Development of radiolabeled bis(zinc(II)-dipicolylamine) complexes for cell death imaging

Although it has been traditionally surmised that phosphatidylserine (PS) externalization is a hallmark of apoptosis, most other non-apoptotic modes of cell death, such as necrosis, are also associated with PS externalization. Bis(zinc-dipicolylamine) (ZnDPA) complexes have been reported to exhibit affinity for PS. The present study aimed to develop novel radiolabeled ZnDPA derivatives for cell death imaging in tumor after treatment with anticancer drugs.

[¹²⁵I]IB-EG₂-ZnDPA and [^99mTc]Tc-MAG3-EG₂-ZnDPA were designed and prepared. The stabilities of these radiotracers were determined in 0.1 M phosphate buffer (pH 7.4) or murine plasma at 37 °C, and their 1-octanol/water partition coefficients (logP) were measured. The uptake of radioactivity in cancer cells, which were preincubated in a normal medium or in a medium containing 5-FU, was measured after incubation with radiotracers. Accumulation of [^99mTc]Tc-MAG3-EG₂-ZnDPA in the tumor was evaluated in tumor-bearing mice treated with or without 5-FU, and then TUNEL staining was performed to detect dead cells in the tumor tissue sections.

The radiochemical purities of [¹²⁵I]IB-EG₂-ZnDPA and [^99mTc]Tc-MAG3-EG₂-ZnDPA exceeded 95%. Although [¹²⁵I]IB-EG₂-ZnDPA gradually decomposing with time, more than 90% of [^99mTc]Tc-MAG3-EG₂-ZnDPA remained in its intact form in phosphate buffer through 6 h of incubation. Neither [¹²⁵I]IB-EG₂-ZnDPA nor [^99mTc]Tc-MAG3-EG₂-ZnDPA decomposed so much after 6-h incubation in murine plasma. [¹²⁵I]IB-EG₂-ZnDPA could not specifically recognize PS on the cell surface because of its high lipophilicity. Conversely, [^99mTc]Tc-MAG3-EG₂-ZnDPA accumulated in cancer cells after treatment with an anticancer drug both in vitro and in vivo, and its accumulation was correlated with the number of TUNEL-positive cells. However, the biodistribution of [^99mTc]Tc-MAG3-EG₂-ZnDPA was not suitable for imaging because of its low accumulation in tumor and high uptake in abdomen organs.

[^99mTc]Tc-MAG3-EG₂-ZnDPA could be useful for the early detection of treatment effects after chemotherapy. Since the signal-to-noise ratio is not enough for single photon emission computed tomography imaging, further modification is needed to improve its biodistribution and affinity for PS.