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

Abstract

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 [18F]fluoroglutamine (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 [18F]FG-avid and TP53–wild-type luminal BC as non-avid, we now propose extending this work to HER2-positive BC to evaluate whether [18F]FG 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 [18F]FG 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 [18F]FG 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. [18F]FG PET imaging represents a promising strategy to identify SSP-active HER2-positive tumors and to guide future clinical evaluation of SSP-targeted therapies.