This study deals with the cytotoxicity of helenanolide-type (10 alpha-methylpseudoguaianolide) sesquiterpene lactones. We determined the influence of substitution patterns on the toxicity of 21 helenanolides to a cloned Ehrlich ascites tumor cell line, EN2. Within a series of helenalin esters, the acetate (2) and isobutyrate (3) were more toxic than helenalin itself (1). Esters with larger acyl groups (tiglate 4 and isovalerate 5) exhibited a decreased toxicity compared with the parent alcohol (1). Similar relationships were observed between the 6,8-diastereomer of helenalin, mexicanin I (6) and its acetate (7) and isovalerate (8). In contrast, cytotoxicity within a series of 11 alpha, 13-dihydrohelenalin esters (9-12) was shown to be directly related to the size and lipophilicity of the ester side chain, dihydrohelenalin (9) being the least toxic compound in this group. Investigation of several 2,3-dihydrohelenalin derivatives (13-21) with 2 alpha-hydroxy-4-oxo- and 2 alpha,4 alpha-dihydroxy- or -O-acyl-substituted cyclopentane rings (arnifolins and chamissonolides, respectively), for which no pharmacological data have been reported so far, revealed further interesting influences of the substitution pattern on cytotoxicity. The results may be interpreted in terms of lipophilicity and steric effects on the accessibility of the reactive sites considered responsible for biological activity.

Aeroplysinin-1 (1) and the structurally related dienone 2 were cytotoxic to Ehrlich ascites tumor (EAT) cells and HeLa tumor cells in the microculture tetrazolium (MTT) and clonogenic assays. Both compounds are bromotyrosine derivatives, isolated from the marine spong Aplysina aerophoba. As the effective concentrations in the MTT assay were lower than in the clonogenic assay, 1 and 2 are able to cause growth inhibition as well as actual cell death in these cell lines. With an IC50 value of 8.2 microM (MTT assay, 2-h incubation, EAT cells), 1 was the more toxic compound. When the cells were depleted of glutathione by pretreatment with buthionine sulfoximine, they were significantly more sensitive toward 1 and 2 in the MTT assay. A dose-enhancement factor as high as 11.8 was found in EAT cells after 2-h incubation with 2. Using electron paramagnetic resonance we were able to measure free radical formation of 1 and 2, yielding the semiquinone structures 3 and 4, respectively, in a culture medium with tumor cells. It is concluded that free radicals are, at least in part, responsible for the cytotoxicity of 1 and 2. This conclusion is in line with expectations derived from the chemical structures of both compounds.

The cytotoxicity of 22 natural and semi-synthetic simple coumarins was evaluated in G LC4, a human small cell lung carcinoma cell line, and in CO LO 320. a human colorectal cancer cell line, using the microculture tetrazolium (MTT) assay. With IC 50 values > 100 j.i m , following a continuous (96h) incubation, most coumarins exhibited only low cytotoxicity. Several com pounds, however, displayed significant potencies. As far as the structure -cytotoxicity rela tionship is concerned, it is conspicuous that all the potentially active natural compounds possess at least two phenolic groups in either the 6,7-or 6,8-positions. In addition, the 5formyl-6-hydroxy substituted semi-synthetic analogue was found to be potent, reflecting the importance of at least two polar functions for high cytotoxicity.