Evaluation of a New ^99mTc-labeled GnRH Analogue as a Possible Imaging Agent for Prostate Cancer Detection

Prostate cancer is a serious threat to men's health so it is necessary to develop technics for early detection of this malignancy. The purpose of this research was the evaluation of a new^99mTc-labeled GnRH analogue as an imaging probe for tumor targeting of prostate cancer.

^99mTc-labeled-DLys6-GnRH analogue was prepared based on HYNIC as a chelating agent and tricine/ EDDA as coligands for labeling with ^99mTc. HYNIC was coupled to epsilon amino group of DLys6 through aminobutyric acid (GABA) as a linker. Radiochemical purity and stability in normal saline and serum, were determined by TLC and HPLC methods. Furthermore, calculation of protein-binding and partition coefficient constant were carried out for ^99mTc labeled peptide. The cellular experiments including receptor binding specificity and affinity were studied using three prostate cancer cell lines LN-CaP, DU-145 and PC-3. Finally, the animal assessment and SPECT imaging of radiolabeled GnRH analogue were evaluated on normal mice and nude mice bearing LN-CaP tumor.

The GnRH conjugate was labeled with high radiochemical purity (~97%). The radiolabeled peptide showed efficient stability in the presence of normal saline and human serum. The in vitro cellular assays on three prostate cancer cell lines indicated that the radiotracer was bound to LN-CaP cells with higher affinity compared to DU-145 and PC-3 cells. The Kd values of ^99mTc- HYNIC (tricine/ EDDA)-Gaba-D-Lys6GnRH were 89.39±26.71, 93.57±30.49 and107.3±18.82 in LN-CaP, PC-3 and DU-145 cells respectively. The biodistribution studies in normal mice and LN-CaP tumor-bearing nude mice showed similar results including rapid blood clearance and low radioactivity accumulation in non-target organs. High kidney uptake proved that the main excretion route of radiopeptide was through the urinary system. The tumor uptake was 1.72±0.45 %ID/g at 1h p.i. decreasing to 0.70±0.06%ID/g at 4h p.i. for ^99mTc-HYNIC-Gaba-D-Lys6GnRH. The maximum tumor/ muscle ratio was 2.30 at 1h p.i. Pre-saturation of receptor using an excess of unlabeled peptide revealed that the tumor uptake was receptor mediated. The results of the SPECT image of LN-CaP tumor were in agreement with the biodistribution data.

Based on this study, we suggest LN-CaP as a favorable cell line for in vivo studies on GnRH analogues. Moreover, this report shows that ^99mTc-HYNIC (tricine/EDDA)-Gaba-D-Lys6GnRH may be a suitable candidate for further evaluation of prostate cancer.