The PRINCE trial showed the clinical activity for 177Lu-PSMA-617 in combination with pembrolizumab for metastatic castration-resistant prostate cancer. To refine patient selection and improve response monitoring strategies to this combination, we investigated candidate tumor-intrinsic biomarkers of treatment response and resistance. Methods: We performed circulating tumor DNA (ctDNA), circulating tumor cell (CTC), and PET imaging analyses at baseline, 12 wk on-treatment, and disease progression in participants enrolled in PRINCE (n = 37). We performed targeted sequencing for ctDNA quantification and genomic analysis of more than 70 prostate cancer genes. CTC enumeration was performed on the EpicSciences platform and was combined with selective single-cell whole-genome sequencing. PET imaging included serial PSMA PET as well as 18F-FDG PET imaging at baseline. Results: A low baseline ctDNA fraction and high PSMA avidity in metastatic lesions were linked to superior treatment responses and may have composite biomarker value. Genomic alterations in tumor suppressor genes TP53, RB1, or PTEN were associated with higher 18F-FDG avidity and metabolic tumor volume on 18F-FDG PET imaging and worse prognosis. At 12-wk on-treatment, both ctDNA detection and PSMA PET imaging were strong indicators of response depth and durability. At disease progression, PSMA expression on PET imaging was lower compared with baseline and supported by subclonal remodeling of ctDNA and CTC copy number profiles and by clonal expansions of tumor suppressor gene mutations. Conclusion: We provide the first integrated molecular and imaging insights into determinants of response and resistance to combined radiopharmaceutical therapy and immunotherapy in prostate cancer and propose biomarker strategies to inform future clinical development.

Abstract Purpose: Bipolar androgen therapy (BAT), which alternates between castrate and supraphysiological testosterone levels, represents a promising alternative to continuous androgen suppression in advanced prostate cancer. However, only a subset of patients’ tumours respond. Using patient-derived models of metastatic castration-resistant prostate cancer, we aimed to identify molecular programs distinguishing BAT responders from non-responders using single-cell transcriptomics. Experimental Procedures: We profiled over 60,000 cells from four patient-derived xenografts (PDXs) using the 10x Genomics single-cell RNA-seq platform. Mouse reads were removed with Xenocell, leaving approximately 40,000 human prostate cancer cells for downstream analysis. Samples included two complete responders, one partial responder, and one non-responder, all evaluated 24 hours after BAT exposure, with responders also assessed at a long-term (6-week) timepoint. Differential expression and gene set enrichment (GSEA) analyses were performed using Hallmark, KEGG, and Gene Ontology collections. Results: BAT triggered robust androgen-responsive transcriptional reprogramming across models, yet the scale and persistence of pathway activation differed by response category. GSEA revealed that responders exhibited suppression of MYC target genes and stress-response pathways, alongside upregulation of metabolic and differentiation programs, including oxidative phosphorylation, cholesterol homeostasis, and cell adhesion. In contrast, the non-responder maintained MYC activation with enrichment of inflammatory, epithelial-mesenchymal transition (EMT), and cell-cycle pathways, suggesting incomplete androgen pathway re-engagement. Over time, complete responders showed reduced proliferative signalling, consistent with stable treatment adaptation. Conclusions: Our single-cell transcriptomic analyses delineate molecular signatures underlying BAT sensitivity and resistance. MYC activity and inflammatory remodelling emerge as potential drivers of resistance, whereas metabolic and differentiation programs define durable response. These insights provide a foundation for identifying biomarkers and designing rational combination strategies to enhance BAT efficacy. Citation Format: Rosalia Quezada Urban, Shivakumar Keerthikumar, Peter Lau, Georgia Cuffe, Linda Teng, Ashlee K. Clark, Gail P. Risbridger, Renea A. Taylor, Megan Crumbaker, ANTHONY JOSHUA, Mitchell G. Lawrence. Single-cell transcriptomic characterisation reveals pathway determinants of bipolar androgen therapy response in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 2697.

[177Lu]Lu-PSMA radioligand therapy targets metastatic castration-resistant prostate cancer by delivering radiation to cells expressing prostate-specific membrane antigen (PSMA). While some patients show remarkable responses, up to 50

Tumor development and progression reshape the physical properties of the surrounding tumor microenvironment (TME), including its biomechanical traits. This is driven by a prominent cell type in the TME, cancer-associated fibroblasts (CAFs), which increases tissue stiffness via extracellular matrix deposition and remodeling. Currently, it is unclear whether there are also physical changes to CAFs at the cellular level and, if so, how they relate to patient outcome. Here, it is shown that CAFs have distinct morphological and biomechanical features from normal fibroblasts. Matched, patient-derived CAFs and non-malignant prostate fibroblasts (NPFs) from 35 patients with primary prostate cancer are examined. Morphologically, CAFs have more aligned stress fibers and larger and more elongated nuclei, based on quantitative image analysis of confocal microscopy images. In addition, single-cell mechanical measurements using real-time deformability cytometry showed that CAFs are larger and stiffer than NPFs. These changes are consistent across patients and validated with atomic force microscopy. A combined morphomechanical score encompassing these features is significantly associated with patient outcome. In transcriptomic analyses, the score is correlated with microtubule dynamics and a myofibroblast phenotype. Importantly, it is also demonstrated that morphomechanical features of prostate fibroblasts are modified by approved treatments for prostate cancer, such as docetaxel, and other small molecular inhibitors, particularly those targeting FGFR. In summary, changes in cellular morphomechanical properties are a consistent feature of CAFs and are associated with patient outcome. Moreover, cellular morphomechanical properties can be therapeutically targeted, potentially providing a new strategy for manipulating the TME to control cancer progression.

PURPOSE:AKT inhibitors, such as capivasertib, have shown activity in specific patients with metastatic castration-resistant prostate cancer when combined with docetaxel although none have been approved. Although PTEN loss is often linked to AKT pathway activation and response to AKT inhibitors, clinical trials show no consistent association. This study uses patient-derived tumor models to identify biomarkers associated with an effective response to AKT inhibitor plus docetaxel. EXPERIMENTAL DESIGN:Targeted DNA sequencing and immunostaining for PTEN and phosphorylated AKT (p-AKT; Ser473) were assessed in 39 patient-derived xenografts (PDX) from patients with prostate cancer, including adenocarcinoma and neuroendocrine (NE) phenotypes. Matching PDX-derived organoids were used to evaluate the functional effects of capivasertib and docetaxel on in vitro tumor growth. RESULTS:p-AKT protein expression varied widely across PDX models and showed no correlation with PTEN/PI3K/AKT mutations or PTEN protein levels. NE tumors displayed higher p-AKT expression than adenocarcinomas. Knockdown of AKT1 in NE organoids increased sensitivity to docetaxel, whereas AKT1 overexpression decreased it. In three of seven organoids tested, the combination of capivasertib and docetaxel produced a synergistic effect, resulting in greater growth inhibition than either agent alone. These responsive organoids exhibited an NE phenotype and high p-AKT expression, consistent with a predictive response. CONCLUSIONS:Our preclinical findings indicate that p-AKT protein expression, rather than PTEN, may be a more reliable predictor of response to AKT inhibition combined with docetaxel. Using p-AKT as a parameter, we uncovered the efficacy of this combination in NE prostate cancer, highlighting the potential to refine patient selection criteria for future clinical trials.

847 Background: Intravesical Bacillus Calmette–Guérin (BCG) remains the gold standard for the management of high-grade non-muscle invasive bladder cancer (NMIBC). However, up to 70% of patients fail BCG therapy. We aimed to define potential mechanisms for BCG resistance and develop an immunohistochemical (IHC) panel to help identify patients likely to respond to intravesical BCG. Methods: Patients with NMIBC undergoing induction intravesical BCG at a tertiary institution were identified. Twelve BCG-responders and 13 non-responders were matched for patient and tumour factors. RNA sequencing was performed with hierarchical clustering and Gene Set Enrichment Analysis to identify response resistance mechanisms. Immune cell subsets were measured using pre and post-BCG therapy for CD4, CD8, T-Bet, GATA-3 and PD-1 antibodies. GATA-3 and T-Bet stains were used as surrogates for Th-2 and Th-1 cells, respectively. T-tests were used to assess differences. An integrated IHC panel using CD4, CD8, T-Bet, GATA and PD1 was developed and correlated with BCG response using Receiver Operator Characteristic (ROC) curves. Results: On hierarchical clustering, BCG-responders and non-responders had distinct gene expression profiles pre- and post-BCG. Prior to exposure to BCG, non-responders had enrichment for a pro-inflammatory gene signature with a higher CD4:CD8 ratio (2.94 vs 1.71, p=0.0003) and a higher Th-2/Th-1 (GATA/T-Bet) ratio (5.95 vs 2.97, p=0.0026) compared to BCG-responders, on IHC. However, on exposure to BCG, non-responders had no changes to the CD4:CD8 or Th-2/Th-1 ratios but had a 78% increase in the PD-1 expression (MD 20.83/5hpf, p=0.016) with BCG, indicating T cell exhaustion. In contrast, upon exposure to BCG, responders had an increase in the Th-2/Th-1 ratio (mean difference 0.9323, p=0.0228) with enrichment of the natural killer cell pathway compared to non-responders (p<0.05). The area under the ROC curve using the integrated IHC panel to predict BCG response was 0.864 (95% CI: 0.663 to 1.000). Conclusions: BCG non-responders had a pro-inflammatory TME, which showed marked T cell exhaustion on exposure to BCG. In contrast, responders had a baseline TME with low CD4:CD8 and low Th-2/Th-1 ratios at baseline, which allowed activation of both humoral and adaptive responses to BCG. These immune changes can be utilised as an IHC panel to predict response to BCG therapy.

Tumour development and progression reshape the physical properties of the surrounding tumour microenvironment (TME) including its biomechanical traits. This is driven by a prominent cell type in the TME, cancer associated fibroblasts (CAFs), which increases tissue stiffness via extracellular matrix deposition and remodelling. Currently, it is unclear whether there are also physical changes to CAFs at the cellular level and, if so, how they relate to patient outcome. Here we show that CAFs have distinct morphological and biomechanical features from normal fibroblasts. We examined matched, patient-derived CAFs and non-malignant prostate fibroblasts (NPFs) from 35 patients with primary prostate cancer. Morphologically, CAFs had more aligned stress fibres, and larger and more elongated nuclei, based on quantitative image analysis of confocal microscopy images. In addition, single-cell mechanical measurements using real-time deformability cytometry showed that CAFs are larger and stiffer than NPFs. These changes were consistent across patients and validated with atomic force microscopy. A combined morphomechanical score encompassing these features was significantly associated with patient outcome. In transcriptomic analyses, the score was correlated with microtubule dynamics and a myofibroblast phenotype. Importantly, we also demonstrated that morphomechanical features of prostate fibroblasts are modified by approved treatments for prostate cancer, such as docetaxel, and other small molecular inhibitors, such as axitinib. In summary, changes in cellular morphomechanical properties are a consistent feature of CAFs and associated with patient outcome. Moreover, cellular morphomechanical properties can be therapeutically targeted, potentially providing a new strategy for manipulating the TME to control cancer progression. ### Competing Interest Statement The authors have declared no competing interest.

Background and objective Under the selective pressure of treatment, prostate cancer cells express constitutively active androgen receptor (AR) variants. Whether AR variants mediate therapy resistance remains contested, because they are often coexpressed with abundant full-length AR. Therefore, we sought to determine how truncated variants shape AR chromatin occupancy and responses to treatments in both the presence and absence of full-length AR. Methods We used a cohort of patient-derived xenografts of metastatic prostate cancer with diverse AR alterations. Chromatin immunoprecipitation and RNA sequencing were used to compare the landscape of AR binding and transcriptomic features. We assessed responses to castration by castrating host mice and evaluated responses to bipolar androgen therapy by administering testosterone cypionate. Key findings and limitations By profiling the AR cistrome, we identified a distinct group of tumours defined by ARv567es expression, a variant arising due to structural rearrangements of the AR gene. ARv567es-positive tumours also had a distinct epigenomic profile and altered transcriptional features, including loss of canonical AR-regulated gene signatures and elevated expression of AR-repressed genes. ARv567es-positive tumours were resistant to castration and bipolar androgen therapy. In tumours that coexpress full-length AR, this involves dampened transcriptional responses and disruption of the autoregulatory loop that modulates AR levels. Study limitations include the need for additional models of AR-driven prostate cancer. Conclusions and clinical implications The emergence of ARv567es via gene rearrangements causes transcriptional reprogramming and therapy resistance. This highlights ARv567es as a potential as a marker to guide treatment decisions.

420 Background: Prostatic ductal adenocarcinoma (DAC) is an aggressive prostate cancer variant, prone to early metastasis at low PSA levels and showing poor response to androgen blockade despite androgen receptor expression. Although DACs do not have any efficacious therapies, DACs frequently harbour DNA damage repair (DDR) mutations. We investigate the efficacy of combined PARP inhibitor (PARPi) and androgen signalling inhibitor (ARSi) therapy in DDR-proficient DAC tumours. Methods: To model DAC, organoids were developed in Matrigel from patient-derived xenografts (PDXs) originating from DDR-proficient radical prostatectomy (287R, 275R) and BRCA2 heterozygous mutant metastatic (201.1) tumours, retaining key histologic and genomic features. These organoids were exposed to different PARP inhibitors (Talazoparib, Saruparib) and androgen signalling inhibitors (Enzalutamide, Apalutamide, Darolutamide) for up to 11 days. Cell viability and growth responses were assessed using PrestoBlue and CellTiter-Glo assays and automated imaging analysis. SynergyFinder software calculated synergy scores for each treatment combination. Results were further validated in vivo using DDR-proficient 287R PDXs. Mechanistic insights were explored through RNA sequencing of 20 DAC and ten acinar radical prostatectomy samples, and four matched DAC and acinar PDXs, with γH2AX immunohistochemistry to examine DNA damage response. Results: Overall, PARPi/ARSi combination treatment significantly reduced organoid viability compared to PARPi alone or ARSi in DDR-proficient and heterozygous BRCA2 -mutant DAC tumours. The levels of synergy were comparable regardless of which PARPi and ARSi agents were combined. In vivo results using DDR-proficient PDX 287R confirmed the efficacy of PARPi + ARSi combination by reducing tumour volume by 58% compared to control ( p =0.0198) and by 40% versus PARPi alone ( p = 0.0326). Mechanistically, RNA sequencing demonstrated upregulation of multiple DDR pathways in DACs compared to acinar prostate tumours, regardless of the DDR status. After treatment with PARPi + ARSI combination, there was an increase in γH2AX compared to the control in the DDR-proficient PDX 287R (mean 1.42 vs 0.5, p=0.0056), suggesting enhanced DNA damage may contribute to the efficacy of the combination treatment. Conclusions: Our results show that PARPi increases the efficacy of ARSi in DAC. This is notable given the poor response of DAC to AR-directed treatments and provides the rationale for a pre-planned phase 1/2 study.

Neuroendocrine prostate cancer (NEPC) tumours are classified by pathology into several distinct subtypes. Gene expression profiling has revealed transcriptional heterogeneity across NEPC, but this is rarely considered in the context of variation between pathologies. Diagnosis typically relies on immunohistochemical markers (CHGA, SYP, NCAM1) and genomic alterations in RB1, PTEN and TP53. We hypothesized that NEPC pathologies have unique transcriptional features. Single‐cell RNA sequencing of 18 632 tumour cells from nine patient‐derived xenograft models representing five pathologies (small‐cell and large‐cell neuroendocrine carcinomas, focal neuroendocrine differentiation (Focal NED), low‐grade neuroendocrine and amphicrine) demonstrated pathway‐specific enrichment. Focal NED and amphicrine tumours exhibited cellular subpopulations enriched for KRAS, IL2‐STAT5 and TNF signalling pathways, absent in small‐ and large‐cell carcinomas, which were instead enriched for Myc and E2F pathways. Furthermore, focal NED cells exhibited minimal clonal divergence from adjacent adenocarcinoma cells, while small cell carcinoma cells were clonally distinct. These data underscore significant transcriptional variation among NEPC pathologies, highlighting focal NED's unique biological context and its clinical implications.

Background Men with neuroendocrine prostate cancer (NEPC) have a poor prognosis. NEPC is commonly diagnosed by immunohistochemical markers (CHGA, SYP and NCAM1) and genomic features (mutations in RB1, PTEN, TP53). But by pathology, NEPC tumours are variable, leading to a classification of NE subtypes such as small cell and large cell neuroendocrine carcinomas, focal neuroendocrine differentiation (Focal NED), and Amphicrine. We postulated the diversity observed in NEPC pathologies might arise from differences in transcriptional profiles and the aim of this study is to utilize single-cell RNA sequencing to define the transcriptional differences between NEPC subtype pathologies. Methods Gene expression profiles were obtained for 18,632 individual tumour cells from 9 patient-derived xenograft (PDX) models representing five distinct neuroendocrine pathologies of prostate cancer. Integration and clustering of cell-level data demarked transcriptionally distinct sub-populations of cells. Differential gene expression, gene set enrichment and transcriptional factor regulon analysis identified expression signatures unique to specific neuroendocrine pathologies. Copy-number estimated from expression data revealed the clonal structure of PDXs with mixed adenocarcinoma and neuroendocrine pathologies. Results Significant differences were observed in the transcriptional profiles of NEPC pathology subtypes. Focal NED cells maintain AR signaling, similar to the amphicrine subtype but different from small and large cell carcinomas. Cellular sub-populations enriched for expression of KRAS, IL2-STAT5 and TNF-signaling genes were found in focal NED and amphicrine pathologies, but not in small or large cell carcinomas. In contrast, sub-populations enriched for the YAP, Myc and E2F pathways were detected in small cell, large cell and amphicrine tumours, but not in focal NED cells. Each pathology showed unique patterns of master regulator activity as well, further implicating focal NED as a transcriptionally distinct entity. Based on copy number alterations within PDXs of mixed pathology, focal NED cells showed little clonal divergence from neighboring adenocarcinoma cells, whereas cells with small cell neuroendocrine pathology were clonally distinct. Conclusions Neuroendocrine prostate cancer subtypes can be identified by pathology and our study shows that transcriptional features identified by single-cell RNA-sequencing also distinguish neuroendocrine subtypes pathologies from each other. In particular, our data redefine focal neuroendocrine differentiation as a pathology expressing androgen receptors (AR), exhibiting its distinctive composition of transcriptionally unique sub-populations. These findings advocate for differences in the treatment of NEPC tumors, particularly those displaying focal NED. ### Competing Interest Statement The authors have declared no competing interest.

There are diverse phenotypes of castration-resistant prostate cancer, including neuroendocrine disease, that vary in their sensitivity to drug treatment. The efficacy of BET and CBP/p300 inhibitors in prostate cancer is attributed, at least in part, to their ability to decrease androgen receptor (AR) signalling. However, the activity of BET and CBP/p300 inhibitors in prostate cancers that lack the AR is unclear. In this study, we showed that BRD4, CBP, and p300 were co-expressed in AR-positive and AR-null prostate cancer. A combined inhibitor of these three proteins, NEO2734, reduced the growth of both AR-positive and AR-null organoids, as measured by changes in viability, size, and composition. NEO2734 treatment caused consistent transcriptional downregulation of cell cycle pathways. In neuroendocrine models, NEO2734 treatment reduced ASCL1 levels and other neuroendocrine markers, and reduced tumour growth in vivo. Collectively, these results show that epigenome-targeted inhibitors cause decreased growth and phenotype-dependent disruption of lineage regulators in neuroendocrine prostate cancer, warranting further development of compounds with this activity in the clinic. © 2024 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Supplementary Table 1: Correlation coefficients and P values of metabolic rates and ssGSEA score. Supplementary Table 2: Correlation coefficients and P values of metabolic rates and genetic mutations. Supplementary Table 3: Correlation coefficients and P values of metabolic rates and copy number changes. Supplementary Table 4: Correlation coefficients and P values of metabolic rates and patient pre-treatments.

Abstract Cancer cells undergo metabolic reprogramming to meet increased bioenergetic demands. Studies in cells and mice have highlighted the importance of oxidative metabolism and lipogenesis in prostate cancer; however, the metabolic landscape of human prostate cancer remains unclear. To address this knowledge gap, we performed radiometric (14C) and stable (13C) isotope tracing assays in precision-cut slices of patient-derived xenografts (PDX). Glucose, glutamine, and fatty acid oxidation was variably upregulated in malignant PDXs compared with benign PDXs. De novo lipogenesis (DNL) and storage of free fatty acids into phospholipids and triacylglycerols were increased in malignant PDXs. There was no difference in substrate utilization between localized and metastatic PDXs and hierarchical clustering revealed marked metabolic heterogeneity across all PDXs. Mechanistically, glucose utilization was mediated by acetyl-CoA production rather than carboxylation of pyruvate, while glutamine entered the tricarboxylic acid cycle through transaminase reactions before being utilized via oxidative or reductive pathways. Blocking fatty acid uptake or fatty acid oxidation with pharmacologic inhibitors was sufficient to reduce cell viability in PDX-derived organoids, whereas blockade of DNL, or glucose or glutamine oxidation induced variable and limited therapeutic efficacy. These findings demonstrate that human prostate cancer, irrespective of disease stage, can effectively utilize all metabolic substrates, albeit with marked heterogeneity across tumors. We also confirm that fatty acid uptake and oxidation are targetable metabolic dependencies in human prostate cancer. Implications: Prostate cancer utilizes multiple substrates to fuel energy requirements, yet pharmacologic targeting of fatty acid uptake and oxidation reveals metabolic dependencies in localized and metastatic tumors.

Supplementary Figure 1. Metabolic profiles of prostate cancer PDX across different pathology. Supplementary Figure 2. Percent labeling of glycolysis end products and TCA cycle intermediates following 4 hours ex vivo labelling of 13C substrates. Supplementary Figure 3. Mass isotopologue distribution of malate following glucose, glutamine, palmitate, and lactate tracing. Supplementary Figure 4. PDXOs responses following metabolic inhibitors treatment. Supplementary Figure 5. Organoids viability following 7 days treatments of metabolic inhibitors. Supplementary Figure 6. Dose responses of metabolic inhibitors treatment in PDXOs.

5064 Background: LuPARP (NCT03874884) is evaluating the safety and efficacy of olaparib in combination with 177 Lu-PSMA-617 in patients (pts) with mCRPC who have progressed on an androgen receptor pathway inhibitors and docetaxel. We aim to identify potential prognostic and predictive biomarkers from serial CTC profiling. Methods: Twenty-six pts with high PSMA expression on PSMA-PET received 7.4 GBq of 177 Lu-PSMA-617 every 6 weeks (wk) together with an escalating dose-schedule of olaparib (50mg BD – 300mg BD, days 2 to 14 or days -4 to 14) for up to 6 cycles. For CTC analysis, 10 mls of blood was collected at baseline, 12-weekly for 48 wk, and thereafter every 24 wk and at disease progression. CK+, CD45-, and DAPI+ CTCs were enumerated from 3 mls of blood and immunoassayed for PSMA expression using the Epic Sciences platform. Correlations between total and PSMA+ CTC counts and PSA50 response (PSA reduction of ≥ 50%) were evaluated. Low pass whole genome sequencing (lpWGS) was performed on baseline and longitudinal CTC samples to identify copy number alterations. Results: At baseline, 23 of 26 pts (88%) had detectable CTCs (median 2.9 CTC/ml, range 0-30), and of these 23 pts, 17 (74%) had detectable PSMA+ CTCs (median 0.9 CTC/ml, range 0.3-27), indicating heterogeneity of PSMA expression in CTCs. The CTC positivity rates (% of cases ≥1 CTC) at week (w)12, w24, w36, w48 and at disease progression were 57%, 58%, 75%, 36% and 100%, and the PSMA positivity rates (% of cases ≥1 PSMA+ CTC) were 38%, 36%, 22%, 50% and 56%, respectively. Fifteen of 20 (75%) evaluable pts with paired baseline and w12 CTCs had a ≥50% decline in total CTC count, of which 9/15 (60%) pts also achieved a PSA50 response. Five of these 9 (56%) pts had 100% CTC clearance at w12. In terms of PSMA+ CTCs, 14/15 (93%) evaluable pts had ≥50% PSMA+ CTC decline with 13 pts (93%) achieving complete CTC clearance by w12. Six of the 13 pts with complete PSMA+ CTC clearance at w12 also achieved a PSA50 response. Beyond PSMA expression, genomic heterogeneity was evident in baseline and on-treatment CTCs, including recurrent loss of PTEN, TP53, BRCA2, ATM, and RB1, and gain of AR and MYC. lpWGS in 10 baseline samples identified 5 cases with BRCA2 loss and 5 cases with ATM loss. Two pts with BRCA2 loss and 3 pts with ATM loss had a PSA50 response by w12. Conclusions: We observed high rates of total and PSMA+ CTCs in PSMA-expressing mCRPC. Total and PSMA+ CTCs decline with combined olaparib and 177 Lu-PSMA-617 treatment. Early declines in total and PSMA+ CTCs including 12w CTC clearance paralleled PSA responses. Notably, total and PSMA+ CTCs rise again in the setting of disease progression. Serial CTC profiling may assist in tracking response to 177 Lu-PSMA-617 therapy. Clinical trial information: NCT03874884 .

Neuroendocrine prostate cancer (NEPC) represent an aggressive malignancy that results in poor patients outcomes. At a histological level, NEPC exhibits heterogeneous pathologies; nevertheless, the molecular features, drivers and therapeutic implications of this heterogeneity are poorly understood. Single cell technology enables the resolution needed to unveil the complex heterogeneity of NEPC that bulk RNA analysis could not previously detect. Therefore, we aim to characterize the molecular heterogeneity of NEPC at a single cell level to detect common phenotypes across patients and identify new therapeutic targets. We performed single cell RNA sequencing on a novel cohort of nine patient derived xenograft (PDX) models that recapitulate the pathological and clinical heterogeneity of NEPC. Downstream analysis of single cell data was first completed on individual samples to identify distinct subpopulations of cells within each tumor. Then, integration analysis was performed to identify common and unique neuroendocrine populations across the different tumors. We profiled the transcriptome of 19,361 cells captured from the eight NEPCs. On an individual sample analysis, we detected 3 to 8 different subpopulations in each tumor, where every subpopulation displayed distinct biological properties such as epithelial mesenchymal transition, quiescence and stemness. Data integration of all samples revealed 16 subpopulations of tumor cells across all patients, of which 10 populations were primarily unique to one patient. Enriched pathway analyses revealed heterogeneous expression of most cancer hallmarks and oncogenic pathways across subpopulations, although some pathways were common to several neuroendocrine subpopulations such as EMT, P53 and KRAS. The detection of heterogeneous subpopulations in NEPC provides novel insight into why finding effective treatments for these aggressive tumors is challenging. Future work should focus on further evaluation of common druggable pathways that are shared across NEPC subpopulations. Alternatively, efforts could look at combination therapies that could effectively target these heterogeneous tumors. Citation Format: Rosalia Quezada Urban, Shivakumar Keerthikumar, Ashlee K. Clark, Laura H. Porter, Mitchell G. Lawrence, Renea A. Taylor, Gail P. Risbridger, Roxanne Toivanen, David L. Goode. Single cell RNA analysis reveals heterogeneous sub populations in aggressive variants of 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 4076.