Radioiodination and Biological Evaluation of Tizanidine as a Potential Brain Imaging Agent

Labeling was carried out by direct iodination of tizanidine (100 μg) with radioiodine (¹²⁵ I) in a fast single step at room temperature, to produce ¹²⁵ I-tizanidine (¹²⁵ I-TZN). 50 μg chloramine-T (CAT) was used as an oxidizing agent to oxidize the iodide ion to the iodonium ion, at neutral ph = 7 within 15 min. A high radiochemical yield of 92.8% ± 0.1 was obtained. ¹²⁵ I-TZN was stable for 2 h without detection of any by-products in the reaction mixture. The partition coefficient value of ¹²⁵ I-TZN was 2.21 ± 0.02, showing that it is very lipophilic and can easily cross the blood brain barrier. Biodistribution studies and in vivo imaging showed that the initial brain uptake correlated fairly well with the brain-binding affinity of the compound. The brain uptake of ¹²⁵ I-TZN was as high as 5.2% and 8.0% in biodistribution studies and in vivo imaging at 120 min post injection, respectively. Thus, ¹²⁵ I-TZN is promising in radioreceptor assays for brain imaging