MP24-02 DEVELOPMENT OF AN IMAGING APPROACH TO DETECT SPLICE VARIANTS OF ANDROGEN RECEPTOR IN PROSTATE CANCER

Abstract

INTRODUCTION AND OBJECTIVES: Resistance to therapies that target the androgen receptor (AR) ligand binding domain (LBD) may be due to expression of constitutively active AR splice variants that lack the LBD. EPI-001/EPI-002 small molecules bind to AR amino-terminal domain (NTD).Wepropose todevelopananalogueof radiolabeled iodine 123(I) EPI-002 for imaging of prostate cancer using single-photon emission computed tomography (SPECT). Currently the approach to image AR uses positron emission tomography with 16b-[18F]-fluoro-5a dihydrotestosterone (F18-FDHT) that binds to AR LBD. F18-FDHT cannot detect AR splice variants lacking LBD. Our approach would employ sequential imaging with F18-FDHT to detect solely full-length (fl)AR and I-EPI-002 to detect NTD of both fl-AR and variant AR. A discordant distribution or discordant level of uptake between 18F-FDHT and I-EPI-002may indicate expression of splice variants lacking LBD. METHODS: Activity and specificity of non-radiolabeled iodine (I)-EPI-002 for inhibiting AR were confirmed in cells using reporter gene constructs for the AR and related steroid hormone receptors, glucocorticoid receptor (GR) and progesterone receptor (PR). Competitive ligand binding assays to detect the displacement of fluorescently labeled ligand from recombinant LBDs of AR, PR, GR and estrogen receptor (ER) by R1881, MDV3100, EPI-002 or I-EPI-002 were performed. EPI002 was successfully radiolabeled with the isotope I. On-going experiments include: binding experiments of I-EPI-002 to recombinant proteins AR activation function-1 and fl-AR, as well as to endogenous flAR and variant AR in cells; evaluation of I-EPI-002 in whole-body distribution including dosimetry in xenografts by small animal SPECT imaging; and evaluation of I-EPI-002 uptake and plasma clearance by quantifying I in harvested organs at various time points. Biological, physical and effective half-life of I-EPI-002 will be determined. RESULTS: In cell-based assays, I-EPI-002 was specific for AR and blocked the induction of AR-driven reporter activity in response to androgen, but did not inhibit either PR or GR transcriptional activities in response to ligand. I-EPI-002 does not bind to LBDs of AR, PR, GR and ER. Ongoing in vivo experiments will reveal the potential of I-EPI-002 for clinical application. CONCLUSIONS: An AR NTD-targeted molecular imaging probe such as I-EPI-002 may be useful to select patients for subsequent antiandrogen therapies, monitor treatment response, and provide insight into the role of all AR species in resistance mechanisms.