Use of antiviral drugs to treat HIV infection is often limited by the inability of compounds to access sites of infection. In this regard, the brain provides a sanctuary for HIV. Three barriers can confound treatment of virus within the brain: the blood-brain barrier at the capillary endothelium (blocks drug entry), the blood-CSF barrier at the choroid plexus (facilitates drug efflux) and the plasma membranes of the glial cells that harbor HIV (limits access to virus). Our experiments with isolated rat and pig brain capillaries indicate that the HIV protease inhibitors, ritonavir and saquinavir, interact with two ATP-driven drug export pumps, p-glycoprotein and Mrp2 and that these transporters are critical impediments to drug penetration. Our recent in vitro and in vivo experiments with animal models indicate that changes in transporter expression could lead to potentially important complications. First, we found increased activity and expression of both transporters in brain capillaries from animals dosed with ligands for the pregnane-X receptor (PXR). Second, we found similar upregulation in capillaries exposed over a period of hours to the proinflammatory cytokine, TNF-alpha. Since many commonly prescribed drugs are PXR ligands and since inflammation often accompanies neuro-AIDS, these findings indicate an even tighter than expected barrier to drug penetration into the CNS. In contrast to the blood-brain barrier, it is still not clear by what mechanisms anti-HIV drugs are cleared from the CSF. Experiments with isolated choroid plexus from mice and rats indicate specific uptake of saquinavir from CSF. We are in the process of functionally mapping the xenobiotic transport pathways in this tissue. Finally, we have determined that both p-glycoprotein and Mrp1 limit accumulation of saquinavir in astrocytes and microglia. Of the two efflux transporters, p-glycoprotein appears to be the most important. In both microglia and astrocytes, p-glycoprotein activity increased upon exposure to low levels of an inflammogen (bacterial endotoxin, LPS) or cytokines (IL-1); Mrp1 activity was not affected. In astrocytes but not microglia, this increase in pump activity was accompanied by a parallel increase in transporter protein expression. These latter observations suggest that inflammation, would reduce the CNS effectiveness of anti-HIV drugs that are p-glycoprotein substrates, even if they first penetrated the blood-brain barrier.
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