By Saumya Joseph

(Reuters) – Alnylam Pharmaceuticals Inc’s late-stage trial data on its new gene silencing drug cast doubt on its safety, even as results revealed its effectiveness in treating a rare, painful disease.

Shares of the company fell nearly 6 percent to $83.41 in early morning trading on Wednesday.

Alnylam said the drug, givosiran, met the main goal of reducing the yearly number of attacks in patients with acute intermittent porphyria (AIP) – a disease that affects the liver and causes debilitating attacks that render most disabled.

Givosiran uses a Nobel prize-winning mechanism, RNA interference (RNAi), to target and “silence” specific genetic material, blocking the production of the deadly protein that causes the disease.

However, a large portion of the patients being treated with givosiran experienced serious side effects, such as renal impairment and elevated liver enzymes, compared to those on placebo.

Givosiran’s efficacy profile looks robust but its safety profile is materially worse than expected, SVB Leerink analyst Mani Foroohar said in a note.

Although the patients did have underlying liver disease which could have affected results, the large number of side effects among patients on the drug was still worrying, Foroohar added.

On a conference call, analysts questioned the drug’s safety but Alnylam executives played down the concerns.

“Chronic administration of givosiran is very, very well tolerated, both from a kidney and liver perspective,” Chief Medical Officer, Pushkal Garg said on the call.

But analysts were not convinced.

“I think what was more concerning was that on the follow up call, the company was very defensive regarding these toxicities and didn’t elaborate on if they were going to provide more data on the disease severity of patients in which these toxicities occurred in,” Nomura Instinet analyst Christopher Marai told Reuters.

Givosiran also did not meet four of its nine secondary goals which tested the reduction in symptoms like daily pain, nausea and fatigue levels.

The drug was tested in patients with acute hepatic porphyria (AHP) – a family of rare diseases that affects the liver. Acute intermittent porphyria is the most common subtype of AHP.

Currently, there are no approved treatments to prevent the attacks and treat the chronic symptoms of the disease, which include intense abdominal pain.

Alnylam estimates about 1,000 patients in the United States and Europe suffer severe, recurrent attacks and nearly 5,000 experience less frequent attacks.

Jefferies analyst Maury Raycroft expects givosiran to bring in peak sales of $600 million by 2030.

Alnylam said it plans to complete its regulatory submissions for the drug to U.S. and European Union regulators in mid-2019, and expects to launch the drug in early 2020.

(Reporting by Saumya Sibi Joseph in Bengaluru; Editing by Arun Koyyur and Shailesh Kuber)