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.