50-64) compared with baseline 34 (26-39), but did not report improved confidence in other educational or leadership skills. CONCLUSION: During crisis, trainees built a multi-institutional virtual education platform for the purposes of sharing pandemic experiences and knowledge. Accomplishment of initiative goals was mixed. Lessons learned from the process and goals may improve future disaster educational initiatives.

Targeting the spectrum of immune checkpoints in prostate cancer.

Introduction: The proven efficacy of the cellular vaccine sipuleucel-T in 2010 led to optimism about immunotherapeutic approaches for the treatment of prostate cancer. Some surmised that prostate cancer might be an ideal target for immune-mediated killing given that the prostate is not an essential organ and expresses unique proteins including prostate-specific antigen, prostate-specific membrane antigen, and prostatic acid phosphatase that could be targeted without side effects. Subsequently, antibodies that inhibit the T cell checkpoints PD1 and CTLA4 were shown to stimulate antitumor immune responses, leading to tumor regression in several cancer types. These therapies have since been tested in several studies as treatments for prostate cancer, but appear to have limited efficacy in molecularly unselected patients.Areas covered: In this review, we discuss these studies and evaluate features of prostate cancer and its host environment that may render it generally resistant to CTLA4 and PD1 blockade. We provide an overview of alternate immune checkpoints that may hold greater significance in this disease.Expert opinion: Combination therapies to target multiple layers of alternate immune checkpoints may be required for an effective immune response to prostate cancer. We discuss combination therapies currently being investigated.

Mismatch repair-deficient prostate cancer with parenchymal brain metastases treated with immune checkpoint blockade.

Parenchymal brain metastases from prostate cancer are unusual and are associated with poor prognosis. Given the rarity of this entity, little is known about its molecular and histologic characteristics. Here we describe a patient with metastatic castration-resistant, mismatch repair-deficient (dMMR) prostate cancer with parenchymal brain metastases. Analysis of a brain metastasis revealed MLH1 loss consistent with dMMR, yet few tumor-infiltrating lymphocytes (TILs). He was treated with immune checkpoint blockade (ICB) and exhibited an extra-central nervous system (CNS) systemic response but CNS progression. Subsequent assessment of a brain metastasis following ICB treatment surprisingly showed increased TIL density and depletion of macrophages, suggestive of an enhanced antitumor immune response. Post-treatment tumoral DNA sequencing did not reveal acquired mutations that might confer resistance to ICB. This is the first description of ICB therapy for a patient with prostate cancer with parenchymal brain metastases, with pre- and post-treatment immunogenomic analyses.

Fatty Acid Synthesis in Prostate Cancer: Vulnerability or Epiphenomenon?

Tumor metabolism supports the energetic and biosynthetic needs of rapidly proliferating cancer cells and modifies intra- and intercellular signaling to enhance cancer cell invasion, metastasis, and immune evasion. Prostate cancer exhibits unique metabolism with high rates of de novo fatty acid synthesis driven by activation of the androgen receptor (AR). Increasing evidence suggests that activation of this pathway is functionally important to promote prostate cancer aggressiveness. However, the mechanisms by which fatty acid synthesis are beneficial to prostate cancer have not been well defined. In this review, we summarize evidence indicating that fatty acid synthesis drives progression of prostate cancer. We also explore explanations for this phenomenon and discuss future directions for targeting this pathway for patient benefit.

Mitochondrial metabolism is essential for invariant natural killer T cell development and function.

Conventional T cell fate and function are determined by coordination between cellular signaling and mitochondrial metabolism. Invariant natural killer T (iNKT) cells are an important subset of "innate-like" T cells that exist in a preactivated effector state, and their dependence on mitochondrial metabolism has not been previously defined genetically or in vivo. Here, we show that mature iNKT cells have reduced mitochondrial respiratory reserve and iNKT cell development was highly sensitive to perturbation of mitochondrial function. Mice with T cell-specific ablation of Rieske iron-sulfur protein (RISP; T-Uqcrfs1-/- ), an essential subunit of mitochondrial complex III, had a dramatic reduction of iNKT cells in the thymus and periphery, but no significant perturbation on the development of conventional T cells. The impaired development observed in T-Uqcrfs1-/- mice stems from a cell-autonomous defect in iNKT cells, resulting in a differentiation block at the early stages of iNKT cell development. Residual iNKT cells in T-Uqcrfs1-/- mice displayed increased apoptosis but retained the ability to proliferate in vivo, suggesting that their bioenergetic and biosynthetic demands were not compromised. However, they exhibited reduced expression of activation markers, decreased T cell receptor (TCR) signaling and impaired responses to TCR and interleukin-15 stimulation. Furthermore, knocking down RISP in mature iNKT cells diminished their cytokine production, correlating with reduced NFATc2 activity. Collectively, our data provide evidence for a critical role of mitochondrial metabolism in iNKT cell development and activation outside of its traditional role in supporting cellular bioenergetic demands.

Perspectives From Underserved African Americans and Their Health Care Providers on the Development of a Diabetes Self-Management Smartphone App: Qualitative Exploratory Study.

BACKGROUND: Type 2 diabetes mellitus (T2DM) affects approximately 10% of the US population, disproportionately afflicting African Americans. Smartphone apps have emerged as promising tools to improve diabetes self-management, yet little is known about the use of this approach in low-income minority communities. OBJECTIVE: The goal of the study was to explore which features of an app were prioritized for people with T2DM in a low-income African American community. METHODS: Between February 2016 and May 2018, we conducted formative qualitative research with 78 participants to explore how a smartphone app could be used to improve diabetes self-management. Information was gathered on desired features, and app mock-ups were presented to receive comments and suggestions of improvements from smartphone users with prediabetes and T2DM, their friends and family members, and health care providers; data were collected from six interactive forums, one focus group, and 15 in-depth interviews. We carried out thematic data analysis using an inductive approach. RESULTS: All three types of participants reported that difficulty with accessing health care was a main problem and suggested that an app could help address this. Participants also indicated that an app could provide information for diabetes education and self-management. Other suggestions included that the app should allow people with T2DM to log and track diabetes care-related behaviors and receive feedback on their progress in a way that would increase engagement in self-management among persons with T2DM. CONCLUSIONS: We identified educational and tracking smartphone features that can guide development of diabetes self-management apps for a low-income African American population. Considering those features in combination gives rise to opportunities for more advanced support, such as determining self-management recommendations based on data in users' logs.

Tumor Frameshift Mutation Proportion Predicts Response to Immunotherapy in Mismatch Repair-Deficient Prostate Cancer.

BACKGROUND: Genomic biomarkers that predict response to anti-PD1 therapy in prostate cancer are needed. Frameshift mutations are predicted to generate more neoantigens than missense mutations; therefore, we hypothesized that the number or proportion of tumor frameshift mutations would correlate with response to anti-PD1 therapy in prostate cancer. METHODS: To enrich for response to anti-PD1 therapy, we assembled a multicenter cohort of 65 men with mismatch repair-deficient (dMMR) prostate cancer. Patient characteristics and outcomes were determined by retrospective chart review. Clinical somatic DNA sequencing was used to determine tumor mutational burden (TMB), frameshift mutation burden, and frameshift mutation proportion (FSP), which were correlated to outcomes on anti-PD1 treatment. We subsequently used data from a clinical trial of pembrolizumab in patients with nonprostatic dMMR cancers of various histologies as a biomarker validation cohort. RESULTS: Nineteen of 65 patients with dMMR metastatic castration-resistant prostate cancer were treated with anti-PD1 therapy. The PSA50 response rate was 65%, and the median progression-free survival (PFS) was 24 (95% confidence interval 16-54) weeks. Tumor FSP, more than overall TMB, correlated most strongly with prolonged PFS and overall survival (OS) on anti-PD1 treatment and with density of CD8+ tumor-infiltrating lymphocytes. High FSP similarly identified patients with longer PFS as well as OS on anti-PD1 therapy in a validation cohort. CONCLUSION: Tumor FSP correlated with prolonged efficacy of anti-PD1 treatment among patients with dMMR cancers and may represent a new biomarker of immune checkpoint inhibitor sensitivity. IMPLICATIONS FOR PRACTICE: Given the modest efficacy of immune checkpoint inhibition (ICI) in unselected patients with advanced prostate cancer, biomarkers of ICI sensitivity are needed. To facilitate biomarker discovery, a cohort of patients with DNA mismatch repair-deficient (dMMR) prostate cancer was assembled, as these patients are enriched for responses to ICI. A high response rate to anti-PD1 therapy in these patients was observed; however, these responses were not durable in most patients. Notably, tumor frameshift mutation proportion (FSP) was identified as a novel biomarker that was associated with prolonged response to anti-PD1 therapy in this cohort. This finding was validated in a separate cohort of patients with nonprostatic dMMR cancers of various primary histologies. This works suggests that FSP predicts response to anti-PD1 therapy in dMMR cancers, which should be validated prospectively in larger independent cohorts.