Synthesis of Halogen-Substituted Pyridyl and Pyrimidyl Derivatives of [3,2-c]Pyrazolo Corticosteroids:  Strategies for the Development of Glucocorticoid Receptor Mediated Imaging Agents

Ligands for the glucocorticoid receptor labeled with high-energy isotopes are highly desired for their potential applications in nuclear medical studies of the brain where the dysregulation of this receptor system is thought to be involved in various neurodegenerative disorders. Analogues of the glucocorticoid cortivazol have previously been prepared as target compounds for labeling with high-energy isotopes. However, the phenyl rings of arylpyrazoles of this type are not sufficiently activated for nucleophilic substitution reactions that are generally required for the synthesis of radiohalogenated analogues. Since suitably substituted aromatic nitrogen heterocyclic groups are amenable to nucleophilic substitution, the goal of this study was the synthesis of pyridylpyrazolo and pyrimidylpyrazolo analogues similar to cortivazol that could be labeled with radiohalogens in the pyridine or pyrimidine rings. We describe the synthesis of several [3,2-c]pyrazolo steroids containing pyridyl, halopyridyl, and pyrimidyl substituents at the 2' position of the pyrazole ring. These compounds were tested for binding to the glucocorticoid receptor and for biological activity in glucocorticoid responsive HeLa cells grown in tissue culture. Of the pyridyl and pyrimidyl derivatives, 2'-(3-pyridyl)-11 beta,17,21-trihydroxy-16 alpha-methyl-20-oxopregn-4-eno[3,2-c]pyrazole showed superior activity in both assays and it was used as the basis for the synthesis of several analogues that were halogenated in the pyridine ring. These halogenated compounds were all tested for their binding to the glucocorticoid receptor and for their biological activity. One, a fluorinated compound 2'-(2-fluoro-5-pyridyl)-11 beta,17,21-trihydroxy-16 alpha-methyl-20-oxopregn-4-eno[3,2-c]pyrazole had excellent activity, considerably better than the potent glucocorticoid dexamethasone. Most importantly, fluorination was achieved using a nucleophilic exchange reaction, a method that is adaptable to radiolabeling with the positron-emitting isotope fluorine-18. Thus, considering its superior biological activity and adaptability for facile radiosynthesis, this target compound has the potential for imaging of glucocorticoid receptor containing tissues using positron emission tomography.