By Bill Berkrot

June 11 (Reuters) - A drug being developed by Amgen Inc for psoriatic arthritis demonstrated sustained effectiveness after a year of treatment with no new safety concerns, according to data from a midstage trial.

Amgen and partner AstraZeneca Plc had previously presented 12-week data from the 168-patient trial of the biotech drug, brodalumab, showing it to be significantly more effective than a placebo.

After 12 weeks, all patients - those who received 140 milligrams of brodalumab every two weeks, those who got the 280-mg dose and the placebo group - were given the high dose over the next 40 weeks with significant disease improvement seen.

At 52 weeks, 71 percent of patients who originally received the 140-mg dose had achieved the study's main goal known as ACR20 - a measure of at least 20 percent improvement in signs and symptoms of the painful condition, such as tender and swollen joints. Among those who got 280 mg from the start, 56 percent were at ACR20 after a year, as was 50 percent of the original placebo group.

After 12 weeks, 37 percent of the low-dose group and 39 percent on the high dose had achieved ACR20 versus 18 percent for placebo.

ACR50, a 50 percent improvement in disease signs and symptoms, was achieved by 47 percent of the original 140-mg patients and 27 percent of the original 280-mg patients at a year, while ACR70 was achieved by 22 percent and 7 percent, respectively. For placebo patients who switched to 280 mg, the ACR50 rate was 38 percent and ACR70, 14 percent.

"From an efficacy point of view the drug worked," Dr. Philip Mease, the study's lead researcher who will present the data at the European League Against Rheumatism meeting in Paris on Saturday, said in a telephone interview.

The results were also published in the New England Journal of Medicine on Wednesday.

Brodalumab belongs to a class of drugs called IL-17 inhibitors that work by blocking a signaling pathway that plays a key role in inducing and promoting inflammatory diseases.

Research firm ISI Group has forecast eventual annual peak sales for the drug of about $2 billion.

Patients in the study had moderate to severe versions of the inflammatory disease related to the skin condition psoriasis, for which the drug is also being tested. They had already failed at least one therapy, including about half who were no longer being helped by injectable biologic medicines known as anti-TNF drugs, such as AbbVie Inc's top-selling Humira and Amgen's Enbrel.

"It's hopeful that this (anti-IL-17) mechanism is central enough that they're going to have responses despite the failure of anti-TNF therapy," Mease said.

Doctors are looking for new treatments for psoriatic arthritis and related diseases as effectiveness of even the powerful anti-TNF drugs tends to diminish over time.

There were four cases of serious adverse events reported, including one serious infection and one patient hospitalized with abdominal pain, but no deaths.

(Reporting by Bill Berkrot in New York; Editing by Lisa Shumaker)