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.

Cancer-Targeted Controlled Delivery of Chemotherapeutic Anthracycline Derivatives Using Apoferritin Nanocage Carriers

The interactions of chemotherapeutic drugs with nanocage protein apoferritin (APO) are the key features in the effective encapsulation and release of highly toxic drugs in APO-based controlled drug delivery systems. The encapsulation enables mitigating the drugs' side effects, collateral damage to healthy cells, and adverse immune reactions. Herein, the interactions of anthracycline drugs with APO were studied to assess the effect of drug lipophilicity on their encapsulation excess n and in vitro activity. Anthracycline drugs, including doxorubicin (DOX), epirubicin (EPI), daunorubicin (DAU), and idarubicin (IDA), with lipophilicity P from 0.8 to 15, were investigated. We have found that in addition to hydrogen-bonded supramolecular ensemble formation with n = 24, there are two other competing contributions that enable increasing n under strong polar interactions (APO(DOX)) or under strong hydrophobic interactions (APO(IDA) of the highest efficacy). The encapsulation/release processes were investigated using UV-Vis, fluorescence, circular dichroism, and FTIR spectroscopies. The in vitro cytotoxicity/growth inhibition tests and flow cytometry corroborate high apoptotic activity of APO(drugs) against targeted MDA-MB-231 adenocarcinoma and HeLa cells, and low activity against healthy MCF10A cells, demonstrating targeting ability of nanodrugs. A model for molecular interactions between anthracyclines and APO nanocarriers was developed, and the relationships derived compared with experimental results.

Selectivity Enhancement of Paclitaxel Liposome Towards Folate Receptor-Positive Tumor Cells by Ligand Number Optimization Approach

The present work aims to develop folate-targeted paclitaxel liposome (F-PTX-LIP), which will selectively target tumor cells overexpressing folate receptor (FR) and leave normal cells. Liposomes were prepared by thin-film hydration method followed by post-insertion of synthesized ligand 1,2-distearoyl-sn-glycero-phosphoethanolamine-polyethyleneglycol 2000-folic acid (DSPE-PEG2000-FA) on the outer surface of the liposome. The synthesized ligand was evaluated for in vivo acute toxicity in Balb/c mice. Developed liposomal formulations were characterized using transmission electron microscopy (TEM) and small-angle neutron scattering (SANS). We have investigated the effect of ligand number on cell uptake and cytotoxicity by confocal laser scanning microscopy (CLSM), competitive inhibition and 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) assay. Compared to lung adenocarcinoma cells (A549), uptake in human ovarian carcinoma cells (SKOV3) was 2.2- and 1.2-fold higher for liposome with 480 and 240 ligand number respectively. Competitive inhibition experiment shows that prior incubation of SKOV3 cells with free folic acid significantly reduced the cell uptake of F-PTX-LIP with 480 ligand number (480 F-PTX-LIP) by 2.6-fold. 480 F-PTX-LIP displays higher cytotoxicity than free drug and PTX liposome. Moreover, it specifically targets the cells with higher folate receptor expression. Optimized 480 F-PTX-LIP formulation can be potentially useful for the treatment of folate receptor-positive tumors.

Theranostic pH-sensitive nanoparticles for highly efficient targeted delivery of doxorubicin for breast tumor treatment

A multifunctional theranostic nanoplatform integrated with environmental responses has been developed rapidly over the past few years as a novel treatment strategy for several solid tumors. We synthesized pH-sensitive poly(β-thiopropionate) nanoparticles with a supermagnetic core and folic acid (FA) conjugation (FA-doxorubicin-iron oxide nanoparticles [FA-DOX@ IONPs]) to deliver an antineoplastic drug, DOX, for the treatment of folate receptor (FR)-overexpressed breast cancer. In addition to an imaging function, the nanoparticles can release their payloads in response to an environment of pH 5, such as the acidic environment found in tumors. After chemical (¹H nuclear magnetic resonance) and physical (morphology and super-magnetic) characterization, FA-DOX@IONPs were shown to demonstrate pH-dependent drug release profiles. Western blotting analysis revealed the expression of FRs in three breast cancer cell lines, MCF-7, BT549, and MD-MBA-231. The cell counting kit-8 assay and transmission electron microscopy showed that FA-DOX@IONPs had the strongest cytotoxicity against breast cancer cells, compared with free DOX and non-FR targeted nanoparticles (DOX@IONPs), and caused cellular apoptosis. The FA-DOX@IONP-mediated cellular uptake and intracellular internalization were clarified by fluorescence microscopy. FA-DOX@IONPs plus magnetic field treatment suppressed in vivo tumor growth in mice to a greater extent than either treatment alone; furthermore, the nanoparticles exerted no toxicity against healthy organs. Magnetic resonance imaging was successfully applied to monitor the nanoparticle accumulation. Our results suggest that theranostic pH-sensitive nanoparticles with dual targeting could enhance the available therapies for cancer.

Surface-enhanced Raman scattering investigation of targeted delivery and controlled release of gemcitabine

Advanced and metastatic cancer forms are extremely difficult to treat and require high doses of chemotherapeutics, inadvertently affecting also healthy cells. As a result, the observed survival rates are very low. For instance, gemcitabine (GEM), one of the most effective chemotherapeutic drugs used for the treatment of breast and pancreatic cancers, sees only a 20% efficacy in penetrating cancer tissue, resulting in <5% survival rate in pancreatic cancer. Here, we present a method for delivering the drug that offers mitigation of side effects, as well as a targeted delivery and controlled release of the drug, improving its overall efficacy. By modifying the surface of gold nanoparticles (AuNPs) with covalently bonded thiol linkers, we have immobilized GEM on the nanoparticle (NP) through a pH-sensitive amide bond. This bond prevents the drug from being metabolized or acting on tissue at physiological pH 7.4, but breaks, releasing the drug at acidic pH, characteristic of cancer cells. Further functionalization of the NP with folic acid and/or transferrin (TF) offers a targeted delivery, as cancer cells overexpress folate and TF receptors, which can mediate the endocytosis of the NP carrying the drug. Thus, through the modification of AuNPs, we have been able to produce a nanocarrier containing GEM and folate/TF ligands, which is capable of targeted controlled-release delivery of the drug, reducing the side effects of the drug and increasing its efficacy. Here, we demonstrate the pH-dependent GEM release, using an ultrasensitive surface-enhanced Raman scattering spectroscopy to monitor the GEM loading onto the nanocarrier and follow its stimulated release. Further in vitro studies with model triple-negative breast cancer cell line MDA-MB-231 have corroborated the utility of the proposed nanocarrier method allowing the administration of high drug doses to targeted cancer cells.

Accessing Structurally Diverse Near-Infrared Cyanine Dyes for Folate Receptor-Targeted Cancer Cell Staining

Folate receptor (FR) targeting is one of the most promising strategies for the development of small-molecule-based cancer imaging agents considering that the FR is highly overexpressed on the surface of many cancer cell types. FR-targeted conjugates of near-infrared (NIR) emissive cyanine dyes are in advanced clinical trials for fluorescence-guided surgery and are valuable research tools for optical molecular imaging in animal models. Only a small number of promising conjugates has been evaluated so far. Analysis of structure-performance relations to identify critical factors modulating the performance of targeted conjugates is essential for successful further optimization. This contribution addresses the need for convenient synthetic access to structurally diverse NIR-emissive cyanine dyes for conjugation with folic acid. Structural variations were introduced to readily available cyanine precursors in particular via C-C-coupling reactions including Suzuki and (for the first time with these types of dyes) Sonogashira cross-couplings. Photophysical properties such as absorbance maxima, brightness, and photostability are highly dependent on the molecular structure. Selected modified cyanines were conjugated to folic acid for cancer cell targeting. Several conjugates display a favorable combination of high fluorescence brightness and photostability with high affinity to FR-positive cancer cells, and enable the selective imaging of these cells with low background.

Folic acid-decorated polyamidoamine dendrimer mediates selective uptake and high expression of genes in head and neck cancer cells

AIM

Folic acid (FA)-decorated polyamidoamine dendrimer G4 (G4-FA) was synthesized and studied for targeted delivery of genes to head and neck cancer cells expressing high levels of folate receptors (FRs).

METHODS

Cellular uptake, targeting specificity, cytocompatibility and transfection efficiency were evaluated.

RESULTS

G4-FA competes with free FA for the same binding site. G4-FA facilitates the cellular uptake of DNA plasmids in a FR-dependent manner and selectively delivers plasmids to FR-high cells, leading to enhanced gene expression.

CONCLUSION

G4-FA is a suitable vector to deliver genes selectively to head and neck cancer cells. The fundamental understandings of G4-FA as a vector and its encouraging transfection results for head and neck cancer cells provided support for its further testing in vivo.

Prediction of Response to Therapy for Autoimmune/Inflammatory Diseases Using an Activated Macrophage-Targeted Radioimaging Agent

The ability to select patients who will respond to therapy is especially acute for autoimmune/inflammatory diseases, where the costs of therapies can be high and the progressive damage associated with ineffective treatments can be irreversible. In this article we describe a clinical test that will rapidly predict the response of patients with an autoimmune/inflammatory disease to many commonly employed therapies. This test involves quantitative assessment of uptake of a folate receptor-targeted radioimaging agent ((99m)Tc-EC20) by a subset of inflammatory macrophages that accumulate at sites of inflammation. Murine models of four representative inflammatory diseases (rheumatoid arthritis, inflammatory bowel disease, pulmonary fibrosis, and atherosclerosis) show markedly decreased uptake of (99m)Tc-EC20 in inflamed lesions upon initiation of successful therapies, but no decrease in uptake upon administration of ineffective therapies, in both cases long before changes in clinical symptoms can be detected. This predictive capability should reduce costs and minimize morbidities associated with failed autoimmune/inflammatory disease therapies.

Folate receptor-targeted multimodality imaging of ovarian cancer in a novel syngeneic mouse model

A new transplantable ovarian tumor model is presented using a novel folate receptor (FR) positive, murine ovarian cancer cell line that emulates the human disease and induces widespread intraperitoneal (i.p.) tumors in immunocompetent mice within 4–8 weeks of implantation. Tumor development was monitored using a new positron emission tomography (PET) FR-targeting reporter with PET/computerized tomography (PET/CT) and fluorescence molecular tomography (FMT) using a commercial FR-targeting reporter. Conventional structural magnetic resonance imaging (MRI) was also performed. Adult female C57BL/6 mice were injected i.p. with 6 × 10⁶ MKP-L FR+ cells. Imaging was performed weekly beginning 2 weeks after tumor induction. The albumin-binding, FR-targeting ligand cm09 was radiolabeled with the positron emitter ⁶⁸Ga and used to image the tumors with a small animal PET/CT. The FR-reporter FolateRSense 680 (PerkinElmer) was used for FMT and flow cytometry. Preclinical MRI (7 T) without FR-targeting was compared with the PET and FMT molecular imaging. Tumors were visible by all three imaging modalities. PET/CT had the highest imaging sensitivity at 3–3.5 h postadministration (mean %IA/g mean > 6) and visualized tumors earlier than the other two modalities with lower kidney uptake (mean %IA/g mean < 17) than previously reported FR-targeting agents in late stage disease. FMT showed relatively low FR-targeted agent in the bladder and kidneys, but yielded the lowest anatomical image resolution. MRI produced the highest resolution images, but it was difficult to distinguish tumors from abdominal organs during early progression since a FR-targeting MRI reporter was not used. Nevertheless, there was good correlation of imaging biomarkers between the three modalities. Tumors in the mouse ovarian cancer model could be detected using FR-targeted imaging as early as 2 weeks post i.p. injection of tumor cells. An imaging protocol should combine one or more of the modalities, e.g., PET/CT or PET/MRI for optimal tumor detection and delineation from surrounding tissues.

Receptor-mediated gene delivery by folic acid-modified stearic acid-grafted chitosan micelles

BACKGROUND

Cationic polymers have been accepted as effective nonviral vectors for gene delivery with low immunogenicity unlike viral vectors. However, the lack of organ or cell specificity sometimes hampers their application and the modification of polymeric vectors has also shown successful improvements in achieving cell-specific targeting delivery and in promoting intracellular gene transfer efficiency.

METHODS

A folic acid-conjugated stearic acid-grafted chitosan (FA-CS-SA) micelle, synthesized by a 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide-coupling reaction, was designed for specific receptor-mediated gene delivery.

RESULTS

Due to the cationic properties of chitosan, the micelles could compact the plasmid DNA (pDNA) to form micelle/pDNA complexes nanoparticles. The particle size and zeta potential of the FA-CS-SA/pDNA complexes with different N/P ratios were 100-200 nm and -20 to -10 mV, respectively. The DNase I protection assay indicated that the complexes can efficiently protect condensed DNA from enzymatic degradation by DNase I. A cytotoxicity study indicated that the micelles exhibited less toxicity in comparison with Lipofectamine™ 2000. Using SKOV3 and A549 as model tumor cells, the cellular uptake of micelles was investigated.

CONCLUSION

It was found that cellular uptake of FA-CS-SA in SKOV3 cells with higher folate receptor expression was faster than that in A549 cells with a short incubation time. Luciferase assay and green fluorescent protein detection were used to confirm that FA-CS-SA could be an effective gene vector. Transfection efficiency of the FA-CS-SA/pDNA complexes in SKOV3 cells was enhanced up to 2.3-fold compared with that of the CS-SA/pDNA complexes. However, there was no significant difference between the transfection efficiencies of the two complexes in A549 cells. Importantly, the transfection efficiency of FA-CS-SA/pDNA decreased with free FA pretreatment in SKOV3 cells. It was concluded that the increase in transfection efficiency of the FA-CS-SA/pDNA complexes was attributed to folate receptor-mediated endocytosis.