ABSTRACT
The effects of zinc pyrithione (ZnPTO) were studied in a series of in vitro tests to determine whether its mode of action is primarily cytostatic or cytotoxic. Sodium pyrithione (NaPTO) was also studied, to check that pyrithione was the active moiety, and the known cytostatic chemical hydroxyurea was included for comparison. ZnPTO had a reversible inhibitory effect on the growth of BHK 21 cells at 0.1 microgram/ml, but had a rapid, irreversible inhibitory effect at 1 microgram/ml associated with cell rounding and detachment. NaPTO produced a similar effect but hydroxyurea produced an essentially reversible inhibition even at a dose well above that producing complete inhibition. ZnPTO and NaPTO both caused contraction, rounding and blebbing of BHK 21 cells in perfusion-chamber tests, at higher levels (1 and 10 micrograms/ml) than required for growth inhibition, but only 10 micrograms ZnPTO/ml caused lactate dehydrogenase (LDH) release. Hydroxyurea had no effects in these tests. ZnPTO and NaPTO also reduced the survival of Chinese hamster V79 cells sharply over a narrow dose range (0.01-0.03 microgram/ml), but the effect of hydroxyurea was not as sharp and occurred at much higher doses. All three showed elements of cytostasis and cytotoxicity as demonstrated by analysis of the relationship between survival and colony area. Of the three, only ZnPTO (at greater than or equal to 5 micrograms/ml) caused significant LDH release from the cells, though both ZnPTO and NaPTO (at 0.1-1 microgram/ml) inhibited cell growth as indicated by total LDH values. In studies with rat peritoneal mast cells, ZnPTO and NaPTO (at 10 ng/ml) both suppressed histamine release induced by 48/80 or Ca ionophore A23187, though neither caused histamine release directly. The combined results of these tests show that ZnPTO is primarily cytotoxic, rather than cytostatic.