Tissue Levels of Cyclic AMP and Tumor Inhibition

The hypothesis that stimulated this investigation is that cyclic AMP may be a regulator of cell division and therefore may control the growth of tumors. Some inhibition of the 6C3HED, L5178Y, and L1210 tumors results from daily administration of 3 μmoles of cyclic AMP or 5.5 μmoles of theophylline. Treatment of the mice with cyclic AMP 48 hr prior to tumor inoculation produced the greatest tumor inhibition. 5′-AMP, if given during or before tumor transplantation, accelerates tumor growth. Intraperitoneal injection of 3 μmoles of cyclic AMP elevates the cyclic AMP level in the tumor for only 2 hr. The reversibility of the inhibition found in cell culture suggests that this is probably inadequate for producing tumor inhibition. The stimulation of tumor growth by 5′-AMP, a metabolite of cyclic AMP, makes multiple injections of cyclic AMP ineffective in tumor inhibition.