Vernolepin inhibits tumor, plant growth Natural product isolated by Wisconsin group has activity against Walker carcinosarcoma 256 Vernolepin, a sesquiterpene dilactone, inhibits tumor growth in rats and also reversibly inhibits plant growth, sci entists at the University of Wisconsin, Madison, have found. These findings stem from a continuing search at Wis consin for tumor inhibitors of plant origin led by the university's Dr. S. Morris Kupchan (C&EN, Dec. 12, 1966, page 6 4 ) . Dr. Kupchan de scribed the findings at the Fifth Inter national Symposium on the Chemistry of Natural Products in London, Eng land. Vernolepin shows significant inhibi tory activity against Walker intramus cular carcinosarcoma 256 in rats at a concentration of 12 mg. of vernolepin per kilogram of animal weight, Dr. Kupchan and his coworkers, Dr. Rich ard J. Hemingway, Dieter Werner, and Dr. Aziz Karim, have found [/. Am. Chem. Soc.} 90, 3596 (1968)]. The compound has been extracted and iso lated from Vernonia hymenolepis leaves. The leaves were collected in Ethiopia and supplied to the Wiscon sin group by Dr. Robert E. Perdue, Jr., of the U.S. Department of Agriculture, Beltsville, Md. The Wisconsin group has isolated vernolepin using ethanol extraction, solvent partition, and silicic acid chro matography. Typically, 1.5 kg. of
powdered leaves yields 1.5 grams of vernolepin. The compound's structure has been determined using infrared, ultraviolet, and nuclear magnetic resonance spec trometry, mass spectrometry, and sin gle crystal x-ray diffraction. A key step in the compound's structure de termination was the x-ray diffraction studies carried out on a derivative, vernolepin p-bromo-benzenesulfonate, by Dr. A. T. McPhail and Dr. G. A. Sim of the University of Sussex, Brighton, England. Like two other active compounds isolated in Dr. Kupchan's laboratory, elephantin and elephantopin (C&EN, Aug. 8, 1966, page 15), vernolepin is a sesquiterpene di lactone. All three compounds contain two «^-unsaturated lactone functional groups which may be significantly re lated to the compounds' biological ac tivity, Dr. Kupchan says. The Wisconsin group's approach to the search for tumor inhibitors in plants differs from the usual classical approach, Dr. Kupchan points out. In the classical or phytochemical ap proach, compounds are studied which are easily separated from a plant and easily crystallized. "But in our isola tion studies, we are guided at every stage by biological assays," Dr. Kup chan tells C&EN staff writer Arnold
BIOASSAY. Dr. S. M. Kupchan (left) and Dr. R. J. Hemingway view a model of vernolepin, a sesquiterpene dilactone. Guided by bioassay rather than the classi cal phytochemical approach, the two scientists isolated the inhibitor. This ap proach enables them to find important minor constituents that they would have missed using the classical approach 36 C&EN JULY 22, 1968
^cH2 0'"\L^X^ ι | j
Vernolepin is a sesquiterpene dilactone with two α,β-unsaturated lactone func tional groups which may relate to the compound's biological activity
Levitt. "We can thus find important minor constituents which would have been missed in the classical approach." By extensive use of biological as says, which measure activity against tumor systems, the Wisconsin scien tists have isolated a wide array of com pounds from plants. After determin ing the structure of these natural prod ucts, they plan to tackle these ques tions: What is the mechanism by which the compounds exert growth in hibition activity? What is the func tion of these compounds in plants? To determine vernolepin's effect on plant growth, Dr. Kupchan and Dr. Hemingway teamed up with Dr. Luis Sequeira of the University of Wiscon sin's department of plant pathology. They find that vernolepin in concen trations of 5 to 50 micrograms per ml. inhibits extension growth (from 20 to 809c) of sections of wheat coleoptile (the rapidly growing sheath at the tip of the plant). (Wheat coleoptile sec tions are used in a standard test for plant growth regulatory activity.) In hibited tissues appear normal and their respiration is unaffected, the Wiscon sin scientists note. The group has washed vernolepintreated wheat coleoptile sections and then treated them with indole-3-acetic acid, a plant growth stimulator. The tissues respond to the growth stimu lator, they find. But the degree of elongation is determined by how long they were treated with vernolepin. If vernolepin and indole-3-acetic acid are administered simultaneously, increas ing concentrations of the stimulator will markedly reduce vernolepin's in hibitory effect. But there is no evi dence for a competitive interaction between the two substances. An account of the plant growth in hibition studies will appear in Science. Vernolepin's reversible inhibitory ef fect, Dr. Kupchan points out, suggests that the compound may have a natural function in the regulation of plant growth.
