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ICANCERRESEARCH28,2338-2349,November1968] Cocarcinogenic Principles from the Seed Oil of Croton tiglium and from Other Euphorbiaceae Erich Hecker Biochemisches Institut am Deutschen Krebsforschungszentrum, Heidelberg, Germany first or "initiation" stage of Berenblum experiments as the Introduction result of essentially irreversible biologic events was readily ac cepted. However, the interpretation of the second or "pro The spurge family or Euphorbiacea includes some 280 gen motion" stage remained controversial for many years, espe era and 8000 species which occur in tropical and in temperate regions all over the world. These succulent or nonsucculent cially after a weak but definite tumorigenic activity of croton plants range from herbs and shrubs to tree and cactus types. oil was detected (see Refs. 10, 36). In case of irreversible Many of them contain a milky juice which is more or less tumorigenic events caused by croton oil, the oil would be just toxic, especially for cold-blooded animals, and can produce a another carcinogen. In case of reversible tumorigenic events dermatitis similar to that from poison ivy. The fruits are usual caused by croton oil, a special type of cocarcinogenic activity ly three-celled capsules, each cell containing a single seed from would have been demonstrated which could be called tumor- which in some species toxic, vesicating, and irritant seed oils promoting activity (10). If the nature of the biologic activity may be obtained. The largest genera of the spurge family are of croton oil could be established definitely in the sense of a those of Croton, with about 700 species, and of spurge or tumor promoter rather than as a carcinogen, Berenblum exper Euphorbia, with about 1600 species. iments might provide useful models for investigations into the biochemical mechanism of tumorigenesis(19, 21). Investigations Concerning the Active Principles from Croton Since croton oil is a multicomponent mixture of lipids, the OU oil as such is not suited for investigations into the nature of its biologic activities and their relation to tumorigenesis (18,19). Croton tiglium is a leafy shrub native to Southeast Asia. Necessarily, then, Berenblum (3) and many others tried to From the seeds of Croton tiglium 50—60%by weight of cro purify and isolate the cocarcinogenic or tumor-promoting prin ciples from the oil. All of these attempts also got stuck with ton oil may be obtained by either extraction or expression. "croton resin," which represents an extraordinary combina Croton oil is toxic to amphibia (9) and fish (41); used inter nally it is a drastic cathartic (9), and on skin it is an irritant tion of nasty biologic and most unusual chemical properties. and vesicant (12). Diluted with a suitable inactive vehicle, cro Some years ago we succeeded in the isolation (13, 18, 24, ton oil was used as a counterirritant. However, it acts so 28, 30) of the active principles from croton oil and its chemi powerfully that the oil was deemed unsafe for use either as a cal as well as biologic characterization (2, 21, 22), employing cathartic or as a counterirritant. Several efforts toward the biologic assays for toxic, irritant, and tumor-promoting activi isolation of the toxic and vesicant principles never went be ties combined with smooth and efficient separation tools such yond a methanol-soluble fraction called "croton resin," which as liquid-liquid extraction methods. As an introduction to accounts for most of the toxic and vesicant activity of croton problems currently under investigation in our laboratory at the oil (9, 12,41). German Cancer Research Center in Heidelberg, I shall sum In 1941, in experiments on the skin of mice, Berenblum marize very briefly some results which have been published detected what he first called the "cocarcinogenic" activity of already. Also it may be noted that the more general term croton oil and of "croton resin" (3). Some years later, after an "cocarcinogen" will be used rather than "tumor-promoter" important modification of this trial by Mottram (34), Beren until it will be possible to definitely characterize the biologic blum and Shubik (6—8)devised their well-known experiment activity of the pure active principles in a satisfactory manner. of treating the skins of mice with one single subcarcinogenic The Hydrophilic Fraction from Croton Oil. From the meth dose of carcinogenic hydrocarbon followed by repeated appli anol-soluble hydrophilic fraction of croton oil, which roughly cations of croton oil. From the results of such and similar resembles what earlier investigators called "croton resin," an other experiments, the "two-stage hypothesis" of skin unexpectedly high number of eleven molecularly uniform carcinogenesis was derived (4). compounds has been obtained. They are highly toxic in frogs In the years to follow, this type of experiment on mouse and represent essentially all the irritant and cocarcinogenic skin became one of the most important although controversial activities of the oil as assayed in mice. These compounds have approaches in the analysis of the mechanism of carcinogenesis. been identified as until then unknown diesters of the same [In the following, such experiments will be referred to as parent alcohol phorbol, each with one short- and one long- "Berenblum experiments" (21)]. The interpretation of the chain fatty acid (see Chart 1). Also these eleven diesters may 2338 CANCER RESEARCH VOL. 28 Downloaded from cancerres.aacrjournals.org on October 1, 2021. © 1968 American Association for Cancer Research. Cocarcinogenic Principles of Croton Oil „OR. lems. The essence of these investigations is summarized in Chart 2. As recorded on the left side of Chart 2, partial synthe sis starts from phorbol (I), which is easily obtained from croton oil (11). Esterification with either long-chain fatty acid chlorides or with acetic anhydride yields phorbol-12,13,20- triacylates ila or phorbol-12,13,20-triacetate lib respectively. In a subsequent step by base-catalyzed transesterification, phorbol-12-monoesters such as Ilia and Ilio are prepared. Acetylation of long-chain fatty acid 12-monoesters (Ilia) yields mixed functional triesters (IVa). In a similar reaction, phorbol-12-monoacetate Ilio is esterified in the 13 and 20 H3C position with long-chain fatty acid chlorides to give mixed functional triesters (IVè). Finally, in the triesters IVa and IVb the ester group at C-20 may be removed selectively by acid- CH2OH R, = R2 = H Phorbol catalyzed transesterification. Depending on the starting mate rial used, this step yields either phorbol-12,13-diesters of Type R^ = long, R2 = short chain Fatty acid residue compound group A Rj = short, R2 = long chain Fatty acid residue compound group B Va with the acetyl residue in the 13 position and the long- chain acyl residue in the 12 position, or Type Vb phorbol- Chart 1. Structure and stereochemistry of the biologically active 12,13-diesters with inverse positions of the acyl residues. phorbol-12,13-diesters from croton oil. Through these and similar routes (11), unlimited amounts of phorbol-12,13-diesters are now available. In order to take care be divided into two groups, A1—A4and Bj—B7the individual of the numerous requests from the scientific community, we diesters within each group all showing practically identical Rf have persuaded a German chemical company Th. Schuchardt, values (0.3 and 0.4 respectively) in thin-layer chromatography. München,to make available Compound A, commercially by The structure of phorbol has been worked out (23, 31) as our partial synthesis (11). shown in Chart 1. Thus phorbol was until now an unknown In order to study systematically their physical, chemical, and tetracyclic diterpene exhibiting 8 centers of asymmetry at C biologic properties, we have prepared a series of each of the atoms 4, 8, 9, 10, 11, 12, 13, and 14. The six oxygen func isomerie phorbol-12,13-diesters of Type Va and Vb (11). In tions reside in a tertiary a-ketol (acyloin) group at C-3 and C-4 their physical and chemical properties, characteristic differ involving the five- and the seven-membered rings, in an ences exist depending clearly upon the type of 12,13-isomer a-glycol group at C-12 and C-13 involving the six- and the under consideration, as summarized in Table 1. For example, three-membered rings and in a tertiary hydroxyl at C-9 inter the diesters of Type Va all show the same RF value; however, connecting the seven- and the six-membered rings. Also, the it is smaller than the RF value of the diesters of Type Ve. relative configuration and the conformation of seven of the There is also a characteristic difference between the two types eight asymmetric centers, excepting C-10, was established of positional isomers with respect to the melting points of (23). Finally, in the fall of last year (32), using a bromine- their 4 -nitroazobenzene-carboxylic acid-{4)-esters and their containing derivative of phorbol prepared in our laboratory, IR-spectra. As a rule, it was found that, in mass-spectrometric the structure shown in Chart 1 was confirmed and completed fragmentation, acyloxy residues attached to C-12 in phorbol- with respect to relative configuration of H-10 by X-ray dif 12,13-diesters leave the molecule as radicals, whereas acyloxy fraction analysis in the laboratory of Prof. Hoppe in Munich. residues attached to C-13 leave the molecule as a complete Furthermore, by this method the absolute configuration of acid F (see Table 1). phorbol was determined as seen in Chart 1. On the lower right These physical and chemical differences may be used as a side of Chart 1, the three-dimensional structure of phorbol is tool for rapid identification of even very small amounts of shown. A British group (37) also confirmed the structure and isomerie phorbol-12,13-diesters.

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