$15 billion biotech stock loses more than 40% of its value after skin cancer drug study falls short

• Nektar's market cap, which was over $15 billion by Friday's close, fell to $9 billion by Monday's close.
• The 41.8 percent drop in stock price was the company's worst one-day fall ever.
• The fall came after data released Saturday suggested patients diagnosed with melanoma aren't reacting to the company's experimental treatment as well as earlier samples did.

President and Chief Executive Officer of Nektar Therapeutics.

Noah Seelam | AFP | Getty Images

Shares of biotech company Nektar Therapeutics lost nearly half their value Monday after the company gave a clinical trial update that fell well short of investor expectations.

The price of the S&P 500 component fell 41.8 percent Monday, down $37.78 from Friday's closing price above $90. Nektar's market cap, which was over $15 billion by Friday's close, was reduced to roughly $9 billion by the end of the session.

The drop in stock price was the company's worst one-day fall ever.

The precipitous fall came after data released Saturday suggested that patients diagnosed with melanoma aren't reacting to the company's experimental treatment as well as earlier samples did. The prior data, released last year, was in part responsible for the company's rally to all-time highs in the first half of 2018.

Bristol-Myers Squibb, Nektar's partner on the skin cancer treatment, fell more than 3 percent Monday.

WATCH: BMY CEO on studies conducted with Nektar