Brain Kp Research Articles

Evaluation of Prediction Accuracy for Volume of Distribution in Rat and Human Using InVitro, InVivo, PBPK and QSAR Methods.

Volume of distribution at steady state (Vss) is an important pharmacokinetic parameter of a drug candidate. In this study, Vss prediction accuracy was evaluated by using: (1) seven methods for rat with 56 compounds, (2) four methods for human with 1276 compounds, and (3) four invivo methods and three Kp (partition coefficient) scalar methods from scaling of three preclinical species with 125 compounds. The results showed that the global QSAR models outperformed the PBPK methods. Tissue fraction unbound (fu,t) method with adipose and muscle also provided high Vss prediction accuracy. Overall, the high performing methods for human Vss prediction are the global QSAR models, Øie-Tozer and equivalency methods from scaling of preclinical species, as well as PBPK methods with Kp scalar from preclinical species. Certain input parameter ranges rendered PBPK models inaccurate due to mass balance issues. These were addressed using appropriate theoretical limit checks. Prediction accuracy of tissue Kp were also examined. The fu,t method predicted Kp values more accurately than the PBPK methods for adipose, heart and muscle. All the methods overpredicted brain Kp and underpredicted liver Kp due to transporter effects. Successful Vss prediction involves strategic integration of in silico, invitro and invivo approaches.

0001 Activation of Nociceptin/Orphanin-FQ Peptide (NOP) Receptors Produces an Increase in Non-REM Sleep in Rats and Constitutes a Novel and Attractive Target for the Treatment of Insomnia

Abstract Introduction Treatments for insomnia have targeted GABA, histamine, serotonin, melatonin and orexin receptors. The nociceptin/orphanin-FQ peptide (NOP) receptor is widely expressed in the nervous system. High doses of NOP agonists administered systemically or locally into the CNS can result in sedation, however, the utility of targeting this receptor to treat insomnia has not been fully described. Methods V117957 is a recently described investigational oral, potent and selective NOP receptor partial agonist. We determined the brain Kp in whole brain and multiple sub-regions (50mg/kg) and receptor occupancy in the hypothalamus (30, 300mg/kg) via in vivo displacement using [3H]-NOP-1A. EEG/EMG were determined in rats chronically implanted with electrodes (cortex and dorsal neck muscle) and recorded via telemetry following dosing (3, 30, 300mg/kg); sleep stage was determined from visual analysis of EEG level. Sleep parameters were also assessed in NOP receptor knock-out rats (300mg/kg). The side-effect profile for V117957 was determined by functional observation battery, whole-body plethysmography, Morris water maze (MWM) (up to 600mg/kg) and conditioned place preference (CPP) assay (up to 300mg/kg). Results V117957 displayed limited distribution into the CNS but achieved a high level of receptor occupancy (75% at 30mg/kg). Administration of V117957 produced dose-dependent and statistically significant increases in non-REM sleep with a minimally efficacious dose of 30mg/kg; a coincident dose-dependent and statistically significant decrease in wakefulness and a non-dose-dependent effect on REM sleep occurred. These changes were not seen in knock-out animals demonstrating effects are via NOP receptors. At doses higher than those that increased non-REM sleep, V117957 had no effects in a functional observational battery, did not affect escape latency in MWM or produce CPP; additionally, V117957 did not affect respiratory parameters. Conclusion We conclude that activation of NOP receptors decreases wakefulness and increases non-REM sleep in rats with an improved preclinical profile compared to historical profiles of current treatments and, therefore, may represent a novel and attractive target for the treatment of insomnia. Support Funded by Shionogi and Imbrium Therapeutics, a subsidiary of Purdue Pharma L.P.

Minimal contribution of P-gp on the low brain distribution of naldemedine, a peripherally acting μ-opioid receptor antagonist

Naldemedine tosylate, a peripherally acting μ-opioid receptor antagonist, is indicated for treatment of opioid induced constipation in both Japan and US. Naldemedine has limited ability to affect the central analgesic effect of opioid analgesics. In this study, we investigated the contribution of P-glycoprotein (P-gp) on the brain distribution of naldemedine. Naldemedine tosylate showed acceptable oral absorption in rats. Following a single oral administration of [14C]-naldemedine tosylate to rats and ferrets, little radioactivity was detected in the region protected by the blood-brain barrier (BBB). In the assessment using Caco-2 cells, it was determined that naldemedine is a substrate for P-gp. The contribution of P-gp to the brain distribution of naldemedine was assessed using multidrug resistance 1a/b (mdr1a/b) knockout mice. While the brain-to-plasma concentration ratio (brain Kp) of naldemedine in the mdr1a/b knockout mice was 4-fold of that in the wild-type mice, the brain Kp in the mdr1a/b knockout mice was quite low (brain Kp < 0.1). These results suggest that the low brain distribution of naldemedine was due to the limited ability to cross the BBB rather than efflux by P-gp and therefore brain distribution of naldemedine would not be affected by concomitant administration of P-gp inhibitors or functional disorder of P-gp.

Examining the Uptake of Central Nervous System Drugs and Candidates across the Blood-Brain Barrier.

Assessing the equilibration of the unbound drug concentrations across the blood-brain barrier (Kp,uu) has progressively replaced the partition coefficient based on the ratio of the total concentration in brain tissue to blood (Kp). Here, in vivo brain distribution studies were performed on a set of central nervous system (CNS)-targeted compounds in both rats and P-glycoprotein (P-gp) genetic knockout mice. Several CNS drugs are characterized by Kp,uu values greater than unity, inferring facilitated uptake across the rodent blood-brain barrier (BBB). Examples are shown in which Kp,uu also increases above unity on knockout of P-gp, highlighting the composite nature of this parameter with respect to facilitated BBB uptake, efflux, and passive diffusion. Several molecules with high Kp,uu values share common structural elements, whereas uptake across the BBB appears more prevalent in the CNS-targeted drug set than the chemical templates being generated within the current lead optimization paradigm. Challenges for identifying high Kp,uu compounds are discussed in the context of acute versus steady-state data and cross-species differences. Evidently, there is a need for better predictive models of human brain Kp,uu.

Brain Exposure of Two Selective Dual CDK4 and CDK6 Inhibitors and the Antitumor Activity of CDK4 and CDK6 Inhibition in Combination with Temozolomide in an Intracranial Glioblastoma Xenograft.

Effective treatments for primary brain tumors and brain metastases represent a major unmet medical need. Targeting the CDK4/CDK6-cyclin D1-Rb-p16/ink4a pathway using a potent CDK4 and CDK6 kinase inhibitor has potential for treating primary central nervous system tumors such as glioblastoma and some peripheral tumors with high incidence of brain metastases. We compared central nervous system exposures of two orally bioavailable CDK4 and CDK6 inhibitors: abemaciclib, which is currently in advanced clinical development, and palbociclib (IBRANCE; Pfizer), which was recently approved by the U.S. Food and Drug Administration. Abemaciclib antitumor activity was assessed in subcutaneous and orthotopic glioma models alone and in combination with standard of care temozolomide (TMZ). Both inhibitors were substrates for xenobiotic efflux transporters P-glycoprotein and breast cancer resistant protein expressed at the blood-brain barrier. Brain Kp,uu values were less than 0.2 after an equimolar intravenous dose indicative of active efflux but were approximately 10-fold greater for abemaciclib than palbociclib. Kp,uu increased 2.8- and 21-fold, respectively, when similarly dosed in P-gp-deficient mice. Abemaciclib had brain area under the curve (0-24 hours) Kp,uu values of 0.03 in mice and 0.11 in rats after a 30 mg/kg p.o. dose. Orally dosed abemaciclib significantly increased survival in a rat orthotopic U87MG xenograft model compared with vehicle-treated animals, and efficacy coincided with a dose-dependent increase in unbound plasma and brain exposures in excess of the CDK4 and CDK6 Ki values. Abemaciclib increased survival time of intracranial U87MG tumor-bearing rats similar to TMZ, and the combination of abemaciclib and TMZ was additive or greater than additive. These data show that abemaciclib crosses the blood-brain barrier and confirm that both CDK4 and CDK6 inhibitors reach unbound brain levels in rodents that are expected to produce enzyme inhibition; however, abemaciclib brain levels are reached more efficiently at presumably lower doses than palbociclib and are potentially on target for a longer period of time.

Methods for estimation of brain active site concentration of drugs and other chemicals

Event Abstract Back to Event Methods for estimation of brain active site concentration of drugs and other chemicals Margareta Hammarlund-Udenaes1* 1 Uppsala University, Division of Pharmacokinetics and Drug Therapy, Department of Pharmaceutical Biosciences, Sweden Brain active site concentrations are defined as the unbound, pharmacologically active concentrations at the site of action. The site of action may be located extracellularly in the brain interstitial fluid (ISF) or at some intracellular target within the parenchymal cells. Unbound blood concentrations are not representative of the brain ISF concentrations when the drug is exposed to active efflux or influx transport at the blood-brain barrier (BBB). The brain ISF concentrations may neither be representative of the intracellular concentrations depending on type of transport to the intracellular target. Methods intended for estimation of brain active site concentrations have to directly measure or estimate the unbound concentration. Recent development has made it possible to measure and/or estimate brain ISF concentrations, and also to estimate average intracellular concentrations. When discussing methods, it is important to distinguish between methods for estimation of rate of transport across the BBB and methods for estimation of the extent of equilibration between brain and blood [1]. Examples of "rate methods" are in situ brain perfusion, brain efflux index and cell culture models that measure the permeability surface area product or uptake clearance into the brain. Only the "extent methods" can be used to estimate active site concentrations. The extent of equilibration can be determined for total drug (Kp) or unbound drug (Kp,uu). Kp,uu is by definition the ratio of unbound concentrations in brain ISF and blood. Total drug ratios can be determined with i.e. logBB methods and with PET. A simple method with which to determine Kp is to administer an i.v. infusion to steady state [1, 2]. A suggestion is to administer an infusion for 4 hours to rats and thereafter take brain and blood samples (Fig. 1). In order to obtain Kp,uu, the infusion should be combined with plasma protein binding determination and brain slice measurement. This method increases the possibility of obtaining good estimations of unbound target concentrations already in the drug discovery setting. Unbound ratios can be measured directly with microdialysis for drugs that do not stick to tubings and probes. The unspecific binding in brain can be determined with Vu,brain or fu,brain measurements, where Vu,brain is the unbound volume of distribution in brain (Fig. 1), and fu,brain is the unbound fraction in brain. Vu,brain is equal to 1/fu,brain under certain circumstances. CSF has for long been valued as a substitute for unbound brain concentrations but with a lack of comparison. This has mainly been due to the lack of methods to measure brain ISF concentrations. The unbound concentration ratio between CSF and blood (Kp,uu,CSF) was recently evaluated as a substitute for Kp,uu [3]. There was a rather good correlation (Fig. 2 A). However, the CSF ratio over-predicted the brain Kp,uu ratio at low values, and underpredicted the ratio at high values. At low values, this can be explained as caused by the presence of P-glycoprotein (P-gp) at the BBB but a different function of P-gp at the bloodcerebrospinal fluid barrier (BCSFB). At high values, it is speculated that active uptake transporters are present in the BBB but not in the BCSFB. Human and rat data had the same rank-order, but human Kp,uu,CSF values were on average 3-fold higher than those in rats (Fig. 2 B). Intracellular exposure can be addressed with a combination of the brain slice and the brain homogenate methods [4]. Development is ongoing to further sub-divide estimations into cellular components. In conclusion, methods for estimation of brain active site concentrations need to measure or estimate the unbound moiety. There is a rather limited availability of methods, but recent development has made it possible to increase the knowledge about the relationship between brain active site concentrations and unbound concentrations in blood. Pic 1 Pic 2

Formulation-dependent brain and lung distribution kinetics of propofol in rats.

Propofol when administered by brief infusion in a lipid-free formulation has a slower onset, prolonged offset and greater potency compared with an emulsion formulation. To understand these findings the authors examined propofol brain and lung distribution kinetics in rats. Rats were infused with equieffective doses of propofol in emulsion (n = 21) or lipid-free formulation (n = 21). Animals were sacrificed at various times to harvest brain and lung. Arterial blood was sampled repeatedly from each animal until sacrifice. Deconvolution and moment analysis were used to calculate the half-life for propofol brain turnover (BT) and brain:plasma partition coefficient (Kp). Lung concentration-time profiles were compared for the two formulations. Peak propofol plasma concentrations for the lipid-free formulation were 50% of that observed for emulsion formulation, whereas peak lung concentrations for lipid-free formulation were 300-fold higher than emulsion formulation. Brain Kp calculated from tissue disposition curve and ratio of brain:plasma area under the curves were 8.8 and 13, and 7.2 and 9.1 for emulsion and lipid-free formulations, respectively. BT were 2.4 and 2.5 min for emulsion and lipid-free formulations, respectively. Significant pulmonary sequestration and slow release of propofol into arterial circulation when administered in lipid-free vehicle accounts for the lower peak arterial concentration and sluggish arterial kinetics relative to that observed with the emulsion formulation. Higher Kp for the lipid-free formulation could explain the higher potency associated with this formulation. BT were independent of formulation and correlated with values reported for effect-site equilibration half-time consistent with a distribution mechanism for pharmacologic hysteresis.

Brain and cerebrospinal fluid distributions of metoprolol in rats.

The distribution of metoprolol tartrate (MPL) into the brain and cerebrospinal fluid (CSF) from plasma was determined at 2 different steady-state plasma concentrations (2 and 20 nmol/ml) and following intravenous bolus administration (100 nmol/kg) in rats. Under steady-state condition, no difference in the brain and CSF distributions of MPL was observed between 2 different plasma MPL concentrations, and the value of brain- or CSF-to-plasma partition coefficients were 5.7 and 1.5, respectively. Following intravenous administration, MPL distributed into CSF very rapidly and its initial uptake phase was not detected. On the other hand, MPL distribution into the brain was relatively slow with the uptake phase. Thus, the brain uptake of MPL from the plasma was pharmacokinetically analyzed using a modified 2-compartment model and deconvolution method, whereas the CSF distribution of MPL was not adequate for kinetic analysis because of the lack of uptake phase. The analysis of brain uptake of MPL by both compartment model and deconvolution gave the same results and the calculated brain Kp value of MPL was almost comparable with that of observed Kp value under steady-state plasma concentration. The distribution of MPL into CSF was considered mainly depending on the pH difference between the plasma and CSF, and the mutual transfer of MPL between CSF and the brain tissue was considered to be negligible from an in vitro study.