Potential Interference of Agents on Radioiodide Thyroid Uptake in the Euthyroid Rat Ming-Der Yu, PhD1; and Stanley M. Shaw, PhD2 1Department of Nuclear Medicine, Tri-Service General Hospital, Taipei, Taiwan; and 2Division of Nuclear Pharmacy, School of Pharmacy and Pharmacal Sciences, Purdue University, West Lafayette, Indiana problems have been documented (1–10), but data are con- The objective of this research was to investigate the merits of flicting in regard to the extent of the problem. The interfer- controlled studies with euthyroid rats as a means of determining ence time of propylthiouracil (PTU) has been reported to the influence of dose and time after administration of agents vary from days to 1–2 wk. The interference time of dia- that may interfere with radioiodide uptake in the thyroid. Meth- trizoate meglumine has been reported to be from 2 wk to ods: Potassium iodide (KI), propylthiouracil (PTU), diatrizoate meglumine, and iohexol were selected to represent interfering about 2 mo. The duration of interference from iodides has agents. Two dose levels per agent were investigated. Doses been noted as 1, 2, or 4 wk. Many reports on drug–radiophar- used were 1 and 2 mg/kg of body weight for KI, 3.5 and 7 mg/kg maceutical interactions are anecdotal and in situations that of body weight for PTU, 1 mL/kg (282 mg I/kg) and 2 mL/kg (564 make comparisons between agents difficult. mg I/kg) of body weight for diatrizoate meglumine, and 1 mL/kg It would appear beneficial to conduct controlled studies (300 mg I/kg) and 2 mL/kg (600 mg I/kg) of body weight for between agents and at various doses to gain additional iohexol. The 24-h radioiodide thyroid uptake was determined knowledge about the length of time and degree of interfer- after 131I was given at 1, 8, 15, and 22 d after administration of interfering agents. Results: The percentage radioiodide uptake ence of agents on RAIU in the thyroid. The focus of this value for the thyroid decreased significantly compared with investigation was on the utilization of the euthyroid rat as a controls for all agents and both doses on day 1 but returned to model for studies and to obtain data on the influence of control levels by day 22 for all agents and both doses The time several agents, each at 2 dose levels, on the degree and to return to normal varied between agents and doses. Conclu- length of time of interference in radioiodide uptake in the sion: We conclude that the interfering agent, the dose given, thyroid. Agents were selected because of problems docu- and the length of time after administration influence the poten- tial for an agent to affect radioiodide uptake in the thyroid. mented in the literature (1–10) and the continued utilization Further studies with the rat, preferably hyperthyroid, would be of the agents in medical practice. A dose representing a beneficial in generating data to reduce confusing contradictory clinical situation, based on weight, was selected as well as information on the length and severity of interference of agents a lower dose to ascertain a dose effect. The length of the in radioiodide thyroid studies. study was based on documented reports, though variable, of Key Words: euthyroid rat; radioiodide thyroid uptake; interfer- the influence of the agents on radioiodide thyroid uptake in ing agents patients. J Nucl Med 2003; 44:832–838 MATERIALS AND METHODS Animals It is well known that radioiodide and radioactive iodide Male Sprague–Dawley rats weighing 220–240 g were obtained uptake (RAIU) studies are important in nuclear medicine to from Harlan Sprague–Dawley, Inc. (Indianapolis, IN). Five ani- mals for each treatment or control group were housed together by aid in the diagnosis of hyperthyroid conditions. Also, the random selection in stainless steel cages measuring 23.5 ϫ 21 ϫ percentage uptake value is known to be important in the 28 cm and maintained at a constant room temperature of 21°C– calculation of the radioiodide dose given to the patient for 23°C with a 12:12 artificial lighting cycle (0600–1800 h). Each treatment of hyperthyroidism. individual cage contained a steel food hopper and water bottle. Various agents have been found to interfere with the Diet (Rodent Laboratory Chow no. 5001; Purina Mills, Inc., St. RAIU study leading to a reduction in the percentage uptake Louis, MO) and tap water were provided ad libitum during a 5-d value because of the presence of the agent. These potential acclimation period. The iodine content of the diet was approxi- mately 0.6 mg/kg. Animals were housed in quarters approved by the American Association for Accreditation of Laboratory Animal 131 Received June 3, 2002; revision accepted Nov. 27, 2002. Care. For each determination of the uptake of I in the thyroid, For correspondence or reprints contact: Stanley M. Shaw, PhD, Division of the rats were fasted overnight and only water was supplied. Over Nuclear Pharmacy, Industrial and Physical Pharmacy, Purdue University, RHPH-308, 575 Stadium Mall Dr., West Lafayette, IN 47907-2051. the time of the entire study, the animals weighed 250–370 g. Each E-mail: [email protected] group remained together throughout the study. The study was 832 THE JOURNAL OF NUCLEAR MEDICINE • Vol. 44 • No. 5 • May 2003 performed with appropriate animal care approved by the Purdue measured in the syringe, was truly administered. As measured by University Animal Research Council. a dose calibrator, the average activity administered to the rats in the study was 148 Ϯ 3.7 kBq. The 100% dose counting rate was Agents and Experimental Design determined for each radioiodide uptake study with three 0.2-mL Potassium iodide ([KI] lot no. 97012-1; Mallinckrodt Chemical radioiodide solution aliquots obtained in the same manner as the Works, St. Louis, MO) was selected as 1 of the interfering agents doses that were delivered to rats. because it is often used in nuclear medicine studies to block radioiodide uptake by the thyroid from 123I- or 131I-labeled radio- Shielding Block pharmaceuticals. PTU (lot no. 76H2500; Sigma Chemical Co., St. To block radiation from regions other than the neck of the rat, Louis, MO) was selected because it inhibits the metabolic synthe- a custom shielding block was prepared. The block was prepared sis of thyroid hormones, and 2 radiopaque contrast agents were with a low-melting-point alloy called a Lipowitz metal with a used in the study because of their high iodide content. Each density of 9.4 g/cm3 and consisting of 50% bismuth, 26.7% lead, treatment or control group was comprised of 5 animals. KI was 13.3% tin, and 10% cadmium. The block can be cast to custom fit given to 1 group of rats orally by intubation needle 2 h before the a region by pouring melted Lipowitz metal into a polystyrene first administration of Na131I solution, whereas PTU was admin- form. Calculations for the thickness of the shielding block were istered orally once daily to another group for 5 d with the last dose based on the activity administered to the rat and the mass attenu- given 2 h before the first administration of radioactive iodide. Two ation coefficients for the materials in the Lipowitz metal. A con- groups of 5 rats received 60% diatrizoate meglumine (Hypaque, lot servative thickness of 1.5 cm was selected. The opening in the no. M270LP; Sanofi Winthrop Pharmaceuticals, New York, NY) shielding block for the determination of activity from the thyroid or iohexol (Omnipaque, lot no. B815NP; Nycomed Inc., New in the neck of the rat was based on actual anatomic evaluations. York, NY) intravenously 24 h before the first administration of 131I. Doses investigated were 1 and 2 mg KI/kg of body weight, 3.5 Geometry and Attenuation Correction 131 and 7 mg/kg of body weight for PTU, 1 mL/kg (282 mg I/kg) and Because the percentage uptake of I in the thyroid of the rat 2 mL/kg (564 mg I/kg) of body weight for diatrizoate meglumine, was calculated based on the relative counts using a sample of the and 1 mL/kg (300 mg I/kg) and 2 mL/kg (600 mg I/kg) of body radioactive solution administered to the rat, the attenuation of weight for iohexol. For each agent, a dose representing a clinical radiation by tissue and the influence of geometry were considered. situation, based on weight, was selected as well as a lower dose to A 0.6-cm thickness of polyacrylamide was used to correct for allow comparisons of dose on radioiodide uptake. geometry and attenuation when obtaining the 100% dose counting Twenty-four hours after the first administration of the Na131I data. The selection of polyacrylamide was based on the mass ␥ solution, percentage thyroid uptake values were determined by attenuation coefficient of the material for the 364-keV -energy 131 counting the neck region of the anesthetized rat with a shielding from I being somewhat equal to tissue. Calculations were con- block (Lipowitz metal) to prevent the detection of radioactivity firmed by actual measurements with tissue covering the thyroid of from other regions within the rat. To correct for geometry and the rat. Additionally, the thickness of the hairy skin and other attenuation, 0.6 cm of polyacrylamide was used to allow for tissue covering the thyroid was measured to determine the influ- accurate counting of the aliquot of radioiodide solution used to ence of geometry. obtain the 100% dose counting rate administered to the rat. The Preliminary Study percentage uptake values in the thyroid were based on relating A preliminary study was performed to assess the accuracy of thyroid counts to the counts from the 100% dose.