Escape mechanisms of human leukemic cells to long-term immunotoxin treatment in an in vitro experimental model

Microplate spectrophotometry for high-throughput screening of cytotoxic molecules

OBJECTIVES

High-throughput chemical and biochemical technologies are now exploited by modern pharmacology and toxicology to synthesize a multitude of new molecules with bioactive potential, or to isolate them from living matter. Testing molecules in cell systems on a large scale, however, is a rate-limiting step in drug discovery or in toxicity assessment. In this study, we developed a low-cost high-throughput method for first-level screening of cytotoxic molecules.

MATERIALS AND METHODS

We used microplate spectrophotometry to measure growth kinetics of human tumour cells that grow in suspension (Molt3) or adherent to the plastic surface of culture wells (HeLa) in standard RPMI medium. Cells were treated with colchicin, idarubicin or paclitaxel under various treatment schedules. The effects were quantified and compared with those measured by optical microscopy using the trypan blue dye exclusion method to reveal dead cells.

RESULTS

Proliferation kinetics of tumour cells can be quantified by measuring variations in optical densities of cell samples at 410 and 560 nm wavelengths. For cells that grow in suspension, one single reading at 730 nm may be sufficient to reconstruct growth curves that parallel those obtained by direct cell counting. Effects of the cytotoxic treatments could also be quantified and results compared very favourably with those obtained using standard techniques.

CONCLUSIONS

Microplate spectrophotometry is a robust and sensitive method to monitor growth of animal cell populations both in the absence and in the presence of cytotoxic drugs. This method implements existing technologies and can be fully automated.

Efficacy of an anti-CD138 immunotoxin and doxorubicin on drug-resistant and drug-sensitive myeloma cells

Multidrug resistance is an increasing problem in the treatment of cancer. We evaluated in vitro the effect of an anti-CD138 plasma-cell-specific immunotoxin (IT, B-B4-SO6) in combination with the chemotherapeutic drug doxorubicin on drug-sensitive and drug-resistant variants of the multiple-myeloma (MM)-derived cell line RPMI8226 and freshly isolated malignant-myeloma cells. Drug-resistant RPMI8226 cells were still sensitive to the IT, although to a lesser extent than drug-sensitive cells. In the clonogenic assay, using 10 nM B-B4-SO6, at least 5 logs kill was found for drug-sensitive RPMI8226 cells, vs. 2.5 logs kill for the drug-resistant RPMI8226 cells. When a sub-optimal dose of 1 nM IT was combined with 3 ng/ml doxorubicin, which was toxic for drug-sensitive but not for drug-resistant cells, an additive effect was found for drug-sensitive RPMI8226 cells. The IT did not influence the sensitivity of resistant cells for doxorubicin. We therefore speculate that this type of IT, may be of more value in combination with primary chemotherapy. The effect of B-B4-SO6 on malignant-myeloma cells of patients was investigated in a viability assay. Both drug-sensitive and drug-resistant cells from MM patients were sensitive to B-B4-SO6. After 2 days, a 50% kill of malignant cells was found when 10 nM IT were used. Doxorubicin was effective only on sensitive cells, and there was a tendency for an additive effect in the combination of these cells.

Monoclonal antibodies in the treatment of human B-cell malignancies

Monoclonal antibody technology emerged in the 1970's and was greeted by a wave of optimism. Many believed this new form of therapy would be effective in the treatment of human cancers. Early clinical trials in B-cell lymphomas demonstrated both the potential and limitations of unlabeled murine monoclonal antibody therapy, and taught us valuable lessons regarding the importance of the antibody structure, and nature of the targeted antigen. Since that time modifications in antibody structure and careful selection of target antigen have improved the clinical efficacy of these agents. Clinical trials using humanized antibodies have demonstrated that human/mouse chimeric antibodies and humanized antibodies have enhanced anti-tumor activity, decreased immunogenicity, and a very favorable toxicity profile. Radiolabeled monoclonal antibodies can induce durable remissions in lymphoma with toxicity limited largely to bone marrow suppression. Clinical trials with immunotoxins have demonstrated anti-tumor activity but also have been associated with significant toxicity. Standard treatment options for B-cell lymphoma will soon include antibody-based therapies. Further basic and clinical research is needed so we can understand more thoroughly the mechanisms responsible for the observed anti-tumor effects, and explore more extensively the best approach to their clinical use.

Quantitative analysis of protein synthesis inhibition and recovery in CRM107 immunotoxin-treated HeLa cells

Previously a mathematical model was proposed that quantitatively related protein synthesis inhibition kinetics of antitransferrin receptor-gelonin immunotoxins to the cellular trafficking of the targeting agent. That work is here extended to describe protein synthesis inhibition kinetics of immunotoxins containing the diphtheria toxin mutant CRM107. CRM107 differs from gelonin in both translocation and ribosomal inactivation mechanisms. Targeting agents used were antitransferrin monoclonal antibodies 5E9 and OKT9, OKT9Fab, and transferrin. CRM107 conjugates inhibited protein synthesis at substantially lower concentrations than gelonin conjugates; this effect was attributed to substantially higher translocation rates for CRM107. However, under certain conditions, CRM107 immunotoxin-treated cells were able to recover completely; this behavior was never observed with gelonin immunotoxins. To quantitatively capture this phenomenon, extracellular and cytosolic degradation of the toxin as well as growth-related recovery from toxin-induced damage were incorporated into the mathematical model. Translocation and cytosolic degradation rate constants were determined for each immunotoxin. Unlike the gelonin conjugates, the translocation rate of CRM107 conjugates depended on the targeting molecule. This provided indirect evidence that CRM107 remains disulfide linked to the targeting agent for at least part of the translocation process. Although the CRM107 conjugates all had higher translocation rates and inhibited protein synthesis at lower concentrations than the gelonin conjugates, the cells' ability to recover from protein synthesis inhibition at low immunotoxin concentrations limits the utility of CRM107 conjugates for targeted cell killing.

Combination immunotoxin treatment and chemotherapy in SCID mice with advanced, disseminated Daudi lymphoma

We describe the use of an immunotoxin (IT) cocktail (anti-CD22- and anti-CD19-ricin A chain) and any 1 of 3 chemotherapeutic drugs (doxorubicin, cytoxan or camptothecin) to treat advanced disseminated Daudi lymphoma in SCID mice (SCID/Daudi). In a previous report, we demonstrated that this regimen was curative when given the day following tumor cell inoculation. Here, we show that combination therapy in mice with advanced tumor significantly increased their survival, although it was not curative. Importantly, the outcome of therapy was dependent upon the temporal order in which IT and chemotherapy were administered. Thus, the best anti-tumor effect was achieved when an IT cocktail was given before or at the same time as chemotherapy. When the IT was given after chemotherapy, there was no additional therapeutic benefit. Our results confirm the rationale of using combination therapy in the treatment of advanced B-cell neoplasia and suggest that ITs should be administered prior to or during chemotherapy.