Oncopharmacological Perspectives of a Plant Lectin (Viscum album Agglutinin-I): Overview of Recent Results from In vitro Experiments and In vivo Animal Models, and Their Possible Relevance for Clinical Applications

An old goal of natural complementary medical therapy has been to aim at a long-term stimulation of natural resistance in order to restrain cancer progression or improve defective immunological conditions without toxic side effects. Mistletoe extracts were applied to a large number of cancer patients because of their modulatory effect on the natural immune system. By carefully removing lectins, an essential group of components, from mistletoe extracts, a significant reduction of their effectiveness on several cellular immune parameters could be observed in vivo (1). That is the reason why, for the last 14 years, biological research of mistletoe extracts has focused on lectins. Meanwhile, the quantitatively dominant lectin, Viscum album agglutinin (VAA)-I has become available in a recombinant form (rVAA). Other constitiuents of plant extracts such as viscotoxins (2,3), polyand oligosaccharides (4), flavonoids (5,6), chitinbinding mistletoe lectin (7) and arginine have also been investigated in connection with the effects of mistletoe extracts on the host defense. However, little evidence has been found that these substances contribute to the effects of mistletoe in vivo (8).