Characterisation of liposomes containing aminolevulinic acid and derived esters

Liposomes of different compositions have been designed to improve delivery of aminolevulinic acid (ALA) and its esterified derivatives ALA-Hexyl ester (He-ALA) and ALA-Undecanoyl ester (Und-ALA) for its use in photodynamic therapy (PDT). Egg yolk phosphatidyl choline (PC), phosphatidic acid (PA) and phosphatidyl glycerol (PG) were employed in the preparation of the liposomes. Sonicated vesicles composed of PC, PC–PG (80:20) or PC–PA (80:20) containing ALA or derivatives were obtained and purified by a minicolumn centrifugation method. PC liposomes presented encapsulation percentages around 6% for 2 mM ALA, 13% for 2 mM He-ALA and 51% for 2 mM Und-ALA. The addition of PG or PA to the formulation, resulted in an increased entrapment: 19% for 2 mM ALA, 69% for 2 mM He-ALA and 87% for 2 mM Und-ALA in PC–PG liposomes and 21% for 2 mM ALA, 60% for 2 mM He-ALA and 87% for 2 mM Und-ALA in PC–PA liposomes. Higher concentrations of ALA or derivatives resulted in lower percentages of entrapment. The three formulations containing ALA or derivatives were stable up to 1 week upon storage at 4 °C. However, upon dilution with medium, ALA leaked from the liposomes, while on the contrary, He-ALA was highly retained, being therefore a good choice for its use in PDT. The stability of Und-ALA upon dilution could not be tested, but Und-ALA proved to have the highest entrapment efficacy.