Electrophilic Analogues of Daunorubicin and Doxorubicin

Daunorubicin (DNR) or doxorubicin (DOX) was modified with one of four "linker reagents" to produce electrophilic drug analogues for synthesis of bioconjugates. Synthesis and characterization of two new reagents [p-isothiocyanatobenzoyl chloride and 3-(p-isothiocyanatophenyl)propionyl chloride] are described here for the first time. Adding one of the new reagents, bromoacetyl bromide, or p-(fluorosulfonyl)benzoyl chloride in chloroform to an alkaline aqueous solution of DNR (or DOX) provided excellent yields of the corresponding, electrophilic 3'-N-amide analogue. The DNR and DOX analogues were characterized by thin-layer chromatography, nuclear magnetic resonance spectroscopy, and infrared spectroscopy. Bioconjugates were produced with the electrophilic DNR or DOX analogues by mixing them with bovine serum albumin (BSA), mouse IgG, or a monoclonal antibody (OC125, which specifically binds to the CA125 antigen from human ovarian carcinoma). The relative reactivity of the 3'-N-substituents toward protein is p-(fluorosulfonyl)benzoyl > phenylisothiocyanato > bromoacetyl. Overall, the new phenyl isothiocyanate acid chlorides are superior to p-(fluorosulfonyl)benzoyl chloride or bromoacetyl bromide as reagents with which to produce electrophilic DNR or DOX analogues for conjugation with monoclonal antibodies. The bioconjugates DNR-OC125 and DOX-OC125 are selectively toxic to two human ovarian cancer cell lines in vitro (1) and bind with high specificity to human ovarian tumor sections (2) that express the CA125 antigen.