Targeting phosphatidylethanolamine and phosphatidylserine for imaging apoptosis in cancer

Abstract

Introduction: Both phosphatidylethanolamine (PE) and phosphatidylserine (PS) can be externalized to the outer cell membrane in apoptosis. Thus the objective was to determine whether PE-targeting ¹⁸F-duramycin and PS-targeting ¹⁸F-Zn-DPA could be used for imaging apoptosis.

Methods: Duramycin and Zn-DPA were labeled with either ¹⁸F-Al or ¹⁸F-SFB. U937 cells were incubated with four different concentrations of camptothecin (CPT). For assessing the effect of incubation time on uptake, 37 MBq of radiotracer was added to cells incubated for 15, 30, 60, and 120 min at 37 °C. For blocking experiments, 150 μg duramycin and 40 μg Zn-DPA were added to cells for 15 min prior to the addition of either duramycin or Zn-DPA labeled with ¹⁸F. Apoptosis was measured by flow cytometry using an annexin-V/PI kit. Cells were co-stained with Hoechst, Cy5-duramycin, and PSVue480 (FITC-Zn-DPA) to localize fluorescent dye uptake in cells.

Results: Apoptosis in cells increased proportionally with CTP as confirmed by both flow cytometry and fluorescent staining. Both FITC-Zn-DPA and FITC-duramycin localized mainly on the cell membrane during early apoptosis and then translocated to the inside during late apoptosis. Uptake of FITC-duramycin, however, was higher than that of FITC-Zn-DPA. Cellular uptake of four different radiotracers was also proportional to the degree of apoptosis, increasing slightly over time and reaching a plateau at about 1 h. The blocking experiments demonstrated that uptake in all the control groups was predominantly non-specific, whereas the specific uptake in all the treated groups was at least 50% for both ¹⁸F labeled duramycin and Zn-DPA.

Conclusion: Both PE-targeting ¹⁸F-duramycin and PS-targeting ¹⁸F-Zn-DPA could be considered as potential radiotracers for imaging cellular apoptosis. Advances in knowledge and implications for patient care: Cellular data support the further development of radiotracers targeting either PE or PS for imaging apoptosis, which can associate with clinical outcome for cancer patients.

Keywords: Apoptosis; Dipicolylamine; Duramycin; PE; PET; PS; Zn-DPA.