Design, synthesis, and biological evaluation of doxorubicin-formaldehyde conjugates targeted to breast cancer cells.

The anthracycline antitumor drug doxorubicin (DOX) has been utilized for decades as a broad-spectrum chemotherapeutic. Recent literature evidence documents the role of formaldehyde in the cytotoxic mechanism, and anthracycline-formaldehyde conjugates possess substantially enhanced activity in vitro and in vivo. Targeting a doxorubicin-formaldehyde conjugate specifically to cancer cells may provide a more efficacious chemotherapeutic. The design and 11-step synthesis of doxorubicin-formaldehyde conjugates targeted to the estrogen receptor, which is commonly overexpressed in breast cancer cells, are reported. The formaldehyde is incorporated in a masked form as an N-Mannich linkage between doxorubicin and salicylamide. The salicylamide triggering molecule, previously developed to release the doxorubicin-formaldehyde active metabolite, is tethered via derivatized ethylene glycols to an E and Z mixture of 4-hydroxytamoxifen. The targeting group, E/Z-4-hydroxytamoxifen, was selected for its ability to tightly bind the estrogen receptor and antiestrogen binding sites. The targeted doxorubicin-formaldehyde conjugates' estrogen receptor binding and in vitro growth inhibition were evaluated as a function of tether length. The lead compound, DOX-TEG-TAM, bearing a triethylene glycol tether, binds the estrogen receptor with a binding affinity of 2.5% relative to E/Z-4-hydroxytamoxifen and inhibits the growth of four breast cancer cell lines with 4-fold up to 140-fold enhanced activity relative to doxorubicin.