P-glycoprotein and mutlidrug resistance-associated proteins limit the brain uptake of saquinavir in mice

Efflux transporters such as P-glycoprotein (P-gp) and multidrug resistance-associated proteins (Mrps) and their contributions to saquinavir (SQV) brain uptake were characterized. Cerebral flow rate was estimated from diazepam uptake and brain vascular volume was assessed using inulin. Mice brains were perfused with buffer containing SQV alone or coperfused with different concentrations of GF120918, a P-gp inhibitor or MK571, a specific Mrp family inhibitor. Inulin, a nonabsorbable marker, was also coperfused in all studies to assess whether the inhibitors altered the physical integrity of the blood-brain barrier (BBB). The estimated cerebral flow rate using diazepam was 250 ml·100g^-1·min^-1. The brain vascular volume, estimated using inulin, was almost constant (0.94 ± 0.03 ml·100 g^-1, n = 12) during the perfusion study. SQV uptake kinetics was linear during the sampling period. Inclusion of 10 μM GF120918 in the perfusate resulted in a more than 7-fold increase in the brain distributional volume (i.e., uptake) of SQV. Inclusion of 100 μM MK571 in the perfusate increased SQV apparent brain uptake by more than 4.4-fold, suggesting, for the first time, that Mrp transporters may play an important role in the brain uptake and retention of SQV. Neither GF120918 nor MK571 altered the integrity of the BBB during the time course of the study. Although the current results reaffirm that SQV is a P-gp substrate, this is the first report implicating the Mrp transporter family in the limited brain uptake and retention of SQV in vivo in mice.