Preclinical Evaluation of Tc-Labeled LHRH Analog as Cancer Receptor Imaging.

[INTRODUCTION] Breast cancer is the main cause of cancer-related mortality in women in the developed world. In particular, receptors of luteinizing hormone-releasing hormone (LHRH or GnRH) are overexpressed in this malignant disease. The aim of this study was to develop a new molecular probe [99mTc]Tc-HYNIC-GSG-LHRH(<sc>d</sc>-Lys6)/tricine/nicotinic acid (NA) as a novel molecular imaging agent for breast cancer.

[METHODS] HYNIC-GSG-LHRH(D-Lys6) was acquired and radiolabeled with [99mTc]Tc. Radiochemical purity and stability under different conditions were evaluated by instant thin-layer chromatography (ITLC) and high-performance liquid chromatography. Lipophilicity was determined by the partition coefficient test. In vitro cell binding studies were performed in different human and mice breast cancer cell lines (MDA-MB-231, MDA-MB-435, MCF-7, BT474, and 4T1) as well as in normal murine fibroblasts (NIH-3T3) and CHO-K1 as a negative control. Biodistribution studies were performed in normal Balb/c mice and 4T1 tumor-bearing Balb/c mice up to 6 h post-injection (pi). SPECT/CT images were performed in 4T1 tumor-bearing Balb/c mice up to 5 h pi.

[RESULTS] [99mTc]Tc-HYNIC-GSG-LHRH(<sc>d</sc>-Lys6)/tricine/NA complex was labeled with a high radiochemical purity (>98%) and remained stable for up to 4 h. It exhibited good hydrophilicity (log p = -2.82 ± 0.04) and also demonstrated significant and specific binding across all evaluated breast cancer cell lines. Biodistribution studies showed a high renal clearance and low nonspecific binding (<2% Act/g) in most organs, as well as appreciable tumor uptake (5.8 ± 0.5 % ID/g 1 h pi) and high tumor-to-muscle ratio (maximum of 30.5 ± 11.2 at 1 h pi). SPECT/CT imaging of 4T1-tumor-bearing Balb/c mice revealed results consistent with the biodistribution studies, showing a tumor-to-non-tumor ratio of greater than 3.5 at all evaluated time points. In vivo blocking studies confirmed specificity for the LHRH receptor.

[CONCLUSIONS] [99mTc]Tc-HYNIC-GSG-LHRH(<sc>d</sc>-Lys6)/tricine/NA complex has shown significant potential for in vivo visualization of LHRH receptors expression in breast cancer.