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

Purpose: 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.

Methods: [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.

Results: 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.

Conclusion: [^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.