The flavanoid catechin and heterogenous samples of oligomeric proanthocyanidins extracted from various sources were compared for their ability to inhibit the biochemical and biological effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse epidermis in vivo. Topical applications of catechin fail to alter the hydroperoxide response to TPA but inhibit the induction of ornithine decarboxylase (ODC) activity and, to a lesser degree, the stimulation of RNA, protein, and DNA synthesis caused by this tumor promoter. Under similar conditions, condensed tannins (CTs) from guamuchil, loblolly pine, and southern red oak barks inhibit to various degrees all these biochemical markers of TPA promotion. The most effective antioxidant, loblolly pine bark CT, also inhibits TPA-induced ODC activity and macromolecule synthesis to a much greater degree than catechin or the other CTs tested. Pecan nut pith CT, however, has no inhibitory activity in this system. Pretreatments with 4 and 12 mg of loblolly pine bark CT remarkably inhibit the incidence and yield of skin tumors promoted by TPA in initiated mice, whereas similar doses of catechin are ineffective. Loblolly pine bark CT inhibits the 2nd rather than the 1st stage of tumor promotion. In contrast to their monomer units, therefore, some naturally occurring polyflavanoids have antioxidant activities and may be valuable against tumor propagation but their efficacy may vary considerably depending on their origin and structure.

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