Introduction: 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 new99mTc-labeled
GnRH analogue as an imaging probe for tumor targeting of prostate cancer.
Methods: 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.
Results: 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.
Conclusion: 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.