Studies of the role of a particulate folate-binding protein in the uptake of 5-methyltetrahydrofolate by cultured human KB cells

Liposomal delivery of gene therapy for ovarian cancer: a systematic review

OBJECTIVE

To systematically identify and narratively synthesize the evidence surrounding liposomal delivery of gene therapy and the outcome for ovarian cancer.

METHODS

An electronic database search of the Embase, MEDLINE and Web of Science from inception until July 7, 2023, was conducted to identify primary studies that investigated the effect of liposomal delivery of gene therapy on ovarian cancer outcomes. Retrieved studies were assessed against the eligibility criteria for inclusion.

RESULTS

The search yielded 564 studies, of which 75 met the inclusion criteria. Four major types of liposomes were identified: cationic, neutral, polymer-coated, and ligand-targeted liposomes. The liposome with the most evidence involved cationic liposomes which are characterized by their positively charged phospholipids (n = 37, 49.3%). Similarly, those with neutrally charged phospholipids, such as 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine, were highly researched as well (n = 25, 33.3%). Eight areas of gene therapy research were identified, evaluating either target proteins/transcripts or molecular pathways: microRNAs, ephrin type-A receptor 2 (EphA2), interleukins, mitogen-activated protein kinase (MAPK), human-telomerase reverse transcriptase/E1A (hTERT/EA1), suicide gene, p53, and multidrug resistance mutation 1 (MDR1).

CONCLUSION

Liposomal delivery of gene therapy for ovarian cancer shows promise in many in vivo studies. Emerging polymer-coated and ligand-targeted liposomes have been gaining interest as they have been shown to have more stability and specificity. We found that gene therapy involving microRNAs was the most frequently studied. Overall, liposomal genetic therapy has been shown to reduce tumor size and weight and improve survivability. More research involving the delivery and targets of gene therapy for ovarian cancer may be a promising avenue to improve patient outcomes.

Formulation Strategies for Folate-Targeted Liposomes and Their Biomedical Applications

The folate receptor (FR) is a tumor-associated antigen that can bind with folic acid (FA) and its conjugates with high affinity and ingests the bound molecules inside the cell via the endocytic mechanism. A wide variety of payloads can be delivered to FR-overexpressed cells using folate as the ligand, ranging from small drug molecules to large DNA-containing macromolecules. A broad range of folate attached liposomes have been proven to be highly effective as the targeted delivery system. For the rational design of folate-targeted liposomes, an intense conceptual understanding combining chemical and biomedical points of view is necessary because of the interdisciplinary nature of the field. The fabrication of the folate-conjugated liposomes basically involves the attachment of FA with phospholipids, cholesterol or peptides before liposomal formulation. The present review aims to provide detailed information about the design and fabrication of folate-conjugated liposomes using FA attached uncleavable/cleavable phospholipids, cholesterol or peptides. Advances in the area of folate-targeted liposomes and their biomedical applications have also been discussed.

Redox-responsive chemosensitive polyspermine delivers ursolic acid targeting to human breast tumor cells: The depletion of intracellular GSH contents arouses chemosensitizing effects

Antitumor efficacy of ursolic acid (UA) is seriously limited due to its low hydrophilicity and needy bioavailability. To overcome these obstacles, chemosensitive polyspermine (CPSP) conjugated with UA and folic acid (FA) as a novel targeted prodrug was designed and successfully synthesized in this investigation. This prodrug not only showed high aqueous solubility, GSH-triggered degradation and good biocompatibility, but also exhibited better inhibition effect on the tumor cells proliferation in comparison with free UA. FA-CPSP-UA could down-regulate the generation of GSH and manifest excellent ability in enhancing antitumor efficacy. In addition, FA-CPSP-UA could inhibit the expression of MMP-9, which led to restricting MCF-7 cells migration. Taken together, the results indicated that FA-CPSP-UA, as a carrier, can efficiently deliver UA to folate receptor positive cancer cells and improve tumor therapy of UA by Chemosensitive effect.

Folate receptor targeted three-layered micelles and hydrogels for gene delivery to activated macrophages

New folic acid (FA) coupled three layered micelles (3LM) were designed to encapsulate DNA, and their application as delivery system that specifically targets activated macrophages was investigated for new treatment options in rheumatoid arthritis (RA). FA coupled poly(l-lactide)-b-poly(ethylene glycol) (FA-PEG-PLLA) was synthesized via the NHS-ester activated/amine coupling method. Fluorescein labeled folic acid was used for flow cytometric detection of the expression of functional folic receptor β in LPS-activated and resting macrophages. FA coupled 3LM were formulated in a two-step procedure and characterized regarding hydrodynamic diameters and zeta potentials. The presence of the targeting ligand was shown not to increase the size of the 3LM compared to their non-targeted counterparts. Targeted and non-targeted 3LM were used in vitro to optimize uptake conditions in the RAW 264.7 macrophage cell line. The amount of FA coupled polymer in the final formulation was found to be optimal at 75% FA-PEG-PLLA and 25% PLLA-PEG-PLLA. Subsequently, transgene expression in vitro in RAW 264.7 cells and ex vivo in primary activated and resting mouse macrophages was determined as a function of FR-mediated internalization of 3LM encapsulating GFP expressing plasmid. FR-overexpressing activated cells, as successfully identified by internalization of FA-fluorescein, showed significantly higher GFP expression in vitro and ex vivo than resting macrophages with only a basal level of FR expression. Lastly, injectable hydrogels as depot formulation were formed by stereocomplexation, and their degradation, DNA release profiles, and dissociation into intact 3LM were found to be beneficial for potential in vivo application. Our findings confirm that FA-3LM are taken up by activated macrophages via folate receptor mediated endocytosis and that their hydrogels release intact 3LM for efficient transfection of primary macrophages. Therefore, FA-3LM could become a promising delivery system for receptor-mediated drug or gene delivery and novel therapy for rheumatoid arthritis in an in situ forming gel formulation.

Targeted Nanotherapies for the Treatment of Surgical Diseases

OBJECTIVE

To describe the components of targeted nanotherapeutics and to review their applications in the treatment of surgical diseases.

BACKGROUND

Targeted nanotherapeutic is a novel strategy for treating a variety of diseases and is an emerging technology that offers advantages over current treatment strategies. The nanoscale size, combined with the ability to surface functionalize the delivery vehicle to enable targeting and incorporate a therapeutic payload, provides a new and innovative therapeutic platform to treat surgical diseases that has yet to be fully realized in the surgical arena.

METHODS

A comprehensive literature review of nanotherapeutics, targeting strategies, and their utility in treating surgical diseases is performed.

RESULTS

Targeted nanotherapeutics have demonstrated safety and biocompatibility in treating surgical diseases. The ability to surface functionalize the nanoparticles affords a unique tailorability that enables targeting specificity and therapeutic payload delivery to treat a variety of surgical diseases. Moreover, the small size and targeting capabilities allow access to biological compartments, such as the blood-brain barrier, that have previously been difficult to treat.

CONCLUSIONS

Targeted nanotherapeutics represent a novel therapeutic platform and have great potential to impact the treatment of surgical diseases.

Folate Receptor β Regulates Integrin CD11b/CD18 Adhesion of a Macrophage Subset to Collagen

Folate, also known as vitamin B9, is necessary for essential cellular functions such as DNA synthesis, repair, and methylation. It is supplied to the cell via several transporters and receptors, including folate receptor (FR) β, a GPI-anchored protein belonging to the folate receptor family. As FRβ shows a restricted expression to cells of myeloid origin and only a subset of activated macrophages and placental cells have been shown to express functional FRβ, it represents a promising target for future therapeutic strategies. In this study, we performed affinity purification and mass spectrometric analysis of the protein microenvironment of FRβ in the plasma membrane of human FRβ(+) macrophages and FRβ-transduced monocytic THP-1 cells. In this manner, we identified a novel role of FRβ: that is, we report functional interactions of FRβ with receptors mediating cellular adhesion, in particular the CD11b/CD18 β2 integrin heterodimer complement receptor type 3/Mac-1. This interaction results in impeded adhesion of FRβ(+) human primary macrophages and THP-1 cells to collagen in comparison with their FRβ(-) counterparts. We further show that FRβ is only expressed by human macrophages when differentiated with M-CSF. These findings thus identify FRβ as a novel CD11b/CD18 regulator for trafficking and homing of a subset of macrophages on collagen.

Folate-Modified Cholesterol-Bearing Pullulan, a New Cancer-Targeted Nanoparticle Drug Carrier: Synthesis and Applications

Folate-modified cholesterol-bearing pullulan (FA-CHP) was synthesized by the reaction of folic acid [.beta]-2-aminoethylamide and 4-nitrophenyl chloroformate-activated cholesterol-bearing pullulan. The substitution ratio was estimated as 10% per glucoside units. Several combinations of FA-CHP, unmodified cholesterol-bearing pullulan (CHP) and doxorubicin (DOX) mixtures were tested for cancer-selective cytotoxicity in vitro. A combination of FA-CHP/CHP/DOX of 1:4:0.02 (weight ratio) had aggressive and selective effects on human epidermoid cancer KB cells, known to express a high level of folate receptor, while the same nanoparticle mixture had a weak effect on the growth of normal human fetal lung fibroblast TIG-1-20 cells.

Expression of functional folate receptors by human parathyroid cells

BACKGROUND

Human pituitary adenomas express folate receptors (FR); therefore, we hypothesized that parathyroid (PT) tumors also might express FR, whereas normal human thyroids might not. The purpose of our study was to characterize the functionality of FRs on human PT tumors, with the goal of developing an imaging tool that would concentrate in PT more than in the thyroid.

METHODS

Human PTs and thyroids were evaluated for FR expression by immunohistochemistry. Expression of genes for FRα and FRβ was measured with the Illumina Human HT-12 Expression Bead Chips and verified by quantitative reverse-transcription polymerase chain reaction. Folate incorporation by PT cells versus normal thyroid cells was determined by incubation with (99m)Technetium ((99m)Tc)(CO)3-folate and (99m)Tc-Etarfolatide, and uptake was determined by gamma counting. Specific targeting of FRs was demonstrated by blocking with cold folate. A549 cells and Jurkat cells served as FR-negative controls, and KB cells and HeLa cells were FR-positive controls.

RESULTS

On immunohistochemistry and Western blotting, human PT cells expressed FRs, whereas human thyroid cells did not. The FRα gene was expressed in all PTs analyzed, and the FRβ gene was expressed by most. Uptake of (99m)Tc(CO)3-folate was increased in PT cells versus thyroid cells. There was dose-dependent uptake of (99m)Tc-etarfolatide, and uptake was inhibited by preincubation with cold folate, confirming FR-mediated binding.

CONCLUSION

This is the first report of the expression and functionality of FRs on human PT cells. These findings suggest that (99m)Tc-folate holds potential for localization of PT tumors preoperatively and their treatment.

Folate-PEG-appended dendrimer conjugate with α-cyclodextrin as a novel cancer cell-selective siRNA delivery carrier

We previously reported that of the various polyamidoamine (PAMAM) STARBURST dendrimer (generation 3, G3) (dendrimer) conjugates with cyclodextrins (CyDs), the dendrimer (G3) conjugate with α-CyD having an average degree of substitution of 2.4 (α-CDE (G3)) has the greatest potential for a novel carrier for siRNA in vitro and in vivo. To improve the siRNA transfer activity and the lack of target specificity of α-CDE (G3), we prepared folate-polyethylene glycol (PEG)-appended α-CDEs (G3) (Fol-PαCs) with various degrees of substitution of folate (DSF) and evaluated their siRNA transfer activity to folate receptor (FR)-overexpressing cancer cells in vitro and in vivo. Of the three Fol-PαCs (G3, DSF 2, 4 and 7), Fol-PαC (G3, DSF 4) had the highest siRNA transfer activity in KB cells (FR-positive). Fol-PαC (G3, DSF 4) was endocytosed into KB cells through FR. No cytotoxicity of the siRNA complex with Fol-PαC (G3, DSF 4) was observed in KB cells (FR-positive) or A549 cells (FR-negative) up to the charge ratio of 100/1 (carrier/siRNA). In addition, the siRNA complex with Fol-PαC (G3, DSF 4) showed neither interferon response nor inflammatory response. Importantly, the siRNA complex with Fol-PαC (G3, DSF 4) tended to show the in vivo RNAi effects after intratumoral injection and intravenous injection in tumor cells-bearing mice. The FITC-labeled siRNA and TRITC-labeled Fol-PαC (G3, DSF 4) were actually accumulated in tumor tissues after intravenous injection in the mice. In conclusion, the present results suggest that Fol-PαC (G3, DSF 4) could potentially be used as a FR-overexpressing cancer cell-selective siRNA delivery carrier in vitro and in vivo.

Low-dose methotrexate inhibits methionine S-adenosyltransferase in vitro and in vivo

Methionine S-adenosyltransferase (MAT) catalyzes the only reaction that produces the major methyl donor in mammals. Low-dose methotrexate is the most commonly used disease-modifying antirheumatic drug in human rheumatic conditions. The present study was conducted to test the hypothesis that methotrexate inhibits MAT expression and activity in vitro and in vivo. HepG2 cells were cultured under folate restriction or in low-dose methotrexate with and without folate or methionine supplementation. Male C57BL/6J mice received methotrexate regimens that reflected low-dose clinical use in humans. S-adenosylmethionine and MAT genes, proteins and enzyme activity levels were determined. We found that methionine or folate supplementation greatly improved S-adenosylmethionine in folate-depleted cells but not in cells preexposed to methotrexate. Methotrexate but not folate depletion suppressed MAT genes, proteins and activity in vitro. Low-dose methotrexate inhibited MAT1A and MAT2A genes, MATI/II/III proteins and MAT enzyme activities in mouse tissues. Concurrent folinate supplementation with methotrexate ameliorated MAT2A reduction and restored S-adenosylmethionine in HepG2 cells. However, posttreatment folinate rescue failed to restore MAT2A reduction or S-adenosylmethionine level in cells preexposed to methotrexate. Our results provide both in vitro and in vivo evidence that low-dose methotrexate inhibits MAT genes, proteins, and enzyme activity independent of folate depletion. Because polyglutamated methotrexate stays in the hepatocytes, if methotrexate inhibits MAT in the liver, then the efficacy of clinical folinate rescue with respect to maintaining hepatic S-adenosylmethionine synthesis and normalizing the methylation reactions would be limited. These findings raise concerns on perturbed methylation reactions in humans on low-dose methotrexate. Future studies on the clinical physiological consequences of MAT inhibition by methotrexate and the potential benefits of S-adenosylmethionine supplementation on methyl group homeostasis in clinical methotrexate therapies are warranted.

Synthesis of folate-functionalized RAFT polymers for targeted siRNA delivery

Receptor-mediated, cell-specific delivery of siRNA enables silencing of target genes in specific tissues, opening the door to powerful therapeutic options for a multitude of diseases. However, the development of delivery systems capable of targeted and effective siRNA delivery typically requires multiple steps and the use of sophisticated, orthogonal chemistries. Previously, we developed diblock copolymers consisting of dimethaminoethyl methacrylate-b-dimethylaminoethyl methacrylate-co-butyl methacrylate-co-propylacrylic acid as potent siRNA delivery systems that protect siRNA from enzymatic degradation and enable its cytosolic delivery through pH-responsive, endosomolytic behavior. (1, 2) These architectures were polymerized using a living radical polymerization method, specifically reversible addition-fragmentation chain transfer (RAFT) polymerization, which employs a chain transfer agent (CTA) to modulate the rate of reaction, resulting in polymers with low polydispersity and telechelic chain ends reflecting the chemistry of the CTA. Here we describe the straightforward, facile synthesis of a folate receptor-targeted diblock copolymer siRNA delivery system because the folate receptor is an attractive target for tumor-selective therapies as a result of its overexpression in a number of cancers. Specifically, we detail the de novo synthesis of a folate-functionalized CTA, use the folate-CTA for controlled polymerizations of diblock copolymers, and demonstrate efficient, specific cellular folate receptor interaction and in vitro gene knockdown using the folate-functionalized polymer.

Targeting the efficacy of a dendrimer-based nanotherapeutic in heterogeneous xenograft tumors in vivo

Our earlier studies have shown the in vitro and in vivo targeting of a generation 5 (G5) dendrimer-based multifunctional conjugate that contained folic acid (FA) as the targeting agent and methotrexate (MTX) as the chemotherapeutic drug. To clinically apply the synthesized G5-FA-MTX nanotherapeutic, it is important that the anticancer conjugate elicits cytotoxicity specifically and consistently. Toward this objective, we evaluated the large-scale synthesis of a G5-FA-MTX conjugate (Lot # 123-34) for its cytotoxic potential and specificity in vitro and in vivo. The cytotoxicity and specificity were tested by using a coculture assay in which FA receptor-expressing and nonexpressing cells (KB and SK-BR-3 cells, respectively) were cultured together and preferential killing was examined. The in-vitro data were compared with the in-vivo data obtained from a heterogeneous xenograft tumor model. The animal model of the artificial heterogeneous xenograft tumor showed that the nanotherapeutic was preferentially cytotoxic to KB cells.

Targeted dendrimeric anticancer prodrug: a methotrexate-folic acid-poly(amidoamine) conjugate and a novel, rapid, "one pot" synthetic approach

A targeted dendrimeric anticancer prodrug, a conjugate of generation 5 (G5) polyamidoamine (PAMAM) dendrimer, folic acid (FA), and methotrexate (MTX), has been successfully synthesized by using a novel "one pot" approach which is simple, reproducible, and feasible for large-scale synthesis. All dendrimer products have been characterized by (1)H NMR, MALDI-TOF, GPC, and HPLC. With this new method, the ratio of FA versus MTX attached to the dendrimer can be easily tuned to achieve the desired therapeutic effect. A new analytical approach for calculating the numbers of FA and MTX attached to the dendrimer has been established. In vitro studies performed on FA receptor-expressing KB cells show that the new conjugate has a similar affinity and cytotoxic potency to G5-FA-MTX synthesized using the traditional multiple-step approach.

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 <or=90 min synthesis time. The new compound was pharmacologically evaluated in vitro and in vivo. The affinity of the non-radioactive (19)F-click folate to the FR was determined in displacement studies with FR expressing KB tumor cells using (3)H-folic acid. In these in vitro binding studies, a nanomolar affinity with a K(i) of 9.76+/-3.13 nM was found for (19)F-click folate. The (18)F-labeled click folate derivative was then applied for in vivo PET studies and ex vivo biodistribution experiments using nude mice bearing KB tumor xenografts. The post mortem dissection experiments showed a high specific uptake of (18)F-click folate derivative in FR-expressing tissues. Uptake in KB tumor xenografts and kidneys (FR-positive tissue) amounted to 3.13%ID/g (94% specific blockade) and 16.53%ID/g (75% specific blockade), respectively. PET imaging using (18)F-click folate permitted a visualization of KB tumors, and blockade studies confirmed the specific accumulation of the radiotracer in vivo. However, strong hepatobiliary excretion of the new tracer led to elevated accumulation of radioactivity in the abdominal region. In conclusion, the click chemistry approach is convenient to accomplish and provided high radiochemical yields of (18)F-click folate. The new tracer showed good in vitro but limited in vivo properties. Ultimately, the (18)F-click folate emphasizes the potential of (18)F-labeled folates for receptor-based tumor PET imaging.

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.

Preclinical antitumor efficacy evaluation of dendrimer-based methotrexate conjugates

Our previous studies have demonstrated the in-vitro and in-vivo targeting of a generation-5 (G5) dendrimer-based multifunctional conjugate, which used folic acid (FA) as the targeting agent and methotrexate (MTX) as the chemotherapeutic drug. For the synthesized G5-FA-MTX nanodevice conjugate to be clinically applicable as a cancer therapeutic drug, it is important that the compound elicits cytotoxicity specifically and consistently. The aim of this work was to evaluate four independently synthesized batches of G5-FA-MTX conjugates for their cytotoxic potential and specificity. For determination of specificity, we have used a unique 'coculture' assay in which FA receptor-positive and FA receptor-negative cells were cultured together and have examined the preferential killing of the former. The results of our study show the batch-to-batch consistency and specificity of the G5-FA-MTX nanodevice in the preferential killing of FA receptor-positive cells. The coculture assay shows the consistency of the four different G5-FA-MTX conjugate lots in the specific killing of targeted cells. Further in-vivo studies are, however, necessary to prove the clinical potential of this targeted therapeutic nanodevice.

A novel monoclonal antibody for detection of folate receptor alpha in paraffin-embedded tissues

Folate biochemical pathway components such as FR-alpha are determinants of response to novel antifolate drugs such as pemetrexed. Knowledge of their level of expression in tumors will enable their optimal use by identifying potentially responsive subgroups of patients. In spite of its importance in the diagnosis and treatment of cancer, monoclonal antibodies to FR-alpha suitable for immunohistochemical analysis of formalin-fixed and paraffin-embedded biopsy samples, or that can be used for Western blot analysis, are not available. The aim of this study was thus to generate a monoclonal antibody that could be used to detect specific expression of FR-alpha in paraffin-embedded tissues. A 189 amino acid region of the FR-alpha sequence was expressed as a recombinant fusion protein and used as antigen to generate monoclonal antibodies. Studies by ELISA and Western blot identified specific reactivity of the BN3.2 antibody to the recombinant protein and a single 40 kD protein in whole cell lysates from cell lines known to express FR-alpha. Immunohistochemical analysis of FR-alpha using hybridoma clone BN3.2 in a panel of normal tissues demonstrated expression limited to ovarian epithelia, placental trophoblasts, and proximal kidney tubules. Analysis of a panel of malignant and benign tissues demonstrated limited expression with variable intensities of staining and patterns of both membrane and cytoplasmic reactivity observed between cases. In the majority of malignant ovarian tumors, high intensity staining was observed, predominantly localized to the plasma membrane. Our results show that clone BN3.2 is a sensitive tool for detection of FR-alpha in paraffin-embedded tissues. This preliminary study also supports its use in immunohistochemical studies to determine the role of FR-alpha as a tumor prognostic marker and a possible therapeutic target.

Exploitation of the folate receptor in the management of cancer and inflammatory disease

Over the last 25+ years, the folate receptor (FR) has emerged as an attractive tumor biomarker with the potential to be exploited for therapeutic purposes. Increasing evidence suggests that this endocytosing protein can functionally mediate the cellular uptake and retention of natural folates, certain antifolates, and folate-drug conjugates; the consequences of the latter two events could result in biological modulation, including (but not limited to) tumor-targeted cytotoxicity. Because its tissue expression profile appears to be somewhat limited to either tissues responsible for whole body retention of folates (e.g., kidney and placenta), or certain pathologic tissues (e.g., tumors or activated macrophages), the FR is believed to be a useful biological target for disease management. Indeed, recent years have been peppered with reports of novel FR-targeted therapies, and many have demonstrated impressive in vivo potency, particularly against tumor xenografts, without the undesirable toxicity that often accompanies nontargeted drug regimens. This chapter will provide essential details on the properties of the FR, including where it is expressed and how it has been successfully manipulated for therapeutic benefit.

Discovery and development of folic-acid-based receptor targeting for imaging and therapy of cancer and inflammatory diseases

In order to avoid the toxicities associated with prescription drug use today, we have explored novel methods for delivering drugs selectively to pathologic cells, thereby avoiding the collateral damage that accompanies their uptake by healthy cells. In this Account, we describe our quest for the ideal targeted therapeutic agent. This effort began with a search for ligands that would bind selectively to pathologic cells, displaying no affinity for healthy cells. After identification of an optimal targeting ligand, effort was focused on construction of linkers that would carry the attached drug to pathologic cells with receptors for the selected ligand. In the case of cancer, we exploited the well-characterized up-regulation of folate receptors on malignant cells to target folate-linked pharmaceuticals to cancer tissues in vivo. Drugs that have been linked to folic acid for tumor-selective drug delivery to date include (i) protein toxins, (ii) chemotherapeutic agents, (iii) gene therapy vectors, (iv) oligonucleotides (including small interfering RNA (siRNA)), (v) radioimaging agents, (vi) magnetic resonance imaging (MRI) contrast agents, (vii) liposomes with entrapped drugs, (viii) radiotherapeutic agents, (ix) immunotherapeutic agents, and (x) enzyme constructs for prodrug therapy. Current clinical trials of four folate-linked drugs demonstrate that folate receptor-targeting holds great promise for increasing the potency while reducing toxicity of many cancer therapies. In the course of developing folate-conjugated drugs for cancer, we discovered that folate receptors are also overexpressed on activated (but not resting or quiescent) macrophages. Recognizing that activated macrophages either cause or contribute to such diseases as rheumatoid arthritis, Crohn's disease, atherosclerosis, lupus, inflammatory osteoarthritis, diabetes, ischemia reperfusion injury, glomerulonephritis, sarcoidosis, psoriasis, Sjogren's disease, and vasculitis, we initiated studies aimed at developing folate-conjugated imaging and therapeutic agents for the diagnosis and treatment of such diseases. In very brief time, significant progress has been made towards identification of clinical candidates for targeted treatment of several inflammatory and autoimmune diseases. This Account summarizes the discovery and development of a variety of folate-targeted drugs for the diagnosis and therapy of cancers and inflammatory/autoimmune diseases.

Dose-dependent effects of (anti)folate preinjection on 99mTc-radiofolate uptake in tumors and kidneys

INTRODUCTION

The folate receptor (FR) is frequently overexpressed in tumors and can be targeted with folate-based (radio)pharmaceuticals. However, significant accumulation of radiofolates in FR-positive kidneys represents a drawback. We have shown that preadministration of the antifolate pemetrexed (PMX) significantly improved the tumor-to-kidney ratio of radiofolates in mice. The aim of this study was to investigate the dose dependence of these effects and whether the same results could be achieved with folic acid (FA) or 5-methyl-tetrahydrofolate (5-Me-THF).

METHODS

Biodistribution was assessed 4 h postinjection of the organometallic (99m)Tc-picolylamine monoacetic acid folate in nude mice bearing FR-positive KB tumor xenografts. PMX (50-400 microg/mouse) was injected 1 h previous to radioactivity. The effects of FA and 5-Me-THF (0.5-50 microg/mouse) were investigated likewise. Tissues and organs were collected and counted for radioactivity and the values tabulated as percentage of injected dose per gram tissue (% ID/g).

RESULTS

PMX administration reduced renal retention (<1.6% ID/g vs. control: >10% ID/g), while the tumor uptake (average 1.35%+/-0.40% ID/g vs. control: 1.79%+/-0.49% ID/g) was only slightly affected independent of the PMX dose. Replacement of PMX by FA or 5-Me-THF (50 microg/mouse) resulted in a significant renal blockade (<0.1% ID/g) but at the same time in an undesired reduction of tumor uptake (<0.2% ID/g).

CONCLUSIONS

Selective reduction of radiofolate uptake in kidneys under retention of high tumor accumulation could be achieved in combination with PMX over a broad dose range but not with FA or 5-Me-THF.

Dendrimer-Based BH3 Conjugate That Targets Human Carcinoma Cells

Our previous studies have demonstrated the efficacy of generation 5 poly(amidoamine) dendrimers (G5-PAMAM) as a platform for the targeted delivery of chemotherapeutics. However, anticancer therapy can be subverted by anti-apoptotic changes in cancer cells. Bcl-2 and several of its peptides are commonly overexpressed in a number of cancers. A means to reverse this anti-apoptotic mechanism is to expose the cells to BH3 peptide, which has a homology to the anti-apoptotic Bcl-2 protein. This cannot be done indiscriminately because it can induce apoptosis in normal cells. In order to specifically target BH3 peptides to cancer cells, we synthesized a trifunctional, G5-PAMAM-based nanodevice to which folic acid was conjugated as a targeting agent, along with fluorescein isothiocyanate as the reporter agent and BH3 peptides that were used to induce apoptosis. The results show, for the first time, the therapeutic potential of targeted BH3 peptides as a means of inducing apoptosis by interfering with anti-apoptotic proteins within specific cells.

Isostructural folate conjugates radiolabeled with the matched pair 99mTc/188Re: a potential strategy for diagnosis and therapy of folate receptor-positive tumors

(99m)Tc-technetium ((99m)Tc) and (188)Re-rhenium ((188)Re) represent an interesting pair of radionuclides for diagnosis and therapy. The aim of this study was to synthesize and characterize in vitro/in vivo the first (188)Re-folate derivative [(188)Re(CO)(3)-picolylamine monoacetic acid 188/Re-OANA-folate (2)] for potential targeted radionuclide therapy of FR-positive tumors. The data were compared with those of the isostructural (99m)Tc-analog [(99m)Tc-PAMA folate (1)] reported previously.

METHODS

In vitro stability of compound 2 was tested in phosphate-buffered saline and human plasma. Cell binding experiments were performed with FR-positive human KB cells. Biodistribution was assessed in female nude mice, bearing KB tumor xenografts.

RESULTS

Cell binding experiments showed high and FR-specific uptake. In vivo, compound 2 accumulated specifically in the FR-positive tumors with maximal values 4 h post injection (p.i.) ['2: 1.87+/-0.04 percent injected dose per gram of weight tissue (% ID/g) vs. '1: 2.33+/-0.36% ID/g]. Unfavorably high retention of radioactivity was found in FR-positive kidneys (12.04+/-0.62% ID/g; 4 h p.i.). Tumor-to-blood ratio of radioactivity ('2: 14.5+/-1.32, 4 h p.i.) was lower than for compound '1 (58.0+/-12.2, 4 h p.i.), whereas tumor-to-kidney ratios were in the same range ('2: 0.15+/-0.01 vs. '1: 0.13+/-0.02, 4 h p.i.). Preadministration of the antifolate pemetrexed significantly improved the tumor-to-kidney ratio (2: 1.59+/-0.30, 4 h p.i.).

CONCLUSIONS

The isostructural radiofolates 1 and '2 displayed almost identical pharmacokinetic profiles and accumulated both specifically in FR-positive tumors. However, only the coapplication of the antifolate pemetrexed improved the biodistribution of the radiotracers in such ways that a potential therapeutic application of compound '2 can be envisaged in the future.

The Folate Receptor α Is Frequently Overexpressed in Osteosarcoma Samples and Plays a Role in the Uptake of the Physiologic Substrate 5-Methyltetrahydrofolate

PURPOSE

Two major systems exist for folate cell entry: the reduced folate carrier (RFC) and the folate receptor (FR). Although defective RFC-mediated transport was frequently identified as a mechanism of methotrexate (MTX) resistance in osteosarcoma, the status of FR and its role in this disease are unknown.

EXPERIMENTAL DESIGN

mRNA for FR alpha was measured in 107 osteosarcoma specimens using quantitative reverse transcription-PCR and was related to RFC expression. The effect of FR alpha overexpression on MTX resistance and natural folate uptake was studied using FR alpha non-expressing osteosarcoma 143B cells transfected with FR alpha cDNA in comparison with those transfected with sense or antisense RFC in the same genetic background.

RESULTS

Eighty-four samples (78.5%) had detectable FR alpha mRNA, and 29.9% had higher levels than the ovarian cancer cell line SKOV-3. No correlation was found between mRNA levels of FR alpha and RFC (r(2)=0.002). FR alpha overexpression had minor effects on the transport of MTX and sensitivity to this drug. Among the transfected 143B sublines, only the 143B-FR alpha was able to uptake 5-methyltetrahydrofolate when the extracellular concentration was reduced to 2 nmol/L, which conferred a growth advantage in physiologic folate concentrations compared with vector-only-transfected cells. Importantly, this was not similarly achieved by RFC overexpression.

CONCLUSIONS

This study suggests that FR alpha plays a role in the uptake of 5-methyltetrahydrofolate when the concentration gradient is insufficient for RFC-mediated transport. FR alpha overexpression is unlikely secondary to the decreased RFC expression in osteosarcoma.

Dendrimer-Based Targeted Delivery of an Apoptotic Sensor in Cancer Cells

Our previous studies have demonstrated the applicability of poly(amidoamine) (PAMAM) dendrimers as a platform for the targeted delivery of chemotherapeutic drugs both in vitro and in vivo. To monitor the rate and extent of cell-killing caused by the delivered chemotherapeutic drug, we wished to analyze the degree of apoptosis in targeted cells on a real-time basis. As the apoptosis-regulating caspases are activated during the apoptotic process, several caspase-hydrolyzable, fluorescence resonance energy transfer (FRET)-based substrates have been marketed for the detection of apoptosis. However, the applicability of these agents is limited because of their nonspecificity and the consequent high background fluorescence in tissues. Here we show the synthesis, characterization, and in vitro targeting of an engineered PAMAM nanodevice in which folic acid (FA) is conjugated as the targeting molecule and a caspase-specific FRET-based agent (PhiPhiLux G1D2) is conjugated as the apoptosis-detecting agent. This conjugate specifically targets FA-receptor-positive, KB cells. In these cells, the apoptosis-inducing agent staurosporine caused a 5-fold increase in the cellular fluorescence. These results show, for the first time, the potential applicability of a targeted apoptosis-measuring nanodevice, which could be used for simultaneously monitoring the apoptotic potential of a delivered drug.

The role of folate receptor alpha in cancer development, progression and treatment: cause, consequence or innocent bystander?

Folate receptor alpha (FRalpha) is a membrane-bound protein with high affinity for binding and transporting physiologic levels of folate into cells. Folate is a basic component of cell metabolism and DNA synthesis and repair, and rapidly dividing cancer cells have an increased requirement for folate to maintain DNA synthesis, an observation supported by the widespread use of antifolates in cancer chemotherapy. FRalpha levels are high in specific malignant tumors of epithelial origin compared to normal cells, and are positively associated with tumor stage and grade, raising questions of its role in tumor etiology and progression. It has been suggested that FRalpha might confer a growth advantage to the tumor by modulating folate uptake from serum or by generating regulatory signals. Indeed, cell culture studies show that expression of the FRalpha gene, FOLR1, is regulated by extracellular folate depletion, increased homocysteine accumulation, steroid hormone concentrations, interaction with specific transcription factors and cytosolic proteins, and possibly genetic mutations. Whether FRalpha in tumors decreases in vivo among individuals who are folate sufficient, or whether the tumor's machinery sustains FRalpha levels to meet the increased folate demands of the tumor, has not been studied. Consequently, the significance of carrying a FRalpha-positive tumor in the era of folic acid fortification and widespread vitamin supplement use in countries such as Canada and the United States is unknown. Epidemiologic and clinical studies using human tumor specimens are lacking and increasingly needed to understand the role of environmental and genetic influences on FOLR1 expression in tumor etiology and progression. This review summarizes the literature on the complex nature of FOLR1 gene regulation and expression, and suggests future research directions.

Clinical utility and pharmacology of high-dose methotrexate in the treatment of primary CNS lymphoma

Primary CNS non-Hodgkin lymphoma (PCNSL) has been shown to be increasing in incidence. This appears to be a consequence of the increasing population of those older than 65 years of age in whom PCNSL occurs most often. PCNSL often has a favorable response to treatment and aggressive management may result in extended survival and, in a proportion of patients less than 65 years of age, cure. The majority of neuro-oncologist's advocate utilizing high-dose methotrexate (HD-MTX) as a platform for the chemotherapy treatment of these neoplasms. In this review, the literature regarding HDHMTX as a treatment for PCNSL is summarized as are the pharmacological principles of HD-MTX.

Preclinical evaluation of novel organometallic 99mTc-folate and 99mTc-pteroate radiotracers for folate receptor-positive tumour targeting

PURPOSE

The folate receptor (FR) is a valuable tumour marker, since it is frequently overexpressed on various cancer types. The purpose of the present study was to pre-clinically evaluate novel site-specifically modified (99m)Tc(CO)(3) folate (gamma-derivative 4, alpha-derivative 5) and pteroate (6) conjugates for FR targeting.

METHODS

The (99m)Tc(CO)(3) radiotracers 4-6 were prepared by a kit-like procedure. In vitro characterisation (K (D) and B (max)) of the radiotracers was performed with FR-positive KB cells. Tissue distribution was studied in tumour-bearing mice. SPECT/CT experiments were performed with a dedicated small animal SPECT/CT scanner.

RESULTS

The complexes 4-6 were formed in high yields (>92%). Binding constants of the radiotracers (K (D) in nM: 4: 2.09; 5: 2.51; 6: 14.52) were similar to those of (3)H-folic acid (K (D) in nM: 7.22). In vivo the folate derivatives showed significantly better tumour uptake (4: 2.3+/-0.4% ID/g and 5: 1.2+/-0.2% ID/g, 4 h p.i.) than the pteroate derivative (6: 0.4+/-0.2% ID/g, 4 h p.i.). Clearance of all radiotracers from the blood pool and from non-targeted tissues was efficient (tumour to blood ratio approx. 200-350, 24 h p.i.). FR-positive tissue and organs were successfully visualised via small animal SPECT/CT.

CONCLUSION

Radiotracers 4-6 are the first (99m)Tc(CO)(3) tracers prepared via a kit formulation which exhibit full biological activity in vitro and in vivo. Folate derivatives 4 and 5 revealed significantly better pharmacokinetic properties than the pteroate derivative 6. Promising pre-clinical SPECT results warrant further assessment of (99m)Tc(CO)(3) radiofolates for detection of FR-positive tumours.

In vitro and in vivo targeting of different folate receptor-positive cancer cell lines with a novel 99mTc-radiofolate tracer

PURPOSE

For the assessment of folate-based radiopharmaceuticals, human nasopharyngeal KB carcinoma cells are traditionally used although nasopharyngeal cancer is rare. On the other hand, the folate receptor (FR) is frequently overexpressed on diverse cancer types, the highest frequency (>90%) being on ovarian carcinomas. The goal of our study was the in vitro and in vivo assessment of different FR-positive human carcinoma cells. In addition, a murine sarcoma cell line was assessed as a pre-clinical alternative to human xenograft models.

METHODS

FR-positive human nasopharyngeal, cervical, ovarian and colorectal cancer cell lines and the transgenic mouse sarcoma (24JK-FBP) cell line were targeted with a novel 99mTc-tricarbonyl folate derivative 2. Comparative in vitro cell binding studies were carried out under standardised folate-deficient conditions. In vivo studies were performed in nude mice and C6 black mice.

RESULTS

The in vitro cell experiments revealed only FR-specific binding (unspecific <0.02%), ranging from 3.5% to 52% of complex 2 owing to variable levels of FR expression of the cell lines. In vivo tumour uptake of radiotracer 2 varied less than in vitro. It ranged from 0.66+/-0.17% ID/g (LoVo) through 1.16+/-0.64% ID/g (IGROV-1) and 1.55+/-0.43% ID/g (24JK-FBP) to 2.33+/-0.36% ID/g (KB) 4 h p.i.

CONCLUSION

These pre-clinical studies indicate that in vitro data obtained in FR-positive cancer cells do not necessarily correspond with or predict in vivo radiofolate uptake in corresponding (xeno)grafts. In addition, the murine 24JK-FBP cell line proved to be a valuable pre-clinical alternative to human tumour models.

Synthesis and in Vitro/in Vivo Evaluation of Novel 99mTc(CO)3-Folates

Novel organometallic 99mTc(I)-folate derivatives have been synthesized and evaluated in vitro and in vivo in order to assess the influence of the overall charge of the radioconjugates and the spacer entity on the affinity and pharmacokinetic profile. Folic acid has been functionalized at the gamma-carboxylate group of the glutamate moiety with (i) a hydrophilic diethoxyethyl spacer bearing a picolylamine monoacetic acid chelate, (ii) a hexyl spacer bearing an iminodiacetic acid chelate, and (iii) a hexyl spacer with a bis(pyridylmethyl)amine chelating system. Coordination of the 99mTc(CO)3-core resulted in neutral complex 21, anionic complex 22, and cationic complex 23 in excellent yields (>90%) at ligand concentrations of 10(-4) M. Complexes 21-23 were HPLC purified for in vitro and in vivo experiments. In the case of 23, separation from the unlabeled folate analogue was incomplete, leading to low specific activity and, hence, significantly inferior in vivo uptake in folate-receptor-positive (FR-positive) organs and tissues (tumors and kidneys). Time dependent in vivo studies were performed in female, athymic nude mice bearing subcutaneous FR-positive human KB cell xenografts at 1, 4, and 24 h post injection (p.i.) of the radiotracers. Tumor uptake ranged between 1.9-2.7% ID/g, 4 h p.i. and 1.6-2.2% ID/g, 24 h p.i. for 21 and 22, and 0.9% ID/g, 4 h p.i. and 1.1% ID/g, 24 h p.i. for 23. Blood clearance was fast for all derivatives (< or =0.2% ID/g 1 h p.i.). Significant fractions of radioactivity were found in nontargeted and FR-negative organs and tissues (particularly in the liver and the intestines/intestinal contents) at early time points p.i. Coadministration of folic acid reduced radioactivity in FR-positive tissues and organs to background levels. In conclusion, overall charge and the nature of the spacer entity seemed to have a relatively minor influence on receptor affinity and the in vivo pharmacokinetic profile of the tested radiofolates.

PAMAM Dendrimer-Based Multifunctional Conjugate for Cancer Therapy: Synthesis, Characterization, and Functionality

Poly(amidoamine) (PAMAM) dendrimer-based multifunctional cancer therapeutic conjugates have been designed and synthesized. The primary amino groups on the surface of the generation 5 (G5) PAMAM dendrimer were neutralized through partial acetylation, providing enhanced solubility of the dendrimer (in conjugation of FITC (fluorescein isothiocyanate)) and preventing nonspecific targeting interactions (in vitro and in vivo) during delivery. The functional molecules fluorescein isothiocyanate (FITC, an imaging agent), folic acid (FA, targets overexpressed folate receptors on specific cancer cells), and paclitaxel (taxol, a chemotherapeutic drug) were conjugated to the remaining nonacetylated primary amino groups. The appropriate control dendrimer conjugates have been synthesized as well. Characterization of the G5 PAMAM dendrimer and its nanosize conjugates, including the molecular weight and number of primary amine groups, has been determined by multiple analytical methods such as gel permeation chromatography (GPC), nuclear magnetic resonance spectroscopy (NMR), potentiometric titration, high-performance liquid chromatography (HPLC), and UV spectroscopy. These multifunctional dendrimer conjugates have been tested in vitro for targeted delivery of chemotherapeutic and imaging agents to specific cancer cells. We present here the synthesis, characterization, and functionality of these dendrimer conjugates.

Multivitamin and alcohol intake and folate receptor alpha expression in ovarian cancer

Folate receptor alpha (FRalpha) expression in epithelial ovarian cancer may be related to folate intake. We examined this association using multivitamin intake, a proxy for folic acid, and assessed whether the relation was modified by alcohol intake, a folate agonist. Cases (n = 148) with suspected epithelial ovarian cancer, of ages > or = 20 years, were seen at Mayo Clinic, Minnesota, between 2000 and 2004; those with tumor specimens (n = 108) were included in analyses. Outpatient controls (n = 148) without cancer and with at least one ovary intact were matched to cases by age (within 5 years) and state of residence. Multivitamin (> or = 4 pills/wk) and weekly alcohol (> or = 5 drinks) intakes were assessed. Tumor specimens were analyzed immunohistochemically for FRalpha. Multivariable rate ratios (RR) and 95% confidence intervals (CI) were estimated using unconditional logistic regression. In case-control analysis, the RRs of multivitamin intake with absent/weak/moderate and strong-expressing FRalpha tumors were 0.30 (95% CI, 0.12-0.70) and 0.47 (95% CI, 0.24-0.91), respectively. For alcohol, the associations were 0.84 (95% CI, 0.24-2.86) and 1.65 (95% CI, 0.69-3.93), respectively. In case-case analysis, the RR associated with developing strong-expressing versus other FRalpha tumors was 3.13 (95% CI, 1.14-8.65) for multivitamins and 1.58 (95% CI, 0.45-5.60) for alcohol. The data did not support evidence for an interaction between multivitamin and alcohol intake with risk of developing a strong-expressing FRalpha tumor. The association of multivitamin intake with ovarian cancer may depend on FRalpha expression level.

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.

Targeting the tumor-associated folate receptor with an 111In-DTPA conjugate of pteroic acid

The cell membrane folate receptor is a potential molecular target for tumor-selective drug delivery. To probe structural requirements for folate receptor targeting with low molecular weight radiometal chelates, specifically the role of the amino acid fragment of folic acid (pteroylglutamic acid) in mediating targeting selectivity, the amide-linked conjugate pteroyl-NHCH(2)CH(2)OCH(2)CH(2)OCH(2)CH(2)NH-DTPA was prepared by a three-step procedure from pteroic acid, 2,2'-(ethylenedioxy)-bis(ethylamine), and t-Bu-protected DTPA. This conjugate, 1-{2-[2-[(2-(biscarboxymethyl-amino)ethyl)-carboxymethyl-amino]ethyl]-carboxymethyl-amino}-acetylamino-3,6-dioxa-8-pteroylamino-octane (1), was employed for synthesis of the corresponding (111)In(III) radiopharmaceutical. Following intravenous administration to athymic mice, the (111)In complex of 1 was found to selectively localize in folate receptor-positive human KB tumor xenografts and to afford prolonged tumor retention of the (111)In radiolabel (5.4 +/- 0.8, 5.6 +/- 1.1, and 3.6 +/- 0.6% of the injected dose per gram of tumor at 1, 4, and 24 h, respectively). The observed tumor localization was effectively blocked by co-administration of folic acid with the (111)In-1 complex, consistent with a folate receptor-mediated targeting process. In control studies, tumor targeting with this pteroic acid conjugate appears as effective as that seen using (111)In-DTPA-folate, a radiopharmaceutical that has progressed to clinical trials for detection of folate receptor-expressing gynecological tumors.

Targeting and inhibition of cell growth by an engineered dendritic nanodevice

The cellular uptake and cytotoxicity of an engineered multifunctional dendritic nanodevice containing folic acid (FA) as the targeting molecule, methotrexate (MTX) as the chemotherapeutic drug, and fluorescein (FI) as the detecting agent were studied in vitro. FI and FA were conjugated to the generation 5 poly(amidoamine) (G5) dendrimer carrier through a thiourea and amide linkage and MTX was conjugated through an ester linkage to the carrier to generate the trifunctional dendritic device, G5-FI-FA-MTX. This trifunctional dendrimer-drug conjugate bound to FA receptor-expressing KB cells in a dose-dependent and saturable manner. Confocal microscopic analysis demonstrated cellular internalization of the conjugate. G5-FI-FA-MTX induced a time- and dose-dependent inhibition of cell growth in KB cells. The targeted dendrimer conjugates G5-FI-FA-MTX and G5-FA-MTX inhibited cell growth in KB cells, whereas the nontargeted G5-MTX failed to induce growth inhibition. These studies show the potential of G5-FI-FA-MTX or G5-FA-MTX for targeting and growth suppression of tumor cells that overexpress FA-receptors.

Nanoparticle Targeting of Anticancer Drug Improves Therapeutic Response in Animal Model of Human Epithelial Cancer

Prior studies suggested that nanoparticle drug delivery might improve the therapeutic response to anticancer drugs and allow the simultaneous monitoring of drug uptake by tumors. We employed modified PAMAM dendritic polymers <5 nm in diameter as carriers. Acetylated dendrimers were conjugated to folic acid as a targeting agent and then coupled to either methotrexate or tritium and either fluorescein or 6-carboxytetramethylrhodamine. These conjugates were injected i.v. into immunodeficient mice bearing human KB tumors that overexpress the folic acid receptor. In contrast to nontargeted polymer, folate-conjugated nanoparticles concentrated in the tumor and liver tissue over 4 days after administration. The tumor tissue localization of the folate-targeted polymer could be attenuated by prior i.v. injection of free folic acid. Confocal microscopy confirmed the internalization of the drug conjugates into the tumor cells. Targeting methotrexate increased its antitumor activity and markedly decreased its toxicity, allowing therapeutic responses not possible with a free drug.

Pteroyl-γ-glutamate-cysteine synthesis and its application in folate receptor-mediated cancer cell targeting using folate-tethered liposomes

Cell membrane-associated folate receptors are selectively overexpressed in certain human tumors. The high affinity of folic acid for folate receptors provides a unique opportunity to use folic acid as a targeting ligand to deliver chemotherapeutic agents to cancer cells. Folate-tethered liposomes bearing pteroyl-gamma-glutamate-cysteine-polyethylene glycol (PEG)-distearoylphosphatidylethanolamine (DSPE) as the targeting component are under investigation as mediators of drug and gene delivery to cancer cells that overexpress folate receptors. Pteroyl-gamma-glutamate-cysteine synthesis is one of the crucial starting steps in the preparation of pteroyl-gamma-glutamate-cysteine-PEG-DSPE. However, published methods for the synthesis of pteroyl-gamma-glutamate-cysteine provide low yields and are not easily reproducible. Therefore, we developed a modified synthetic method for the removal of the N(10)-trifluoroacetyl group after cleavage/deprotection that is reliable, is easily reproducible, and has high yield (38%) compared with an unreliable yield of 3-20% with the earlier methods. Folate-tethered liposomes containing calcein or doxorubicin were prepared using pteroyl-gamma-glutamate-cysteine-PEG-DSPE as the targeting component along with nontargeted liposomes with PEG-DSPE. The results of the uptake of calcein and cytotoxicity of doxorubicin in human cervical cancer HeLa-IU(1) cells and human colon cancer Caco-2 cells demonstrated that folate-tethered liposomes were efficient in selective delivery to cancer cells overexpressing folate receptors. The improvement in yield of the targeting component can significantly facilitate "scale up" of folate receptor-mediated liposomal cancer therapy to the preclinical and clinical levels of investigations.

The Role of α-Folate Receptor-Mediated Transport in the Antitumor Activity of Antifolate Drugs

PURPOSE

Raltitrexed, pemetrexed, lometrexol, and ZD9331 are antifolate drugs transported into cells via the ubiquitously expressed reduced-folate carrier. They display also high affinity for the alpha-folate receptor (alpha-FR), a low capacity folate transporter that is highly overexpressed in some epithelial tumors. The role of alpha-FR in the activity of the antifolates has been evaluated in two alpha-FR-overexpressing cell lines grown in a physiological concentration of folate (20 nM R,S-Leucovorin).

EXPERIMENTAL DESIGN AND RESULTS

A431-FBP cells (transfected with the alpha-FR) were 3-5-fold more sensitive to the antifolates than A431 cells. KB cells (constitutive alpha-FR overexpression) were less sensitive to the drugs when coexposed to 1 microM folic acid to competitively inhibit binding to the alpha-FR. Raltitrexed, pemetrexed, and lometrexol are polyglutamated in cells leading to drug retention, e.g., the raltitrexed 4- and 24-h IC(50)s in A431 cells were approximately 0.6 and 0.008 microM, respectively, compared with 0.003 microM for 72-h continuous exposure. A431-FBP cells were approximately 3-fold more sensitive to raltitrexed and pemetrexed at all exposure times. ZD9331 is not polyglutamated, and the 4- and 24-h IC(50)s in A431 cells were >100 and approximately 100 microM, respectively, reducing to 2 and 0.1 microM, respectively, in A431-FBP cells. The ZD9331 4- and 24-h IC(50)s in KB cells were 20 and 1 microM, respectively, and reversible by coaddition of 1 microM folic acid. An in situ thymidylate synthase assay demonstrated continued thymidylate synthase inhibition after ZD9331-treated A431-FBP and KB, but not A431, cells were placed in drug-free medium for 16 h. A model is proposed in which the antifolates accumulate in the alpha-FR/endosomal apparatus, leading to slow release into the cytoplasm. In particular, this leads to cellular retention of the nonpolyglutamatable ZD9331.

CONCLUSIONS

Antifolate drugs, particularly ZD9331, have the potential for increased efficacy in tumors that highly overexpress the alpha-FR.

The folate receptor as a molecular target for tumor-selective radionuclide delivery

The cell-membrane folate receptor is a potential molecular target for tumor-selective drug delivery, including radiolabeled folate-chelate conjugates for diagnostic imaging. We review here some background on the folate receptor as tumor-associated molecular target for drug delivery, and briefly survey the literature on tumor-targeting with radiolabeled folate-chelate conjugates.

Folic acid

Folic acid is an essential nutrient from the B complex group of vitamins. Folate, as a cofactor, is involved in numerous intracellular reactions, and this is reflected in the various derivatives that have been isolated from biological sources. Folic acid is involved in single carbon transfer reactions and serves as a source of single carbon units in different oxidative states. The processes involved in the absorption, transport, and intracellular metabolism of this cofactor are complex. Much of folate is bound tightly to enzymes, indicating that there is not excess of this cofactor and that its cellular availability is protected as well as being strictly regulated. In animals, the liver controls the supply of folate through first pass metabolism, biliary secretion, enterohepatic recirculation, as well as through senescent erythrocyte recycling. Deficiencies of folate can occur for many reasons, including reduced intake, increased metabolism, and/or increased requirements as well as through genetic defects. The effects of folate deficiency include hyperhomocysteinemia, megaloblastic anemia, and mood disorders. Folate deficiency has also been implicated in disorders associated with neural tube defects. Supplementation of grain products such as cereals has been undertaken in several countries as a cost-effective means of reducing the prevelance of neural tube defects. Recently, common polymorphisms have been discovered in several genes associated with folate pathways that may play a role in diseases associated with folate deficiency, particularly mild folate deficiency.