154 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table IV Permeability Constants (K v) of the Selected 12 Hydrocortisone 17-Valerate 0.2% o/w Emulsions (n = 3) Formula no. Kp, cm/hr 1 3.20 X 10-5(3.97 x 10-6) * 0.06 2 2.40 x 10 -5 (8.66 x 10 -7) 0.19 6 4.34 X 10-5(5.54 X 10 -6 ) 1.34 10 3.57 X 10-5(9.01 X 10 -6 ) 0.35 13 4.14 X 10 -5 (1.85 X 10 -6 ) 1.34 17 3.08 X 10-5(4.89 X 10 -6 ) 0.57 19 5.11 X 10-5(3.03 X 10 -7 ) 1.34 23 3.56 X 10-5(7.10 X 10 -6 ) 0.88 24 4.78 X 10-5(3.85 X 10 -6 ) 1.34 28 5.71 X 10-5(1.05 X 10 -7 ) 1.34 31 5.46 X 10-5(8.31 X 10 -7 ) 1.34 36 3.10 X 10-5(3.65 X 10 -6 ) 3.59 * Standard error of the means (n = 3). study indicate that the permeability constant 3.50 x 10 -5 cm/hr (s.e. = 5.10 x 10 -7 cm/hr) of the marketed HCV 0.2% cream was 0.6 times that of 5.71 X 10 -5 cm/hr (s.e. = 1.05 x 10 -7 cm/hr) of formula #28. Correspondence between the in vitro skin permeation rate and the in vivo vasoconstrictor activity is indicated in this study. It is therefore reasonable to assume that the enhanced vasoconstrictor activity of formula #28 can be attributed to its higher occlusivity. i Y= 2__.0:35 X 10 -5 + 2.446 X lO -5 X r: 0.902:1 0 0.25 0,50 0,75 1.00 1.25 1.50 1.75 Oi[/W(]ter Ph(]se-Vo[ume R(]tio (•) Figure 7. Correlation of the permeability constants and phase-volume ratios (4)) of the selected o/w emul- sions. The bar indicates the standard error of the means (n = 3).

EMULSION VEHICLES AND VASOCONSTRICTOR ACTIVITY 15 5 6 o#28 #3i 5 m. t#13 #i9Q #24 Y=4.24 x iO -5 + 2.77 x 10 -7 (r= 0.8514) o lo 20 30 40 50 x % Petrotatum = 34) Figure 8. Linear correlation of permeability constants and % of petrolatum of the selected o/w emulsions with 4) = 1.34. The bar indicates the standard error of the means (n = 3). CONCLUSIONS In conclusion, the findings of this study suggest that the occlusivity of an o/w emulsion system can be enhanced by adjusting its o/w phase-volume ratio with petrolatum. This enhanced occlusivity will, in turn, promote the in vitro human skin permeation rate. However, occlusivity provided by too high a concentration of petrolatum did not en- hance the permeation of HCV. This study shows that petrolatum, not mineral oil, is the key ingredient providing the occlusivity of the o/w emulsion system. In vivo vaso-