The influence of a broad concentration range of naturally
occurring naphthoquinones and other cytotoxic or cytostatic
agents (azathioprine, colchicine, cyclophosphamide, fluorouracil,
methotrexate and vincristine) on human granulocytes and lymphocytes
was investigated in a systematic in vitro study. At high
concentrations (100 micrograms-10 ng/ml) nearly all substances
showed the known cytotoxic or immunosuppressive effects, whereas
most of the same compounds displayed immunostimulating activities
at very low concentrations (10 ng-10 fg/ml). After a cold shock or
heat treatment lymphocytes became more sensitive against these
compounds in the active concentration range than untreated cells.
These results possibly give an explanation for the antitumour
activity of some plant extracts, for which a direct cytotoxicity
due to the applied low dose can be excluded.
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