SAN DIEGO (AP) — Shares of Vical Inc. plunged 51 percent Monday after the drug developer said its potential cancer treatment failed in a late-stage study, and that it would now shift its focus to infectious disease vaccines.

Allovectin did not show a statistically significant improvement in either response rate or overall survival when compared with chemotherapy in patients with metastatic melanoma, which is an advanced form of the deadliest type of skin cancer.

Vical has several independent and collaborative infectious disease vaccine programs. The San Diego company is planning to start an early-stage study of a vaccine against herpes simplex virus type 2 before the end of the year.

Vical said it had $70 million in cash and investments as of June 30, which it believes will meet its needs at least through the end of 2014.

“In the coming weeks, we will make the necessary changes to focus resources on our infectious disease vaccine programs and reduce expenses to conserve cash,” CEO Vijay B. Samant said in a statement from the company.

Vical’s shares $1.83 to hit $1.75 before the opening bell. The shares had climbed 23 percent so far in 2013 as of Friday’s closing price of $3.58.