ALKYLATING-AGENTS AND IMMUNOTOXINS EXERT SYNERGISTIC CYTOTOXIC ACTIVITY AGAINST OVARIAN-CANCER CELLS - MECHANISM OF ACTION

Alkylating agents can be administered in high dosage to patients with ovarian cancer using autologous bone marrow support, but drug-resistan t tumor cells can still persist. Immunotoxins provide reagents that mi ght eliminate drug resistant cells. In the present study, concurrent t reatment with alkylators and immunotoxins proved superior to treatment with each agent alone. Toxin immunoconjugates prepared from different monoclonal antibodies and recombinant ricin A chain (rRTA) inhibited clonogenic growth of ovarian cancer cell lines in limiting dilution as says. When alkylating agents and toxin conjugates were used in combina tion, the addition of the immunotoxins to cisplatin, or to cisplatin a nd thiotepa, produced synergistic cytotoxic activity against the OVCA 432 and OVCAR III cell lines. Studies performed to clarify the mechani sm of action showed that cisplatin and thiotepa had no influence on in ternalization and binding of the 317G5-rRTA immunotoxin. Intracellular uptake of [195m]Pt-cisplatin was not affected by the immunoconjugate and thiotepa. The combination of the 317G5-rRTA and thiotepa, as well as 317G5-rRTA alone, increased [195m]Pt cisplatin-DNA adduct levels. T he immunotoxin alone and in combination with the alkylators decreased intracellular glutathione levels and reduced glutathione-S-transferase activity. Repair of DNA damage induced by the combination of alkylato rs and 317G5-rRTA was significantly reduced when compared to repair af ter damage with alkylators alone. These findings suggest that immunoto xins affect levels and activity of enzymes required for the prevention and repair of alkylator damage.