How does saquinavir's absorption change when co-administered with ritonavir?

When saquinavir is co-administered with ritonavir, the absorption of saquinavir is enhanced due to ritonavir's ability to inhibit the metabolism of saquinavir. Ritonavir is a potent inhibitor of cytochrome P450 enzymes, particularly CYP3A4, which plays a significant role in the metabolism of many drugs, including saquinavir. By inhibiting this metabolic pathway, ritonavir increases the plasma concentration of saquinavir, allowing for greater absorption and bioavailability of the drug.

This interaction is particularly valuable in clinical settings, as it enables the use of lower doses of saquinavir while maintaining effective antiviral activity, thus reducing potential side effects. In the absence of ritonavir, saquinavir may be rapidly metabolized, resulting in suboptimal drug levels. Therefore, the co-administration of ritonavir effectively enhances the therapeutic effects of saquinavir, making this approach beneficial in HIV treatment regimens.