A combination of the drugs ritonavir and lopinavir. It is used to treat infection with HIV (the virus that causes AIDS). It is also being studied in the treatment of some types of cancer. Kaletra blocks the ability of HIV to make copies of itself and may help other anticancer drugs work better or may block the growth of cancer cells. Ritonavir blocks the breakdown of lopinavir. Kaletra is a type of anti-HIV agent and a type of protease inhibitor. Also called lopinavir/ritonavir.