Scott Summerfield

Central Nervous System Drug Disposition: The Relationship between in Situ Brain Permeability and Brain Free Fraction

The dispositions of 50 marketed central nervous system (CNS) drugs into the brain have been examined in terms of their rat in situ (P) and in vitro apparent membrane permeability (Papp) alongside lipophilicity and free fraction in rat brain tissue. The inter-relationship between these parameters highlights that both permeability and brain tissue binding influence the uptake of drugs into the CNS. Hydrophilic compounds characterized by low brain tissue binding display a strong correlation (R2 = 0.82) between P and Papp, whereas the uptake of more lipophilic compounds seems to be influenced by both Papp and brain free fraction. A nonlinear relationship is observed between logPoct and P over the 6 orders of magnitude range in lipophilicity studied. These findings corroborate recent reports in the literature that brain penetration is a function of both rate and extent of drug uptake into the CNS.

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.

Toward an improved prediction of human in vivo brain penetration

The penetration of drugs into the central nervous system is a composite of both the rate of drug uptake across the blood–brain barrier and the extent of distribution into brain tissue compartments. Clinically, positron emission tomography (PET) is the primary technique for deriving information on drug biodistribution as well as target receptor occupancy. In contrast, rodent models have formed the basis for much of the current understanding of brain penetration within pharmaceutical Drug Discovery. Linking these two areas more effectively would greatly improve the translation of candidate compounds into therapeutic agents. This paper examines two of the major influences on the extent of brain penetration across species, namely plasma protein binding and brain tissue binding. An excellent correlation was noted between unbound brain fractions across species (R2 > 0.9 rat, pig, and human, n = 21), which is indicative of the high degree of conservation of the central nervous system environment. In vitro estimates of human brain–blood or brain–plasma ratios of marketed central nervous system drugs and PET tracers agree well with in vivo values derived from clinical PET and post-mortem studies. These results suggest that passive diffusion across the blood–brain barrier is an important process for many drugs in humans and highlights the possibility for improved prediction of brain penetration across species.

Assessment of the blood–brain barrier in CNS drug discovery

A wide variety of models have been developed over the years to predict blood-brain barrier (BBB) penetration, most of them have focussed on predicting total concentrations of drug and then expressing this as a brain:blood (or plasma) ratio. This approach is somewhat flawed and fails to address the critical issue of understanding the relationship between access of free drug to the requisite site of action. In this short review, we highlight the need for an integrated approach and whilst blood-brain barrier permeability is an important determinant in achieving efficacious CNS drug concentrations it should not be viewed or measured in isolation. Optimal CNS penetration is achieved through the correct balance of permeability, a low potential for active efflux and the appropriate physicochemical properties that allow for drug partitioning and distribution into brain tissue. Such an approach should enhance and accelerate our understanding and ability to predict CNS efficacy in terms of free drug concentrations and the rate at which they are achieved.

Investigation into the role of P2X3/P2X2/3 receptors in neuropathic pain following chronic constriction injury in the rat: an electrophysiological study

1 Two P2X3/P2X2/3 receptor antagonists with different potencies were profiled electrophysiologically in a rat model of nerve injury. 2 A‐317491 has poor CNS penetrance (blood : brain, 1 : <0.05), and was therefore administered intravenously in chronic constriction injury (CCI)‐ and sham‐operated rats to study the involvement of P2X3 subunit‐containing receptors in the periphery in neuropathic pain. A‐317491 and Compound A were administered topically to the spinal cord to investigate the central contribution. 3 There were no significant inhibitory effects of A‐317491 intravenous (i.v.) seen in sham‐operated animals compared to vehicle controls. In CCI‐operated animals, there were significant inhibitory effects of 3 mg kg−1 A‐317491 i.v. on C fibre‐evoked responses, and with 10 mg kg−1 A‐317491 i.v. on Aδ and C fibre‐evoked responses. No significant effects of A‐317491 were observed after topical application to the spinal cord. In contrast, when Compound A was administered spinally in CCI animals, there was a decrease in Aδ and C fibre‐evoked responses, and wind up. 4 These changes indicate that A‐317491 has a selective effect on neuronal responses in CCI animals compared to sham, demonstrating an increased involvement of P2X3/P2X2/3 receptors in sensory signalling following nerve injury. In addition, the more potent antagonist Compound A was effective spinally, unmasking a potential central role of P2X3/P2X2/3 receptors at this site post nerve injury. These data support a role for P2X3/P2X2/3 antagonists in the modulation of neuropathic pain.

Combining PET biodistribution and equilibrium dialysis assays to assess the free brain concentration and BBB transport of CNS drugs.

The passage of drugs in and out of the brain is controlled by the blood—brain barrier (BBB), typically, using either passive diffusion across a concentration gradient or active transport via a protein carrier. In-vitro and preclinical measurements of BBB penetration do not always accurately predict the in-vivo situation in humans. Thus, the ability to assay the concentration of novel drug candidates in the human brain in vivo provides valuable information for derisking of candidate molecules early in drug development. Here, positron emission tomography (PET) measurements are combined with in-vitro equilibrium dialysis assays to enable assessment of transport and estimation of the free brain concentration in vivo. The PET and equilibrium dialysis data were obtained for 36 compounds in the pig. Predicted P-glycoprotein (P-gp) status of the compounds was consistent with the PET/equilibrium dialysis results. In particular, Loperamide, a well-known P-gp substrate, exhibited a significant concentration gradient consistent with active efflux and after inhibition of the P-gp process the gradient was removed. The ability to measure the free brain concentration and assess transport of novel compounds in the human brain with combined PET and equilibrium dialysis assays can be a useful tool in central nervous system (CNS) drug development.

Discovery DMPK: changing paradigms in the eighties, nineties and noughties

Background: The science of drug metabolism and pharmacokinetics (DMPK) plays a critical role in supporting the selection of potent, selective leads that retain the appropriate physicochemical properties to ensure distribution from the site of administration into the tissue or target of interest. Historically, Discovery DMPK has bridged the gap between the disciplines of biology and medicinal chemistry thereby ensuring a clinical focus during the discovery and early development phases. Objective: Here we discuss the fundamentals of DMPK screening in drug discovery from a historical perspective, highlighting DMPKs part in improving the chances of success for novel drug candidates and suggesting new and exciting areas for future development. Conclusions: Such a broad remit has resulted in the development of a wide variety of assays, both in vitro and in vivo, focused on assessing the developability risks associated with a molecules progression into clinical development, such as likely bioavailability in humans, the potential for drug–drug interactions, human metabolism, interactions with transporters and the potential for metabolism-mediated idiosyncratic toxicity. Arguably DMPK has already adopted many of the concepts now of interest in translational medicine and quantitative pharmacology while scientific and regulatory pressures continue to drive the subject towards better and more integrated approaches, such as systems thinking.

In vitro, in vivo and in silico models of drug distribution into the brain

Achieving sufficient brain penetration to elicit efficacy in humans is one of the most challenging tasks for scientists in CNS Drug Discovery. Substantial progress has been made in the past decade in understanding the factors influencing the rate and extent of brain distribution via a variety of in vivo, in vitro and in silico methodologies, and hence, predict their likelihood of success in man. This purpose of this review is to summarize the current approaches with a special focus on parameters related to free drug concentrations in brain which are the most pharmacologically relevant for the majority of CNS disease targets. Due to the dynamic and complex nature of this targeted organ, it is inevitable that these approaches have not been able to provide a fully comprehensive assessment of brain distribution and are expected to evolve further in the years to come.

Characterization of three diaminopyrimidines as potent and selective antagonists of P2X3 and P2X2/3 receptors with in vivo efficacy in a pain model

BACKGROUND AND PURPOSE P2X3 and P2X2/3 receptors are highly localized on the peripheral and central pathways of nociceptive signal transmission. The discovery of A‐317491 allowed their validation as chronic inflammatory and neuropathic pain targets, but this molecule has a very limited oral bioavailability and CNS penetration. Recently, potent P2X3 and P2X2/3 blockers with a diaminopyrimidine core group and better bioavailability were synthesized and represent a new opportunity for the validation of P2X3‐containing receptors as targets for pain. Here we present a characterization of three representative diaminopyrimidines.

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.