Multiple therapeutic options have been shown to improve survival in patients with advanced prostate cancer. PCa is a highly heterogeneous disease
[39][40][41], yet it is still managed as a single, homogenous disease, mainly due to the lack of validated biomarkers needed to correlate the molecular biology of the disease with the expected clinical course. Although the androgen receptor (AR) has long been a primary target of treatment, approximately 60% of patients present alterations in other molecular pathways, representing potentially-actionable novel treatment targets
[41]. Numerous candidate biomarkers have been identified as potential prognostic or predictive indicators, which—if validated—could be used to guide treatment selection in clinical trials, potentially improving clinical outcomes in patients with advanced PCa.
3.1. DNA Repair Defects
In patients with metastatic castration-resistant PCa (mCRPC), approximately 25% of tumours present mutations in the genes involved in DNA repair, known as DDR genes
[42][43][39][41][44][45][46]. From 12% to 16% of these are germline mutations
[42][43]. In the studies published to date, BRCA2 is the most commonly mutated gene (both germline and somatic)
[1][42][43][41][44]. The phase III PROFOUND trial included the largest number of prostate tumour specimens (
n = 2792) screened for mutations in DDR-related genes. The findings of the PROFOUND trial demonstrated the efficacy of olaparib in the treatment of mCRPC in patients who failed to prior chemotherapy and androgen signalling inhibitor (ARSi) treatments
[47]. Moreover, 28% of the tumour samples presented alterations in at least one of the 15 genes involved in the homologous recombination repair (HRR) pathway, with mutations detected both in primary tumours (27%) and metastatic lesions (32%). These data, considered together with the findings recently described by Mateo et al.—who found that DDR defects were present in a similar percentage of localised tumours (diagnostic biopsies) and in biopsied samples obtained from men (
n = 61) with mCRPC
[48]—confirm that alterations in the HRR pathway occur early in the course of disease in certain prostate tumours. Although HRR alterations are an early event, their prevalence is markedly higher in castration-resistant disease
[42][43] compared to localised PCa
[39][46], suggesting a correlation between HRR mutations and more aggressive forms of PCa.
In patients with mCRPC, the presence of DDR alterations has been identified as a biomarker of response to poly ADP-ribose polymerase (PARP) inhibitors
[47][49][50][51][52][53], and to platinum-based chemotherapy
[54][55][56]. Multiple clinical trials are currently underway to evaluate the efficacy of these drugs at different stages of the disease (
Table 2). Nonetheless, the findings described above suggest that screening for these mutations could provide data that are valuable both for genetic counselling and to guide treatment selection.
Table 2. Ongoing clinical trials to evaluate poly ADP-ribose polymerase (PARP) inhibitors in patients with mCRPC.
PARP Inhibitor |
Trial |
Phase |
Regimen |
Patient Population |
Rucaparib |
TRITON2 (NCT02952534) |
II |
Rucaparib monotherapy |
Post-abiraterone/enzalutamide and post-chemotherapy with DNA-repair abnormalities |
(NCT03442556) |
II |
Rucaparib |
Patients who are responding after docetaxel + carboplatin with DNA-repair abnormalities |
TRITON3 (NCT02975934) |
III |
Rucaparib vs. abiraterone or enzalutamide or docetaxel |
Patients with DNA-repair abnormalities (2L mCRPC) |
Niraparib |
BEDIVERE (NCT02924766) |
I |
Niraparib + apalutamide or abiraterone + prednisone |
Post AR-targeted therapy and post-taxane |
QUEST (NCT03431350) |
I/II |
Niraparib + abiraterone or JNJ-63723283 |
Post AR-targeted therapy |
GALAHAD (NCT02854436) |
II |
Niraparib monotherapy |
Post-chemotherapy with DNA-repair abnormalities |
MAGNITUDE (NCT03748641) |
III |
Niraparib + abiraterone vs. placebo + abiraterone |
Patients with or without DNA-repair defects |
Talazoparib |
TALAPRO-1 (NCT03148795) |
II |
Talazoparib monotherapy |
Post-abiraterone/enzalutamide and post-chemotherapy with DNA-repair abnormalities |
TALAPRO-2 (NCT03395197) |
III |
Talazoparib + enzalutamide vs. placebo + enzalutamide |
First line mCRPC |
Olaparib |
(NCT01972217) |
II |
Olaparib + abiraterone vs. placebo + abiraterone |
Post docetaxel mCRPC |
PROpel (NCT03732820) |
III |
Olaparib + abiraterone vs. placebo + abiraterone |
First line mCRPC |
PROfound (NCT02987543) |
III |
Olaparib vs. abiraterone/enzalutamide |
Post-abiraterone/enzalutamide mCRPC with HRR gene alterations |
KEYLINK-010 (NCT05834519) |
III |
Olaparib + pembrolizumab vs. abiraterone/enzalutamide |
Post AR-targeted therapy and post-taxane |
The TOPARP-B trial evaluated the efficacy of olaparib in patients (
n = 98) with mCRPC and DDR gene aberrations who had been previously treated with one or two taxane chemotherapy regimens. Patients were randomised to receive either 400 mg or 300 mg of olaparib twice daily, with a higher overall response rate (54.3% vs. 39.1%) in the higher dose group. However, due to treatment-related toxicity, 37% of patients in the 400 mg group required a dose reduction to 300 mg. Patients with BRCA1/2 mutations presented the best treatment response, with radiological and biochemical response rates of 52% and 77%, respectively, versus only 5% and 11.3% for other DDR gene mutations
[49].
Preliminary results from the phase II TRITON2
[51] and GALAHAD trials
[53] in patients with treatment-refractory mCRPC confirm the efficacy of two other PARP inhibitors, rucaparib and niraparib, respectively. Although both trials included patients with DDR defects, they used different gene panels and techniques to determine the presence of these defects. For example, the GALAHAD trial only enrolled patients with biallelic mutations. The preliminary data show that the highest objective response rate was obtained in patients with BRCA1/2 mutations, especially BRCA2. The data from these two trials suggest that response does not appear to be conditioned by the type of mutation, since response was similar among patients with somatic, germline, monoallelic, and biallelic mutations
[51][53] a finding that was confirmed in the recent study by Jonsson et al.
[57], who evaluated the role of BRCA1 and BRCA2 mutations in more than 17,000 patients (including 1042 with PCa). That study showed that, in certain tumours such as PCa, the clinical benefit of PARP inhibitors is similar, regardless of the type of mutation (germline, somatic, monoallelic, or biallelic).
A prospective analysis of 78 patients included in the TRITON2 trial with mutations in DDR genes other than BRCA1/2 showed that the presence of these alterations—particularly ATM, CDK12 and CHEK2—was associated with poor radiological and biochemical response rates (<11%)
[52]. These results underscore the need for more studies to better elucidate how mutations in genes other than BRCA1/2 influences the activity of PARP inhibitors in PCa.
The phase III PROFOUND trial
[47] was performed to evaluate patients with mCRPC and DDR mutations who had failed previous treatment with an ARSi (either abiraterone acetate or enzalutamide). Patients were randomised to receive olaparib 300 mg b.i.d. versus the ARSi that they had not previously received. Patients were stratified into two cohorts according to the type of gene mutations (Cohort A: BRCA1/2 and ATM vs. Cohort B: BARD1, BRIP1, CDK12, CHEK1, CHEK2, FANCL, PALB2, PPP2R2A, RAD51B, RAD51C, RAD51D, and RAD54L). In cohort A, radiological progression-free survival (the primary endpoint) was 7.4 months with olaparib versus 3.9 months for patients treated with abiraterone or enzalutamide (HR 0.34; 95% CI 0.25–0.47,
p < 0.001). An exploratory analysis carried out to evaluate the individual effect of each gene found that BRCA2 seems to be the best predictor of response to olaparib. Importantly, 28% of the samples were, for various reasons, not suitable for sequencing
[47] which is why alternative approaches, such as circulating DNA analysis, are being used in some studies with promising early results
[58].
The presence of DDR gene alterations may have clinical implications not only for treatment with PARP inhibitors, but also for response to current treatments for mCRPC, such as taxanes or ARSi. The available retrospective evidence is controversial due to conflicting results
[59][60][61]. PROREPAIR-B is the only prospective trial
[43] to date to evaluate patients with mCRPC (
n = 419) with germline DDR mutations (16% of the sample). In that study, the presence of a germline mutation in BRCA2 (gBRCA2) was confirmed as an independent prognostic factor of cause-specific survival (CSS). Moreover, the findings of that study also suggest that patients with gBRCA2 have worse CSS when treated with first-line taxanes followed by ARSi (10.7 vs. 28.4 months,
p < 0.001), but not when the sequence is reversed (ARSi followed by taxanes: 24 vs. 31.3 months,
p = 0.901). Those data, if confirmed, could position gBRCA2 as the first biomarker to guide first-line treatment selection in patients with mCRPC.
In other DNA repair pathways, such as DNA mismatch repair (MMR), the presence of a mutation (generally in MSH2, MSH6, MLH1, and PMS2) has been associated with microsatellite instability (MSI). Depending on the study, these alterations have been detected in 3%–12% of patients with mCRPC
[42][43][41][61][62][63][64]. Abida et al. evaluated more than 1000 patients with PCa, finding that the most commonly altered gene was MSH2. Moreover, in 22% of patients with MMR alterations, the MSH2 mutation was present in the germline
[64].
The presence of MSI is associated with genetically unstable tumours that have a tendency to accumulate mutations. This, in turn, leads to a higher burden of neoantigens, which may increase (hypothetically) the probability of response to immunotherapy
[65]. In 2017, the Food and Drug Administration (FDA) approved pembrolizumab for the treatment of patients with alterations in this pathway (deficient-MMR or MSI), regardless of the tumour origin. However, a recent exploratory analysis of data from the phase II KEYNOTE-199 trial (performed to evaluate the role of pembrolizumab—an anti-programmed death receptor-1 (PD1)—in patients with mCRPC) was unable to find a clear association between the presence of alterations in DDR or MMR genes and response to pembrolizumab
[66].
The phase III IMbassador250 trial (NCT 03016312) compared atezolizumab plus enzalutamide versus enzalutamide alone in patients with mCRPC. The trial, however, was discontinued early due to lack of efficacy, underscoring the need for more specific studies to determine the role of immunotherapy in advanced PCa.
3.3. Androgen Receptor
Other common alterations observed in patients with PCa are those directly associated with AR signalling. Several studies have demonstrated the prognostic—and potential predictive—value of AR amplification, which is present in more than 50% of patients with castration-resistant PCa
[75][76][77]. An exploratory analysis performed by Conteduca et al. found a lower risk of death for patients with AR amplification who received first-line docetaxel versus patients treated with ARSi, leading the authors to hypothesise that this alteration may be associated with resistance to ARSi, but not to taxanes in the first-line treatment of mCRPC
[77]. Mutations in AR are common in castration-resistant disease, but not in early stage disease
[40][41] The vast majority of these mutations appear after exposure to treatments that inhibit androgen signalling (T878A, F877L, among others), or after prolonged exposure to corticosteroid treatment (L702H)
[68]. The presence of these alterations in liquid biopsy has been linked to resistance to treatments involved in androgen signalling, such as abiraterone and enzalutamide
[75][76][78]. Similarly, numerous studies have found that the AR splice variant 7 (AR-V7) may also be important
[79][80][81][82], as AR-V7 expression has been associated with worse clinical outcomes in patients treated with ARSi (both first- and second-line), but not with taxanes
[79][80][81][82]. The PROPHECY trial (NCT02269982) is a prospective, multicentre study in patients with mCRCP treated with abiraterone/enzalutamide. That trial compared the two most important platforms for detecting AR-V7 in CTC: AR-V7 mRNA (AdnaTest) or AR-V7 nuclear localisation protein (Epic Sciences). The results of that trial showed that men with AR-V7–positive mCRPC had shorter PFS and OS outcomes, as well as fewer confirmed treatment responses (PSA and soft tissue)
[83]. Although the prognostic value of this biomarker seems clear, its predictive value has not yet been clearly demonstrated and thus cannot be relied upon for clinical decision-making. Other alterations such as TP53 mutations have also been associated with poor clinical outcomes in mCRPC patients treated with ARSi
[84][85].
In recent years, research into the biology of advanced PCa has helped us to better understand this disease. In this setting, several studies have identified the prognostic and/or predictive role of different biomarkers. For example, AR aberrations, including AR-V7, PTEN loss and/or TP53 mutations have all been associated with poor clinical outcomes in advanced PCa (prognostic biomarker). DDR alterations, particularly BRCA2 mutations, have also been correlated with more aggressive disease; more importantly, recent studies have shown that patients with these alterations may benefit from PARP inhibitors and platinum-based chemotherapy (prognostic and predictive role). Ideally, in the near future, several of the biomarkers described here will be incorporated into routine clinical practice, thereby improving the clinical management of patients. Until then, more research is needed to better elucidate the molecular biology of PCa.