Target site occupancy: emerging generalizations from clinical and preclinical studies.

What percentage of receptors, ion channels or transporters must be occupied by drugs to trigger therapeutic effects in patients, or by drugs and other ligands to induce physiological effects in humans or animals? Human studies utilizing Positron Emission Tomography and Single Photon Emission Computed Tomography, along with data from an array of preclinical methodologies, have begun to provide consistent answers to this question. The required target occupancy is dependent upon the molecular class of both target and ligand, and appears to be similar for both patient therapy and human or animal physiology. In the case of antagonists, approximately 60-90% target occupancy is required for G protein-coupled receptors, neurotransmitter transporters, and ligand-gated ion channels. Effective doses of agonists occupy a wider range of their target sites, dependent upon the intrinsic activity of the agonist, the receptor or ion channel reserve of the target site, and the response that is measured, with low efficacy agonists generally requiring high degrees of occupancy while high efficacy agonists generally require low degrees of occupancy. Target desensitization, competition by endogenous ligands, and regional target differences all influence target occupancy requirements. Measurements of target occupancy can help assure proper dosing and targeting of compounds in preclinical and clinical drug development as well as in basic research. Target occupancy generalizations can be especially important in establishing initial dosing recommendations for the many new drug targets provided by genomic and proteomic initiatives, where little data is available on their functional responses.

Anxiolytic-like effects of the corticotropin-releasing factor1 (CRF1) antagonist DMP904 [4-(3-pentylamino)-2,7-dimethyl-8-(2-methyl-4-methoxyphenyl)-pyrazolo-[1,5-a]-pyrimidine] administered acutely or chronically at doses occupying central CRF1 receptors in rats.

Corticotropin-releasing factor(1) (CRF(1)) antagonists may be effective in the treatment of anxiety disorders with fewer side effects compared with classic benzodiazepines. The behavioral effects of DMP904 [4-(3-pentylamino)-2,7-dimethyl-8-(2-methyl-4-methoxyphenyl)-pyrazolo-[1,5-a]-pyrimidine] and its effects on the hypothalamic-pituitary-adrenal (HPA) axis were related to its levels in plasma and estimated occupancy of central CRF(1) receptors. DMP904 (10-30 mg/kg, p.o.) and alprazolam (10 mg/kg, p.o.) increased time spent in open arms of an elevated-plus maze. In addition, acutely or chronically (14 days) administered DMP904 (1.0-30 mg/kg, p.o.) and acute alprazolam (1.0-3.0 mg/kg, p.o.) significantly reduced exit latency in the defensive withdrawal model of anxiety in rats, suggesting that tolerance may not develop to the anxiolytic-like effects of DMP904 in this model of anxiety. Acutely, DMP904 reversed the stress-induced increase in plasma corticosterone levels in defensive withdrawal at doses of 3.0 mg/kg and higher. These doses also resulted in levels of DMP904 in plasma similar to (for anxiolytic-like effects) or 4-fold higher (for effects on the HPA axis) than the in vitro IC(50) value for binding affinity at CRF(1) receptors and greater than 50% occupancy of CRF(1) receptors. Unlike alprazolam, DMP904 did not produce sedation, ataxia, or chlordiazepoxide-like subjective effects (as measured by locomotor activity, rotorod performance, and chlordiazepoxide discrimination assays, respectively) at doses at least 3-fold higher than anxiolytic-like doses. In conclusion, anxiolytic-like effects and effects on the stress-activated HPA axis of DMP904 in the defensive withdrawal model of anxiety required 50% or greater occupancy of central CRF(1) receptors. This level of CRF(1) receptor occupancy resulted in fewer motoric side effects compared with classic benzodiazepines.

Imidazo[4,5-b]pyridines as corticotropin releasing factor receptor ligands.

A series of high affinity CRF receptor ligands with an imidazo[4,5-b]pyridine core is described. Individual analogues were synthesized and tested in a rat CRF receptor binding assay. The best compounds were further tested in the dog N-in-1 pharmacokinetic model to assess plasma levels at 1mg/kg (po) and in the rat situational anxiety model to assess anxiolytic efficacy at 3mg/kg (po). The structure-activity relationships for good receptor binding affinity are described herein.

Imidazo[4,5-c]pyridines as corticotropin releasing factor receptor ligands.

A series of high affinity CRF receptor ligands with an imidazo[4,5-c]pyridine core is described. Individual analogues were synthesized and tested in vitro in rat brain receptors to determine binding affinity. The best compound was further tested in the dog N-in-1 pharmacokinetic model to assess oral bioavailability at 1 mg/kg po.