https://orcid.org/0000-0002-2512-6131
1. Introduction
Prostate cancer is the second most commonly diagnosed malignancy and the fifth most frequent cause of cancer-related death in males worldwide [Citation1]. Although most cases of prostate cancer are diagnosed and treated while the disease is localized, some men have evidence of metastatic prostate cancer at presentation or following localized disease. The management of advanced prostate cancer has considerably improved over the last two decades. It involves a combination of androgen deprivation therapy (ADT) sequence of hormonal therapies and chemotherapies. More recently, large-scale sequencing efforts have allowed a better understanding of the genomic landscape of prostate cancer. Beyond maintenance of androgen receptor (AR) signaling in a castration setting, alternative mechanisms have been proposed for disease progression. In particular, germline or somatic aberrations in the DNA damage repair (DDR) genes are found in 19% of primary prostate cancer and almost 23% of metastatic castration-resistant prostate cancer (mCRPC) and compromise genomic integrity [Citation2]. Patients with BRCA2 pathogenic sequence variants have increased levels of serum prostate-specific antigen (PSA) at diagnosis, an increased proportion of high Gleason tumors, elevated rates of nodal and distant metastases, and high recurrence rates.
DDR-targeting agents are being evaluated either as single agents or in combination with treatments eliciting DNA damage in clinical studies enrolling patients with prostate cancer. Furthermore, according to the National Comprehensive Cancer Network (NCCN) guidelines, PSA screening is indicated from 45 years of age for men with BRCA2 germline mutation (gBRCA2). However, further follow-up is required to assess the role of screening in those with gBRCA1. The IMPACT study proposed annual PSA screening in the population with gBRCA1/2 and prostate biopsy if PSA >3.0 ng/mL [Citation3].
Overall, cells with gBRCA1/2 have an impaired double-strand DNA breaks (DSBs). The efficacy of poly (ADP-ribose) polymerase (PARP) inhibitors in BRCA-deficient cancers supports that additional factors within DNA repair represent synthetic lethality targets. Furthermore, synthetic lethality represents the therapeutic strategy of ‘BRCAness’ molecular signature, demonstrating the shared phenotype between sporadic and familial cancers with BRCA1/2 mutations. This is based on the evidence that tumors with deficiency in additional genes implicated in homologous recombination (HR) may also respond to treatment similar to BRCA1/2 mutated tumors. Proteins involved in HR repair include CDK12, ATM, FANCD2, RAD51C, CHEK2, PALB2, BRIP1, and HDAC2. Consequently, alterations in DDR genes, particularly in those involved in HR repair, are predictors of response to PARP inhibition. Finally, single-nucleotide polymorphisms (SNPs) in several DDR genes have been reported with increased risk of prostate cancer, including base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), and non-homologous end joining (NHEJ) pathway (). However, their use is limited due to the lack of functional studies.
Table 1. SNPs and gDDR genes mutations in prostate cancer
2. DDR mutations in prostate cancer
Prostate cancer is enriched for genomic alterations in DDR pathways. According to The Cancer Genome Atlas (TCGA) integrative assessment of 333 primary prostate cancers published in 2015, alterations in DDR genes were common, affecting almost 20% of samples through mutations or deletions in BRCA2, BRCA1, CDK12, ATM, FANCD2, or RAD51C [Citation4]. In terms of the metastatic setting, the Prostate Cancer Foundation-Stand Up To Cancer (SU2C-PCF) team analyzed prospectively whole-exome and transcriptome sequencing data from a cohort of 150 mCRPC patients [Citation2]. The investigators demonstrated higher prevalence of aberrations in AR (63%), TP53 (53%), RB1 (21%), and in the PI3K/AKT pathways (49%) in mCRPC compared to primary tumor (). An enrichment in DDR gene mutations was also reported in 23% of cases, including HR mediated repair genes (mostly BRCA2 [13%] and ATM [7.3%]) and MMR genes (MSH2 [2%]). Similarly, the large phase III PROfound study enrolled patients with mCRPC who had DDR defects and had failed previous treatment with AR signaling inhibitors (ARSi) [Citation5]. Alterations in DDR genes were identified in 28% of the samples analyzed, with a frequency of metastatic sites and primary disease of 32% and 27% respectively. The most commonly altered genes were BRCA2 (8.7%), CDK12 (6.3%), ATM (5.9%), CHEK2 (1.2%), and BRCA1 (1%), while concurrent aberrations in two or more DDR genes were found in 2.2% of cases.
Table 2. Genomic aberrations in primary and mCRPC
PROREPAIR-B is the first prospective study designed to assess the prevalence and impact of germline DDR mutations in patients with mCRPC [Citation6]. Among 419 patients screened for 107 DDR mutations, 16.2% were found to be DDR carriers, including 6.2% who carried mutations in BRCA2, ATM, or BRCA1. Those with gBRCA2 mutations showed a cancer-specific survival that was approximately halved compared with wild-type patients, which reached statistical significance (17.4 vs. 33.2 months respectively; p = 0.027).
Potential differences between patients with germline mutations and somatic aberrations are yet to be elucidated. Strict separation of somatic and germline variants is not regularly performed. Preliminary results suggest that somatic BRCA mutations are more often observed in late stages of prostate cancer. Consequently, genomic re-assessment with a new solid or liquid biopsy is highly recommended to offer an updated snapshot of the tumor.
3. Implications for the treatment
Synthetic lethality arises due to the combined effect of two genetic variations, which when occurring in isolation, are non-lethal [Citation7]. PARP inhibitors are synthetically lethal in HR deficient cells. Indeed, pharmacological PARP inhibition leads to the development of DSBs at the replication fork and blocks alternative repair pathways like NER, BER, and NHEJ, which leads to loss of genome integrity in cells with HR deficiency. PARP inhibition specifically in BRCA1/2 deficient tumor cells may result in up to a 1000-fold increased sensitivity as compared to wild-type tumor cells.
Apart from breast and ovarian cancer, several PARP inhibitors have been investigated in mCRPC patients () [Citation8]. The currently approved by the FDA PARP inhibitors for the treatment of mCRPC are olaparib, rucaparib, and niraparib. Differences exist in their metabolism; olaparib and rucaparib are metabolized by cytochrome P450 enzymes, whilst niraparib by carboxylesterase-catalyzed amide hydrolysis. The synthetic lethality mechanism of action may have protective effect against severe PARP inhibitor toxicity. The most common side effects include gastro-intestinal manifestations, myelosuppression, and fatigue. Trapped PARP-DNA complexes interfere with DNA replication forks leading to subsequent instability and replication stress. PARP trapping may account for the cytotoxic effects of PARP inhibitors, which do not exert the same extent of cytotoxicity. Among tested PARP inhibitors, olaparib, rucaparib, and niraparib are 100-fold more efficient at trapping than veliparib, which does not yet have an approved label. Olaparib was the first that demonstrated efficacy in patients with disease progression following standard treatments. The first stage of the TOPARP study (TOPARP-A) identified an association between DDR gene aberrations and response to olaparib in 49 molecularly unselected patients pre-treated with docetaxel and/or ARSi [Citation9]. In 16 of them, the estimated treatment response was 33% (95% confidence interval [95% CI], 20–48). The demonstrated antitumor activity was more pronounced in the presence of BRCA1/2, ATM, Fanconi’s anemia genes and CHEK2 mutations. The most frequent grade ≥ 3 adverse events in the TOPARP-A trial were the anemia and fatigue (20% and 12%, respectively) that were dose-limiting. These results led on to the start of TOPARP-B, in which only patients with DDR gene aberrations were randomly assigned (1:1) to receive either 400 mg or 300 mg olaparib twice daily [Citation10]. A confirmed composite response was observed in 25 (54.3%; 95% CI 39.0–69.1) of 46 patients in the 400 mg cohort and 18 (39.1%; 25.1–54.6) of 46 patients in the 300 mg cohort (p = 0.14). Among all DDR gene aberration subgroups, the greatest antitumor activity was demonstrated in the subgroup with BRCA1/2 alterations. More specifically, tumors with BRCA2 homozygous deletions had the best outcomes with a median radiographic progression-free survival (rPFS) of 16.4 months, as compared to the patients with deleterious BRCA1/2 germline and somatic mutations (5.6 and 8.2 months respectively). The median overall survival (OS) from the initiation of olaparib was 22.2 months for the BRCA2 homozygous deletion group, whilst in the deleterious and somatic BRCA1/2 mutations this was shorter (14.7 and 14.6 months respectively). Furthermore, among the treated TOPARP-B patients, 21 had ATM altered tumors (15 somatic mutations, 5 germline mutations, and 1 homozygous deletion). In this subset, 12/21 patients (57.1%) had a detectable second event and were predicted to have biallelic loss of ATM. They had longer rPFS (9.5 vs 5.2 months); nevertheless, this did not translate into improved OS (13.5 vs 16.6 months). In terms of tolerance, almost 30% of patients treated with a higher dose of olaparib discontinued the treatment or underwent dose modifications due to the grade 3/4 adverse events.
Table 3. Clinical trials of PARP inhibitors in locally advanced and mCRPC
Finally, in the PROfound study, patients were randomly assigned in a 2:1 ratio to receive olaparib (300 mg twice daily) or second-line ARSi. The median OS was 19.1 months with olaparib and 14.7 months with ARSi treatment (hazard ratio [HR]) 0.69; p = 0.02) [Citation5]. In the overall population (cohort A with BRCA1, BRCA2 and ATM alterations and cohort B with alterations in BARD1, BRIP1, CDK12, CHEK1, CHEK2, FANCL, PALB2, PPP2R2A, RAD51B, RAD51C, RAD51D, and RAD54L), the corresponding durations were 17.3 and 14.0 months (HR 0.79). The median total duration of treatment was 7.4 months in the olaparib arm and 3.9 months in the control group. The incidence of adverse events of grade ≥ 3 was higher in the experimental arm and included anemia and nausea, while fatigue was frequent in both arms. Moreover, 4% of patients in the olaparib group developed pulmonary embolism as compared to 1% in the control group. On 19 May 2020, the US Food and Drug Administration (FDA) approved olaparib for the treatment of patients with germline or somatic HR repair gene-mutated mCRPC previously treated with ARSi.
Beyond olaparib, the predictive value of DDR mutations was also estimated in two ongoing phase II trials. TRITON2 evaluates rucaparib in patients with mCRPC and a deleterious germline or somatic alteration in BRCA1/2, ATM, CDK12, or other prespecified DDR gene. The preliminary results confirmed radiological objective response rate (ORR) of 43.5% (95% CI, 31.0–56.7), whereas the biochemical response rate was 54.8% (95% CI, 45.2–64.1) [Citation11]. Moreover, ORRs were similar for patients with a somatic or gBRCA or other DDR gene alteration, while a higher PSA response rate was observed in those with a BRCA2 alteration. The most frequent grade ≥ 3 adverse event was anemia, reported in 25.2% of the participants. Based on this encouraging efficacy, rucaparib was granted accelerated approval by the US FDA on 15 May 2020, for treatment of mCRPC patients with BRCA1/2 after progression on taxane or ARSi.
The GALAHAD is being conducted to assess niraparib in patients with defined biallelic alterations in BRCA1/2, ATM, FANCA, PALB2, CHEK2, BRIP1, or HDAC2. In those with BRCA1/2 alterations, the ORR and the composite response rate were 41% and 63% respectively, whilst the median duration of objective response was 5.5 months (range 3.5–9.2 months). PSA decline of greater than 50% was observed in 50% of patients with BRCA and 3% of those with non-BRCA biallelic DDR gene alterations [Citation12]. Grade 3/4 adverse events included anemia, thrombocytopenia, and neutropenia (29, 15, and 7% respectively), which were managed with dose modification.
Platinum-based agents could be used to generate synthetic lethality in DDR deficient cancers [Citation7]. There is evidence that this therapeutic strategy may be effective in mCRPC patients harboring gBRCA2 mutations. Indeed, in a cohort of 141 mCRPC patients with docetaxel-resistant disease, 75% of the gBRCA2 patients demonstrated a > 50% PSA decline within 12 weeks of treatment initiation, compared to 17% in those lacking a gBRCA2 variant (p < 0.001). The median OS was 18.9 and 9.5 months respectively (p = 0.03) [Citation13].
MMR deficiency leads to the accumulation of somatic mutations that generate neoantigens. The correlation of neoantigens with cancer immunity provides the therapeutic rationale of immune checkpoint inhibitors in this subset of MMR-deficient tumors. CDK12 has a role in controlling genomic stability through regulation of DDR genes. Translational studies have shown that CDK12 mutations may delineate an immuno-responsive subgroup of prostate cancer, characterized by increased levels of T cell infiltration and high neoantigen burden. In this regard, anti-programmed cell death 1 (PD1) monotherapy may also be effective. However, exploratory analysis of data from the phase 2 KEYNOTE-199 trial of pembrolizumab in mCRPC patients did not correlate potential efficacy of pembrolizumab with DDR defects or MMR deficiency [Citation14]. A combined treatment with CDK12 and immune checkpoint inhibitors for prostate cancer may be suggested by the immune phenotype of CDK12-mutated tumors.
4. Expert opinion
A combination of alterations in several signaling pathways is crucial for prostate cancer initiation, progression, and therapeutic resistance. DNA sequencing of primary and metastatic prostate cancer has improved the understanding of the genomic and transcriptomic landscape, elucidating the prognosis and guiding precision oncology approaches.
The incidence of DDR alterations among patients with metastatic prostate cancer is much higher as compared to those with localized disease. Indeed, mCRPC tumors have approximately five times as many mutations as primary tumors and also include several new mutations. BRCA2 is the most commonly altered DDR gene. The identification of germline mutations in DDR genes in prostate cancer patients provides a firmly established basis for clinical decisions. PARP1 inhibition significantly increases levels of DNA damage in patients with germline mutations in DDR genes and induces substantial objective responses, whereas knowledge of somatic mutations in BRCA1/2 has promoted genetic counseling and testing of those patients. gBRCA2 are associated with poor clinical outcome and resistance to second-generation ARSi, independent of other prognostic factors [Citation6]. Expanding the criteria for genetic testing will increase the yield of identified actionable mutations. This is based on the paradigm of the inclusion of the entire population of epithelial ovarian cancer patients for germline testing, independent of the family history [Citation15]. The Philadelphia prostate cancer consensus conference specifically recommended germline testing in all prostate cancer patients at any stage with a broad gene panel, or if not available at least testing for BRCA1/2 mutations [Citation16]. However, there is still a lack of clarity about: a) the stage of the disease at which patients should be tested (initial diagnosis, mCRPC), b) the tissue that should be used for the analysis, and c) the performance of germline testing first followed by somatic testing or vice-versa. Another emerging question is whether circulating tumor DNA (ctDNA) can be used in place of tumor tissue at any time point. Early studies have confirmed a good concordance of mutation profiles and copy number alterations between tumor tissue and ctDNA. Technically, the evolution of next-generation sequencing (NGS) allows rapid evaluation of variants in multiple cancer susceptibility genes at similar costs to single gene sequencing, optimizing the precise diagnostic and therapeutic approach of prostate cancer patients. Moreover, liquid biopsy is an efficient, cost-, and tissue-saving analysis technique, providing a real-time evaluation of the tumor clonal evolution and acquired resistance.
PARP inhibitor monotherapy induces an objective antitumor activity in patients with PALB2, BRIP1, or FANCA aberrations. In contrast, those with ATM and CDK12 alterations do not seem to benefit. The data from TOPARP-B indicate that olaparib may be therapeutically effective in ATM-altered mCRPC. However, detection of ATM alterations alone might be insufficient to identify this subset of patients and further studies are required to elucidate the potentially additional genomic alterations, essential to sensitize to PARP inhibition. The fact that a fraction of biomarker-negative patients respond to PARP inhibitors is indicative of undetected DDR mutations that may exist in genomic regions less effectively covered by NGS. Olaparib has been approved by the FDA for mCRPC with genomic alterations in 14 different DNA repair genes, whilst only for BRCA-mutant cancers by the European Medicines Agency (EMA). This discordance in approvals underlines that it is crucial to further study biomarkers that predict clinical benefit from PARP inhibition. Rucaparib has been approved by the FDA for the treatment of mCRPC patients with somatic and/or germline DDR alterations who have progressed through enzalutamide or abiraterone treatment [Citation17]. Different trials are using different panels of DDR genes and yielded variable results and inconsistent outcomes in terms of specific gene alterations. This may be partly due to the differences in the PARP inhibitors themselves.
The evaluation of the combination of a PARP inhibitor with either ADT or ADT plus ARSi is currently ongoing in the mCRPC setting. Upon the observed synergy between the AR and DDR pathways, there is a plausible rationale to use the combination of ARSi with PARP inhibitors in patients even unselected for DDR alterations.
Finally, CDK12-altered prostate cancer represents an aggressive subtype. Biallelic inactivation of CDK12 is associated with a unique genome instability phenotype. The CDK12-specific focal tandem duplications can lead to the differential expression of oncogenic drivers, such as CCND1 and CDK4. As such, there is a possibility of vulnerability to CDK4/6 inhibitors for CDK12-mutated tumors. Moreover, the CDK12 aberrations may be used next to MMR deficiency, as a biomarker of treatment response. This highlights the rationale for the combination therapeutic strategy of immune checkpoint blockade and CDK4/6 inhibition in clinical trials.
Declaration of interest
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
Reviewer disclosures
Peer reviewers in this manuscript have no relevant financial or other relationships to disclose
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