Cytotoxic and cytostatic effects of the streptococcal preparation OK-432 and its subcellular fractions on human ovarian tumor cells

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Abstract
Previous studies have indicated that OK-432 is a potent biologic response modifier (BRM) and that it augments immune responses to tumor cells. We studied the direct effect of OK-432 on tumor cells. Established and freshly derived human ovarian carcinoma lines were examined for their susceptibility to OK-432 or its subcellular fractions in direct cytotoxicity, cytostatic activity, and inhibition of metabolic activity. OK-432 was cytotoxic to 13 of 15 freshly derived ovarian carcinoma lines in a 24-hour chromium-51 (51Cr) release assay. The optimal effect was noticed at OK-432 concentrations between 0.1 and 1.0 Klinishe Einhert (KE) per milliliter. The cytostatic effect on two established lines and one fresh line correlated with the cytotoxic activity. In all three lines, however, the metabolic activities (DNA, RNA, and protein synthesis) were inhibited by OK-432, suggesting that cell lysis by OK-432 may not be directly correlated with the inhibition of metabolic activities. Several subcellular fractions were derived from OK-432 and only the cytoplasmic and protoplast membrane fractions showed cytotoxic activity against the OK-432-sensitive tumor cell lines, although the cytotoxicity obtained was greatly less than the whole microorganism OK-432. The direct binding of 14C-OK-432 to tumor cells was examined. Binding took place rapidly after 1 hour of incubation and reached a maximum activity at 37°C. Binding in all three lines ranged between 1.7 and 2.7 pg/cell. These results demonstrate the direct cytotoxic effect of OK-432 and some subcellular fractions on human ovarian carcinoma lines. These results also show that the BRM OK-432 may exert its effect by both potentiating the antitumor response and directly inhibiting tumor cell growth.