POTENCIAL OF POLYAMIDOAMINE DENDRIMER AS A DRUG CARRIER FOR PROTOPORPHYRIN IX : SOLUBILITY STUDIES

Introduction: Polyamidoamine dendrimers (PAMAM) are hyperbranched, ordered, monodispersed polymers. Above the fourth generation, dendrimers adopt a globular structure with large hollow cavities inside. These molecular boxes may be utilized as drug delivery vehicles because of their ability to form complexes via molecular encapsulation, covalent and non-covalent interactions. PAMAM dendrimer have high surface charge density that make possible that each macromolecule attach numerous drug molecule. Futhermore, their hydrophobic interior can bind hydrophobic drugs, increasing in this way their water solubility. Protoporphyrin IX (PpIX) is an efficient photosensitizer for Photodynamic Therapy (PDT). However, it is highly hydrophobic and forms aggregates in water environment. Therefore, PpIX has a limite skin penetration for the treatment of topical skin tumours by PDT. The present study evaluates the efficacy of dendrimer PAMAM G4.5 to form complexes with PpIX and increase its solubility for further skin penetration studies. Methods: Solubility studies were carried out using the Higuchi rotating bottle method. Solubility of PpIX in the presence of 0,2% of PAMAM G4.5 was first studied in function of time and pH (7 and 4). Then the solubility profile of PpIX as function of PAMAM G4.5 concentration was obtained adding an excess of PpIX to HEPES buffer solutions at pH 7 in vials containing increasing amounts of the dendrimer. The vials were rotated at 37º C for 4 days. After equilibration, the samples were filtered and analyzed by HPLC.  Results: Solubility studies of PpIX in the presence of PAMAM G- 4.5 0.2% in function of time showed that the complex reaction attain its equilibrium in 4 days. After this time, PAMAM G4.5 increased around 32 times the aqueous solubility of PpIX at pH 7. At this pH, PAMAM G4.5 has its carboxylic groups at the surface negatively ionized but the tertiary amines inside its cavity are positively charged. Therefore, its probably that the negative PpIX interact eletrostatically with the positive groups inside the dendrimer. At pH 4 a slight, but not statistically significant, increase in the PpIX solubility was observed. This may occurs because the internal and central amine groups are protonated, favouring eletrostatic interactions between PpIX and the amine groups. The solubility profile of PpIX as a function of increasing concentration of the G-4.5 PAMAM dendrimer in aqueous vehicle at pH 7.0. The phase–solubility profile showed a trend similar to the relatively infrequent Higuchi’s AN-type curve (negative deviation from the linearity). The solubility of PpIX increased linearly at lower concentrations (up to 0.05% w/v) and above this concentration the increase was no longer linear, presumably due to self-association of dendrimers at higher concentrations. Conclusion: In conclusion, PAMAM G4.5 increased significantly the solubility of the high lipophilic PpIX in buffer solution at pH 7 and 4.