Alexza Pharmaceuticals Inc. said Thursday it is offering about $18 million in stock and warrants to institutional investors.

The company said it agreed to sell a total of 6,685,183 units, with each unit consisting of one share of common stock and a warrant to purchase a half-share of common stock. Each unit is being offered at $2.70 per unit.

The warrants will be exercisable six months after issuance at $3.30 per share and will expire five years from the date of issuance.

The company, which is based in CourtMountain View, Calif., expects to use about $16.4 million in net proceeds for general corporate purposes, including clinical trial, research and development, general and administrative and manufacturing expenses.

RBC Capital Markets Corporation is acting as the sole lead placement agent.

Shares of Alexza fell 46 cents, or 15.3 percent, to $2.54 a share in morning trading Thursday.