Elongated multiple electronic cascade and cyclization spacer systems in activatible anticancer prodrugs for enhanced drug release.

The design and synthesis of several novel elongated self-elimination spacer systems for application in prodrugs is described. These elongated spacer systems can be incorporated between a cleavable specifier and the parent drug. Naphthalene- and biphenyl-containing spacers were synthesized but did not eliminate. Prodrugs of the anticancer agents doxorubicin and paclitaxel are reported that contain two or three electronic cascade spacers. A novel catalytic application of HOBt was found for the synthesis of N-aryl carbamates through reacting a 4-nitrophenyl carbonate with an aniline derivative, to connect the 1,6-elimination spacers via a carbamate linkage. In addition, a double spacer-containing paclitaxel prodrug was synthesized, comprising a 1,6-elimination spacer and a bis-amine linker connected to paclitaxel via a 2'-carbamate linkage. Prodrugs in which the novel spacer systems were incorporated between a specific tripeptide specifier and the parent drug doxorubicin or paclitaxel proved to be significantly faster activated by plasmin in comparison with prodrugs containing conventional spacer systems. It is expected that the generally applicable novel spacer systems reported herein will contribute to future development of improved enzymatically activated prodrugs.

Synthesis and biological evaluation of 2'-carbamate-linked and 2'-carbonate-linked prodrugs of paclitaxel: selective activation by the tumor-associated protease plasmin.

The nontoxic paclitaxel-2'-carbamate prodrugs 2-5 and paclitaxel-2'-carbonate prodrug 6 were synthesized and tested for activation by the tumor-associated enzyme plasmin. A generally applicable method for the synthesis of paclitaxel-2'-carbamates was developed. In buffer solution, prodrug 2, which contained an unsubstituted ethylenediamine spacer, was not stable, whereas prodrugs 3-6 were highly stable. Prodrugs 3-6 showed on average a decrease in cytotoxicity of more than 8000-fold in comparison with the parent drug in seven human tumor cell lines. Prodrugs 5 and 6 are the most nontoxic prodrugs of paclitaxel that yield the free parent drug upon selective activation currently reported. Enzyme hydrolysis and spacer elimination rates were determined by incubation of prodrugs 5 and 6 in the presence of human plasmin. From these results, prodrug 6 was selected as the promising prodrug for further in vivo studies.