Folate receptor-targeted aminopterin therapy is highly effective and specific in experimental models of autoimmune uveitis and autoimmune encephalomyelitis

Targeting folate receptor beta on monocytes/macrophages renders rapid inflammation resolution independent of root causes

Provoked by sterile/nonsterile insults, prolonged monocyte mobilization and uncontrolled monocyte/macrophage activation can pose imminent or impending harm to the affected organs. Curiously, folate receptor beta (FRβ), with subnanomolar affinity for the vitamin folic acid (FA), is upregulated during immune activation in hematopoietic cells of the myeloid lineage. This phenomenon has inspired a strong interest in exploring FRβ-directed diagnostics/therapeutics. Previously, we have reported that FA-targeted aminopterin (AMT) therapy can modulate macrophage function and effectively treat animal models of inflammation. Our current investigation of a lead compound (EC2319) leads to discovery of a highly FR-specific mechanism of action independent of the root causes against inflammatory monocytes. We further show that EC2319 suppresses interleukin-6/interleukin-1β release by FRβ⁺ monocytes in a triple co-culture leukemic model of cytokine release syndrome with anti-CD19 chimeric antigen receptor T cells. Because of its chemical stability and metabolically activated linker, EC2319 demonstrates favorable pharmacokinetic characteristics and cross-species translatability to support future pre-clinical and clinical development.

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Functional folate receptor beta is transiently expressed on inflammatory monocytes

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EC2319 is an enhancement of traditional dihydrofolate reductase inhibitors

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EC2319 anti-monocyte activity correlates with local/systemic therapeutic benefit

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EC2319 inhibition of cytokine release suggests emergency use for hyperinflammation

A Novel Treatment for Glomerular Disease: Targeting the Activated Macrophage Folate Receptor with a Trojan Horse Therapy in Rats

Since activated macrophages express a functional folate receptor β (FRβ), targeting this macrophage population with folate-linked drugs could increase selectivity to treat inflammatory diseases. Using a macrophage-mediated anti-glomerular basement membrane (anti-GBM) glomerulonephritis (GN) in WKY rats, we investigated the effect of a novel folic acid-aminopterin (AMT) conjugate (EC2319) designed to intracellularly deliver AMT via the FR. We found that treatment with EC2319 significantly attenuated kidney injury and preserved renal function. Kidney protection with EC2319 was blocked by a folate competitor, indicating that its mechanism of action was specifically FRβ-mediated. Notably, treatment with methotrexate (MTX), another folic acid antagonist related to AMT, did not protect from kidney damage. EC2319 reduced glomerular and interstitial macrophage infiltration and decreased M1 macrophage recruitment but not M2 macrophages. The expression of CCL2 and the pro-fibrotic cytokine TGF-β were also reduced in nephritic glomeruli with EC2319 treatment. In EC2319-treated rats, there was a significant decrease in the deposition of collagens. In nephritic kidneys, FRβ was expressed on periglomerular macrophages and macrophages present in the crescents, but its expression was not observed in normal kidneys. These data indicate that selectively targeting the activated macrophage population could represent a novel means for treating anti-GBM GN and other acute crescentic glomerulonephritis.

Targeted delivery of mPGES-1 inhibitors to macrophages via the folate receptor-β for inflammatory pain

Activated macrophages overexpress the folate receptor β (FR-β) that can be used for targeted delivery of drugs conjugated to folic acid. FR-expressing macrophages contribute to arthritis progression by secreting prostaglandin E₂ (PGE₂). Non-steroidal anti-inflammatory drugs (NSAIDs) block PGs and thromboxane by inhibiting the cyclooxygenase (COX) enzymes and are used for chronic pain and inflammation despite their well-known toxicity. New NSAIDs target an enzyme downstream of COXs, microsomal prostaglandin E synthase-1 (mPGES-1). Inhibition of mPGES-1 in inflammatory macrophages promises to retain NSAID efficacy while limiting toxicity. We conjugated a potent mPGES-1 inhibitor, MK-7285, to folate, but the construct released the drug inefficiently. Folate conjugation to the primary alcohol of MK-7285 improved the construct's stability and the release of free drug. Surprisingly, the drug-folate conjugate potentiated PGE₂ in FR-positive KB cells, and reduced PGE₂ in macrophages independently of the FR. Folate conjugation of NSAIDs is not an optimal strategy for targeting of macrophages.

A Role for Folate in Microbiome-Linked Control of Autoimmunity

The microbiome exerts considerable control over immune homeostasis and influences susceptibility to autoimmune and autoinflammatory disease (AD/AID) such as inflammatory bowel disease (IBD), multiple sclerosis (MS), type 1 diabetes (T1D), psoriasis, and uveitis. In part, this is due to direct effects of the microbiome on gastrointestinal (GI) physiology and nutrient transport, but also to indirect effects on immunoregulatory controls, including induction and stabilization of T regulatory cells (T _reg). Secreted bacterial metabolites such as short-chain fatty acids (SCFA) are under intense investigation as mediators of these effects. In contrast, folate (vitamin B9), an essential micronutrient, has attracted less attention, possibly because it exerts global physiological effects which are difficult to differentiate from specific effects on the immune system. Here, we review the role of folate in AD/AID with some emphasis on sight-threatening autoimmune uveitis. Since folate is required for the generation and maintenance of T _{reg ,} we propose that one mechanism for microbiome-based control of AD/AID is via folate-dependent induction of GI tract T _{reg ,} particularly colonic T _reg, via anergic T cells (T _an). Hence, folate supplementation has potential prophylactic and/or therapeutic benefit in AID/AD.

Efficacy and tolerability of folate-aminopterin therapy in a rat focal model of multiple sclerosis

BACKGROUND

Activated macrophages in the experimental model of multiple sclerosis (MS) express folate receptor-β (FR-β), representing a promising target for the treatment of MS. Here, we both evaluated the efficacy of a novel folate-aminopterin construct (EC2319) in a rat focal model of multiple sclerosis (MS) and investigated the utility of ⁶⁸Ga-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid-conjugated folate (⁶⁸Ga-FOL) for assessing inflammatory lesions. In addition, we investigated whether FR-β is expressed in the brain of patients with MS.

METHODS

Focal delayed-type hypersensitivity experimental autoimmune encephalomyelitis (fDTH-EAE) was induced in 40 Lewis rats; 20 healthy Lewis rats were used as controls. Rats were divided into six groups according to the duration of disease (control, acute, or chronic) and intervention (vehicle versus EC2319). ⁶⁸Ga-FOL analyses, histology, and immunofluorescence of the brain were performed to evaluate the efficacy of subcutaneously administered EC2319 on lesion development. Immunofluorescence was used to assess FR-β expression in postmortem brain samples from 5 patients with MS and 5 healthy controls.

RESULTS

Immunofluorescence and histological analyses revealed significant reductions in FR-β expression (P < 0.05) and lesion size (P < 0.01), as well as improved inducible nitric oxide synthase/mannose receptor C type 1 ratios (P < 0.01) in macrophages and microglia during the chronic but not acute phase of fDTH-EAE in EC2319-treated rats. The uptake of IV-injected ⁶⁸Ga-FOL in the brain was low and did not differ between the groups, but the in vitro binding of ⁶⁸Ga-FOL was significantly lower in EC2319-treated rats (P < 0.01). FR-β positivity was observed in chronically active lesions and in normal-appearing white matter in MS brain samples.

CONCLUSIONS

EC2319 was well tolerated and attenuated inflammation and lesion development in a rat model of a chronic progressive form of MS. Human MS patients have FR-β-positive cells in chronically active plaques, which suggests that these results may have translational relevance.

Folate receptor-targeted positron emission tomography of experimental autoimmune encephalomyelitis in rats

BACKGROUND

Folate receptor-β (FR-β) is a cell surface receptor that is significantly upregulated on activated macrophages during inflammation and provides a potential target for folate-based therapeutic and diagnostic agents. FR-β expression in central nervous system inflammation remains relatively unexplored. Therefore, we used focally induced acute and chronic phases of experimental autoimmune encephalomyelitis (EAE) to study patterns of FR-β expression and evaluated its potential as an in vivo imaging target.

METHODS

Focal EAE was induced in rats using heat-killed Bacillus Calmette-Guérin followed by activation with complete Freund's adjuvant supplemented with Mycobacterium tuberculosis. The rats were assessed with magnetic resonance imaging and positron emission tomography/computed tomography (PET/CT) at acute (14 days) and chronic (90 days) phases of inflammation. The animals were finally sacrificed for ex vivo autoradiography of their brains. PET studies were performed using FR-β-targeting aluminum [¹⁸F]fluoride-labeled 1,4,7-triazacyclononane-1,4,7-triacetic acid conjugated folate ([¹⁸F]AlF-NOTA-folate, ¹⁸F-FOL) and 18 kDa translocator protein (TSPO)-targeting N-acetyl-N-(2-[¹¹C]methoxybenzyl)-2-phenoxy-5-pyridinamine (¹¹C-PBR28). Post-mortem immunohistochemistry was performed using anti-FR-β, anti-cluster of differentiation 68 (anti-CD68), anti-inducible nitric oxide synthase (anti-iNOS), and anti-mannose receptor C-type 1 (anti-MRC-1) antibodies. The specificity of ¹⁸F-FOL binding was verified using in vitro brain sections with folate glucosamine used as a blocking agent.

RESULTS

Immunohistochemical evaluation of focal EAE lesions demonstrated anti-FR-β positive cells at the lesion border in both acute and chronic phases of inflammation. We found that anti-FR-β correlated with anti-CD68 and anti-MRC-1 immunohistochemistry; for MRC-1, the correlation was most prominent in the chronic phase of inflammation. Both ¹⁸F-FOL and ¹¹C-PBR28 radiotracers bound to the EAE lesions. Autoradiography studies verified that this binding took place in areas of anti-FR-β positivity. A blocking assay using folate glucosamine further verified the tracer's specificity. In the chronic phase of EAE, the lesion-to-background ratio of ¹⁸F-FOL was significantly higher than that of ¹¹C-PBR28 (P = 0.016).

CONCLUSION

Our EAE results imply that FR-β may be a useful target for in vivo imaging of multiple sclerosis-related immunopathology. FR-β-targeted PET imaging with ¹⁸F-FOL may facilitate the monitoring of lesion development and complement the information obtained from TSPO imaging by bringing more specificity to the PET imaging armamentarium for neuroinflammation.

Rhesus Brain Transcriptomic Landscape in an ex vivo Model of the Interaction of Live Borrelia Burgdorferi With Frontal Cortex Tissue Explants

Lyme neuroborreliosis (LNB) is the most dangerous manifestation of Lyme disease caused by the spirochete Borrelia burgdorferi which can reach the central nervous system most commonly presenting with lymphocytic meningitis; however, the molecular basis for neuroborreliosis is still poorly understood. We incubated explants from the frontal cortex of three rhesus brains with medium alone or medium with added live Borrelia burgdorferi for 6, 12, and 24 h and isolated RNA from each group was used for RNA sequencing with further bioinformatic analysis. Transcriptomic differences between the ex vivo model of live Borrelia burgdorferi with rhesus frontal cortex tissue explants and the controls during the progression of the infection were identified. A total of 2249, 1064, and 420 genes were significantly altered, of which 80.7, 52.9, and 19.8% were upregulated and 19.3, 47.1, 80.2% were downregulated at 6, 12, and 24 h, respectively. Gene ontology and KEGG pathway analyses revealed various pathways related to immune and inflammatory responses during the spirochete infection were enriched which is suggested to have a causal role in the pathogenesis of neurological Lyme disease. Moreover, we propose that the overexpressed FOLR2 which was demonstrated by the real-time PCR and western blotting could play a key role in neuroinflammation of the neuroborreliosis based on PPI analysis for the first time. To our knowledge, this is the first study to provide comprehensive information regarding the transcriptomic signatures that occur in the frontal cortex of the brain upon exposure to Borrelia burgdorferi, and suggest that FOLR2 is a promising target that is associated with neuroinflammation and may represent a new diagnostic or therapeutic marker in LNB.

Folic acid deficiency and vision: a review

Folic acid (FA), also termed folate, is an essential vitamin for health at all ages since it participates in the biosynthesis of nucleotides, amino acids, neurotransmitters, and certain vitamins. It is therefore crucial for rapidly growing tissues such as those of the fetus. It is becoming clear that FA deficiency and impaired folate pathways are implicated in many diseases of both early life and old age. FA can be transported into the cell by the folate receptor, the reduced folate transporter, and proton-coupled folate transporter. Folate transport proteins are present in certain eye tissues, which explains why FA plays an important role in eye development. The purpose of this literature review is to investigate the evidence relating FA deficiency to eye diseases.

The folate receptor β as a macrophage-mediated imaging and therapeutic target in rheumatoid arthritis

Macrophages play a key role in the pathophysiology of rheumatoid arthritis (RA). Notably, positive correlations have been reported between synovial macrophage infiltration and disease activity as well as therapy outcome in RA patients. Hence, macrophages can serve as an important target for both imaging disease activity and drug delivery in RA. Folate receptor β (FRβ) is a glycosylphosphatidyl (GPI)-anchored plasma membrane protein being expressed on myeloid cells and activated macrophages. FRβ harbors a nanomolar binding affinity for folic acid allowing this receptor to be exploited for RA disease imaging (e.g., folate-conjugated PET tracers) and therapeutic targeting (e.g., folate antagonists and folate-conjugated drugs). This review provides an overview of these emerging applications in RA by summarizing and discussing properties of FRβ, expression of FRβ in relation to macrophage polarization, FRβ-targeted in vivo imaging modalities, and FRβ-directed drug targeting.

Histone deacetylases in monocyte/macrophage development, activation and metabolism: refining HDAC targets for inflammatory and infectious diseases

Macrophages have central roles in danger detection, inflammation and host defense, and consequently, these cells are intimately linked to most disease processes. Major advances in our understanding of the development and function of macrophages have recently come to light. For example, it is now clear that tissue-resident macrophages can be derived from either blood monocytes or through local proliferation of phagocytes that are originally seeded during embryonic development. Metabolic state has also emerged as a major control point for macrophage activation phenotypes. Herein, we review recent literature linking the histone deacetylase (HDAC) family of enzymes to macrophage development and activation, particularly in relation to these recent developments. There has been considerable interest in potential therapeutic applications for small molecule inhibitors of HDACs (HDACi), not only for cancer, but also for inflammatory and infectious diseases. However, the enormous range of molecular and cellular processes that are controlled by different HDAC enzymes presents a potential stumbling block to clinical development. We therefore present examples of how classical HDACs control macrophage functions, roles of specific HDACs in these processes and approaches for selective targeting of drugs, such as HDACi, to macrophages. Development of selective inhibitors of macrophage-expressed HDACs and/or selective delivery of pan HDACi to macrophages may provide avenues for enhancing efficacy of HDACi in therapeutic applications, while limiting unwanted side effects.

Antiinflammatory Activity of a Novel Folic Acid Targeted Conjugate of the mTOR Inhibitor Everolimus

Folate receptor (FR)-β has been identified as a promising target for antimacrophage and antiinflammatory therapies. In the present study, we investigated EC0565, a folic acid-derivative of everolimus, as a FR-specific inhibitor of the mammalian target of rapamycin (mTOR). Because of its amphiphilic nature, EC0565 was first evaluated for water solubility, critical micelle formation, stability in culture and FR-binding specificity. Using FR-expressing macrophages, the effect of EC0565 on mTOR signaling and cellular proliferation was studied. The pharmacokinetics, metabolism and bioavailability of EC0565 were studied in normal rats. The in vivo activity of EC0565 was assessed in rats with adjuvant arthritis, a "macrophage-rich" model with close resemblance to rheumatoid arthritis. EC0565 forms micellar aggregates in physiological buffers and demonstrates good water solubility as well as strong multivalent FR-binding capacity. EC0565 inhibited mTOR signaling in rat macrophages at nanomolar concentrations and induced G0/G1 cell cycle arrest in serum-starved RAW264.7 cells. Subcutaneously administered EC0565 in rats displayed good bioavailability and a relatively long half-life (~12 h). When given at 250 nmol/kg, EC0565 selectively inhibited proliferating cell nuclear antigen expression in thioglycollate-stimulated rat peritoneal cells. With limited dosing regimens, the antiarthritic activity of EC0565 was found superior to that of etanercept, everolimus and a nontargeted everolimus analog. The in vivo activity of EC0565 was also comparable to that of a folate-targeted aminopterin. Folate-targeted mTOR inhibition may be an effective way of suppressing activated macrophages in sites of inflammation, especially in nutrient-deprived conditions, such as in the arthritic joints. Further investigation and improvement upon the physical and biochemical properties of EC0565 are warranted.

Positron-emission tomography molecular imaging of glia and myelin in drug discovery for multiple sclerosis

INTRODUCTION

Therapies acting on glial cells are being explored for new drug development for multiple sclerosis. Molecular imaging using positron-emission tomography (PET) could address relevant questions in early phase clinical trials.

AREAS COVERED

In this article, the authors critically review human PET methods that can be applied in specialised centres for imaging activated microglia and astrocytes and myelin.

EXPERT OPINION

Strengths of PET lie in the molecular selectivity, sensitivity and potential for absolute quantitation. Even now, translocator protein PET radioligands could be used in exploratory studies for interventions targeting brain microglial activation. The clinical and neuropathological meaningfulness of signal from PET radioligands reporting on astrocyte activation through cellular expression of either monoamine oxidase B or the I2-imidazoline receptor or metabolism of [(11)C]acetate can now explored. [(11)C] N-methyl-4,4'-diaminostilbene, a PET marker for myelin, could soon enter first human trials. However, use of any of these PET glial markers demands a well-focused hypothesis and a commitment to validation in the context of use. Enhanced access to these radioligands, standardisation of analyses and lowering the costs of using them are needed if their full promise is to be realised.

Folate receptor-β in activated macrophages: ligand binding and receptor recycling kinetics

Activated macrophages overexpress a receptor for the vitamin folic acid termed the folate receptor β (FR-β). Because conjugation of folate to low molecular weight drugs, genes, liposomes, nanoparticles, and imaging agents has minor effects on FR binding, the vitamin can be exploited to target both therapeutic and imaging agents to activated macrophages without promoting their uptake by other healthy cells. In this paper, we characterize the binding, internalization, and recycling kinetics of FR-β on activated macrophages in inflamed tissues of rats with adjuvant-induced arthritis. Our results demonstrate that saturation of macrophage FR is achieved at injection doses of ∼150-300 nmol/kg, with more rapidly perfused tissues saturating at lower doses than inflamed appendages. After binding, FR-β internalizes and recycles back to the cell surface every ∼10-20 min, providing empty receptors for additional folate conjugate uptake. Because the half-life of low molecular weight folate conjugates in the vasculature is usually <1 h, these data suggest that targeting of folate conjugates to activated macrophages in vivo can be maximized by frequent dosing at conjugate concentrations that barely saturate FR (∼150 nmol/kg), thereby minimizing nonspecific binding to receptor-negative tissues and maximizing the probability that unoccupied cell surface receptors will be exposed to folate-drug conjugate.