The success of chemotherapy is limited by poor selectivity of active drugs combined with occurrence of tumor resistance. New star-like structured N-(2-hydroxypropyl) methacrylamide (HPMA) copolymer-based drug delivery systems containing Doxorubicin (Dox) attached via a pH-sensitive hydrazone bond were designed and investigated for their ability to overcome chemotherapy resistance. These conjugates combine two strategies to achieve a high drug concentration selectively at the tumor site: (I) high accumulation by passive tumor targeting based on enhanced permeability and retention (EPR) - effect and (II) pH-sensitive site-specific drug release due to an acidic tumor microenvironment. Mice bearing Dox resistant xenograft tumors were treated with Dox, PBS, pHPMA precursor or pHPMA-Dox conjugate at different equivalent doses of 5mg/kg bodyweight Dox up to a 7-fold total dose using different treatment schedules. Intratumoral drug accumulation was analyzed by fluorescence imaging utilizing intrinsic fluorescence of Dox. Free Dox induced significant toxicity but hardly any tumor inhibiting effects. Administering of at least 3-fold dose of pHPMA-Dox conjugate was necessary to induce a transient response whereas doses of about 5- to 6-fold induced strong regressions. Tumors completely disappeared in some cases. The onset of response was differential delayed depending on the tumor model which could be ascribed to distinct characteristics of the microenvironment. Further fluorescence imaging based analyses regarding underlying mechanisms of the delayed response revealed a related switch to a more supporting intratumoral microenvironment for effective drug release. In conclusion, the present study demonstrates that the concept of tumor site-restricted high-dose chemotherapy is able to overcome therapy resistance.
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