Practical considerations for optimising homologous recombination repair mutation testing in patients with metastatic prostate cancer

Analysis of the genomic landscape of prostate cancer has identified different molecular subgroups with relevance for novel or existing targeted therapies. The recent approvals of the poly(ADP-ribose) polymerase (PARP) inhibitors olaparib and rucaparib in the metastatic castration-resistant prostate cancer (mCRPC) setting signal the need to embed molecular diagnostics in the clinical pathway of patients with mCRPC to identify those who can benefit from targeted therapies. Best practice guidelines in overall biospecimen collection and processing for molecular analysis are widely available for several tumour types. However, there is no standard protocol for molecular diagnostic testing in prostate cancer. Here, we provide a series of recommendations on specimen handling, sample pre-analytics, laboratory workflow, and testing pathways to maximise the success rates for clinical genomic analysis in prostate cancer. Early involvement of a multidisciplinary team of pathologists, urologists, oncologists, radiologists, nurses, molecular scientists, and laboratory staff is key to enable optimal workflow for specimen selection and preservation at the time of diagnosis so that samples are available for molecular analysis when required. Given the improved outcome of patients with mCRPC and homologous recombination repair gene alterations who have been treated with PARP inhibitors, there is an urgent need to incorporate high-quality genomic testing in the routine clinical pathway of these patients.

Circulating Tumor DNA Testing for Homology Recombination Repair Genes in Prostate Cancer: From the Lab to the Clinic

Approximately 23% of metastatic castration-resistant prostate cancers (mCRPC) harbor deleterious aberrations in DNA repair genes. Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) therapy has shown improvements in overall survival in patients with mCRPC who harbor somatic and/or germline alterations of homology recombination repair (HRR) genes. Peripheral blood samples are typically used for the germline mutation analysis test using the DNA extracted from peripheral blood leucocytes. Somatic alterations can be assessed by extracting DNA from a tumor tissue sample or using circulating tumor DNA (ctDNA) extracted from a plasma sample. Each of these genetic tests has its own benefits and limitations. The main advantages compared to the tissue test are that liquid biopsy is a non-invasive and easily repeatable test with the value of better representing tumor heterogeneity than primary biopsy and of capturing changes and/or resistance mutations in the genetic tumor profile during disease progression. Furthermore, ctDNA can inform about mutation status and guide treatment options in patients with mCRPC. Clinical validation and test implementation into routine clinical practice are currently very limited. In this review, we discuss the state of the art of the ctDNA test in prostate cancer compared to blood and tissue testing. We also illustrate the ctDNA testing workflow, the available techniques for ctDNA extraction, sequencing, and analysis, describing advantages and limits of each techniques.

Circulating tumor DNA analysis in the era of precision oncology

The spatial and temporal genomic heterogeneity of various tumor types and advances in technology have stimulated the development of circulating tumor DNA (ctDNA) genotyping. ctDNA was developed as a non-invasive, cost-effective alternative to tumor biopsy when such biopsy is associated with significant risk, when tumor tissue is insufficient or inaccessible, and/or when repeated assessment of tumor molecular abnormalities is needed to optimize treatment. The role of ctDNA is now well established in the clinical decision in certain alterations and tumors, such as the epidermal growth factor receptor (EGFR) mutation in non-small cell lung cancer and the v-Ki-ras2 kirsten rat sarcoma viral oncogene homolog (KRAS) mutation in colorectal cancer. The role of ctDNA analysis in other tumor types remains to be validated. Evolving data indicate the association of ctDNA level with tumor burden, and the usefulness of ctDNA analysis in assessing minimal residual disease, in understanding mechanisms of resistance to treatment, and in dynamically guiding therapy. ctDNA analysis is increasingly used to select therapy. Carefully designed clinical trials that use ctDNA analysis will increase the rate of patients who receive targeted therapy, will elucidate our understanding of evolution of tumor biology and will accelerate drug development and implementation of precision medicine. In this article we provide a critical overview of clinical trials and evolving data of ctDNA analysis in specific tumors and across tumor types.

A systematic review of the prevalence of DNA damage response gene mutations in prostate cancer

Several ongoing international prostate cancer (PC) clinical trials are exploring therapies that target the DNA damage response (DDR) pathway. This systematic review summarizes the prevalence of DDR mutation carriers in the unselected (general) PC and familial PC populations. A total of 11 electronic databases, 10 conference proceedings, and grey literature sources were searched from their inception to December 2017. Studies reporting the prevalence of somatic and/or germline DDR mutations were summarized. Metastatic PC (mPC), castration‑resistant PC (CRPC) and metastatic CRPC (mCRPC) subgroups were included. A total of 11,648 records were retrieved, and 80 studies (103 records) across all PC populations were included; 59 records were of unselected PC and 13 records of familial PC. Most data were available for DDR panels (n=12 studies), ataxia telangiectasia mutated (ATM; n=13), breast cancer susceptibility gene (BRCA)1 (n=14) and BRCA2 (n=20). ATM, BRCA2 and partner and localizer of BRCA2 (PALB2) had the highest mutation rates (≥4%). Median prevalence rates for DDR germline mutations were 18.6% in PC (range, 17.2‑19%; three studies, n=1,712), 11.6% in mPC (range, 11.4‑11.8%; two studies, n=1,261) and 8.3% in mCRPC (range, 7.5‑9.1%; two studies, n=738). Median prevalence rates for DDR somatic mutations were 10.7% in PC (range, 4.9‑22%; three studies, n=680), 13.2% in mPC (range, 10‑16.4%; two studies, n=105) and not reported (NR) in mCRPC. The prevalence of DDR germline and/or somatic mutations was 27% in PC (one study, n=221), 22.67% in mCRPC (one study, n=150) and NR in mPC. In familial PC, median mutation prevalence was 12.1% (range, 7.3‑16.9%) for germline DDR (two studies, n=315) and 3.7% (range, 1.3‑7.9%) for BRCA2 (six studies, n=945). In total, 88% of studies were at a high risk of bias. The prevalence of DDR gene mutations in PC varied widely within somatic subgroups depending on study size, genetic screening techniques, DDR mutation definition and PC diagnosis; somatic and/or germline DDR mutation prevalence was in the range of 23‑27% in PC. These findings support DDR mutation testing for all patients with PC (including those with mCRPC). With the advent of the latest clinical practice PC guidelines highlighting the importance of DDR mutation screening, and ongoing mCRPC clinical trials evaluating DDR mutation‑targeted drugs, future larger epidemiological studies are warranted to further quantify the international burden of DDR mutations in PC.

Relationship between serum markers and volume of liver metastases in castration-resistant prostate cancer

BACKGROUND

Prostate cancer patients with liver metastases have a poor prognosis. To date, no study exists investigating the relationship between liver tumor burden and clinical laboratory markers.

MATERIALS AND METHODS

Metastatic castrate-resistant prostate cancer (mCRPC) patients with radiographic evidence of liver metastases were selected for this study. Volumetric measurements of liver metastases were ascertained for all available patients. Prostate specific antigen (PSA), lactate dehydrogenase (LDH), alkaline phosphatase (ALP), aspartate aminotransferase (AST), alanine aminotransferase (ALT), albumin (ALB), total bilirubin and hemoglobin (HGB) levels were then assessed to coincide with the scan dates. Univariate and multivariate mixed-model regression analysis were performed to evaluate the relationship between laboratory markers and liver lesion volume. Data sets with non-normal distribution were logarithmically transformed. Akaike information criteria (AIC) was used to identify the most reliable multivariate model.

RESULTS

In our heavily pretreated liver-metastatic patient population, univariate analysis demonstrated a statistically significant positive correlation between PSA (p = 0.0002), ALP (p = 0.0305), AST (p < 0.0001), ALT (p = 0.0049), and LDH (p = 0.0019) and liver lesion volume. Additionally, ALB (p = 0.0006) and HGB (p = 0.0103) had statistically significant negative correlation. Multivariate analysis identified AST and hemoglobin assessments as the best predictors of increasing liver lesion burden. Preliminary data on circulating tumor DNA (ctDNA) mutational and amplification findings are also reported.

CONCLUSIONS

Analysis identified AST and hemoglobin as optimal predictors of liver lesion volume. These patients have a heavy burden of ctDNA abnormalities. Further studies with a larger patient population are needed to verify these results. Micro Abstract: This study investigates the association between liver lesion burden and clinical laboratory markers in castrate-resistant prostate cancer patients with hepatic metastases. Our univariate analysis identified multiple laboratory markers as significant indicators of worsening hepatic disease. Multivariate analysis demonstrated that AST and hemoglobin were the most effective predictors of change in liver lesion volume.

Optimised ARID1A immunohistochemistry is an accurate predictor of ARID1A mutational status in gynaecological cancers

ARID1A is a tumour suppressor gene that is frequently mutated in clear cell and endometrioid carcinomas of the ovary and endometrium and is an important clinical biomarker for novel treatment approaches for patients with ARID1A defects. However, the accuracy of ARID1A immunohistochemistry (IHC) as a surrogate for mutation status has not fully been established for patient stratification in clinical trials. Here we tested whether ARID1A IHC could reliably predict ARID1A mutations identified by next-generation sequencing. Three commercially available antibodies - EPR13501 (Abcam), D2A8U (Cell Signaling), and HPA005456 (Sigma) - were optimised for IHC using cell line models and human tissue, and screened across a cohort of 45 gynaecological tumours. IHC was scored independently by three pathologists using an immunoreactive score. ARID1A mutation status was assessed using two independent sequencing platforms and the concordance between ARID1A mutation and protein expression was evaluated using Receiver Operating Characteristic statistics. Overall, 21 ARID1A mutations were identified in 14/43 assessable tumours (33%), the majority of which were predicted to be deleterious. Mutations were identified in 6/17 (35%) ovarian clear cell carcinomas, 5/8 (63%) ovarian endometrioid carcinomas, 2/5 (40%) endometrial carcinomas, and 1/7 (14%) carcinosarcomas. ROC analysis identified greater than 95% concordance between mutation status and IHC using a modified immunoreactive score for all three antibodies allowing a definitive cut-point for ARID1A mutant status to be calculated. Comprehensive assessment of concordance of ARID1A IHC and mutation status identified EPR13501 as an optimal antibody, with 100% concordance between ARID1A mutation status and protein expression, across different gynaecological histological subtypes. It delivered the best inter-rater agreement between all pathologists, as well as a clear cost-benefit advantage. This could allow patients to be accurately stratified based on their ARID1A IHC status into early phase clinical trials.

Utility of cell-free nucleic acid and circulating tumor cell analyses in prostate cancer

Prostate cancer is characterized by bone metastases and difficulty of objectively measuring disease burden. In this context, cell-free circulating tumor DNA (ctDNA) and circulating tumor cell (CTC) quantitation and genomic profiling afford the ability to noninvasively and serially monitor the tumor. Recent data suggest that ctDNA and CTC quantitation are prognostic for survival. Indeed, CTC enumeration using the CellSearch® platform is validated as a prognostic factor and warrants consideration as a stratification factor in randomized trials. Changes in quantities of CTCs using CellSearch also are prognostic and may be employed to detect a signal of activity of new agents. Molecular profiling of both CTCs and ctDNA for androgen receptor (AR) variants has been associated with outcomes in the setting of novel androgen inhibitors. Serial profiling to detect the evolution of new alterations may inform drug development and help develop precision medicine. The costs of these assays and the small quantities in which they are detectable in blood are a limitation, and novel platforms are required to address this challenge. The presence of multiple platforms to assay CTCs and ctDNA also warrants the consideration of a mechanism to allow comparison of data across platforms. Further validation and the continued development and standardization of these promising modalities will facilitate their adoption in the clinic.

Taxane-based Combination Therapies for Metastatic Prostate Cancer

CONTEXT

Multiple single-agent therapies improving survival are approved for the treatment of metastatic castration-resistant prostate cancer (mCRPC), including two chemotherapies, two androgen-signaling axis-targeting agents, an immunotherapeutic vaccine, and a radiopharmaceutical. Combination therapy can target multiple oncogenic pathways simultaneously, while potentially curbing the development of treatment resistance.

OBJECTIVE

To provide a succinct overview of taxane-based combination therapies currently being evaluated for the treatment of metastatic prostate cancer.

EVIDENCE ACQUISITION

We searched MEDLINE/PubMed^® and relevant congress databases for literature focused on taxane-based combination therapies being evaluated for the treatment of metastatic prostate cancer. In addition, a systematic search of www.clinicaltrials.gov was performed to gather information regarding ongoing taxane-based combination trials in prostate cancer. This search included phase II or III trials starting after January 1, 2010, which included the terms "docetaxel" or "cabazitaxel" and "prostate", and was then manually filtered for combination studies.

EVIDENCE SYNTHESIS

Single-agent therapy yields modest increments in survival. The success of combining docetaxel with androgen deprivation to improve overall survival (OS) for metastatic hormone-sensitive disease suggests the potential of similar approaches in mCRPC. Several classes of biological drugs have previously been combined with docetaxel for mCRPC in clinical trials without improvement in OS. However, combining docetaxel or cabazitaxel with newer agents with established single-agent benefit, such as radium-223, second-generation androgen pathway-targeted agents, or other chemotherapies, has the potential to benefit patients when compared with taxane chemotherapy alone. Our search revealed that the majority of trials currently assessing taxanes are focused on combination therapies: a combination approach is being evaluated in 37 of 47 trials assessing docetaxel and in 18 of 34 trials assessing cabazitaxel.

CONCLUSIONS

Despite prior failures, novel taxane-based combination therapies have the potential to improve outcomes in mCRPC. Challenges include the absence of validated predictive biomarkers for the selection of suitable patients and the potential for enhanced toxicity.

PATIENT SUMMARY

Patients with metastatic prostate cancer have access to multiple therapies improving survival. Many advanced epithelial cancers are treated with combinations of drugs; however, prostate cancer has remained an exception. A number of clinical studies have shown that combining chemotherapy with other classes of therapy may improve patient outcomes in prostate cancer. Here, we summarize the various combinations that are tested in the clinic and review the results.