The Na(+)/I(-) symporter (NIS) mediates iodide uptake in the thyroid gland as well as in other NIS-expressing cells. This transport is the basis for an emerging approach to selective cancer cell destruction by using radioiodide after targeted NIS gene transfer. Therapeutic efficacy requires that radioiodide retention be maximized in tumor cells. A first generation of forty imidazo[2,1-b]thiazole derivatives as iodide efflux inhibitors is described along with the evaluation of their biological properties. Structure-activity relationship studies by using radioiodide uptake in rat thyroid-derived cells (FRTL5) revealed that the 5,6-dihydroimidazo[2,1-b]thiazole heterocycle is required for activity. Introduction of electron-donor substituents on the 3-biphenyl moiety led to the discovery of novel potent compounds. A compound was identified with enhanced potency compared to reference 1. These molecules give the possibility to increase the cellular retention of radioiodide in NIS-expressing tumors, leading to higher absorbed doses and killing efficacy.