Drug delivery systems: anticancer prodrugs and their polymeric conjugates

Docetaxel encapsulation in nanoscale assembly micelles of folate-PEG-docetaxel conjugates for targeted fighting against metastatic breast cancer in vitro and in vivo

Due to the high frequency and mortality of breast cancer, developing efficient targeted drug delivery systems for frightening against this malignancy is among cancer research priorities. The aim of this study was to synthesize a targeted micellar formulation of docetaxel (DTX) using DTX, folic acid (FA) and polyethylene glycol (PEG) conjugates as building blocks. In the current study, two therapeutic polymers consisting of FA-PEG-DTX and PEG-DTX conjugates were synthesized and implemented to form folate-targeted and non-targeted micelles. Dissipative particle dynamics (DPD) method was used to simulate the behavior of the nanoparticle. The anti-cancer drug, DTX was loaded in to the micelles via solvent switching method in order to increase its solubility and stability. The cytotoxicity of the targeted and non-targeted formulations was evaluated against 4T1 and CHO cell lines. In vivo therapeutic efficiency was studied using ectopic tumor model of metastatic breast cancer, 4T1, in Female BALB/c mice. The successful synthesis of therapeutic polymers, FA-PEG-DTX and PEG-DTX were confirmed implementing ¹HNMR spectral analysis. The size of DTX-loaded non-targeted and targeted micelles were 176.3 ± 8.3 and 181 ± 10.1 nm with PDI of 0.23 and 0.17, respectively. Loading efficiencies of DTX in non-targeted and targeted micelles were obtained to be 85% and 82%, respectively. In vitro release study at pH = 7.4 and pH = 5.4 showed a controlled and continuous drug release for both formulations, that was faster at pH = 5.4 (100% drug release within 120 h) than at pH = 7.4 (80% drug release within 150 h). The targeted formulation showed a significant higher cytotoxicity against 4T1 breast cancer cells (high expression of folate receptor) within the range of 12.5 to 200 μg/mL in comparison with no-targeted one. However, there was no significant difference between the cytotoxicity of the targeted and non-targeted formulations against CHO cell line as low-expressed cell line. In accordance with in vitro investigation, in vivo studies verified the ideal anti-tumor efficacy of the targeted formulation compared to Taxotere and non-targeted formulation. Based on the obtained data, FA-targeted DTX-loaded nano-micelles significantly increased the therapeutic efficacy of DTX and therefore can be considered as a new potent platform for breast cancer chemotherapy.

Well-defined podophyllotoxin polyprodrug brushes: preparation via RAFT polymerization and evaluation as drug carriers

Novel poly(triethylene glycol methacrylate)-b-poly(podophyllotoxin methacrylate) copolymers (PTP) with a well-defined structure were designed and synthesized by direct RAFT polymerization with the hydrophobic monomer derivative from the anticancer drug podophyllotoxin.

The road to the synthesis of "difficult peptides"

The last decade has witnessed a renaissance of peptides as drugs. This progress, together with advances in the structural behavior of peptides, has attracted the interest of the pharmaceutical industry in these molecules as potential APIs. In the past, major peptide-based drugs were inspired by sequences extracted from natural structures of low molecular weight. In contrast, nowadays, the peptides being studied by academic and industrial groups comprise more sophisticated sequences. For instance, they consist of long amino acid chains and show a high tendency to form aggregates. Some researchers have claimed that preparing medium-sized proteins is now feasible with chemical ligation techniques, in contrast to medium-sized peptide syntheses. The complexity associated with the synthesis of certain peptides is exemplified by the so-called "difficult peptides", a concept introduced in the 80's. This refers to sequences that show inter- or intra-molecular β-sheet interactions significant enough to form aggregates during peptide synthesis. These structural associations are stabilized and mediated by non-covalent hydrogen bonds that arise on the backbone of the peptide and-depending on the sequence-are favored. The tendency of peptide chains to aggregate is translated into a list of common behavioral features attributed to "difficult peptides" which hinder their synthesis. In this regard, this manuscript summarizes the strategies used to overcome the inherent difficulties associated with the synthesis of known "difficult peptides". Here we evaluate several external factors, as well as methods to incorporate chemical modifications into sequences, in order to describe the strategies that are effective for the synthesis of "difficult peptides". These approaches have been classified and ordered to provide an extensive guide for achieving the synthesis of peptides with the aforementioned features.

Poly(ethylene glycol)-Prodrug Conjugates: Concept, Design, and Applications

Poly(ethylene glycol) (PEG) is the most widely used polymer in delivering anticancer drugs clinically. PEGylation (i.e., the covalent attachment of PEG) of peptides proteins, drugs, and bioactives is known to enhance the aqueous solubility of hydrophobic drugs, prolong circulation time, minimize nonspecific uptake, and achieve specific tumor targetability through the enhanced permeability and retention effect. Numerous PEG-based therapeutics have been developed, and several have received market approval. A vast amount of clinical experience has been gained which has helped to design PEG prodrug conjugates with improved therapeutic efficacy and reduced systemic toxicity. However, more efforts in designing PEG-based prodrug conjugates are anticipated. In light of this, the current paper highlights the synthetic advances in PEG prodrug conjugation methodologies with varied bioactive components of clinical relevance. In addition, this paper discusses FDA-approved PEGylated delivery systems, their intended clinical applications, and formulations under clinical trials.

Synthesis and biological studies of 5-aminolevulinic acid-containing dendrimers for photodynamic therapy

Using a convergent growth approach, a series of novel 5-aminolevulinic acid (ALA)-containing dendrimers have been synthesized. In these molecules, ALA residues are attached to the periphery by ester linkages, with amide bonds connecting the dendrons. Three first-generation dendrimers, bearing either 6 or 9 ALA residues, were synthesized by attachment of a tris(Boc-protected ALA)-containing wedge (1) to a di- or tripodent aromatic, or tripodent aliphatic core. Two second generation 18-ALA-containing dendrimers were also synthesized using a 3,3'-iminodipropionic acid spacer unit between wedge 1 and the aromatic core. These compounds differed only in the distance between the core and the linker unit. The Boc-protected dendrimers were deprotected using trifluoroacetic acid and isolated as their TFA salts. The potential of these ALA ester dendrimers as macromolecular prodrugs for photodynamic therapy has been demonstrated in the tumorigenic keratinocyte PAM 212 cell line.

Peptide and protein PEGylation: a review of problems and solutions

The paper discusses general problems in using PEG for conjugation to high or low molecular weight molecules. Methods of binding PEG to different functional groups in macromolecules is reported together with their eventual limitations. Problems encountered in conjugation, such as the evaluation of the number of PEG chains bound, the localisation of the site of conjugation in polypeptides and the procedure to direct PEGylation to the desired site in the molecule are discussed. Finally, the paper reports on more specific methods regarding reversible PEGylation, cross-linking reagents with PEG arms, PEG for enzyme solubilization in organic solvent and new polymers as alternative to PEG.

Drug delivery systems employing 1,4- or 1,6-elimination: poly(ethylene glycol) prodrugs of amine-containing compounds

A general methodology for synthesizing poly(ethylene glycol) (PEG) prodrugs of amino-containing compounds has been developed and constitutes the basis for solubilization of insoluble drugs, extending plasma circulating half-lives and, in the case of anticancer agents, apparent tumor accumulation. Thus, we have successfully designed PEG conjugated specifiers or "triggers" as part of a double-prodrug strategy that relies, first, on enzymatic separation of PEG followed by the classical and rapid 1,4- or 1, 6-benzyl elimination reaction releasing the amine (drug) bound in the form of a carbamate. The prodrug trigger was comprised of ester, carbonate, carbamate, or amide bonds in order to secure predictable rates of hydrolysis. Further refinement of the hydrolysis was accomplished by the introduction of steric hindrance through the use of ortho substituents on the benzyl component of the prodrug. This modification led to longer circulating plasma half-lives of the final tripartate form. The "ortho" effect also had the beneficial effect of directing nucleophilic attack almost exclusively to the activated benzyl 6-position of the heterobifunctional intermediates. In vivo testing of the PEG daunorubicin prodrugs (transport forms) prepared in the course of this study ultimately identified the type 1 carbamate (34b), with a circulating t(1/2) of 4 h, as the most effective derivative for solid tumor growth inhibition.

Receptor-mediated and enzyme-dependent targeting of cytotoxic anticancer drugs

This review is a survey of various approaches to targeting cytotoxic anticancer drugs to tumors primarily through biomolecules expressed by cancer cells or associated vasculature and stroma. These include monoclonal antibody immunoconjugates; enzyme prodrug therapies, such as antibody-directed enzyme prodrug therapy, gene-directed enzyme prodrug therapy, and bacterial-directed enzyme prodrug therapy; and metabolism-based therapies that seek to exploit increased tumor expression of, e.g., proteases, low-density lipoprotein receptors, hormones, and adhesion molecules. Following a discussion of factors that positively and negatively affect drug delivery to solid tumors, we concentrate on a mechanistic understanding of selective drug release or generation at the tumor site.