Application of natural and semi-synthetic polymers for the delivery of sensitive drugs

This review summarises recent developments in the application of natural and semi-synthetic polymers for the delivery of sensitive drugs. Peptides, proteins and nucleic acids are drugs of increasing relevance potentially offering treatment options in indications with high unmet medical need. However, these drugs are characterised by high molecular weight, high sensitivity to enzymatic degradation, unfavourable pharmacokinetics and often require targetting to specific cell types or cellular compartments. To successfully transform these drug molecules into efficacious therapies, advanced drug delivery systems must be developed that protect the drug, control drug release to improve pharmacokinetics and allow efficient targetting. Synthetic, semisynthetic or natural polymers or inorganic materials are frequently used for the development of drug delivery systems. Considering factors such as biocompatibility, biodegradability, solvent-free processing and availability from renewable resources, natural and semi-synthetic polymers are often advantageous compared to synthetic alternatives. On the other hand, material heterogeneity and purity of natural materials are concerns that need to be addressed. In this review, authors focus on frequently used biopolymers such as polysaccharides like chitosan and hyaluronan and proteins like silk fibroin (SF) and collagen and their semi-synthetic derivatives. Special emphasis will be put on material properties of such polymers rendering them suitable for drug delivery purposes and allow tight control to assure product quality and proper release characteristics. Natural polymers are frequently synthetically modified to alter or improve their characteristics. Such semi-synthetic derivatives and their advantages and disadvantages are critically discussed. Furthermore, the biocompatibility of natural materials and their derivatives is discussed.