David J. Spalding

Improving the in Vitro Prediction of in Vivo Central Nervous System Penetration: Integrating Permeability, P-glycoprotein Efflux, and Free Fractions in Blood and Brain

This work examines the inter-relationship between the unbound drug fractions in blood and brain homogenate, passive membrane permeability, P-glycoprotein (Pgp) efflux ratio, and log octanol/water partition coefficients (cLogP) in determining the extent of central nervous system (CNS) penetration observed in vivo. The present results demonstrate that compounds often considered to be Pgp substrates in rodents (efflux ratio greater than 5 in multidrug resistant Madin-Darby canine kidney cells) with poor passive permeability may still exhibit reasonable CNS penetration in vivo; i.e., where the unbound fractions and nonspecific tissue binding act as a compensating force. In these instances, the efflux ratio and in vitro blood-brain partition ratio may be used to predict the in vivo blood-brain ratio. This relationship may be extended to account for the differences in CNS penetration observed in vivo between mdr1a/b wild type and knockout mice. In some instances, cross-species differences that might initially seem to be related to differing transporter expression can be rationalized from knowledge of unbound fractions alone. The results presented in this article suggest that the information exists to provide a coherent picture of the nature of CNS penetration in the drug discovery setting, allowing the focus to be shifted away from understanding CNS penetration toward the more important aspect of understanding CNS efficacy.

Correlation between in vitro and in vivo concentration–effect relationships of naproxen in rats and healthy volunteers

1 Understanding the mechanisms underlying the analgesic effect of new cyclooxygenase inhibitors is essential to identify dosing requirements in early stages of drug development. Accurate extrapolation to humans of in vitro and in vivo findings in preclinical species is needed to optimise dosing regimen in inflammatory conditions. 2 The current investigation characterises the inhibition of prostaglandin E2 (PGE2) and thromboxane B2 (TXB2) by naproxen in vitro and in vivo in rat and human blood. The inhibition of PGE2 in the absence or presence of increasing concentrations of naproxen (10−8–10−1 M) was measured by ex vivo whole blood stimulation with LPS, whereas inhibition of TXB2 was measured in serum following blood clotting. In further experiments, inhibition of PGE2 and TXB2 levels was also assessed ex vivo in animals treated with naproxen (2.5, 10, 25 mg kg−1). Subsequently, pharmacokinetic (PK)/pharmacodynamics (PD) modelling of in vitro and in vivo data was performed using nonlinear mixed effects in NONMEM (V). 3 Inhibition of PGE2 and TXB2 was characterised by a sigmoid Emax model. The exposure–response relationships in vitro and in vivo were of the same order of magnitude in both species. IC80 estimates obtained in vitro were similar for PGE2 inhibition (130.8±11 and 131.9±19 10−6 M, mean±s.d. for humans and rats, respectively), but slightly different for TXB2 inhibition (103.9±15 and 151.4±40 10−6 M, mean±s.d. for humans and rats, respectively, P< 0.05). These differences, however, may not be biologically relevant. 4 The results confirm the value of exposure–effect relationships determined in vitro as a means to predict the pharmacological activity in vivo. This analysis also highlights the need to parameterise concentration–effect relationships in early drug development, as indicated by the estimates of IC80 for PGE2 and TXB2 inhibition.

Differences in the sensitivity of behavioural measures of pain to the selectivity of cyclo‐oxygenase inhibitors

Objectives: Freunds complete adjuvant (FCA) is an animal model of inflammatory pain commonly used in the screening of COX‐inhibitors. However, there is little understanding of how behavioural measures of the anti‐inflammatory effect in the FCA model correlate to differences in mechanism of action and whether such endpoints equally reflect drug activity in humans. In the current investigation we evaluate the time course of the analgesic effect for different endpoints after treatment with drugs with varying degrees of selectivity for COX‐1 and COX‐2. We also assess prostaglandin (PGE2) and thromboxane (TXB2) inhibition to establish the correlation between behavioural measures and the degree of selectivity for COX‐1 and COX‐2.

Population pharmacokinetic modelling of the enterohepatic recirculation of diclofenac and rofecoxib in rats: Modelling of enterohepatic recirculation in rats

Enterohepatic recirculation (EHC) is a common pharmacokinetic phenomenon that has been poorly modelled in animals. The presence of EHC leads to the appearance of multiple peaks in the concentration‐time profile and increased exposure, which may have implications for drug effect and extrapolation across species. The aim of this investigation was to develop a population pharmacokinetic model for diclofenac and rofecoxib that describes EHC and to assess its consequence for the pharmacodynamics of both drugs.

Orally active C-6 heteroaryl- and heterocyclyl-substituted imidazo[1,2-a]pyridine acid pump antagonists (APAs).

Acid pump antagonists (APAs) such as the imidazo[1,2-a]pyridine AZD-0865 2 have proven efficacious at low oral doses in acid related gastric disorders. Herein we describe some of the broader SAR in this class of molecule and detail the discovery of an imidazo[1,2-a]pyridine 15 which has excellent efficacy in animal models of gastric acid secretion following oral administration, as well as a good overall developability profile. The discovery strategy focuses on use of heteroaryl and heterocyclic substituents at the C-6 position and optimization of developability characteristics through modulation of global physico-chemical properties.

Impact of chronic inflammation on the pharmacokinetic–pharmacodynamic relationship of naproxen

Objectives The use of biomarkers for predicting the clinical doses of analgesic drugs relies on the understanding of the relationship between drug exposure and response under disease conditions. In this study, we demonstrate the relevance of such a relationship for COX‐inhibitors by modelling the effect of naproxen on prostaglandin E2 and thromboxane B2 in a chronic inflammation model in rats.

Discovery of {4-[4,9-bis(ethyloxy)-1-oxo-1,3-dihydro-2H-benzo[f]isoindol-2-yl]-2-fluorophenyl}acetic acid (GSK726701A), a novel EP 4 receptor partial agonist for the treatment of pain

A novel series of EP4 agonists and antagonists have been identified, and then used to validate their potential in the treatment of inflammatory pain. This paper describes these novel ligands and their activity within a number of pre-clinical models of pain, ultimately leading to the identification of the EP4 partial agonist GSK726701A.