Absence of tumor growth stimulation in a panel of 26 human tumor cell lines by mistletoe (Viscum album L.) extracts Iscador in vitro

Mistletoe (Viscum album L.) extracts exhibit antitumor activity based on direct inhibition of tumor growth as well as modulation of immune response. Recent reports suggested potential stimulation of tumor growth at low doses of mistletoe extracts, particularly in hematological tumors and tumors responding to immunotherapy. Therefore, the direct effect of the three mistletoe extracts Iscador (R) M Spezial, Iscador Qu Spezial and Iscador P on tumor growth was investigated in a panel of 26 human tumor cell lines in vitro using cellular proliferation assays. Antitumor activity of the three preparations at high concentrations was investigated in a panel of 12 cell lines. The results showed no evidence of stimulation of tumor growth by any of the three extracts, in particular the five tumor cell lines previously reported to be sensitive to direct mistletoe lectin stimulation. On the contrary, the lectin containing preparations Iscador M Spezial and Iscador Qu Spezial expressed a pronounced antitumor activity exhibiting a nearly identical antitumor profile compared to isolated mistletoe lectin 1.