Synthesis, F-18 radiolabeling, and microPET evaluation of 3-(2,4-dichlorophenyl)-N-alkyl-N-fluoroalkyl-2,5-dimethylpyrazolo[1,5-a]pyrimidin-7-amines as ligands of the corticotropin-releasing factor type-1 (CRF1) receptor

Abstract A series of 3-(2,4-dichlorophenyl)- N -alkyl- N -fluoroalkyl-2,5-dimethylpyrazolo[1,5- a ]pyrimidin-7-amines were synthesized and evaluated as potential positron emission tomography (PET) tracers for the corticotropin-releasing factor type-1 (CRF 1 ) receptor. Compounds 27 , 28 , 29 , and 30 all displayed high binding affinity (⩽1.2 nM) to the CRF 1 receptor when assessed by in vitro competition binding assays at 23 °C, whereas a decrease in affinity (⩾10-fold) was observed with compound 26 . The log  P 7.4 values of [ 18 F] 26 –[ 18 F] 29 were in the range of ∼2.2–2.8 and microPET evaluation of [ 18 F] 26 –[ 18 F] 29 in an anesthetized male cynomolgus monkey demonstrated brain penetrance, but specific binding was not sufficient enough to differentiate regions of high CRF 1 receptor density from regions of low CRF 1 receptor density. Radioactivity uptake in the skull, and sphenoid bone and/or sphenoid sinus during studies with [ 18 F] 28 , [ 18 F] 28- d 8 , and [ 18 F] 29 was attributed to a combination of [ 18 F]fluoride generated by metabolic defluorination of the radiotracer and binding of intact radiotracer to CRF 1 receptors expressed on mast cells in the bone marrow. Uptake of [ 18 F] 26 and [ 18 F] 27 in the skull and sphenoid region was rapid but then steadily washed out which suggests that this behavior was the result of binding to CRF 1 receptors expressed on mast cells in the bone marrow with no contribution from [ 18 F]fluoride.