Targeted therapy with immunotoxins in a nude rat model for leptomeningeal growth of human small cell lung cancer.

Metastasis to the central nervous system in patients with small cell lung cancer is not uncommon, and a fraction of the cases have leptomeningeal disease for which no effective therapy is available. To establish an experimental model for evaluation of new therapeutic approaches for such tumor lesions, 1 x 10(6) human H-146 cells were injected directly into the cerebrospinal fluid in the cisterna magna of nude rats. Small, superficial leptomeningeal tumors developed, consistently resulting in symptoms of central nervous system involvement after a mean latency of 20 days. The model was used to study the efficacy of intrathecal targeted therapy with immunotoxins. The monoclonal anti-carcinoma antibodies MOC-31 and NrLu10 and the growth factor transferrin were conjugated to Pseudomonas exotoxin A (PE), and 1 day after tumor cell inoculation instilled in the cisterna magna as a single bolus dose of 1.5 micrograms. The antibody conjugates, which were highly cytotoxic to target cells in a protein synthesis inhibition assay in vitro, increased the symptom-free latency by 35-46%. PE had no effect, reflecting a lower in vitro cytotoxicity and possibly also a down-regulation of transferrin-receptor expression in the meningeal H-146 tumors. Delayed or repeated treatment with MOC-31-PE was less effective than day 1 administration, whereas the addition of 10% glycerol to the injection solution increased the symptom-free period to 72%. The efficacy of MOC-31-PE is superior to reported effects obtained in similar models with other therapies, and the results support the development of this immunotoxin towards clinical evaluation in small cell lung cancer patients with leptomeningeal carcinomatosis.