Zn2+-exchanged clinoptilolite-rich rock as active carrier for antibiotics in anti-acne topical therapy: In-vitro characterization and preliminary formulation studies

The combination of erythromycin and zinc in the topical antibiotic therapy of acne has been proved useful to reduce the development of antibiotic-resistant strains. An Italian zeolite-rich rock, containing 66 wt.% of clinoptilolite, has been used to prepare an active carrier for erythromycin. The zeolite has been conditioned in zinc form by a set of cation exchange processes, then erythromycin has been adsorbed onto the micronized rock by vacuum drying. After 6 month ageing, almost 95% of the loaded drug was found, indicating good stability of the Zn-carrier-erythromycin system. Minimum inhibitory concentration (M.I.C.) assay on Propionibacterium acnes showed that the process of adsorption of erythromycin does not affect its antimicrobial activity. Both drug and zinc release were evaluated from the carrier powder: the erythromycin release was more than 80% in the first hour and independent on hydrodynamic conditions (United States Pharmacopoeia dissolution apparatus or Franz cell). The release of zinc was fast, and more dependent on the hydrodynamic conditions. The influence of formulating the carrier by dispersion in two anhydrous bases, an absorption petrolatum base (HP) and a water soluble polyethylenglycol base (PEG), was evaluated. In both cases erythromycin and zinc showed a release quantitatively similar to that of the powder, although the release rate was slightly lower from the more hydrophobic HP base.