Z.M.F. de Freitas et al. / European Journal of Pharmaceutical Sciences 25 (2005) 67–72
71
Table 2
ing the complete interaction between glyceryl esters and stra-
tum corneum.
Hydrolysis rates by R. miehei lipase
Ester
Hydrolysis rate (mol fatty acid/min)
Considering possible sunscreen/formulation and sun-
screen/skin molecular interactions, it is reasonable to admit
the importance of investigating the effect of the formulation
excipients in the sunscreen agents partition coefficient and its
adherence to the skin. Based in this hypothesis, we intend to
investigate a more suitable formulation that evidences in vivo
to the results obtained by enzymatic hydrolysis evaluation.
Several in vitro and in vivo studies have shown that certain
sunscreen agents are absorbed through human skin (Simeoni
the physicochemical properties of the active compound and
also of the nature of the vehicle in which the sunscreen is ap-
plied, e.g. polarity of the solvent, particle size, type of vehicle
Tributyrin
Triolein
EHPM
6a
0.46
0.31
1.3
0.46
1.2
6b
ilar hydrolysis rates as tributyrin, both showing 1.5 times
higher hydrolysis rates than triolein.
Differences on hydrolysis rates between tributyrin and tri-
olein can be attributed to different acyl chain lengths. On the
other hand, the lower hydrolysis rate (higher stability) of sun-
screen 6a, compared to ester 6b, in addition to different acyl
chain lengths, can also be possibly due to the higher lipofilic-
ityof1,3-dipalmitoylglycerol, which could hinderinteraction
between the sunscreen and the enzyme.
It is interesting to point out that the ester 6a presented a 2.8
times lower hydrolysis rate by the lipase than the commercial
sunscreen EHPM. Thus, it can be suggested that the synthetic
triacylglycerol 6a could also present a longer retention time
on the skin than EHPM, which would lead to a more intense
and effective sun protection.
The effectiveness of a sunscreen implies that it will adhere
to the skin forming a protective film and therefore this sub-
stance should have a high affinity for the stratum corneum.
Hence, the difference of in vitro hydrolysis rates observed be-
tween 6a and EHPM could be due to the fact that the triacyl-
glycerol presents a structural analogy with skin constituents
effective antisolar action in vivo.
¨
(Wissing and Muller, 2002).
Additional studies must be undertaken to determine the
percutaneous absorption properties of these glyceryl esters,
in order to rule out the possibility of absorption following
topical application.
Therefore, owing to their higher enzymatic stability and
more adequate physicochemical properties, long chain glyc-
eryl esters can be considered more advantageous than alkyl
esters for use as sunscreens.
Acknowledgements
The authors would like to thank Dr. Joaquim Fernando
Mendes da Silva and Dra. Adriane Regina Todeschini for
critical reading of the manuscript, Analytical Laboratory of
The lack of phototoxic activity of the synthesised 1,3-
diacylglycerols and triacylglycerols was previously assessed
by in vitro and in vivo methods (Freitas et al., 2000). The
commercial sunscreen EHPM, employed as standard, vali-
´
Nucleo de Pesquisa de Produtos Naturais, Universidade Fed-
eral do Rio de Janeiro for 13C NMR spectra analyses, Ana-
lytical Laboratory of Souza Cruz Industry for mass spectra
In a previous paper, triacylglycerols were incorporated in
oil/water emulsions and these emulsions had their SPF val-
ues determined through in vitro and in vivo methods (Freitas
et al., 2001). In both experiments, the commercial sunscreen
EHPM was employed as standard. In vivo SPF values of the
sunscreens under glyceryl ester form did not differ from the
EHPM SPF values. Evidently, they should not be different,
because the 1,3-diacylglycerol system does not exert elec-
tronic effects that enable alteration in the UVB absorption
process. It was expected, however, that the in vivo protection
ofthesenewsunscreenswouldbehigherduetophysicochem-
icalpropertiesofthesystem, higherlipofilicityandenzymatic
stability, however, these properties although present, did not
result in an increase of SPF.
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