DIPHTHERIA-TOXIN EFFECTS ON BRAIN-TUMOR XENOGRAFTS - IMPLICATIONS FOR PROTEIN-BASED BRAIN-TUMOR CHEMOTHERAPY

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 K(i) for DT in small N417D tumors was 0.49 μl/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 μg/animal, DT increased the median survival time to 19 days (p<0.0016) while 1.0-μg doses extended median survival times to 26.5 days (p<0.0002). A higher dose of DT (3.0 μg) 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.