Department of Pharmacology & Therapeutics, University of Liverpool, Ashton Street, Liverpool L69 3GE, UK
Keywords: P-glycoprotein, lipophilicity, physiochemical, transport, efflux
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Background |
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Plasma drug concentrations |
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Protein binding |
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Intracellular PI concentrations |
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The degree to which each PI is associated with the cell is a dynamic balance between influx, efflux and sequestration. In another study, the co-administration of lopinavir and ritonavir resulted in a greater accumulation of ritonavir in cells compared with other PI regimens containing ritonavir,8 suggesting that some of the processes that govern intracellular accumulation may be amenable to therapeutic manipulation.
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Mechanisms of intracellular accumulation |
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Lipophilicity
In order to penetrate the cell, the PIs must either passively diffuse through the lipid membrane or enter by active transport. The partition coefficient is a measure of lipophilicity, the more lipid soluble the PI, the greater its ability to cross the cell or cellular compartment barrier. All the PIs, except indinavir, are highly lipophilic and easily traverse cellular membranes.3
Intracellular protein binding
In the plasma, the PIs are highly bound to proteins such as 1-acid glycoprotein; however, within the cell there are also many protein-rich areas (e.g. microfilaments, microtubules, proteins embedded in cellular, nuclear and mitochondrial membranes, and phospholipids and proteins in the endoplasmic reticulum). As the PIs are highly bound to extracellular proteins, it seems feasible that the PIs may also be highly bound to intracellular proteins. However, ritonavir, saquinavir, nelfinavir and lopinavir show similar plasma protein binding, yet their hierarchy of cellular accumulation is different. These data imply that protein-mediated sequestration is not solely responsible for the intracellular localization of the PIs.
Ion trapping
As weak bases,3 the PIs are more likely to cross the lipid membranes of cells and organelles when the pH is greater than the pKa of the PIs. Secondly, there is a potential for ion trapping in acidic compartments, resulting in accumulation.
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Drug transporters |
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P-glycoprotein |
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P-gp distribution
P-gp has been observed in liver, colon, jejunum, kidney, pancreatic ductules and adrenal.13 More recently, P-gp expression has been observed in components of the bloodbrain and bloodtestis barriers and this may have implications for limiting PI penetration to these organs, with the resultant formation of sanctuary sites for the virus.14 Normal human lymphocytes express P-gp; we have recently confirmed that different cell subsets express variable levels of the protein.15 CD56+ cells have the highest level of expression followed by CD8+ then CD4+ lymphocytes.
Impact of P-gp on intracellular accumulation
P-gp expression in the lymphocyte alters the in vivo accumulation of PIs,16 raising the possibility that variation in P-gp expression may reflect treatment outcome on highly active antiretroviral therapy. These issues have fuelled the investigation of the role of genetic variations in the ABCB1 gene on P-gp, drug exposure and immune recovery in HIV and a correlation between the C3435T single nucleotide polymorphism (SNP) was recently observed.17 The C3435T SNP is a non-coding, non-promoter SNP at a wobble position in exon 26. It is thus unlikely to affect ABCB1 gene expression per se, but may be linked to functionally important SNPs in the promoter or enhancer regions of the ABCB1 gene, or in sequences important for messenger RNA processing. Interestingly, this SNP has also been related to the extent of induction (in response to P-gp inducers) of P-gp, and recent studies in our laboratory have indicated that some PIs induce this protein in vitro. Further investigations are now required to determine whether PIs cause a reduction in their own accumulation via this mechanism.
Relationship between P-gp and CYP 3A4
When considering drug accumulation in the lymphocyte it is also important to remember that the PIs are metabolized by CYP 3A4.1,2 CYP 3A4 is expressed in lymphocytes and a synergic effect between P-gp and CYP 3A4 has been reported for indinavir in the intestine. This raises the possibility that CYP 3A4 may be an important consideration for PI lymphocytic accumulation. It is interesting to note that the C3435T polymorphism in ABCB1 has been reported to impact on CYP 3A4 expression in intestine. Clearly, these investigations must now be extended in order to assess interplay at the level of the lymphocyte.
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MRP1 |
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Distribution within the cell |
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Summary |
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Acknowledgements |
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Footnotes |
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References |
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