Diab, Sami

3134 Background: Mucinous breast cancer (MuBC) is a rare subtype of invasive ductal carcinoma (IDC) that accounts for less than 2% of all breast cancers and is associated with a favorable prognosis. Since MuBCs are rare in clinical trials, current treatment guidelines are extrapolated from IDC-no special type (IDC-NST). To provide better understanding of MuBCs and factors contributing to their clinical behavior, we examined the transcriptomic profiles of MuBCs in our FLEX study. Methods: The prospective, observational FLEX Study (NCT03053193) includes stage I-III breast cancer patients who receive MammaPrint (MP)/BluePrint (BP) testing and consent to full transcriptome and clinical data collection. For this study, histologically confirmed MuBCs (n = 102) in the FLEX database were included. All patients examined were ER+/HER2- by immunohistochemistry and Luminal by BP. MuBC was compared with IDC matched for Age, MP, and BP index (n = 97). Differential gene expression analyses (DGEA) were performed with R package ‘limma’ and differentially expressed genes (DEGs) were considered significant if they had an adjusted p < 0.05 and fold change ≥ 2. Results: DGEA comparing MuBC (n = 102) with IDC (n = 97) revealed 60 DEGs, regardless of the genomic risk, of which 42 genes were upregulated and 18 were downregulated in MuBC relative to IDC. Genes associated with MuBC, such as MUC2, TFF1, CARTPT were among the upregulated genes. Of the 102 MuBC patients, 56 were Luminal A (MP Low Risk-LR) and 46 were Luminal B (MP High Risk-HR) by MammaPrint and BluePrint. Comparison of LR MuBC with LR IDC revealed 111 DEGs. Functional enrichment showed upregulation of pathways involved in estrogen response (early & late) and androgen response and a downregulation of the epithelial to mesenchymal transition (EMT) and E2F pathways in LR MuBC compared to LR IDC. DGEA between HR MuBC and HR IDC revealed only 22 DEGs with immune pathways being downregulated in HR MuBC. DGEA comparing LR MuBC with HR MuBC resulted in 63 DEGs, indicating LR and HR MuBC are biologically distinct types. Interestingly in LR MuBC, the tumor suppressor marker SCUBE2 is upregulated. Over expression of SCUBE2 is associated with better prognosis. Conclusions: Although MuBCs are often expected to have low clinical risk, MP revealed that half of the MuBCs examined in this study were MP High Risk (Luminal B). MP low risk MuBC is biologically different from MP low risk IDC, and downregulation of E2F and EMT pathways might lead to favorable prognoses in MP low risk MuBC. MP high risk MuBC showed limited DEGs compared to high-risk IDCs indicating these tumor types are highly genomically similar and likely to benefit from chemotherapy. The downregulation of immune pathways in MP high risk MuBC may lead to immune surveillance escape resulting in metastasis and further investigation is needed. Clinical trial information: NCT03053193.

Abstract Background LIV-1, a transmembrane protein and downstream target of STAT3, is highly expressed in breast cancer cells. It is associated with lymph node involvement and metastatic progression. SGN-LIV1A is an anti-LIV-1 antibody conjugated via a protease-cleavable linker to monomethyl auristatin E (MMAE). Upon binding to cell-surface LIV-1, SGN-LIV1A is internalized and releases MMAE, which disrupts microtubulin and induces apoptosis. Methods This ongoing, phase 1 study evaluates safety, tolerability, pharmacokinetics, and antitumor activity of SGN-LIV1A (q3wks IV) in women with LIV-1-positive, unresectable, locally advanced or metastatic breast cancer (LA/MBC) (NCT01969643). Patients (pts) with measurable disease and ≥2 prior cytotoxic regimens for LA/MBC are eligible. Pts with ≥ Grade 2 neuropathy are excluded. Response is assessed per RECIST v1.1; pts with stable disease (SD) or better can continue treatment until disease progression or intolerable toxicity. At completion of dose escalation in hormone receptor-positive/HER2-negative (HR+/HER2–) and triple-negative (TN) pts, expansion cohorts were opened to further evaluate safety and antitumor activity of monotherapy in TN pts. Tumor biopsies are evaluated for LIV-1 expression. Results To date, 69 pts (18 HR+/HER2–, 51 TN) have received a median of 3 cycles (range, 1–12) of SGN-LIV1A at doses of 0.5–2.8 mg/kg. Median age was 56 yrs. Pts had a median of 3 prior cytotoxic regimens for LA/MBC; 58 had visceral disease and 37 had bone metastases. No dose-limiting toxicities (DLTs) occurred in 19 DLT-evaluable pts; maximum tolerated dose was not exceeded at 2.8 mg/kg. Expansion cohorts of TN pts were opened at 2.0 and 2.5 mg/kg. Treatment-emergent adverse events (AEs) reported in ≥25% of pts were fatigue (59%), nausea (51%), peripheral neuropathy (44%), alopecia (36%), decreased appetite (33%), constipation (30%), abdominal pain, diarrhea, and neutropenia (25% each). Most AEs were Grade 1/2; AEs ≥ Grade 3 included neutropenia (25%) and anemia (15%). Febrile neutropenia occurred in 2 pts whose total dose exceeded 200 mg per cycle, including 1 treatment-related death due to sepsis. No other treatment-related deaths occurred on-study. Seven pts discontinued treatment due to AEs. In dose escalation, activity was observed in 17 efficacy evaluable (EE) HR+/HER2- pts, with a disease control rate (DCR= CR+PR+SD) of 59% (10 SD), including 1 pt with SD ≥24 wks. Among the 44 EE TN pts (dose escalation plus expansion cohorts), the objective response rate (ORR) was 32% (14 PR) with a confirmed PR rate of 21%, DCR was 64% (14 PR, 14 SD), and clinical benefit rate (CBR=CR+PR+SD ≥24 wks) was 36% (16 pts). For TN pts, median PFS was 11.3 wks (95% CI: 6.1, 17.1); 10 pts remain on treatment. Of 631 MBC tumor samples of all clinical subtypes evaluated for LIV-1, 91% were positive; 75% had moderate-to-high expression (H-score ≥100). Conclusions LIV-1 is expressed in almost all MBC tumors. SGN-LIV1A monotherapy was generally well tolerated and showed encouraging antitumor activity in heavily pretreated TN MBC, with a PR rate of 32%, confirmed PR rate of 21%, and CBR (≥24 wks) of 36%. Response duration data continue to evolve. Enrollment continues in the TN monotherapy expansion cohort. Citation Format: Modi S, Pusztai L, Forero A, Mita M, Miller KD, Weise A, Krop I, Burris III H, Kalinsky K, Tsai M, Liu MC, Hurvitz SA, Wilks S, Ademuyiwa F, Diab S, Han HS, Kato G, Nanda R, O'Shaughnessy J, Kostic A, Li M, Specht J. Phase 1 study of the antibody-drug conjugate SGN-LIV1A in patients with heavily pretreated triple-negative metastatic breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr PD3-14.