Transport of propranolol and lidocaine through the rat blood-brain barrier. Primary role of globulin-bound drug

Plasma Protein-Mediated Uptake and Contradictions to the Free Drug Hypothesis: A Critical Review

According to the free drug hypothesis (FDH), only free, unbound drug is available to interact with biological targets. This hypothesis is the fundamental principle that continues to explain the vast majority of all pharmacokinetic and pharmacodynamic processes. Under the FDH, the free drug concentration at the target site is considered the driver of pharmacodynamic activity and pharmacokinetic processes. However, deviations from the FDH are observed in hepatic uptake and clearance predictions, where observed unbound intrinsic hepatic clearance (CL_int,u) is larger than expected. Such deviations are commonly observed when plasma proteins are present and form the basis of the so-called plasma protein-mediated uptake effect (PMUE). This review will discuss the basis of plasma protein binding as it pertains to hepatic clearance based on the FDH, as well as several hypotheses that may explain the underlying mechanisms of PMUE. Notably, some, but not all, potential mechanisms remained aligned with the FDH. Finally, we will outline possible experimental strategies to elucidate PMUE mechanisms. Understanding the mechanisms of PMUE and its potential contribution to clearance underprediction is vital to improving the drug development process.

Physiologically Based Pharmacokinetic Model of Brain Delivery of Plasma Protein Bound Drugs

INTRODUCTION

A physiologically based pharmacokinetic (PBPK) model is developed that focuses on the kinetic parameters of drug association and dissociation with albumin, alpha-1 acid glycoprotein (AGP), and brain tissue proteins, as well as drug permeability at the blood-brain barrier, drug metabolism, and brain blood flow.

GOAL

The model evaluates the extent to which plasma protein-mediated uptake (PMU) of drugs by brain influences the concentration of free drug both within the brain capillary compartment in vivo and the brain compartment. The model also studies the effect of drug binding to brain tissue proteins on the concentration of free drug in brain.

METHODS

The steady state and non-steady state PBPK models are comprised of 11-12 variables, and 18-23 parameters, respectively. Two model drugs are analyzed: propranolol, which undergoes modest PMU from the AGP-bound pool, and imipramine, which undergoes a high degree of PMU from both the albumin-bound and AGP-bound pools in plasma.

RESULTS

The free propranolol concentration in brain is under-estimated 2- to fourfold by in vitro measurements of free plasma propranolol, and the free imipramine concentration in brain is under-estimated by 18- to 31-fold by in vitro measurements of free imipramine in plasma. The free drug concentration in brain in vivo is independent of drug binding to brain tissue proteins.

CONCLUSIONS

In vitro measurement of free drug concentration in plasma under-estimates the free drug in brain in vivo if PMU in vivo from either the albumin and/or the AGP pools in plasma takes place at the BBB surface.

Developmental changes in the extent of drug binding to rat plasma proteins

Binding of therapeutics to proteins in blood plasma is important in influencing their distribution as it is their free (unbound) form that is able to cross cellular membranes to enter tissues and exert their actions. The concentration and composition of plasma proteins vary during pregnancy and development, resulting in potential changes to drug protein binding. Here, we describe an ultrafiltration method to investigate the extent of protein binding of six drugs (digoxin, paracetamol, olanzapine, ivacaftor, valproate and lamotrigine) and two water soluble inert markers (sucrose and glycerol) to plasma proteins from pregnant and developing rats. Results showed that the free fraction of most drugs was lower in the non-pregnant adult plasma where protein concentration is the highest. However, plasma of equivalent protein concentration to younger pups obtained by diluting adult plasma did not always exhibit the same extent of drug binding, reinforcing the likelihood that both concentration and composition of proteins in plasma influence drug binding. Comparison between protein binding and brain drug accumulation in vivo revealed a correlation for some drugs, but not others. Results suggests that plasma protein concentration should be considered when using medications in pregnant and paediatric patients to minimise potential for fetal and neonatal drug exposure.

Natural Blockers of PD-1/PD-L1 Interaction for the Immunotherapy of Triple-Negative Breast Cancer-Brain Metastasis

The limited treatment options for triple-negative breast cancer with brain metastasis (TNBC-BM) have left the door of further drug development for these patients wide open. Although immunotherapy via monoclonal antibodies has shown some promising results in several cancers including TNBC, it cannot be considered the most effective treatment for brain metastasis. This is due to the protective role of the blood-brain barrier (BBB) which limits the entrance of most drugs, especially the bulky ones such as antibodies, to the brain. For a drug to traverse the BBB via passive diffusion, various physicochemical properties should be considered. Since natural medicine has been a key inspiration for the development of the majority of current medicines, in this paper, we review several naturally-derived molecules which have the potential for immunotherapy via blocking the interaction of programmed cell death protein-1 (PD-1) and its ligand, PD-L1. The mechanism of action, physicochemical properties and pharmacokinetics of these molecules and their theoretical potential to be used for the treatment of TNBC-BM are discussed.

A Historical Review of Brain Drug Delivery

The history of brain drug delivery is reviewed beginning with the first demonstration, in 1914, that a drug for syphilis, salvarsan, did not enter the brain, due to the presence of a blood-brain barrier (BBB). Owing to restricted transport across the BBB, FDA-approved drugs for the CNS have been generally limited to lipid-soluble small molecules. Drugs that do not cross the BBB can be re-engineered for transport on endogenous BBB carrier-mediated transport and receptor-mediated transport systems, which were identified during the 1970s-1980s. By the 1990s, a multitude of brain drug delivery technologies emerged, including trans-cranial delivery, CSF delivery, BBB disruption, lipid carriers, prodrugs, stem cells, exosomes, nanoparticles, gene therapy, and biologics. The advantages and limitations of each of these brain drug delivery technologies are critically reviewed.

Blockade of β-Adrenergic Receptors by Propranolol Disrupts Reconsolidation of Drug Memory and Attenuates Heroin Seeking

Persistent traces of drug reward memories contribute to intense craving and often trigger relapse. A number of pharmacological interventions on drug-associated memories have shown significant benefits in relapse prevention at a preclinical level but their translational potential is limited due to deleterious side effects. Propranolol, a non-specific β-adrenergic receptors antagonist, is known for its ability to erase maladaptive memories associated with nicotine or cocaine in rodents and humans. However, little is known about its effect on reconsolidation of heroin memory and heroin seeking. In the present study, rats with a history of intravenous heroin self-administration received the propranolol treatment (10 mg/kg; i.p.) at different time windows with or without CS (conditioned stimulus) exposure. Our results showed that propranolol, when administered immediately after CS exposure but not 6 h later, can significantly attenuate cue-induced and drug-primed reinstatement of heroin seeking, suggesting that propranolol has the ability to disrupt heroin memory and reduce relapse. The propranolol treatment without retrieval of drug memory had no effect on subsequent reinstatement of heroin seeking, suggesting that its interfering effects are retrieval-dependent. Importantly, the effects of propranolol were long lasting as rats showed diminished drug seeking even 28 days after the treatment. Altogether, our study suggests that propranolol can interfere with reconsolidation of heroin memory and reduce subsequent drug seeking, making it an attractive therapeutic candidate for the treatment of opioid addiction and relapse prevention.

Corneal sensitivity is required for orientation in free-flying migratory bats

The exact anatomical location for an iron particle-based magnetic sense remains enigmatic in vertebrates. For mammals, findings from a cornea anaesthesia experiment in mole rats suggest that it carries the primary sensors for magnetoreception. Yet, this has never been tested in a free-ranging mammal. Here, we investigated whether intact corneal sensation is crucial for navigation in migrating Nathusius' bats, Pipistrellus nathusii, translocated from their migratory corridor. We found that bats treated with corneal anaesthesia in both eyes flew in random directions after translocation and release, contrasting bats with a single eye treated, and the control group, which both oriented in the seasonally appropriate direction. Using a Y-maze test, we confirmed that light detection remained unaffected by topical anaesthesia. Therefore our results suggest the cornea as a possible site of magnetoreception in bats, although other conceivable effects of the anaesthetic are also explored. Furthermore, we demonstrate that the corneal based sense is of bilateral nature but can function in a single eye if necessary.

Alpha-1 Acid Glycoprotein as a Biomarker for Subclinical Illness and Altered Drug Binding in Rats

Alpha-1 acid glycoprotein (AGP) is a significant drug binding acute phase protein that is present in rats. AGP levels are known to increase during tissue injury, cancer and infection. Accordingly, when determining effective drug ranges and toxicity limits, consideration of drug binding to AGP is essential. However, AGP levels have not been well established during subclinical infections. The goal of this study was to establish a subclinical infection model in rats using AGP as a biomarker. This information could enhance health surveillance, aid in outlier identification, and provide more informed characterization of drug candidates. An initial study (n = 57) was conducted to evaluate AGP in response to various concentrations of Staphylococcus aureus (S. aureus) in Sprague-Dawley rats with or without implants of catheter material. A model validation study (n = 16) was then conducted using propranolol. Rats received vehicle control or S. aureus and when indicated, received oral propranolol (10 mg/kg). Health assessment and blood collection for measurement of plasma AGP or propranolol were performed over time (days). A dose response study showed that plasma AGP was elevated on day 2 in rats inoculated with S. aureus at 10⁶, 10⁷ or, 10⁸ CFU regardless of implant status. Furthermore, AGP levels remained elevated on day 4 in rats inoculated with 10⁷ or 10⁸ CFUs of S. aureus. In contrast, significant increases in AGP were not detected in rats treated with vehicle or 10³ CFU S. aureus. In the validation study, robust elevations in plasma AGP were detected on days 2 and 4 in S. aureus infected rats with or without propranolol. The AUC levels for propranolol on days 2 and 4 were 493 ± 44 h × ng/mL and 334 ± 54 h × ng/mL, respectively), whereas in noninfected rats that received only propranolol, levels were 38 ± 11 h × ng/mL and 76 ± 16. h × ng/mL, respectively. The high correlation between plasma propranolol and AGP demonstrated a direct impact of AGP on drug pharmacokinetics and pharmacodynamics. The results indicate that AGP is a reliable biomarker in this model of subclinical infection and should be considered for accurate data interpretation.

The search for brain-permeant NKCC1 inhibitors for the treatment of seizures: Pharmacokinetic-pharmacodynamic modelling of NKCC1 inhibition by azosemide, torasemide, and bumetanide in mouse brain

Because of its potent inhibitory effect on the Na⁺-K⁺-2Cl^- symporter isotype 1 (NKCC1) in brain neurons, bumetanide has been tested with varying results for treatment of seizures that potentially evolve as a consequence of abnormal NKCC1 activity. However, because of its physicochemical properties, bumetanide only poorly penetrates into the brain. We previously demonstrated that NKCC1 can be also inhibited by azosemide and torasemide, which lack the carboxyl group of bumetanide and thus should be better brain-permeable. Here we studied the brain distribution kinetics of azosemide and torasemide in comparison with bumetanide in mice and used pharmacokinetic-pharmacodynamic modelling to determine whether the drugs reach NKCC1-inhibitory brain concentrations. All three drugs hardly distributed into the brain, which seemed to be the result of probenecid-sensitive efflux transport at the blood-brain barrier. When fractions unbound in plasma and brain were determined by equilibrium dialysis, only about 6-17% of the brain drug concentration were freely available. With the systemic doses (10 mg/kg i.v.) used, free brain concentrations of bumetanide and torasemide were in the NKCC1-inhibitory concentration range, while levels of azosemide were slightly below this range. However, all three drugs exhibited free plasma levels that would be sufficient to block NKCC1 at the apical membrane of brain capillary endothelial cells. These data suggest that azosemide and torasemide are interesting alternatives to bumetanide for treatment of seizures involving abnormal NKCC1 functionality, particularly because of their longer duration of action and their lower diuretic potency, which is an advantage in patients with seizures.

Eyes are essential for magnetoreception in a mammal

Several groups of mammals use the Earth's magnetic field for orientation, but their magnetosensory organ remains unknown. The Ansell's mole-rat (Fukomys anselli, Bathyergidae, Rodentia) is a microphthalmic subterranean rodent with innate magnetic orientation behaviour. Previous studies on this species proposed that its magnetoreceptors are located in the eye. To test this hypothesis, we assessed magnetic orientation in mole-rats after the surgical removal of their eyes compared to untreated controls. Initially, we demonstrate that this enucleation does not lead to changes in routine behaviours, including locomotion, feeding and socializing. We then studied magnetic compass orientation by employing a well-established nest-building assay under four magnetic field alignments. In line with previous studies, control animals exhibited a significant preference to build nests in magnetic southeast. By contrast, enucleated mole-rats built nests in random magnetic orientations, suggesting an impairment of their magnetic sense. The results provide robust support for the hypothesis that mole-rats perceive magnetic fields with their minute eyes, probably relying on magnetite-based receptors in the cornea.

Biological barriers, and the influence of protein binding on the passage of drugs across them

Drug-protein binding plays a key role in determining the pharmacokinetics of a drug. The distribution and protein binding ability of a drug changes over a lifetime, and are important considerations during pregnancy and lactation. Although proteins are a significant fraction in plasma composition, they also exist beyond the bloodstream and bind with drugs in the skin, tissues or organs. Protein binding influences the bioavailability and distribution of active compounds, and is a limiting factor in the passage of drugs across biological membranes and barriers: drugs are often unable to cross membranes mainly due to the high molecular mass of the drug-protein complex, thus resulting in the accumulation of the active compounds and a significant reduction of their pharmacological activity. This review describes the consequences of drug-protein binding on drug transport across physiological barriers, whose role is to allow the passage of essential substances-such as nutrients or oxygen, but not of xenobiotics. The placental barrier regulates passage of xenobiotics into a fetus and protects the unborn organism. The blood-brain barrier is the most important barrier in the entire organism and the skin separates the human body from the environment.

The Presence of a Transporter-Induced Protein Binding Shift: A New Explanation for Protein-Facilitated Uptake and Improvement for In Vitro-In Vivo Extrapolation

Accurately predicting hepatic clearance is an integral part of the drug-development process, and yet current in vitro to in vivo (IVIVE) extrapolation methods yield poor predictions, particularly for highly protein-bound transporter substrates. Explanations for error include inaccuracies in protein-binding measurements and the lack of recognition of protein-facilitated uptake, where both unbound and bound drug may be cleared, violating the principles of the widely accepted free drug theory. A new explanation for protein-facilitated uptake is proposed here, called a transporter-induced protein binding shift High-affinity binding to cell-membrane proteins may change the equilibrium of the nonspecific binding between drugs and plasma proteins, leading to greater cellular uptake and clearance than currently predicted. The uptake of two lower protein-binding organic anion transporting polypeptide substrates (pravastatin and rosuvastatin) and two higher binding substrates (atorvastatin and pitavastatin) were measured in rat hepatocytes in incubations with protein-free buffer versus 100% plasma. Decreased unbound K _m values and increased intrinsic clearance values were seen in the plasma incubations for the highly bound compounds, supporting the new hypothesis and mitigating the IVIVE underprediction previously seen for highly bound transporter substrates.

An examination of protein binding and protein-facilitated uptake relating to in vitro-in vivo extrapolation

As explained by the free drug theory, the unbound fraction of drug has long been thought to drive the efficacy of a molecule. Thus, the fraction unbound term, or f_u, appears in equations for fundamental pharmacokinetic parameters such as clearance, and is used when attempting in vitro to in vivo extrapolation (IVIVE). In recent years though, it has been noted that IVIVE does not always yield accurate predictions, and that some highly protein bound ligands have more efficient uptake than can be explained by their unbound fractions. This review explores the evolution of f_u terms included when implementing IVIVE, the concept of protein-facilitated uptake, and the mechanisms that have been proposed to account for facilitated uptake.

Lidocaine is a nocebo treatment for trigeminally mediated magnetic orientation in birds

Even though previously described iron-containing structures in the upper beak of pigeons were almost certainly macrophages, not magnetosensitive neurons, behavioural and neurobiological evidence still supports the involvement of the ophthalmic branch of the trigeminal nerve (V1) in magnetoreception. In previous behavioural studies, inactivation of putative V1-associated magnetoreceptors involved either application of the surface anaesthetic lidocaine to the upper beak or sectioning of V1. Here, we compared the effects of lidocaine treatment, V1 ablations and sham ablations on magnetic field-driven neuronal activation in V1-recipient brain regions in European robins. V1 sectioning led to significantly fewer Egr-1-expressing neurons in the trigeminal brainstem than in the sham-ablated birds, whereas lidocaine treatment had no effect on neuronal activation. Furthermore, Prussian blue staining showed that nearly all iron-containing cells in the subepidermal layer of the upper beak are nucleated and are thus not part of the trigeminal nerve, and iron-containing cells appeared in highly variable numbers at inconsistent locations between individual robins and showed no systematic colocalization with a neuronal marker. Our data suggest that lidocaine treatment has been a nocebo to the birds and a placebo for the experimenters. Currently, the nature and location of any V1-associated magnetosensor remains elusive.

Computational investigations of physicochemical, pharmacokinetic, toxicological properties and molecular docking of betulinic acid, a constituent of Corypha taliera (Roxb.) with Phospholipase A2 (PLA2)

BACKGROUND

Betulinic acid (BA) is a natural triterpenoid compound and exhibits a wide range of biological and medicinal properties including anti-inflammatory activity. Therefore, this theoretical investigation is performed to evaluate (a) physicochemical properties such as acid dissociation constant (pKa), distribution coefficient (logD), partition coefficient (logP), aqueous solubility (logS), solvation free energy, dipole moment, polarizability, hyperpolarizability and different reactivity descriptors, (b) pharmacokinetic properties like human intestinal absorption (HIA), cellular permeability, skin permeability (P_Skin), plasma protein binding (PPB), penetration of the blood brain barrier (BBB), (c) toxicological properties including mutagenicity, carcinogenicity, risk of inhibition of hERG gene and (d) molecular mechanism of anti-inflammatory action which will aid the development of analytical method and the synthesis of BA derivatives.

METHODS

The physicochemical properties were calculated using MarvinSketch 15.6.29 and Gaussian 09 software package. The pharmacokinetic and toxicological properties were calculated on online server PreADMET. Further, the molecular docking study was conducted on AutoDock vina in PyRx 0.8.

RESULTS

The aqueous solubility increased with increasing pH due to the ionization of BA leading to decrease in distribution coefficient. The solvation energies in water, dimethyl sulfoxide (DMSO), acetonitrile, n-octanol, chloroform and carbon tetrachloride were - 41.74 kJ/mol, - 53.80 kJ/mol, - 66.27 kJ/mol, - 69.64 kJ/mol, - 65.96 kJ/mol and - 60.13 kJ/mol, respectively. From the results of polarizability and softness, it was clear that BA is less stable and hence, kinetically more reactive in water. BA demonstrated good human intestinal absorption (HIA) and moderate cellular permeability. Further, BA also exhibited positive CNS activity due to high permeability through BBB. The toxicological study revealed that BA was a mutagenic compound but noncarcinogenic in mice model. Moreover, molecular docking study of BA with PLA2 revealed that BA interacts with GLY22 & GLY29 through hydrogen bond formation and LEU2, PHE5, HIS6, ALA17, ALA18, HIS47 and TYR51 through different types of hydrophobic interactions. The binding affinity of BA was - 41.00 kJ/mol which is comparable to the binding affinity of potent inhibitor 6-Phenyl-4(R)-(7-Phenyl-heptanoylamino)-hexanoic acid (BR4) (- 33.89 kJ/mol).

CONCLUSIONS

Our computed properties may assist the development of analytical method to assay BA or to develop BA derivatives with better pharmacokinetic and toxicological profile.

Intracellular Unbound Atorvastatin Concentrations in the Presence of Metabolism and Transport

Accurate prediction of drug target activity and rational dosing regimen design require knowledge of drug concentrations at the target. It is important to understand the impact of processes such as membrane permeability, partitioning, and active transport on intracellular drug concentrations. The present study aimed to predict intracellular unbound atorvastatin concentrations and characterize the effect of enzyme-transporter interplay on these concentrations. Single-pass liver perfusion studies were conducted in rats using atorvastatin (ATV, 1 µM) alone at 4°C and at 37°C in presence of rifampin (RIF, 20 µM) and 1-aminobenzotriazole (ABT, 1 mM), separately and in combination. The unbound intracellular ATV concentration was predicted with a five-compartment explicit membrane model using the parameterized diffusional influx clearance, active basolateral uptake clearance, and metabolic clearance. Chemical inhibition of uptake and metabolism at 37°C proved to be better controls relative to studies at 4°C. The predicted unbound intracellular concentration at the end of the 50-minute perfusion in the +ABT , +ABT+RIF, and the ATV-only groups was 6.5 µM, 0.58 µM, and 5.14 µM, respectively. The predicted total liver concentrations and amount recovered in bile were within 0.94-1.3 fold of the observed value in all groups. The fold difference in total liver concentration did not always extrapolate to the fold difference in predicted unbound concentration across groups. Together, these results support the use of compartmental modeling to predict intracellular concentrations in dynamic organ-based systems. These predictions can provide insight into the role of uptake transporters and metabolizing enzymes in determining drug tissue concentrations.

Mechanism of brain targeting by dexibuprofen prodrugs modified with ethanolamine-related structures

The first molecular insights into how prodrugs modified with ethanolamine-related structures target the brain were generated using an in vitro BBB model and in situ perfusion technique. Prodrugs were delivered safely and efficiently to the brain through tight interaction with the anionic membrane of brain capillary endothelial cells, observed as a shift in zeta potential, followed by uptake into the cells. Prodrugs III and IV carrying primary and secondary amine modifications appeared to enter the brain via energy-independent passive diffusion. In contrast, besides the passive diffusion, prodrugs I and II carrying tertiary amine modifications also appeared to enter via an active process that was energy and pH dependent but was independent of sodium or membrane potential. This active process involved, at least in part, the pyrilamine-sensitive H(+)/OC antiporter, for which the N,N-diethyl-based compound II showed a much lower affinity than the N,N-dimethyl-based compound I, likely due to steric hindrance. These new insights into brain-targeting mechanisms may help guide efforts to design new prodrugs.

Brain-specific delivery of dopamine mediated by n,n-dimethyl amino group for the treatment of Parkinson's disease

Parkinson's disease (PD) has become one of the most deadly diseases due to a lack of effective treatment. Herein, N-3,4-bis(pivaloyloxy)dopamine-3-(dimethylamino)propanamide (PDDP), a brain-specific derivative of dopamine, was designed and synthesized, which consists of a brain targeted ligand, N,N-dimethyl amino group, and two dipivaloyloxy groups for lipophilic modification. PDDP was investigated both in vitro and in vivo by comparing with L-DOPA and another derivative (BPD) without N,N-dimethyl amino group. PDDP showed a more pronounced accumulation in mouse brain microvascular endothelial cells (bEnd.3) than BPD via an active transport process. The increased cellular uptake of PDDP was proven to be mediated by putative pyrilamine cationic transporters. Following intravenous administration, the concentration of PDDP in the brain was 269.28-fold and 6.41-fold higher than that of L-DOPA and BPD at 5 min, respectively. Additionally, PDDP effectively attenuated the striatum lesion caused by 6-hydroxydopamine (6-OHDA) in rats. More importantly, PDDP presented antioxidant and antiapoptotic effects on 6-OHDA-induced toxicity in human neuroblastoma cells (SH-SY5Y). Thus, N,N-dimethyl amino group-based PDDP represents an effective and safe treatment for PD.

Adrenergic and glucocorticoid modulation of the sterile inflammatory response

Exposure to an intense, acute stressor, in the absence of a pathogen, alters immune function. Exposure to a single bout of inescapable tail shock increases plasma and tissue concentrations of cytokines, chemokines, and the danger associated molecular pattern (DAMP) Hsp72. Although previous studies have demonstrated that adrenergic receptor (ADR) and glucocorticoid receptor (GCR)-mediated pathways alter pathogen or microbial associated molecular pattern (MAMP)-evoked levels of cytokines, chemokines, and Hsp72, far fewer studies have tested the role of these receptors across multiple inflammatory proteins or tissues to elucidate the differences in magnitude of stress-evoked sterile inflammatory responses. The goals of the current study were to (1) compare the sterile inflammatory response in the circulation, liver, spleen, and subcutaneous (SQ) adipose tissue by measuring cytokine, chemokine, and DAMP (Hsp72) responses; and (2) to test the role of alpha-1 (α1), beta-1 (β1), beta-2 (β2), and beta-3 (β3) ADRs, as well as GCRs in signaling the sterile inflammatory response. The data presented indicate plasma and SQ adipose are significantly more stress responsive than the liver and spleen. Further, administration of ADR and GCR-specific antagonists revealed both similarities and differences in the signaling mechanisms of the sterile inflammatory response in the tissues studied. Finally, given the selective increase in the chemokine monocyte chemotactic protein-1 (MCP-1) in SQ tissue, it may be that SQ adipose is an important site of leukocyte migration, possibly in preparation for infection as a consequence of wounding. The current study helps further our understanding of the tissue-specific differences of the stress-induced sterile inflammatory response.

Isoflurane induces learning impairment that is mediated by interleukin 1β in rodents

Postoperative cognitive decline is a clinical syndrome. Volatile anesthetics are commonly used during surgery. It is conceivable that volatile anesthetics may contribute to postoperative cognitive decline. Isoflurane can impair cognitive functions of animals under certain conditions. However, the mechanisms for this impairment are not clear. Here, male 18-month old Fisher 344 rats or 10-week old mice were exposed to 1.2 or 1.4% isoflurane for 2 h. Our studies showed that isoflurane impaired the cognitive functions of the rats in Barnes maze. Isoflurane-exposed rats had reduced freezing behavior during the training sessions in the fear conditioning test. This isoflurane effect was attenuated by lidocaine, a local anesthetic with anti-inflammatory property. Rats that had training sessions and were exposed to isoflurane 30 min later had freezing behavior similar to that of control animals. Isoflurane increased the expression of interleukin 1β (IL-1β), interleukin-6 and activated caspase 3 in the hippocampus of the 18-month old rats. IL-1β positive staining was co-localized with that of NeuN, a neuronal marker. The increase of IL-1β and activated caspase 3 but not interleukin-6 was attenuated by lidocaine. Isoflurane also impaired the cognitive functions of 10-week old C57BL/6J mice and increased IL-1β in their hippocampi. However, isoflurane did not affect the cognitive functions of IL-1β deficient mice. Our results suggest that isoflurane impairs the learning but may not affect the recall of the aged rats. IL-1β may play an important role in this isoflurane effect.

Overton's rule helps to estimate the penetration of anti-infectives into patients' cerebrospinal fluid

In 1900, Ernst Overton found that the entry of anilin dyes through the cell membranes of living cells depended on the lipophilicity of the dyes. The brain is surrounded by barriers consisting of lipid layers that possess several inward and outward active transport systems. In the absence of meningeal inflammation, the cerebrospinal fluid (CSF) penetration of anti-infectives in humans estimated by the ratio of the area under the concentration-time curve (AUC) in CSF (AUC(CSF)) to that in serum (AUC(CSF)/AUC(S)) correlated positively with the lipid-water partition coefficient at pH 7.0 (log D) (Spearman's rank correlation coefficient r(S) = 0.40; P = 0.01) and negatively with the molecular mass (MM) (r(S) = -0.33; P = 0.04). The ratio of AUC(CSF) to the AUC of the fraction in serum that was not bound (AUC(CSF)/AUC(S,free)) strongly correlated with log D (r(S) = 0.67; P < 0.0001). In the presence of meningeal inflammation, AUC(CSF)/AUC(S) also correlated positively with log D (r(S) = 0.46; P = 0.002) and negatively with the MM (r(S) = -0.37; P = 0.01). The correlation of AUC(CSF)/AUC(S,free) with log D (r(S) = 0.66; P < 0.0001) was as strong as in the absence of meningeal inflammation. Despite these clear correlations, Overton's rule was able to explain only part of the differences in CSF penetration of the individual compounds. The site of CSF withdrawal (lumbar versus ventricular CSF), age of the patients, underlying diseases, active transport, and alterations in the pharmacokinetics by comedications also appeared to strongly influence the CSF penetration of the drugs studied.

Dibucaine mitigates spreading depolarization in human neocortical slices and prevents acute dendritic injury in the ischemic rodent neocortex

Background

Spreading depolarizations that occur in patients with malignant stroke, subarachnoid/intracranial hemorrhage, and traumatic brain injury are known to facilitate neuronal damage in metabolically compromised brain tissue. The dramatic failure of brain ion homeostasis caused by propagating spreading depolarizations results in neuronal and astroglial swelling. In essence, swelling is the initial response and a sign of the acute neuronal injury that follows if energy deprivation is maintained. Choosing spreading depolarizations as a target for therapeutic intervention, we have used human brain slices and in vivo real-time two-photon laser scanning microscopy in the mouse neocortex to study potentially useful therapeutics against spreading depolarization-induced injury.

Methodology/Principal Findings

We have shown that anoxic or terminal depolarization, a spreading depolarization wave ignited in the ischemic core where neurons cannot repolarize, can be evoked in human slices from pediatric brains during simulated ischemia induced by oxygen/glucose deprivation or by exposure to ouabain. Changes in light transmittance (LT) tracked terminal depolarization in time and space. Though spreading depolarizations are notoriously difficult to block, terminal depolarization onset was delayed by dibucaine, a local amide anesthetic and sodium channel blocker. Remarkably, the occurrence of ouabain-induced terminal depolarization was delayed at a concentration of 1 µM that preserves synaptic function. Moreover, in vivo two-photon imaging in the penumbra revealed that, though spreading depolarizations did still occur, spreading depolarization-induced dendritic injury was inhibited by dibucaine administered intravenously at 2.5 mg/kg in a mouse stroke model.

Conclusions/Significance

Dibucaine mitigated the effects of spreading depolarization at a concentration that could be well-tolerated therapeutically. Hence, dibucaine is a promising candidate to protect the brain from ischemic injury with an approach that does not rely on the complete abolishment of spreading depolarizations.

Influence of changes in protein binding on the central activity of antidepressants

The central effect (expressed as analgesic response), protein binding and brain uptake of mianserin were measured in mice receiving drug intraperitoneally. A significant decrease of the central effect of mianserin (30 mg kg−1) was seen in mice with experimental inflammation when compared with control animals (reaction time (s)= 12·12 ± 1·22 vs 25·56 ± 2·92; P &lt; 0·001) and the dose-analgesia response curve (10−60 mg kg−1) was significantly shifted to the right in mice with inflammation. In serum of mice with inflammation, unbound concentration of mianserin was decreased from 19·37 ± 0·73 to 17·83 ± 0·30% (P &lt; 0·05) and seromucoid levels were significantly increased (P &lt; 0·001). Following the intraperitoneal administration of 30 mg kg−1 of mianserin, brain uptake decreased in diseased mice when compared with control animals (P &lt; 0·02), suggesting that the decrease in analgesia was secondary to a decrease in drug delivery to the brain because of increased protein binding.

Penetration of drugs through the blood-cerebrospinal fluid/blood-brain barrier for treatment of central nervous system infections

The entry of anti-infectives into the central nervous system (CNS) depends on the compartment studied, molecular size, electric charge, lipophilicity, plasma protein binding, affinity to active transport systems at the blood-brain/blood-cerebrospinal fluid (CSF) barrier, and host factors such as meningeal inflammation and CSF flow. Since concentrations in microdialysates and abscesses are not frequently available for humans, this review focuses on drug CSF concentrations. The ideal compound to treat CNS infections is of small molecular size, is moderately lipophilic, has a low level of plasma protein binding, has a volume of distribution of around 1 liter/kg, and is not a strong ligand of an efflux pump at the blood-brain or blood-CSF barrier. When several equally active compounds are available, a drug which comes close to these physicochemical and pharmacokinetic properties should be preferred. Several anti-infectives (e.g., isoniazid, pyrazinamide, linezolid, metronidazole, fluconazole, and some fluoroquinolones) reach a CSF-to-serum ratio of the areas under the curves close to 1.0 and, therefore, are extremely valuable for the treatment of CNS infections. In many cases, however, pharmacokinetics have to be balanced against in vitro activity. Direct injection of drugs, which do not readily penetrate into the CNS, into the ventricular or lumbar CSF is indicated when other effective therapeutic options are unavailable.

In vivo methods to study uptake of nanoparticles into the brain

Several in vivo techniques have been developed to study and measure the uptake of CNS compounds into the brain. With these techniques, various parameters can be determined after drug administration, including the blood-to-brain influx constant (K_in), the permeability-surface area (PS) product, and the brain uptake index (BUI). These techniques have been mostly used for drugs that are expected to enter the brain via transmembrane diffusion or by carrier-mediated transcytosis. Drugs that have limitations in entering the brain via such pathways have been encapsulated in nanoparticles (based on lipids or synthetic polymers) to enhance brain uptake. Nanoparticles are different from CNS compounds in size, composition and uptake mechanisms. This has led to different methods and approaches to study brain uptake in vivo. Here we discuss the techniques generally used to measure nanoparticle uptake in addition to the techniques used for CNS compounds. Techniques include visualization methods, behavioral tests, and quantitative methods.

Prediction of human pharmacokinetics—evaluation of methods for prediction of hepatic metabolic clearance

Methods for prediction of hepatic clearance (CLH) in man have been evaluated. A physiologically-based in-vitro to in-vivo (PB-IVIV) method with human unbound fraction in blood (fu,bl) and hepatocyte intrinsic clearance (CLint)-data has a good rationale and appears to give the best predictions (maximum ∼2-fold errors; &lt; 25% errors for half of CL-predictions; appropriate ranking). Inclusion of an empirical scaling factor is, however, needed, and reasons include the use of cryopreserved hepatocytes with low activity, and inappropriate CLint- and fu,bl-estimation methods. Thus, an improvement of this methodology is possible and required. Neglect of fu,bl or incorporation of incubation binding does not seem appropriate. When microsome CLint-data are used with this approach, the CLH is underpredicted by 5- to 9-fold on average, and a 106-fold underprediction (attrition potential) has been observed. The poor performance could probably be related to permeation, binding and low metabolic activity. Inclusion of scaling factors and neglect of fu,bl for basic and neutral compounds improve microsome predictions. The performance is, however, still not satisfactory. Allometry incorrectly assumes that the determinants for CLH relate to body weight and overpredicts human liver blood flow rate. Consequently, allometric methods have poor predictability. Simple allometry has an average overprediction potential, &gt; 2-fold errors for ∼1/3 of predictions, and 140-fold underprediction to 5800-fold overprediction (potential safety risk) range. In-silico methodologies are available, but these need further development. Acceptable prediction errors for compounds with low and high CLH should be ∼50 and ∼10%, respectively. In conclusion, it is recommended that PB-IVIV with human hepatocyte CLint and fu,bl is applied and improved, limits for acceptable errors are decreased, and that animal CLH-studies and allometry are avoided.

Prediction of antiepileptic drug efficacy: the use of intracerebral microdialysis to monitor biophase concentrations

Biophase concentrations of antiepileptic drugs can differ significantly from pharmacokinetics in plasma. A crucial determinant in the disposition of antiepileptic drugs to the brain is represented by the blood-brain barrier. There is growing evidence that this barrier can alter the availability of antiepileptic drugs at the target site. The permeability of the blood-brain barrier becomes particularly relevant in epileptic conditions and in drug refractory situations. In vivo, intracerebral microdialysis is a valuable technique to determine biophase drug concentrations as it enables investigation of antiepileptic drug transport and distribution in the brain as a function of time. The present review illustrates that intracerebral microdialysis is an indispensable tool for the assessment of the pharmacokinetics of antiepileptic drugs. In addition, we demonstrate how microdialysis data can be used in conjunction with mechanism-based pharmacokinetic/pharmacodynamic modeling for dose selection and optimization of the therapeutic regimen for novel compounds.

Brain penetration of local anaesthetics in the rat: Implications for experimental neuroscience

Multiple experimental neuroscience techniques rely on the use of general anaesthesia to minimize the discomfort associated to animal restraint and to achieve a more effective control of relevant physiological parameters. In order to minimise potential interference on brain neuronal activity, such studies are typically conducted at low anaesthetic doses. This practice is often coupled to peripheral infiltration of local anaesthetics to provide supplementary analgesia and prevent sub-threshold activation of pain pathways that may confound central measurements of brain function. However, little is known of the effect of peripheral anaesthesia on central measurements of brain activity in small laboratory animal species. In order to begin to address this question, we measured total and free brain exposure of five different local anaesthetics following subcutaneous infiltration of analgesic doses in a surgical protocol widely used in rodent neuroimaging and electrophysiology studies. Notably, all the anaesthetics exhibited detectable total and free brain concentrations at all the time points examined. Lidocaine and mepivacaine showed the highest free brain exposures (>525 ng/g), followed by bupivacaine and ropivacaine (>70 ng/g). The ester-type local anaesthetic tetracaine produced the lowest free brain exposure (<8.6 ng/g). Our data suggest that peripheral administration of local anaesthetics in small laboratory animals could result in pharmacologically active brain exposures that might influence and confound central measurements of brain function. The use of the ester-type anaesthetic tetracaine produced considerably lower brain exposure, and may represent a viable experimental option when local anaesthesia is required.

Measurement of drug-protein dissociation rates by high-performance affinity chromatography and peak profiling

The rate at which a drug or other small solute interacts with a protein is important in understanding the biological and pharmacokinetic behavior of these agents. One approach that has been developed for examining these rates involves the use of high-performance affinity chromatography (HPAC) and estimates of band-broadening through peak profiling. Previous work with this method has been based on a comparison of the statistical moments for a retained analyte versus nonretained species at a single, high flow rate to obtain information on stationary phase mass transfer. In this study an alternative approach was created that allows a broad range of flow rates to be used for examining solute-protein dissociation rates. Chromatographic theory was employed to derive equations that could be used with this approach on a single column, as well as with multiple columns to evaluate and correct for the impact of stagnant mobile phase mass transfer. The interaction of L-tryptophan with human serum albumin was used as a model system to test this method. A dissociation rate constant of 2.7 (+/-0.2) s(-1) was obtained by this approach at pH 7.4 and 37 degrees C, which was in good agreement with previous values determined by other methods. The techniques described in this report can be applied to other biomolecular systems and should be valuable for the determination of drug-protein dissociation rates.

Beta-adrenergic-mediated inhibition of feeding by mercaptoacetate in food-deprived rats

This study investigated the effect of intraperitoneal (IP) injections of the fatty acid oxidation (FAO) inhibitor mercaptoacetate (MA, 45.6 mg/kg) on feeding in food-deprived rats. As previously, MA significantly stimulated feeding in ad libitum-fed rats. MA, however, reduced feeding in 18 and 36 h-fasted rats despite apparently antagonizing the fasting-induced increase in hepatic FAO. To test whether this anorectic effect involves beta-adrenergic stimulation, 36 h-fasted rats were IP injected with the nonspecific beta-adrenergic receptor antagonist propranolol (PROP, 0.5 mg/kg) just before MA injection. PROP attenuated MA's feeding-inhibitory effect, suggesting that MA anorexia is at least partially mediated by beta-adrenergic stimulation. Finally, we evaluated the role of subdiaphragmatic vagal afferent fibers in MA's feeding-inhibitory effect by testing the ability of MA to inhibit food intake in fasted rats after subdiaphragmatic vagal deafferentation (SDA). MA inhibited feeding similarly in SDA rats and sham-operated rats. These data demonstrate that subdiaphragmatic vagal afferents are not necessary for the feeding-inhibitory effect of peripheral MA. These results suggest that the FAO inhibitor MA elicits a feeding-inhibitory effect in fasted rats that is mediated by a different mechanism than its feeding-stimulatory effect.

Toward the prediction of CNS drug-effect profiles in physiological and pathological conditions using microdialysis and mechanism-based pharmacokinetic-pharmacodynamic modeling

Our ultimate goal is to develop mechanism-based pharmacokinetic (PK)-pharmacodynamic (PD) models to characterize and to predict CNS drug responses in both physiologic and pathologic conditions. To this end, it is essential to have information on the biophase pharmacokinetics, because these may significantly differ from plasma pharmacokinetics. It is anticipated that biophase kinetics of CNS drugs are strongly influenced by transport across the blood-brain barrier (BBB). The special role of microdialysis in PK/PD modeling of CNS drugs lies in the fact that it enables the determination of free-drug concentrations as a function of time in plasma and in extracellular fluid of the brain, thereby providing important data to determine BBB transport characteristics of drugs. Also, the concentrations of (potential) extracellular biomarkers of drug effects or disease can be monitored with this technique. Here we describe our studies including microdialysis on the following: (1) the evaluation of the free drug hypothesis; (2) the role of BBB transport on the central effects of opioids; (3) changes in BBB transport and biophase equilibration of anti-epileptic drugs; and (4) the relation among neurodegeneration, BBB transport, and drug effects in Parkinson's disease progression.

Factors affecting delivery of antiviral drugs to the brain

Although the CNS is in part protected from peripheral insults by the blood-brain barrier and the blood-cerebrospinal fluid barrier, a number of human viruses gain access to the brain, replicate within this organ, or sustain latent infection. The efficacy of antiviral drugs towards the cerebral viral load is often limited as both blood-brain interfaces impede their cerebral distribution. For polar compounds, the major factor restricting their entry lies in the tight junctions that occlude the paracellular pathway across these barriers. For compounds with more favourable lipid solubility properties, CNS penetration will be function of a number of physicochemical factors that include the degree of lipophilicity, size and ability to bind to protein or red blood cells, as well as other factors inherent to the vascular and choroidal systems, such as the local cerebral blood flow and the surface area available for exchange. In addition, influx and efflux transport systems, or metabolic processes active in both capillary endothelial cells and choroid plexus epithelial cells, can greatly change the bioavailability of a drug in one or several compartments of the CNS. The relative importance of these various factors with respect to the CNS delivery of the different classes of antiviral drugs is illustrated and discussed.

Involvement of P-glycoprotein in blood-brain barrier transport of pentazocine in rats using brain uptake index method

The involvement of P-glycoprotein (P-gp) in pentazocine (PTZ) transport at the blood-brain barrier (BBB) in rats was evaluated by means of an in vivo study using the brain uptake index (BUI) method. The amount of radioactivity in the brain was estimated at different intervals (up to 240 s) after carotid injection in rats. The apparent elimination rate constant (k(test)) due to efflux of PTZ from the brain was calculated as 0.22 min(-1). The observed BUI values of [(3)H]-PTZ (0.35 microM) were not significantly different between 5 and 15 s after the carotid injection. The concentration-dependent uptake of PTZ by the brain was increased gradually by increasing the concentration (0.01-1 mM) of PTZ in the injection solution. The apparent uptake of PTZ by the brain increased in the presence of P-gp inhibitors such as cyclosporin A, quinidine, verapamil and vinblastine after the carotid injection. These results suggest that the increment of PTZ uptake by the brain could be explained by the saturable efflux transport system involving a P-gp-mediated efflux mechanism of PTZ transport at the BBB.

The P-glycoprotein inhibitor quinidine decreases the threshold for bupivacaine-induced, but not lidocaine-induced, convulsions in rats

PURPOSE

To examine whether inhibition of P-glycoprotein (P-gp) activity by quinidine affects the central nervous system toxicity of lidocaine and racemic bupivacaine (bupivacaine).

METHODS

Forty male Sprague-Dawley rats were randomly divided into four groups (n = 10). Fifteen minutes following administration of 15 mg x kg(-1) of quinidine (QL and QB groups) or saline (L and B groups), lidocaine (L and QL groups, 4 mg x kg(-1) x min(-1)) or bupivacaine (B and QB groups, 1 mg x kg(-1) x min(-1)) was infused until convulsions occurred. Concentrations of lidocaine and its primary metabolite, monoethylglycinexylidide (MEGX) and bupivacaine in plasma and in the brain at the onset of convulsions were measured by high-performance liquid chromatography.

RESULTS

There were no differences in the dose of lidocaine required to induce convulsions between the L and QL groups. There were no differences in the concentrations of total (L = 17.2 +/- 3.5, QL = 16.6 +/- 2.6 micro g x mL(-1)) or unbound lidocaine (L = 7.8 +/- 2.5, QL = 7.3 +/- 2.3 micro g x mL(-1)), total (L = 1.2 +/- 0.5, QL = 1.3 +/- 0.7 micro g x mL(-1)) or unbound MEGX (L = 0.9 +/- 0.5, QL = 0.8 +/- 0.4 micro g x mL(-1)) in plasma, total lidocaine or MEGX in the brain at the onset of convulsions between the L and QL groups. The dose of bupivacaine required to induce convulsions was comparable in the B and QB groups. At the onset of convulsions, plasma concentrations of both total (B = 4.9 +/- 1.1, QB = 4.0 +/- 0.6 micro g x mL(-1), P = 0.03) and unbound bupivacaine (B = 1.4 +/- 0.6, QB = 0.9 +/- 0.2 micro g x mL(-1), P = 0.02) were significantly lower in the QB group than in the B group. There were no differences in concentration of total bupivacaine in the brain between the B and QB groups.

CONCLUSION

These results suggest that quinidine inhibited P-gp activity, resulting in increased brain/plasma concentration ratio of bupivacaine, but not of lidocaine, and decreased the threshold of plasma concentration for bupivacaine-induced convulsions.

Considerations in the use of cerebrospinal fluid pharmacokinetics to predict brain target concentrations in the clinical setting: implications of the barriers between blood and brain

In the clinical setting, drug concentrations in cerebrospinal fluid (CSF) are sometimes used as a surrogate for drug concentrations at the target site within the brain. However, the brain consists of multiple compartments and many factors are involved in the transport of drugs from plasma into the brain and the distribution within the brain. In particular, active transport processes at the level of the blood-brain barrier and blood-CSF barrier, such as those mediated by P-glycoprotein, may lead to complex relationships between concentrations in plasma, ventricular and lumbar CSF, and other brain compartments. Therefore, CSF concentrations may be difficult to interpret and may have limited value. Pharmacokinetic data obtained by intracerebral microdialysis monitoring may be used instead, providing more valuable information. As non-invasive alternative techniques, positron emission tomography or magnetic resonance spectroscopy may be of added value.

Albumin binding of acylated insulin (NN304) does not deter action to stimulate glucose uptake

NN304 [Lys(B29)-tetradecanoyl des(B30) human insulin] is a potentially therapeutic insulin analog designed to exhibit protracted glucose-lowering action. In dogs with infusion rates similar to insulin itself, NN304 exhibits similar glucose uptake (R(d)) stimulation with delayed onset of action. This compartmental modeling study was to determine if NN304 action could be accounted for by the approximately 2% unbound NN304 concentration. NN304 (or human insulin) (n = 6 each) was infused at 10.2 pmol center dot min(-1) center dot kg(-1) under euglycemic clamp conditions in anesthetized dogs. NN304 appearance in lymph, representing interstitial fluid (ISF), was slow compared with insulin (t(1/2) = 70 +/- 7 vs. 14 +/- 1 min, P < 0.001). R(d) was highly correlated with the ISF concentration for insulin and NN304 (r = 0.86 and 0.93, respectively), suggesting that slow transendothelial transport (TET) is responsible for sluggish NN304 action. Insulin and NN304 concentration data were fit to a two-compartment (plasma and ISF) model. NN304 plasma elimination and TET were reduced to 10 and 7% of insulin, respectively. Thus, there was reduction of NN304 transport, but not to the degree expected. In ISF, there was no reduction in NN304 elimination. Thus, this acylated insulin analog demonstrates blunted kinetics in plasma, and full efficacy in the compartment of action, ISF.

Carrier proteins determine local pharmacokinetics and arterial distribution of paclitaxel

The growing use of local drug delivery to vascular tissues has increased interest in hydrophobic compounds. The binding of these drugs to serum proteins raises their levels in solution, but hinders their distribution through tissues. Inside the arterial interstitium, viscous and steric forces and binding interactions impede drug motion. As such, this might be the ideal scenario for increasing the amount of drug delivered to, and residence time within, arterial tissues. We quantified carrier-mediated transport for paclitaxel, a model hydrophobic agent with potential use in proliferative vascular diseases, by determining, in the presence or absence of carrier proteins, the maximum concentration of drug in aqueous solution, the diffusivity in free solution, and the diffusivity in arterial tissues. Whereas solubility of paclitaxel was raised 8.1-, 21-, and 57-fold by physiologic levels of alpha(1)-acid glycoproteins, bovine serum albumin, and calf serum over that in protein-free solution, diffusivity of paclitaxel in free solution was reduced by 41, 49, and 74%, respectively. When paclitaxel mixed in these solutions was applied to arteries both in vitro and in vivo, drug was more abundant at the tissue interface, but protein carriers tended to retain drug in the lumen. Once within the tissue, these proteins did not affect the rate at which drug traverses the tissue because this hydrophobic drug interacted with the abundant fixed proteins and binding sites. The protein binding properties of hydrophobic compounds allow for beneficial effects on transvascular transport, deposition, and distribution, and may enable prolonged effect and rationally guide local and systemic strategies for their administration.

Altered disposition and effect of lerisetron in rats with elevated alpha 1-acid glycoprotein levels

PURPOSE

To examine the effect of changes in plasma alpha1-acid glycoprotein (AAG) levels on the pharmacokinetics (PK) and pharmacodynamics (PD) of lerisetron, a novel serotonin 5-HT3 receptor antagonist, in the rat.

METHODS

After subcutaneous administration of turpentine oil, AAG was significantly elevated compared with controls. The PK of unchanged lerisetron (UL; high-performance liquid chromatography with radioactivity monitoring) and total lerisetron (TL; unchanged + changed, scintillation counting) was characterized post intravenous (i.v.) 14C lerisetron (50 microg/kg) in control and turpentine oil pretreated rats. The PK (0-180 min) was described by a two-compartmental model. Protein binding of lerisetron in vitro was measured using an ultrafiltration technique. The effect of lerisetron (5 microg/kg, i.v.) over 180 min was measured in anesthetized rats (control and pretreated) with the Bezold-Jarisch reflex (inhibition of bradycardia after 16 microg/kg serotonin i.v.) as the endpoint. PD parameters were estimated by sigmoid Emax models.

RESULTS

The unbound fraction was significantly diminished in pretreated rats (mean +/- SEM) (6.60 +/- 1.23% vs. control 14.4 +/- 1.40%, P < 0.05). Volume of distribution (V) and clearance for UL and TL were significantly decreased when compared to the controls (P < 0.0001 for UL and P < 0.05 for TL). Plasma clearance based on unbound concentration for UL did not differ between groups but the unbound V and steady-state unbound V remained decreased (P < 0.05 and P < 0.0001). Pretreated rats showed a significantly diminished drug effect: the area under the E-t curve over 180 min was (mean +/- SEM) 5,189 +/- 657.7 in control animals vs. 3,486 +/- 464.4 in the pretreated group (P < 0.05). The EC50 (concentration at half maximum effect) for UL and TL were increased in pretreated rats and were not compensated when the unbound concentration was used.

CONCLUSIONS

An increase in AAG causes alterations in the PK and PD of lerisetron, and because this is not compensated with the unbound concentration, we suggest that mechanisms not linked to protein binding may be involved.

Comparison of serum, cerebrospinal fluid and brain extracellular fluid pharmacokinetics of lamotrigine

We investigated the rate of penetration into and the intra-relationship between the serum, cerebrospinal fluid (CSF) and regional brain extracellular fluid (bECF) compartments following systemic administration of lamotrigine in rat. The serum pharmacokinetics were biphasic with an initial distribution phase, (half-life approximately 3 h), and then a prolonged elimination phase of over 30 h. The serum pharmacokinetics were linear over the range 10 - 40 mg kg(-1). Using direct sampling of CSF with concomitant serum sampling, the calculated penetration half-time into CSF was 0.42+/-0.15 h. At equilibrium, the CSF to total serum concentration ratio (0.61+/-0.02) was greater than the free to total serum concentration (0.39+/-0.01). Using in vivo recovery corrected microdialysis sampling in frontal cortex and hippocampus with concomitant serum sampling, the calculated penetration half-time of lamotrigine into bECF, 0.51+/-0.11 h, was similar to that for CSF and was not area or dose dependent. At equilibrium, the bECF to total serum concentration ratio (0.40+/-0.04) was similar to the free to total serum concentration (0.39+/-0.01), and did not differ between hippocampus and frontal cortex. The species specific serum kinetics can explain the prolonged action of lamotrigine in rat seizure models. Lamotrigine has a relatively slow penetration into both CSF and bECF compartments compared with antiepileptic drugs used in acute seizures. Furthermore, the free serum drug concentration is not the sole contributor to the CSF compartment, and the CSF concentration is an overestimate of the bECF concentration of lamotrigine.

Pharmacokinetic optimisation of the treatment of bacterial central nervous system infections

Central nervous system (CNS) infections caused by bacteria with reduced sensitivity to antibacterials are an increasing worldwide challenge. In successfully treating these infections the following conditions should be considered: (i) Antibacterials do not distribute homogeneously in the central nervous compartments [cerebrospinal fluid (CSF), extracellular space of the nervous tissue, intracellular space of the neurons, glial cells and leucocytes]. Even within the CSF, after intravenous administration, a ventriculo-lumbar concentration gradient is often observed. (ii) Valid parameters of drug entry into the CSF are the CSF: serum concentration ratio in steady state and the CSF: serum ratio of the area under the concentration-time curves (AUCCSF/AUCS). Frequently, the elimination half-life (t1/2 beta) in CSF is longer than t1/2 beta in serum. (iii) For most antibacterials, lipophilicity, molecular weight and serum protein binding determine the drug entry into the CSF and brain tissue. With an intact blood-CSF and blood-brain barrier, the entry of hydrophilic antibacterials (beta-lactam antibacterials, glycopeptides) into the CNS compartments is poor and increases during meningeal inflammation. More lipophilic compounds [metronidazole, quinolones, rifampicin (rifampin) and chloramphenicol] are less dependent on the function of the blood-CSF and blood-brain barrier. (iv) Determination of the minimal inhibitory concentrations (MIC) of the causative organism is necessary for optimisation of treatment. (v) For rapid sterilisation of CSF, drug concentrations of at least 10 times MIC are required. The minimum CSF concentration: MIC ratio ensuring successful therapy is unknown. Strategies to achieve optimum antibacterial concentrations in the presence of minor disturbances of the blood-CSF and blood-brain barrier include, the increased use of low toxicity antibacterials (e.g., beta-lactam antibiotics), the use of moderately lipophilic compounds, and the combination of intravenous and intraventricular administration. Antibacterials which do not interfere with bacterial cell wall synthesis delay and/or decrease the liberation of proinflammatory bacterial products, delay or inhibit tumour necrosis factor release, and may reduce brain oedema in experimental meningitis. Conclusive evidence of the reduction of neuronal damage by this approach, however, is lacking.

Comparison of single- and repeated-dose pharmacokinetics of diazepam

PURPOSE

To determine whether repeat boluses of diazepam (DZP) lead to significant accumulation in the central nervous system and/or peripheral compartments, as repeat intravenous boluses of diazepam are commonly used in the treatment of status epilepticus (SE).

METHODS

In a rat model that permits simultaneous serum and cerebrospinal fluid (CSF) sampling, we characterized the pharmacokinetics of DZP and its metabolite, desmethyldiazepam, in CSF and blood using HPLC. DZP was administered by intraperitoneal injection as either a single dose (20 or 30 mg/kg) or repeat doses (10 or 20 mg/kg x 3, 1 h apart).

RESULTS

After a single intraperitoneal dose, DZP was rapidly absorbed with a time to maximum concentration of 10 min. The serum concentrations then declined biexponentially. DZP rapidly entered the CSF, the CSF to serum ratio reached equilibrium within 10 min, and was equivalent to the ratio of free to total serum concentration. Repeated DZP dosing resulted in a threefold decrease in volume of distribution and clearance (p < 0.001). This was reflected in the CSF concentration data; however, after the third dose, the ratio of CSF to serum concentration, also increased greatly, representing further persistence of DZP in the CSF compartment.

CONCLUSIONS

Repeat dosing of DZP leads to substantial accumulation, and high, persistent serum and CSF concentrations, which may explain the toxic effects of repeat DZP dosing. Repeat dosing of DZP using a tapering protocol, however, may increase the effectiveness of DZP in treating SE by preventing relapses without substantially increasing toxicity.

Blood-to-brain transfer of various oxicams: effects of plasma binding on their brain delivery

PURPOSE

The objective of this work was to assess the influence of binding to plasma proteins and to serum on the brain extraction of four antiinflammatory oxicams.

METHODS

The brain extraction of isoxicam, tenoxicam, meloxicam and piroxicam was investigated in rats using the carotid injection technique. Blood protein binding parameters were determined by equilibrium dialysis using human serum, human serum albumin (HSA) and alpha-l-acid glycoprotein (AAG) solutions at various concentrations.

RESULTS

All oxicams had low values of brain extraction, between 19% and 39% when dissolved in serum, i.e. under physiological conditions. Brain efflux rate constants calculated from the wash-out curves were the same in the absence or presence of serum. Brain efflux was inversely related to the polarity of the oxicams, such that the higher their H-bonding capacity, the lower their brain efflux. The free dialyzable drug fraction was inversely related to protein concentration. However, rat brain extraction was always higher than expected from in vitro measurements of the dialyzable fraction.

CONCLUSIONS

Except for piroxicam whose brain extraction was partially decreased in the presence of proteins, the serum unbound and initially bound fractions of oxicams both seem available for transfer into the brain. Modest affinities for AAG rule out any related effect. More surprising is the apparent lack of effect on brain transfer of the high-affinity binding to HSA and serum. The enhanced brain uptake of meloxicam in the presence of AAG could be a result of interactions between this globular protein and the endothelial wall.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.