Prostate cancer (PCa) clinical heterogeneity underscores tumor heterogeneity, which may be best defined by cell subtypes. To test if cell subtypes contributing to progression can be assessed noninvasively, we investigated whether 14 genes representing luminal, neuroendocrine, and stem cells are detectable in whole blood RNA of patients with advanced PCa. For each gene, reverse transcription quantitative polymerase chain reaction assays were first validated using RNA from PCa cell lines, and their traceability in blood was assessed in cell spiking experiments. These were next tested in blood RNA of 40 advanced PCa cases and 40 healthy controls. Expression in controls, which was low or negative, was used to define stringent thresholds for gene overexpression in patients to account for normal variation in white blood cells. Thirty-five of 40 patients overexpressed at least one gene. Patients with more genes overexpressed had a higher risk of death (hazard ratio 1.42, range 1.12-1.77). Progression on androgen receptor inhibitors was associated with overexpression of stem (odds ratio [OR] 7.74, range 1.68-35.61) and neuroendocrine (OR 13.10,) genes, while luminal genes were associated with taxanes (OR 2.7, range 1.07-6.82). Analyses in PCa transcriptomic datasets revealed that this gene panel was most prominent in metastases of advanced disease, with diversity among patients. Collectively, these findings support the contribution of the prostate cell subtypes to disease progression. Cell-subtype specific genes are traceable in blood RNA of patients with advanced PCa and are associated with clinically relevant end points. This opens the door to minimally invasive liquid biopsies for better management of this deadly disease.

The androgen receptor (AR) plays a crucial role in the development and homeostasis of the prostate and is a key therapeutic target in prostate cancer (PCa). The gold standard therapy for advanced PCa is androgen deprivation therapy (ADT), which targets androgen production and AR signaling. However, resistance to ADT develops via AR‐dependent and AR‐independent mechanisms. As reports on AR expression patterns in PCa have been conflicting, we performed cell‐by‐cell AR quantification by immunohistochemistry in the benign and malignant prostate to monitor changes with disease development, progression, and hormonal treatment. Prostates from radical prostatectomy (RP) cases, both hormone‐naïve and hormone‐treated, prostate tissues from patients on palliative ADT, and bone metastases were included. In the normal prostate, AR is expressed in >99% of luminal cells, 51% of basal cells, and 61% of fibroblasts. An increase in the percentage of AR negative (%AR−) cancer cells along with a gradual loss of fibroblastic AR were observed with increasing Gleason grade and hormonal treatment. This was accompanied by a parallel increase in staining intensity of AR positive (AR+) cells under ADT. Staining AR with N‐ and C‐terminal antibodies yielded similar results. The combination of %AR− cancer cells, %AR− fibroblasts, and AR intensity score led to the definition of an AR index, which was predictive of biochemical recurrence in the RP cohort and further stratified patients of intermediate risk. Lastly, androgen receptor variant 7 (ARV7)+ cells and AR− cells expressing neuroendocrine and stem markers were interspersed among a majority of AR+ cells in ADT cases. Altogether, the comprehensive quantification of AR expression in the prostate reveals concomitant changes in tumor cell subtypes and fibroblasts, emphasizing the significance of AR− cells with disease progression and palliative ADT.

Prostate cancer (PCa) is curable in most men but becomes lethal for patients with metastases at diagnosis and those who experience biochemical recurrence (BCR) after curative therapies. Androgen deprivation therapy is given upon BCR, but patients inevitably fail and become castration resistant (CRPC), dying from metastatic (m)CRPC despite receiving 2nd and 3rd line therapies (androgen receptor inhibitors/ARIs and/or taxanes). Drugs targeting other pathways have been tested in clinical trials. Few are implemented in practice due to low response rates, although some show a significant benefit in subsets of unselected patients. This clinical heterogeneity underscores the cellular and functional heterogeneity of malignant cells, highlighting the need for biomarkers of response to therapies. In line with the growing interest in using non-invasive liquid biopsies to monitor disease progression, we hypothesized that genes encoding proteins representative of prostate epithelial cell-subtypes may be detectable in blood as predictive biomarkers. Our objective was to identify such representative genes and test them in the blood of patients to determine whether they can stratify patients for optimal disease management. 14 genes pertaining to cell subtypes were chosen based on a thorough literature review. The panel was validated in transcriptomic datasets showing their predominant overexpression in metastases of mCRPC cases compared to primary tumours and benign prostate from diverse cohorts of patients. TaqMan assays were designed and optimized in serial dilutions of RNA from five prostate cancer cell lines. The panel was tested in the blood of healthy controls (n=9) and patients prior to prostatectomy (n=8), post-curative therapies (n=7), or mCRPC (n=19). In control men with no prostatic disease in their lifetime, we see low or no expression of most genes, with no correlation with age (29-71 years old). No association was seen between the proportions of different white blood cells and genes of interest expressed at varying levels in the blood of mCRPC patients. The threshold for overexpression in patients was defined as 2.58 standard deviation above the mean expression from controls (99.5% confidence interval). Phenotypic and functional diversity was seen in all categories of patients. Changes in genes patterns were significant in mCRPC cases based on current treatments (at time of blood draw) and the choice of initial curative therapy. For example, neuroendocrine genes were predominantly overexpressed in patients who underwent curative radiation therapy, whereas stem cell genes arose in cases under AR-Is at blood draw. In conclusion, we identified circulating genes that may be clinically meaningful to stratify and follow patients and predict response to therapies. Genes encoding drug targets may allow patient-tailored clinical trials for personalized treatments to impact on this unpredictable and lethal disease. Citation Format: Seta Derderian, Edouard Jarry, Arynne Santos, Mohanachary Amaravadi, Quentin Vesval, Lucie Hamel, Nathalie Cote, Marie Vanhuyse, Armen Aprikian, Simone Chevalier. Identifying therapeutic options for patients with advanced prostate cancer through genes in liquid biopsies [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr CC06-01.