Agenus Inc. fell sharply after an unsuccessful clinical trial of a cancer treatment that includes its compound that is designed to make drugs more effective. British drug maker GlaxoSmithKline is studying cancer treatments that include Agenus’s SQ-21 Stimulon vaccine adjuvant. Glaxo said an experimental melanoma treatment did not meet its first goal in a mid-stage clinical trial. Glaxo is studying an immune therapy that targets MAGE-A3, an antigen found in various cancers. The therapy includes Agenus’ SQ-21 Stimulon adjuvant, extracted from the bark of an evergreen called Soapbark.