Purpose Morphologically heterogeneous prostate cancers (PC) that behave clinically like small cell PC (SCPC) share their chemotherapy responsiveness. We asked whether these clinically defined, morphologically diverse, ‘aggressive variant PC’ (AVPC) also share molecular features with SCPC. Experimental Design 59 PC samples from 40 clinical trial participants meeting AVPC criteria, and 8 patient-tumor derived xenografts (PDX) from 6 of them, were stained for markers aberrantly expressed in SCPC. DNA from 36 and 8 PDX was analyzed by Oncoscan® for copy number gains (CNG) and losses (CNL). We used the AVPC PDX to expand observations and referenced publicly available data sets to arrive at a candidate molecular signature for the AVPC. Results Irrespective of morphology, Ki67 and Tp53 stained ≥ 10% cells in 80% and 41% of samples respectively. RB1 stained <10% cells in 61% of samples and AR in 36%. MYC (surrogate for 8q) CNG and RB1 CNL showed in 54% of 44 samples each and PTEN CNL in 48%. All but 1 of 8 PDX bore Tp53 missense mutations. RB1 CNL was the strongest discriminator between unselected castration resistant PC (CRPC) and the AVPC. Combined alterations in RB1, Tp53 and/or PTEN were more frequent in the AVPC than in unselected CRPC and in The Cancer Genome Atlas samples. Conclusions Clinically defined AVPC share molecular features with SCPC and are characterized by combined alterations in RB1, Tp53 and/or PTEN.

Background Poly(ADP-ribose) polymerase (PARP) inhibitors have antitumour activity against metastatic castrationresistant prostate cancers with DNA damage response (DDR) alterations in genes involved directly or indirectly in homologous recombination repair (HRR). In this study, we assessed the PARP inhibitor talazoparib in metastatic castration-resistant prostate cancers with DDR-HRR alterations.Methods In this open-label, phase 2 trial (TALAPRO-1), participants were recruited from 43 hospitals, cancer centres, and medical centres in Australia,

119 Background: Phase 2 and 3 studies with poly(ADP-ribose) polymerase inhibitors (PARPi) have demonstrated antitumor activity in patients (pts) with mCRPC with DDRmut who were previously treated with novel hormonal therapy (NHT). We report the first IA of a Phase 2 study of TALA, a potent inhibitor and trapper of PARP. Methods: TALAPRO-1 (NCT03148795) is enrolling pts (N ≈ 100) with measurable soft tissue disease, progressive mCRPC, and DDRmut likely to sensitize to PARPi (including ATM, ATR, BRCA1, BRCA2, CHEK2, FANCA, MLH1, MRE11A, NBN, PALB2, RAD51C), who received 1–2 chemotherapy regimens (≥1 taxane-based) and progressed on ≥1 NHT (enzalutamide/abiraterone acetate). Pts receive oral TALA 1 mg/d (moderate renal impairment, 0.75 mg/d) until radiographic progression, unacceptable toxicity, or consent withdrawal. Primary endpoint is objective response rate (ORR; blinded independent review). Secondary endpoints are time to OR, duration of response, prostate-specific antigen (PSA) decrease ≥50%, circulating tumor cell (CTC) count conversion (to CTC = 0 and <5 per 7.5 mL of blood), time to PSA progression, radiographic progression-free survival (rPFS), overall survival, safety, pt-reported outcomes, and pharmacokinetics. A planned IA of safety and efficacy was performed after 20 pts with BRCA1/2 mutations were on treatment for ≥8 wks. Results: 81 pts received TALA as of June 5, 2019; 43 pts enrolled by Feb 12, 2019 were evaluable for the primary endpoint (20 BRCA1/2, 2 PALB2, 14 ATM, 7 other). All had received docetaxel and 49% prior cabazitaxel. Overall ORR (95% CI) was 25.6% (13.5–41.2), ORRBRCA1/2 50.0% (27.2–72.8), ORRATM 7.1% (0.2–33.9). Overall median (95% CI) rPFS was 5.6 months (mo) (3.5–8.2), rPFSBRCA1/2 8.2 mo (5.6–NE), rPFSATM 3.5 mo (1.7–8.1). Most common treatment-emergent adverse events (≥20%) were anemia, nausea, asthenia, decreased appetite, constipation, and platelet count decreased. Conclusions: TALA monotherapy demonstrates encouraging antitumor activity in docetaxel-pretreated mCRPC pts, especially those with BRCA1/2mut, and was generally well tolerated. This study was sponsored by Pfizer Inc. Clinical trial information: NCT03148795.

Summary Chronic lymphocytic leukaemia (CLL) is a genetically heterogeneous disease characterised by genomic alterations and gene mutations that may portend worse survival or resistance to treatments. A total of 680 blood or bone marrow samples underwent targeted sequencing of 29 genes previously identified as being mutated in CLL, which were correlated to known prognostic clinical characteristics. Overall, 400 (59%) patients were treatment‐naïve (TN) and 280 (41%) were relapsed/refractory (R/R). Most patients (70%) had ≥1 mutation, with TP53 (22%), SF3B1 (18%), NOTCH1 (13%) and ATM (13%) being the most commonly mutated genes. A higher proportion of R/R patients had mutations in SF3B1 (P = 0·01) and TP53 (P < 0·001). Patients with mutated IGHV CLL more often had mutations in KLHL6 (P = 0·001) and MYD88 (P < 0·001). Pairwise associations showed mutational co‐occurrences in the TN group including SF3B1/ATM [false discovery rate (FDR) < 0·05] and NOTCH1/POT1 (FDR < 0·01). Recurrent mutations resulting in premature truncation prior to the ubiquitination domains of NOTCH1 in its PEST domain and BIRC3 in its RING domain can produce proteins that constitutively activate CLL. Frequent missense mutations, such as K700E in SF3B1 and E571K in XPO1, have unknown function but are most likely to be activating mutations. Future directions include using these mutations to identify pathways for therapeutic targeting and rational drug design.

93 Background: Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) show antitumor activity in mCRPC/DDRm pts treated with novel hormonal therapy (NHT). TALAPRO-1 is an open-label study evaluating TALA (a potent PARP inhibitor/trapper) in men with mCRPC/DDRm. We report a planned interim analysis (IA; Dec 2019). Updated results at a Sep 4 2020 cut-off, available in November 2020, will be presented at the meeting. Methods: TALAPRO-1 (NCT03148795) is enrolling pts (N ≈ 100) with measurable soft tissue disease, progressive mCRPC, and DDRm likely to sensitize to PARPi ( ATM, ATR, BRCA1/2, CHEK2, FANCA, MLH1, MRE11A, NBN, PALB2, RAD51C), who received 1–2 chemotherapy regimens (≥1 taxane-based) for metastatic disease and progressed on ≥1 NHT (enzalutamide/abiraterone acetate) given for mCRPC. DDRm are defined as known/likely pathogenic variants or homozygous deletions. Pts receive oral TALA 1 mg/day (moderate renal impairment 0.75 mg/day) until radiographic progression, unacceptable toxicity, consent withdrawal, or death. Primary endpoint is objective response rate (ORR). Secondary endpoints: time to objective response; response duration; prostate-specific antigen (PSA) decrease ≥50%; circulating tumor cell (CTC) count conversion (to CTC = 0 and <5 per 7.5 mL blood); time to PSA progression; radiographic progression-free survival (rPFS); overall survival; safety. A planned efficacy/safety IA was done when 60 pts with DDRm and measurable disease completed ≥6 months of TALA/no longer followed. Radiographic responses are based on investigator assessments. Results: 113 pts received TALA (cut-off Dec 12 2019); 75 pts were evaluable for IA, with DDRm, had measurable disease, received ≥16 weeks’ treatment, and were assessed for ORR (54.7% BRCA1/2, 4.0% PALB2, 22.7% ATM; 18.7% other DDRm).All pts evaluable for IA had prior docetaxel; 45.3% cabazitaxel. Confirmed ORR, rPFS, and composite response (investigator-assessed) in pts who received TALA for ≥16 weeks are in the table. Most common treatment-emergent adverse events: anemia (42.5%); nausea (32.7%). Conclusions: TALA monotherapy has encouraging antitumor activity in docetaxel-pretreated mCRPC pts with BRCA1/2 alterations and was generally well tolerated. Funding: Pfizer Inc. Clinical trial information: NCT03148795. [Table: see text]