Facile synthesis of SSR180575 and discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide, a potent pyridazinoindole ligand for PET imaging of TSPO in cancer

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Abstract

A novel synthesis of the translocator protein (TSPO) ligand 7-chloro-N,N,5-trimethyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (SSR180575, 3) was achieved in four steps from commercially available starting materials. Focused structure–activity relationship development about the pyridazinoindole ring at the N3 position led to the discovery of 7-chloro-N,N,5-trimethyl-4-oxo-3(6-fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide (14), a novel ligand of comparable affinity. Radiolabeling with fluorine-18 (18F) yielded 7-chloro-N,N,5-trimethyl-4-oxo-3(6-[18F]fluoropyridin-2-yl)-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide ([18F]-14) in high radiochemical yield and specific activity. In vivo studies of [18F]-14 revealed this agent as a promising probe for molecular imaging of glioma.

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Acknowledgment

We acknowledge funding from the National Institutes of Health (K25 CA127349, P50 CA128323, S10 RR17858, U24 CA126588, 1R01 CA163806, P50 CA095103, P30 DK058404), The Kleberg Foundation, and The Lustgarten Foundation. Dr. M. Noor Tantawy, George H. Wilson, and Dr. Daniel Colvin are acknowledged for support of the imaging studies.

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