Initial characterization of an antineoplastic, polysaccharide-rich extract of Mycobacterium bovis BCG, Tice® substrain

A purified hot-water extract from Mycobacterium bovis (BCG vaccine) has been found to have significant antitumor activity against a murine sarcoma in vivo but not in vitro suggesting that the active compound is behaving as an immunostimulant. The material, termed PSI, has an average molecular weight of 22.4 kDa is freely soluble in water, but has low solubility in acetone or ethanol and is remarkably heat-stable as is the parent BCG vaccine in terms of high-dose antitumor activity. PSI contains at least 50% carbohydrate consisting mainly of glucose galactose and mannose and about 10% lipid that may correspond to phosphatidylinositol. It shares chemical and biological properties with an arabinomannan isolated from M. tuberculosis but it contains only trace quantities of lipoarabinomannan (LAM). Crossed immunoelectophoresis indicated that PSI contains the mycobacterial antigen 89 but only a single, non-migrating precipitin arc appeared on immunoelectrophoresis against a standard anti-BCG serum. PSI appears to be non-toxic in mice up to a dose of 5 mg/kg while as little as 70 μg/kg is sufficient to inhibit tumor formation significantly.