PET Imaging of CRF1 with [11C]R121920 and [11C]DMP696: is the target of sufficient density?

Abstract

Aim-Overstimulation of the CRF type 1 receptor (CRF1) is implicated in anxiety and depressive disorders. The aim of this study was to investigate the in vivo binding characteristics of [ 11 C]R121920 and [ 11 C]DMP696 in the non-human primate for application in positron emission tomography (PET) studies of CRF1.Methods-PET imaging with the two novel CRF1 radioligands was performed in baboon. In vitro binding studies for CRF1 were performed in postmortem brain tissue of baboon and human to assess sufficiency of receptor density for PET.Results-Both [ 11 C]R121920 and [ 11 C]DMP696 distributed rapidly and uniformly throughout brain. Washout was comparable across brain regions, without differences in volume of distribution between regions reported to have high and low in vitro CRF1 binding. Membrane-enriched tissue homogenate assay using [ 125 I]Tyr 0 -sauvagine and specific CRF1 antagonists CP154,526 and SN003 in human occipital cortex yielded maximal binding (B max ) of 63.3 and 147.3 fmol/mg protein, respectively, and in human cerebellar cortex yielded B max of 103.6 and 64.6 fmol/mg protein, respectively. Dissociation constants (K D ) were subnanomolar. In baboon, specific binding was not detectable in the same regions; therefore B max and K D were not measurable. Autoradiographic results were consistent except there was also detectable CRF1-specific binding in baboon cerebellum. Conclusion-Neither[ 11 C]R121920 nor [ 11 C]DMP696 demonstrated quantifiable regional binding in vivo in baboon. In vitro results suggest CRF1 density in baboon may be insufficient for PET. Studies in man may generate more promising results due to the higher CRF1 density compared with baboon in cerebral cortex and cerebellum.

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

“…82 Compounds [ 11 C] 6 , [ 11 C] 7 , and [ 11 C] 8 all penetrated the BBB of a baboon but did not display specific binding and were rapidly metabolized. 84, 85 Compounds [ 18 F] 9 – [ 18 F] 11 and [ 11 C] 12 have recently been reported, and these compounds were able to penetrate the BBB of a monkey but they did not display specific binding. 87, 88 Thus, attempts to develop a viable CRF 1 receptor radiotracer have been hampered by low brain entry, little or no in vivo specific binding when brain entry is achieved, and rapid metabolism.…”

Non-peptide ligands in the characterization of peptide receptors at the interface between neuroendocrine and mental diseases

“…11 C]R121919 were not successful, probably due to low receptor density of CRF1 subtype in these animals [62].…”

“…in the pituitary gland [59,60]. It is released from the neurons of the paraventricular region of the hypothalamus into the median eminence region which is one of the seven known circumventricular regions of the brain with interruption of the blood brain barrier [8,19,44,61,62]. Here, the paraventricular region is in close contact to the capillary plexus drained by long portal veins into the anterior pituitary a main target region of CRF [22].…”

“…Two G-protein coupled receptor subtypes CRF1 and CRF2, which are members of the B-receptor family [64][65][66][67][68][69], as well as the CRF-binding protein [60] [62,63,74] corresponding to the expression levels of the CRF1-R (see also Table 1). Hypersecretion of CRF has been postulated to be involved in the pathogenesis of several mood and anxiety disorders [5,15,75,76].…”

A Biomathematical Modeling Approach to Central Nervous System Radioligand Discovery and Development

“…B max values were obtained from the literature (9/10 targets) except for the B max of GlyT1 in the human cortex, which was measured in-house using homogenate binding assays. Among those obtained from literature, the density of 5-HT 2c was measured from rabbit homogenate (17), the density of PDE4 was from rat homogenate (18), and the density of the other targets was measured in human homogenate (19)(20)(21)(22)(23)(24) (one other exception is that the B max for D 3 in human pallidum was determined using autoradiography (25) in the absence of any homogenate binding data). Free fractions f P and f ND of all the molecules were measured in Yorkshire/Danish Landrace pig blood and brain tissue using equilibrium dialysis (12).…”