Influence of nicotinamide on the radiosensitivity of normal and goitrous thyroid in the rat

Radioiodine is used to treat thyroid cancer and hyperthyroidism. In order t o reduce radiation hazard to the patient and to people in contact with the patient it would be desirable to obtain the same therapeutic effect with lo wer activities of the radioisotope. This could be achieved by the simultane ous administration of a compound that increases tissue radiosensitivity. In this study we analyzed the use of nicotinamide (NA) as a radiosensitizer t o radioiodine, to increase I-131 efficacy. NA administered during 30 days t o Wistar rats failed to alter thyroid weight. The influence of NA on radiot hyroidectomy induced by increasing doses of I-131 was examined in otherwise nontreated rats. NA produced a significant increase in the ablation caused by radioiodine. Goiter was then induced by the administration of methylmer captoimidazol (MMI) to rats, followed by the treatment with radioiodine wit h and without simultaneous administration of NA. Thyroid weight per 100 g o f body weight ratio was not changed by NA alone; I-131 administration cause d a 25% decrease in goiter size, while I-131 plus NA produced a reduction o f the ratio of 46% (p < 0.01 vs. NA). No changes were observed in adenosine diphosphate (ADP)-ribosilation of thyroid nuclear protein in NA-treated ra ts. Thyroid blood flow (determined by Rb-86 uptake) was increased by 84% by NA. In conclusion, nicotinamide has a significant radiosensitizing effect to I-131 both in normal and goitrous rats. This action is because of an inc rease in thyroid blood flow, which probably enhances tissue oxgenation.