Folate-mediated targeting of antisense oligodeoxynucleotides to ovarian cancer cells

Lipid and Peptide-Oligonucleotide Conjugates for Therapeutic Purposes: From Simple Hybrids to Complex Multifunctional Assemblies

Antisense and small interfering RNA (siRNA) oligonucleotides have been recognized as powerful therapeutic compounds for targeting mRNAs and inducing their degradation. However, a major obstacle is that unmodified oligonucleotides are not readily taken up into tissues and are susceptible to degradation by nucleases. For these reasons, the design and preparation of modified DNA/RNA derivatives with better stability and an ability to be produced at large scale with enhanced uptake properties is of vital importance to improve current limitations. In the present study, we review the conjugation of oligonucleotides with lipids and peptides in order to produce oligonucleotide conjugates for therapeutics aiming to develop novel compounds with favorable pharmacokinetics.

Targeted Drug Delivery: Trends and Perspectives

BACKGROUND

Due to various limitations in conventional drug delivery system, it is important to focus on the target-specific drug delivery system where we can deliver the drug without any degradation. Among various challenges faced by a formulation scientist, delivering the drug to its right site, in its right dose, is also an important aim. A focused drug transport aims to extend, localize, target and have a safe drug interaction with the diseased tissue.

OBJECTIVE

The aim of targeted drug delivery is to make the required amount of the drug available at its desired site of action. Drug targeting can be accomplished in a number ways that include enzyme mediation, pH-dependent release, use of special vehicles, receptor targeting among other mechanisms. Intelligently designed targeted drug delivery systems also offer the advantages of a low dose of the drug along with reduced side effects which ultimately improves patient compliance. Incidences of dose dumping and dosage form failure are negligible. A focused drug transport aims to have a safe drug interaction with the diseased tissue.

CONCLUSION

This review focuses on the available targeting techniques for delivery to the colon, brain and other sites of interest. Overall, the article should make an excellent read for the researchers in this area. Newer drug targets may be identified and exploited for successful drug targeting.

A Bibliometric analysis of folate receptor research

BACKGROUND

The objective of this study was to conduct a bibliometric analysis of the entire field of folate receptor research. Folate receptor is expressed on a wide variety of cancers and certain immune cells.

METHODS

A Web of Science search was performed on folate receptor or folate binding protein (1969-to June 28, 2019). The following information was examined: publications per year, overall citations, top 10 authors, top 10 institutions, top 10 cited articles, top 10 countries, co-author collaborations and key areas of research.

RESULTS

In total, 3248 documents for folate receptor or folate binding protein were retrieved for the study years outlined in the methods section search query. The range was 1 per year in 1969 to 264 for the last full year studied (2018). A total of 123,720 citations for the 3248 documents retrieved represented a mean citation rate per article of 38.09 and range of 1667 citations (range 0 to 1667). Researchers in 71 countries authored publications analyzed in this study. The US was the leader in publications and had the highest ranking institution. The top 10 articles have been cited 7270 times during the time frame of this study. The top cited article had an average citation rate of 110 citations per year. Network maps revealed considerable co-authorship among several of the top 10 authors.

CONCLUSION

Our study presents several important insights into the features and impact of folate receptor research. To our knowledge, this is the first bibliometric analysis of folate receptor.

To Conjugate or to Package? A Look at Targeted siRNA Delivery Through Folate Receptors

RNA interference (RNAi) applications have evolved from experimental tools to study gene function to the development of a novel class of gene-silencing therapeutics. Despite decades of research, it was not until August 2018 that the US FDA approved the first-ever RNAi drug, marking a new era for RNAi therapeutics. Although there are many limitations associated with the inherent structure of RNA, delivery to target cells and tissues remains the most challenging. RNAs are unable to diffuse across cellular membranes due to their large size and polyanionic backbone and, therefore, require a delivery vector. RNAi molecules can be conjugated to a targeting ligand or packaged into a delivery vehicle. Alnylam has used both strategies in their FDA-approved formulations to achieve efficient delivery to the liver. To harness the full potential of RNAi therapeutics, however, we must be able to target additional cells and tissues. One promising target is the folate receptor α, which is overexpressed in a variety of tumors despite having limited expression and distribution in normal tissues. Folate can be conjugated directly to the RNAi molecule or used to functionalize delivery vehicles. In this review, we compare both delivery strategies and discuss the current state of research in the area of folate-mediated delivery of RNAi molecules.

Biomedical and Pharmacological Uses of Fluorescein Isothiocyanate Chitosan-Based Nanocarriers

Chitosan-based nanocarriers (ChNCs) are considered suitable drug carriers due to their ability to encapsulate a variety of drugs and cross biological barriers to deliver the cargo to their target site. Fluorescein isothiocyanate-labeled chitosan-based NCs (FITC@ChNCs) are used extensively in biomedical and pharmacological applications. The main advantage of using FITC@ChNCs consists of the ability to track their fate both intra and extracellularly. This journey is strictly dependent on the physico-chemical properties of the carrier and the cell types under investigation. Other applications make use of fluorescent ChNCs in cell labeling for the detection of disorders in vivo and controlling of living cells in situ. This review describes the use of FITC@ChNCs in the various applications with a focus on understanding their usefulness in labeled drug-delivery systems.

Improving cellular uptake of therapeutic entities through interaction with components of cell membrane

Efficient cellular delivery of biologically active molecules is one of the key factors that affect the discovery and development of novel drugs. The plasma membrane is the first barrier that prevents direct translocation of chemic entities, and thus obstructs their efficient intracellular delivery. Generally, hydrophilic small molecule drugs are poor permeability that reduce bioavailability and thus limit the clinic application. The cellular uptake of macromolecules and drug carriers is very inefficient without external assistance. Therefore, it is desirable to develop potent delivery systems for achieving effective intracellular delivery of chemic entities. Apart from of the types of delivery strategies, the composition of the cell membrane is critical for delivery efficiency due to the fact that cellular uptake is affected by the interaction between the chemical entity and the plasma membrane. In this review, we aimed to develop a profound understanding of the interactions between delivery systems and components of the plasma membrane. For the purpose, we attempt to present a broad overview of what delivery systems can be used to enhance the intracellular delivery of poorly permeable chemic entities, and how various delivery strategies are applied according to the components of plasma membrane.

Folate-functionalized dendrimers for targeting Chlamydia-infected tissues in a mouse model of reactive arthritis

Chlamydia trachomatis is an intracellular human pathogen that causes a sexually transmitted disease which may result in an inflammatory arthritis designated Chlamydia-induced reactive arthritis (ReA). The arthritis develops after dissemination of infected cells from the initial site of chlamydial infection. During Chlamydia-associated ReA, the organism may enter into a persistent infection state making treatment with antibiotics a challenge. We hypothesize that folate receptors (FR), which are overexpressed in Chlamydia-infected cells, and the associated inflammation would allow folate-targeted nanodevices to better treat infections. To investigate this, we developed a folate-PAMAM dendrimer-Cy5.5 conjugate (D-FA-Cy5.5), where Cy5.5 is used as the near-IR imaging agent. Uptake of D-FA-Cy5.5 upon systemic administration was assessed and compared to non-folate conjugated controls (D-Cy5.5), using a mouse model of Chlamydia-induced ReA, and near-IR imaging. Our results suggested that there was a higher concentration of folate-based nanodevice in sites of infection and inflammation compared to that of the control nanodevice. The folate-conjugated nanodevices localized to infected paws and genital tracts (major sites of inflammation and infection) at 3-4 fold higher concentrations than were dendrimer alone, suggesting that the overexpression of folate receptors in infected and inflamed tissues enables higher dendrimer uptake. There was an increase in uptake into thymus, spleen, and lung, but no significant differences in the uptake of the folate nanodevices in other organs including kidney and heart, indicating the 'relative specificity' of the D-FA-Cy5.5 conjugate nanodevices. These results suggest that folate targeting dendrimers are able to deliver drugs to attenuate infection and associated inflammation in Chlamydia-induced ReA.

Cellular delivery and photochemical activation of antisense agents through a nucleobase caging strategy

Antisense oligonucleotides are powerful tools to regulate gene expression in cells and model organisms. However, a transfection or microinjection is typically needed for efficient delivery of the antisense agent. We report the conjugation of multiple HIV TAT peptides to a hairpin-protected antisense agent through a light-cleavable nucleobase caging group. This conjugation allows for the facile delivery of the antisense agent without a transfection reagent, and photochemical activation offers precise control over gene expression. The developed approach is highly modular, as demonstrated by the conjugation of folic acid to the caged antisense agent. This enabled targeted cell delivery through cell-surface folate receptors followed by photochemical triggering of antisense activity. Importantly, the presented strategy delivers native oligonucleotides after light-activation, devoid of any delivery functionalities or modifications that could otherwise impair their antisense activity.

Cellular uptake and intracellular trafficking of antisense and siRNA oligonucleotides

Significant progress is being made concerning the development of oligonucleotides as therapeutic agents. Studies with antisense, siRNA, and other forms of oligonucleotides have shown promise in cellular and animal models and in some clinical studies. Nonetheless, our understanding of how oligonucleotides function in cells and tissues is really quite limited. One major issue concerns the modes of uptake and intracellular trafficking of oligonucleotides, whether as "free" molecules or linked to various delivery moieties such as nanoparticles or targeting ligands. In this review, we examine the recent literature on oligonucleotide internalization and subcellular trafficking in the context of current insights into the basic machinery for endocytosis and intracellular vesicular traffic.

Versatile site-specific conjugation of small molecules to siRNA using click chemistry

We have previously demonstrated that conjugation of small molecule ligands to small interfering RNAs (siRNAs) and anti-microRNAs results in functional siRNAs and antagomirs in vivo. Here we report on the development of an efficient chemical strategy to make oligoribonucleotide-ligand conjugates using the copper-catalyzed azide-alkyne cycloaddition (CuAAC) or click reaction. Three click reaction approaches were evaluated for their feasibility and suitability for high-throughput synthesis: the CuAAC reaction at the monomer level prior to oligonucleotide synthesis, the solution-phase postsynthetic "click conjugation", and the "click conjugation" on an immobilized and completely protected alkyne-oligonucleotide scaffold. Nucleosides bearing 5'-alkyne moieties were used for conjugation to the 5'-end of the oligonucleotide. Previously described 2'- and 3'-O-propargylated nucleosides were prepared to introduce the alkyne moiety to the 3' and 5' termini and to the internal positions of the scaffold. Azido-functionalized ligands bearing lipophilic long chain alkyls, cholesterol, oligoamine, and carbohydrate were utilized to study the effect of physicochemical characteristics of the incoming azide on click conjugation to the alkyne-oligonucleotide scaffold in solution and on immobilized solid support. We found that microwave-assisted click conjugation of azido-functionalized ligands to a fully protected solid-support bound alkyne-oligonucleotide prior to deprotection was the most efficient "click conjugation" strategy for site-specific, high-throughput oligonucleotide conjugate synthesis tested. The siRNA conjugates synthesized using this approach effectively silenced expression of a luciferase gene in a stably transformed HeLa cell line.

Folate-mediated solid-liquid lipid nanoparticles for paclitaxel-coated poly(ethylene glycol)

BACKGROUND

As a promising anticancer drug, severe side-effects of current clinical formulations for paclitaxel have restricted its use, developing a better technical-economical formulation for paclitaxel delivery is needed.

METHOD

In this study, the compound of folate-poly(ethylene glycol) (PEG)-phosphatidylethanolamine was synthesized and characterized with Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy. The solid-liquid lipid nanoparticle (SLLN) for paclitaxel modified with folate and poly(ethylene glycol) (folate-PEG-SLLN) was prepared and characterized. Morphology of folate-PEG-SLLN was examined by transmission electron microscopy. The particle size and zeta potential were performed by Zetapals. Encapsulation efficiency was analyzed by HPLC. The in vitro drug release of paclitaxel was investigated via membrane dialysis. The in vivo pharmacokinetics was measured with male Sprague-Dawley rats. Treatment efficiency was investigated with the mouse with sarcoma180 ascites tumor.

RESULTS

Paclitaxel loaded on the newly designed binary SLLN showed a longer and sustained in vitro releasing property. More importantly, S180 tumor-bearing mice treated with paclitaxel-loaded SLLN exhibited higher tumor inhibition rate, comparing with animals administered with paclitaxel injection alone (45.3% and 37.3%, respectively).

CONCLUSION

The newly developed paclitaxel delivery system may have improved in vivo antitumor activity. The results demonstrated a great interest to use folate-mediated SLLN as a prospective drug delivery system for paclitaxel.

Targeted drug delivery via folate receptors

Targeted delivery via selective cellular markers can potentially increase the efficacy and reduce the toxicity of therapeutic agents. The folate receptor (FR) has two glycosyl phosphatidylinositol (GPI)-anchored isoforms, alpha and beta. FR-alpha expression is frequently amplified in epithelial cancers, whereas FR-beta expression is found in myeloid leukemia and activated macrophages associated with chronic inflammatory diseases. Conjugates of folic acid and anti-FR antibodies can be taken up by cancer cells via receptor-mediated endocytosis, thus providing a mechanism for targeted delivery to FR+ cells. The aim of this article is to provide a brief overview of applications of FR targeting in drug delivery, with an emphasis on the strategy of using folate as a targeting ligand. In order to do this, recent literature is surveyed on targeted delivery via both FR sub-types, as well as new findings on selective receptor upregulation in the targeted cells. A wide variety of molecules and drug carriers, including imaging agents, chemotherapeutic agents, oligonucleotides, proteins, haptens, liposomes, nanoparticles and gene transfer vectors have been conjugated to folate and evaluated for FR-targeted delivery. Substantial targeting efficacy has been found both in vitro and in vivo. In addition, mechanisms and methods for selective FR upregulation have been uncovered, which might enhance the effectiveness of the FR-targeted delivery strategy. FR-alpha serves as a useful marker for cancer, whereas FR-beta serves as a marker for myeloid leukemia and chronic inflammatory diseases. FR-targeted agents have shown promising efficacy in preclinical models and significant potential for future clinical application in a wide range of diseases.

Some novel adenosine mimics: synthesis and anticancer potential against cervical cancer caused by human papilloma virus

Two novel adenosine analogs, viz. 9-(1'-beta-D-arabinofuranosyl)-6-nitro-1,3-dideazapurine or Ara-NDDP (1) and 9-(5'-deoxy-5'-S-(propionic acid) (1'-beta-D-ribofuranosyl) adenine or SAH analog (2), indigenously synthesized, have been found to be potential anticancer agents against cervical cancer caused by human papilloma virus.

A folic acid-based functionalized surface for biosensor systems

The performance of a biosensor depends largely on its interface with the biological system. This interface imparts a biologically relevant function to the device and provides a measure of specificity towards the biological analyte of interest. This paper documents the choice of folic acid as the functional component of a cantilever sensor to recognize nasopharyngeal (KB) cancer cells. A conjugation chemistry protocol has been outlined to deploy folic acid onto a titanium-coated sensor surface using a silane linker. The presence and biological activity of the sensor was verified by means of an immunospecific (ELISA) procedure. The overall performance of the folic acid-based cantilever sensor was measured using cancerous KB cell-binding experiments.

Design and regioselective synthesis of a new generation of targeted chemotherapeutics. Part 1: EC145, a folic acid conjugate of desacetylvinblastine monohydrazide

An efficient synthesis of the folate receptor (FR) targeting conjugate EC145 is described. EC145 is a water soluble derivative of the vitamin folic acid and the potent cytotoxic agent, desacetylvinblastine monohydrazide. Both molecules are connected in regioselective manner via a hydrophilic peptide spacer and a reductively labile disulfide linker.

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.

In vivo tumor targeting of ODN-PEG-folic acid/PEI polyelectrolyte complex micelles

A tumor-targeting antisense oligodeoxynucleotide (ODN) delivery system based on polyelectrolyte complex (PEC) micelles is demonstrated. ODN-PEG-folic acid (ODN-PEG-FA) was synthesized using a heterofunctional PEG linker. The PEC micelles for the targeted ODN delivery to tumor cells were produced by ionic interactions between the ODN-PEG-FA and polyethylenimine (PEI). The in vivo targeting properties of the PEC micelles were assessed using a mouse tumor model. The size of ODN-PEG-FA/PEI PEC micelles was 92.3 nm with a relatively narrow distribution. Cellular uptake of the ODN-PEG-FA/PEI PEC micelles by folic acid receptor over-expressing cells (KB) was greatly enhanced compared to that of ODN-PEG/PEI PEC micelles. When the ODN-PEG-FA/PEI PEC micelles were systemically administered to the mice bearing KB cell xenograft tumor, ODN was accumulated to the solid tumor in a target specific manner. This study suggests that the PEC micelles with a receptor-recognizable targeting ligand on the surface have potential for passive and active targeted delivery of ODN drugs to cancer cells.

Folate-mediated targeting of polymeric conjugates of gemcitabine

The synthesis of two new macromolecular prodrugs for active tumor targeting was set up. Gemcitabine (2'-deoxy-2',2'-difluorocytidine) was conjugated to alpha,beta-poly(N-2-hydroxyethyl)-DL-aspartamide (PHEA) through succinyl or diglycolyl hydrolysable spacers. The targeting agent folic acid was attached to the macromolecular backbone through the aminocaproic spacer. The two conjugates [PHEA-(5'-succinylgemcitabine)-1'-carboxypentyl-folamide and PHEA-(5'-diglycolyl-gemcitabine)-1'-carboxypentyl-folamide], were purified and extensively characterised by spectroscopic (UV, IR and NMR) and chromatographic analyses to determine the correct chemical structure, the purity degree and the reaction yield. In vitro studies demonstrated that the drug release depends on the spacer arm (diglycolyil or succinyl) and incubation pH. After 30 h incubation at pH 7.4, mimicking the plasma and extracellular compartments, the gemcitabine release from the succinyl and diglycolyl derivatives was 28 and 31%, respectively. After 30 h incubation at pH 5.5, mimicking the lisosomial compartment, the drug released from both bioconjugates was lower than 13%. In plasma, the polymer conjugation increased the drug stability and provided for a sustained drug release. In vitro citotoxicity studies performed using human nasopharyngeal epidermal carcinoma KB cells demonstrated that PHEA-(5'-succinylgemcitabine)-1'-carboxypentyl-folamide displays an higher dose dependent cytotoxic effect with respect to PHEA-(5'-diglycolyl-gemcitabine)-1'-carboxypentyl-folamide.

Tumor-targeted and activated bioconjugates for improved camptothecin delivery

Earlier reports from our laboratory described bioconjugates of camptothecin (CPT) for tumor targeting. In the current work, the rate and site of CPT release from the bioconjugates were modulated using increasingly sterically hindered amino acids and cysteine proteinase-sensitive peptide linkers, respectively. Polyethylene glycol served as a spacer/scaffold between CPT and folic acid. The folic acid receptor, overexpressed on many cancer cells, was targeted using folate. The delivery system was tested in vitro for hydrolytic stability, enzyme-mediated cleavage, cytotoxicity and targeting potential. The linkers successfully modulated the hydrolysis rate (around 1--100 h) and potential site (tumor microenvironment) of CPT release. Preliminary molecular modeling approaches were utilized to assess the influence of molecular volume on hydrolysis half-life (i.e. CPT release). There was a clear, but non-linear, relationship between in vitro CPT release and increasing steric hindrance offered by the peptide linker. The efficacy of four conjugates was studied in a syngeneic rat breast cancer model. Histopathological analysis on treated tumors was performed to evaluate disease prognosis. The results demonstrate that programmed bioconjugates may provide superior efficacy and greater control over the rate and site of CPT release, resulting in higher anti-tumor efficacy and lower toxicity.

Folate receptor mediated intracellular protein delivery using PLL–PEG–FOL conjugate

To develop a receptor-mediated intracellular delivery system that can transport therapeutic proteins or other bioactive macromolecules into a specific cell, a di-block copolymer conjugate, poly(L-lysine)-poly(ethylene glycol)-folate (PLL-PEG-FOL), was synthesized. The PLL-PEG-FOL conjugate was physically complexed with fluorescein isothiocyanate conjugated bovine serum albumin (FITC-BSA) in an aqueous phase by ionic interactions. Cellular uptake of PLL-PEG-FOL/FITC-BSA complexes was greatly enhanced against a folate receptor over-expressing cell line (KB cells) compared to a folate receptor deficient cell line (A549 cells). The presence of an excess amount of free folate (1 mM) in the medium inhibited the intracellular delivery of PLL-PEG-FOL/FITC-BSA complexes. This suggests that the enhanced cellular uptake of FITC-BSA by KB cells in a specific manner was attributed to folate receptor-mediated endocytosis of the complexes having folate moieties on the surface. The PLL-PEG-FOL di-block copolymer could be potentially applied for intracellular delivery of a wide range of other biological active agents that have negative charges on the surface.

Tumor-targeted bioconjugate based delivery of camptothecin: design, synthesis and in vitro evaluation

Camptothecin (CPT) presents numerous challenges associated with optimal transport and delivery including variability in clinically observed effects, low target tissue concentrations and severe and unpredictable toxicity. The objective of the present study was to optimize the delivery of CPT by targeting it to cancer cells using an endogenous receptor system. A novel CPT bioconjugate was synthesized using carbodiimide chemistry with a linear poly(ethylene glycol) (PEG) and amino acid glycine as the spacer and linker respectively. Folic acid was used as the targeting ligand to take advantage of folate receptor mediated endocytosis. The bioconjugate was extensively characterized using MALDI, proton NMR, FT-IR and amino acid analysis. Furthermore, the bioconjugate was evaluated in vitro for specific targeting to folate receptor-expressing KB cells, a human nasopharyngeal carcinoma. Finally, the delivery system was evaluated for cytotoxicity using a MTT based assay. The results indicate significantly higher efficacy of the bioconjugate in comparison to CPT. A control conjugate without PEG demonstrated no improvement in efficacy over untargeted CPT emphasizing the importance of spacer between the anticancer compounds and targeting moiety. This bioconjugate represents the 'first-in-series' of targeted bioconjugates and serves as prototype for improving tumor cell concentration and efficacy.

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.

Preparation of 66Ga- and 68Ga-labeled Ga(III)-deferoxamine-folate as potential folate-receptor-targeted PET radiopharmaceuticals

A folate-receptor-targeting radiopharmaceutical, Ga(III)-deferoxamine-folate (Ga-DF-Folate), was radiolabeled with two positron-emitting isotopes of gallium, cyclotron-produced (66)Ga (9.5 hour half-life) and generator-produced (68)Ga (68 minute half-life). The [(66)Ga]Ga-DF-Folate was administered to athymic mice with folate-receptor-positive human KB cell tumor xenografts to demonstrate that microPET mouse tumor imaging is feasible with (66)Ga, despite the relatively high positron energy of this radionuclide. Using the athymic mouse KB tumor xenograft model, dual-isotope autoradiography was also performed following i.v. co-administration of [(18)F]-FDG, a marker of regional metabolic activity, and folate-receptor-targeted [(111)In]In-DTPA-Folate. The autoradiographic images of 1 mm tumor sections demonstrate the gross heterogeneity of the KB cell tumor xenograft, as well as subtle disparity in the regional accumulation of the two radiotracers.

HPMA copolymer delivery of chemotherapy and photodynamic therapy in ovarian cancer

Our studies document a unique and unexpected advantage of the combination of HPMA copolymer bound doxorubicin with mesochlorin e6/photodynamic therapy in the treatment of ovarian cancer. Each drug's activity is individually enhanced when compared with free (low molecular weight) drugs, furthermore, in combination these HPMA copolymer bound agents act synergistically to create an unexpected biological effect. Figure 8 depicts the known activities of each agent which may play synergistic roles. HPMA copolymer-doxorubicin has been widely evaluated in preclinical and clinical studies. It demonstrates marked advantages over free doxorubicin: control of biodistribution and accumulation via molecular weight restrictions, biodegradability, minimal immunogenicity, subcellular localization, anticancer activity, enhanced permeability and retention, increased apoptosis, lipid peroxidation, DNA damage, and reduced nonspecific toxicity. Recent clinical trials in the UK provide "proof of principle" of the "enhanced permeability and retention effect" for solid tumors and the unique advantages of this novel drug delivery system for the treatment of ovarian cancer. With regards to photodynamic therapy using the photosensitizer mesochlorin e6, the preclinical evaluations thus far document: control of biodistribution and accumulation via molecular weight restrictions, biodegradability, subcellular localization, anticancer activity, enhanced permeability and retention, and reduced nonspecific toxicity. Ongoing microarray studies document unique cellular pathways and new pharmaceutical properties which are initiated by the HPMA copolymer delivery delivery of these agents, and predict an exciting future for this novel drug delivery system.

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.

Oligonucleotide conjugates as potential antisense drugs with improved uptake, biodistribution, targeted delivery, and mechanism of action

This review summarizes the effect of conjugating small molecules and large biomacromolecules to antisense oligonucleotides to improve their therapeutic potential. In many cases, favorable changes in pharmacokinetic and pharmacodynamic properties were observed. Opportunities exist to change the terminating mechanism of antisense action or to enhance the RNase H mode of action via conjugate formation.

The cellular delivery of antisense oligonucleotides and ribozymes

The design and development of antisense oligonucleotides and ribozymes for the treatment of diseases arising from genetic abnormalities has become a real possibility over the past few years. Improvements in oligonucleotide chemistry have led to the synthesis of nucleic acids that are relatively stable in the biological milieu. However, advances in cellular targeting and intracellular delivery will probably lead to more widespread clinical applications. This review looks at recent advances in the in vitro and in vivo delivery of antisense oligodeoxynucleotides and ribozymes.

Discovery and analysis of antisense oligonucleotide activity in cell culture

In the past decade antisense oligonucleotides (ASOs) have proven to be a useful tool for dissection of gene function in molecular cell biology (Koller, E., Gaarde, W. A., and Monia, B. P. (2000) Trends Pharm. Sci., 21, 142-148), and validation of gene targets in animal models (Crooke, S. T. (1998) Biotechnol. Gen. Eng. Rev. 15, 121-157), as well as a means for therapeutic treatment of human diseases (Bennett, C. F. (1999) Exp. Opin. Invest. Drugs 8, 237-253). An important step toward usage of ASOs in the described applications is identification of an active ASO. This article describes the underlying basis and means for achieving this goal in cell culture.

Folate-mediated targeting: from diagnostics to drug and gene delivery

The covalent attachment of the vitamin folic acid to almost any molecule yields a conjugate that can be endocytosed into folate receptor-bearing cells. Because folate receptors are significantly overexpressed in the majority of human cancers, this methodology is currently being investigated for the selective delivery of imaging and therapeutic agents to tumor tissue. Phase I and II clinical studies for the first folate-containing imaging agent were initiated in 1999, and clinical trials of folate-targeted therapeutic agents should soon follow. This review will summarize folate-mediated drug delivery and highlight those techniques undergoing active preclinical or clinical investigation.

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.

Receptor-targeted gene delivery via folate-conjugated polyethylenimine

A novel synthetic gene transfer vector was evaluated for tumor cell-specific targeted gene delivery. The folate receptor is a tumor marker overexpressed in more than 90% of ovarian carcinomas and large percentages of other human tumors. Folic acid is a high affinity ligand for the folate receptor that retains its binding affinity upon derivatization via its gamma carboxyl. Folate conjugation, therefore, presents a potential strategy for tumor-selective targeted gene delivery. In the current study, we investigated a series of folate conjugates of the cationic polymer polyethylenimine (PEI) for potential use in gene delivery. A plasmid containing a luciferase reporter gene (pCMV-Luc) and the folate receptor expressing human oral cancer KB cells were used to monitor gene transfer efficiency in vitro. Transfection activity of polyplexes containing unmodified polyethylenimine was highly dependent on the positive to negative charge (or the N/P) ratio. Folate directly attached to PEI did not significantly alter the transfection activity of its DNA complexes compared to unmodified PEI. Modification of PEI by polyethyleneglycol (PEG) led to a partial inhibition of gene delivery compared to unmodified PEI. Attaching folates to the distal termini of PEG-modified PEI greatly enhanced the transfection activity of the corresponding DNA complexes over the polyplexes containing PEG-modified PEI. The enhancements were observed at all N/P ratios tested and could be blocked partially by co-incubation with 200 microM free folic acid, which suggested the involvement of folate receptor in gene transfer. Targeted vectors based on the folate-PEG-PEI conjugate are potentially useful as simple tumor-specific vehicles of therapeutic genes.