A "click chemistry" approach to the efficient synthesis of multiple imaging probes derived from a single precursor.
Different imaging modalities can provide complementary information on biological processes at the cellular or molecular level in vitro and in vivo. However, specific molecular probes suitable for a comparison of different imaging modalities are often not readily accessible because their preparation is usually accomplished by individually developed and optimized syntheses. Herein, we present a general, modular synthetic approach that provides access to multiple probes derived from a single precursor by application of the same, efficient functionalization strategy, the Cu(I)-catalyzed cycloaddition of terminal alkynes and azides (click chemistry). To demonstrate the viability and efficiency of this approach, folic acid (FA) was selected as a targeting vector because the preparation of FA-based imaging probes used for SPECT, PET, MRI, and NIRF by reported synthetic strategies is usually difficult to achieve and often results in low overall yields. We prepared a versatile γ-azido-FA precursor as well as a set of alkyne functionalized probes and precursors including ligand systems suitable for the chelation of various (radio)metals, an NIR dye and (18)F- and (19)F-derivatives, which enabled the parallel development of new FA-imaging probes. The Cu(I)-mediated coupling of the alkynes with the γ-azido-FA precursor was accomplished in high yields and with minimal use of protective groups. The various probes were fully characterized spectroscopically as well as in vitro and in vivo. In vitro, all new FA-derivatives exhibited high affinity toward the folic acid receptor (FR) and/or were specifically internalized into FR-overexpressing KB cells. In vivo experiments with nude mice showed that all probes (except the MRI probes which have not been tested yet) accumulated specifically in FR-positive organs and human KB-cell xenografts. However, in vivo imaging revealed significant differences between the various FA-derivatives with respect to unspecific, off-target localization. In general, the comparison of different probes proved the superiority of the more hydrophilic, radiometal-based imaging agents, a result which will guide future efforts for the development of FA-based imaging probes and therapeutic agents. In addition, the strategy presented herein should be readily applicable to other molecules of interest for imaging and therapeutic purposes and thus represents a valuable alternative to other synthetic approaches.
Department of Chemistry and Applied Biosciences, ETH Zurich, Switzerland.
Thomas L
TL
Mindt
Cristina
C
Müller
Florian
F
Stuker
Jean-Frédéric
JF
Salazar
Alexander
A
Hohn
Thomas
T
Mueggler
Markus
M
Rudin
Roger
R
Schibli
D000818:Animals; D002614:Chelating Agents / Q000737:chemistry* / Q000378:metabolism*; D057930:Click Chemistry / Q000379:methods; D058973:Folate Receptors, GPI-Anchored / Q000737:chemistry* / Q000378:metabolism*; D005492:Folic Acid / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D007624:KB Cells; D008024:Ligands; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D008819:Mice, Nude; D057054:Molecular Imaging / Q000379:methods*; D015335:Molecular Probes / Q000138:chemical synthesis; D009368:Neoplasm Transplantation / Q000000981:diagnostic imaging; D011877:Radionuclide Imaging; D019275:Radiopharmaceuticals / Q000493:pharmacokinetics; D014018:Tissue Distribution
D002614:Chelating Agents; D058973:Folate Receptors, GPI-Anchored; D008024:Ligands; D015335:Molecular Probes; D019275:Radiopharmaceuticals; D005492:Folic Acid