Influence of RU 41.740 on human monocytes in vitro: release of soluble factors which retard multiplication of tumor cells in culture.

RU 41.740 (Biostim) is an immunostimulating substance of bacterial origin with a reported protective activity against bacterial infections in man. The aim of the present investigation was to determine if Biostim-exposed blood mononuclear cells release soluble factors in vitro which may inhibit growth of tumor cells in culture. It was observed that exposure of mononuclear cells to Biostim at concentrations as low as 1 ng/ml for 24 h resulted in the release of factors capable of retarding growth of K562 cells. Three human glioma cell lines tested also appeared to be sensitive whereas three other cell lines were less sensitive or resistant. The growth inhibitory factors were shown to be produced by monocytes which is in line with previous findings showing that Biostim is a potent activator of monocytes-macrophages. Biostim-exposed monocytes, however, did not express any cytotoxic activity for K562 cells using a short-term 51Cr-release assay. It is concluded that Biostim may trigger monocytes to release factors with anti-tumor activity. These factors have not yet been identified and it remains to be determined if their production increases during Biostim treatment and whether this is correlated to an anti-tumor activity in man.