Investigation of the role and quantitative impact of breast cancer resistance protein on drug distribution into brain and CSF in rats.

Breast cancer resistance protein (BCRP) expressed in the blood-brain barrier plays a major role in limiting drug distribution into the central nervous system (CNS). However, functional involvement of BCRP in drug distribution into the brain and cerebrospinal fluid (CSF) remains unclear. The aim of present study was to reveal the role and quantitative impact of BCRP on CNS distribution. The brain-to-plasma unbound concentration ratio (Kp,uu,brain) and CSF-to-plasma unbound concentration ratio (Kp,uu,CSF) values of BCRP-specific substrates were determined in rats. The Kp,uu,brain values decreased, as the in vitro BCRP corrected flux ratio (CFR) increased. The Kp,uu,CSF values of BCRP-specific substrates were greater than the Kp,uu,brain values. Increase in the Kp,uu,brain values induced by co-administration of BCRP inhibitor correlated with the in vitro BCRP CFR and were greater than the increase in Kp,uu,CSF values induced by BCRP inhibitor except nebicapone. The contribution of BCRP to the brain and CSF distribution of the dual P-glycoprotein/BCRP substrates, imatinib and prazosin, was similar to that of BCRP-specific substrates. Thus, we revealed that the impact of in vivo BCRP on CNS distribution is correlated with in vitro BCRP CFR, and that BCRP limits drug distribution into the brain more strongly than into the CSF.

Quantitative investigation of the brain-to-cerebrospinal fluid unbound drug concentration ratio under steady-state conditions in rats using a pharmacokinetic model and scaling factors for active efflux transporters.

A pharmacokinetic model was constructed to explain the difference in brain- and cerebrospinal fluid (CSF)-to-plasma and brain-to-CSF unbound drug concentration ratios (Kp,uu,brain, Kp,uu,CSF, and Kp,uu,CSF/brain, respectively) of drugs under steady-state conditions in rats. The passive permeability across the blood-brain barrier (BBB), PS1, was predicted by two methods using log(D/molecular weight(0.5)) for PS1(1) or the partition coefficient in octanol/water at pH 7.4 (LogD), topologic van der Waals polar surface area, and van der Waals surface area of the basic atoms for PS1(2). The coefficients of each parameter were determined using previously reported in situ rat BBB permeability. Active transport of drugs by P-glycoprotein (P-gp) and breast cancer resistance protein (Bcrp) measured in P-gp- and Bcrp-overexpressing cells was extrapolated to in vivo by introducing scaling factors. Brain- and CSF-to-plasma unbound concentration ratios (Kp,uu,brain and Kp,uu,CSF, respectively) of 19 compounds, including P-gp and Bcrp substrates (daidzein, dantrolene, flavopiridol, genistein, loperamide, quinidine, and verapamil), were simultaneously fitted to the equations in a three-compartment model comprising blood, brain, and CSF compartments. The calculated Kp,uu,brain and Kp,uu,CSF of 17 compounds were within a factor of three of experimental values. Kp,uu,CSF values of genistein and loperamide were outliers of the prediction, and Kp,uu,brain of dantrolene also became an outlier when PS1(2) was used. Kp,uu,CSF/brain of the 19 compounds was within a factor of three of experimental values. In conclusion, the Kp,uu,CSF/brain of drugs, including P-gp and Bcrp substrates, could be successfully explained by a kinetic model using scaling factors combined with in vitro evaluation of P-gp and Bcrp activities.

Use of cassette dosing approach to examine the effects of P-glycoprotein on the brain and cerebrospinal fluid concentrations in wild-type and P-glycoprotein knockout rats.

The study objectives were 1) to test the hypothesis that the lack of P-glycoprotein (P-gp) and the inhibition of breast cancer resistance protein (Bcrp) at the blood-brain barrier after cassette dosing of potent P-gp and Bcrp inhibitors were due to low plasma concentrations of those inhibitors and 2) to examine the effects of P-gp on the unbound brain (C(u,brain)) and cerebrospinal fluid (CSF) concentrations (C(u,CSF)) of P-gp substrates in rats. In vitro inhibition of 11 compounds (amprenavir, citalopram, digoxin, elacridar, imatinib, Ko143 [(3S,6S,12aS)-1,2,3,4,6,7,12,12a-octahydro-9-methoxy-6-(2-methylpropyl)-1,4-dioxopyrazino[1',2':1,6]pyrido[3,4-b]indole-3-propanoic acid 1,1-dimethylethyl ester], loperamide, prazosin, quinidine, sulfasalazine, and verapamil) on P-gp and Bcrp was examined in P-gp- and Bcrp-expressing Madin-Darby canine kidney cells, respectively. An in vivo study was conducted in wild-type and Mdr1a(-/-) rats after subcutaneous cassette dosing of the 11 compounds at 1-3 mg/kg, and the brain, CSF, and plasma concentrations of these compounds were determined. At the maximal unbound concentrations observed in rats at 1-3 mg/kg, P-gp and Bcrp were not inhibited by a cassette of the 11 compounds. For non-P-gp/Bcrp substrates, similar C(u,brain), C(u,CSF), and unbound plasma concentrations (C(u,plasma)) were observed in wild-type and P-gp knockout rats. For P-gp/Bcrp substrates, C(u,brain) ≤ C(u,CSF) ≤ C(u,plasma) in wild-type rats, but C(u,brain) and C(u,CSF) increased in the P-gp knockout rats and were within 3-fold of C(u,plasma) for six of the seven P-gp substrates. These results indicate that P-gp and Bcrp inhibition at the blood-brain barrier is unlikely in cassette dosing and also suggest that P-gp and Bcrp activity at the blood-CSF barrier is functionally not important in determination of the CSF concentration for their substrates.

Cerebrospinal fluid can be used as a surrogate to assess brain exposures of breast cancer resistance protein and P-glycoprotein substrates.

The objectives of the study were to characterize the selectivity of dantrolene to breast cancer resistance protein (Bcrp) and to evaluate whether cerebrospinal fluid (CSF) can be used as a surrogate to assess brain exposures of BCRP and P-glycoprotein (Pgp) substrates. The impact of Bcrp and Pgp on dantrolene exposures in brain and CSF was examined in Bcrp and Mdr1a/1b knockout mice and was further investigated in wild-type mice in the presence of the Bcrp inhibitor (3S,6S,12aS)-1,2,3,4,6,7,12,12a-octahydro-9-methoxy-6-(2-methylpropyl)-1,4-dioxopyrazino[1',2':1,6]pyrido[3,4-b]indole-3-propanoic acid 1,1-dimethylethyl ester (Ko143), the Pgp inhibitor 6-[(2S,4R,6E)-4-methyl-2-(methylamino)-3-oxo-6-octenoic acid]-7-l-valine-cyclosporine A (PSC833), and the dual inhibitor N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918). The effect of Bcrp and Pgp on digoxin exposures in brain and CSF was investigated in wild-type mice in the presence of the inhibitors. In vivo studies showed dantrolene exposures in brain and CSF, but not the blood, increased in Bcrp(-/-) and Mdr1a/1b(-/-)/Bcrp(-/-) mice, or in the presence of the Bcrp inhibitors Ko143 or GF120918. Inhibition of Pgp by GF120918 and PSC833 significantly increased digoxin exposures in brain, CSF, and blood to a lesser extent. Results from the present study demonstrated that inhibition of Bcrp and Pgp increased not only the exposures of dantrolene and digoxin in brain, but also the exposures in CSF. In addition, the change of exposures in CSF reflected the changes in brain. The present study strongly suggests that the dantrolene and digoxin exposures in CSF are primarily determined by the rapid transport from brain to CSF, and inhibition of Bcrp and Pgp exhibits little impact on using CSF as surrogates to assess brain exposures of Bcrp and Pgp substrates.

Evaluation of the relationship between a pharmaceutical compound's distribution coefficient, log D and adsorption loss to polypropylene in urine and CSF.

BACKGROUND: The distribution coefficient, D, is a physicochemical property used to determine the partitioning of compounds between aqueous and hydrophobic media at a given pH. RESULTS: A clear relationship was observed between the calculated pH-dependent distribution coefficient of six representative pharmaceutical probe compounds and their propensity to partition between a relatively hydrophobic polypropylene surface and the aqueous matrices, human urine or human cerebrospinal fluid (CSF). Compound log D cut-off values of 1.5 and 3.8 for urine and CSF, respectively, were determined using a threshold of less than 20% adsorption to the polypropylene surface. CONCLUSION: The ability to forecast the adsorption of a given compound to a polypropylene container with urine and CSF offers an effective means for screening potential issues and identifying when additional testing and corrective measures may need to be applied.

Subarachnoid hemorrhage and the distribution of drugs delivered into the cerebrospinal fluid. Laboratory investigation.

OBJECT: Investigators in experimental and clinical studies have used the intrathecal route to deliver drugs to prevent or treat vasospasm. However, a clot near an artery or arteries after subarachnoid hemorrhage (SAH) may hamper distribution and limit the effects of intrathecally delivered compounds. In a primate model of right middle cerebral artery (MCA) SAH, the authors examined the distribution of Isovue-M 300 and 3% Evans blue after infusion into the cisterna magna CSF. METHODS: Ten cynomolgus monkeys were assigned to SAH and sham SAH surgery groups (5 in each group). Monkeys received CSF injections as long as 28 days after SAH and were killed 3 hours after the contrast/Evans blue injection. The authors assessed the distribution of contrast material on serial CT within 2 hours after contrast injection and during autopsy within 3 hours after Evans blue staining. RESULTS: Computed tomography cisternographies showed no contrast in the vicinity of the right MCA (p < 0.05 compared with left); the distribution of contrast surrounding the entire right cerebral hemisphere was substantially reduced. Postmortem analysis demonstrated much less Evans blue staining of the right hemisphere surface compared with the left. Furthermore, the Evans blue dye did not penetrate into the right sylvian fissure, which occurred surrounding the left MCA. The authors observed the same pattern of changes and differences in contrast distribution between SAH and sham SAH animals and between the right and the left hemispheres on Days 1, 3, 7, 14, 21, and 28 after SAH. CONCLUSIONS: Intrathecal drug distribution is substantially limited by SAH. Thus, when using intrathecal drug delivery after SAH, vasoactive drugs are unlikely to reach the arteries that are at the highest risk of delayed cerebral vasospasm.

Drug delivery to the central nervous system: a review.

The brain is a delicate organ, and evolution built very efficient ways to protect it. Unfortunately, the same mechanisms that protect it against intrusive chemicals can also frustrate therapeutic interventions. Many existing pharmaceuticals are rendered ineffective in the treatment of cerebral diseases due to our inability to effectively deliver and sustain them within the brain. General methods that can enhance drug delivery to the brain are, therefore, of great interest. Despite aggressive research, patients suffering from fatal and/or debilitating central nervous system (CNS) diseases, such as brain tumors, HIV encephalopathy, epilepsy, cerebrovascular diseases and neurodegenerative disorders, far outnumber those dying of all types of systemic cancer or heart disease. The clinical failure of much potentially effective therapeutics is often not due to a lack of drug potency but rather to shortcomings in the method by which the drug is delivered. Treating CNS diseases is particularly challenging because a variety of formidable obstacles often impede drug delivery to the brain and spinal cord. By localizing drugs at their desired site of action one can reduce toxicity and increase treatment efficiency. In response to the insufficiency in conventional delivery mechanisms, aggressive research efforts have recently focused on the development of new strategies to more effectively deliver drug molecules to the CNS. This review intends to detail the recent advances in the field of brain-targeting, rational drug design approach and drug delivery to CNS. To illustrate the complexity of the problems that have to be overcome for successful brain targeting, a brief intercellular characterization of the blood-brain barrier (BBB) is also included.

Pharmacokinetic approaches to drug distribution in the cerebrospinal fluid based on ventricular administration in beagle dogs.

Two mathematical approaches are described to approximate the distribution of compounds (e.g., drugs) in the cerebrospinal fluid (CSF) downstream of or distal to both the ventricular injection site and the cisterna magna sampling site. The first approach uses a graphic representation and is, in essence, model independent; the second approach considers the geometry and physiology of CSF distribution and clearance. In all studies, radiolabeled inulin was used as an "internal standard" since it is not metabolized and is eliminated from the CSF primarily by bulk flow. Temporal comparison of the study compound to radiolabeled inulin in the cisternal CSF allowed testing of these models in beagle dogs. One use of this data is in the estimation of the drug exposure integral for antineoplastic drugs administered in the CSF to treat leptomeningeal neoplasia.

Clinical pharmacokinetics of cerebrospinal fluid.

The distribution of drugs into the cerebrospinal fluid has long been considered a challenging field of investigation in 2 major respects: (a) understanding how the physicochemical properties (molecular weight, pKa, plasma protein binding) of various molecules influence their movements across such a specific structure as the blood-brain barrier; and (b) defining the relationship between cerebrospinal fluid concentrations of various drugs and their central (side) effects. An attempt has been made to review the very dispersed information presently available to offer a clinically orientated picture of this area of pharmacokinetics. Drugs acting on the central nervous system (benzodiazepines, tricyclic antidepressants, anticonvulsants, opioids), antibacterial agents, cardiovascular drugs (beta-adrenoceptor blockers and digoxin), antineoplastic drugs (mainly methotrexate), and other miscellaneous agents (corticosteroids, cimetidine, methylxanthines) are reviewed. The available evidence seems to support the conclusion that only for methotrexate and antibacterial agents does knowledge of cerebrospinal fluid pharmacokinetics have direct therapeutic implications, while the mosaic of information available for other drugs does little more than provide a partially satisfactory picture.