Sirna Therapeutics announced plans Monday for a secondary stock offering, sending shares of the biotech company down more than 8 percent.

Sirna plans to sell a total of 8 million shares. An additional 2 million shares are being sold by venture capital firms Oxford Bioscience Partners, The Sprout Group, and Venrock Associates.

“It’s very important for us to have a significant balance sheet to be able to pursue our programs,” said Rebecca Galler Robison, Sirna’s senior director of corporate strategy. “This is a very big year for us… so this money is going to be very instrumental.”

Shares of Sirna slipped $0.58 to $6.18 in recent trading as investors worried how the additional shares would dilute profit per share.

Based in San Francisco, Sirna focuses on a technology called RNA interference. This involves readying a very efficient enzyme nicknamed “splicer” to seek out and destroy specific messenger RNA molecules, stopping them from instructing the cell to make certain proteins.

The result is that specific genes are silenced (see Top 10 Trends: Silencing the genes).

Ms. Galler Robison said the proposed public offering will enable Sirna to start an early-stage clinical trial for an experimental treatment for hepatitis C and to select a clinical candidate from its hair loss drug development program by the end of the year. The company hopes to find out late this year whether it will be permitted to proceed with the hepatitis C trial.

AMD Drug

Allergan, based in Irvine, California, is developing Sirna’s lead drug candidate for the treatment of age-related macular degeneration (AMD), a common cause of blindness among the elderly.

AMD has been considered the low-hanging fruit of the RNAi space, because potential RNAi treatments could be delivered straight to the eye, with an injection. Developing RNAi therapeutics to travel around the body in the blood to their anticipated site of action has proven more difficult (see Q&A: Sirna’s Howard Robin).

Allergan’s program is due to enter mid-stage clinical trials soon. The company could not immediately confirm when the trial is due to start.