Biodegradation and Biodistribution

The enzymatic degradability of PG-drug conjugates is influenced by their structure, composition, and charge as well as the physicochemical properties of the drug attached to PG.22-25 PG is more susceptible to lysosomal degradation than are poly(aspartic acid) and poly(d-glutamic acid).26 Cysteine proteases, particularly cathepsin B, play key roles in the lysosomal degradation of PG.22,27 Although researchers have identified oligomeric glutamic acids as the primary degradation products of PG,28 more recent results demonstrated that monomeric l-glutamic acid is produced in the lysosomal degradation of PG.29 Degradation of the PG backbone may not necessarily lead to the release of free drug in every case.

The biodistribution of PG and its drug conjugates depends on the molecular weight of PG. Polymers with a molecular weight lower than the renal clearance threshold are rapidly removed from the blood circulation by glomerular filtration. McCormick-Thomson and Duncan27 found that the biodistribution of copolymers of glutamic acid and other hydrophobic amino acids was markedly influenced by the composition of the copolymers. These results suggest that the biodistribution of PG-drug conjugates is a function of the drugs used, the degree of modification, and the molecular weight of PG.

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