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Background Cabazitaxel was shown to improve overall survival (OS) in patients with metastatic castration-resistant prostate cancer (mCRPC) after abiraterone/enzalutamine and docetaxel failure, though benefit by the presence of DNA damage repair (DDR) defects is unknown. With the advent of poly(adenosine diphosphate–ribose) polymerase inhibitors (PARPi) in partially overlapping indications with cabazitaxel, we aimed to determine cabazitaxel activity in men with mCRPC according to their DDR status. Methods This is a retrospective multicenter study that enrolled patients with mCRPC treated with cabazitaxel who had undergone DDR tumour tissue profiling. Patients with at least one deleterious germline or somatic alterations were considered DDR positive (DDR+). Each DDR + patient has been matched with a DDR negative (DDR-) from the same institution who underwent the same test. An exploratory cohort of patients found to be DDR + by liquid biopsy was also included. Prostate specific antigen (PSA) decline≥50% (PSA50), PSA progression-free survival (PFS, PSA-PFS), radiographic PFS (rPFS), clinical PFS or radiographic PFS (c/rPFS) and OS were evaluated. Results Among 190 men (95 DDR+, 95 DDR-) with tissue sequencing, PSA50 was achieved with cabazitaxel in 29/92 (32%) and 33/92 (36%) in patients with DDR+ and DDR- (P = 0.64). The median rPFS was 5.33 months [95%CI 4.34–7.04] versus 5.75 months [95%CI 4.67–7.27] (P = 0.55). The median OS was 15.4 months [95%CI 12.16–26.6] and 11.5 months [95%CI 9.76–14.4] (P = 0.036), respectively. No PSA50 responses on cabazitaxel were observed in BRCA1/2 patients previously treated with PARPi (n = 10). Similar outcomes with cabazitaxel were observed in the liquid biopsy cohort (n = 63 DDR+). Conclusions Our study suggests that cabazitaxel is active in patients with mCRPC regardless of their DDR status, although its activity in men pretreated with a PARPi may be lower.

9008 Background: Amivantamab, a fully human bispecific antibody targeting epidermal growth factor receptor (EGFR) and MET, is approved for the treatment of non-small cell lung cancer (NSCLC) with EGFR exon 20 insertion after prior platinum-based chemotherapy. Given its bispecific nature, amivantamab is being explored in patients (pts) with primary MET exon 14 skipping mutation (METex14) in the MET-2 cohort of the CHRYSALIS study. Methods: CHRYSALIS (NCT02609776) is an ongoing phase 1 dose escalation/dose expansion study of amivantamab in pts with advanced NSCLC. Pts with primary METex14 whose disease progressed on or who declined current standard of care therapy were treated with amivantamab 1050 mg (pts 54% regardless of prior treatment (Table). Median duration of response (DOR) was not reached (range, 2.1-12.2 months); 67% (8/13) had DOR ≥6 months. Ten of the 12 responders remain on treatment (6.0-14.4 months) with ongoing responses; 2 discontinued after 2 and 12 months, respectively. Safety profile was consistent with previously reported experience of amivantamab (Sabari 2021 JTO 16(3):S108-109). Treatment-related adverse events leading to dose reduction or discontinuation occurred in 3 pts, each. Conclusions: Amivantamab demonstrates anti-tumor activity in primary METex14 NSCLC including after prior MET inhibitor treatment. Enrollment is ongoing and updated data will be shown. Clinical trial information: NCT02609776. [Table: see text]

Circulating tumor DNA (ctDNA) has emerged as a non-invasive &ldquo, liquid biopsy&rdquo, for early breast cancer diagnosis. We evaluated the suitability of ctDNA analysis in the diagnosis of early breast cancer after mammography findings, comparing PIK3CA and TP53 mutations between tumor biopsies and pre-biopsy circulating DNA. Matched plasma and frozen fresh tissue biopsies from patients with Breast Imaging-Reporting and Data System (BIRADS) 4c/5 mammography findings and subsequent diagnosis of primary breast cancer were analyzed using NGS TruSeq Custom Amplicon Low Input Panel (Illumina) and plasma SafeSEQ (Sysmex Inostics). The same plasma and tumor mutations were observed in eight of 29 patients (27.6%) with four in TP53 and five in PIK3CA mutations. Sequencing analysis also revealed four additional ctDNA mutations (three in TP53 and one in PIK3CA) previously not identified in three patients tissue biopsy. One of these patients had mutations in both genes. Age, tumor grade and size, immunohistochemical (IHC) subtype, BIRADS category, and lymph node positivity were significantly associated with the detectability of these blood tumor-derived mutations. In conclusion, ctDNA analysis could be used in early breast cancer diagnosis, providing critical clinical information to improve patient diagnosis.

5024 Background: PSA value is widely used for the monitoring of treatment outcome in mCRPC in the clinical real-world setting. Early PSA changes are not considered in the definition of PSAProg due to the potential for spurious “flare” reactions. We aimed to evaluate the significance of an early PSA increase in mCRPC patients (pts) treated with enzalutamide or abiraterone (Enz/Abi). Methods: We retrospectively evaluated Enz/Abi-treated mCRPC pts from 11 hospitals between 2011-2020. Early PSAProg was defined as a 25% increase in PSA from baseline at 4 (PSAProg4) or 8 (PSAProg8) weeks after treatment initiation. PSA progression at 12 weeks (PSAProg12) was confirmed by a second reading. Uni- and multivariable (MV) Cox regression models were conducted to explore the association of PSAProg and overall survival (OS) in chemotherapy naïve patients treated with Abi or Enz. Interaction tests were conducted to explore differences in the impact of PSA progression on OS in Abi or Enz-treated pts. Results: We analyzed 511 chemotherapy-naïve mCRPC pts treated with Abi (N=391; 76.5%) or Enz (N=120; 23.5%). Median follow-up: 30.2 months. OS was longer in Enz-treated pts (38.1 vs 29m; HR 1.4; p=0.027). 59 (15.1%), 70 (17.9%) and 48 (12.3%) of Abi-treated and 9 (7.5%), 11 (9.2%) and 10 (8.3%) of Enz-treated pts experienced PSAProg4, PSAProg8 and PSAProg12, respectively, although differences were not statistically significant. PSAProg was associated with worse OS at all 3 timepoints only in Abi-treated pts. In Enz-treated pts, PSAProg4 had a large impact on OS, not observed in PSAProg8 or PSAProg12. We observed no significant interaction between agent (Enz/Abi) and PSA progression (Table). Conclusions: PSA progression at 4 weeks after Enz/Abi is significantly associated with shorter OS and may help identify pts not benefitting from Abi/Enz before clinical or radiographic progression. PSA pattern progression and its association with OS might differ depending on the drug used (Enz/Abi). Prospective validation studies are needed.[Table: see text]

5554 Background: The CARD trial proved that in mCPRC patients (pts), previously treated with docetaxel and an androgen-receptor signaling inhibitor (ARSi), cabazitaxel (CBZ) significantly improves progression-free (PFS) and Overall Survival (OS) compared with the alternative ARSi. Concurrently, the PROFOUND study showed the superiority of olaparib vs. ARSi in pts with similar prior treatment history and genetic alterations in Homologus Recombination DNA repair related genes (HRR). Methods: PROREPAIR-B (NCT03075735) is a prospective study which aimed to demonstrate the prognostic role of germline deleterious mutations in (g)HRR genes and the benefit of first (1L), second (2L) and subsequent therapy lines for mCRPC. Outcomes with 1-2L have been previously reported. Here we evaluated radiographic (r)-PFS, clinical (c)-PFS, and OS in PROREPAIR-B pts who meet CARD study eligibility criteria and who received CBZ and/or ARSi. Survival analysis were performed using Kaplan Meier method and Cox regression models. Results: 95 out of 419 mCRPC pts included in PROREPAIR-B meet CARD eligibility criteria and received CBZ (n=60) or ARSi (n=35) including 14 gHRR carriers, 8/6 treated with CBZ/ARSi, respectively. Visceral metastases were more frequent among pts treated with CBZ (p=0.01). ECOG 2, M1 at diagnosis, Abiraterone as 1st ARSi and prior radiographic PD (all p

5579 Background: IDC histology in PC has been suggested to associate with germline BRCA2 mutations (gBRCA2) in small series, leading to the potential recommendation of genetic testing for all PC patients with IDC in the primary tumor. Methods: We conducted a case-control study in which primary PC from 58 germline BRCA2 mutation carriers ( gBRCA2) and 116 from non-carriers (NC) were matched 1:2 by Gleason score and specimen type (core biopsy/prostatectomy). Samples were independently reviewed by two expert pathologists blinded to genetic status who established the presence of IDC and/or CRIB morphology with supportive immunohistochemical stains in a subset of cases. Next-generation sequencing, aCGH and/or FISH were used to assess for somatic mono-/bi-allelic BRCA2 alterations. PTEN protein loss was determined by IHC, and TMPRSS2-ERG was detected by FISH/qRT-PCR. Chi-square tests were used to compare the frequency of IDC and cribriform histology in gBRCA2 vs NC, as well as to assess the associations with other variables. Logistic regression models were built to identify independent factors associated with IDC and CRIB histology. Results: gBRCA2 cases were younger at diagnosis (median 61.3 vs 64, p < 0.01) and had T3-4 disease more often than NC cases (31% vs 10.5%, p < 0.01), but the two groups did not differ in any other clinical-pathologic characteristics. After independent histopathological review, 79 cases demonstrated IDC histology and 81 had CRIB histology. No differences in the prevalence of IDC (50% NC vs 36% gBRCA2, p = 0.09) or CRIB (43% NC vs 53% gBRCA2, p = 0.20) patterns were observed. The probability of IDC was higher in PC with bi-allelic BRCA2 alterations (OR 5.1, 95%CI 2.1-12.6), PTEN loss (OR 5.1, 95%CI 1.9-13.5) or both (OR 23.0, 95%CI 4.9-107.2) compared to those without these alterations. Bi-allelic BRCA2 alteration was also associated with higher probability of CRIB histology (OR 7.2, 95%CI 3.1-16.4). TMPRSS2-ERG fusions were not associated with IDC or CRIB histology. MVA confirmed the independent association of bi-allelic BRCA2 alteration (p < 0.01) and PTEN loss (p < 0.01) with IDC histology. Bi-allelic BRCA2 alteration (p < 0.01) and Gleason >8 (p < 0.01) were independent risk factors for CRIB histology. Conclusions: Primary PC with bi-allelic BRCA2 alterations was significantly associated with IDC and CRIB histology, independent of other clinical-pathologic factors (while gBRCA2 status alone was not). PTEN loss in primary PC was also independently associated with IDC, but not CRIB, histology.

e16530 Background: Changes in PSA are widely used as a biomarker for the monitoring of treatment outcome in Metastatic Castration-Resistant Prostate Cancer (mCRPC) in the clinical real-world setting. Early PSA changes (before 12 weeks) are not considered in the definition of PSA Progression (PSAProg) due to the potential for spurious “flare” reactions. We aimed to evaluate the significance of an early PSA increase in Abiraterone/Enzalutamide (Abi/Enz)-treated mCRPC patients (pts). Methods: We retrospectively evaluated Abi/Enz-treated mCRPC pts from 11 hospitals between 2011-2018. Early PSAProg was defined as a 25% increase in PSA from baseline at 4 (PSAProg4) or 8 (PSAProg8) weeks after treatment initiation. PSA progression at 12 weeks (PSAProg12) was confirmed by a second reading. Uni- and multivariable (MV) Cox regression models were conducted to explore the association of PSAProg and overall (OS) and radiographic progression-free (rPFS) survival. Sensitivity (Se), specificity (Sp) and predictive values (PPV, NPV) for the association of early PSAProg with PSAProg12 were calculated. Results: We analyzed 581 mCRPC pts; median follow-up: 19.1 months. 96 (17.1%); 105 (21.6%) and 85 (16.9%) pts had PSAprog at 4, 8 and 12 wks. PSAProg4 and PSAProg8 were significantly associated with confirmed PSAProg12. 55.3% of pts with PSAProg4 and 66.7% of pts with PSAProg8 had a confirmed PSAProg12. Only 9% of pts with no PSA prog at 4 wks and 4.1% of pts with no PSAProg8 had a confirmed PSAProg12. PSAProg4 had Se: 56.6%, Sp: 90.5%, PPV: 55.2%, NPV: 91% for the detection of PSAProg12. PSAProg8 had Se: 81.9%, Sp: 91.2%, PPV: 66.7%, NPV: 95.9% for the detection of PSAProg12. PSAprog at 4, 8 and 12 wks was significantly associated with OS and rPFS in uni- and MV Cox models (Table). Conclusions: Early PSAProg after Abi/Enz is significantly associated with both confirmed PSA Prog at 12 wks and outcome, and may help identify pts not benefitting from Abi/Enz before clinical or radiographic progression. Prospective validation studies are needed. [Table: see text]

267 Background: Abiraterone, enzalutamide and docetaxel represent first-line (1L) treatment options in mCRPC. A significant correlation between rPFS and OS has been reported for patients treated with 1L abi and enza in mCRPC. It is however unclear whether TTPP or rPFS present a similar magnitude of correlation with OS in Doc-treated pts. Methods: We evaluated the association of TTPP and rPFS with OS in pts treated with 1L Abi/Enza or Doc in a prospective multicenter observational cohort study. TTPP and rPFS were defined as per PCWG2. Correlation between TTPP and rPFS with OS was evaluated with Spearman rho coefficients (r), and by calculating the concordance index (c-index) in Cox-regression models. Results: 406 out of 419 pts received 1L Abi/Enza or Doc. After a median follow-up of 40 months (m), 253 mCRPC-related deaths were observed, with a median OS of 31.3 m (95% CI: 27.6-35). Median rPFS and TTPP were 10.8 m (95% CI:9.7-11.9) and 7.2 m (95% CI:6.7-7.7), respectively. Significant correlations between rPFS/TTPP and OS were observed in all pts treated at 1L, as well as in Abi/Enza and Doc treated pts (Table). R and c-index were consistently higher in Abi/Enza treated pts than in Doc treated pts, with a higher difference in predictive accuracy of the Cox regression model observed when comparing the association between TTPP and OS (c-index 0.788 in Abi/Enza treated pts vs 0.627 in Doc treated pts). Conclusions: Differences in r and c-index were observed when evaluating the association between TTPP/rPFS and OS in Abi/Enza and Doc treated pts, suggesting rPFS and TTPP may better predict OS in Abi/Enza than in Doc-treated pts. Indirect comparisons of TTPP in Abi/Enza vs Doc pts may therefore not reflect their true impact on OS. Further insight on the exact significance of TTPP is needed. Clinical trial information: NCT03075735. [Table: see text]

1585Background: Male breast cancer (MBC) is a rare disease that is still poorly understood. It is mainly classified by immunohistochemistry (IHQ) as a luminal disease. In this study, we use for the first time the PAM50 signature to unravel the molecular genetic heterogeneity in MBC. Methods: We collected surgical specimens of invasive MBC from four different Spanish pathology laboratories. IHQ staining for estrogen receptor (ER), progesterone receptor (PR), androgen receptor (AR), human epidermal growth factor receptor 2 (HER2), cytokeratin 5/6 (CK5/6), epidermal growth factor receptor (EGFR) and Ki-67 was performed on tissue microarrays composed of 3 cores sized 0.6 mm per case by standard methods. Breast cancer molecular subtypes were classified according to Cheang et al. (2009). Gene-expression profiling was performed on a research-use-only (RUO) nCounter Analysis System using the RUO PAM50 assay analyzed with the Prosigna algorithm. We explored the association of IHQ and PAM50 subtypes using Fisher’s ...

11049 Background: Prosigna has been clinically validated in 2 large randomized studies to predict the risk of distant and late recurrence in HR+/HER2- patients treated with endocrine therapy.NAC ef...