Emodin, a protein tyrosine kinase inhibitor from Polygonum cuspidatum

Bioassay-directed fractionation of a Chinese medicinal plant, Polygonum cuspidatum (Polygonaceae), has led to the discovery of an anthraquinone, emodin [1], as a strong inhibitor of a protein tyrosine kinase (p56lck) partially purified from bovine thymus. Comparison of the IC50 values of emodin for protein tyrosine kinase inhibitory activity with physcion [2] and emodin-O8-D-glucoside [3], also isolated from the same plant, reveal the importance of the hydroxyl groups at C-6 and C-8 for the observed activity.