Purpose: The 2018 American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guideline on HER2 testing in breast cancer permits reclassification of cases with HER2-equivocal results by FISH. The impact of such reclassification is unclear. We sought to determine the proportion of HER2-equivocal cases that are reclassified as HER2-negative and the impact of anti-HER2 therapy on survival in HER2-equivocal cases. Methods: We reviewed medical records of breast cancer patients who had HER2 testing by fluorescence in stitu hybridization (FISH) and immunohistochemistry (IHC) performed or verified at The University of Texas MD Anderson Cancer Center during April 2014 through March 2018 and had equivocal results according to the 2013 ASCO/CAP guideline. The population was divided into 2 cohorts according to whether the biopsy specimen analyzed came from primary or from recurrent or metastatic disease. HER2 status was reclassified according to the 2018 ASCO/CAP guideline. Overall survival (OS) and event-free survival (EFS) were calculated using the Kaplan-Meier method, and the relationship between anti-HER2 therapy and clinical outcomes was assessed. Results: We identified 139 cases with HER2-equivocal results according to the 2013 ASCO/CAP guideline: 90 cases of primary disease and 49 cases of recurrent/metastatic disease. Per the 2018 ASCO/CAP guideline, these cases were classified as follows: overall, HER2-negative 112 cases (80%), HER2-positive 1 (1%), and unknown 26 (19%); primary cohort, HER2-negative 85 (94%), HER2-positive 1 (1%), unknown 4 (4%); and recurrent/metastatic, HER2-negative 27 (55%) and unknown 22 (45%). Five patients in the primary-disease cohort and 1 patient in the recurrent/metastatic-disease cohort received anti-HER2 therapy. There was no significant association between anti-HER2 therapy and OS or EFS in either cohort (primary disease: OS, p = 0.67; EFS, p = 0.49; recurrent/metastatic-disease, OS, p = 0.61; EFS, p = 0.78. Conclusions: The majority of HER2-equivocal breast cancer cases were reclassified as HER2-negative per the 2018 ASCO/CAP guideline. No association between anti-HER2 therapy and OS or EFS was observed. HER2-equivocal cases seem to have clinical behavior similar to that of HER2-negative breast cancers.

The interpretation of human epidermal growth factor receptor 2 (HER2) fluorescence in situ hybridization (FISH) results may be challenging in tumors with polysomy 17, which is defined as increased signals of chromosome enumeration probe 17 (CEP17). The effect of polysomy 17 on HER2 protein expression and tumor treatment response has not been established. In this retrospective study, we investigated the clinicopathological features of breast cancer with polysomy 17 and determined the tumors’ response to neoadjuvant chemotherapy (NACT). The study included 366 patients with primary breast cancer whose tumors had a CEP17 count of ≥ three/nucleus based on HER2 FISH studies. These cases were categorized according to HER2/CEP17 ratio and HER2 signals/nucleus using the American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines. We compared the clinicopathological characteristics and tumor response to NACT among different groups. There was a statistically significant difference in patients’ age at diagnosis, tumor pathological grade, estrogen and progesterone receptor status, and NACT response among different HER2 FISH groups. Polysomy 17 tumors in group 1 had a higher rate of response (pathological complete response and residual cancer burden class I) to NACT containing anti-HER2 reagent than did those in other groups (p = 0.004), whereas polysomy 17 tumors in group 3 did not show a significant response to anti-HER2 treatment. Polysomy 17 tumors in different HER2 FISH groups have different pathological features and respond to NACT differently. These results may help us identify patients who will benefit from anti-HER2 therapy.

595 Background: Androgen-receptor-like (LAR) triple-negative breast cancer (TNBC) is a subtype identified using Vanderbilt’s molecular signature. LAR subtype has the lowest pCR rate for NACT among all TNBC subtypes (10% vs. 28% for TNBC in general). We launched a clinical trial to determine the effectiveness of enzalutamide and paclitaxel (ZT) in improving this poor chemo. response in the neoadjuvant setting for pts with anthracycline-refractory, androgen receptor (AR)+ TNBC (NCT02689427). However, we do not yet have a robust predictive biomarker to detect an activated AR pathway and have not seen a robust correlation between molecular LAR subtype and AR IHC staining intensity. Methods: Molecular profiling and immunohistochemical analysis of key biomarkers (LAR, Ki67, and vimentin) was performed for all pts enrolled in A Randomized triple negative breast cancer enrolling Trial to Confirm Molecular Profiling Improves Survival (ARTEMIS; NCT02276443). Patients receive 4 cycles of AC, followed by an experimental arm or standard taxane, tailored using nuclear IHC staining. IHC staining of ≥30% AR+ was used as a threshold for selection for enzalutamide combination arm. We evaluated the concordance between LAR-subtype using molecular profiling vs % AR+ cells via IHC. Results: As part of the clinical trial, tumors with ≥30% AR+ cells were classified as LAR. In addition, we used RNA profiling to assign Vanderbilt subtype scores, resulting in classification of 15 tumors as LAR+. We observed a significant correlation (r=0.75) between LAR score and %AR+ cells, with 13 of 15 LAR tumors having ≥30% AR+ cells. Among patients with high % of AR+ tumor cells, 11 received enzalutamide, with 43% (3/7) having responses (pCR or RCB-I). Conclusions: Comparison on numerical scores for Vanderbilt subtype and IHC scores suggests ≥30% AR+ IHC staining as the threshold (ppv=0.65, npv=0.98, Table) to identify the molecular LAR subtype. We observed a trend where response rate was higher in patients with ≥ AR+ IHC scores treated with enzalutamide; however, these results need confirmation in a larger cohort of patients. Clinical trial information: NCT02689427, NCT02276443. [Table: see text]

Triple-negative Breast cancer is a highly heterogeneous tumor subtype lacking universal therapeutic targets, a factor that frequently renders standard pharmacological approaches ineffective. To address the urgent clinical need for precision strategies, we developed a computational framework centered on personalization. Utilizing specific gene expression data, we initially constructed and calibrated 41 individualized regulatory networks, thereby capturing the precise molecular signature of each patient. Then, we applied a hybrid approach to these models—integrating Boolean logical determinism with the exploratory optimization of the combinatorial search space via a Genetic Algorithm—and identified the minimal sets of driver nodes capable of irreversibly driving the tumor cell toward apoptosis. Subsequently, a rigorous optimization process translated these targets into actionable clinical options by mapping them to optimal combinations of oncological drugs under stringent therapeutic constraints. The apoptotic transition was successfully achieved in 92.7% of the samples. Results indicate that effective tumor reprogramming relies on manipulating critical nodes that orchestrate the network's logical dynamics. By minimizing unnecessary perturbations to the overall signaling architecture, this methodology demonstrates the computational feasibility of identifying robust, non-obvious therapeutic targets through Boolean modeling. Ultimately, it offers an adaptive and scalable platform for the rational design of minimally disruptive, patient-specific combinatorial therapies.

E14729 Background: The best targeted therapeutic approach for HER2-equivocal cases remains unclear. New ASCO/CAP HER2 guidelines are intended to decrease this ambiguity by combining immunohistochemistry and in situ hybridization to resolve equivocal cases as positive or negative. However, the benefit of anti-HER2 therapy in HER2-equivocal cases is unknown. Methods: We retrospectively reviewed patients who visited MD Anderson from April 2017 to March 2018 with equivocal HER2 results based on the 2013 ASCO/CAP guidelines. The population was divided into 2 cohorts according to biopsy origin (primary cohort: biopsy from breast or axilla; recurrent/metastatic cohort: biopsy from recurrent or metastatic site). HER2 status was redefined using the 2018 ASCO/CAP guidelines. OS and PFS were calculated (Kaplan-Meier method) based on redefined HER2 status and use of HER2 targeted therapy. Results: A total of 139 equivocal results were found. Primary cohort had 90 patients (33 received neoadjuvant and 57 adjuvant therapy). HER2 IHC results were 0 (6.6%), 1+ (37.7%), 2+ (50%), 3+ (1.1%), and no IHC (4.4%). 94% of HER2-equivocal results became HER2 negative. Only 5 patients received anti-HER2 therapy, all of them in the HER2-negative group. After median follow-up of 1.91 yrs, 3 deaths and 8 progressions had occurred. There was no statistically significant association between anti-HER2 therapy and OS (p = 0.67) or PFS (p = 0.49). The recurrence/metastatic cohort had 49 cases with equivocal results. HER2 IHC results were 0 (6.1%), 1+ (22.4%), 2+ (26.5%), and no IHC (44.9%). 55% of HER2-equivocal results became HER2 negative, and only 1 patient received anti-HER2 therapy. After median follow-up of 2.96 yrs, 15 deaths and 35 progressions had occurred. There was no statistically significant association between anti-HER2 therapy and OS (p = 0.61) or PFS (p = 0.78). Conclusions: Most HER2-equivocal results were redefined as HER2 negative using the new ASCO/CAP guidelines. Association between anti-HER2 therapy and OS or PFS according to the new HER2 status was not observed. Although this is a small sample with short follow-up, patients with HER2-equivocal breast cancers seem to have clinical behavior similar to HER2-negative breast cancer.