Shares of Agenus fell sharply Thursday on unsuccessful clinical trial results for a cancer treatment that included its compound that is designed to make drugs more effective.

British drugmaker GlaxoSmithKline PLC is studying cancer treatments that include Agenus’ SQ-21 Stimulon vaccine adjuvant. Glaxo said Thursday that an experimental melanoma treatment did not meet its first goal in a mid-stage clinical trial, as patients who received the therapy did not have greater disease-free survival than patients who were given a placebo.

GlaxoSmithKline is studying an immune therapy that targets MAGE-A3, an antigen found in cancers including melanoma, non-small cell lung cancer, head and neck cancer, and bladder cancer. It does not occur in healthy cells. The therapy includes Agenus’ SQ-21 Stimulon adjuvant, which is extracted from the bark of an evergreen tree called Soapbark and is being studied as a component in a variety of cancer treatments.

The study is not finished because it has a second goal: GlaxoSmithKline wants to see if patients with a particular genetic signature benefit from the treatment. Results from that analysis are expected in 2015.

Agenus Inc., of Lexington, Mass., does not have any approved products.

Its shares of lost 81 cents, or 21.6 percent to $2.91 in afternoon trading after trading as low as $2.45 , its lowest level since January 2012. Shares of GlaxoSmithKline fell 40 cents to $51.56.