Cambridge Neoadjuvant Cancer of the Prostate (CANCAP03): A Window Study into the Effects of Olaparib ± Degarelix in Primary Prostate Cancer

Abstract

Purpose: The purpose was to investigate combined PARP and androgen inhibition in primary prostate cancer and understand the biological mechanisms underlying clinical efficacy, especially in the absence of mutations in homologous recombination (HR) repair pathways.

Patients and methods: The primary objective was to measure PARP inhibition, and the secondary objectives were to assess safety and feasibility. Participants received olaparib for 2 weeks before prostatectomy and were randomly assigned or not assigned (1:1) to degarelix. We analyzed diagnostic biopsy and radical prostatectomy samples for PARylated protein expression using IHC. Exploratory analyses included tumor gene sequencing, mutation analysis, and RNA sequencing (RNA-seq) using both bulk and single-cell RNA-seq performed on pretreatment and posttreatment tissues.

Results: PARylated protein expression was significantly reduced in both cohorts, with no drug-related delays in radical prostatectomy. The gene set enrichment analysis identified distinct treatment response signatures related to olaparib in both cohorts and showed downregulation of androgen response genes after olaparib + degarelix treatment.Transcript profiling revealed an upregulation of the p53 hallmark, which was more pronounced with the combination treatment. Canonical cell-cycle progression hallmarks, including E2F targets and the G2-M checkpoint, were suppressed across all cases, correlating with a HR-deficient transcriptional signature. Single-nuclear RNA-seq indicated a greater increase in inflammatory response pathway activity within tumor epithelia after combination treatment.

Conclusions: Transcriptomic analysis identified common hallmark alterations reflecting the combined impact of PARP inhibitor and androgen blockade on cell-cycle progression. We observed a shared phenotypic response to combination therapy across prostate cancers without known HR repair gene alterations. This suggests alternative mechanisms rather than antiandrogen-induced HR deficiency.

Figure 1. CONSORT diagram for CANCAP03 study.

Figure 2. Representative IHC sections (x400 magnification) stained with H&E and for PAR protein expression at baseline (diagnostic prostate biopsy) compared to after treatment (radical prostatectomy specimen).

A. olaparib and B. olaparib + degarelix. C. PAR protein expression, H-score, for individual participant samples measured before (diagnostic) and after (prostatectomy) olaparib monotherapy or olaparib + degarelix combination treatment. D. Mean H score for olaparib monotherapy and olaparib+degarelix combination cohorts at baseline and after treatment.

Figure 3

Left: Graph with individual participants, treated with olaparib, ordered by degree of change/ fall in PSA plotted with % change PAR expression (H-score), and significant DDR genetic alterations listed per participant. Right: Graph with individual participants, treated with olaparib + degarelix; ordered by degree of change/ fall in PSA plotted with % change PAR expression. Bottom summary table of clinical features (based on diagnostic biopsy/ baseline staging) per patient including significant genetic changes listed per participant – pathogenic variants in BRCA1, BRCA2 or ATM marked. NB. Participant 1002 did not have a PSA result from the day of radical prostatectomy.

Figure 4. Transcriptomic analysis of CANCAP03 tumours.

(A) Gene set enrichment analysis for participants treated with olaparib and olaparib + degarelix cohorts. Genes were ranked by Log2Fold change, with respect to pre-treatment samples. Enrichment analysis was performed using the MSigDB cancer hallmarks and expression signatures relating to homologous recombination deficiency (see supplementary for details). (B) Gene set enrichment plots revealing the direction of modulation for signatures of interest. (C) Heatmap showing the magnitude of expression changes across the participant cohorts for pathways of interest. Patient 1013 excluded from RNAseq analysis due to insufficient tumour content available. (D) Expression changes of Prolaris signatures following treatment, expressed as a Z-score with respect to pre-treatment samples, and ordered by PSA change. Olaparib either PSA decline (reduction > 10% from pre-treatment) or not and olaparib + degarelix (all participants had PSA decline). (E) Correlation between Prolaris and HRD down expression changes (with removal of outlier patient 1018). (F) Cell type annotation using single-nuclear RNA sequencing of n=6 patients from both monotherapy and combination treatment group. (G) Gene set enrichment analysis showing differences between olaparib + degarelix and olaparib treatment groups in the labelled pathways; arrows denote the trend of expression as upregulated (arrow up) or downregulation (arrow down), while numbers represent q values (NS – not significant).