Imaging of activated macrophages in experimental osteoarthritis using folate-targeted animal single-photon-emission computed tomography/computed tomography.

OBJECTIVE: Evaluation of macrophage activation may provide essential information about the etiology and progression rate of osteoarthritis (OA). Activated macrophages abundantly express folate receptor ß (FRß), which can be targeted using radioactive-labeled folic acid. This study was undertaken to investigate whether macrophage activation can be monitored in small animal models of OA using a folate radiotracer and to determine whether macrophage activation differs in different models of OA with different OA progression. METHODS: Two rat models of OA were used: the mono-iodoacetate (MIA) model, which is a fast-progressing biochemically induced model, and the anterior cruciate ligament transection (ACLT) model, which induces OA at a slower pace. Images were obtained using high-resolution small animal single-photon-emission computed tomography/computed tomography. The specificity of the technique was tested by eradicating macrophages using clodronate-laden liposomes and blockade of FRß by cold folic acid. RESULTS: The MIA model had high initial macrophage activation, with a peak after 2 weeks which disappeared after 8 weeks. The ACLT model showed less activation but was still active 12 weeks after induction. The technique allowed monitoring of the disease process over time, in which late stages of the disease showed less macrophage activation than early stages, especially in the fast-progressing MIA model of OA. CONCLUSION: Our findings indicate that macrophage activation in experimental OA can clearly be demonstrated and monitored by the folate radiotracer. The high resolution, high sensitivity, and high specificity of the technique allow clear localization of macrophage activity in a disease model that is not known for abundant macrophage involvement.

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.

Fluorine-18 click radiosynthesis and preclinical evaluation of a new 18F-labeled folic acid derivative.

The folate receptor (FR) is highly expressed on most epithelial cancer cells, while normal cells show only restricted expression of FR. As a result, the FR is an ideal target for receptor-based molecular imaging and therapy of cancer and has become a promising target in oncology. To date, several folate-based chemotherapeutics and imaging probes such as radiopharmaceuticals for single photon emission computed tomography (SPECT) have been developed. However, an (18)F-labeled folic acid derivative suitable for positron emission tomography (PET) imaging that can be routinely applied is still lacking. In this study, a new fluorinated and radiofluorinated folic acid derivative, (18/19)F-click folate, was synthesized using click chemistry. In a convenient and very efficient two-step radiosynthesis, the isolated (18)F-click folate was obtained in good radiochemical yields of 25-35% with a specific activity of 160+/-70 GBq/micromol after