Synthesis and preliminary evaluation of radiolabeled bis(zinc(II)-dipicolylamine) coordination complexes as cell death imaging agents

Abstract

The aim of this study was the development of (⁹⁹m)Tc labeled bis(zinc(II)-dipicolylamine) (Zn²⁺-DPA) coordination complexes, and the in vivo evaluation of their usefulness as radiotracers for the detection of cell death. DPA ligand 1 was labeled with (⁹⁹m)Tc via the (⁹⁹m)Tc-tricarbonyl core ([(⁹⁹m)Tc(CO)₃-1]³⁺) or via HYNIC ((⁹⁹m)Tc-HYNIC-1) in good radiochemical yields. Highest in vitro stabilities were demonstrated for [(⁹⁹m)Tc(CO)₃-1]³⁺. A mouse model of hepatic apoptosis (anti-Fas mAb) was used to demonstrate binding to apoptotic cells. (⁹⁹m)Tc-HYNIC-1 showed the best targeting of apoptotic hepatic tissue with a 2.2 times higher liver uptake in anti-Fas treated mice as compared to healthy animals. A rat model of ischemia-reperfusion injury was used to further explore the ability of the (⁹⁹m)Tc-labeled Zn²⁺-DPA coordination complexes to target cell death. Selective accumulation could be detected for both tracers in the area at risk, correlating with histological proof of cell death. Area at risk to normal tissue uptake ratios were 3.82 for [(⁹⁹m)Tc(CO)₃-1]³⁺ and 5.45 for (⁹⁹m)Tc-HYNIC-1.

Figure 1

Structure of compound 1 and putative structures of [^99mTc(CO)₃-1]³⁺ and ^99mTc-HYNIC-1, complexed with Zn²⁺.

Figure 2

Representative radiochromatograms of [^99mTc(CO)₃-1]³⁺ (A) and ^99mTc-HYNIC-1 (B) before C18 Sep-Pak purification.

Figure 3

Tissue distribution (%ID/g) at different time points post injection of [^99mTc(CO)₃-1]³⁺ (A) or ^99mTc-HYNIC-1 (B) in normal BALB-c mice (n = 3 per time point).

Figure 4

Planar images 50 min after ^99mTc-HYNIC-1 injection in anti-Fas treated (D) or normal mice (E). Liver, intestines and bladder are indicated by L, I and Bl. 60 min pi livers were excised, and analyzed by autoradiography (A: anti-Fas treated, F: normal), H&E (B:anti-Fas treated, G: normal) and immunostaining (C: anti-Fas treated, H: normal) using an anti-cleaved caspase-3 mAb (brown color in C).

Figure 5

Uptake of radioactivity (%ID/g) in livers of control mice or mice treated with anti-Fas mAb, 60 min post iv injection of [^99mTc(CO)₃-1]³⁺ and ^99mTc-HYNIC-1.

Figure 6

Digital photographs of 2mm myocardial slices from rats injected iv with 111 MBq of [^99mTc(CO)₃-1]³⁺ (A) or with ^99mTc-HYNIC-1 (C). Normal myocardium is colored with Evans blue (blue-grey zones); the area at risk is pink-red. Corresponding autoradiograms with [^99mTc(CO)₃-1]³⁺ (B) or ^99mTc-HYNIC-1 (D) uptake matching the area at risk.