Route of administration may determine the biological effects of RU 41,740 (Biostim). Trapping of 99mTc-labelled human albumin colloids in mice.

Experiments have been conducted in mice to examine whether treatment with the immunostimulating drug RU 41,740 (Biostim) may change the distribution of i.v. injected radiolabelled human albumin colloids. It was observed that a single i.p. injection of Biostim (1 ng-1000 micrograms) significantly enhanced trapping of the particles in spleen and lungs. The effect, which was dose-dependent, was most pronounced in the lungs where more than a 10-fold increase could take place. On the contrary, oral administration of Biostim caused a dose-dependent decrease of colloid trapping in the lungs. Further experiments showed that oral administration of Biostim resulted in the appearance of soluble factors in the blood which inhibited the phagocytic activity of lung macrophages, for example, sera from such mice inhibited lung colloid trapping when injected into new hosts. I.p. administration of Biostim, however, resulted in the appearance of factors in the blood which enhanced phagocytosis of lung macrophages. Our conclusion is that the biological activity of Biostim in vivo may be highly dependent on its route of administration.