Antitumor activity of plant cannabinoids with emphasis on the effect of cannabidiol on human breast carcinoma. 03 Jun Dr D The psychoactive plant cannabinoid, Delta9-tetrahydrocannabinol, is antagonized by Delta8- and Delta9-tetrahydrocannabivarin in mice in vivo. Plant-derived cannabinoids modulate the activity of transient receptor potential channels of ankyrin type-1 and melastatin type-8.