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Diphtheria toxin effects on brain-tumor xenografts. Implications for protein-based brain-tumor chemotherapy
C J Wrobel1, D C Wright, R L Dedrick
1Surgical Neurology Branch, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland.
Abstract:
A model was developed to determine whether protein-based chemotherapeutic agents can cross the blood-brain barrier and successfully treat brain tumors. The human small-cell lung carcinoma N417D was grown as a solid tumor in the nude rat brain, and diphtheria toxin (DT) was administered intravenously as therapy. Because rat cells lack functional DT receptors and are 1000 to 10,000 times less sensitive to DT than human cells, a therapeutic window exists between the implanted human tumor and the nude rat host. The pharmacokinetic and pharmacodynamic characteristics of DT were defined. Within 6 hours, more than 90% of the initial DT concentration was removed from the blood. The blood-to-tumor transfer constant Ki for DT in small N417D tumors was 0.49 microliters/gm-min, one-fourth to one-fifth the reported values for permeability to proteins in other experimental tumor models. Despite the toxin's short plasma half-life and the relatively intact blood-tumor barrier, DT administered intravenously as a single dose significantly extended animal survival. Untreated nude rats developed solid parenchymal tumors and died in 11 to 16 days (median 15 days). When administered at 0.1 micrograms/animal, DT increased the median survival time to 19 days (p less than 0.0016) while 1.0-microgram doses extended median survival times to 26.5 days (p less than 0.0002). A higher dose of DT (3.0 micrograms) had no further beneficial effect on survival (26.1 days). Blood-brain barrier constraints to successful monoclonal antibody-based therapies of brain tumors may have been overestimated since antibody conjugates have plasma half-lives longer than DT, and the permeability of N417D tumors to DT is equal to or less than the permeability of other experimental tumors to large proteins. Recently developed immunotoxins that have the higher potency of DT and a therapeutic window as wide as DT has in this nude rat/human tumor paradigm may be effective in treating brain tumors despite limited blood-tumor permeability.
Insights
Diphtheria toxin (DT) effectively treated brain tumors in rats by leveraging a therapeutic window, extending survival despite blood-brain barrier challenges. This suggests protein-based therapies for brain tumors may be more feasible than previously thought.
Area of Science:
- Oncology
- Pharmacology
- Neuroscience
Background:
- The blood-brain barrier (BBB) poses a significant challenge for delivering protein-based chemotherapeutics to brain tumors.
- Diphtheria toxin (DT) is a potent protein toxin that can be used therapeutically.
- A model system was needed to assess the efficacy of protein-based agents across the BBB.
Purpose of the Study:
- To develop and validate a model for evaluating protein-based chemotherapeutic agents, specifically DT, in treating brain tumors.
- To determine the pharmacokinetic and pharmacodynamic properties of DT in the context of brain tumor treatment.
- To assess the therapeutic potential of DT against human small-cell lung carcinoma xenografts in nude rats.
Main Methods:
- Human small-cell lung carcinoma (N417D) xenografts were established in the brains of nude rats.
- Intravenous administration of diphtheria toxin (DT) was used as the therapeutic intervention.
- Pharmacokinetic analysis of DT distribution and clearance was performed.
- Survival rates of DT-treated rats were compared to untreated controls.
Main Results:
- DT exhibited a short plasma half-life, with over 90% removed from circulation within 6 hours.
- The blood-to-tumor transfer constant (Ki) for DT was relatively low, indicating limited permeability across the blood-tumor barrier.
- Single intravenous doses of DT significantly extended survival in tumor-bearing rats, with optimal effects observed at 1.0 microgram.
- Higher doses of DT did not provide additional survival benefits.
Conclusions:
- Despite BBB and blood-tumor barrier limitations, DT demonstrated significant efficacy in extending survival for brain tumor models.
- The findings suggest that the constraints imposed by the BBB on protein-based therapies, such as antibody conjugates, may have been overestimated.
- Novel immunotoxins with high potency and a wide therapeutic window hold promise for treating brain tumors, even with restricted permeability.