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The hydrophilic character and aggregation phenomena exhibited by the photosensitizer zinc phthalocyanine tetrasulfonate (ZnPcSO4) make it difficult for this compound to penetrate the skin, and reduce the compound's photodynamic efficacy. A microemulsion (ME) was developed to increase the skin penetration of ZnPcSO4 while avoiding its aggregation. Ternary phase diagrams composed of surfactants (Span® 80/Tween® 80), canola oil and a propylene glycol (PG)/water mixture (3:1) were constructed as a basis for choosing an adequate ME preparation. Rheological, electrical conductivity, dynamic light scattering and zeta potential studies were carried out to characterize the ME formulations. Monomerization of ZnPcSO4 in the ME was determined photometrically and fluorometrically. In vitro skin penetration and retention of the compound in the skin were measured using porcine ear skin mounted on a diffusion cell apparatus. The in vivo accumulation 6 h after ZnPcSO4 application was determined fluorometrically in hairless mice skin. Confocal laser scanning microscopy was used to visualize ZnPcSO4 distribution in the skin. A ME composed of canola oil:surfactant:PG-water at 38:47:15 (w/w/w) was chosen for ZnPcSO4. This was oil-in-water with internal phase diameter of 15.7 ± 0.15 nm. Spectroscopic techniques confirmed that the ME was able to keep ZnPcSO4 in its monomeric form. In the in vitro penetration of ZnPcSO4 in the stratum corneum (SC) and in epidermis (without stratum corneum) with dermis ([E + D]) was 33.0- and 28.0-fold higher, respectively compared to the control solution of the drug. In vivo studies, confirmed that when the ME was used as carrier, ZnPcSO4 concentrations in the SC and [E + D] were about 1.6- and 5.6-fold higher, respectively, than controls. Visualization of ZnPcSO4 skin penetration by confocal laser scanning microscopy confirmed that the ME increased both penetration and biodistribution of this photosensitizer in the skin.In vitro and in vivo experiments showed increased ZnPcSO4 penetration in the stratum corneum (SC) and in epidermis (without stratum corneum) and dermis (E + D) for the microemulsion preparation compared to the control.