Dual fluorescent HPMA copolymers for passive tumor targeting with pH-sensitive drug release: synthesis and characterization of distribution and tumor accumulation in mice by noninvasive multispectral optical imaging.

Preclinical in vivo characterization of new polymeric drug conjugate candidates is crucial for understanding the effects of certain chemical modifications on distribution and elimination of these carrier systems, which is the basis for rational drug design. In our study we synthesized dual fluorescent HPMA copolymers of different architectures and molecular weights, containing one fluorescent dye coupled via a stable hydrazide bond functioning as the carrier label and the other one modeling the drug bound to a carrier via a pH-sensitive hydrolytically cleavable hydrazone bond. Thus, it was possible to track the in vivo fate, namely distribution, elimination and tumor accumulation, of the polymer drug carrier and a cleavable model drug simultaneously and noninvasively in nude mice using multispectral optical imaging. We confirmed our in vivo results by more detailed ex vivo characterization (imaging and microscopy) of autopsied organs and tumors. There was no significant difference in relative biodistribution in the body between the 30 KDa linear and 200 KDa star-like polymer, but the star-like polymer circulated much longer. We observed a moderate accumulation of the polymeric carriers in the tumors. The accumulation of the pH-sensitive releasable model drug was even higher compared to the polymer accumulation. Additionally, we were able to follow the long-term in vivo fate and to prove a time-dependent tumor accumulation of HPMA copolymers over several days.