Sienna Biopharmaceuticals Inc.’s announcement that it will conduct a secondary stock offering this week to raise $20 million sent shares down 18 percent on Wednesday.

The Westlake Village company, a clinical-stage biotech focused on inflammatory skin conditions, will sell 8 million shares at $2.50 each. The underwriters will have a 30-day option to purchase 1.2 million extra shares at the same price.

The offering is expected to close on Friday. Cowen and BMO Capital Markets are joint book-running managers for the offering.

Shares of Sienna (SNNA) closed Wednesday down 54 cents, or 18 percent, to $2.45 on the Nasdaq.