Polymers containing enzymatically degradable bonds, 7. Design of oligopeptide side‐chains in poly[N‐(2‐hydroxypropyl)methacrylamide] copolymers to promote efficient degradation by lysosomal enzymes
Abstract N ‐(2‐Hydroxypropyl)methacrylamide copolymers are considered to be a potential drug delivery system. To fulfil this role the drug‐polymer linkage must be susceptible to intralysosomal hydrolysis. Taking p ‐nitroanilide as a drug analogue, copolymers were synthesized bearing oligopeptidyl‐ p ‐nitroanilide side‐chains designed to match known specificities of the lysosomal enzymes cathepsin L or cathepsin D. Degradation of side‐chains by rat liver lysosomal enzymes (measured by monitoring terminal p ‐nitroaniline release) occurred only in the presence of reduced glutathione (5 mmol/l) and was effectively inhibited by leupeptin, indicating the involvement of thiol‐proteinases in every case. Depending on side‐chain composition, between 20 and more than 50% of the terminal p ‐nitroaniline residues were liberated during a 5 h incubation. It has also been shown that 1) a polymer molecule may contain side‐chains of a single type that are nevertheless differentially susceptible to lysosomal hydrolysis; 2) two of the side‐chains studied liberate only a p ‐nitroaniline residue, whereas the others also release amino‐acyl‐ p ‐nitroanilides; 3) the cleavage of all side‐chains displays a broad pH optimum pH 5 to pH 7; 4) the Michaelis‐Menten constant K m for side‐chain cleavage varied between 26,1 and 143,2 mg/ml, depending on the amino acid sequence of the side‐chain.