SummaryMultidrug resistance (MDR), which is due, in part, to the overexpression of P-glycoprotein, confers resistance to a variety of natural product chemotherapeutic agents such as daunorubicin, vincristine, and colchicine. RV + cells are a P-glycoprotein overexpressing variant of the HL60 myeloid leukemia cell line. In addition to classic MDR, IkV + cells displayed relative resistance to complement-mediated cytotoxicity with both immunoglobulin G and M antibodies against different cell surface antigens, but not to antibody-dependent cellular cytotoxicity and lymphokine-activated killing. Complement resistance was reversed both by treatment with verapamil and with specific monoclonal antibodies (mAbs) capable of binding to P-glycoprotein and blocking its function. To further confirm that the resistance of IkV + cells was not a consequence of the selection of the cells on vincristine, a second system involving P-glycoprotein infectants was also investigated. K562 cells infected with the MDR1 gene, which were never selected on chemotherapeutic drugs, also displayed relative resistance to complementmediated cytotoxicity. This MDR1 infection-induced resistance was also reversed by mAbs that bind to P-glycoprotein. Therefore, the MDI< phenotype as mediated by P-glycoprotein provides resistance to complement-mediated cytotoxicity. The increased intracellular pH and the decreased membrane potential due to the MDI< phenotype may result in abnormal membrane attack complex function. This observation may have implications for the possible mechanisms of action of P-glycoprotein and for a possible physiologic role for P-glycoprotein in protection against complement-mediated autolysis.M ultidrug resistance (MDP,.) is characterized by broad resistance to several pharmacologically and chemically distinct chemotherapeutic compounds, in particular, the vinca alkaloids and the anthracyclines (1). Tumor cells that are resistant to these compounds exhibit decreased retention of drugs over time (2). The MDk phenotype (3) is the result of the overexpression of the mdrl gene product at least in part, a 170--180-kD glycoprotein known as P-glycoprotein. A "drug pump" model has been proposed in which the P-glycoprotein serves as an active transporter that pumps drugs out of the cell (1, 3, 4).The cytotoxic abilities of mAbs M195 (anti-CD33) (5) and M31 (anti-CD15) against HL60 myeloid leukemia cells and a MDt< HL60 variant that overexpresses the MDR protein (RV + [6]), were examined. In this paper, we describe the relative resistance of RV + ceils to specific mAbs that react against cell surface protein or carbohydrate targets, and mediate cytotoxicity via complement fixation. MDP,. reversal studies using verapamil and with specific mAbs to P-glycoprotein showed that a significant portion of this newly described immunological resistance is mediated by P-glycoprotein overexpression. To test whether these observations were not partly due to the selection of the tLV + cells on vincristine or a spurious phenomenon for a single cell line, K56...

This trial studied the biodistribution, pharmacology, toxicity, immunogenicity, and biologic characteristics of a trace-labeled, anti- CD33, humanized monoclonal antibody M195 (Hu-M195) in patients with relapsed and refractory myeloid leukemia. Hu-M195 is a computer- modeled, “complementarity-determining region-grafted,” IgG1, humanized version of M195. M195 is a murine monoclonal antibody that reacts with CD33, a 67-kD glycoprotein expressed on early myeloid progenitor cells and myeloid leukemia (acute myelogenous leukemia and chronic myelogenous leukemia) cells, but not normal stem cells. 131I-murine- M195 has already shown significant ability to cytoreduce patients with relapsed or refractory myeloid leukemias. Hu-M195 has higher avidity than the original mouse monoclonal antibody and, unlike murine M195, has the capability to mediate antibody-dependent cellular cytotoxicity against leukemia targets. Thirteen patients with relapsed or refractory myelogenous leukemia were treated with Hu-M195 at 4 levels of 0.5, 1.0, 3.0, and 10.0 mg/m2 in a phase I trial. Patients received a total of 6 doses per patient over 18 days. Two patients were retreated for a total of 12 doses. The first dose of Hu-M195 was trace-labeled with 131I to allow detailed pharmacokinetic and biodistribution studies by serial sampling of blood, radioimmunoassays of cells, and whole-body gamma- camera imaging. Cumulative total doses of up to 216 mg of Hu-M195 were administered safely. Reversible fever and rigors were observed after infusion at the highest dose levels. The entire bone marrow was specifically and clearly imaged within hours after infusion, with optimal biodistribution occurring at the 3 mg/m2 level. Adsorption of Hu-M195 onto targets in vivo was demonstrated by flow cytometry; near saturation of available sites occurred at the 3 mg/m2 dose level. Plasma and whole body half lives were 38 and 51 hours, respectively, which may reflect continual replenishment of target sites on new leukemia cells. 131I-Hu-M195 was rapidly internalized into the target cells in vivo within 1 hour. Human antihuman antibody responses were not observed. In conclusion, Hu-M195 can be administered safely in multiple doses, without significant toxicity or any evidence of immunogenicity, and can localize rapidly and efficiently to the bone marrow in patients with myeloid leukemias. Additional phase II trials with this agent alone or in combination with cytokines or isotopes are warranted at the optimal biologic dose.

This trial studied the biodistribution, pharmacology, toxicity, immunogenicity, and biologic characteristics of a trace-labeled, anti- CD33, humanized monoclonal antibody M195 (Hu-M195) in patients with relapsed and refractory myeloid leukemia. Hu-M195 is a computer- modeled, “complementarity-determining region-grafted,” IgG1, humanized version of M195. M195 is a murine monoclonal antibody that reacts with CD33, a 67-kD glycoprotein expressed on early myeloid progenitor cells and myeloid leukemia (acute myelogenous leukemia and chronic myelogenous leukemia) cells, but not normal stem cells. 131I-murine- M195 has already shown significant ability to cytoreduce patients with relapsed or refractory myeloid leukemias. Hu-M195 has higher avidity than the original mouse monoclonal antibody and, unlike murine M195, has the capability to mediate antibody-dependent cellular cytotoxicity against leukemia targets. Thirteen patients with relapsed or refractory myelogenous leukemia were treated with Hu-M195 at 4 levels of 0.5, 1.0, 3.0, and 10.0 mg/m2 in a phase I trial. Patients received a total of 6 doses per patient over 18 days. Two patients were retreated for a total of 12 doses. The first dose of Hu-M195 was trace-labeled with 131I to allow detailed pharmacokinetic and biodistribution studies by serial sampling of blood, radioimmunoassays of cells, and whole-body gamma- camera imaging. Cumulative total doses of up to 216 mg of Hu-M195 were administered safely. Reversible fever and rigors were observed after infusion at the highest dose levels. The entire bone marrow was specifically and clearly imaged within hours after infusion, with optimal biodistribution occurring at the 3 mg/m2 level. Adsorption of Hu-M195 onto targets in vivo was demonstrated by flow cytometry; near saturation of available sites occurred at the 3 mg/m2 dose level. Plasma and whole body half lives were 38 and 51 hours, respectively, which may reflect continual replenishment of target sites on new leukemia cells. 131I-Hu-M195 was rapidly internalized into the target cells in vivo within 1 hour. Human antihuman antibody responses were not observed. In conclusion, Hu-M195 can be administered safely in multiple doses, without significant toxicity or any evidence of immunogenicity, and can localize rapidly and efficiently to the bone marrow in patients with myeloid leukemias. Additional phase II trials with this agent alone or in combination with cytokines or isotopes are warranted at the optimal biologic dose.