Shares of Axovant Sciences nearly doubled in value after the dementia drug developer said its initial public offering raised $315 million, more than Axovant had expected.

The company expanded its offering to 21 million shares and said the offering priced at $15 a share, at the high end of its estimates.

The stock climbed $14.90, or 99.3 percent, to end its first trading day at $29.90. It peaked at $31.17.

The offering values Axovant at around $1.4 billion. The Bermuda-based company is controlled by Roivant Sciences Ltd., which invests in biotech companies with late-stage experimental drugs. Roivant owns about 80 percent of Axovant's shares.

The shares are trading under the symbol "AXON."

Axovant Sciences Ltd.'s main drug candidate is called RVT-101. The company is preparing to start late-stage trials of RVT-101 as a once-per-day treatment for dementia caused by Alzheimer's disease and also wants to study it as a treatment for other types of dementia.

Axovant says that in one clinical study, patients who were treated with RVT-101 and Aricept, an older drug, had bigger improvements in cognition and daily living measurements than patients who took only Aricept.

The CEO of Axovant is Vivek Ramaswamy, 29, and the company said he is the second-youngest CEO of any company listed on the New York Stock Exchange. Ramaswamy is also the chairman of Tekmira Pharmaceuticals Corp., a company that is studying treatments for Ebola and hepatitis B. Roivant Sciences owns a minority stake in Tekmira.