Experimental approaches to measuring brain exposure: rate, extent and regional distribution Public Deposited

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  • March 20, 2019
Creator
  • Zhao, Rong
    • Affiliation: Eshelman School of Pharmacy
Abstract
  • The objective of this project was to understand the influence of physicochemical properties of compounds, biologic factors and physiologic factors on the kinetics (rate, extent and regional distribution) of brain uptake. The initial rate of brain uptake was determined using in situ brain perfusion in mice, and was found to vary widely among different compounds. Lipophilicity and ionization state, brain tissue unbound fraction, and P-gp-mediated efflux were important determinants of the rate of brain uptake. Fexofenadine was poorly permeable at the blood-brain barrier (BBB) and the slow equilibration of this compound between brain tissue and blood influenced the accuracy of measurement of kinetic parameters such as the P-gp efflux ratio. Pharmacokinetic simulations and continuous subcutaneous infusion with osmotic minipumps provided evidence of time-dependency in fexofenadine P-gp efflux ratio at the BBB. The more permeable precursor, terfenadine, was biotransformed to fexofenadine in brain tissue and increased fexofenadine brain exposure. The potential functional efficiency of breast cancer resistance protein (Bcrp), which shares similar expression pattern and substrate specificity with P-gp, was evaluated using in vitro, in situ and in vivo experimental approaches. While Bcrp effectively mediated substrate flux in vitro, it had limited impact on brain tissue exposure in situ or in vivo. The impact of local perfusion flow rate, capillary density, and P-gp activity on regional exposure to colchicine, quinidine, verapamil, diazepam and loperamide was evaluated in mdr1a(+/+) and mdr1a(-/-) mice using the in situ brain perfusion and in vivo administration together with brain microdissection technique. The regional perfusion rate varied 7.5-fold, and capillary density varied 3.7-fold, among the thirteen brain regions examined. Regional flow correlated with capillary density (R2=0.955). Regional P-gp-mediated efflux activity for colchicine and verapamil was directly proportional to local capillary density. Verapamil brain uptake was reduced with decreased blood perfusion rate, and P-gp-mediated efflux in less-perfused brain regions was influenced by changes in flow. In well-perfused brain structures, flow had no or little effect on P-gp activity. Taken together, these results provide a framework for understanding the multiplicity of interactive factors that determine regional brain exposure in intact animals.
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  • Pollack, Gary M.
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