Kaletra contains lopinavir and ritonavir as its active ingredients.
Lopinavir is a protease inhibitor. When viral RNA gets transcribed into DNA and DNA gets translated into proteins, the viral proteins produced are joined together, forming a long polypeptide sequence. In order for the different viral proteins to function properly, they must first separate from the long polypeptide sequence. HIV proteases take over the task of breaking the long polypeptide into functional proteins. Of course, this is bad news for us. Lopinavir functions by inhibiting HIV proteases and thus preventing the formation of functional viral proteins.
Lopinavir gets metabolised in the liver almost exclusively by CYP3A, a cytochrome P450 enzyme. This is why ritonavir is also administered with lopinavir; as it is a CYP3A inhibitor, and therefore has the potential to suppress lopinavirís metabolism, increasing its levels in the body and thus prolonging and potentiating its effects.