Injections of the pineal hormone melatonin reduced growth and increased doubling time of the R3327H Dunning prostatic adenocarcinoma in the Copenhagen X Fisher rat. This occurred even though testosterone levels (50% of normal) were ostensibly high enough to maintain normal tumor growth. Melatonin may act directly upon the conversion of testosterone to dihydrotestosterone to inhibit tumor growth. Alternatively, a more complicated mechanism may be involved since studies on MCF breast cancer tissue indicate an estrogen requirement for inhibition of growth by melatonin.
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