Stereospecific modulation of tumorigenicity by opioid antagonists.

The effects of (+) and (-) isomers of naloxone in altering tumor response in A/Jax mice inoculated with S20Y neuroblastoma were determined. Mice given daily subcutaneous injections of 15 mg/kg (-)naloxone had a 71% tumor incidence, a 33% delay in the time before tumor appearance, and a 28% increase in survival time. Inoculation of neuroblastoma cells in control subjects resulted in 100% tumor incidence within 13 days. Animals given 15 mg/kg (+)naloxone were comparable to control subjects in all aspects of neural neoplasia. These results show that opioid antagonists exert a stereospecific action on neural cancers, and provide further evidence that endogenous opioid systems play an important role in neuro-oncogenic expression.