A panel of monoclonal antibodies (MAbs) to P-glycoprotein was developed by immunization of mice with multidrug-resistant human neuroepithelioma and neuroblastoma cells. All the anti-P-glycoprotein MAbs reacted with the extracellular portion of P-glycoprotein. The MAbs were examined for their ability to enhance accumulation of actinomycin D, vincristine, vinblastine, and doxorubicin in the human mdr1 transfectant cell line, BRO/pFRmdr1.6. HYB-241, an IgG1 anti-P-glycoprotein MAb, was the most

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... ective modulator, increasing actinomycin D levels in the transfectant line by 6-fold, vincristine by 2-fold, and vinblastine levels by 3-fold. None of the MAbs were capable of modifying the accumulation of doxorubicin. HYB-241 lowered the 50% inhibitory concentration values of actinomycin D by 11-fold, vincristine by 6-fold, and vinblastine by 2-fold. No effect on the 50% inhibitory concentration values of doxorubicin or gramicidin were seen. 111In-labeled HYB-241 localized in human tumor xenografts of BRO/pFRmdr1.6 in nude mice (25% injected dose/g at 120 h). Mice with established drug-resistant xenografts were treated with antibody 24 h prior to the injection of Vinca alkaloid at concentrations known to be non-growth inhibitory. The addition of HYB-241 at 25 mg/kg per injection prior to drug resulted in a significant inhibition of growth of this drug-resistant tumor.