Saquinavir is an HIV protease inhibitor with no, or limited, effect on the activity of other structurally related human aspartic proteinases. As with other HIV protease inhibitors, saquinavir inhibits the cleavage of the gag-pol protein substrate leading to the release of structurally defective and functionally inactive viral particles. It is active on both HIV-1 and HIV-2, and also has activity on chronically infected cells and HIV strains resistant to reverse transcriptase inhibitors. Synergy of action has been observed with other antiretroviral drugs. Saquinavir is characterised by a low bioavailability which is further reduced in the fasting state. Metabolism is mainly hepatic through cytochrome P450 (CYP) 3A4, but intestinal metabolism through the same system has also been reported. To achieve higher drug plasma concentrations and increase the antiviral effect, a new formulation of saquinavir with a higher bioavailability has recently been introduced. Higher plasma drug concentrations may also be obtained by combining the drug with CYP blockers, such as ritonavir or ketoconazole. Because of its metabolic interference with the CYP system, saquinavir cannot be coadministered with astemizole, terfenadine or cisapride. Rifampicin (rifampin) is also contraindicated because coadministration can lead to decreases in saquinavir concentrations. Interactions have also been reported with other drugs metabolised through the same system, including non-nucleoside reverse transcriptase inhibitors and HIV protease inhibitors. Resistance has been observed after both in vitro and in vivo drug exposure, with a relatively specific mutation profile compared with other protease inhibitors. Saquinavir is generally well tolerated, with mild gastrointestinal symptoms representing the most commonly observed adverse effects. Although characterized by low bioavailability, in phase III trials saquinavir has been shown to have clinical efficacy in terms of survival and progression rate. As with the other protease inhibitors, saquinavir should be used in combination with other antiretroviral drugs. Current therapeutic guidelines, however, recommend the selection of an initial treatment regimen with other protease inhibitors with higher in vivo activity in terms of RNA and CD4 response. The results of ongoing studies will clarify to what extent a new saquinavir formulation, recently introduced, is superior to the previous one in terms of antiviral activity and to provide comparisons with other protease inhibitors. Further studies are also needed to define the best place of saquinavir within treatment strategies based on protease inhibitors, particularly in respect to the optimal sequence for its use with other protease inhibitors, and the dynamics of cross-resistance and its role within regimens based on the combination of protease inhibitors.
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