Many conjugates of water-soluble polymers with biologically active molecules were developed during the last two decades. Although, therapeutic effects of these conjugates are affected by the properties of carriers, the properties of the attached drugs appear more important than the same carrier polymer in this case. Pirarubicin (THP), a tetrahydropyranyl derivative of doxorubicin (DOX), demonstrated more rapid cellular internalization and potent cytotoxicity than DOX. Here, we conjugated the THP or DOX to N-(2-hydroxypropyl)methacrylamide copolymer via a hydrazone bond. The polymeric prodrug conjugates, P-THP and P-DOX, respectively, had comparable hydrodynamic sizes and drug loading. Compared with P-DOX, P-THP showed approximately 10 times greater cellular uptake during a 240 min incubation and a cytotoxicity that was more than 10 times higher during a 72-h incubation. A marginal difference was seen in P-THP and P-DOX accumulation in the liver and kidney at 6 h after drug administration, but no significant difference occurred in the tumor drug concentration during 6-24 h after drug administration. Antitumor activity against xenograft human pancreatic tumor (SUIT2) in mice was greater for P-THP than for P-DOX. To sum up, the present study compared the biological behavior of two different drugs, each attached to an N-(2-hydroxypropyl)methacrylamide copolymer carrier, with regard to their uptake by tumor cells, body distribution, accumulation in tumors, cytotoxicity, and antitumor activity in vitro and in vivo. No differences in the tumor cell uptake of the polymer-drug conjugates, P-THP and P-DOX, were observed. In contrast, the intracellular uptake of free THP liberated from the P-THP was 25-30 times higher than that of DOX liberated from P-DOX. This finding indicates that proper selection of the carrier, and especially conjugated active pharmaceutical ingredient (API) are most critical for anticancer activity of the polymer-drug conjugates. THP, in this respect, was found to be a more preferable API for polymer conjugation than DOX. Hence the treatment based on enhanced permeability and retention (EPR) effect that targets more selectively to solid tumors can be best achieved with THP, although both polymer conjugates of DOX and THP exhibited the EPR effects and drug release profiles in acidic pH similarly.
Keywords: EPR effect; HPMA polymer conjugate; acid-cleavable linkage; doxorubicin (DOX); pirarubicin (THP).