6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS x5x10 3 3 1 Control 3Omi Z PT 1.0,•g/•l 3H TdR 1/tCi pulse-label ing 3 4 5 6 7 8 hours after removal of Amethopterin Figure 4. Effect of ZPT on synchronizedJTC-17 cells at early S phase as shown by pulse 3H-TdR uptake for 30 min. ZPT was added at 1.0/•g/ml immediately after removal of amethopterin. No lethal effect on the treated cells was observed with phase contrast microscopy during 8 hr. 3H-TdR incorporation, only 40% and 55% inhibitions of UR and leucine incorporations respectively were seen at the concentration of 1.0 !ag/ml of ZPT and DS. INHIBITORY EFFECT ON CELL GROWTH Addition of ZPT to normal medium in the range of 0.04-0.4/xg/ml caused JTC-17 cells to have a growth inhibition in 5 to 14 days after initiation of culture in comparison with growth in normal medium (Figure 6). This growth inhibition varied depending on the initial concentrations of the seeded cells. Thus, at a low concentration of 3 x 104 cells/dish, about 27.9% growth inhibition was observed with 0.2/xg/ml of ZPT (Figure 6A) while a much lower inhibitory effect of 12.7% was demonstrated at a higher seed concentration of 3.2 x 105 cells/dish (Figure 6B). In the latter case, even 0.4/xg/ml of ZPT exhibits less growth inhibition than is seen at the 3 x 104 cells/dish concentra- tion. DISCUSSION The present study using cultured human skin cells has demonstrated that ZPT has a capacity to inhibit reversibly mammalian DNA synthesis by influencing a cellular

EFFECT OF ZINC PYRITHIONE ON SKIN CELLS 7 5 xlO Z PT(1.O/jg/ml) j' 2 3 4 5 6 7 hours after removal of Amethopterin Figure 5. Effect of ZPT on synchronized JTC-17 cells at early S phase as shown by cumulative incorporation of 3H-TdR. 1.0/ag/ml of ZPT was added to the synchronized culture at the times indicated by arrows.