360 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table VI Substantivities of PABA, and Its Esters Determined by the In Vivo Method, for Different Concentrations of Each Sunscreen PABA Escalol 507 © Amerscreen "P"© Concen- Concen- Concen- tration Substantivity tration Substantivity tration Substantivity (mg%) (%) (mg%) (%) (mg%) (%) Mean - SD 85 0.33 -+ 100 50.08 -+ 80 0.44 _+ 0.05 2.16 0.04 90 0.29 -+ 110 51.20 -+ 110 0.49 -+ 0.04 3.33 0.04 105 0.30 -+ 120 51.23 -+ 120 0.49 -+ 0.04 2.13 0.05 0.30 + 50.84 + 0.47 + 0.02 0.66 0.03 eficial properties to a product is a question still open to discussion (1). An excessive substantivity might result in irreversible binding to constituents of living tissues, thus introducing a distinct risk of toxicity (11). On the other hand, if substantivity is accepted as arising from adsorption of the molecule on active sites of the skin barrier, a high substantivity might induce a delayed percutaneous absorption, resulting possibly in a reduced systemic toxicity (1). The solubility in water of the sunscreen has been recognized as a major determinant in establishing its retention on the skin after sweating or swimming (4). The results of the present study support this conclusion. Inspection of the values reported in Table V reveals that the substantivities of the three agents studied increase with their de- creasing the water solubility. These results are in agreement with those reported by Cumpelik (4). The sorption isotherms of the three sunscreens under study in ethanol were in all cases linear, having the origin as intercept. The straight lines did not show a final inflection or plateau within the concentration range studied. This would indicate a constant partition of the solute between substrate and solution, analogous to the partitioning between two immiscible solvents, and corresponds to a type C isotherm. In such a linear plot, K corresponds to the slope of the isotherm. Table IV shows the K values for the three sunscreen agents at the different concentrations studied. These values represent the partition coefficients of the sunscreen substances between the keratin and the solvents and indicate that K PABA K Amerscreen "P"© K Escalol 507 ©. Thus PABA is shown to have the highest interaction with keratin when the solvent used is alcohol and is followed by Amerscreen "P"© and Escalol 507 © in decreasing order, which is in conformity with the solubility of the substances in eth- anol, i.e. Escalol 507 © Amerscreen "P"© PABA. The substantivies of the three sunscreens were determined at three concentrations these concentrations were selected at levels where the affinities of the three substances for the keratin (K values) were constants (Table VI). Hydration favors the adherence of the sunscreen to the keratin, which results in higher K values. Thus, vehicles that modify the skin hydration may affect its affinity for a

SUNSCREEN SUBSTANTIVITY 361 compound. It has been found that an increased hydration of the keratin molecules parallels an increased affinity for various substances (12). Apparently hydration causes unfolding of the keratin molecules, thus exposing more binding sites (1,12). In ad- dition, according to Scheuplein (13), it seems likely that the swelling and softening of the keratin filaments in water is accompanied by a partial dissolution of the cell mem- branes which open larger holes, which thus facilitates diffusion. Table VI contains the percentage of substantivity obtained in the in vitro studies. The results are almost identical to those determined using the in vivo method, as can be seen in Figure 1, where a comparison is made of the substativities of the three sunscreen substances obtained by the two methods. It seems desirable to establish a laboratory method for evaluating the relative resistance of sunscreen products to wash off, as a preliminary "screen" prior to testing on humans under actual use conditions. The static water bath immersion procedure carried out by Cumpelik (4) is one such method which, however, has all the limitations and problems o 5O 10 %S 1 2 3 10 50 %s "IN Vi TR O" Nlethod Figure 1. Comparison of substantivities of sunscreens obtained by in vitro and in vivo methods. O = PABA, ß = Amerscreen "P"©, ß = Escalol 507 ©.