Longitudinal Molecular Imaging of Progesterone Receptor Reveals Early Differential Response to Endocrine Therapy in Breast Cancer with an Activating <i>ESR1</i> Mutation

Abstract

Activating mutations in the estrogen receptor (ER) α-gene (<i>ESR1</i>) result in constitutive transcriptional activity in the absence of estrogen and are associated with endocrine resistance in metastatic ER-positive (+) breast cancer. It is not known how activating <i>ESR1</i> mutations may alter the predictive values of molecular imaging agents for endocrine therapy response. This study investigated the effect of an activating <i>ESR1</i> mutation on pretreatment ¹⁸F-fluoroestradiol (¹⁸F-FES) uptake and early assessment of endocrine therapy response using ¹⁸F-FDG and ¹⁸F-fluorofuranylnorprogesterone (¹⁸F-FFNP) PET/CT imaging of tumor glucose metabolism and progesterone receptor (PR) expression, respectively. <b>Methods:</b> ER+, PR+ T47D breast cancer cells expressing wild-type (WT) ER or an activating <i>ESR1</i> mutation, Y537S-ER, were used to generate tumor xenografts in ovariectomized female immunodeficient mice supplemented with 17β-estradiol. Tumor growth curves were determined in the presence or absence of estrogen and for ethanol vehicle control or fulvestrant treatment, a selective ER degrader. Pretreatment ¹⁸F-FES uptake was compared between Y537S-ER and WT-ER tumors. Longitudinal PET/CT imaging with ¹⁸F-FFNP and ¹⁸F-FDG was performed before and 7-9 d after the start of endocrine therapy with fulvestrant. Radiopharmaceutical uptake in Y537S-ER and WT-ER tumors was compared between baseline and follow-up scans. Statistical significance was determined using paired <i>t</i> testing for longitudinal imaging and 2-way ANOVA for the ¹⁸F-FFNP tissue biodistribution assay. <b>Results:</b> Y537S-ER xenografts showed estrogen-independent growth, whereas WT-ER tumors grew only with estrogen. Fulvestrant treatment for 28 d significantly reduced tumor volumes for WT-ER but only stabilized volumes for Y537S-ER. Baseline ¹⁸F-FES uptake did not significantly differ between WT-ER and Y537S-ER tumors. Fulvestrant treatment induced a similar early metabolic response for both WT-ER and Y537S-ER tumors. ¹⁸F-FFNP uptake in WT-ER tumors was significantly reduced after 7 d of fulvestrant treatment; however, this reduction did not occur in Y537S-ER tumors, which showed no significant change between baseline and follow-up PET/CT. <b>Conclusion:</b> Molecular imaging of PR expression dynamics could be a noninvasive approach for early identification of reduced effectiveness of endocrine therapy resulting from activating <i>ESR1</i> mutations.