P-glycoprotein expression in HTLV-III cells after treatment with HIV-1 protease inhibitors

P-glycoprotein (P-gp), a membrane protein that pumps drugs out of cells, affects the availability and effectiveness of drugs and their extrusion from cells. HIV-1 protease inhibitors (PIs), part of the antiretroviral treatment known as highly active antiretroviral treatment, are substrates and possibly inhibitors of the P-gp pump. Their interaction may represent a potential effect on treatment efficiency. Our objective is to evaluate how the P-gp/PI interaction limits drug effectiveness.

HTLV IIIcc cell cultures were exposed to ritonavir and saquinavir for 24 hours. Supernatant solution was recovered for viral load assessment. Cells were labeled with monoclonal antibody against P-gp and analyzed by flow cytometery.

Upregulation in P-gp expression from 1% to 7% was observed when cells were exposed to PIs, compared with cells not exposed to PIs (P = .05). Ritonavir 10 microg/mL caused a similar P-gp increment as did 20 microg/ mL saquinavir. To evaluate P-gp functionality, cells were exposed to rhodamine-123, a fluorescent dye that is also a P-gp substrate. Its accumulation was measured by flow cytometry. Slightly more rhodamine was observed in cells treated with higher PI concentration (P = .05). Higher viral load was obtained in suspension of cells with upregulated P-gp. Statistically significant decreased viral load was obtained in supernatants of cells expressing less P-gp (P < .04). Ritonavir 20 microg/mL caused the most marked reduction in viral load.

Our results suggest that the use of PIs upregulates the expression of P-glycoprotein on HTLV IIIcc cells, showing slightly inhibited functionality for those treated with higher concentrations. The rapid extrusion of the drug by P-gp seems to limit its action. Decreased viral load in suspensions with ritonavir 20 microg/mL may represent the inactivation of the transport pump, allowing the drug to work more efficiently.