LJPC La Jolla Pharmaceutical Company

On March 22, 2017, La Jolla Pharmaceutical Company (“ we ,” “ La Jolla ” or the “ Company ”) entered into an underwriting agreement (the “ Underwriting Agreement ”) with J.P. Morgan Securities LLC and Cowen and Company, LLC, as representatives of the several underwriters (the “ Underwriters ”), pursuant to which we agreed to issue and sell an aggregate of 3,731,344 shares of our common stock (the “ Shares ”) to the Underwriters (the “ Offering ”). The Shares were sold at a public offering price of $33.50 per Share, and were purchased by the Underwriters from us at a price of $31.49 per Share. Under the terms of the Underwriting Agreement, we granted the Underwriters an option for 30 days from the date of the Underwriting Agreement to purchase up to an additional 559,700 shares of our common stock (the “ Optional Shares ”).

We estimate that the net proceeds we will receive from the Offering, not including the proceeds from the sale of any of the Optional Shares that may be purchased by the Underwriters following the exercise of their option, will be approximately $117,250,023, after deducting the Underwriters’ discounts and commissions and estimated offering expenses payable by us, and without regard to any reimbursement of certain offering-related expenses by the Underwriters.

The Offering was made pursuant to La Jolla’s shelf registration statement (File No. 333-214721), including a base prospectus, which was declared effective by the United States Securities and Exchange Commission (the “ SEC ”) on December 8, 2016. The specific terms of the offering are described in a prospectus supplement to be filed with the SEC in connection with the offering.

The Offering is expected to close on March 28, 2017, subject to the satisfaction of customary closing conditions. In the Underwriting Agreement, the Company agreed to indemnify the Underwriters against certain liabilities, including liabilities under the Securities Act of 1933, as amended, or to contribute to payments that the Underwriters may be required to make because of such liabilities.

A copy of the Underwriting Agreement is attached as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing description of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to such exhibit.

A copy of the opinion of Gibson, Dunn & Crutcher LLP relating to the validity of the securities issued in the Offering is filed herewith as Exhibit 5.1.