The protease inhibitors cause resistant variants of the original (called "wild type") virus to form. The hope is to knock out the wild type virus quickly, before this happens. If you do relapse, and resistant variants have formed, you can't use the available PIs again to treat because it won't work -- your virus has now mutated to be resistant to those drugs.
I do think they are trying to find new PIs with different resistance profiles (i.e. they cause different types of resistant strains to form). If this succeeded, I think it would theoretically be possible to re-treat with a different PI that had a different resistance profile from the one you originally took.
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