Synthesis of folate-conjugated amphiphiles for tumor-targeted drug delivery

Design of Folate-Containing Liposomal Nucleic Acid Delivery Systems for Antitumor Therapy

The delivery of therapeutic nucleic acids is a prospective method for the treatment of both inherited and acquired diseases including cancer. To achieve maximal delivery efficiency and selectivity, nucleic acids should be targeted to the cells of interest. In the case of cancer, such targeting may be provided through folate receptors overexpressed in many tumor cells. For this purpose, folic acid and its lipoconjugates are used. Compared to other targeting ligands, folic acid provides low immunogenicity, rapid tumor penetration, high affinity to a wide range of tumors, chemical stability, and easy production. Different delivery systems can utilize targeting by folate ligand including liposomal forms of anticancer drugs, viruses, and lipid and polymer nanoparticles. This review focuses on the liposomal gene delivery systems that provide targeted nucleic acid transport into tumor cells due to folate lipoconjugates. Moreover, important development step, such as rational design of lipoconjugates, folic acid content, size, and ζ-potential of lipoplexes are discussed.

Design and preparation of nanostructures based on Krafft point of nonionic amphiphiles for delivery of poorly water-soluble compounds

Micellar solubilization can effectively dissolve low water-soluble compounds in aqueous environment, however, the micellar systems are not able to withstand dilution and maintain solubilization of poorly water-soluble drugs below critical micelle concentration. To overcome the drawbacks of conventional micellar solubilization, nonionic polyoxyethylated surfactants with Krafft points at or higher than body temperature were chosen to create novel micelle-based nanostructures as a delivery vehicle for poorly water-soluble compounds. A technique "thermo-spray process" was developed for the preparation of the nanostructures-containing formulation, in which the drug-containing micelle solution was first prepared and maintained at the elevated temperature above the Krafft point of the surfactant, then spray dried to solidify the obtained micelle-like nanostructure at room temperature. Lactose was used as an excipient to embed the nanostructures in the thermo-spray products. Water insoluble spherical nanoparticles with size range from 80 to 250 nm were obtained after reconstitution of the product at the temperature lower than Krafft point. When paclitaxel was used as model drug, the micelle-like nanostructures exhibited similar drug entrapment efficiency, solubility enhancement and drug release facilitation as conventional micelles, but provided lower critical micellar concentration at body temperature, and good encapsulation stability upon storage and dilution. These findings indicated that the developed thermo-spray product can serve as a promising delivery platform for drugs with low aqueous solubility.

Development of a novel folate-modified nanobubbles with improved targeting ability to tumor cells

Conjugation of folate (FOL) to nanobubbles could enhance the selective targeting to tumors expressing high levels of folate receptor (FR). To further improve the selective targeting ability of FOL-modified nanobubbles, a novel FOL-targeted nanobubble ((FOL)₂-NB) with increasing FOL content (accomplished by linking two FOL molecules per DSPE-PEG2000 chain) was synthesized, through the methods of mechanical shaking and low-speed centrifugation based on lipid-stabilized perfluoropropane. The bubble size and distribution range were measured by dynamic light scattering (DLS). Enhanced imaging ability was evaluated using a custom-made agarose mold with a clinical US imaging system at mechanical indices of up to 0.12 at a center frequency of 9.0MHz. Targeted ability was also carried out in human breast cancer MCF-7 cells, which over-express the FR, by fluorescence activated cell sorting (FACS) and fluorescence microscopy, respectively. (FOL)₂-NB with a particle size of 286.87±22.96nm were successfully prepared, and they exhibited superior contrast imaging effect. FACS and fluorescence microscopy studies showed greater cellular targeting ability in the group of (FOL)₂-NB than in their control group of Non-targeted-NB (no FOL targeted nanobubbles) and FOL-NB (one FOL molecule per DSPE-PEG2000 chain). These results suggest that a new type of stronger targeted nanobubble was successfully prepared by increasing the FOL content per DSPE-PEG2000 chain. This novel (FOL)₂-NBs are potentially useful for ultrasound molecular imaging and treatment of FR-positive tumors and are worthy for further investigation.

Novel serum-tolerant lipoplexes target the folate receptor efficiently

Gene transfer using non-viral vectors is a promising approach for the safe delivery of nucleic acid therapeutics. In this study, we investigate a lipid-based system for targeted gene delivery to malignant cells overexpressing the folate receptor (FR). Cationic liposomes were formulated with and without the targeting ligand folate conjugated to distearoylphosphatidyl ethanolamine polyethylene glycol 2000 (DSPE-PEG2000), the novel cytofectin 3β[N(N(1),N(1)-dimethlaminopropylsuccinamidoethane)-carbamoyl]cholesterol (SGO4), which contains a 13atom, 15Å spacer element, and the helper lipid, dioleoylphosphatidylethanolamine (DOPE). Physicochemical parameters of the liposomes and lipoplexes were obtained by zeta sizing, zeta potential measurement and cryo-TEM. DNA-binding and protection capabilities of liposomes were confirmed by gel retardation assays, EtBr intercalation and nuclease protection assays. The complexes were assessed in an in vitro system for their effect on cell viability using the MTT assay, and gene transfection activity using the luciferase assay in three cell lines; HEK293 (FR-negative), HeLa (FR(+)-positive), KB (FR(++)-positive). Low cytotoxicities were observed in all cell lines, while transgene activity promoted by folate-tagged lipoplexes in FR-positive lines was tenfold greater than that by untargeted constructs and cell entry by folate complexes was demonstrably by FR mediation. These liposome formulations have the design capacity for in vivo application and may therefore be promising candidates for further development.

Dual-Modal Tumor Imaging via Long-Circulating Biodegradable Core-Crosslinked Polymeric Micelles

We present a long-circulating biodegradable core-crosslinked polymeric micelle (d-CCPM) for the nuclear/optical imaging of tumors. The d-CCPM was derived from an amphiphilic block-copolymer consisting of a hydrophilic block of brush-like poly(ethylene glycol) and a hydrophobic block containing cleavable pendent triethoxysilane. The resultant imaging tracer had prolonged circulation in the blood (half-life of clearance phase = 36.5 hrs), substantial accumulation in tumor (% injected dose per gram of tissue = 8.5% ± 1.0% at 24 hrs post-injection) and minimal uptake in the liver (5.0% ± 0.1%) or spleen (5.1% ± 0.3%). Both nuclear and near-infrared fluorescence imaging revealed strong signals in tumor regions. At 48 hrs, nuclear imaging exhibited tumor-to-liver and tumor-to-blood ratios of 1.4 and 1.1, respectively. The degradation of d-CCPM was studied in vitro at pH 5.0 and 37°C; and confirmed by transmission electron microscopy confirmed. Our study indicates that the d-CCPM system is an effective probe for dual-modal cancer imaging and a potential safe platform nanocarrier for the delivery of anti-cancer drugs and cancer therapy.

Targeting an ultrasound contrast agent to folate receptors on ovarian cancer cells: feasibility research for ultrasonic molecular imaging of tumor cells

OBJECTIVE

The purpose of the study was to synthesize and characterize folate-targeted microbubbles (MB(F)) as an ultrasound contrast agent and to evaluate their affinity to the folate receptor (FR) in vitro.

METHODS

Folate-targeted microbubbles were prepared by incorporating 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene glycol)-2000]-folate into the lipid membrane of microbubbles. The diameter and concentration of the MB(F) were determined by a cell counter and sizer. The MB(F), control microbubbles (MB(C)), and MB(F) with a free folic acid block-ade were tested for binding specificity to human ovarian carcinoma SKOV3 cells, which overexpress the FR, by microscopy and confocal imaging, respectively.

RESULTS

The basic physical characteristics of MB(F) were similar to those of MB(C). In the cell binding test, the adherence efficiency of MB(F) to the SKOV3 cells (mean +/- SD, 16 +/- 5 microbubbles per cell; P < .01) was significantly higher than that of MB(C) (0.7 +/- 0.4 microbubbles per cell) or MB(F) with the free folic acid blockade (0.7 +/- 0.6 microbubbles per cell).

CONCLUSIONS

Folate-targeted microbubbles showed high affinity to SKOV3 cells with FR overexpression. They are potentially useful for ultrasonic molecular imaging and treatment of FR-positive tumors and warrant further investigation.

Functional polymers for targeted delivery of nucleic acid drugs

Cationic polymers have been chemically modified with a variety of targeting molecules such as peptides, proteins, antibodies, sugars and vitamins for targeted delivery of nucleic acid drugs to specific cells. Stimuli-sensitive polymers exhibiting different size, charge and conformation in response to physiological signals from specific cells have also been utilized for targeted delivery. To achieve target-specific delivery of nucleic acids, conjugation chemistry is critical to produce stable nanosized polyplexes tethered with cell-recognizable ligands for facile cellular uptake via a receptor-mediated endocytic pathway. In this review, synthetic strategies of functional cationic polymers with various targeting ligands are presented.