Antitumor activity of naltrexone and correlation with steroid hormone receptors

We have evaluated the opiate peptide antagonist, naltrexone, for its effectiveness as an antitumor agent. For this evaluation, we tested the effect of naltrexone given daily in the diet on the growth of established 7,12-dimethylbenz(a)anthracene-induced mammary tumors. Tumors continued to grow actively in rats fed chow diet only (control group). In contrast, the naltrexone-supplemented diet (75 mg/kg diet) significantly decreased the size of the established mammary tumors in rats over the 25 day observation period, resulting in an average decrease in tumor volume by approximately 23% compared with their sizes at the beginning of the treatment. Tumor regression occurred in 70% of the rats. Tumors that respond to naltrexone showed appreciable amounts of estrogen and progesterone receptors while unresponsive tumors were negative for estrogen and progesterone receptors. For the first time, we report that naltrexone can regress established mammary tumors and that the inhibitory effect of naltrexone appears to be restricted to the hormonally responsive mammary tumors.