On the active principles of the spurge family (Euphorbiaceae). XI. [1] The skin irritant and tumor promoting diterpene esters of Euphorbia tirucalli L. originating from South Africa.

The irritant and tumor-promoting constituents of latex of Euphorbia tirucalli L. originating from South Africa were isolated. They were identified as irritant ingenane and tigliane type diterpene esters derived from unsaturated aliphatic acids and acetic acid and the polyfunctional diterpene parent alcohols 4-deoxyphorbol, phorbol and ingenol, respectively. The irritant and tumor-promoting esters of 4-deoxyphorbol are predominant and were fully characterized chemically and biologically. They are positionally isomeric 12,13-acylates, acetates e.g. Euphorbiafactors Ti1-Ti4. As acyl groups they carry homologous, highly unsaturated aliphatic acids of the general structure CH3-(CH2)m-(CH = CH)n-COOH (m = 2,4; n = 1,2, 3,4,5; N = 2n + m + 2). Corresponding diesters of 4-deoxy-4 alpha-phorbol are also present which are biologically inactive. Comparison of structures and biological activities of 12,13-diesters of 4-deoxyphorbol indicates that--for a distinct total number of C-atoms (N) in the acyl moiety--an increasing number of conjugated double bonds (n) may increase the irritant but decrease the tumor-promoting activity. Replacement of the hydroxyl function at C-4 (phorbol-12,13-diesters) by hydrogen (corresponding 4-deoxyphorbol-12,13-diesters) does not essentially alter biological activities. Epimerization of 4-deoxyphorbol-12,13-diesters at C-4 abolishes biological activities. The specific chemical properties demonstrated for the diterpene ester irritants contained in the latex of E. tirucalli and hence in all plant parts may be useful in trials to abolish the potential risk of cancer involved especially in occupational mass production and handling of the plant. Some of the structure activity relations of the Euphorbia factors isolated made them excellent tools in experimental cancer research for the analysis of mechanisms of tumorigenesis.