Radioiodinated L-703,606: a potent, selective antagonist to the human NK1 receptor

A new, radioiodinated, NK1 selective radiotracer ([125I]L-703,606) was prepared. L-703,606 is an iodinated analog of the NK1 antagonist CP-96,345 in which the methoxy group has been replaced by an iodine substituent. [125I]L-703,606 was made from the corresponding trimethylsilyl compound by treatment with no carrier added Na 125I and an Iodobead in TFA. The tracer was prepared at a specific activity of approx. 1100 Ci/mmol and preliminary binding studies demonstrated that [125I]L-703,606 binds selectively to NK1 receptors with a kd = 0.3 nM. These results suggest that this radioligand will be useful for the biochemical and pharmacological characterization of the human NK1 receptor and, if labeled with I-123, may be useful for non-invasive NK1 receptor imaging via SPECT.