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12100 Background: Rash associated with increased peripheral eosinophils develops in approximately 50% of metastatic breast cancer patients receiving alpelisib. Antihistamines and corticosteroids have limited benefit. Refractory rash may lead to decreased dose intensity and affect clinical outcome. Benralizumab is an anti-IL-5Rα chimeric monoclonal antibody that depletes peripheral eosinophils and has demonstrated benefit in eosinophilic asthma and hypereosinophilic syndrome. We investigate the efficacy and safety of benralizumab for the treatment of alpelisib rash. Methods: We performed a single-center, single-arm, prospective phase 2 study to evaluate the efficacy and safety of benralizumab in cancer patients who developed CTCAE grade 2/3 skin events resulting from immunotherapy or targeted therapies with absolute blood eosinophil counts of ≥300/mcl. While remaining on culprit drugs, patients were treated with benralizumab 30mg once every 4 weeks for the first 3 doses followed by once every 8 weeks for 3 additional doses (approved dosing for eosinophilic asthma). Primary endpoint was clinical response measured as reduction in CTCAE grade 2/3 skin event to grade ≤1 by week 4. Secondary endpoints were patient quality of life (QoL) measured by skindex16, safety data, need for supportive oral corticosteroids, and changes in cytokines and eosinophil biomarkers. This interim analysis focuses on patients with PIK3CA-mutant metastatic breast cancer receiving alpelisib. Results: Between September 16th 2020 and January 1st 2022, we enrolled 10 metastatic breast cancer patients with grade 2/3 rash attributed to alpelisib (5 pts with G3). All patients had a reduction of rash to grade ≤1 (n = 10, p < 0.0001), and a decrease in peripheral absolute eosinophils (mean 500/mcl to 0, p < 0.0001). Of these, 6 patients had been on prophylactic oral antihistamines and 2 had oral steroid coadministration. QoL significantly improved (Skindex16 mean score 58 to 16, p = 0.0001) and eosinophils in skin histology decreased per HPF (mean 6.25 to 0.25, n = 8, p = 0.2) by week 4. An increase in IL-5 > 600% and reduction IL-6 and TNF-α > 50% were reported by week 4 and 8. Grade 1/2 mucositis in 4 patients were reported as adverse events. Conclusions: Our findings suggest that benralizumab is safe and effective for the treatment of grade 2/3 rash with eosinophilia related to alpelisib in patients with breast cancer. A reduction in rash severity was evidenced in all patients, along with improved QoL. Larger controlled studies are in development to evaluate the efficacy of benralizumab for the prevention of alpelisib rash. Clinical trial information: NCT04552288.

e15016 Background: Neratinib (N) is a potent irreversible inhibitor of HER1, HER2, and HER4. Capecitabine (X) at optimal dose of 7 days on and 7 days off schedule (7/7) is well-tolerated with low rates of ≥ grade (G3) diarrhea. Methods: We conducted phase Ib/II study of N with X (7/7) with loperamide and colestipol prophylaxis in patients (pts) with pretreated HER2+ MBC (NCT03377387). Eligible pts had normal left ventricular ejection fraction (≥ 50%), any and < 4 prior chemotherapy-based treatments in phase Ib and II, respectively. Primary endpoint of phase Ib is maximum tolerated dose (MTD) and phase II is response rate. Secondary endpoints are safety, tolerability, and progression-free survival. Exploratory endpoint is to quantify cell-free DNA (cfDNA), correlating with response. Phase Ib follows traditional 3+3 design with 4 dose levels. In phase II, if > 3/9 respond, study is expanded to 24. If > 10/24 respond, study is deemed successful. Results: As of 2-4-2021, 10 pts were enrolled in phase Ib. 4 pts were treated at dose level 1 (X at 1500 mg BID at 7/7 and N at 240 mg daily); 2/4 pts experienced with G3 diarrhea during cycle 1. Six pts were treated at dose level -1 (X at 1000 mg BID 7/7 and N at 240 mg daily); 1 (17%) developed G3 diarrhea. The MTD is X at 1000 mg BID 7/7 and N at 240 mg daily. Twenty-two of 24 pts have been enrolled in phase II. Of 22 pts, data show 6 with partial response, 8 with stable disease, 3 with progressive disease, and 5 have not been assessed radiographically. Overall, 6/22 (27.3%) and 1/22 (4.5%) had all G and G3 diarrhea, respectively. Other significant toxicities at MTD included G2 hand foot syndrome (n = 1, 4.5%), G1 fatigue (n = 1, 4.5%) and G1 nausea (n = 4, 18%). Conclusions: The MTD is X at 1000 mg BID at 7/7 and N at 240 mg daily. This combination is safe and well tolerated with G3 diarrhea rate of 4.5%, which is significantly lower than the X 14/7 schedule in NALA study. The phase II portion of the study is near completion and updated result will be presented. Analysis of cfDNA, to correlate with response for phase II portion, is ongoing. Clinical trial information: NCT03377387.

1017 Background: Genome-wide transcriptional analysis has identified a unique subset of androgen receptor (AR) +, estrogen receptor (ER)/progesterone receptor (PR)- breast cancer (BC). The functional role of AR was confirmed initially in preclinical models demonstrating that androgen-driven growth could be abrogated by antiandrogen therapy. TBCRC011 established the safety and efficacy of inhibiting AR with bicalutamide (B) in patients (pts) with AR+/ER/PR- metastatic BC (MBC) with a median progression free survival (PFS) of 12 weeks (wks) (95% CI, 11–22 wks). In preclinical data, palbociclib (P) has been shown to reduce growth of AR+/ER/PR- MDA-MB-453 BC cells. It has been shown that AR+ triple negative BC (TNBC) expresses a luminal profile and has intact Rb protein, the target of P activity. We conducted this Phase I/II trial of the AR inhibitor B in combination with the CDK4/6 inhibitor P in pts with AR+/ER/PR/HER2- BC (NCT02605486) to test the hypothesis that androgen blockade, paired with CDK4/6 inhibition would have increased efficacy in pts with androgen-dependent BC. Methods: Postmenopausal pts with AR+ TN MBC defined as IHC ≥ 1% nuclear staining (DAKO, Clone AR441 (5/2016-11/2016) then Ventana AR SP107 (11/2016-6/2018), ECOG ≤2, measurable/non-measurable disease were eligible for enrollment. Any number of prior regimens was permitted. Pts received B 150 mg daily and P 125 mg daily 3 wks on 1 wk off. Pts were evaluated for toxicity every 2-4 wks and for response every 8-12 wks. Primary endpoint: 6 month (mo) PFS. Secondary endpoints: clinical benefit rate, toxicity, correlative studies to better characterize AR+ TNBC. A Simon 2-stage minimax design that discriminates between 6 mo PFS rates of 20% and 40% was used. If ≥ 11/33 pts were PF at 6 mo then B+P would warrant further study. Results: As of 1.1.20 33 pts were enrolled on study with median (med) age 67 (42-79), performance status 0 (0-1). Number of pts with visceral metastases: 20, measurable disease: 22. AR% 1-9: 3, 10-50: 6; 51-100: 24. Med prior lines for MBC: 3 (0-9). Best response: (31 evaluable pts): 11 pts PF at 6mo: 10 SD > 6mo, 1 PR. Med wks on study: 14 (2-101). Toxicity > 10% grade >3 related: Number of pts with leukopenia: 21, neutropenia: 21, lymphocytopenia: 6, thrombocytopenia: 3. One pt with febrile neutropenia. One death due to disease progression within 30 days off study. Conclusions: In this selected subset of pts with AR+ TN MBC, this study met its prespecified endpoint with 11 pts PF at 6 mo on B 150 mg + P 125 mg. B+P has been well tolerated with no unexpected toxicity observed. Clinical trial information: NCT02605486 .

2582 Background: Veliparib (V) is a potent PARP inhibitor that delays repair of DNA damage induced by chemotherapeutic agents. In metastatic breast cancer pts, we evaluated V with doxorubicin (A) and cyclophosphamide (C) given both on day 1 with V at 100 mg po BID for 7 days post-chemotherapy every 21 days, to increase DNA damage. We report use of γH2AX, (phosphorylated histone protein), a marker of DNA double-strand breaks, in circulating tumor cells (CTCs) to assess DNA damage. We evaluated number of CTCs and percentage of γH2AX-positive CTCs pre- and post-treatment. Methods: Eligibility included prior A ≤ 300 mg/m2 and EF ≥ 50%. Further A was omitted after a cumulative dose of 420 mg/m2. Primary objective was to assess DNA damage response to treatment by measuring γH2AX-positive CTCs during cycle 1 on days 1 (pre-treatment), 2, 7, and 14. Cell Search System was used to enumerate CTCs. γH2AX was quantitated using a validated assay. Results: Eleven pts enrolled. Median age was 53 (34 – 73); median ECOG PS 1 (0 – 2); there were 1 ER-negative/HER2+, 4 triple-negative (BRCA2+, n =1), and 6 ER+/PR+/HER2-negative (BRCA2+, n =2) tumors. Most common drug-related toxicities were grade (gr) 4 neutropenia, gr 2 anemia and thrombocytopenia, and gr 1 nausea and vomiting. In BRCA2+ pts, there were 2 PRs and 1 SD. In BRCA wt or unknown status, 5 pts had SD ≥ 3 mo and there were 3 PDs. CTCs (≥ 8) were detected in 10/11 pts on days 1 and 2. Day 7 samples were not obtainable in 2 pts. On day 7, 1/8 pts had 0 CTCs and rest had ≥ 3 CTCs. A decrease in CTCs (p < 0.0001) occurred from day 1 (median: 22, range, 8-1216) to day 7 (median: 5, range 3-37). At baseline, 7 pts had ≥ 10% γH2AX-positive CTCs. Fraction of CTCs positive for γH2AX increased to ≥ 50% by day 7 in 6/7 pts and persisted to day 14 in 5 pts. Conclusions: The toxicity profile of V 100 mg BID days 1-7 with AC (60/600 mg/m2) on day 1 on a 21-day cycle was expected. Objective antitumor activity was seen in BRCA mutation carriers. CTCs decreased and percentage of γH2AX-positive CTCs increased after combination treatment with a PARP inhibitor and chemotherapy. This observation is notable and we plan to extend dosing of V to 14 days. This work is supported by NCI U01-CA132194. Clinical trial information: NCT00740805.

1091 Background: Gemcitabine (GEM) as a standard 30-min infusion has activity as a single-agent in metastatic breast cancer (mbc). Prolonged infusion of GEM at a fixed dose rate (FDR) of 10 mg/m2/min is associated with greater formation of active metabolite and may result in improved antitumor activity. Imatinib mesylate (IM)-mediated inhibition of PDGFR reduces tumor interstitial fluid pressure allowing for improved chemotherapy penetration into tumors. We evaluated the addition of IM to FDR GEM in patients (pts) with previously treated mbc. Methods: This was a randomized phase II trial in mbc pts who progressed after 1 but no more than 2 prior cytotoxic regimens. Eligibility included measurable disease; no prior GEM or IM exposure. Group 1 received FDR GEM IV 1250 mg/m2 at 10 mg/m2/min (over 120 min) on days 3 and 10 every 21 days; Group 2 received the same FDR GEM dose and schedule plus IM 400 mg orally daily on days 1-5 and 8-12 every 21 days. Primary endpoint was time to progression (TTP). Sample size of 40 pts per group was needed to detect an 8 mo increase in TTP with the combination (80% power, α = .05, 2-sided). Secondary endpoints included ORR and safety. Results: Between 5/2006-4/2011, 44 pts were randomized (22 per group). Study closed early due to slow accrual. Median age 54 (31-75); median ECOG PS 1 (0-2); 52% were hormone receptor-positive and HER2-negative; 27% triple-negative, and 20% HER2-positive. Median number of cycles was 2 in Group 1 (range 1-12) and 3.5 in Group 2 (range 1-13). Most common adverse events (%) of any grade, Group 1 vs Group 2 were neutropenia (68 vs. 78), anemia (48 vs. 65), thrombocytopenia (24 vs. 47), nausea (40 vs. 52) and vomiting (24 vs. 21). One gr 4 thrombotic thrombocytopenic purpura occurred in Group 2. Median TTP were 2 mo (95% CI: 1-5 mo) in Group 1 and 2.5 mo (95% CI: 1-5 mo) in Group 2 (p = 0.3 log rank test). ORR was 9.1% in each group (95% CI: 1.6-30.6%), with 2 PRs in each group. Stable disease (≥ 3 mo) was 32% in Group 1 and 41% in Group 2. Conclusions: This study was underpowered to draw any conclusion regarding a difference in TTP between the two groups at the time it was prematurely closed. Combination of IM and FDR GEM showed a trend to increased toxicity compared to FDR GEM alone. Clinical trial information: NCT00323063.