The basis of combining ritonavir with lopinavir:

Five protease inhibitors-Indinavir (IDV), Nelfinavir (NFV), Saquinavir (SQV), Ritonavir (V) and Lopinavir (in combination with ritonavir LPV /r) have been marketed in India for use against HIV.All are extensively metabolized by CYP3A4 and other CYP isoenzymes. All (especially ritonavir and lopinavir) are potent inhibitors of CYP3A4, while few other CYP isoenzymes are induced. The Pis interact with many drugs. Nelfinavir and ritonavir induce their own metabolism.Ritonavir: It is a potent PI; also a potent CYP3A4 inhibitor. .ESSENTIALS OF MEDICAL PHARMACOLOGY K.D.TRIPATHI SIXTH EDITION PAGE NO:772,773,774