Cancer Care Team
Authors: A. Gulland
Journal: BMJ (2014)

<b>733</b> <h3><b>Objectives:</b></h3> Prostate specific membrane antigen (PSMA) is increasingly utilized in prostate cancer imaging. Recently, we developed a novel fluorine-18 labeled PSMA PET imaging agent, CTT1057, based on a phosphoramidate core that targets an extracellular region of the PSMA with high affinity, pronounced cellular internalization and possesses excellent imaging characteristics as seen in preclinical models. Herein, we present our preliminary experience in comparison to gallium-68 labeled PSMA-11, a urea-based agent. <h3><b>Methods:</b></h3> Five patients (average age: 66 ±19.2 years, range 35 to 85) with metastatic prostate cancer were imaged after intravenous administration of 10 mCi (370 MBq) CTT1057 after one-hour uptake time. All studies were performed on a SIGNA PET/MR (GE Healthcare, Waukesha, WI). The list mode data of 3 minutes per bed position were used for reconstruction to produce a comparable image quality to that of <sup>68</sup>Ga-PSMA-11. In addition, 1.5 minutes per bed data reconstruction was performed to simulate a 50% dose reduction. For comparison, ten randomly selected, consecutive patients (average age: 68.5 ± 4.5 years, range 60 to 76) with biochemical recurrent prostate cancer were retrospectively identified, who underwent <sup>68</sup>Ga-PSMA-11 PET. Scans were performed after intravenous administration of 5.98 ± 1.45 mCi (221.4 ± 53.8 MBq) after 66.7 ± 10.5 minutes uptake time. The list mode data of 3 minutes per bed position were reconstructed. A nuclear medicine physician blinded to the radiotracer and reconstruction time performed PET qualitative assessment by scoring each study with a Visual Analog Scoring (VAS) system of 1 to 100 (1 = non-diagnostic, 100= perfect study). SUVmeans for liver, blood pool and muscle were identified for each patient. The anatomic locations and SUVmax values were determined for each identifiable lesion. Lesion to background ratios (i.e., tumor SUVmax to SUVmean for liver, blood pool, and muscle) were calculated. Finally, SUVmeans were calculated for salivary glands and kidneys, the organs associated with high uptake of 68Ga-PSMA-11 PET. <h3><b>Results:</b></h3> Average VAS was 72.5±5.4 and 76±8.5 for <sup>68</sup>Ga-PSMA-11 and CTT1057, respectively. The 1.5 minutes per bed reconstruction of CTT1057 yielded average VAS of 71[RF1] ±7.42. Ratios of lesion to background (liver, muscle, and blood pool) for all PSMA-avid lesions were 2.76±2.34, 14.86±11.61, and 3.16±2.42 for CTT1057, and 3.53±2.71, 27.11±22.03, 12.56±8.56 for <sup>68</sup>Ga-PSMA-11. SUVmeans of salivary glands and kidneys were 3.64±0.36 and 9.12±0.82 for CTT1057 and 15.79±2.72 and 35.89±9.58 for <sup>68</sup>Ga-PSMA-11, respectively. <h3><b>Conclusion:</b></h3> Our early experience demonstrates that CTT1057 PET produces image quality similar to that of the urea-based <sup>68</sup>Ga-PSMA-11 PET. Additionally, overall lesion-to-background ratios are comparable with an exception of higher lesion-to-blood ratio for CTT1057. Uptake of CTT1057 in dose-limiting organs of PSMA imaging agents such as kidneys and salivary glands is different than that of 68Ga-PSMA-11. Overall, it is clear that CTT1057 is a promising new imaging agent that warrants further clinical investigation. <b>Research Support:</b><b>NIH grant</b> - R44 CA192451-01 <b>DoD</b> - W81XWH-14-1-0603