Characterization of diverse targetable ERBB2 alterations in 512,993 patients with solid tumors.

3129 Background: Trastuzumab deruxtecan (T-DXd) is an antibody-drug conjugate (ADC) that has demonstrated activity against a range of HER2-expressing and HER2-mutant (mut) solid tumors. We present an extensive landscape of ERBB2 alterations (alt) that may predict sensitivity to HER2-targeted therapies in development with a focus on mutations and cancer types without approved indications. Methods: We queried institutional databases (MSKCC and Foundation Medicine, Inc.) of comprehensive genomic profiling (CGP) performed on solid tumor tissue samples using either MSK-IMPACT (N=83,332; MSK) or FoundationOne/FoundationOne CDx (N=429,661; FMI) and on patient-matched liquid biopsies using FoundationOne Liquid CDx (N=1,922). ctDNA tumor fraction (ctDNA TF) on FoundationOne Liquid CDx was estimated using a composite algorithm. Clinicopathological characteristics and treatment outcomes were annotated for MSK-IMPACT samples. Results: ERBB2 activating alt were detected in 6.6% (n=28,508) and 5.0% (n=4,133) of tumors in the FMI and MSK cohorts, respectively. In FMI tissue samples, ERBB2 alt were most prevalent in gastroesophageal (GEC; 18.1%), bladder (18.0%), salivary gland (14.1%), breast (12.7%), and uterine (12.1%) cancers. The cancer types with the largest number of ERBB2mut tumors, specifically, included NSCLC (n=1,618, 1.9%), breast (n=1,565, 3.5%), colorectal (CRC; n=1,221, 2.3%), bladder (n=855, 8.8%), and GEC (n=660, 3.4%). Across these 5 cancers, ERBB2 mut were most commonly located in the kinase domain (KD; 58%), followed by the extracellular domain (ECD; 28%) and transmembrane domain (TMD; 2%). Rare splice site mut and in-frame deletions resulting in exon 16 loss (Ex16Alt) were also observed (n=76, <0.1%). ERBB2 mut were found to be mutually exclusive with other RTK/MAPK driver oncogenes including: EGFR, KRAS, ALK, MET, BRAF, and RET in NSCLC; FGFR1 in breast cancer; BRAF in CRC; and FGFR3 in bladder cancer. In patient-matched liquid biopsy samples, sensitivity for detecting ERBB2 amplifications (amp; 33.3%, 43/129) was lower than for detecting ERBB2 mut (72.3%, 47/65) across select cancers. However, sensitivity for detection of both alts was improved in liquid samples with elevated ctDNA TF (≥1%), increasing to 52.6% (40/76) for amp and 97.3% (36/37) for mut. In the MSK cohort, 110 of 986 (11.2%) patients with ERBB2 mut non-NSCLC tumors received one or more HER2-targeted therapies, including ADCs. Clinical responses to HER2-targeted therapies were observed in patients harboring KD mut (n=10), ECD mut (n=6), TMD mut (V659E, n=1), and in 1 patient with a V697L mut. Conclusions: CGP detects diverse ERBB2activating alt at a high incidence in solid tumor types beyond NSCLC. These represent patient populations who may benefit from HER2-directed therapies. Trials of novel HER2-targeted agents are necessary to refine our understanding of the biological dependency of HER2 alterations.