l-METHYL-2-MERCAPTOIMIDAZOLE: A NEW ACTIVE ANTITHYROID AGENT*

SINCE thiouracil, the first antithyroid drug, became available, additional related compounds have received clinical trial. Chief among the group (Table 1) are thiourea, thiobarbital, aminothiouracil, propylthiouracil, methylthiouracil, iodothiouracil, Meprocil (a combination of methylthiouracil and propylthiouracil) and the newest member, 1-methyl- 2-mercaptoimidazole (Tapazole).1 As each new antithyroid drug becomes available it not only promptly receives clinical trial to determine its antithyroid effectiveness and dosage but, more important, the incidence of toxicity, since there is a constant effort to find a drug which can be safely administered. When propylthiouracil became available and the incidence of reactions was found to be only 1.6 per cent, the use of thiouracil and thiobarbital was abandoned because of their higher reaction rates. Similarly, methylthiouracil is not routinely used because it has a reaction incidence of 13 per cent. Clinical experience is now being gained with the newest antithyroid drug, l-methyl-2-mercaptoimidazole (Tapazole). This substance was investigated by Stanley and Astwood (1) and found to have pronounced antithyroid activity. In testing this drug by comparing its inhibitory effect on radioactive iodine uptake with that of thiouracil, it was found to be 100 times as effective as thiouracil. These workers reported that preliminary studies on 30 patients with hyperthyroidism treated with this compound confirmed this high degree of effectiveness.