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Radium-223 is a targeted alpha radiation therapy for metastatic castration-resistant prostate cancer. DNA damage repair (DDR) defective prostate cancers, specifically genetic aberrations leading to homologous recombination deficiency (HRD), accumulate irreparable DNA damage following genotoxic treatment. This retrospective study assessed DDR mutation status in patients treated with radium-223, investigating their association with efficacy and overall survival (OS). Patients and methods: Included patients were treated with radium-223 and had results from primary or metastatic tumour tissue of a comprehensive next-generation sequencing panel of DDR genes, including canonical HRD genes. Patients were grouped by presence (DDRþ) or absence (DDRÀ) of pathogenic somatic or germline aberrations in DDR genes. We evaluated OS, time to ALP progression (TAP), time to initiation of subsequent systemic therapy (TST) and biochemical responses between DDR groups.

Platinum-based chemotherapy is not standard of care for unselected or genetically selected metastatic castration-resistant prostate cancer (mCRPC) patients. A retrospective assessment of 71 patients was performed on platinum use in the Netherlands. Genetically unselected patients yielded low response rates. For a predefined subanalysis of all patients with comprehensive next-generation sequencing, 30 patients were grouped based on the presence of pathogenic aberrations in genes associated with DNA damage repair (DDR) or aggressive variant prostate cancer (AVPC). Fourteen patients (47%) were DDR deficient (DDRd), of which seven with inactivated BRCA2 (BRCA2mut). Six patients classified as AVPC. DDRd patients showed beneficial biochemical response to carboplatin, largely driven by all BRCA2mut patients having >50% prostate-specific antigen (PSA) decline and objective radiographic response. In the wild-type BRCA2 subgroup, 35% had a >50% PSA decline (P = .006) and 16% radiographic response (P < .001). Median overall survival was 21 months for BRCA2mut patients vs 7 months (P = .041) for those with functional BRCA2. AVPC patients demonstrated comparable responses to non-AVPC, including a similar overall survival, despite the poor prognosis for this subgroup. In the scope of the registration of poly-(ADP)-ribose polymerase inhibitors (PARPi) for mCRPC, we provide initial insights on cross-resistance between PARPi and platinum compounds. By combining the literature and our study, we identified 18 patients who received both agents. In this cohort, only BRCA2mut patients treated with

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Background Molecular tumour boards (MTB) optimally match oncological therapies to patients with genetic aberrations. Prostate cancer (PCa) is underrepresented in these MTB discussions. This study describes the impact of routine genetic profiling and MTB referral on the outcome of PCa patients in a tertiary referral centre. Methods All PCa patients that received next-generation sequencing results and/or were discussed at an MTB between Jan 1, 2017 and Jan 1, 2020 were included. Genetically matched therapies (GMT) in clinical trials or compassionate use were linked to actionable alterations. Response to these agents was retrospectively evaluated. Results Out of the 277 genetically profiled PCa patients, 215 (78%) were discussed in at least one MTB meeting. A GMT was recommended to 102 patients (47%), of which 63 patients (62%) initiated the GMT. The most recommended therapies were PARP inhibitors (n = 74), programmed death-(ligand) 1 inhibitors (n = 21) and tyrosine kinase inhibitors (n = 19). Once started, 41.3% had a PFS of ≥6 months, 43.5% a PSA decline ≥50% and 38.5% an objective radiographic response. Conclusion Recommendation for a GMT is achieved in almost half of the patients with advanced prostate cancer, with GMT initiation leading to durable responses in over 40% of patients. These data justify routine referral of selected PCa patients to MTB’s.

121 Background: Ra223 is a therapeutic option for mCRPC patients (pts) with symptomatic bone metastases. DDR-defective prostate cancers, specifically those with homologous recombination deficiency (HRD), accumulate irreparable DNA damage following genotoxic treatment. This study assessed presence or absence of DDR alterations in mCRPC pts treated with Ra223, investigating the effect on efficacy and overall survival (OS). Methods: All pts included were treated with Ra223 and had genomic results from a comprehensive next-generation sequencing panel of DDR genes that directly or indirectly led to HRD, from primary or metastatic tissue. Exclusion criteria were prior platinum-based chemotherapy or treatment with poly-ADP ribose polymerase inhibitors (PARPi). Pts were grouped by presence (DDR+) or absence (DDR-) of pathogenic somatic and/or germline aberrations in DDR genes. Primary endpoint was OS, and secondary endpoints were time to alkaline phosphatase (ALP) progression, time to next systemic therapy and biochemical responses; comparing DDR+ and DDR– groups. Results: 93 pts were included in this two-centre retrospective study. Median age was 68 years. 56% received prior chemotherapy. Baseline characteristics where comparable between DDR status subgroups. 28 (30%) pts had mutations in DDR genes, most frequently occurring in ATM (8.6%), BRCA2 (6.5%), and CDK12 (4.3%) genes. DDR+ pts showed prolonged OS (median 36.3 vs. 17.0 months; HR 2.29; 95% CI 1.21-4.32; P= 0.01). Median time to alkaline phosphatase progression was 6.9 months for DDR+ pts and 5.8 months for DDR- pts (HR 1.48; 95% CI 0.87-2.50; P =0.15), and median time to next systemic therapy was 8.9 months for DDR+ pts and 7.3 months for DDR- pts (HR 1.58; 95% CI 0.94-2.64; P =0.08). A higher proportion of DDR+ pts completed Ra223 therapy (79% vs 47%; P= 0.05). No differences in biochemical (prostate-specific antigen, ALP) responses were seen. Conclusions: Pts harboring deleterious DDR aberrations more commonly completed Ra223, and derived a greater OS benefit. These findings need prospective confirmation, but support combination of Ra223 with PARPi or ATR inhibitors in DDR-defective mCRPC pts.

Homologous recombination repair deficiency (HRD) is observed in 10% of patients with castrate-resistant prostate cancer (PCa). Preliminary data suggest that HRD-PCa might be more responsive to immune checkpoint inhibitors (ICIs). In this study, we compare the tumor immune landscape and peripheral T cell receptor (TCR) repertoire of patients with and without HRD-PCa to gain further insight into the immunogenicity of HRD-PCa. Immunohistochemistry was performed on tumor tissue of 81 patients, including 15 patients with HRD-PCa. Peripheral TCR sequencing was performed in a partially overlapping cohort of 48 patients, including 16 patients with HRD-PCa. HRD patients more frequently had intratumoral CD3 + , CD3 + CD8 − FoxP3 − or Foxp3 + TILs above median compared to patients without DNA damage repair alterations (DDRwt; CD3 + and Foxp3 + : 77% vs 35%, p = .013; CD3 + CD8 − FoxP3 − : 80% vs 44%, p = .031). No significant difference in CD8 + TILs or PD-L1 expression was observed. In peripheral blood, HRD patients displayed a more diverse TCR repertoire compared to DDRwt patients (p = .014). Additionally, HRD patients shared TCR clusters with low generation probability, suggesting patient-overlapping T cell responses. A pooled analysis of clinical data from 227 patients with molecularly characterized PCa suggested increased efficacy of ICIs in HRD-PCa. In conclusion, patients with HRD-PCa display increased TIL density and an altered peripheral TCR repertoire. Further research into the efficacy of ICIs and the presence of shared neoantigens in HRD-PCa is warranted.

end points are safety and tolerability, PSA response rate, and ORR per RECIST v1.1 by blinded independent central review (BICR). Secondary end points are time to PSA progression, ORR, and rPFS per PCWG3-modified RECIST v1.1 by BICR; DOR and DCR per RECIST v1.1 and PCWG3-modified RECIST v1.1 by BICR; and OS.