Influence of breast cancer resistance protein (Abcg2) and p-glycoprotein (Abcb1a) on the transport of imatinib mesylate (Gleevec) across the mouse blood-brain barrier.

Imatinib, a protein tyrosine kinase inhibitor, may prevent the growth of glioblastoma cells. Unfortunately, its brain distribution is restricted by p-glycoprotein (p-gp or multidrug resistance protein Mdr1a), and probably by breast cancer resistance protein (Bcrp1), two efflux pumps expressed at the blood-brain barrier (BBB). We have used in situ brain perfusion to investigate the mechanisms of imatinib transport across the mouse BBB. The brain uptake of imatinib in wild-type mice was limited by saturable efflux processes. The inhibition of p-gp, by valspodar and zosuquidar, increased imatinib uptake (2.5-fold), as did the deficiency of p-gp in Mdr1a/1b(-/-) mice (5.5-fold). Perfusing imatinib with the p-gp/Bcrp1 inhibitor, elacridar, enhanced the brain uptake of imatinib in wild-type (4.1-fold) and Mdr1a/1b(-/-) mice (1.2-fold). However, the brain uptake of imatinib was similar in wild-type and Bcrp1(-/-) mice when it was perfused at a non-saturating concentration. The brain uptake of CGP74588, an active metabolite of imatinib, was low. It was increased by perfusion with elacridar (twofold), but not with valspodar and zosuquidar. CGP74588 uptake was 1.5 times greater in Bcrp1(-/-) mice than in wild-type mice. These data suggest that imatinib transport at the mouse BBB is limited by p-gp and probably by Bcrp1, and that CGP74588 transport is restricted by Bcrp1.

Modulation of the brain distribution of imatinib and its metabolites in mice by valspodar, zosuquidar and elacridar.

PURPOSE: The selective protein tyrosine kinase inhibitor, imatinib, inhibits the growth of glioma cells in preclinical models, but its poor brain distribution limits its efficacy in patients. P-glycoprotein (P-gp, rodent Mdr1a/1b or Abcb1a/1b) and Breast cancer resistance protein (rodent Bcrp1 or Abcg2) were suggested to restrict the delivery of imatinib to the brain. This study evaluates the effect of administering selective inhibitors of these transporters together with imatinib on the systemic and cerebral disposition of imatinib in mice. MATERIALS AND METHODS: Wild-type, Mdr1a/1b(-/-) and Bcrp1(-/-) mice were given imatinib intravenously, either alone, or with valspodar, zosuquidar (P-gp inhibitors), or elacridar (a P-gp and Bcrp1 inhibitor). The blood and brain concentrations of [(14)C]imatinib and its radioactive metabolites were determined. RESULTS: The blockade of P-gp by valspodar or zosuquidar (>3 mg/kg) enhanced the brain uptake of imatinib ( approximately 4-fold) in wild-type mice, but not that of its metabolites. Blockade of both P-gp and Bcrp1 by elacridar (>3 mg/kg) produced significantly greater brain penetration of imatinib (9.3-fold) and its metabolites (2.8-fold). In contrast, only the lack of P-gp enhanced imatinib brain penetration (6.4-fold) in knockout mice. These results of brain uptake correlated reasonably well with those obtained previously by our group using in situ brain perfusion. CONCLUSIONS: Imatinib and its metabolites penetrate into the brain poorly and their penetration is limited by P-gp and (probably) Bcrp1. Administering imatinib together with P-gp (and Bcrp1) transporter inhibitors such as elacridar may improve the delivery of imatinib to the brain, making it potentially more effective against malignant gliomas.

ABP688, a novel selective and high affinity ligand for the labeling of mGlu5 receptors: identification, in vitro pharmacology, pharmacokinetic and biodistribution studies.

[(11)C]ABP688 (2) has recently been demonstrated to be a useful PET tracer for in vivo imaging of the metabotropic glutamate receptors type 5 (mGluR5) in rodents. We describe here the identification and preclinical profiling of ABP688 and its tritiated version [(3)H]ABP688, and show that its high affinity (K(d)=2nM), selectivity, and pharmacokinetic properties fulfill all requirements for development as a PET tracer for clinical imaging of the mGlu5 receptor.

Influence of hydroxyurea on imatinib mesylate (gleevec) transport at the mouse blood-brain barrier.

The combination of imatinib mesylate and hydroxyurea provides a therapeutic benefit in patients with glioblastoma, although each drug is not effective when used alone. The increase of brain delivery of one or both drugs has been suggested to be a potential cause of this therapeutic benefit. The cross-influence of hydroxyurea and imatinib on their respective brain distribution was examined in mice and rats. We used in situ brain perfusion in mice to determine whether these two drugs have an influence on their respective initial transport across the blood-brain barrier. The brain penetration of hydroxyurea, assessed by its brain uptake clearance, Knet, was low in mice (approximately 0.10 microl/g/s) and not modified by coperfusion of imatinib (0.5-500 microM). Likewise, the brain penetration of imatinib was low (Knet, 1.39 +/- 0.17 microl/g/s) and not modified by direct coperfusion of hydroxyurea (0.2-1000 microM) or by intravenous pretreatment with 15 or 1000 mg/kg hydroxyurea. We also examined a potential time-dependent influence of hydroxyurea on imatinib brain distribution after sustained subcutaneous administration in rats using an implantable osmotic pump. The brain penetration of imatinib in rats increased with time, approximately 1.6-fold (p < 0.01) after 7 and 14 days' infusion of imatinib (3 mg/day) with or without hydroxyurea (15 mg/day), and was not influenced by hydroxyurea. The results of these two sets of experiments indicate that hydroxyurea has no significant influence on the brain distribution of imatinib in mice and rats.

Distribution of STI-571 to the brain is limited by P-glycoprotein-mediated efflux.

The adequate distribution of STI-571 (Gleevec) to the central nervous system (CNS) is critical for its effective use in CNS tumors. P-glycoprotein-mediated efflux in the blood-brain barrier may play a role in the CNS delivery of this drug. Whether STI-571 is a substrate of P-glycoprotein was determined by examining the directional flux of [(14)C]STI-571 in parental and MDR1-transfected Madin-Darby canine kidney (MDCK) II epithelial cell monolayers. The basolateral-to-apical flux of STI-571 was 39-fold greater than the apical-to-basolateral flux in the MDR1-transfected cells and 8-fold greater in the parental cell monolayers. This difference in directional flux was significantly reduced by a specific P-glycoprotein inhibitor (2R)-anti-5-[3-[4-(10,11-difluoromethanodibenzo-suber-5-yl)piperazin-1-yl]-2-hydroxypropoxy]quinoline trihydrochloride (LY335979). The role of P-glycoprotein in the CNS distribution of STI-571 was examined in vivo, using wild-type and mdr1a/b (-/-) knockout mice that were orally administered 25 mg/kg [(14)C]STI-571. In the wild-type mice, the brain-to-plasma STI-571 concentration ratio at all time points was low (1-3%); however, there was an 11-fold greater brain partitioning of STI-571 at 1 h postdose in the mdr1a/b (-/-) mice compared with the wild-type mice. When 12.5 mg/kg STI-571 was given intravenously, the brain-to-plasma ratio of STI-571 in the mdr1a/b (-/-) mice was approximately 7-fold greater than that of wild-type mice up to 120 min postdose. These data indicate that STI-571 is a substrate of P-glycoprotein, and that the inhibition of P-glycoprotein affects the transport of STI-571 across MDCKII monolayers. Moreover, P-glycoprotein plays an important role in limiting the distribution of STI-571 to the CNS.

Brain pharmacokinetics of a nonpeptidic corticotropin-releasing factor receptor antagonist.

Corticotropin-releasing factor (CRF) is known to play an important role in the body response to stress. Butyl-[2,5-dimethyl-7-(2,4,6-trimethylphenyl)-7H-pyrrolo[2,3-d]pyrimidin-4-yl]-ethylamine (CP-154,526) is a CRF(1) antagonist showing anxiolytic activities in rats in behavioral models, suggesting that CP-154,526 crosses the blood-brain barrier. However, there is no direct evidence for this. This study determined the pharmacokinetic profile of CP-154,526 in rats after i.v. and p.o. application. After i.v. bolus, the concentration declined in a biphasic manner, the first half-life being 0.9 h and the terminal one being 51 h. Systemic clearance was 36 ml/min/kg, and the volume of distribution was 105 l/kg. Oral bioavailability reached 27%. To study brain pharmacokinetics, rats were given a single dose of CP-154,526 p.o. or i.v. and sacrificed after different post-treatment times. Plasma, cortex, striatum, hypothalamus, hippocampus, and cerebellum concentrations were measured. After i.v. bolus, maximal brain concentration was reached after 20 min. The hypothalamus displayed significantly lower concentrations compared with the other brain tissues. In the p.o. study, the maximal plasma concentration was reached after 30 min, whereas the maximal brain concentration was observed after 1 h and remained stable until 2 h post-treatment, without significant differences between the brain tissues. The unidirectional brain extraction ratio was 27 and 9% at vascular concentrations of 0.08 and 16 nmol/ml, respectively. These results demonstrate a large brain penetration of CP-154,526 after i.v. and p.o. applications and a comparable distribution among the brain regions, except for the hypothalamus, which displayed lower concentrations after i.v. bolus.