The epidermal growth factor receptor and human topoisomerases represent potential cellular targets of oligomeric procyanidins.

Oligomeric procyanidins were found to inhibit the protein tyrosine kinase activity of the epidermal growth factor receptor (EGFR). The inhibitory potency was found to increase with the degree of oligomerisation (PA2 > PC1 >> PB1 = PB2 = PB3 = PB4 >> (–)-epicatechin). To address the question whether the interference with the activity of isolated EGFR preparations also plays a role within intact cells, effects on the phosphorylation status of the EGFR, as a measure of its activity, were determined in human colon carcinoma cells. Treatment of HT29 cells with the trimeric procyanidin PC1 resulted in a decrease of the EGFR autophosphorylation already at low micromolar concentrations. A respective procyanidin tetramer (PA2) failed to affect substantially the receptor phosphorylation status by up to 50 μM, indicating that the effectiveness of oligomeric procyanidins against EGFR activity within intact cells might be limited with increasing degree of polymerisation. Nevertheless, oligomeric procyanidins were identified as potential inhibitors of the EGFR, which might be of interest with respect to chemoprevention. However, PC1 and PA2 were also identified as potent inhibitors of the catalytic activity of human topoisomerase I and II, demanding further studies to elucidate whether the interference of procyanidins with topoisomerases might impair DNA integrity, thus limiting their usefulness in terms of chemoprevention.