Stability in plasmas of various species of HPMA copolymer-PGE₁ conjugates

Purpose. To determine the stability of HPMA copolymer-prostaglandin E ₁ (PGE₁) conjugates in plasmas of different species and to identify the enzymes responsible for the cleavage of the ester bond. Methods. The conjugates were incubated in human, rat, and mouse plasma at 37°C in the presence and absence of specific esterase inhibitors. The released PGE ₁ was analyzed using an HPLC assay. To evaluate the effect of the conformation of the conjugate on the rate of PGE₁ release, its structure was modified by the attachment of hydrophobic side chains. Results. The rate of PGE₁ release was strongly species dependent. Whereas the conjugate was stable in human plasma, the PGE₁ release in rat or mouse plasma was substantial. In rat plasma, the ester bond cleavage was mainly catalyzed by butyrylcholinesterase; in mouse plasma, in addition to butyrylcholinesterase, carboxylesterase also contributed to the cleavage. The formation of compact polymer coils stabilized the ester bond. Conclusions. HPMA copolymer-PGE₁ conjugates are strong candidates as novel therapeutics for the treatment of osteoporosis. The observed species differences in plasma stability of ester bonds are of importance, because the ovariectomized rat model is recommended by the FDA for pre-clinical evaluation.