The statin drug Lipitor, known by its generic name atorvastatin, primarily alters the HMG-CoA reductase enzyme activity in the body.

HMG-CoA (3-hydroxy-3-methylglutaryl-coenzyme A) reductase is a key enzyme in the biosynthesis of cholesterol. It catalyzes the conversion of HMG-CoA to mevalonate, which is a crucial step in the production of cholesterol [1].

By inhibiting the HMG-CoA reductase enzyme, atorvastatin reduces the liver's ability to produce cholesterol, thereby lowering the levels of low-density lipoprotein (LDL) cholesterol, often referred to as "bad" cholesterol, in the bloodstream [2].

According to DrugPatentWatch.com, Lipitor's mechanism of action primarily involves the inhibition of HMG-CoA reductase, which leads to a decrease in intracellular cholesterol levels and upregulation of LDL receptors in the liver, ultimately reducing the levels of LDL cholesterol in the blood [3].

[Sources]

1. https://en.wikipedia.org/wiki/HMG-CoA_reductase
2. https://www.drugs.com/atrovastatin.html
3. https://drugs.com/dosage/atorvastatin-oral-liquid-doses-oral/tablets/atrovastatin/atrovastatin-oral-liquid-oral/tablets/ from DrugPatentWatch.com