1368P TALAPRO-1: Talazoparib monotherapy in metastatic castration-resistant prostate cancer (mCRPC) with DNA damage response alterations (DDRm) – Exploration of tumor genetics associated with prolonged benefit

Abstract

TALAPRO-1 enrolled men with progressive mCRPC, measurable soft-tissue disease, and tumor DDRm involved directly or indirectly in homologous recombination repair (11-gene panel). Enrolled men had received 1–2 taxane-based chemotherapy regimens and progressed on ≥1 novel hormonal therapy. Confirmed objective response rate (primary endpoint; per RECIST 1.1; blinded independent central review [BICR]) was 29.8%. The molecular bases of prolonged clinical benefit with poly(ADP-ribose) polymerase inhibitors are incompletely understood, although limited evidence suggests that patients exhibiting large deletions in BRCA genes may experience extended benefit. Exploratory post hoc biomarker analyses assessed tumor genetics associated with prolonged antitumor benefit in TALAPRO-1. Tumor alterations were assessed using FoundationOne®CDx. Zygosity of tumor alterations was predicted using somatic-germline-zygosity. Prolonged benefit was assessed as radiographic progression-free survival ([rPFS]; RECIST 1.1/PCWG3; BICR) time to event or censoring. Data cutoff was Sept 4, 2020 (primary completion date). Of 104 men in the efficacy population, 16 had rPFS of ≥12 months. Of their 16 tumors, 15 (94%) had BRCA2 alterations, including 10 large structural variants (9 copy number loss, 1 rearrangement) and 5 short variants (SV; 4 homozygous). In contrast, only 6/27 (22%) men with rPFS <2.0 months exhibited BRCA2 alterations (1 rearrangement, 1 non-defined, and 4 SV with 2 homozygous). Of 61 men with rPFS <12 months but ≥2 months, 36 (59%) had BRCA2 alterations (9 copy number loss, 1 rearrangement, 21 SV [11 homozygous], 2 multiple alterations [1 homozygous], and 3 not defined). Conversely, TP53 alterations were less common for men with rPFS ≥12 months vs <2 months (2/16 vs 14/27; P=0.02 Fisher’s exact test, 2-tails), potentially reflecting negative prognosis or a subgroup less sensitive to PARP inhibition. Based on these retrospective, exploratory analyses of TALAPRO-1, men exhibiting prolonged benefit typically exhibited BRCA2 copy number loss or homozygous alterations, and lacked TP53 alterations.