Shares of Arrowhead Pharmaceuticals Inc. fell about 15% on Monday, sliding on the same day that rival Ionis Pharmaceuticals sharply cut its sales forecast for a competing triglyceride drug.

Arrowhead shares fell $8.95, or 15.4%, to $49.21 in Monday trading, with about 3.2 million shares traded, nearly double the average. The decline cut the company's market value to about $7 billion. The stock is still up about 46% over the past year.

Arrowhead had closed at $58.16 on Nov. 6.

Arrowhead did not issue a price-sensitive announcement explaining Monday's move.

The decline........

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