Molecular vehicles for targeted drug delivery

Targeted drug delivery by cell-specific cytokines and antibodies promises greater drug efficacy and reduced side effects. We describe a novel strategy for assembly of drug delivery vehicles that does not require chemical modification of targeting proteins. The strategy relies on a noncovalent binding of standardized "payload" modules to targeting proteins expressed with a "docking" tag. The payload modules are constructed by linking drug carriers to an adapter protein capable of binding to a docking tag. Using fragments of bovine ribonuclease A as an adapter protein and a docking tag, we have constructed vascular endothelial growth factor (VEGF) based vehicles for gene delivery and for liposome delivery. Assembled vehicles displayed remarkable selectivity in drug delivery to cells overexpressing VEGF receptors. We expect that our strategy can be employed for targeted delivery of many therapeutic or imaging agents by different recombinant targeting proteins.     

D-甘露糖修饰聚合物胶束的制备及其在靶向药物输送中的应用

聚合物胶束作为药物载体具有良好的稳定性和生物相容性,提高疏水性药物溶解性等优势,是一类很有应用潜力的药物传输系统。本研究以合成的共价键连D-甘露糖的双亲性聚合物分子(PGMA-Mannose)为药物载体,包载抗癌药物阿霉素(DOX)制备具有甘露糖受体靶向性和pH敏感药物释放特性的新型载药聚合物胶束。利用激光共聚焦显微镜和MTT细胞毒性评价方法对载药胶束的细胞内吞摄取和毒性进行评价。实验结果表明,载药胶束能特异性识别人乳腺癌细胞MDA-MB-231表面过度表达的甘露糖受体,被癌细胞大量摄取并在细胞溶酶体酸性环境内释放药物,而载药胶束在表面甘露糖受体低表达的HEK293细胞中只有少量摄取。与原药DOX相比,该载药胶束对癌细胞的毒性显著提高,而对正常细胞的毒性较低。因此,该PGMA-Mannose聚合物胶束有望成为一种新型的靶向药物输送系统应用于癌症的治疗。     

Cell-specific aptamer-mediated targeted drug delivery

Nucleic acid aptamers are in vitro-selected small, single-stranded DNA or RNA oligonucleotides that can specifically recognize their target on the basis of their unique 3-dimensional structures. Recent advances in the development of escort aptamers to deliver and enhance the efficacy of other therapeutic agents have drawn enthusiasm in exploiting cell-type-specific aptamers as drug delivery vehicles. This review mainly focuses on the recent developments of aptamer-mediated targeted delivery systems. We also place particular emphasis on aptamers evolved against cell membrane receptors and possibilities for translation to clinical applications.     

Nanoparticle-mediated targeted drug delivery for breast cancer treatment

Breast cancer (BC) is the most common malignancy in women worldwide, and one of the deadliest after lung cancer. Currently, standard methods for cancer therapy including BC are surgery followed by chemotherapy or radiotherapy. However, both chemotherapy and radiotherapy often fail to treat BC due to the side effects that these therapies incur in normal tissues and organs. In recent years, various nanoparticles (NPs) have been discovered and synthesized to be able to selectively target tumor cells without causing any harm to the healthy cells or organs. Therefore, NPs-mediated targeted drug delivery systems (DDS) have become a promising technique to treat BC. In addition to their selectivity to target tumor cells and reduce side effects, NPs have other unique properties which make them desirable for cancer treatment such as low toxicity, good compatibility, ease of preparation, high photoluminescence (PL) for bioimaging in vivo, and high loadability of drugs due to their tunable surface functionalities. In this study, we summarize with a critical analysis of the most recent therapeutic studies involving various NPs-mediated DDS as alternatives for the traditional treatment approaches for BC. It will shed light on the significance of NPs-mediated DDS and serve as a guide to seeking for the ideal methodology for future targeted drug delivery for an efficient BC treatment.     

Local and targeted drug delivery for bone regeneration

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Synthesis of cholesterol-carborane conjugate for targeted drug delivery

The cholesterol-carborane conjugate has been designed and synthesized to selectively deliver boron to tumor cells by means of reconstituted low-density lipoprotein. The chemical stability and cytotoxicity of the new compound have been examined. Several methods have been evaluated for incorporation of the compound into LDL.     

Functionalized amphiphilic hyperbranched polymers for targeted drug delivery

Amphiphilic hyperbranched core-shell polymers with folate moieties as the targeting groups were synthesized and characterized. The core of the amphiphilic polymers was hyperbranched aliphatic polyester Boltorn H40. The inner part and the outer shell of the amphiphilic polymers were composed of hydrophobic poly(epsilon-caprolactone) segments and hydrophilic poly(ethylene glycol) (PEG) segments, respectively. To achieve tumor cell targeting property, folic acid was further incorporated to the surface of the amphiphilic polymers via a coupling reaction between the hydroxyl group of the PEG segment and the carboxyl group of folic acid. The polymers were characterized by (1)H NMR, (13)C NMR, and combined size-exclusion chromatography and multiangle laser light scattering analysis. The nanoparticles of the amphiphilic polymers prepared by dialysis method were characterized by transmission electron microscopy and particle size analysis. Two antineoplastic drugs, 5-fluorouracil and paclitaxel, were encapsulated into the nanoparticles. The drug release property and the targeting of the drug-loaded nanoparticles to different cells were evaluated in vitro. The results showed the drug-loaded nanoparticles exhibited enhanced cell inhibition because folate targeting increased the cytotoxicity of drug-loaded nanoparticles against folate receptor expressing tumor cells.     

Microparticulate systems for targeted drug delivery to phagocytes

Comment on: Jhunjhunwala S, et al. J Control Release 2009;133:191-7.     

Silk-fibroin-coated liposomes for long-term and targeted drug delivery

Many barriers to drug delivery into a tumor site require careful consideration when designing a new drug. In this study, the adhesive targeting and drug specificity of modified liposomal vesicles on human-scar-producing cells, keloid fibroblasts, were investigated. Keloids express abundant levels of mucopolysaccharides and receptor tyrosine kinase (RTK). In this report, the structural properties, drug release kinetics, and therapeutic availability of silk-fibroin-coated, emodin-loaded liposomes (SF-ELP), compared with uncoated, emodin-loaded liposomes (ELP), were investigated. SF-ELP had a highly organized lamellae structure, which contributed to 55% of the liposomal diameter. This modified liposomal structure decreased emodin release rates by changing the release kinetics from a swelling and diffusional process to a purely diffusional process, probably due to steric hindrance. SF-ELP also increased adhesion targeting to keloid fibroblasts. Increased retention of SF-ELP is most likely due to the interaction of the fibrous protein coating around the ELP with the pericellular molecules around the cell. SF-ELP also decreased survival rate of keloids that expressed high levels of RTK. These results demonstrated that SF-ELP enhanced emodin delivery by improved diffusion kinetics and specific cell targeting.     

A photochemical approach for controlled drug release in targeted drug delivery

Photochemistry provides a unique mechanism that enables the active control of drug release in cancer-targeting drug delivery. This study investigates the light-mediated release of methotrexate, an anticancer drug, using a photocleavable linker strategy based on o-nitrobenzyl protection. We evaluated two types of the o-nitrobenzyl-linked methotrexate for the drug release study and further extended the study to a fifth-generation poly(amidoamine) dendrimer carrier covalently conjugated with methotrexate via the o-nitrobenzyl linker. We performed the drug release studies by using a combination of three standard analytical methods that include UV/vis spectrometry, (1)H NMR spectroscopy, and anal. HPLC. This article reports that methotrexate is released by the photochemical mechanism in an actively controlled manner. The rate of the drug release varies in response to multiple control parameters, including linker design, light wavelength, exposure time, and the pH of the medium where the drug release occurs.     

A new method for targeted drug delivery using polymeric microcapsules

Recent research and clinical evidence suggest that thalidomide could potentially be used to treat inflammation associated with Crohn's disease. However, systemic side effects associated with large doses of this drug have limited its widespread use. Treatment, with thalidomide would prove more efficacious if the drug could be delivered directly to target areas in the gut, thereby reducing systemic circulation. Microcapsule encapsulation could enable direct delivery of the drug. To assess the latter, we designed and tested drug-targeting release characteristics of alginate-poly-l-lysine-alginate (APA) microcapsules in simulated gastrointestinal environments. The results show that APA capsules enabled delivery of thalidomide in the middle and distal portions of the small intestine. We also compared the APA membrane formulation with an earlier designed alginate chitosan (AC) membrane thalidomide formulation. The results show that both APA and AC capsules allow for successful delivery of thalidomide in the gut and could prove beneficial in the treatment of Crohn's disease. However, further research is required.     

Gold nanoparticles: Emerging paradigm for targeted drug delivery system

The application of nanotechnology in medicine, known as nanomedicine, has introduced a plethora of nanoparticles of variable chemistry and design considerations for cancer diagnosis and treatment. One of the most important field is the design and development of pharmaceutical drugs, based on targeted drug delivery system (TDDS). Being inspired by physio-chemical properties of nanoparticles, TDDS are designed to safely reach their targets and specifically release their cargo at the site of disease for enhanced therapeutic effects, thereby increasing the drug tissue bioavailability. Nanoparticles have the advantage of targeting cancer by simply being accumulated and entrapped in cancer cells. However, even after rapid growth of nanotechnology in nanomedicine, designing an effective targeted drug delivery system is still a challenging task. In this review, we reveal the recent advances in drug delivery approach with a particular focus on gold nanoparticles. We seek to expound on how these nanomaterials communicate in the complex environment to reach the target site, and how to design the effective TDDS for complex environments and simultaneously monitor the toxicity on the basis of designing such delivery complexes. Hence, this review will shed light on the research, opportunities and challenges for engineering nanomaterials with cancer biology and medicine to develop effective TDDS for treatment of cancer.     

Adapter protein for site-specific conjugation of payloads for targeted drug delivery

High-affinity interactions of two fragments of human RNase I (1-15-aa Hu-tag and 21-125-aa HuS adapter protein) can be used for assembly of targeting drug delivery complexes. In this approach, a targeting protein is expressed as a fusion protein with a 15-aa Hu-tag, while HuS is conjugated to a drug (or a drug carrier) creating a "payload" module, which is then bound noncovalently to the Hu-tag of the targeting protein. Although this approach eliminates chemical modifications of targeting proteins, the payload modules are still constructed by random cross-linking of drugs or drug carriers to an adapter protein that might lead to functional heterogeneity of the complexes. To avoid this problem, we engineered an adapter protein HuS(N88C) with an unpaired cysteine in position 88 that can be directly modified without interference with activity of assembled targeting complexes. HuS(N88C) binds Hu-tagged annexin V with K(D) of 50 +/- 6 nM, which is comparable to that of wild-type HuS. To demonstrate the utility of HuS(N88C) for developing uniform payload modules, we constructed a HuS(N88C)-lipid conjugate and inserted it into preformed liposomes loaded with a fluorescent dye. Targeting proteins, Hu-tagged vascular endothelial growth factor or Hu-tagged annexin V, were docked to liposomes decorated with HuS, and the assembled complexes delivered liposomes selectively to target cells.     

Shear targeted drug delivery to stenotic blood vessels

In this review we focus on shear targeted drug delivery as a novel strategy to selectively deliver drugs to sites of vascular obstruction. We review the physics of stenotic (abnormally narrowed) blood vessels, while focusing mainly on the hemodynamics and transport phenomena at these sites. We then discuss how the local abnormal levels of shear stress, which can mechanically activate platelets, can be leveraged for localized drug delivery. We describe the development of Shear Activated Nano-particle Aggregates (SA-NPAs) that are designed to release and localize their nanoparticle drug carriers at sites of vascular stenosis. We present results in a variety of in vivo models, demonstrating the superiority of SA-NPAs carrying a thrombolytic drug compared to conventional treatment with the free drug. We also describe, shear-stress sensitive lenticular liposomes, which also show selective release under stenotic flow conditions. We then discuss limitations of both technologies, challenges in this new field and potential future applications. Altogether, we believe that mechano-sensitive therapeutics may offer a potential new approach for treatment of cardiovascular diseases.     

Biophysical characterization of a riboflavin-conjugated dendrimer platform for targeted drug delivery

The present study describes the biophysical characterization of generation-five poly(amidoamine) (PAMAM) dendrimers conjugated with riboflavin (RF) as a cancer-targeting platform. Two new series of dendrimers were designed, each presenting the riboflavin ligand attached at a different site (isoalloxazine at N-3 and d-ribose at N-10) and at varying ligand valency. Isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC) were used to determine the binding activity for riboflavin binding protein (RfBP) in a cell-free solution. The ITC data shows dendrimer conjugates have K(D) values of ≥ 465 nM on a riboflavin basis, an affinity ~93-fold lower than that of free riboflavin. The N-3 series showed greater binding affinity in comparison with the N-10 series. Notably, the affinity is inversely correlated with ligand valency. These findings are also corroborated by DSC, where greater protein-conjugate stability is achieved with the N-3 series and at lower ligand valency.     

Synthesis and characterization of a hemoglobin-ribavirin conjugate for targeted drug delivery

A novel conjugate of human hemoglobin (Hb) and the nucleoside analogue ribavirin (RBV) was synthesized to demonstrate the utility of Hb as a biocompatible drug carrier for improved drug delivery in the treatment of liver disease. RBV is used in combination with interferon for the treatment of hepatitis C, but its side effects can result in dose limitation or discontinuation of treatment. Targeted delivery of RBV may help to prevent or minimize its toxicity. The hemoglobin-ribavirin conjugate (Hb-RBV) was designed to release bioactive drug upon endocytosis by cells and tissues involved in extracellular Hb catabolism and clearance. Ribavirin-5'-monophosphate (RBV-P) was prepared from RBV and activated as the 5'-monophosphorimidazolide (RBV-P-Im) for reaction with carbonmonoxyhemoglobin to yield Hb-RBV consisting of multiple RBV drugs covalently attached as physiologically labile phosphoramidates via their 5'-hydroxyl groups. A molar drug ratio of six to eight RBV molecules per Hb tetramer was obtained with near complete haptoglobin (Hp) binding of the drug modified Hb maintained. The conjugate complex (Hp-Hb-RBV) was selectively taken up in vitro by cells that express the hemoglobin-haptoglobin receptor, CD163. Recovered ribavirin enzymatically cleaved from Hb-RBV showed equipotent antiproliferative activity compared to control unconjugated RBV against human HepG2 and mouse AML12 liver cell lines. Based upon the reported high level of Hb uptake in the liver, Hb-RBV may be useful in the treatment of certain liver diseases, as well as inflammatory disorders associated with CD163-positive macrophages.     

Synthesis of RGD analogs as potential vectors for targeted drug delivery

RGD analogs bind to integrin receptors with high affinity and therefore have the potential to be used as vectors for the targeted delivery of pharmaceutical agents to designated sites. Critical to this application is the ability to synthesize RGD analogs with different side chain functional groups that allow for the ready tethering of pharmaceutical agents without sacrificing their affinity for the target receptor significantly. A series of RGD analogs intended to be used as delivery vectors of pharmaceutical agents were prepared and evaluated for their ability to inhibit platelet aggregation by binding to glycoprotein IIb/IIIa. Among them, compound 11 showed the lowest IC50 against platelets activated by ADP. It was found that such RGD analogs could tolerate side chain modification fairly well with various functional groups attached such as amide, amine, ester, protected amine and poly(ethylene glycol). The fact that the compound with a side chain modification of poly(ethylene glycol) retained high affinity for glycoprotein IIb/IIIa (IC50 150 nM) suggests the feasibility of tethering fairly large pharmaceutical agents to such RGD analogs without significant sacrifice of their affinity to the intended receptor.     

Engineering S-protein fragments of bovine ribonuclease A for targeted drug delivery

High affinity interaction between S-protein and S-peptide fragments of bovine pancreatic RNase A has been recently used for construction of molecular vehicles for targeted drug delivery. The vehicle is assembled as a complex of drug carrier conjugated S-protein with S-peptide-tagged targeting protein. To avoid random chemical crosslinking of drug carriers to S-protein, we constructed a mutant 16-124aa fragment of RNase A in which 122ala is replaced with a cysteine residue. The mutant and the corresponding wild type fragments expressed in Escherichia coli are refolded into functional conformations only in the presence of S-peptide. After the removal of S-peptide, both fragments retain the ability to bind S-peptide and S-peptide-tagged proteins. The 122cys residue in the mutant fragment is available for site-specific conjugation.