Abstract only 216 Background: 18 FFluciclovine PET is clinically approved for detection of biochemically recurrent prostate cancer (PCa). However, its use for initial staging and treatment monitoring remains uncertain. No published data exists evaluating the use of fluciclovine-PET for monitoring response to androgen deprivation therapy. Thus, the purpose of this study is to evaluate the potential use of fluciclovine-PET in monitoring response to androgen deprivation therapy (ADT) during initial treatment of patients with newly diagnosed high risk PCa. Methods: A prospective study enrolled patients with high-risk PCa who had no imaging evidence of metastatic disease. All patients underwent a pretreatment fluciclovine-PET/MRI for primary staging, 6 weeks of ADT, and had a follow-up PET/MRI. All exams were interpreted by 3 nuclear medicine physicians. Identification of the primary intraprostatic lesion and nodal metastatic disease was performed with mean and maximum SUV of the MRI-defined primary prostatic lesions and lymph node metastases measured before and after ADT. Results: A total of 10 patients were enrolled. Gleason scores of their primary tumor were 3+4 (n = 2), 4+3 (n=2), 8 (n = 3), and 9 (n = 3). The average pretreatment serum PSA value was 33.04 ng/mL (range 2.83–76.5). The average serum PSA 6 weeks following initiation of ADT was 2.13 ng/mL (range 0.29–5.66). The primary intraprostatic lesion was accurately identified in all 10 patients and 7/10 patients demonstrated suspicious lymph nodes on the pretreatment PET/MRI. Following ADT, all 10 patients demonstrated a decrease in tracer activity both within the primary intraprostatic lesion and suspicious lymph nodes. The primary lesion mean SUV prior to treatment was 4.46±1.14 and 2.37±1.07 following initiation of ADT (p=0.0007). The primary lesion maximum SUV prior to treatment was 7.13±1.70 and 3.54±2.04 following initiation of ADT (p=0.0006). Conclusions: Fluciclovine-PET tracer activity in patients with PCa undergoing ADT appears to correlate with decrease in serum PSA. Fluciclovine-PET imaging may be useful in monitoring response to ADT, particularly if there is a failure of appropriate PSA response. Clinical trial information: NCT03264456.