Poly(amino acid) Copolymers as a Potential Soluble Drug Delivery System. 1. Pinocytic Uptake and Lysosomal Degradation Measured In Vitro

The suitability of poly(amino acid) copolymers as soluble drug- carriers is determined in part by their rate of pinocytic uptake by cells and intracellular degradation. Poly(amino acid) copolymers, poly(GluNa,Tyr) (1:1) or (4:1) and poly(GluNa,Tyr,Ala) (1:1:1) or (6:3:1) were radioiodinated and their rate of uptake and intracellular degradation measured using rat visceral yolk sacs cultured in vitro as a model system. Pinocytic uptake of these poly(amino acid) copolymers was 10-100 times greater than the rate measured for fluid- phase pinocytosis in the yolk sac, and it was dependent on their composition, poly(GluNa,Tyr) (4:1) being captured fastest. Inhibition of tissue association of radioactivity by incubation at 4°C confirmed uptake to be an active process. Intracellular degradation proceeded rapidly in all cases and was similarly related to copolymer amino acid composition. When the ¹²⁵ I-labelled poly- (amino acid) copolymers were incubated with isolated rat liver lysosomal en zymes they were degraded to yield TCA soluble radioactivity, poly(GluNa, Tyr,Ala) (6:3:1) being degraded most rapidly (completely degraded within 2h) and poly(GluNa,Tyr) (1:1) degraded very slowly. Lysosomal thiol-dependent (cysteine) proteinases were partly responsible for the observed degradation as judged by leupeptin (24μg/ml) inhibition of degradation.