Alnylam Pharmaceuticals Inc. said Wednesday it is abandoning a late-stage clinical trial of an experimental drug to treat a rare life-threatening heart and nerve disease, the second development setback in the past week for the Cambridge company.

In a statement issued after the close of financial markets, Alnylam said it was following the recommendation of an independent data monitoring committee that cited the onset or worsening of nerve damage in patients taking the Alnylam drug, called revusiran, in the study. Revusiran was the considered the second most advanced drug candidate in Alnylam’s pipeline.

Alnylam officials said 18 patients have died in the study involving more than 200 patients. But investigators have not concluded that any of the deaths were drug-related. Some patients in the study were taking a placebo rather than the drug, but the company did not disclose how many of the patients who died had taken the drug and how many had taken the placebo.

Patients with the disease being treated by Alnylam’s compound — a condition called hereditary ATTR amyloidosis with cardiomyopathy — have a high morbidity rate and often die of heart failure. The condition affects about 40,000 people worldwide and has no approved treatment.

“This is a disappointing outcome,’’ Alnylam chief executive John Maraganore said in a late afternoon conference call with stock analysts. He said the clinical study was discontinued because company executives determined that “patients come first.’’

Shares of Alnylam were down 42 percent in after-hours trading.

The company is one of the highest valued biotech companies in Massachusetts, with a market capitalization of more than $5.8 billion, despite not yet having a drug on the market.

On Sept. 29, the company said it halted development of a liver disease drug candidate, called ALN-AAT, after an earlier-stage clinical study showed that three patients had liver enzyme elevation at the highest dose given in the trial.

Maraganore said the latest setback doesn’t affect any of seven other Alnylam drug candidates in clinical development, including its lead experimental drug, called patisiran, which treats another form of the disease called hereditary ATTR amyloidosis with polyneuropathy.

He said Alnylam, which is developing a class of drugs based on a gene-silencing technology known as RNA interference, still expects to have three treatments on the market by 2020.

Robert Weisman can be reached at robert.weisman@globe.com. Follow him on Twitter @GlobeRobW.