Anthracycline–Formaldehyde Conjugates and Their Targeted Prodrugs

Abstract

The sequence of research leading to a proposal for anthracycline cross-linking of DNA is presented. The clinical anthracycline antitumor drugs are anthraquinones, and as such are redox active. Their redox chemistry leads to induction of oxidative stress and drug metabolites. An intermediate in reductive glycosidic cleavage is a quinone methide, once proposed as an alkylating agent of DNA. Subsequent research now im- plicates formaldehyde as a mediator of anthracycline-DNA cross-linking. The cross-link at 5?-GC-3? sites consists of a covalent linkage from the amino group of the anthra- cycline to the 2-amino group of the G-base through a methylene from formaldehyde, hydrogen bonding from the 9-OH to the G-base on the opposing strand, and hydropho- bic interactions through intercalation of the anthraquinone. The combination of these interactions has been described as a virtual cross-link ofDNA. The origin of the formalde- hyde in vivo remains a mystery. In vitro, doxorubicin reacts with formaldehyde to give firstly a monomeric oxazolidine, doxazolidine, and secondly a dimeric oxazolidine, doxo- form. Doxorubicin reacts with formaldehyde in the presence of salicylamide to give the N-Mannich base conjugate, doxsaliform. Doxsaliform is several fold more active in tu- mor cell growth inhibition than doxorubicin, but doxazolidine and doxoform are orders of magnitude more active than doxorubicin. Exploratory research on the potential for doxsaliform and doxazolidine as targeted cytotoxins is presented. A promising lead de- sign is pentyl PABC-Doxaz, targeted to a carboxylesterase enzyme overexpressed in liver cancer cells and/or colon cancer cells