Engineered Peptides for Applications in Cancer-Targeted Drug Delivery and Tumor Detection.

Cancer-targeting peptides as ligands for targeted delivery of anticancer drugs or drug carriers have the potential to significantly enhance the selectivity and the therapeutic benefit of current chemotherapeutic agents. Identification of tumor-specific biomarkers like integrins, aminopeptidase N, and epidermal growth factor receptor as well as the popularity of phage display techniques along with synthetic combinatorial methods used for peptide design and structure optimization have fueled the advancement and application of peptide ligands for targeted drug delivery and tumor detection in cancer treatment, detection and guided therapy. Although considerable preclinical data have shown remarkable success in the use of tumor targeting peptides, peptides generally suffer from poor pharmacokinetics, enzymatic instability, and weak receptor affinity, and they need further structural modification before successful translation to clinics is possible. The current review gives an overview of the different engineering strategies that have been developed for peptide structure optimization to confer selectivity and stability. We also provide an update on the methods used for peptide ligand identification, and peptide- receptor interactions. Additionally, some applications for the use of peptides in targeted delivery of chemotherapeutics and diagnostics over the past 5 years are summarized.

Pharmacokinetics of PSC 833 (valspodar) in its Cremophor EL formulation in rat.

Valspodar is a P-glycoprotein inhibitor widely used in preclinical and clinical studies for overcoming multidrug resistance. Despite this, the pharmacokinetics of valspodar in rat, a commonly used animal model, have not been reported. Here, we report on the pharmacokinetics of valspodar in Sprague-Dawley rats following intravenous and oral administration of its Cremophor EL formulation, which has been used for humans in clinical trials. After intravenous doses, valspodar displayed properties of slow clearance and a large volume of distribution. Its plasma unbound fraction was around 15% in the Cremophor EL formulation used in the study. After 10 mg kg(-1) orally it was rapidly absorbed with an average maximal plasma concentration of 1.48 mg l(-1) within approximately 2 h. The mean bioavailability of valspodar was 42.8%. In rat, valspodar showed properties of low hepatic extraction and wide distribution, similar to that of its structural analogue cyclosporine A.

Micelles self-assembled from poly(ethylene oxide)-block-poly(N-hexyl stearate L-aspartamide) by a solvent evaporation method: effect on the solubilization and haemolytic activity of amphotericin B.

The goal of this study was to assess a solvent evaporation method for the encapsulation of amphotericin B (AmB) in poly(ethylene oxide)-block-poly(N-hexyl stearate L-aspartamide) (PEO-b-PHSA) micelles. By the solvent evaporation method, PEO-b-PHSA self-assembled into small spherical micelles with a high AmB content based on transmission electron microscopy, size exclusion chromatography and absorption spectroscopy. The encapsulation of AmB was slightly better than an earlier method based on dialysis. Importantly, AmB in PEO-b-PHSA micelles encapsulated by the solvent evaporation method was non-haemolytic at 15 microg/ml, whereas AmB in PEO-b-PHSA micelles encapsulated by the dialysis method caused 50% haemolysis at the level of 3.8 microg/ml, and AmB itself caused 100% haemolysis at 1.0 microg/ml. Thus, PEO-b-PHSA micelles could effectively encapsulate AmB, increase the overall water solubility of AmB and reduce the toxicity of the membrane-acting drug, particularly by a solvent evaporation method.

The effect of alkyl core structure on micellar properties of poly(ethylene oxide)-block-poly(L-aspartamide) derivatives.

Block copolymers based on fatty acid esters of poly(ethylene oxide)-block-poly(hydroxy alkyl L-aspartamide) were prepared and characterized by 1H-NMR. The structure of the core-forming block was changed through application of different lengths of the poly(L-aspartamide) (PLAA) block, spacer group or fatty acid and varying the substitution level of the side chain on the polymeric backbone. Transmission electron microscopy and fluorescent probe studies provided evidence for the formation of supramolecular core/shell architectures with nanoscopic dimensions. The same techniques were used to study the effect of hydrophobic block structure on micellar size, critical micelle concentration (CMC), core polarity and viscosity of the polymeric micelles. Among the structural factors studied, it was revealed that the length of the PLAA block and the level of fatty acid attached to the polymeric backbone are the major factors affecting micellar properties. An increase in micellar size and reduction in CMC were observed when the level of fatty acid attachment to the polymeric backbone was raised. The elongation of the PLAA block, on the other hand, resulted in an increase in micellar size and core viscosity. Micellar size was the only characteristic being affected by the length of the attached fatty acid. However, a decrease in microviscosity was revealed when behenic acid (22 carbons) was attached to the core-forming block in a high level of substitution. The length of spacer group was also found to be a useful means by which the level of side chain attachment could be controlled. Chemical tailoring of the core structure in polymeric micelles may be used as an efficient means to change micellar properties. As a result, nanoscopic, spherical and stable micelles with improved core properties may be achieved to insure efficient loading and controlled release of an individual drug from the delivery system.

Micelles of poly(ethylene oxide)-block-poly(N-alkyl stearate L-aspartamide): synthetic analogues of lipoproteins for drug delivery.

Stearic acid esters of poly(ethylene oxide)-block-poly(hydroxyethyl L-aspartamide) and poly(ethylene oxide)-block-poly(hydroxyhexyl L-aspartamide) have been synthesized from poly(ethylene oxide)-block-poly(beta-benzyl L-aspartate) by polymer-analogous reactions and self-assembled into a micelle. Transmission electron microscopy and fluorescent probe studies reveal that the micelle mimics structural features of serum lipoproteins: it is nanoscopic, spherical, and has a supramolecular core-shell architecture, where the core is rich in fatty acid esters. As a result, the polymeric micelles effectively solubilize amphotericin B, a key drug for systemic mycoses. Serum lipoproteins solubilize many hydrophobic drugs as a biological transport system besides amphotericin B. A synthetic polymeric analogue may achieve the same aim, but with the ease of structural modification, safety, and stability.

The effect of serum albumin on the aggregation state and toxicity of amphotericin B.

Studies have shown that the dose-limiting toxicity of amphotericin B (AmB), a key drug for systemic mycoses, depends on its self-aggregation state. In a step toward understanding the various factors in blood mediating the toxicity of AmB, we have investigated the effect of serum albumin, the most abundant plasma protein, on the aggregation state of AmB using absorption spectroscopy. The critical aggregation concentration (CAC) of AmB, which coincides with its concentration at the onset of toxicity (hemolysis), was 1.1 microM, but rose in proportion to the level of serum albumin (1.0 to 4.0% w/v). The CAC of AmB was 8.0 microM at 4.0% w/v serum albumin, which is considerably higher than peak therapeutic levels of AmB in plasma (i.e., 2.0 microM). Serum albumin (4.0% w/v) lowered the degree of aggregation of AmB (size of aggregates) above the CAC and increased its solubility. The results suggest that serum albumin attenuates the toxicity of AmB at a membrane level by affecting its aggregation state. In this way, serum albumin in blood may balance deleterious effects of AmB mediated by serum low-density lipoproteins.

Microencapsulation of theophylline using ethylcellulose: in vitro drug release and kinetic modelling.

This study details the process of coating theophylline with ethylcellulose using the coacervation technique of microencapsulation. Microencapsulation of theophylline not only renders it sustained-release, but also decreases its gastric irritation and masks the bitter taste (Lin and Yang 1987). The non-solvent addition method was chosen through a literature survey to ascertain various phases of the coacervate (Robinson 1989, Nixon and Wong 1990). A three-phase diagram was used to determine the optimum quantity of each component required. Steps were then carried out to optimize the production. Drug release rates of the prepared microcapsules were determined over 12-h cycles using the U.S.P. dissolution apparatus and the results obtained were compared with those of Knoll's (Germany) sustained-release theophylline capsules. Significant control over the rate of drug released from the developed dosage form was achieved during the experiment time (12 h). It is concluded that the method employed in this study could be effectively used in the preparation of sustained-release theophylline microcapsules capable of releasing their drug content for an extended period of time. Kinetic studies suggested that both the prepared microcapsules and Knoll's product followed Higuchi's model for drug release. Particle size and release data analysis from five consecutive batches prepared in the laboratory indicated suitable reproducibility of the coacervation process.