The basis of combining ritonavir with lopinavir is ?

Ans. is 'b' i.e., CyP3A4 inhibition by ritonavir o Ritonavir is a potent inhibitor of CyP3A4, as such, co-administration with agents heavily metabolized by CyP3A4 must be approached with caution. o This propey of ritonavir can be exploited as a benefit also. o Other less toxic proton pump inhibitors (lopinavir, indinavir, amprenavir) are also metabolized by CyP3A4. So, if lower than therapeutic dose of ritonavir is given along with any of these PIs, it inhibits the metabolism of these drugs. o This results in following benefits for these PIs (lopinavir, indinavir, amprenavir): i) Increased time of drug exposure --> reduced risk of resistance ii) Prolonged half life and less frequent dosing ---> increased compliance.