Model based analysis of antagonist binding kinetics at CRF-1 receptors in vitro and in vivo
Neil J Attkins(1), Simeon Ramsey(2), Rebecca Fish(2), Piet H. van der Graaf(1)
Pharmacokinetics, Dynamics and Metabolism (1) & Discovery Biology(2), Sandwich Research Unit, Pfizer, Ramsgate Road, Sandwich, Kent, UK, CT13 9NJ
Objectives: Both in-house and literature1data show that CRF-1 antagonists display differing degrees of insurmountable antagonism in standard competition binding assays. We have optimised the use and analysis of data from a non-equilibrium binding assay to measure the kinetics of these compounds2. The kinetically derived association and dissociation rates were used in conjunction with the compounds pharmacokinetic parameters in the rat to simulate the receptor occupancy vs. time profiles. Our aim was to use these simulations as a replacement for in vivo receptor occupancy studies to enable faster triage of compounds with slow offset at an early stage of discovery, to enable quicker progression to compound selection and first time in man. Compounds with slower off-set from the receptor have the potential to sustain the duration of efficacy due to an increased residence time at the receptor.
Methods: Data from the non-equilibrium binding assays was fitted in NONMEM v6.2 to obtain estimates of the compounds association and dissociation rates3. The receptor occupancy versus time simulations were performed in Berkley Madonna.
Results: The kinetic parameters derived from the non-equilibrium binding assay made it possible to classify compounds as slow, moderate or fast dissociating compounds. With the exception of DMP-904, all of the in vitro association rates were slower than in vivo and the simulations tended to either under-estimate the degree of occupancy or take longer to reach the maximum occupancy level. In contrast to this, the dissociation rates were better estimated and the rank order of the compounds dissociation half-life was translatable in vitro to in vivo.
Conclusions: Whilst there appeared to be a consistent discrepancy between in vitro and in vivo receptor association rates, the rank order of the compounds in terms of their rate of dissociation from the CRF-1 receptor translated well. Therefore, this PKPD model based approach proved to be useful to triage between compounds at an early stage of the project where it is not feasible to perform in vivo receptor occupancy studies on a large number of compounds.
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