Folate receptor-ß constitutes a marker for human proinflammatory monocytes.

Activated macrophages are commonly involved in the pathogenesis of inflammatory and autoimmune diseases and have been frequently reported to overexpress FR-ß. Although FR-targeted therapies aimed at eliminating activated macrophages have shown promise for treating inflammatory diseases, little work has been performed to evaluate whether other hematopoietic cells might also express FR-ß. Analysis of peripheral blood cells with a mAb to human FR-ß reveals that only monocytes express FR-ß. Molecular characterization of these circulating monocytes further demonstrates that solely the classic/proinflammatory subset (CD14(high)CD16(-)) expresses the FR and that only CD14(high)CD16(-) FR-ß(+) monocytes also display the ability to bind folate-linked molecules. Confirmation that this subset of monocytes indeed constitutes the proinflammatory subpopulation was obtained by demonstrating coexpression of FR-ß with other proinflammatory markers, including CCR2 and HLA-DR. Synovial monocytes from the joints of patients with RA were also shown to express FR-ß. As inhibition of the chemotaxis of proinflammatory monocytes into sites of inflammation has been explored frequently as a means of controlling autoimmune diseases, demonstration that FR-ß is uniquely expressed on this proinflammatory subpopulation offers a new strategy to suppress migration of inflammatory monocytes into sites of inflammation.

A functional folate receptor is induced during macrophage activation and can be used to target drugs to activated macrophages.

Previous work has demonstrated that a subset of macrophages expresses a folate receptor (FR) that can mediate internalization of folate-linked molecules, including imaging and therapeutic agents. To characterize this subset, macrophages were collected from peritoneal cavities of mice injected with saline, thioglycolate, zymosan, heat-killed or live bacteria, and cell-surface markers that coexpress with FR were identified. Virtually no F4/80(+) peritoneal macrophages from saline-injected mice expressed FR, whereas numerous macrophages from mice injected with each inflammatory stimulus expressed FR. Examination of cell differentiation antigens that are up-regulated in FR(+) macrophages revealed markers characteristic of an activated state (CD80, CD86, Ly-6C/G), whereas macrophages lacking these activation markers expressed few or no FR. FR(+) macrophages also produced tumor necrosis factor-alpha (TNF-alpha) and reactive oxygen species, and production of reactive oxygen species correlated linearly with expression of FR. Synovial macrophages collected from arthritic patients were found to bind and internalize folate-linked dyes. Moreover, a folate-linked radioimaging agent was shown to image inflamed joints of rheumatoid arthritic patients. These results suggest that FR constitutes a marker for macrophage activation and that FR(+) macrophages can be targeted with folate-linked drugs without promoting drug uptake by nonactivated macrophages. This trial was registered at www.clinicaltrials.gov as #NCT00588393.

Synthesis and activity of a folate peptide camptothecin prodrug.

A folate receptor targeted camptothecin prodrug was synthesized using a hydrophilic peptide spacer linked to folate via a releasable disulfide carbonate linker. The conjugate was found to possess high affinity for folate receptor-expressing cells and inhibited cell proliferation in human KB cells with an IC(50) of 10nM. Activity of the prodrug was completely blocked by excess folic acid, demonstrating receptor-mediated uptake.

Folate receptor-mediated drug targeting: from therapeutics to diagnostics.

Folate targeted drug delivery has emerged as an alternative therapy for the treatment and imaging of many cancers and inflammatory diseases. Due to its small molecular size and high binding affinity for cell surface folate receptors (FR), folate conjugates have the ability to deliver a variety of molecular complexes to pathologic cells without causing harm to normal tissues. Complexes that have been successfully delivered to FR expressing cells, to date, include protein toxins, immune stimulants, chemotherapeutic agents, liposomes, nanoparticles, and imaging agents. This review will summarize the applications of folic acid as a targeting ligand and highlight the various methods being developed for delivery of therapeutic and imaging agents to FR-expressing cells.