Abstract LB-37: In vitro and in vivo studies of novel acid-sensitive prodrugs derived from dendritic polyglycerol
Authors: Calderón Marcelo , Welker Pia , Licha Kai , Graeser Ralph , Haag Rainer , Kratz Felix 
Published: February 2011

Abstract
Research in the development of tumor-specific nanocarriers is currently one of the major sources of innovation for improved anticancer chemotherapy. Due to their low degree of molecular weight dispersity, their flexible design and their biocompatible nature, dendritic polyglycerols (PGs) have a broad range of potential applications in medicine and pharmacology. Recently, we demonstrated that drugs could be bound to the thiolated hyperbranched polymers incorporating enzymatically cleavable spacers that were efficiently released by the target enzyme cathepsin B. These initially developed dug carriers with PGs, however, did not have an optimal molecular weight for EPR-mediated tumor targeting and remaining amino groups on the surface of the dendritic scaffold showed acute toxicity properties.
As a consequence, we set out to improve the conjugation method and fine-tune the architecture of the polyglycerol drug conjugates for optimal tumor targeting. A logically consistent option was to mask remaining and potentially toxic amino groups with the polymer polyethylene glycol which would have the additional advantage of increasing the molecular weight of drug polyglycerol conjugate to &gt; 40 kDa which is a pre-requisite for passive targeting. Thus, we developed a macromolecular prodrug derived from hyperbranched amino-bearing polyglycerol that in a first step was partially thiolated with iminothiolane followed by a Michael addition to the maleimide group of the acid-sensitive 6-maleimidocaproyl(hydrazone) derivative of doxorubicin, and in a second step the maleimide group of 2 and 5 kDa PEG chains were reacted with the remaining sulfhydryl groups. Thus, the drug was covalently attached to the hyperbranched polyglycerol core and surrounded by a “shell” of long, solubilizing PEG chains which radiate from the core. By varying the amount of iminothiolane, of doxorubicin prodrug and of PEG-maleimide, drug PG conjugates with different drug loading (1 to 10) were isolated. Analysis by dynamic light scattering measurements (DLS) and z-potential showed that average diameters were in the range of 12 to 16 nm and the conjugates showed a slightly negative surface charge. The in vitro release studies showed that the release of doxorubicin was minimal at pH 7.4 after 22 h (less than 10%), while at acidic pH half-lives were below 4 h. Although free doxorubicin was more active than the PG doxorubicin conjugates against A2780 ovarian carcinoma cells, in the respective xenograft model the pegylated PG doxorubicin conjugates with 5 to 10 bound molecules of doxorubicin showed a distinct increase in tolerability and antitumor efficacy in comparison to the parent drug producing excellent antitumor effects with complete and transient tumor remissions (at 24 mg/kg doxorubicin equivalents). Doxorubicin treated animals only showed a moderate antitumor effect at its optimal dose of 2 × 8 mg/kg.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr LB-37.