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Monotherapy with PARP inhibitors is effective for the subset of castrate-resistant prostate cancer (CRPC) with defects in homologous recombination (HR) DNA repair. New treatments are required for the remaining tumors, and an emerging strategy is to combine PARP inhibitors with other therapies that induce DNA damage. Here we tested whether PARP inhibitors are effective for HR-proficient CRPC, including androgen receptor (AR)-null tumors, when used in combination with CX-5461, a small molecule that inhibits RNA polymerase I transcription and activates the DNA damage response, and has antitumor activity in early phase I trials. The combination of CX-5461 and talazoparib significantly decreased in vivo growth of patient-derived xenografts of HR-proficient CRPC, including AR-positive, AR-null, and neuroendocrine tumors. CX-5461 and talazoparib synergistically inhibited the growth of organoids and cell lines, and significantly increased the levels of DNA damage. Decreased tumor growth after combination therapy was maintained for 2 weeks without treatment, significantly increasing host survival. Therefore, combination treatment with CX-5461 and talazoparib is effective for HR-proficient tumors that are not suitable for monotherapy with PARP inhibitors, including AR-null CRPC. This expands the spectrum of CRPC that is sensitive to PARP inhibition.Copyright © 2021 American Association for Cancer Research

Background: Studies investigating immune checkpoint inhibitors (ICI) in mCRPC have produced modest results. Radiation therapy may be synergistic with ICIs. We hypothesised that SABR would enhance anti-tumour activity of PD-L1 inhibitor avelumab in patients (pts) with progressive mCRPC. Method(s): This phase II, single arm, multicentre study enrolled mCRPC pts following progression on >=1 novel androgen receptor-directed therapy. Up to two lines of prior taxane chemotherapy were permitted. Pts received avelumab 10mg/kg IV q2weeks for a total of 24 weeks (12 cycles). A single fraction of 20Gy SABR was administered to 1-2 disease sites within five days prior to first and second doses of avelumab. Primary endpoint was disease control rate (DCR); secondary endpoints were PSA response (PSA ), overall response rate (ORR), radiographic progression-free survival (rPFS), overall survival (OS) and safety. Radiographic disease assessment (CT and bone scintigraphy) was performed after cycles 6 and 12 of avelumab treatment. Following enrolment of 14 pts, a protocol amendment allowed avelumab beyond 12 cycles in pts with disease control at 24 weeks. Result(s): Thirty-one pts were enrolled, with 30 evaluable for the primary endpoint. Median follow-up was 18 months (mo). Pt characteristics: median age 71 years (IQR 64-75), bone-only disease 42%, visceral disease 16%, prior taxane chemotherapy 84%, treatment with both abiraterone and enzalutamide 13%. Seventy metastatic sites received SABR, most frequently to bone (90%) and soft tissue (29%) disease. Avelumab was given as second-line, third-line and fourth- or greater line systemic therapy in 29%, 42% and 29% of pts, respectively. Median cycles of avelumab administered was 9 (IQR 5-13). DCR (95% CI) was 50% (31-69) and 60% (32-84) in allcomers and soft tissue disease subgroup, respectively. Following protocol amendment, 7/17 pts (41%) received avelumab beyond 12 cycles. Incidence of grade 3-4 treatment-related AEs was 16% (no grade 5

Preclinical testing is a crucial step in evaluating cancer therapeutics. We aimed to establish a significant resource of patient-derived xenografts (PDXs) of prostate cancer for rapid and systematic evaluation of candidate therapies. The PDX collection comprises 59 tumors collected from 30 patients between 2012-2020, coinciding with availability of abiraterone and enzalutamide. The PDXs represent the clinico-pathological and genomic spectrum of prostate cancer, from treatment-naive primary tumors to castration-resistant metastases. Inter- and intra-tumor heterogeneity in adenocarcinoma and neuroendocrine phenotypes is evident from bulk and single-cell RNA sequencing data. Organoids can be cultured from PDXs, providing further capabilities for preclinical studies. Using a 1 x 1 x 1 design, we rapidly identify tumors with exceptional responses to combination treatments. To govern the distribution of PDXs, we formed the Melbourne Urological Research Alliance (MURAL). This PDX collection is a substantial resource, expanding the capacity to test and prioritize effective treatments for prospective clinical trials in prostate cancer.Copyright © 2021, The Author(s).

Monotherapy with PARP inhibitors is effective for the subset of castrate-resistant prostate cancer (CRPC) with defects in homologous recombination (HR) DNA repair. New treatments are required for the remaining tumors, and an emerging strategy is to combine PARP inhibitors with other therapies that induce DNA damage. Here we tested whether PARP inhibitors are effective for HR-proficient CRPC, including androgen receptor (AR)-null tumors, when used in combination with CX-5461, a small molecule that inhibits RNA polymerase I transcription and activates the DNA damage response, and has antitumor activity in early phase I trials. The combination of CX-5461 and talazoparib significantly decreased in vivo growth of patient-derived xenografts of HR-proficient CRPC, including AR-positive, AR-null, and neuroendocrine tumors. CX-5461 and talazoparib synergistically inhibited the growth of organoids and cell lines, and significantly increased the levels of DNA damage. Decreased tumor growth after combination therapy was maintained for 2 weeks without treatment, significantly increasing host survival. Therefore, combination treatment with CX-5461 and talazoparib is effective for HR-proficient tumors that are not suitable for monotherapy with PARP inhibitors, including AR-null CRPC. This expands the spectrum of CRPC that is sensitive to PARP inhibition.Copyright © 2021 American Association for Cancer Research

Background: Studies investigating immune checkpoint inhibitors (ICI) in mCRPC have produced modest results. Radiation therapy may be synergistic with ICIs. We hypothesised that SABR would enhance anti-tumour activity of PD-L1 inhibitor avelumab in patients (pts) with progressive mCRPC. Method(s): This phase II, single arm, multicentre study enrolled mCRPC pts following progression on >=1 novel androgen receptor-directed therapy. Up to two lines of prior taxane chemotherapy were permitted. Pts received avelumab 10mg/kg IV q2weeks for a total of 24 weeks (12 cycles). A single fraction of 20Gy SABR was administered to 1-2 disease sites within five days prior to first and second doses of avelumab. Primary endpoint was disease control rate (DCR); secondary endpoints were PSA response (PSA ), overall response rate (ORR), radiographic progression-free survival (rPFS), overall survival (OS) and safety. Radiographic disease assessment (CT and bone scintigraphy) was performed after cycles 6 and 12 of avelumab treatment. Following enrolment of 14 pts, a protocol amendment allowed avelumab beyond 12 cycles in pts with disease control at 24 weeks. Result(s): Thirty-one pts were enrolled, with 30 evaluable for the primary endpoint. Median follow-up was 18 months (mo). Pt characteristics: median age 71 years (IQR 64-75), bone-only disease 42%, visceral disease 16%, prior taxane chemotherapy 84%, treatment with both abiraterone and enzalutamide 13%. Seventy metastatic sites received SABR, most frequently to bone (90%) and soft tissue (29%) disease. Avelumab was given as second-line, third-line and fourth- or greater line systemic therapy in 29%, 42% and 29% of pts, respectively. Median cycles of avelumab administered was 9 (IQR 5-13). DCR (95% CI) was 50% (31-69) and 60% (32-84) in allcomers and soft tissue disease subgroup, respectively. Following protocol amendment, 7/17 pts (41%) received avelumab beyond 12 cycles. Incidence of grade 3-4 treatment-related AEs was 16% (no grade 5

Preclinical testing is a crucial step in evaluating cancer therapeutics. We aimed to establish a significant resource of patient-derived xenografts (PDXs) of prostate cancer for rapid and systematic evaluation of candidate therapies. The PDX collection comprises 59 tumors collected from 30 patients between 2012-2020, coinciding with availability of abiraterone and enzalutamide. The PDXs represent the clinico-pathological and genomic spectrum of prostate cancer, from treatment-naive primary tumors to castration-resistant metastases. Inter- and intra-tumor heterogeneity in adenocarcinoma and neuroendocrine phenotypes is evident from bulk and single-cell RNA sequencing data. Organoids can be cultured from PDXs, providing further capabilities for preclinical studies. Using a 1 x 1 x 1 design, we rapidly identify tumors with exceptional responses to combination treatments. To govern the distribution of PDXs, we formed the Melbourne Urological Research Alliance (MURAL). This PDX collection is a substantial resource, expanding the capacity to test and prioritize effective treatments for prospective clinical trials in prostate cancer.Copyright © 2021, The Author(s).

Background: There are fewer preclinical models of prostate cancer compared to other common tumours. New models that represent the diverse features of castrate-sensitive and castration-resistant prostate cancer (CRPC) are required for thorough preclinical testing of novel treatments. Therefore, the goal of the Melbourne Urological Research Alliance (MURAL) is to develop patient-derived xenografts (PDXs) spanning the clinical trajectory of prostate cancer. Method(s): We grafted >200 surgery, biopsy or rapid autopsy samples into testosterone-supplemented or castrated male NSG mice. Actively growing tumours were serially transplanted or grown as explants or organoids. PDXs were analysed using RNAseq, targeted genomic sequencing and histopathology review. Result(s): We previously reported 4 serially transplantable PDXs (Lawrence, et al., 2018, European Urology). Now we have established ~30 additional models spanning treatment naive primary disease to CRPC. PDXs of CRPC were often from soft tissue metastases of patients who had failed docetaxel, cabazitaxel, enzalutamide, abiraterone and other contemporary treatments. Accordingly, they had diverse mechanisms of resistance, including AR mutations, genomic structural rearrangements, gene amplifications and expression of AR variants. In addition, several PDXs had AR-null phenotypes, including small cell prostate cancer. All PDXs closely reflected the genomic, transcriptomic and histopathological characteristics of the original patient tumours. As renewable sources of tissue, the PDXs could also be grown as ex vivo slice cultures and in vitro organoids, providing complementary models with different timescales and endpoints. Conclusion(s): We have developed a new collection of castrate-sensitive and castrate-resistant PDXs of prostate cancer, providing diverse tumours for preclinical testing of candidate treatments.

Background: There are fewer preclinical models of prostate cancer compared to other common tumours. New models that represent the diverse features of castrate-sensitive and castration-resistant prostate cancer (CRPC) are required for thorough preclinical testing of novel treatments. Therefore, the goal of the Melbourne Urological Research Alliance (MURAL) is to develop patient-derived xenografts (PDXs) spanning the clinical trajectory of prostate cancer. Method(s): We grafted >200 surgery, biopsy or rapid autopsy samples into testosterone-supplemented or castrated male NSG mice. Actively growing tumours were serially transplanted or grown as explants or organoids. PDXs were analysed using RNAseq, targeted genomic sequencing and histopathology review. Result(s): We previously reported 4 serially transplantable PDXs (Lawrence, et al., 2018, European Urology). Now we have established ~30 additional models spanning treatment naive primary disease to CRPC. PDXs of CRPC were often from soft tissue metastases of patients who had failed docetaxel, cabazitaxel, enzalutamide, abiraterone and other contemporary treatments. Accordingly, they had diverse mechanisms of resistance, including AR mutations, genomic structural rearrangements, gene amplifications and expression of AR variants. In addition, several PDXs had AR-null phenotypes, including small cell prostate cancer. All PDXs closely reflected the genomic, transcriptomic and histopathological characteristics of the original patient tumours. As renewable sources of tissue, the PDXs could also be grown as ex vivo slice cultures and in vitro organoids, providing complementary models with different timescales and endpoints. Conclusion(s): We have developed a new collection of castrate-sensitive and castrate-resistant PDXs of prostate cancer, providing diverse tumours for preclinical testing of candidate treatments.

Background: The expression of androgen receptor splice variants (AR-V) in mCRPC patients has widely been regarded as a predictive biomarker for non-response to AR-targeted therapies. However, recent data from our group and others has questioned this association. We report new data from our whole blood assay, correlating baseline and end-of-treatment (EOT) AR-V7 and AR-V9 expression with clinical outcomes in mCRPC patients treated with abiraterone or enzalutamide. Method(s): We have previously developed a whole-blood, quantitative polymerase chain reaction assay for detecting circulating AR-V7 and AR-V9. The assay was applied to samples prospectively collected from 48 mCRPC patients immediately prior to commencing abiraterone or enzalutamide, and where available, at treatment cessation. Patients positive for either AR-V7 or AR-V9 were defined as AR-V-positive, and AR-Vnegative if neither variant was detected. All AR-V-positive samples underwent confirmatory DNA sequencing to confirm variant expression. AR-V expression was correlated with PSA response rate (chi-square test) and PSA progression-free survival (PSA-PFS) (logrank test). Result(s): The median follow-up was 10.6 mo. Twelve out of 48 patients (25%) were AR-V-positive, with no samples positive for both variants at baseline. Similar response rates were observed in AR-V-positive (7/12) and AR-V-negative (23/36) patients (50% vs. 64%, p = 0.7). PSA-PFS did not differ significantly between groups (8.1 mo vs. not reached, p = 0.5). EOT samples were available in nine patients, with four exhibiting AR-V-positivity. Of these, two (50%) patients were AR-V-positive at baseline, and two (50%) patients converted from AR-V-negative to AR-V-positive at treatment cessation (PSA-PFS 3.4 and 2.6 mo respectively). One of the conversion patients transitioned from V7-/V9- to V7+/V9+. Neither AR-V7 nor AR-V9 were detected in any of 13 normal male controls. Conclusion(s): With extended follow-up, whole blood expression of AR-V7 an

Background: Identifying predictive biomarkers for mCRPC patients receiving androgen receptor signalling inhibitors (ARSI) or chemotherapy remains an unmet clinical need. FOLH1 encodes for ProstateSpecific Membrane Antigen (PSMA), a type II glycoprotein highly expressed on prostate cancer cells. We designed a whole blood assay to detect FOLH1 mRNA, and correlated expression with clinical outcomes in patients commencing ARSI (abiraterone or enzalutamide) or chemotherapy (docetaxel or cabazitaxel). Method(s): mCRPC patients commencing ARSI or chemotherapy were prospectively recruited at three Australian centres from June 2016 to July 2018. A quantitative reverse transcription polymerase chain reaction assay was used to detect FOLH1 transcript from whole blood samples collected in PAXgene RNA tubes. Pre-treatment FOLH1 expression was correlated with PSA response rate (Fisher's exact test) and PSA progression-free survival (PSA-PFS) (log-rank test). Result(s): Median follow-up was 13.6 months (IQR 9.7-19.3). In total, 88 pretreatment samples were analysed, of which 75 (85%) were FOLH1-positive. In patients receiving ARSI, outcomes favoured FOLH1-positive patients compared to FOLH1negative patients, with higher PSA response rates (39/60, 65% vs. 2/7, 29%; p = 0.1) and longer PSA-PFS (median 9.0 months [95% CI, 7.210.8] vs. 2.8 months [95% CI, 2.33.3] ; p = 0.03). Conversely, in chemotherapy-treated patients, inferior outcomes were observed in FOLH1-positive patients compared to FOLH1negative patients, with lower PSA response rates (4/15, 27% vs. 5/6, 83%, p = 0.05) and shorter PSA-PFS (median 2.9 months [95% CI, 2.83.0] vs. 4.1 months [95% CI, 3.74.5] ; p = 0.32). Conclusion(s): Pre-treatment FOLH1 expression may differentiate between outcomes on ARSI vs. chemotherapy in mCRPC patients. The utility of FOLH1 as a predictive biomarker in mCRPC warrants further evaluation in larger, independent cohorts. (Table Presented) .

Background: The expression of androgen receptor splice variants (AR-V) in mCRPC patients has widely been regarded as a predictive biomarker for non-response to AR-targeted therapies. However, recent data from our group and others has questioned this association. We report new data from our whole blood assay, correlating baseline and end-of-treatment (EOT) AR-V7 and AR-V9 expression with clinical outcomes in mCRPC patients treated with abiraterone or enzalutamide. Method(s): We have previously developed a whole-blood, quantitative polymerase chain reaction assay for detecting circulating AR-V7 and AR-V9. The assay was applied to samples prospectively collected from 48 mCRPC patients immediately prior to commencing abiraterone or enzalutamide, and where available, at treatment cessation. Patients positive for either AR-V7 or AR-V9 were defined as AR-V-positive, and AR-Vnegative if neither variant was detected. All AR-V-positive samples underwent confirmatory DNA sequencing to confirm variant expression. AR-V expression was correlated with PSA response rate (chi-square test) and PSA progression-free survival (PSA-PFS) (logrank test). Result(s): The median follow-up was 10.6 mo. Twelve out of 48 patients (25%) were AR-V-positive, with no samples positive for both variants at baseline. Similar response rates were observed in AR-V-positive (7/12) and AR-V-negative (23/36) patients (50% vs. 64%, p = 0.7). PSA-PFS did not differ significantly between groups (8.1 mo vs. not reached, p = 0.5). EOT samples were available in nine patients, with four exhibiting AR-V-positivity. Of these, two (50%) patients were AR-V-positive at baseline, and two (50%) patients converted from AR-V-negative to AR-V-positive at treatment cessation (PSA-PFS 3.4 and 2.6 mo respectively). One of the conversion patients transitioned from V7-/V9- to V7+/V9+. Neither AR-V7 nor AR-V9 were detected in any of 13 normal male controls. Conclusion(s): With extended follow-up, whole blood expression of AR-V7 an

Background: Identifying predictive biomarkers for mCRPC patients receiving androgen receptor signalling inhibitors (ARSI) or chemotherapy remains an unmet clinical need. FOLH1 encodes for ProstateSpecific Membrane Antigen (PSMA), a type II glycoprotein highly expressed on prostate cancer cells. We designed a whole blood assay to detect FOLH1 mRNA, and correlated expression with clinical outcomes in patients commencing ARSI (abiraterone or enzalutamide) or chemotherapy (docetaxel or cabazitaxel). Method(s): mCRPC patients commencing ARSI or chemotherapy were prospectively recruited at three Australian centres from June 2016 to July 2018. A quantitative reverse transcription polymerase chain reaction assay was used to detect FOLH1 transcript from whole blood samples collected in PAXgene RNA tubes. Pre-treatment FOLH1 expression was correlated with PSA response rate (Fisher's exact test) and PSA progression-free survival (PSA-PFS) (log-rank test). Result(s): Median follow-up was 13.6 months (IQR 9.7-19.3). In total, 88 pretreatment samples were analysed, of which 75 (85%) were FOLH1-positive. In patients receiving ARSI, outcomes favoured FOLH1-positive patients compared to FOLH1negative patients, with higher PSA response rates (39/60, 65% vs. 2/7, 29%; p = 0.1) and longer PSA-PFS (median 9.0 months [95% CI, 7.210.8] vs. 2.8 months [95% CI, 2.33.3] ; p = 0.03). Conversely, in chemotherapy-treated patients, inferior outcomes were observed in FOLH1-positive patients compared to FOLH1negative patients, with lower PSA response rates (4/15, 27% vs. 5/6, 83%, p = 0.05) and shorter PSA-PFS (median 2.9 months [95% CI, 2.83.0] vs. 4.1 months [95% CI, 3.74.5] ; p = 0.32). Conclusion(s): Pre-treatment FOLH1 expression may differentiate between outcomes on ARSI vs. chemotherapy in mCRPC patients. The utility of FOLH1 as a predictive biomarker in mCRPC warrants further evaluation in larger, independent cohorts. (Table Presented) .

Background: Androgen receptor splice variant (AR-V) expression has previously been regarded as a negative predictive biomarker for response to abiraterone and enzalutamide in mCRPC patients. However, recent data questions this association. We designed a whole blood assay to detect AR-V7 and AR-V9, the two most abundantly expressed AR-Vs, and correlated expression with clinical outcomes in patients commencing abiraterone or enzalutamide. Method(s): We developed a quantitative real-time polymerase chain reaction assay to detect AR-V7 and AR-V9 from whole blood collected in PAXgene tubes. The assay was applied to samples prospectively collected from 37 mCRPC patients prior to commencing abiraterone or enzalutamide, and at treatment cessation. Patients positive for either AR-V7 or AR-V9 were defined as AR-V-positive, and AR-V-negative if neither variant was detected. AR-V expression was correlated with PSA response rate (chi-square test) and PSA progression-free survival (PSA-PFS) (log-rank test). Assay sensitivity was determined by serially diluting RNA from VCaP prostate cancer cells (known to express AR-V7) to establish a lower limit of detection. Result(s): The median follow-up was 7.29 months (IQR 4.21-10.55); 9 of 37 patients (24%) were AR-V-positive. We observed similar response rates in AR-V-positive (6/9) and AR-V-negative (18/28) patients (66% vs. 64%, p = 0.896). PSA-PFS did not differ significantly between groups (9.2 months vs. not reached, p = 0.355). Two patients converted from AR-V-negative to AR-V-positive (PSA-PFS 3.35 and 0.60 months respectively), and one patient remained AR-V-positive at baseline and end-of-treatment sampling. The lower limit of detection for AR-V7 was 0.1%, and AR-V7/V9 was not detected in any of the 13 normal male controls. Conclusion(s): We developed a sensitive and specific whole blood assay for AR-V7 and AR-V9 detection in patients with mCRPC. Neither PSA response rates nor PSA-PFS differed significantly between AR-V-positive a

Background: The intractability of castration-resistant prostate cancer (CRPC) is exacerbated by tumour heterogeneity, including diverse alterations to the androgen receptor (AR) axis and AR-independent phenotypes. The availability of additional models encompassing this heterogeneity would facilitate the identification of more effective therapies for CRPC. Objective(s): To discover therapeutic strategies by exploiting patient-derived models that exemplify the heterogeneity of CRPC. Design, setting, and participants: Four new patient-derived xenografts (PDXs) were established from independent metastases of two patients and characterised using integrative genomics. A panel of rationally selected drugs was tested using an innovative ex vivo PDX culture system. Intervention(s): The following drugs were evaluated: AR signalling inhibitors (enzalutamide and galeterone), a PARP inhibitor (talazoparib), a chemotherapeutic (cisplatin), a CDK4/6 inhibitor (ribociclib), bromodomain and extraterminal (BET) protein inhibitors (iBET151 and JQ1), and inhibitors of ribosome biogenesis/function (RNA polymerase I inhibitor CX-5461 and pan-PIM kinase inhibitor CX-6258). Outcome measurements and statistical analysis: Drug efficacy in ex vivo cultures of PDX tissues was evaluated using immunohistochemistry for Ki67 and cleaved caspase-3 levels. Candidate drugs were also tested for antitumour efficacy in vivo, with tumour volume being the primary endpoint. Two-tailed t tests were used to compare drug and control treatments. Results and limitations: Integrative genomics revealed that the new PDXs exhibited heterogeneous mechanisms of resistance, including known and novel AR mutations, genomic structural rearrangements of the AR gene, and a neuroendocrine-like AR-null phenotype. Despite their heterogeneity, all models were sensitive to the combination of ribosome-targeting agents CX-5461 and CX-6258. Conclusion(s): This study demonstrates that ribosome-targeting drugs may be effective again

Background: The intractability of castration-resistant prostate cancer (CRPC) is exacerbated by tumour heterogeneity, including diverse alterations to the androgen receptor (AR) axis and AR-independent phenotypes. The availability of additional models encompassing this heterogeneity would facilitate the identification of more effective therapies for CRPC. Objective(s): To discover therapeutic strategies by exploiting patient-derived models that exemplify the heterogeneity of CRPC. Design, setting, and participants: Four new patient-derived xenografts (PDXs) were established from independent metastases of two patients and characterised using integrative genomics. A panel of rationally selected drugs was tested using an innovative ex vivo PDX culture system. Intervention(s): The following drugs were evaluated: AR signalling inhibitors (enzalutamide and galeterone), a PARP inhibitor (talazoparib), a chemotherapeutic (cisplatin), a CDK4/6 inhibitor (ribociclib), bromodomain and extraterminal (BET) protein inhibitors (iBET151 and JQ1), and inhibitors of ribosome biogenesis/function (RNA polymerase I inhibitor CX-5461 and pan-PIM kinase inhibitor CX-6258). Outcome measurements and statistical analysis: Drug efficacy in ex vivo cultures of PDX tissues was evaluated using immunohistochemistry for Ki67 and cleaved caspase-3 levels. Candidate drugs were also tested for antitumour efficacy in vivo, with tumour volume being the primary endpoint. Two-tailed t tests were used to compare drug and control treatments. Results and limitations: Integrative genomics revealed that the new PDXs exhibited heterogeneous mechanisms of resistance, including known and novel AR mutations, genomic structural rearrangements of the AR gene, and a neuroendocrine-like AR-null phenotype. Despite their heterogeneity, all models were sensitive to the combination of ribosome-targeting agents CX-5461 and CX-6258. Conclusion(s): This study demonstrates that ribosome-targeting drugs may be effective again

Background: Androgen receptor splice variant (AR-V) expression has previously been regarded as a negative predictive biomarker for response to abiraterone and enzalutamide in mCRPC patients. However, recent data questions this association. We designed a whole blood assay to detect AR-V7 and AR-V9, the two most abundantly expressed AR-Vs, and correlated expression with clinical outcomes in patients commencing abiraterone or enzalutamide. Method(s): We developed a quantitative real-time polymerase chain reaction assay to detect AR-V7 and AR-V9 from whole blood collected in PAXgene tubes. The assay was applied to samples prospectively collected from 37 mCRPC patients prior to commencing abiraterone or enzalutamide, and at treatment cessation. Patients positive for either AR-V7 or AR-V9 were defined as AR-V-positive, and AR-V-negative if neither variant was detected. AR-V expression was correlated with PSA response rate (chi-square test) and PSA progression-free survival (PSA-PFS) (log-rank test). Assay sensitivity was determined by serially diluting RNA from VCaP prostate cancer cells (known to express AR-V7) to establish a lower limit of detection. Result(s): The median follow-up was 7.29 months (IQR 4.21-10.55); 9 of 37 patients (24%) were AR-V-positive. We observed similar response rates in AR-V-positive (6/9) and AR-V-negative (18/28) patients (66% vs. 64%, p = 0.896). PSA-PFS did not differ significantly between groups (9.2 months vs. not reached, p = 0.355). Two patients converted from AR-V-negative to AR-V-positive (PSA-PFS 3.35 and 0.60 months respectively), and one patient remained AR-V-positive at baseline and end-of-treatment sampling. The lower limit of detection for AR-V7 was 0.1%, and AR-V7/V9 was not detected in any of the 13 normal male controls. Conclusion(s): We developed a sensitive and specific whole blood assay for AR-V7 and AR-V9 detection in patients with mCRPC. Neither PSA response rates nor PSA-PFS differed significantly between AR-V-positive a