Enhancement of the in vitro and in vivo antitumor activities of phosphorylated mitomycin C and etoposide derivatives by monoclonal antibody-alkaline phosphatase conjugates.

Alkaline phosphatase (AP) was covalently linked to the two antitumor monoclonal antibodies, L6 (anticarcinoma) and 1F5 (anti-B lymphoma), forming conjugates that could bind to antigen-positive tumor cells. The conjugates were able to convert the prodrugs, mitomycin phosphate (MOP) and etoposide phosphate (EP), into an active mitomycin C derivative, mitomycin alcohol, and etoposide, respectively. MOP and EP were less toxic to cultured cells from the H2981 lung adenocarcinoma than their respective hydrolysis products, mitomycin alcohol and etoposide, by a factor greater than 100, and they were also less toxic in mice. Pretreatment of H2981 cells with L6-AP greatly enhanced the cytotoxic effects of MOP and EP, while 1F5-AP caused no such enhancement. A strong antitumor response was observed in H2981-bearing mice that were treated with L6-AP followed 24 h later by either MOP or a combination of MOP and EP. This response was superior to that of MOP or combinations of MOP and EP given alone.