The rats were then killed by exsanguination for organ weight measurement and histopathological/immunohistochemical examinations. == Hematology == The parameters determined were red (RBC) and white WDR5-0103 blood cell (WBC) counts, hemoglobin (HGB), hematocrit (HCT), mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), mean corpuscular hemoglobin concentration (MCHC) and platelet (PTL) count, which were determined using an automated hematology analyzer (K-4500, Sysmex Corp., Hyogo, Japan). and an increase in the number of IgG-positive plasma cells in the red pulp; these changes were most prevalent in the MMI-treated female rats. Total red and white blood cell counts were decreased in the MMI-treated male and female rats; lymphocytes and monocytes were lower in male and female rats, respectively. Bone marrow nucleated cells were significantly lower in the MMI-treated males. This is the first study to demonstrate that ATDs induce spleen specific B-cell reactions in rats Keywords:antithyroid drug, methimazole, propylthiouracil, spleen, germinal center, autoantibody == Introduction == Methimazole (MMI) and propylthiouracil (PTU) are antithyroid drugs (ATDs) that have been used to treat hyperthyroidism for more than several decades. However, ATDs are associated with potentially life-threatening adverse drug reactions (ADRs), including agranulocytosis, hepatotoxicity and vasculitis that is often associated with acute renal and respiratory symptoms; minor ADRs include skin reactions and arthralgia1. The incidence of agranulocytosis and vasculitis is rare; both are thought to be autoimmune mediated since anti-granulocyte autoantibodies, such as anti-neutrophil cytoplasmic antibodies (ANCAs), and myeloperoxidase (MPO), one of the major antigens of ANCA, are often detected in hyperthyroid patients taking ATDs1,2. Typically, drug-induced autoimmunity is idiosyncratic; because the incidence is very low and highly species specific, it is difficult to predict in preclinical studies3,4and is therefore excluded from the scope of the ICH S8 guideline on immunotoxicity studies for human pharmaceuticals5. Germinal centers are main sites in which high affinity Rabbit polyclonal to NOTCH4 antibody-secreting plasma cells and memory B cells are generated6. The development of germinal centers is therefore reflective of enhanced antibody responses to antigens draining the regional lymphoid tissues. To date, there are no reports that ATDs induce germinal center development in lymphoid tissues in rodent preclinical studies, although only a few compounds have been reported to cause such lesions7. Thus, the aim of this study was to examine the effect of ATDs on histopathology in peripheral lymphoid organs in rats. We also examined the effects of the drugs on hematologic parameters. == Materials and WDR5-0103 Methods == == Test substances == MMI and PTU were purchased from Sigma-Aldrich Japan (Tokyo, Japan), dissolved in propylene glycol at the dosing volume of 5 mL/kg body weight, stored at 4C in the dark and used within a week after preparation. == Animals == Male and female Crl:CD(SD) rats were purchased from Charles River Laboratories Japan Inc. (Kanagawa, Japan) at five weeks of age. The rats were housed individually in plastic cages (265 425 200 mm) embedded with ALPHA-dri (Shepherd Specialty Papers Inc., Kalamazoo, MI, USA), fed CE-2 solid chow (Oriental Yeast Co., Ltd., Chiba, Japan) and given tap waterad libitum. The animal room conditions were as follows: 1) temperature, 2026C; 2) relative humidity, 4075%; 3) ventilation, 15 to 25 air changes per hour; and 4) lighting, 07:00 to 19:00. Serological, bacteriological and microscopic examinations of sentinel rats kept simultaneously in the same animal room for health monitoring revealed no signs of major viral, bacterial or parasitic infections. == Treatment == After one week of acclimatization, rats were divided into the vehicle- (propylene glycol; control) and ATD-treated groups (n=5/sex/group) for each of the following three experiments. In experiments 1 and 2, MMI (2, 20 and 200 mg/kg/day) and PTU WDR5-0103 (25 and 250 mg/kg/day), respectively, were administered by gavage once daily for 14 days. In experiment 3, according to the results of experiments 1 and 2, MMI and PTU were administered to rats of both sexes at the dose levels.