Lapachol [2-hydroxy-3-(3-methyl-2-butenyl)-l,4-naphthoquinone] was identified as an active substance in extracts of the roots of Stereospermum suaveolens and showed anti-tumor activity.However, a phase Ⅰ clinical trial of lapachol revealed an anticoagulant side effect and an effective plasma concentration for anti-tumor activity could not be achieved without toxicity in cancer patients receiving lapachol.For these reasons, the clinical studies with lapachol were terminated.The mechanism underlying the anti-tumor effect of lapachol is still unknown.However, the anticoagulant activity of lapachol arises from its potent inhibition of vitamin K epoxide reductase and vitamin K quinone reductase.Other vitamin K antagonists have been shown to be potent anti-tumor drugs, but the anti-tumor effect is not mediated solely via their anticoagulant activity.