Figure 5 (a) NHS esters compounds react with nucleophiles to release the NHS leaving group and form an acetylated product. (b) PEG can be succinylated to form –COOH group, which can further form amide or ester bond with biomolecules. Figure 6 (a) Active and passive targeting by nanocarriers ; (b) (1) polymer-conjugated drug is internalized by tumor cells
through receptor-mediated endocytosis following ligand-receptor docking, (2) transport of DDS in membrane limited organelles; (3) fusion … The reactive PEG and its derivatives succinimidyl succinate and succinimidyl Inhibitors,research,lifescience,medical glutamate are used for conjugation with drugs or proteins. The coupling reactions involving amine groups are usually of two types: (a) acylation, (b) alkylation. These reactions are comparatively efficient to form a stable amide bond. In addition, carbodiimide coupling reactions or zero lengths crosslinkers are widely used for coupling or condensation reactions. Most of the coupling methodologies involve use of heterobifunctional reagent Inhibitors,research,lifescience,medical to couple via modified lysine residues on one protein to sulphydryl groups on the second protein , while modification of lysine residues involves the use of a heterobifunctional reagent comprising an NHS functional group, together with a maleimide Inhibitors,research,lifescience,medical or protected sulphydryl group. The linkage formed is either a disulphide bridge or as a thioether bond, depending if the introduced group is either a sulphydryl or maleimide,
respectively. The thiol group on the second protein may be an endogenous free sulphydryl, or chemically introduced by modification of lysine residues. 4. PEG Prodrug Conjugates as Drug-Delivery Systems In general, low-molecular-weight compounds diffuse Inhibitors,research,lifescience,medical into normal and tumor tissue through endothelia cell layer of blood capillaries Inhibitors,research,lifescience,medical . Conjugation of low-molecular-weight drugs with high-molecular-weight
polymeric carriers results in high-molecular weight prodrugs (Figure 1). However, such conjugation substantially alters the mechanism of cellular internalization and accumulation. High-molecular-weight drugs are internalized mainly by endocytosis, which is a much slower internalization process over to simple diffusion. Hence in case of endocytosis higher drug concentration outside the cell is required to produce tuclazepam the same cellular effect as corresponding low-molecular-weight drug . Therefore, higher-molecular-weight prodrugs displays lower specific activity compared to its free form of drugs. For example, polymeric anticancer prodrugs are generally less toxic when compared with its free form, yet require substantially higher concentrations inside the tumor to be cytotoxic. Compensation for this decrease in drug efficacy can be achieved by targeting a polymeric drug to the specific organ, tissue, and/or cell . Following two Fasudil in vivo approaches is generally used to target polymeric anticancer drugs to the tumor or cancer cells [25, 26]: passive targeting, active targeting.