LIMITATIONS OF PEGYLATED NANOCARRIERS: UNFAVOURABLE PHYSICOCHEMICAL PROPERTIES, BIODISTRIBUTION PATTERNS AND CELLULAR AND SUBCELLULAR FATES

Assuming that polyethylene glycol (PEG)-conjugated or PEGylated nanocarriers always offer outstanding physicochemical properties and pharmacokinetics profiles when compared to non-PEGylated ones, is not always accurate. For example, drug-loaded PEGylated nanocarriers for the treatment of cancer will not magically escape the reticuloendothelial system (RES) sequestration and clearance, benefit from the enhanced permeability and retention (EPR) effect of the tumor leaky vasculature and preferentially accumulate in the target tissue or cells. This is too good to be true. In this review, several drawbacks of PEGylation will be discussed; for example, how PEGylation can give rise to unfavourable physicochemical characteristics (e. g. particle size and release patterns) and post in vivo administration limitations of the formulated nanocarriers (e. g. limited evasion of RES uptake, development of hypersensitivity reactions, reduced intracellular accumulation and interference with the subcellular processing of nanocarriers necessary to produce the intended pharmacological effect).This review aims at providing better understanding of the pros and cons of PEGylation, encouraging the use of PEGylation with caution, avoiding the assumption that PEGylation will provide all advantages needed to deliver nanocarriers to the target tissue and looking for alternatives to optimize nanocarriers' utilization especially in the delivery of chemotherapeutic agents for the treatment of different types of cancer. This review comprises a summary of some of the reported literature between 2013 and 2018 using different search engines; PubMed, Science Direct and Google Scholar, and the keywords listed below

A Naturally Derived Carrier for Photodynamic Treatment of Squamous Cell Carcinoma: In Vitro and In Vivo Models

Photodynamic therapy (PDT) is a non-invasive treatment strategy that includes the combination of three components—a photosensitizer, a light source, and tissue oxygen. PDT can be used for the treatment of skin diseases such as squamous cell carcinoma. The photosensitizer used in this study is the naturally derived chlorophyll derivative chlorin e6 (Ce6), which was encapsulated in ultradeformable ethosomes. Singlet oxygen production by Ce6 upon laser light irradiation was not significantly affected by encapsulation into ethosomes. PDT of squamous cell carcinoma cells treated with Ce6 ethosomes triggered increased mitochondrial superoxide levels and increased caspase 3/7 activity, resulting in concentration- and light-dose-dependent cytotoxicity. Ce6 ethosomes showed good penetration into 3D squamous cell carcinoma spheroids, which upon laser light irradiation exhibited reduced size, proliferation, and viability. The PDT effect of Ce6 ethosomes was specific and showed higher cytotoxicity against squamous cell carcinoma spheroids compared to normal skin fibroblast spheroids. In addition, PDT treatment of squamous cell carcinoma xenografts grown on chorioallantoic membranes of chick eggs (CAM) exhibited reduced expression of Ki-67 proliferation marker and increased terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) staining, indicating reduced proliferation and activation of apoptosis, respectively. The results demonstrate that Ce6-loaded ethosomes represent a convenient formulation for photodynamic treatment of squamous cell carcinoma