Theoretical aspects of P-glycoprotein mediated drug efflux on the distribution volume of anaesthetic-related drugs in the brain

Blood-brain barrier: mechanisms governing permeability and interaction with peripherally acting μ-opioid receptor antagonists

The blood-brain barrier (BBB) describes the unique properties of endothelial cells (ECs) that line the central nervous system (CNS) microvasculature. The BBB supports CNS homeostasis via EC-associated transport of ions, nutrients, proteins and waste products between the brain and blood. These transport mechanisms also serve as physiological barriers to pathogens, toxins and xenobiotics to prevent them from contacting neural tissue. The mechanisms that govern BBB permeability pose a challenge to drug design for CNS disorders, including pain, but can be exploited to limit the effects of a drug to the periphery, as in the design of the peripherally acting μ-opioid receptor antagonists (PAMORAs) used to treat opioid-induced constipation. Here, we describe BBB physiology, drug properties that affect BBB penetrance and how data from randomized clinical trials of PAMORAs improve our understanding of BBB permeability.

Knockout of P-glycoprotein does not alter antiepileptic drug efficacy in the intrahippocampal kainate model of mesial temporal lobe epilepsy in mice

Pharmacoresistance to antiepileptic drugs (AEDs) is a major challenge in epilepsy therapy, affecting at least 30% of patients. Thus, there is considerable interest in the mechanisms responsible for such pharmacoresistance, with particular attention on the specific cellular and molecular factors that lead to reduced drug sensitivity. Current hypotheses of refractory epilepsy include the multidrug transporter hypothesis, which posits that increased expression or function of drug efflux transporters, such as P-glycoprotein (Pgp), in brain capillaries reduces the local concentration of AEDs in epileptic brain regions to subtherapeutic levels. In the present study, this hypothesis was addressed by evaluating the efficacy of six AEDs in wildtype and Pgp deficient Mdr1a/b(-/-) mice in the intrahippocampal kainate model of mesial temporal lobe epilepsy. In this model, frequent focal electrographic seizures develop after an initial kainate-induced status epilepticus. These seizures are resistant to major AEDs, but the mechanisms of this resistance are unknown. In the present experiments, the focal nonconvulsive seizures were resistant to carbamazepine and phenytoin, whereas high doses of valproate and levetiracetam exerted moderate and phenobarbital and diazepam marked anti-seizure effects. All AEDs suppressed generalized convulsive seizures. No significant differences between wildtype and Pgp-deficient mice were observed in anti-seizure drug efficacies. Also, the individual responder and nonresponder rates in each experiment did not differ between mouse genotypes. This does not argue against the multidrug transporter hypothesis in general, but indicates that Pgp is not involved in the mechanisms explaining that focal electrographic seizures are resistant to some AEDs in the intrahippocampal mouse model of partial epilepsy. This was substantiated by the finding that epileptic wildtype mice do not exhibit increased Pgp expression in this model.

Challenges of using in vitro data for modeling P-glycoprotein efflux in the blood-brain barrier

The efficacy of central nervous system (CNS) drugs may be limited by their poor ability to cross the bloodbrain barrier (BBB). Transporters, such as p-glycoprotein, may affect the distribution of many drugs into the CNS in conjunction with the restricted paracellular pathway of the BBB. It is therefore important to gain information on unbound drug concentrations in the brain in drug development to ensure sufficient drug exposure from plasma at the target site in the CNS. In vitro methods are routinely used in drug development to study passive permeability and p-glycoprotein efflux of new drugs. This review discusses the challenges in the use of in vitro data as input parameters in physiologically based pharmacokinetic (PBPK) models of CNS drug disposition of p-glycoprotein substrates. Experience with quinidine demonstrates the variability in in vitro parameters of passive permeability and active pglycoprotein efflux. Further work is needed to generate parameter values that are independent of the model and assay. This is a prerequisite for reliable predictions of drug concentrations in the brain in vivo.

Pharmacokinetic consequences of active drug efflux at the blood-brain barrier

PURPOSE

The objective of this simulation study was to investigate how the nature, location, and capacity of the efflux processes in relation to the permeability properties influence brain concentrations.

METHODS

Reduced brain concentrations can be due to either influx hindrance, a gatekeeper function in the luminal membrane, which has been suggested for ABCB1 (P-glycoprotein), or efflux enhancement by transporters that pick up molecules on one side of the luminal or abluminal membrane and release them on the other side. Pharmacokinetic models including passive transport, influx hindrance, and efflux enhancement were built using the computer program MATLAB. The simulations were based on experimentally obtained parameters for morphine, morphine-3-glucuronide, morphine-6-glucuronide, and gabapentin.

RESULTS

The influx hindrance process is the more effective for keeping brain concentrations low. Efflux enhancement decreases the half-life of the drug in the brain, whereas with influx hindrance the half-life is similar to that seen with passive transport. The relationship between the influx and efflux of the drug across the blood-brain barrier determines the steady-state ratio of brain to plasma concentrations of unbound drug, K(p,uu).

CONCLUSIONS

Both poorly and highly permeable drugs can reach the same steady-state ratio, although the time to reach steady state will differ. The volume of distribution of unbound drug in the brain does not influence K(p,uu), but does influence the total brain-to-blood ratio K(p) and the time to reach steady state in the brain.

Role of drug efflux transporters in the brain for drug disposition and treatment of brain diseases

The blood-brain barrier (BBB) serves as a protective mechanism for the brain by preventing entry of potentially harmful substances from free access to the central nervous system (CNS). Tight junctions present between the brain microvessel endothelial cells form a diffusion barrier, which selectively excludes most blood-borne substances from entering the brain. Astrocytic end-feet tightly ensheath the vessel wall and appear to be critical for the induction and maintenance of the barrier properties of the brain capillary endothelial cells. Because of these properties, the BBB only allows entry of lipophilic compounds with low molecular weights by passive diffusion. However, many lipophilic drugs show negligible brain uptake. They are substrates for drug efflux transporters such as P-glycoprotein (Pgp), multidrug resistance proteins (MRPs) or organic anion transporting polypeptides (OATPs) that are expressed at brain capillary endothelial cells and/or astrocytic end-feet and are key elements of the molecular machinery that confers the special permeability properties to the BBB. The combined action of these carrier systems results in rapid efflux of xenobiotics from the CNS. The objective of this review is to summarize transporter characteristics (cellular localization, specificity, regulation, and potential inhibition) for drug efflux transport systems identified in the BBB and blood-cerebrospinal fluid (CSF) barrier. A variety of experimental approaches available to ascertain or predict the impact of efflux transport on brain access of therapeutic drugs also are described and critically discussed. The potential impact of efflux transport on the pharmacodynamics of agents acting in the CNS is illustrated. Furthermore, the current knowledge about drug efflux transporters as a major determinant of multidrug resistance of brain diseases such as epilepsy is reviewed. Finally, we summarize strategies for modulating or by-passing drug efflux transporters at the BBB as novel therapeutic approaches to drug-resistant brain diseases.

Blood-brain barrier active efflux transporters: ATP-binding cassette gene family

The blood-brain barrier (BBB) contributes to brain homeostasis by protecting the brain from potentially harmful endogenous and exogenous substances. BBB active drug efflux transporters of the ATP-binding cassette (ABC) gene family are increasingly recognized as important determinants of drug distribution to, and elimination from, the CNS. The ABC efflux transporter P-glycoprotein (Pgp) has been demonstrated as a key element of the BBB that can actively transport a huge variety of lipophilic drugs out of the brain capillary endothelial cells that form the BBB. In addition to Pgp, other ABC efflux transporters such as members of the multidrug resistance protein (MRP) family and breast cancer resistance protein (BCRP) seem to contribute to BBB function. Consequences of ABC efflux transporters in the BBB include minimizing or avoiding neurotoxic adverse effects of drugs that otherwise would penetrate into the brain. However, ABC efflux transporters may also limit the central distribution of drugs that are beneficial to treat CNS diseases. Furthermore, neurological disorders such as epilepsy may be associated with overexpression of ABC efflux transporters at the BBB, resulting in pharmacoresistance to therapeutic medication. Therefore, modulation of ABC efflux transporters at the BBB forms a novel strategy to enhance the penetration of drugs into the brain and may yield new therapeutic options for drug-resistant CNS diseases.

Determinants of drug onset

PURPOSE OF REVIEW

The timing and magnitude of drug onset can be influenced by factors in the chain of drug delivery from the site of administration to the site of effect. This review examines recent evidence regarding the contribution and significance of these factors.

RECENT FINDINGS

It is apparent that drug formulations and mixtures can play a significant role in drug onset. An extension of this is the effect of coadministration of drugs, which can influence drug effect both by altering the physiology underlying drug delivery and by an effect at the target organ. Of the physiological variables, cardiac output and its distribution are clearly important. Cardiac output is a significant source of variability in drug response, and indeed has been successfully incorporated into pharmacokinetic models. The pattern of cardiac output distribution is also relevant. In particular, the blood flow to target organs will influence both the timing and magnitude of the effect of some anaesthetic drugs. In addition, the role of the lung in affecting drug distribution may be important for some drugs. At the site or organ of effect itself, variability in drug distribution, drug-receptor interactions, and the influence of other drugs, can all impact on the profile of drug onset.

SUMMARY

Factors in the chain of drug delivery have been demonstrated to affect the nature of drug onset, and can account for some of the observed variability in response. The significance to dosing guidelines and strategies, and to predictions of variability in response, remains to be explored.