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5566 Background: As the prognosis of BRCA-related advanced ovarian cancer (AOC) continues to improve thanks to the introduction of targeted therapies like poly ADP-ribose polymerase inhibitors (PARPi) and bevacizumab, the occurrence of second primary tumors, in particular breast cancer (BC), is assumed to be more common. Despite this, data on the incidence of BC in this population are poor and guidelines for BC screening or risk reducing strategies are lacking. Methods: Published data from recent PARPi-based, phase III clinical trials involving germline BRCA mutated AOC were reviewed and analyzed. Only studies in which the incidence of at least one second primary solid tumor was reported were included. Cases of second primary BC cancers were pooled to calculate cumulative incidence according to the median follow-up period for each trial. 95% confidence interval (CI) was considered. Results: Four trials (SOLO1, PAOLA1, SOLO2 and SOLO3) involving 1186 germline BRCA 1-2 mutated AOC patients were included ( BRCA1, n = 816; BRCA2, n = 360; BRCA1 + BRCA2, n = 4; missing, n = 6). In all trials, patients were randomized to receive olaparib +/- bevacizumab (n = 788) versus placebo or standard therapies (n = 398). Median age at diagnosis of AOC was 56 years (range, 29-87). With a follow-up ranging from 3.9 months to 5 years, 16 new cases of BC were recorded, 10 in PARPi-based arms and 6 in control arms. Only two patients had a previous history of BC. The total cumulative incidence of BC was 1.37% (95% CI, 0.77-2.18), while cumulative incidences were 1.26% (95% CI, 0.61-2.31) and 1.51% (95% CI, 0.55-3.25) in PARPi-based arms and control arms, respectively. In a sensitivity analysis excluding the SOLO3 trial with the shortest follow-up, the total cumulative incidence was 1.63% (95% CI, 0.92-2.67). Other 20 cases of second primary solid tumors were registered, including 4 cases of non-small cell lung cancer. Conclusions: Patients with germline BRCA-related AOC receiving PARPi-based maintenance therapy can be reassured about the risk of second primary BC and intensive screening should be avoided at least in early treatment phase. More data with long term follow-up are needed.

e13008 Background: Liquid biopsy-based biomarkers, including circulating tumor DNA (ctDNA), are increasingly important for the characterization of metastatic breast cancer (MBC). Currently, CA15.3 is the most commonly used serum marker for monitoring disease burden. To date, no liquid biopsy-based biomarkers have been proposed for this scope in clinical practice. Methods: The CRO-2018-56 multicenter study prospectively enrolled 83 patients (pts) with luminal-like MBC treated with first line endocrine therapy (ET) and CDK4/6 inhibitors. All pts were characterized for ctDNA through droplet digital PCR (ddPCR) in from 2018 to 2021. Clinicopathological and laboratory characteristics at baseline were tested for associations with tumor markers, ACTB fragments lengths, methylation status of ESR1 main promoters (expressed as promA and promB ratio, i.e., met_ratio) and ESR1/PIK3CA mutational status through Kruskal-Wallis test and Chi-square test. Prognosis was tested in terms of Progression Free Survival (PFS) and Overall Survival (OS) through log-rank test. Results: At baseline, in 26 (31%) pts disease was diagnosed as de novo metastatic, 66 (80%) pts had < 3 of metastatic sites, and 41 (49%) pts had < 5 of metastatic lesions. Bone metastases were detected in 53 (64%) pts, liver metastases in 21 (25%) pts, and lung lesions in 30 (36%) pts. A ctDNA-detected ESR1 mutation and a PIK3CA mutation were found in 15% of pts and in 34% pts, respectively. Met_ratio was > 1.5 in 35 (42%) of pts. Median CA15.3 was 48.2 U/mL and median CEA was 3.8 U/mL. Number of liver metastases and number of metastatic sites were significantly higher in pts with ESR1 mutation (respectively, P = 0.0055 and P = 0.0208). CA15.3 and ctDNA yield were significantly higher in pts with number of metastatic sites ≥ 3, (respectively, P = 0.0164, and P = 0.0239), while number of metastatic sites ≥ 3 and number of metastatic lesions ≥ 5 were significantly associated with CEA > 3.8 U/mL (respectively, P = 0.039, and P = 0.029). Presence of bone metastases was significantly associated with PIK3CA mutation (P = 0.040), while number of metastatic sites ≥ 3 was significantly associated with ESR1 mutation (P = 0.022). No association with tumor burden was observed for different ACTB DNA fragments lengths. Met_ratio > 1.5 was significantly associated with lower number of metastatic lesions (P = 0.031). Number of metastatic sites ≥ 3, high ctDNA yield and CEA were associated with worse OS (respectively P = 0.0465, P = 0.0250 and P = 0.0474), while only CEA impacted on PFS (P = 0.0097). Conclusions: In pts with luminal-like MBC, some liquid biopsy-based biomarkers (i.e., ctDNA-detected ESR1 and PIK3CA mutations, ctDNA yield) were significantly associated with the burden of disease. The potential clinical validity and utility of these results deserve to be tested in an expansion cohort.

577 Background: It is matter of current debate which would be the best chemotherapy backbone of neoadjuvant HER2-targeted therapy for HER2+ BC. The TRAIN 2 trial showed no significant difference in terms of pathological complete response (pCR) when anthracyclines–based (CTA) or anthracyclines–free regimens (CT) were combined with dual HER2 blockade. However, it remains unclear how anthracyclines may influence the relative benefit across different anti-HER2 treatments. Methods: A systematic review was conducted which included all phase II/III randomized clinical trials (RCTs) comparing different neoadjuvant regimens for HER2+ BC. pCR (yT0/isN0) was the outcome of interest. Indirect comparisons of all combination of anti-HER2 agents with CTA or CT were estimated with a random-effects frequentist NMA. Estimated pCR rates were inferred adopting a Bayesian NMA. Results: 17 RCTs (3933 patients) were included. Overall, 8 arms were identified, comprising all possible combinations of CTA and CT with trastuzumab (H), lapatinib (L) and dual HER2 blockade (D) but also CTA and D only. Odds ratios (OR) for pCR and 95% confidence interval (CI) of selected NMA comparisons are shown in the table. Estimated rates of pCR for each treatment and 95% credible interval (CrI) are reported in the table. Conclusions: Through indirect comparisons, no significant pCR gain was found for CTA vs CT when combined to D, H and L. In particular, considering double vs single-agent anti-HER2 regimens, D-CT remains superior to H-CTA, supporting a possible omission of anthracyclines when dual anti-HER2 block is used. On the contrary, our pooled estimate suggests a more relevant role for anthracyclines when comparing H-CT/A vs CTA. Moreover, we estimated a 4% pCR gain for D-CTA vs D-CT, and an 8% higher pCR rate for H-CTA vs H-CT. [Table: see text]

5564 Background: Patients (pts) experiencing a PS rOC are generally re-exposed to a platinum based-chemotherapy (CT). In this setting, the addition of a targeted agent like bevacizumab (BEV) or PARP inhibitors (PARPi) as concomitant and/or maintenance therapy has shown to improve progression free survival (PFS). In the absence of direct comparison in randomized trials (RCTs), we have performed a NMA to evaluate differences in terms of efficacy between BEV and PARPi in pts with PS rOC, according to BRCA status. Methods: We searched PubMed, Embase and Medline for RCTs involving pts with PS rOC treated with BEV (n = 3, 1563 pts) or PARPi (n = 5, 1839 pts). Only trials with PFS as primary endpoint were included. Trials in first line setting were excluded. Analyses have been done pooling pts who had received PARPi in three groups, according to the available data on BRCA genes status: all comers (AC), BRCA mutated pts (BRCAm) and BRCA wild-type pts (BRCAwt). A frequentist approach has been used with R statistical software. To rank the effect size of treatments, surface under the cumulative ranking value (SUCRA) has been applied. Results: In AC pts, PARPi improved PFS compared to BEV (hazard ratio [HR] = 0.70, 95% CI 0.54-0.91, test of heterogeneity [I2] = 40.5%). In BRCAm pts the gain in PFS for PARPi was even higher compared to BEV (HR = 0.46, 95% CI 0.36-0.59, I2= 17.2%). In BRCAwt pts the benefit of PARPi over BEV was not statistically significant (HR = 0.87, 95% CI 0.63-1.20, I2= 35.7%) but PARPi had the highest likelihood of being ranked as the best treatment in terms of efficacy according to SUCRA (90% and 60%, respectively for PARPi and BEV). Hazard ratio for PFS between PARPi, BEV and CT in the three cohorts are reported in the table. Conclusions: According to indirect comparisons, PARPi performed the best for the treatment of PS rOC, especially in BRCAm pts who had not previously received PARPi. BEV could be still an option in BRCAwt pts. [Table: see text]