The folate receptor as a potential therapeutic anticancer target

Single- versus Dual-Targeted Nanoparticles with Folic Acid and Biotin for Anticancer Drug Delivery

Cancer is one of the major causes of death worldwide and its treatment remains very challenging. The effectiveness of cancer therapy significantly depends upon tumour-specific delivery of the drug. Nanoparticle drug delivery systems have been developed to avoid the side effects of the conventional chemotherapy. However, according to the most recent recommendations, future nanomedicine should be focused mainly on active targeting of nanocarriers based on ligand-receptor recognition, which may show better efficacy than passive targeting in human cancer therapy. Nevertheless, the efficacy of single-ligand nanomedicines is still limited due to the complexity of the tumour microenvironment. Thus, the NPs are improved toward an additional functionality, e.g., pH-sensitivity (advanced single-targeted NPs). Moreover, dual-targeted nanoparticles which contain two different types of targeting agents on the same drug delivery system are developed. The advanced single-targeted NPs and dual-targeted nanocarriers present superior properties related to cell selectivity, cellular uptake and cytotoxicity toward cancer cells than conventional drug, non-targeted systems and single-targeted systems without additional functionality. Folic acid and biotin are used as targeting ligands for cancer chemotherapy, since they are available, inexpensive, nontoxic, nonimmunogenic and easy to modify. These ligands are used in both, single- and dual-targeted systems although the latter are still a novel approach. This review presents the recent achievements in the development of single- or dual-targeted nanoparticles for anticancer drug delivery.

Folic acid modified TPGS as a novel nano-micelle for delivery of nitidine chloride to improve apoptosis induction in Huh7 human hepatocellular carcinoma

Background

The development of novel and effective drugs for targeted human hepatocellular carcinoma still remains a great challenge. The alkaloid nitidine chloride (NC), a component of a traditional Chinese medicine, has been shown to have anticancer properties, but doses at therapeutic levels have unacceptable side effects. Here we investigate folic acid modified D-α-tocopheryl polyethylene glycol 1000 succinate (TPGS-FA) as a potential carrier for controlled delivery of the drug.

Methods

Synthesized TPGS-FA was characterized by FTIR, UV-visible and ¹H NMR spectroscopy, and TPGS loaded with NC was evaluated for its ability to induce apoptosis in Huh7 cells by Annexin V/PI and MTT assays, and observed by laser scanning confocal microscopy and inverted phase contrast microscopy.

Results

TPGS-FA/NC complexes were prepared successfully, and were homogenious with a uniform size of ~ 14 nm diameter. NC was released from the TPGS-FA/NC complexes in a controlled and sustained manner under physiological conditions (pH 7.4). Furthermore, its cytotoxicity to hepatocarcinoma cells was greater than that of free NC.

Conclusions

TPGS-FA is shown to be useful carrier for drugs such as NC, and TPGS-FA/NC could potentially be a potent and safe drug for the treatment of hepatocellular carcinoma.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40360-020-00461-y.

Structural tuning of acridones for developing anticancer agents targeting dihydrofolate reductase

Targeting dihydrofolate reductase, here, we report the tumor growth inhibitory activity of substituted acridones. The screening of the molecules over 60 cell line panel of human cancer cells identified (S)-oxiran-2-ylmethyl 9-oxo-9,10-dihydroacridine-4-carboxylate (19) with average GI₅₀ 0.3 μM. The specificity of the compound to CCRF-CEM, MOLT-4 and SR cell lines of leukemia and SW-620, SF268, LOXIMVI, ACHN and MCF7 cancerous cells exhibiting GI₅₀ in the nM range was observed. C6 Glioma cells treated with compound 19 showed differentiated cell morphology and cell cycle arrest in G2/M phase. The interactions of the compound with dihydrofolate reductase were ascertained with the help of enzyme immunoassays, molecular docking and molecular dynamic studies.

Bioresorbable filomicelles for targeted delivery of betulin derivative - In vitro study

Filomicelles (worm-like micelles) possess high drug loading capacity and long circulation time in the bloodstream. A novel approach can be filomicelles with folic acid (FA) as a targeting moiety. Folate-drug delivery systems can target FA receptors (FAR) that are overexpressed in several human carcinomas, which can potentially maximize therapeutic efficacy while minimizing side effects. The aim of this study was to develop filomicelles from combination of poly(L-lactide)-Jeffamine-folic acid and poly(L-lactide)-poly(ethylene glycol) for delivery of betulin derivative. Phosphate derivative of betulin reveals high cytotoxicity against cancer cells, however its application is restricted due to poor solubility in water. Incorporation into hydrophobic core of micelles can effectively solubilize the drug. Three kinds of micelles were obtained with high drug loading capacity. Based on TEM analysis, the copolymers formed exclusively filomicelles or mixture of filomicelles and spherical micelles. All kinds of micelles provided release of betulin derivative for over 9 days and apart the very initial phase displayed similar release profile. The influence of PLA block on initial burst effect was revealed. The in vitro cytotoxicity of betulin derivative loaded micelles against FAR-positive HeLa cells was confirmed, which proves their usefulness for targeted delivery of cytostatic drug.

Inihibition of Glycolysis by Using a Micro/Nano-Lipid Bromopyruvic Chitosan Carrier as a Promising Tool to Improve Treatment of Hepatocellular Carcinoma

Glucose consumption in many types of cancer cells, in particular hepatocellular carcinoma (HCC), was followed completely by over-expression of type II hexokinase (HKII). This evidence has been used in modern pharmacotherapy to discover therapeutic target against glycolysis in cancer cells. Bromopyruvate (BrPA) exhibits antagonist property against HKII and can be used to inhibit glycolysis. However, the clinical application of BrPA is mostly combined with inhibition effect for healthy cells particularly erythrocytes. Our strategy is to encapsulate BrPA in a selected vehicle, without any leakage of BrPA out of vehicle in blood stream. This structure has been constructed from chitosan embedded into oleic acid layer and then coated by dual combination of folic acid (FA) and bovine serum albumin (BSA). With FA as specific ligand for cancer folate receptor and BSA that can be an easy binding for hepatocytes, they can raise the potential selection of carrier system.

Rational modification of the lead molecule: Enhancement in the anticancer and dihydrofolate reductase inhibitory activity

By using molecular docking studies, the practice of fragment based drug discovery is conceptualized by introducing oxindole and iso-propanol moieties in our previous lead molecule 1. The resulting compound 2 exhibited competitive inhibition and favorable Ka and Ki for hDHFR. The screening of compound 2 at 60 cell line panel of human tumor cell lines showed its considerably better efficacy than compound 1 and hence put the candidature of 2 on stronghold for further studies.

Baicalin loaded in folate-PEG modified liposomes for enhanced stability and tumor targeting

Bioavailability of baicalin (BAI), an example of traditional Chinese medicine, has been modified by loading into liposome. Several liposome systems of different composition i.e., lipid/cholesterol (L), long-circulating stealth liposome (L-PEG) and folate receptor (FR)-targeted liposome (L-FA) have been used as the drug carrier for BAI. The obtained liposomes were around 80 nm in diameter with proper zeta potentials about -25 mV and sufficient physical stability in 3 months. The entrapment efficiency and loading efficiency of BAI in the liposomes were 41.0-46.4% and 8.8-10.0%, respectively. The morphology details of BAI lipsosome systems i.e., formation of small unilamellar vesicles, have been determined by cryogenic transmission electron microscopy (cryo-TEM) and small angle X-ray scattering (SAXS). In vitro cytotoxicity of BAI liposomes against HeLa cells was evaluated by MTT assay. BAI loaded FR-targeted liposomes showed higher cytotoxicity and cellular uptake compared with non-targeted liposomes. The results suggested that L-FA-BAI could enhance anti-tumor efficiency and should be an effective FR-targeted carrier system for BAI delivery.

Folic acid-conjugated iron oxide porous nanorods loaded with doxorubicin for targeted drug delivery

Iron oxide porous nanorods (IOPNR) with lengths ranging from 40nm to 60nm and pore diameters ranging from 5nm to 10nm were prepared, and further modified with NH2-PEG-FA (FA-PEG-IOPNR) for ligand targeting and modified with NH2-PEG-OCH3 (PEG-IOPNR) as a control. Instead of chemical bonding, doxorubicin (DOX), a low water solubility anticancer drug, was loaded in the pores of the modified IOPNR because of their porous structure and high porosity. The release of DOX in acidic PBS solution (pH 5.3) was faster than that in neutral (pH 7.4) solution. The analysis results from TEM, inductively coupled plasma emission spectroscopy, confocal laser scanning microscopy, and flow cytometry analyses indicated that the presence of FA on the surface of the nanorods increase the cellular uptake of nanorods in the case of HeLa cells, a folate receptor (FR)-positive cell line. In contrast, for COS 7 cells, a FR-negative cell line, FA ligand on the surface of the nanorods showed no effect on the cellular uptake. MTT assay indicated that the cytotoxicity of DOX loaded in FA-PEG-IOPNR to HeLa cells was higher than that of DOX in PEG-IOPNR. In the case of COS 7 cells, no significant difference between the cytotoxicity of DOX loaded in FA-PEG-IOPNR and PEG-IOPNR was found. These results suggested that FA-PEG-IOPNR had the potential for target delivery of chemotherapeutic into cancer cells.

In vitro and in vivo anticancer activity of surface modified paclitaxel attached hydroxyapatite and titanium dioxide nanoparticles

Targeted drug delivery using nanocrystalline materials delivers the drug at the diseased site. This increases the efficacy of the drug in killing the cancer cells. Surface modifications were done to target the drug to a particular receptor on the cell surface. This paper reports synthesis of hydroxyapatite and titanium dioxide nanoparticles and modification of their surface with polyethylene glycol (PEG) followed by folic acid (FA). Paclitaxel, an anticancer drug, is attached to functionalized hydroxyapatite and titanium dioxide nanoparticles. The pure and functionalised nanoparticles are characterised with XRD, TEM and UV spectroscopy. Anticancer analysis was carried out in DEN induced hepatocarcinoma animals. Biochemical, hematological and histopathological analysis show that the surface modified paclitaxel attached nanoparticles have an higher anticancer activity than the pure paclitaxel and surface modified nanoparticles without paclitaxel. This is due to the targeting of the drug to the folate receptor in the cancer cells.

Targeted gene delivery by new folate-polycationic amphiphilic cyclodextrin-DNA nanocomplexes in vitro and in vivo

AIM

Development and evaluation of a new targeted gene delivery system by first preforming self-assembled nanocomplexes from a polycationic amphiphilic cyclodextrin (paCD) and pDNA and then decorating the surface of the nanoparticles with folic acid (FA).

EXPERIMENTAL SECTION

The cyclodextrin derivative (T2) is a tetradecacationic structure incorporating 14 primary amino groups and 7 thioureido groups at the primary face of a cyclomaltoheptaose (β-CD) core and 14 hexanoyl chains at the secondary face.

RESULTS AND CONCLUSIONS

T2 complexed and protected pDNA (luciferase-encoding plasmid DNA, pCMVLuc) and efficiently mediated transfection in vitro and in vivo with no associated toxicity. The combination of folic acid with CDplexes afforded ternary nanocomplexes (Fol-CDplexes) that enhanced significantly the transfection activity of pCMVLuc in human cervix adenocarcinoma HeLa cells, especially when formulated with 1 μg FA/μg DNA. The observed transfection enhancement was associated to specific folate receptor (FR)-mediated internalization of Fol-CDplexes, as corroborated by employing a receptor-deficient cell line (HepG2) and an excess of free folic acid. The in vivo studies, including luciferase reporter gene expression and biodistribution, indicated that 24h after intravenous administration of the T2-pDNA nanocomplexes, transfection takes part mainly in the liver and partially in the lung. Interestingly, the corresponding Fol-CDplexes lead to an increase in the transfection activity in the lung and the liver compared to non-targeted CDplexes. Folate-CDplexes developed in this study have improved transfection efficiency and although various methods have been used for the preparation of ligand-DNA-complexes, covalent binding is usually needed and insoluble aggregates are formed unless the concentration of the components is minimized. However, the complexes developed by first time in this work were prepared by simple mixing. The synthetic nature of this formulation provides the potential of flexibility in terms of composition and the capability of inexpensive and large-scale production of the complexes. These nanovectors may be an adequate alternative to viral vectors for gene therapy in the future.

Significance of folate receptor alpha and thymidylate synthase protein expression in patients with non-small-cell lung cancer treated with pemetrexed

INTRODUCTION

Folate receptor alpha (FRA) regulates cellular uptake of folates and antifolates. Information about FRA protein expression in metastatic non-small-cell lung cancer (NSCLC) is limited. We investigated FRA as a biomarker for pemetrexed-based chemotherapy and compared it with thymidylate synthase (TS), the main target of pemetrexed.

METHODS

Pretreatment tumor specimens from 207 patients with advanced NSCLC were assessed for FRA and TS protein expression by immunohistochemistry using the H-score (range, 0-300) and correlated to patients' clinicopathological data, radiographic response, progression-free survival (PFS), and overall survival (OS).

RESULTS

Low total (cytoplasmic and nuclear) TS protein expression (H-score < 210) was associated with improved PFS (median: 5.6 versus 3.5 months; hazard ratio [HR] = 0.6379, p = 0.0131) and prolonged OS (median: 22.5 versus 11.5 months; HR = 0.5680,p = 0.0107). An association between lower TS levels and response to pemetrexed-based therapy was found-mean H-score 187 ± 5, median 180 for responders versus mean H-score 201 ± 4, median 210, for non-responders, p = 0.0244. High intracellular FRA expression (H-score ≥110) was associated with prolonged OS (28.9 versus 11.7 months, HR = 0.5316, p = 0.0040) and a trend for association with PFS (5.6 versus 4.1 months, HR = 0.7395, p = 0.0801) was noted. Membranous FRA expression was seen in 83% of patients, moreover, high membranous expression (H-score ≥20) was associated with improved PFS (5.6 versus 3.7 months, HR = 0.6445, p = 0.0306) and OS (22.1 versus 11.5 months, HR = 0.5378, p = 0.0131).

CONCLUSIONS

A large number of NSCLC patients have high expression of FRA and/or a low level of TS expression. Expression levels of FRA and TS were associated with clinical benefit from pemetrexed therapy.

Process optimization for the preparation of oligomycin-loaded folate-conjugated chitosan nanoparticles as a tumor-targeted drug delivery system using a two-level factorial design method

Oligomycin-A (Oli-A), an anticancer drug, was loaded to the folate (FA)-conjugated chitosan as a tumor-targeted drug delivery system for the purpose of overcoming the nonspecific targeting characteristics and the hydrophobicity of the compound. The two-level factorial design (2-LFD) was applied to modeling the preparation process, which was composed of five independent variables, namely FA-conjugated chitosan (FA-CS) concentration, Oli-A concentration, sodium tripolyphosphate (TPP) concentration, the mass ratio of FA-CS to TPP, and crosslinking time. The mean particle size (MPS) and the drug loading rate (DLR) of the resulting Oli-loaded FA-CS nanoparticles (FA-Oli-CSNPs) were used as response variables. The interactive effects of the five independent variables on the response variables were studied. The characteristics of the nanoparticles, such as amount of FA conjugation, drug entrapment rate (DER), DLR, surface morphology, and release kinetics properties in vitro were investigated. The FA-Oli-CSNPs with MPS of 182.6 nm, DER of 17.3%, DLR of 58.5%, and zeta potential (ZP) of 24.6 mV were obtained under optimum conditions. The amount of FA conjugation was 45.9 mg/g chitosan. The FA-Oli-CSNPs showed sustained-release characteristics for 576 hours in vitro. The results indicated that FA-Oli-CSNPs obtained as a targeted drug delivery system could be effective in the therapy of leukemia in the future.

A smart multifunctional nanocomposite for intracellular targeted drug delivery and self-release

A multifunctional 'all-in-one' nanocomposite is fabricated using a colloid, template and surface-modification method. This material encompasses magnetic induced target delivery, cell uptake promotion and controlled drug release in one system. The nanocomposite is characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, N(2) adsorption and vibrating sample magnetometry. The prepared material has a diameter of 350-400 nm, a high surface area of 420.29 m(2) g(-1), a pore size of 1.91 nm and a saturation magnetization of 32 emu g(-1). Doxorubicin (DOX) is loaded in mesopores and acid-sensitive blockers are introduced onto the orifices of the mesopores by a Schiff base linker to implement pH-dependent self-release. Folate was also introduced to improve DOX targeted delivery and endocytosis. The linkers remained intact to block pores with ferrocene valves and inhibit the diffusion of DOX at neutral pH. However, in lysosomes of cancer cells, which have a weak acidic pH, hydrolysis of the Schiff base group removes the nanovalves and allows the trapped DOX to be released. These processes are demonstrated by UV-visible absorption spectra, confocal fluorescence microscopy images and methyl thiazolyl tetrazolium assays in vitro, which suggest that the smart nanocomposite successfully integrates targeted drug delivery with internal stimulus induced self-release and is a potentially useful material for nanobiomedicine.

Preparation, characterization, and in vitro targeted delivery of folate-decorated paclitaxel-loaded bovine serum albumin nanoparticles

Paclitaxel (Taxol^®) is an important anticancer drug in clinical use for treatment of a variety of cancers. Because of its low solubility, it is formulated in high concentration in Cremophor EL^® which induces hypersensitivity reactions. In this study, targeted delivery of paclitaxel-loaded nanoparticles was prepared by a desolvation procedure, crosslinked on the wall material of bovine serum albumin, and subsequently decorated by folic acid. The characteristics of the nanoparticles, such as amount of folate conjugation, surface morphology, drug entrapment efficiency, drug loading efficiency, and release kinetics were investigated in vitro. The targeting effect was investigated in vitro by cancer cell uptake of fluorescein isothiocyanate-labeled nanoparticles. The spherical nanoparticles obtained were negatively charged with a zeta potential of about −30 mV, and characterized around 210 nm with a narrow size distribution. Drug entrapment efficiency and drug loading efficiency were approximately 95.3% and 27.2%, respectively. The amount of folate conjugation was 9.22 μg/mg of bovine serum albumin. The folate-decorated nanoparticles targeted a human prostate cancer cell line effectively.

Differential usage of the transport systems for folic acid and methotrexate in normal human T-lymphocytes and leukemic cells

Methotrexate (MTX) has been used as an effective anti-cancer drug for a long time. Conceptually, it is accepted that MTX and folic acid are transported by folate receptors (FRs) in cancerous cells, but the exact mechanism of MTX uptake in human leukemia is unknown. The objective of this study was to investigate different transport systems for FA and MTX, and to delineate their uptake mechanism in MOLT4, K562, Hut78 leukemia cells and normal human T cells. In MOLT4, uptake of MTX was higher than FA, similar to that of K562, Hut78 and normal T cells. In MOLT4 cells, MTX uptake was maximum at pH 7.4 whereas FA uptake was maximum at pH 4.5. Uptake of FA and MTX was significantly inhibited by anions, suggesting anion-dependent transport system. FA uptake was found to be energy dependent whereas MTX uptake was energy independent. RT-PCR and immunofluorescence results demonstrated the presence of reduced folate carrier as well as proton coupled folate transporter and absence of FR in MOLT4 and normal T cells. These data suggest the existence of two separate and independent carrier-mediated transport systems for the uptake of FA and MTX in normal and leukemic human T cells.

Folate status in various pathophysiological conditions

Folate is the generic term for compounds that have vitamin activity similar to that of pteroylglutamic acid. Folate acts as a coenzyme in several single carbon transfers involved in biosynthesis of purine nucleotides and deoxythymidylic acid essential for DNA and RNA synthesis. In addition, folate provides one-carbon unit for methylation of a wide variety of biological substances including DNA, proteins, phospholipids, and neurotransmitters, thereby regulating their function. Recent epidemiological-clinical and experimental studies suggest the association of folate deficiency with the risk of various cancers, birth defects, and cardiovascular diseases. Thus, it is important to consider the conditions that are associated with altered folate status and their consequences. The impairment in folate status has been found in number of pathophysiological conditions like inflammatory bowel disease, cancer, alcoholism, pregnancy, neonatal growth, and during administration of some drugs. The recent advances dealing with mechanistic aspects of impaired folate status in these conditions have been discussed in this review.

Expression status of folate receptor alpha is significantly correlated with prognosis in non-small-cell lung cancers

BACKGROUND

To evaluate the prognostic value of folate receptor alpha (FOLR1) and/or reduced folate carrier (RFC1) expression, which are well-characterized folate transporters, in completely resected non-small-cell lung cancer (NSCLC).

METHODS

We quantitatively examined gene expression of FOLR1 and RFC1 in surgical specimens resected from NSCLC patients. A total of 119 consecutive patients from January 2003 to June 2004 were included.

RESULTS

In adenocarcinoma, the FOLR1 gene expression was downregulated in smokers and male patients (P = 0.006 and P < 0.001, respectively). In addition, FOLR1 expression values in patients with well-differentiated, early p-stage, pT1, pN0, EGFR mutant, and p53 wild-type cancers were significantly higher than those for poorly differentiated, advanced p-stage, pT2-4, pN1-3, EGFR wild-type, and p53 mutant (P < 0.001, P < 0.001, P < 0.001, P = 0.002, P < 0.001 and P = 0.023, respectively). In squamous cell carcinoma, FOLR1 expression values in patients with pN1-3 was significantly higher than those with pN0 (P = 0.037). Moreover, the 3-year survival rate and disease-free survival rate of high-FOLR1-expressing patients (94.7% and 75.4%) were significantly higher than those of low-FOLR1-expressing patients (80.9% and 60.8%) (P = 0.008 and P = 0.038). A multivariate analysis confirmed that high FOLR1 expression was an independent and significant factor predicting a favorable prognosis (P = 0.043).

CONCLUSIONS

Higher levels of FOLR1 appear to be associated with better prognoses for patients with lung adenocarcinomas.

Folic acid-PEG conjugated superparamagnetic nanoparticles for targeted cellular uptake and detection by MRI

We report the development and in vitro study of a nanoconjugate serving as a targeted magnetic resonance imaging (MRI) contrast enhancement agent for detection of cancer cells overexpressing the folate receptor. The nanoconjugate was synthesized by coating superparamagnetic iron oxide nanoparticles with covalently bound bifunctional poly(ethylene glycol) (PEG), followed by conjugation with folic acid (FA). The specificity of the nanoconjugate targeting cancerous cells was demonstrated by comparative intracellular uptake of the nanoconjugate and PEG-/dextran-coated nanoparticles by human adenocarcinoma HeLa cells. Preferential targeting to cancerous cells was studied by comparing the uptake of the nanoconjugate by HeLa cells and by non-FR expressing osteosarcoma MG-63 cells. Uptake of the nanoconjugate by HeLa cells after 4 h incubation was found to be a 12-fold higher than that of PEG- or dextran-coated nanoparticles as quantified by inductively coupled plasma spectroscopy. A significant negative contrast enhancement was observed with magnetic resonance (MR) phantom imaging for HeLa cells over MG-63 cells, when both were cultured with the nanoconjugate. Specificity of the nanoconjugate for folate receptors was also verified with a competitive inhibition assay, in which HeLa cells were incubated with both NP-PEG-FA and free FA. The bifunctional PEG used has amide linkages within the PEG chains that can form interchain hydrogen bonding, leading to improved stability of the PEG coating. Self-assembled PEG can be controlled at the molecular level and are suitable for nanoscale coatings.

Design, synthesis, and biological evaluation of folic acid targeted tetraphenylporphyrin as novel photosensitizers for selective photodynamic therapy

Photodynamic therapy (PDT) is a cancer treatment involving systemic administration of a tumor-localizing photosensitizer; this, when activated by the appropriate light wavelength, interacts with molecular oxygen to form a toxic, short-lived species known as singlet oxygen, which is thought to mediate cellular death. Targeted PDT offers the opportunity of enhancing photodynamic efficiency by directly targeting diseased cells and tissues. Two new conjugates of three components, folic acid/hexane-1,6-diamine/4-carboxyphenylporphyrine 1 and folic acid/2,2'-(ethylenedioxy)-bis-ethylamine/4-carboxyphenylporphyrine 2 were synthesized. The conjugates were characterized by 1H NMR, MALDI, UV-visible spectroscopy, and fluorescence quantum yield. The targeted delivery of these photoactive compounds to KB nasopharyngeal cell line, which is one of the numerous tumor cell types that overexpress folate receptors was studied. It was found that after 24 h incubation, conjugates 1 and 2 cellular uptake was on average 7-fold higher than tetraphenylporphyrin (TPP) used as reference and that 1 and 2 cellular uptake kinetics increased steadily over the 24 h period, suggesting an active transport via receptor-mediated endocytosis. In corresponding results, conjugates 1 and 2 accumulation displayed a reduction of 70% in the presence of a competitive concentration of folic acid. Survival measurements demonstrated that KB cells were significantly more sensitive to conjugated porphyrins-mediated PDT. Under the same experimental conditions and the same photosensitizer concentration, TPP displayed no photocytotoxicity while conjugates 1 and 2 showed photodynamic activity with light dose values yielding 50% growth inhibition of 22.6 and 6.7 J/cm2, respectively.

Folate receptor-targeted immunotherapy: Induction of humoral and cellular immunity against hapten-decorated cancer cells

We previously exploited the frequent overexpression of folate receptors on cancer cells to decorate malignant cell surfaces selectively with folate-hapten conjugates. In antihapten-immunized hosts, this targeted localization of foreign haptens to tumor cells led to rapid accumulation of autologous antihapten IgG, which in turn yielded potent antitumor activity upon stimulation with cytokines (IL-2, IFN-alpha). In an effort to understand the effector mechanisms responsible for tumor regression, we have now investigated the involvement of both humoral and cellular immune components in the tumor destruction process. We report that the dependence of therapeutic efficacy on folate-hapten concentration is bimodal, suggesting that the conjugate must bridge between a cell surface FR and an antihapten IgG in order to mediate killing. Studies with cancer cells in vitro further demonstrate that folate-fluorescein-marked tumor cells are killed primarily by antibody-dependent cellular cytotoxicity and phagocytosis, with no contribution from complement-dependent mechanisms. Investigations of specific immune cell involvement also reveal that asialo-GM1(+)-natural killer cells, macrophages, CD4+ T cells and CD8+ T cells contribute significantly to recognition/removal of the cancer mass, and that elimination of these cell types markedly compromises the therapy. Because the initial antibody-dependent stage of tumor cell killing is shown to lead to a long-term antibody-independent cellular immunity that involves both CD4+ and CD8+ T cells, we propose that F(c) receptor-expressing immune cells not only initiate destruction of the IgG-marked tumor cells, but also participate in presentation of endogenous tumor antigens in a manner that leads to long-term cellular immunity.

Mechanisms of drug resistance in cancer chemotherapy

The management of cancer involves procedures, which include surgery, radiotherapy and chemotherapy. Development of chemoresistance is a persistent problem during the treatment of local and disseminated disease. A plethora of cytotoxic drugs that selectively, but not exclusively, target actively proliferating cells include such diverse groups as DNA alkylating agents, antimetabolites, intercalating agents and mitotic inhibitors. Resistance constitutes a lack of response to drug-induced tumour growth inhibition; it may be inherent in a subpopulation of heterogeneous cancer cells or be acquired as a cellular response to drug exposure. Resistance varies. Although regulatory approval may require efficacy in as few as 20% of trial cohorts, a drug may subsequently be used in unselected patients displaying resistance to the treatment. Principal mechanisms may include altered membrane transport involving the P-glycoprotein product of the multidrug resistance (MDR) gene as well as other associated proteins, altered target enzyme (e.g. mutated topoisomerase II), decreased drug activation, increased drug degradation due to altered expression of drug-metabolising enzymes, drug inactivation due to conjugation with increased glutathione, subcellular redistribution, drug interaction, enhanced DNA repair and failure to apoptose as a result of mutated cell cycle proteins such as p53. Attempts to overcome resistance mainly involve the use of combination drug therapy using different classes of drugs with minimally overlapping toxicities to allow maximal dosages and with narrowest cycle intervals, necessary for bone marrow recovery. Adjuvant therapy with P-glycoprotein inhibitors and, in specific instances, the use of growth factor and protein kinase C inhibitors are newer experimental approaches that may also prove effective in abrogating or delaying onset of resistance. Gene knockout using antisense molecules may be another effective way of blocking drug resistance genes. Conversely, drug resistance may also be used to good purpose by transplanting retrovirally transformed CD34 cells expressing the MDR gene to protect the bone marrow during high-dose chemotherapy.

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.

Folate receptor-targeted liposomes as vectors for therapeutic agents

The folate receptor is a cell surface protein that has recently been identified as a tumor marker, due to its differential overexpression in several malignancies. Current research indicates that folate can be covalently attached to the surface of liposomes to mediate their selective internalization by tumor cells through the folate receptor-mediated endocytic pathway. Optimized liposome formulations, characterized by improvements in drug loading, extended residence times in the circulation and improved drug release, have been developed to improve the biodistribution of therapeutic molecules. Theoretically, folate receptor-targeting can be combined with liposome encapsulation to synergistically affect disease outcome by enhancing the delivery of chemotherapeutic agents to neoplastic cells, while reducing systemic toxicities to normal tissues. The purpose of this chapter is to characterize the components of folate receptor-targeted liposomes, and summarize their applications in gene and drug delivery.

Folate-mediated delivery of macromolecular anticancer therapeutic agents

The receptor for folic acid constitutes a useful target for tumor-specific drug delivery, primarily because: (1) it is upregulated in many human cancers, including malignancies of the ovary, brain, kidney, breast, myeloid cells and lung, (2) access to the folate receptor in those normal tissues that express it can be severely limited due to its location on the apical (externally-facing) membrane of polarized epithelia, and (3) folate receptor density appears to increase as the stage/grade of the cancer worsens. Thus, cancers that are most difficult to treat by classical methods may be most easily targeted with folate-linked therapeutics. To exploit these peculiarities of folate receptor expression, folic acid has been linked to both low molecular weight drugs and macromolecular complexes as a means of targeting the attached molecules to malignant cells. Conjugation of folic acid to macromolecules has been shown to enhance their delivery to folate receptor-expressing cancer cells in vitro in almost all situations tested. Folate-mediated macromolecular targeting in vivo has, however, yielded only mixed results, largely because of problems with macromolecule penetration of solid tumors. Nevertheless, prominent examples do exist where folate targeting has significantly improved the outcome of a macromolecule-based therapy, leading to complete cures of established tumors in many cases. This review presents a brief mechanistic background of folate-targeted macromolecular therapeutics and then summarizes the successes and failures observed with each major application of the technology.

Mechanisms of the growth inhibitory effects of the isoflavonoid biochanin A on LNCaP cells and xenografts

BACKGROUND

Isoflavones inhibit the growth of some types of tumor cells, including prostate adenocarcinoma. This study used LNCaP cells and xenografts to investigate the mechanisms of the antiproliferative effects of biochanin A, a major isoflavone present in red clover but not soy-derived products.

METHODS

LNCaP cells were exposed to varying doses of biochanin A to evaluate viability, DNA synthesis, and DNA fragmentation (TUNEL) analysis. Regulation of gene expression was determined by using Western immunoblotting and cDNA microarrays. Anti-tumorigenic effects were evaluated by using athymic mice with LNCaP flank tumors.

RESULTS

Biochanin A induced a dose-dependent inhibition of proliferation and [(3)H]thymidine incorporation that correlated with increased DNA fragmentation, indicative of apoptosis. Western blot analyses of cell cycle regulatory proteins revealed that biochanin A significantly decreased expression of cyclin B and p21, whereas flow cytometry showed that cells were accumulating in the G(0)/G(1) phase. cDNA microarray analyses identified 29 down-regulated genes with six reduced below assay detection limits. Eleven genes were up-regulated, including 9 that were undetectable in controls. In mice with LNCaP xenografts, biochanin A significantly reduced tumor size and incidence.

CONCLUSION

These results indicate that biochanin A inhibits prostate cancer cell growth through induction of cell cycle arrest and apoptosis. Biochanin A-regulated genes suggest multiple pathways of action. Biochanin A inhibits the incidence and growth of LNCaP xenograft tumors in athymic mice.

New perspectives on folate catabolism

Folate catabolism has been assumed to result from the nonenzymatic oxidative degradation of labile folate cofactors. Increased rates of folate catabolism and simultaneous folate deficiency occur in several physiological states, including pregnancy, cancer, and when anticonvulsant drugs are used. These studies have introduced the possibility that folate catabolism may be a regulated cellular process that influences intracellular folate concentrations. Recent studies have demonstrated that the iron storage protein ferritin can catabolize folate in vitro and in vivo, and increased heavy-chain ferritin synthesis decreases intracellular folate concentrations independent of exogenous folate levels in cell culture models. Ferritin levels are elevated in most physiological states associated with increased folate catabolism. Therefore, folate catabolism is emerging as an important component in the regulation of intracellular folate concentrations and whole-body folate status.

Heavy chain ferritin enhances serine hydroxymethyltransferase expression and de novo thymidine biosynthesis

We have elucidated a biochemical mechanism whereby changes in iron metabolism cause changes in folate-dependent one-carbon metabolism. Although animal and clinical studies have demonstrated that perturbations in iron status and metabolism alter folate metabolism, the biochemical mechanisms underlying these associations have yet to be identified. The effect of altered ferritin expression on folate metabolism was determined in human MCF-7 cells and SH-SY5Y neuroblastoma. Cells expressing rat heavy chain ferritin (HCF) exhibited markedly increased expression of the folate-dependent enzyme cytoplasmic serine hydroxymethyltransferase (cSHMT). These effects were not seen when rat light chain ferritin was expressed. Additionally, cSHMT expression was not altered when HCF expression was induced in MCF-7 cells cultured with supplemental ferric citrate. This indicates that cSHMT expression is increased by elevated HCF concentrations, independent of increased iron availability, suggesting that cSHMT expression may respond to HCF-induced chelation of the regulatory iron pool. Increased HCF expression did not alter cSHMT mRNA levels, but did increase translation rates of cSHMT mRNA. The increase in translation was mediated, at least in part, through the cSHMT 5'-untranslated region of the transcript. MCF-7 cells with increased expression of cSHMT displayed increased efficiency of de novo thymidylate biosynthesis, indicating that thymidylate synthesis is normally limited by cSHMT activity in MCF-7 cells. Our data suggest that the iron regulatory pool may play an important role in regulating folate metabolism and thereby thymidine biosynthesis.

Targeted drug delivery via the folate receptor

The folate receptor is a highly selective tumor marker overexpressed in greater than 90% of ovarian carcinomas. Two general strategies have been developed for the targeted delivery of drugs to folate receptor-positive tumor cells: by coupling to a monoclonal antibody against the receptor and by coupling to a high affinity ligand, folic acid. First, antibodies against the folate receptor, including their fragments and derivatives, have been evaluated for tumor imaging and immunotherapy clinically and have shown significant targeting efficacy in ovarian cancer patients. Folic acid, a high affinity ligand of the folate receptor, retains its receptor binding properties when derivatized via its gamma-carboxyl. Folate conjugation, therefore, presents an alternative method of targeting the folate receptor. This second strategy has been successfully applied in vitro for the receptor-specific delivery of protein toxins, anti-T-cell receptor antibodies, interleukin-2, chemotherapy agents, gamma-emitting radiopharmaceuticals, magnetic resonance imaging contrast agents, liposomal drug carriers, and gene transfer vectors. Low molecular weight radiopharmaceuticals based on folate conjugates showed much more favorable pharmacokinetic properties than radiolabeled antibodies and greater tumor selectivity in folate receptor-positive animal tumor models. The small size, convenient availability, simple conjugation chemistry, and presumed lack of immunogenicity of folic acid make it an ideal ligand for targeted delivery to tumors.