By Tess Stynes

Ionis Pharmaceuticals Inc. said partner GlaxoSmithKline PLC has opted against initiating a Phase 3 study of a drug they are developing for treatment of a rare, potentially fatal organ condition.

The Carlsbad, Calif., drug developer's shares, down 65% this year, fell 38% to $21.96 in midafternoon trading in New York.

The U.S. Food and Drug Administration in April placed a clinical hold on GlaxoSmithKline's Phase 3 study of Ionis-TTR as a potential treatment for patients with transthyretin amyloid cardiomyopathy, a condition in which the misfolded transthyretin protein accumulates in the cardiac muscle.

On a conference call, Ionis confirmed that severe declines in platelets were observed in a small number of patients in two other Ionis studies, one involving patients with transthyretin amyloid polyneuropathy and the other in a Phase 3 study of another treatment, volanesorsen. The latter drug is meant for patients with familial chylomicronemia syndrome, a condition that results in very high levels of triglycerides.

Ionis didn't give a specific number of cases or disclose the specific medical effects of the platelet declines.

Leerink in a research note said that regardless of whether the safety issues are related to the drug class or to a specific compound, "we believe these observations clearly increase the risk around [Ionis's] platform." Leerink said that Ionis's transthyretin amyloidosis program comprises roughly 36% of its valuation.

Ionis said that GlaxoSmithKline would consider options for the transthyretin amyloid cardiomyopathy study once additional clinical data are available from other studies that are under way.

Ionis is evaluating the treatment in patients with a hereditary version of transthyretin amyloidosis in continuing late-stage study, which the company said remains on track, with clinical data expected in the first half of next year.

Shares of Alnylam Pharmaceuticals Inc., which also has a treatment for hereditary transthyretin amyloidosis in its late-stage development pipeline, rose 11% to $71.24 in midafternoon trading.

Write to Tess Stynes at tess.stynes@wsj.com

(END) Dow Jones Newswires

May 26, 2016 15:23 ET (19:23 GMT)

Copyright (c) 2016 Dow Jones & Company, Inc.
GSK (NYSE:GSK)
Historical Stock Chart
From Mar 2024 to Apr 2024 Click Here for more GSK Charts.
GSK (NYSE:GSK)
Historical Stock Chart
From Apr 2023 to Apr 2024 Click Here for more GSK Charts.