An ethyl acetate-soluble extract of the combined leaves and twigs of Thuja occidentalis was found to inhibit 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced ornithine decarboxylase (ODC) in cultured mouse epidermal ME 308 cells. Bioassay-guided fractionation of this extract led to the isolation of six active constituents (1-6), namely, (+)-7-oxo-13-epi-pimara-14,15-dien-18-oic acid (1), (+)-7-oxo-13-epi-pimara-8,15-dien-18-oic acid (2), (+)-isopimaric acid (3), (1S,2S,3R)-(+)-isopicrodeoxypodophyllotoxin (4), (-)-deoxypodophyllotoxin (5), and (-)-deoxypodorhizone (6). Compounds 1 and 4 are new natural products, and their structures and stereochemistry were determined using spectroscopic methods. Compounds 1-6 were evaluated for inhibition of the transformation of murine epidermal JB6 cells, inhibition of ornithine decarboxylase induction with murine epidermal ME 308 cells, and cytotoxic activity against KB cells.

The alpha-helical antibiotic peptide (P18: KWKLFKKIPKFLHLAKKF-NH2) designed from the cecropin A(1-8)-magainin 2 (1-12) hybrid displayed strong bactericidal and tumoricidal activity without inducing hemolysis. The effect of the Pro9 residue at central position of P18 on cell selectivity was investigated by Pro9 --> Leu or Pro9 --> Ser substitution. Either substitution markedly reduced the antibacterial activity of P18 and increased hemolysis, although it did not significantly affect cytotoxicity against human transformed tumor and normal fibroblast cells. These results suggest that a proline kink in alpha-helical antibiotic peptide P18 serves as a hinge region to facilitate ion channel formation on bacterial cell membranes and thus plays an important role in providing high selectivity against bacterial cells. Furthermore, to investigate the structure-antibiotic activity relationships of P18, a series of N- or C-terminal deletion and substitution analogs of P18 were synthesized. The C-terminal region of P18 was related to its antibiotic activity and alpha-helical conformation on lipid membranes rather than N-terminal one. Higher alpha-helicity of the peptides was involved in the hemolytic and antitumor activity rather than antibacterial activity. Except for [L9]-P18 and [S9]-P18, all the designed peptides containing a Pro residue showed potent antibacterial activity, although they did not induce a cytolytic effect against human erythrocyte and normal fibroblast cells at the concentration required to kill bacteria. In particular, P18 and some analogs (N-1, N-2, N-3, N-3L and N-4L) with potent bactericidal and tumoricidal activity and little or no normal cell toxicity may serve as an attractive candidate for the development of novel anti-infective or antitumor agents.