The in vitro antiproliferative or stimulatory activity of an aqueous mistletoe extract (
AME) with a defined content of bioactive mistletoe
lectin (ML) was investigated in 6 human tumor cell lines, including two
melanomas and
leiomyosarcomas, each of which had previously been reported to show evidence of growth stimulation if treated with low concentrations of isolated ML. The effects of
AME were compared to that of the standard
cytotoxic agent adriamycin (ADR) using the well established
propidium iodide and sulforhodamin B proliferation assays. The
AME concentrations used ranged from 0.5 pg to 5 ng (0.82 fMol-85 pM) bioactive ML/ml in
melanoma (HT-144, SK-MEL-28) and
leiomyosarcoma (SK-MLS-1, S-UT-1B) cell lines and from 0.1-100 ng ML/ml (1.7 pM-1.7 nM) in MCF-7
breast cancer and SW620 colon
carcinoma cell lines, respectively. The influence of
AME on cell growth was determined at various time-points from 24 hours to 6 days of exposure. We found a time- and cell line-dependent inhibition of
tumor cell growth, but no reproducible stimulation of
tumor cell proliferation. Inhibitory concentrations 50% (IC50) for e.g. the SK-MEL-28
melanoma cell line, decreased from 4.1 ng ML/ml at 24 hours to 0.16 ng ML/ml at 72 hours and 0.18 ng ML/ml at 5 days. Our data clearly demonstrate that, by applying scientifically valid methods and procedures, the standardized
AME did not stimulate
tumor cell proliferation but showed time- and concentration-dependent antiproliferative effects.