By Bill Berkrot

NEW ORLEANS (Reuters) - A new type of heart drug developed by Medicines Co significantly lowered "bad" LDL cholesterol in high-risk patients with no major safety issues, according to data from a midstage trial that helped the company identify the dose it will move into advanced testing.

The drug, now called inclisiran, belongs to a new class of drugs that inhibit a protein known as PCSK9, which prevents the removal of LDL from the blood. But the Medicines Co drug works in a different way than the two already on the market and is given far less frequently, an important potential commercial difference if it eventually wins approval.

The study tested several strengths of inclisiran against a placebo in about 500 patients, many of whom had had a prior heart attack or stroke. All had high LDL levels despite most being on high doses of cholesterol-lowering statins.

The study found 300 milligrams to be the minimum dose to achieve maximum efficacy, having shown itself to lower LDL about as much as the 500 mg strength. That is the dose that will be used in larger Phase III trials aimed at winning regulatory approval, the company said.

One 300 mg dose of inclisiran led to an average lowering of LDL by 51 percent at Day 60, which was maintained over three months. Two 300 mg injections given on Day 1 and Day 90 on average took LDL down by 57 percent at Day 120, and maintained its effectiveness over six months.

The data indicates the drug "is likely to ensure significant and durable reductions in LDL cholesterol and could potentially impact cardiovascular outcomes," said Dr. Kausik Ray, who presented the data at the American Heart Association scientific meeting in New Orleans on Tuesday.

Dr. Borge Nordestgaard, who was not involved in the study, called the data "really, really encouraging."

Only 189 patients were available for evaluation at 180 days. Six-month data for all patients is expected by year-end.

Inclisiran is designed to be taken just twice or three times a year. Amgen Inc's Repatha and Praluent from Regeneron Pharmaceuticals Inc and Sanofi SA are injected either every two or four weeks.

Those drugs are antibodies that block PCSK9 from doing its job in the blood. Inclisiran utilizes a technology known as RNA interference, or RNAi, that blocks PCSK9 production at its source in the liver.

Medicines Co licensed inclisiran from Alnylam Pharmaceuticals Inc, which would be entitled to royalty payments on future sales.

(Editing by Matthew Lewis)