^99mTc(CO)₃-labeled 1-(2-Pyridyl)piperazine derivatives as radioligands for 5-HT₇ receptors

The 5-hydroxytryptamine receptor (5-HTR) family includes seven classes of receptors. The 5-HT₇R is the newest member of this family and contributes to different physiological and pathological processes. As a pathology, glioblastoma multiform (GBM) overexpresses 5-HT₇R; hence, this study aims to develop radiolabeled aryl piperazine derivatives as 5-HT₇R imaging agents.  METHODS: Compounds 6 and 7 as 1-(3-nitropyridin-2-yl)piperazine derivatives were radiolabeled with fac-[^99mTc(CO)₃(H₂O)₃]⁺ and ^99mTc(CO)₃-[6] and ^99mTc(CO)₃-[7] were obtained with high radiochemical purity (RCP > 94%). The stability of the radiotracers was evaluated in both saline and mouse serum. Specific binding on different cell lines including U-87 MG, MCF-7, SKBR3, and HT-29 was performed. The biodistribution of these radiotracers was evaluated in normal and U-87 MG Xenografted models. Finally, ^99mTc(CO)₃-[6] and ^99mTc(CO)₃-[7] were applied for in vivo imaging in U-87 MG Xenografted models.

Specific binding study indicates that ^99mTc(CO)₃-[6] and ^99mTc(CO)₃-[7] can recognize 5-HT₇R of U87-MG cell line. The biodistribution study in normal mice indicates that the brain uptake of ^99mTc(CO)₃-[6] and ^99mTc(CO)₃-[7] is the highest at 30 min post-injection (0.8 ± 0.25 and 0.64 ± 0.18%ID/g, respectively). The data of the biodistribution study in the U87-MG xenograft model revealed that these radiotracers could accumulate in the tumor site, and the highest tumor uptake was observed at 60 min post-injection (3.38 ± 0.65 and 3.27 ± 0.5%ID/g, respectively). The injection of pimozide can block the tumor's radiotracer uptake, indicating the binding of these radiotracers to the 5-HT₇R. The imaging study in the xenograft model also confirms the biodistribution data. The acquired images clearly show the tumor site, and the tumor-to-muscle ratio for ^99mTc(CO)₃-[6] and ^99mTc(CO)₃-[7] at 60 min was 3.33 and 3.88, respectively.  CONCLUSIONS: ^99mTc(CO)₃-[6] and ^99mTc(CO)₃-[7] can visualize tumor in the U87-MG xenograft model  due to their affinity toward 5-HT₇R.