Folate and macrophage folate receptor-ß in idiopathic pulmonary fibrosis disease: the potential therapeutic target?

Idiopathic pulmonary fibrosis (IPF) is a chronic, fatal disease with high mortality and poor prognosis. It is characterized by a gradual decline in lung function, and there are currently no effective therapeutic methods. Folate is a water-soluble B vitamin that plays an important role in one-carbon transfer reactions, nucleic acid biosynthesis and methylation reactions. Studies have shown that folate may participate in the pathogenesis of IPF through ways of DNA repair, methylation, and reactive oxygen species. Macrophage activation is an important early cellular event in IPF and the inflammatory response that they trigger is a significant feature of IPF. Folate receptor-ß (FR-ß) is a cell surface glycosylphosphatidylinositol-anchored glycoprotein that can mediate the unidirectional transport of folate into cells. And it has been found in previous studies that FR-ß is usually overexpressed on activated macrophages, but the expression on resting macrophages was undetectable. Therefore, targeting FR-ß may have potential value for the early diagnosis and therapy of IPF. Our goal is to highlight the biological role of folate and FR-ß in IPF, and we hope to provide helpful insight for clinical treatment strategies.

Design, synthesis and biological evaluation of novel pyrrolo[2,3-d]pyrimidine as tumor-targeting agents with selectivity for tumor uptake by high affinity folate receptors over the reduced folate carrier.

Tumor-targeted 6-substituted pyrrolo[2,3-d]pyrimidine benzoyl compounds based on 2 were isosterically modified at the 4-carbon bridge by replacing the vicinal (C11) carbon by heteroatoms N (4), O (5) or S (6), or with an N-substituted formyl (7), trifluoroacetyl (8) or acetyl (9). Replacement with sulfur (6) afforded the most potent KB tumor cell inhibitor, ~6-fold better than the parent 2. In addition, 6 retained tumor transport selectivity via folate receptor (FR) α and -ß over the ubiquitous reduced folate carrier (RFC). FRα-mediated cell inhibition for 6 was generally equivalent to 2, while the FRß-mediated activity was improved by 16-fold over 2. N (4) and O (5) substitutions afforded similar tumor cell inhibitions as 2, with selectivity for FRα and -ß over RFC. The N-substituted analogs 7-9 also preserved transport selectivity for FRα and -ß over RFC. For FRα-expressing CHO cells, potencies were in the order of 8 > 7 > 9. Whereas 8 and 9 showed similar results with FRß-expressing CHO cells, 7 was ~16-fold more active than 2. By nucleoside rescue experiments, all the compounds inhibited de novo purine biosynthesis, likely at the step catalyzed by glycinamide ribonucleotide formyltransferase. Thus, heteroatom replacements of the CH2 in the bridge of 2 afford analogs with increased tumor cell inhibition that could provide advantages over 2, as well as tumor transport selectivity over clinically used antifolates including methotrexate and pemetrexed.

A facile one-step folic acid modified partially oxidized graphene for high sensitivity tumor cell sensing.

A highly sensitive and selective tumor cell sensor based on partially oxidized graphene (POG) and folate acid (FA) composite was constructed. The POG was prepared through a modified Hummers method and characterized by means of Raman spectroscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, atomic force microscopy and transmission electron microscopy. The as-prepared POG exhibited the advantages of high electrochemical activity and a good capacity of linking amine derivatives. Using a facile one step reaction, the FA-modified POG was endowed with a more sensitive response to folate-expressing tumor cells than those sensors constructed by the two-step reaction, as well as high selectivity, good reproducibility and long-term stability.

Fabrication of folic acid-sensitive gold nanoclusters for turn-on fluorescent imaging of overexpression of folate receptor in tumor cells.

Based on the fluorescence quenching of folic acid-sensitive bovine serum albumin-directed gold nanoclusters (BSA-AuNCs) via folic acid-induced the change of environment around BSA-AuNCs, we have constructed a turn on fluorescence imaging of folate receptor overexpressed tumor cells. In this paper, the primary fluorescence intensity of BSA-AuNCs was quenched via self-assembly of folic acid onto BSA-AuNCs to produce negligible fluorescence background, the linear range of the method was 0.1-100µg/mL with the limit of detection (LOD) of 30ng/mL (S/N=3); In the presence of overexpression of folate receptor on the surface of tumor cells, the primary fluorescence intensity of BSA-AuNCs turned on by folic acid desorbing from BSA-AuNCs, the linear range of method was 0.12-2µg/mL with the LOD of 20ng/mL (S/N=3). Additionally, due to specific and high affinity of folic acid and folate receptor, the probe had high selectivity for folate receptor, other interferences hardly changed the fluorescence intensity of the probe. Moreover, the text for cytotoxicity implied that the probe had no toxicity for tumor cells. Consequently, using the fluorescence probe, satisfactory results for the turn on imaging of folate receptor overexpressed tumor cells were obtained. A novel turn-on and red fluorescent probe for folate receptor overexpressed tumor cells was developed based on the recovery of fluorescence intensity of folic acid-sensitive BSA-AuNCs.

Imaging atherosclerotic plaque inflammation via folate receptor targeting using a novel 18F-folate radiotracer.

Folate receptor ß (FR-ß) is overexpressed on activated, but not resting, macrophages involved in a variety of inflammatory and autoimmune diseases. A pivotal step in atherogenesis is the subendothelial accumulation of macrophages. In nascent lesions, they coordinate the scavenging of lipids and cellular debris to define the likelihood of plaque inflammation and eventually rupture. In this study, we determined the presence of FR-ß-expressing macrophages in atherosclerotic lesions by the use of a fluorine-18-labeled folate-based radiotracer. Human endarterectomized specimens were used to measure gene expression levels of FR-ß and CD68. Increased FR-ß and CD68 levels were found in atherosclerotic plaques compared to normal artery walls by quantitative real-time polymerase chain reaction. Western blotting and immunohistochemistry demonstrated prominent FR-ß protein levels in plaques. FR-ß-positive cells colocalized with activated macrophages (CD68) in plaque tissue. Carotid sections incubated with 3'-aza-2'-[18F]fluorofolic acid displayed increased accumulation in atherosclerotic plaques through in vitro autoradiography. Specific binding of the radiotracer correlated with FR-ß-expressing macrophages. These results demonstrate high FR-ß expression in atherosclerotic lesions of human carotid tissue correlating with CD68-positive macrophages. Areas of high 3'-aza-2'-[18F]fluorofolic acid binding within the lesions represented FR-ß-expressing macrophages. Selectively targeting FR-ß-positive macrophages through folate-based radiopharmaceuticals may be useful for noninvasive imaging of plaque inflammation.