Selective liposome targeting of folate receptor positive immune cells in inflammatory diseases.

Activated macrophages play a key role in the development and maintenance of inflammatory diseases such as atherosclerosis, lupus, psoriasis, rheumatoid arthritis, ulcerative colitis, and many others. These activated macrophages, but not resting or quiescent macrophages highly up-regulate folate receptor beta (FR-ß). This differential expression of FR-ß provides a mechanism to selectively deliver imaging and therapeutic agents utilizing folate as a targeting molecule. In an effort to determine whether inflammatory diseases can be targeted utilizing a folate-linked nanosize carrier, a PEG-coated liposome was prepared that incorporated a folate conjugated PEG that also could transport imaging or therapeutic cargo. We demonstrate that these folate-liposomes specifically bind to folate receptor positive cells and accumulate at sites of inflammation in mouse models of colitis and atherosclerosis. These two animal models show that folate-targeted liposomes could be successfully utilized to deliver fluorescent molecules and an anti-inflammatory drug (betamethasone) for diagnostic and therapeutic applications.

Folate-conjugated liposomes target and deliver therapeutics to immune cells in a rat model of rheumatoid arthritis.

AIM: We endeavored to create a folate-targeted liposome (Fol-liposome) that could selectively target areas of inflammation. MATERIALS & METHODS: Fol-liposomes were prepared with encapsulated DiD fluorophore or betamethasone (BM) to image and treat an adjuvant-induced rat model of rheumatoid arthritis. RESULTS: Fol-liposomes selectively accumulated in arthritic rat paws to a greater extent than nontargeted liposomes. When these Fol-liposomes were used to encapsulate BM and administered to arthritic rats, animals exhibited less paw swelling, lower arthritis scores, a reduction in bone erosion, less splenomegaly and better maintenance of body weight when compared with nontreated or nontargeted BM-containing liposome groups. CONCLUSION: Fol-liposomes can selectively deliver imaging and therapeutic agents to sites of inflammation in a rat model of rheumatoid arthritis.

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

Activated macrophages overexpress a receptor for the vitamin folic acid termed the folate receptor ß (FR-ß). 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-ß 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-ß 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.

A folate receptor-α-specific ligand that targets cancer tissue and not sites of inflammation.

UNLABELLED: Folic acid has been frequently exploited to target attached drugs to cells that overexpress a folate receptor (FR). Unfortunately, folic acid and folate-linked drugs bind equally well to both major isoforms of the FR-that is, FR-α, which is primarily expressed on malignant cells, and FR-ß, which is upregulated on activated monocytes and macrophages. Because both major isoforms of FR can be expressed simultaneously in the same organism, folic acid cannot enable selective targeting of therapeutic and imaging agents to either tumor masses or sites of inflammation. In an effort to develop a targeting ligand that can selectively deliver attached imaging and therapeutic agents to tumor cells, we constructed a reduced and alkylated form of folic acid, N(5), N(10)-dimethyl tetrahydrofolate (DMTHF) that exhibits selectivity for FR-α. METHODS: DMTHF-(99m)Tc was injected into mice bearing FR-α-expressing tumor xenografts and imaged by γ-scintigraphy. The selectivity for FR-α over FR-ß in vivo was examined by γ-scintigraphic images of animal models of various inflammatory diseases such as apolipoprotein E-deficient mice with atherosclerosis, DBA/1 LacJ mice with induced arthritis, C57BL/6J mice with muscle injury, and BALB/C mice with both FR-α tumor and ulcerative colitis, by administration of equal doses of DMTHF-(99m)Tc and EC20-(99m)Tc. The uptake of radiochelates in various organs was quantified by biodistribution studies. DMTHF-near-infrared dye conjugate and DMTHF-Oregon green dye conjugates were synthesized and evaluated for FR-α selectivity over FR-ß in rat peritoneal macrophages and human peripheral blood monocytes, respectively, by flow cytometry. Fluorescence-guided imaging was also performed using folate and DMTHF dye conjugates. RESULTS: The new targeting ligand was found to bind malignant cells in mice with solid tumor xenografts but not peripheral blood monocytes or inflammatory macrophages in animal models of atherosclerosis, rheumatoid arthritis, muscle injury, or ulcerative colitis. Results from optical and radioimaging studies and biodistribution experiments confirm the differential specificity of this new ligand for malignant masses. CONCLUSION: The new targeting ligand DMTHF enables selective noninvasive imaging and therapy of tumor tissues in the presence of inflammation.

Imaging sites of infection using a 99mTc-labeled folate conjugate targeted to folate receptor positive macrophages.

EC20, a folate-targeted (99m)Tc based radioimaging agent with a high folate receptor (FR) binding affinity, has been used for both the diagnosis and the staging of FR positive malignancies (currently in phase III trials) and also for the localization of inflamed lesions characterized by the accumulation of FR+ macrophages. Because recent evidence has suggested that FR+ macrophages might accumulate at sites of infectious disease, this study evaluated whether EC20 might prove similarly useful for imaging bacterial infection foci. Using gamma scintigraphic imaging, it was demonstrated that EC20 accumulated at sites of Staphylococcus aureus infection with a significant difference (P < 0.0001, n = 12) in enrichment noted between infected and noninfected limbs. Confirmation that the elevated uptake of EC20 in infected limbs was FR-mediated was supported by suppression of EC20 accumulation in the presence of a 200-fold excess of free folic acid (P < 0.0001, n = 12). This study establishes for the first time the use of EC20 to image and localize sites of infectious disease.

Synthesis and activity of folate conjugated didemnin B for potential treatment of inflammatory diseases.

A folate receptor targeted didemnin B conjugate was synthesized using a hydrophilic peptide spacer linked to folate via a releasable disulfide carbonate linker. Cell cytotoxicity and TNF-α inhibition in RAW264.7 macrophage-like cells exhibited IC(50)s of 13 and 5 nM, respectively. Folate didemnin B was found to be ∼50-100 fold more potent than didemnin B itself. More importantly, activity of the prodrug was blocked by excess folic acid, demonstrating receptor-mediated cellular uptake of the conjugate.

Imaging of atherosclerosis in apoliprotein e knockout mice: targeting of a folate-conjugated radiopharmaceutical to activated macrophages.

UNLABELLED: Early detection of heart disease is essential for the implementation of intervention strategies that reduce the risk of cardiovascular events. Radioimaging methods that have been explored for this purpose include (18)F-FDG, which measures sites of elevated metabolic activity; (99m)Tc-annexin A5, which reveals regions of enhanced apoptosis and thrombosis; and (99m)Tc-labeled anti-lectinlike oxidized low-density lipoprotein receptor 1 antibody, which detects the lectinlike oxidized low-density lipoprotein receptor 1 that is overexpressed on a variety of vasculature-associated cells. In this study, we examine the use of a folate-targeted chelate of (99m)Tc, termed (99m)Tc-EC20, for imaging of folate receptor (FR)-expressing macrophages that accumulate in atherosclerotic plaques, internalize cholesterol-rich lipoprotein particles, and evolve into foam cells that form components of vulnerable atherosclerotic lesions. METHODS: (99m)Tc-EC20 was injected into apoliprotein E knockout (apoE-/-) mice fed a normal or Western (high-fat) diet for 25 wk and imaged by gamma-scintigraphy. Treated mice were also dissected, and radioactivities in excised aortas were quantified by gamma-counting and imaged by autoradiography. The role of FR-expressing macrophages in uptake of (99m)Tc-EC20 was also examined by comparing images of apoE-/- mice before and after treatment with clodronate liposomes to deplete tissue macrophages, comparing the sites of (99m)Tc-EC20 enrichment with sites of macrophage accumulation in thin sections of atherosclerotic tissues, and examining the expression of FRs on atherosclerotic plaque-derived macrophages by flow cytometry. RESULTS: ApoE-/- mice on Western chow exhibited significantly greater accumulation of (99m)Tc-EC20 in atherosclerotic lesions than their counterparts on normal chow. The aortas of apoE-/- mice on a Western diet demonstrated greater numbers of FR-positive macrophages by flow cytometry than did those of apoE-/- mice on a normal diet. Clodronate liposome treatment significantly reduced the accumulation of (99m)Tc-EC20 in atherosclerotic tissues, suggesting that macrophages or monocytes are responsible for uptake of the folate-linked radioimaging agent. Histologic and autoradiographic analysis of tissue sections demonstrated that macrophage accumulation correlated with regions of (99m)Tc-EC20 uptake. CONCLUSION: (99m)Tc-EC20 can be used for the imaging of atherosclerosis by selectively targeting FR-positive activated macrophages.

Folate-targeted hapten immunotherapy of adjuvant-induced arthritis: comparison of hapten potencies.

We have previously reported that disease symptoms can be greatly ameliorated in rodents with adjuvant-induced arthritis (AIA) by first immunizing the rodents against fluorescein and then treating the animals with folate-fluorescein. In this targeted hapten therapy, folate-fluorescein was shown to decorate folate receptor (FR)-expressing activated macrophages with fluorescein (an immunogenic hapten), leading to binding of antifluorescein antibodies and the consequent elimination of the activated macrophages by Fc receptor-expressing immune cells. In the current study, we compare the therapeutic potencies of a variety of FR-targeted haptens in treating the symptoms of AIA in rats. Rats were immunized with either dinitrophenyl (DNP) or trinitrophenyl (TNP) conjugated to keyhole limpet hemocyanin followed by induction of AIA with heat-inactivated Mycobacterium butyricum. Following development of arthritis, rats were treated with one of five folate-hapten conjugates (folate-DNP1, folate-DNP2, folate-DNP3, folate-FITC, or folate-TNP) at two different doses (30 nmol/kg or 200 nmol/kg) 5x/week for 25 days. Symptoms of AIA in treated rats, including paw swelling, arthritis score, splenomegaly, bone erosion, and FR(+) activated macrophage density in inflamed tissues, were quantitated over the course of therapy. Although all folate-hapten conjugates promoted a reduction in disease symptoms, folate-TNP and folate-FITC proved to be more potent than any of the 3 folate-DNP conjugates. We conclude that both folate-TNP and folate-FITC constitute promising haptens for use in FR-targeted immunotherapy of arthritis.

A second-site suppressor significantly improves the defective phenotype imposed by mutation of an aromatic residue in the N-terminal domain of the HIV-1 capsid protein.

The HIV-1 capsid (CA) protein plays an important role in virus assembly and infectivity. Previously, we showed that Ala substitutions in the N-terminal residues Trp23 and Phe40 cause a severely defective phenotype. In searching for mutations at these positions that result in a non-lethal phenotype, we identified one candidate, W23F. Mutant virions contained aberrant cores, but unlike W23A, also displayed some infectivity in a single-round replication assay and delayed replication kinetics in MT-4 cells. Following long-term passage in MT-4 cells, two second-site mutations were isolated. In particular, the W23F/V26I mutation partially restored the wild-type phenotype, including production of particles with conical cores and wild-type replication kinetics in MT-4 cells. A structural model is proposed to explain the suppressor phenotype. These findings describe a novel occurrence, namely suppression of a mutation in a hydrophobic residue that is critical for maintaining the structural integrity of CA and proper core assembly.