Development of an 18F-labeled glutamine PET radiotracer to detect serine synthesis pathway activity in HER2-positive breast cancer.

e15070 Background: Serine synthesis pathway (SSP) enzymes PSAT1 and PHGDH are highly upregulated in TP53-mutant breast cancer (BC), particularly basal-type triple-negative disease (TNBC). However, HER2-positive BC also frequently harbors TP53 mutations and is known to exhibit glutamine dependence. Since glutamine is imported into cancer cells and converted to glutamate, which serves as a substrate for PSAT1, glutamine dependence implies SSP activity. We are developing the PET radiotracer 18Ffluoroglutamine (FG) to identify tumors with elevated SSP activity and, ultimately, patients who may benefit from SSP inhibition. While our initial hypothesis focused on TP53-mutant TNBC as 18FFG-avid and TP53–wild-type luminal BC as non-avid, we now propose extending this work to HER2-positive BC to evaluate whether 18FFG avidity correlates with TP53 status and/or other molecular features, including ER and PIK3CA status. Methods: Public datasets (METABRIC, TCGA, SCAN-B, GTEx) were interrogated using Breast Cancer Gene-Expression Miner v5.2, muTarget, and GEPIA to examine associations between SSP gene expression and TP53 mutation status. We propose in vitro studies and in vivo xenograft models using HER2-amplified BC cell lines stratified by TP53 mutation status and SSP activity. In parallel, we propose development and validation of cGMP-grade 18FFG for future clinical PET studies in patients with HER2-positive BC stratified by TP53 status, ER status, and other molecular markers of cancer aggressiveness. Results: In silico analyses demonstrated upregulation of PSAT1 (2.64-fold) and PHGDH (2.37-fold) in TP53-mutant versus TP53–wild-type BC. In contrast, ESR1, GATA3, and FOXA1 expression was reduced (5.4-, 3.3-, and 3.1-fold, respectively). GATA3-mutant tumors, largely ER-positive, showed increased ESR1, GATA3, and FOXA1 expression with corresponding downregulation of PHGDH and PSAT1. PSAT1 represented the largest gene-expression difference between TNBC and non-TNBC. Development of cGMP-grade 18FFG is underway and is expected to require approximately 12 months for regulatory approval. In the interim, laboratory studies using research-grade tracers are ongoing to assess whether HER2-associated glutamine dependence reflects SSP activity and its relationship to TP53 status. Conclusions: HER2-positive BC may exhibit elevated SSP activity, potentially driven by glutamine dependence and frequent TP53 mutations. 18FFG PET imaging represents a promising strategy to identify SSP-active HER2-positive tumors and to guide future clinical evaluation of SSP-targeted therapies.