In order to find radiosensitizers with low toxicity, radiation enhancing effects of the pentoxifylline (PENTO), which is known to bear so little toxicity as to be widely used for clinical purpose to improve peripheral blood circulation for vascular disorders, was compared with the nitroimidazole derivative SR 2508 (SR), which is an effective radiosensitizer but has severe neurotoxicity as do other nitroimidazole derivatives. We used an experimental tumor in mice and calculated an-enhancement ratio (ER) using a growth delay method. ER of PENTO and SR were 1.6 and 1.4, respectively, showing PENTO has stronger radiosensitizing effect than SR. Also combined effects with nicotinamide (NA) were compared between PENTO and SR. When combined with NA, the enhancing effects of PENTO was enhanced especially at lower radiation doses, whereas that of SR was decreased. Together with the fact that both PENTO and NA have very low toxicity, the usefulness of PENTO as a radiosensitizer either alone or combined with NA is suggested.

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