Characterization of (18)F-dipicolylamine (DPA) derivatives in cells infected with influenza virus

Bis(Zn-dipicolylamine (Zn-DPA)) coordination complexes represent a new class of synthetic small molecules that can target anionic phosphatidylserine (PS) in the apoptotic cells with high affinity and specificity. In this study, we labeled Zn-DPA and Cy7-Zn-DPA with different (18)F-prosthetic groups and characterized their uptake in A549 cells infected with influenza A virus from the 2009 pandemic (H1N1pdm).

DPA was labeled with N-succinimidyl 4-(18)F-fluorobenzoate ((18)F-SFB), 4-nitrophenyl 2-(18)F-fluoropropionate ((18)F-NFP), 2-(18)F-Fluoroethyl toslyate ((18)F-FET), and (18)F-aluminum (Al(18)F), respectively. Cy7-DPA was labeled with (18)F-SFB and (18)F-NFP only. The tracers were reconstituted with zinc nitrate before use. Apoptosis in A549 cells was induced by infection with the H1N1pdm virus for 48 h. Three μCi of each tracer was added to each well and incubated at 37 °C. The effect of different prosthetic groups, different MOI, and incubation time on percent cellular uptake was studied. Cell internalization and efflux was evaluated within 2h of incubation. The competitive binding assay was performed with increasing concentration (10(-12)-10(-5)M) of Zn-DPA or Cy7-Zn-DPA prior to the addition of either (18)F-FB-Zn-DPA or (18)F-FB-Cy7-Zn-DPA into each well. IC50 values for the two Zn-DPA analogues were estimated by GraphPad Prism 6.0.

Among all the four prosthetic groups, the (18)F-SFB method provided the highest conjugation yield for DPA and the highest uptake ratio between the infection cells and the control when both Zn-DPA and Cy7-Zn-DPA were present in the complex. The uptake ratio was similar for (18)F-FB-Zn-DPA and (18)F-FB-Cy7-Zn-DPA. Uptake of (18)F-FB-Zn-DPA and (18)F-FB-Cy7-Zn-DPA was proportional to the degree of apoptosis with a plateau at MOI 3. Uptake of (18)F-FB-Cy7-Zn-DPA also increased over incubation time and reached a plateau at 1h, whereas uptake of (18)F-FB-Zn-DPA did not show any significant change over time. Cell internalization studies showed that more than 70% of (18)F-FB-Zn-DPA remained on the cell surface over a time course of 2 hr in the cell media, but over 90% of (18)F-FB-Cy7-Zn-DPA was internalized within 15 min of incubation. IC50 values were estimated to be 1.5±0.3 nM and 26.2±5.1 nM for Zn-DPA and Cy7-Zn-DPA, respectively.

(18)F-SFB was the optimal labeling method for Zn-DPA and Cy7-Zn-DPA with respect to radiochemistry and provided complexes with high target-to-background ratios. (18)F-FB-Zn-DPA and (18)F-FB-Cy7-Zn-DPA appeared to have a completely different internalization mechanism, while Zn-DPA showed higher binding affinity than Cy7-Zn-DPA. Based on these favorable characteristics, (18)F-FB-Zn-DPA and (18)F-FB-Cy7-Zn-DPA should be further evaluated as potential imaging agents for viral infection.