Effect of blood, ascites, and tumor cell density on cytocidal action of neocarzinostatin.

Cytocidal activity of Neocarzinostatin (NCS) was analyzed in vitro by using cultured and ascitic L1210 cells. NCS shows rapid and typical concentration-dependent cytocidal action against L1210 cells. The concentration required for 90% cell-kill (MLD90) and for all 10(6) cell-kill (MIC/10(6)) was 5 x 10(-2) and 4 x 10(-1) microgram/ml, respectively, when L1210/C cells were exposed to NCS at the concentration of 2 x 10(5)/ml in RPMI-1640 medium. When L1210/C cells at the concentration of 1.3 x 10(7)/ml were exposed to NCS, cytocidal activity of NCS decreased, and MLD90 and MIC/10(6) increased to 1.75 x 10(-1) microgram/ml (3.4x) and 2.83 microgram/ml (6.5x), respectively. Also, when a small fraction of whole BDF1 mouse blood, red blood cells, or spleen cells was present in the reaction mixture, cytocidal activity of NCS appeared to decrease. Furthermore, when washed hemorrhagic ascitic L1210 cell suspension was exposed to NCS, 100 times or more concentration of NCS was required for 90% cell-kill or all 10(6) cell-kill. The effect of plasma or serum on cytocidal activity of NCS was minimum. These results indicate that cytocidal activity of NCS is greatly inhibited through contact with tumor cells, blood cells, spleen cells, and/or hemorrhagic ascites. This may be one of the reasons why L1210 cells, which show high sensitivity against NCS in vitro, are less sensitive in vivo. In order to explain these findings, the possibility of inactivation of NCS by the cells or reduction of free active NCS molecules by the binding or adsorption with the cells is discussed. This characteristics in action of NCS should be take into account in clinical use.