Pfizer Inc. said on Wednesday it has begun production of a key new drug that combines its best-selling cholesterol drug Lipitor with a new compound designed to raise levels of “good” HDL cholesterol.

Pfizer said the pill combines Lipitor, which lowers levels of “bad” LDL cholesterol, with its experimental compound torcetrapib, which is designed to raise levels of HDL, providing even more protection from heart attacks and stroke.

The combination drug will be produced at an expanded plant in Loughbeg, Ireland, Pfizer said in a release. Pfizer has previously said the combination product has the potential to outstrip Lipitor’s current $10 billion in annual sales.

Lipitor’s U.S. patent expires in 2011. The combination drug, as a new product, would have many years of patent protection and help preserve Pfizer’s earnings growth.

Pfizer is spending an estimated $800 million on late-stage trials of the combination pill, the most expensive phase III trial ever conducted by a drugmaker.

Torcetrapib has been shown in earlier trials to boost levels of HDL by over 50 percent, far better than the 20 percent to 30 percent elevations seen with standard medicines like niacin. Potency of the combo pill, however, will depend on the dosage of torcetrapib used in the ongoing trial.

For decades, cardiologists have focused on preventing heart attacks and strokes by reducing levels of LDL cholesterol. Interest in HDL-raising medicines, however, is growing thanks to studies that have suggested low levels of protective HDL might be an even bigger cause of heart disease than high levels of harmful LDL.

But Wall Street analysts have cautioned that the safety of torcetrapib, which works differently than existing treatments, has not yet conclusively been established.