Synthesis of a high specific activity 125I-labeled analog of PK 11195, potential agent for SPECT imaging of the peripheral benzodiazepine binding site.

The peripheral benzodiazepine binding site ligand PK 11195 has been 125I-labeled by direct displacement of aromatic chlorine under solid-state conditions in 50-76% radiochemical yield and greater than 94% radiochemical purity. Purification by high pressure liquid chromatography increased the specific activity of the product from an initial 15-17 Ci/mmol to a final activity of 260-910 Ci/mmol. To determine the affinity of this [125I]PK 11195 analog for human glioma cells, saturation experiments were performed on monolayers of U251 human glioblastoma cells. Scatchard analysis of saturation data demonstrated that the [125I]PK 11195 analog binds to a single class of sites with a KD of 8.0 +/- 1.7 nM and maximal binding of 3.8 +/- 0.1 pmol/mg protein. These values are similar to those obtained when [3H]PK 11195 was assayed in U251 cells (KD = 14 +/- 3.4, Bmax = 4.1 +/- 1.3) suggesting that iodination does not appreciably alter the binding of PK 11195 to human glioma cells. In vivo autoradiographic studies of brain in C6 glioma bearing rats demonstrate selective binding of the radioligand to the tumor. These results suggest that this [125I]PK 11195 analog may be a useful radiotracer for the study of peripheral benzodiazepine binding sites.