Determining androgen inactivation status in prostate cancer by [18F] DHT PET.

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191 Background: Castration-resistant prostate cancer occurs in part due to increased tumor tissue testosterone (T) and dihydrotestosterone (DHT) that sustain tumor growth. T and DHT are converted to inactive T- and DHT-glucuronide (T/DHT-G) by uridine 5'-diphospho-glucuronosyltransferase family genes (UGT2B15 and UGT2B17) in glucuronidation-competent cells and excreted, but not in glucuronidation-deficient cells. Thus, low glucuronidation activity enables prostate tumors to preserve androgens, which increases hormone treatment resistance and may be detectable by functional imaging. Methods: We knocked out (KO) UGT2B15 and 17 in LNCaP cells and tested the DHT retention rate in the cell lines by pulse-chase using [3H]DHT as a probe. Free and DHT-G retention rates were separately determined after 5 to 60 min. To increase the signal difference between control and KO cells, we screened several ATP-binding cassette transporter inhibitors to block DHT-G excretion. We performed [18F]DHT PET/CT in castrated mice having a control and a KO xenograft on contralateral flanks ( n = 3). The ratio of the standard uptake value (SUV) in control to KO xenografts in each mouse was calculated. To increase the ratio between control and KO tumors, 50µg cyclosporin A (CSA) was injected 30 min before injecting [18F]DHT. Results: After 5 minutes of chase, control cells retained twice the DHT of KO cells. In control cells, 50%-70% DHT was glucuronidated. Almost no DHT-G was detected in KO cells, and free DHT was similar to control. Of the inhibitors, only CSA increased DHT-G (but not free DHT) in control cells, resulting in a 3-4-fold increase in overall signal. In vivo PET/CT showed control xenografts had higher peak SUV but also a higher elution rate. CSA increased the SUV ratio by 1.5-2. Conclusions: We developed a PET/CT modality to detect androgen inactivation in a prostate cancer xenograft model. Androgen-glucuronidation-proficient tumors give off a stronger signal that is increased by ATP transporter inhibition. Our method can provide a noninvasive means of determining androgen metabolism status and therefore could possibly predict effectiveness of potential therapies in a subgroup of tumors predisposed to androgen deprivation resistance.