Histamine blockers alter biodistribution of radioiodine

Objectives Gastric retention of Tc-99m O4-, similar in distribution to radioiodine, is increased by histamine-2 (H2) blockers and proton pump inhibitors (PPI’s). H1 and H2 blockers may alter TSH levels. The objective was to determine whether short term use of these drugs alters radioiodine biodistribution.

Methods Male rats received no drug (controls) or daily SC injections of promethazine (1 mg/kg daily), esomeprazole (5 mg/kg daily), or famotidine (3 mg/kg twice daily) commencing 1 day prior to IP injection of 1 uCi I-131 and continuing to day of euthanasia ( 1d or 8d post I-131 injection). Organ uptake of I-131 by control (C) and drug-treated (D) rats was assayed by gamma well counting and compared by 1-way ANOVA with Dunnett error protection (significant p < .05).

Results There was no difference in blood levels of I-131 at 1d or 8d post I-131 administration for control, famotidine or promethazine treated rats. Both famotidine and promethazine (respectively) resulted in significantly increased salivary gland uptake of I-131 (D:C ratios) at 1d (1.37, 1.40) and 8d (4.52, 5.57) post I-131. Both famotidine and promethazine (respectively) demonstrated a significant decrease in uptake of I-131 by the liver (D:C ratios) at 1d (0.60, 0.71) post radioiodine administration but a marked increase over control levels (11.21, 9.28) at 8d post I-131. Famotidine did not alter gastric uptake of I-131 either at 1 or 8d post I-131. Promethazine significantly increased gastric I-131 uptake (D:C ratios) at 1d (1.47), and at 8d (1.46) post I-131. Famotidine did not alter uptake of I-131 by the thyroid either at 1d or 8d. However, promethazine resulted in a significant increase in uptake of I-131 by the thyroid (D:C ratios) both at 1d (1.32) and 8d (1.52) post I-131 administration. Esomeprazole did not alter biodistribution of radioiodine compared to controls for any organs or time points studied.

Conclusions The short term use of H1 and H2 blockers alter the biodistribution of radioiodine. These drugs could result in increased irradiation of non-target tissues during I-131 therapy for hyperthyroidism or thyroid cancer.