An assessment of beclomethasone dipropionate clathrate formation in a model suspension metered dose inhaler

The aims of this study were to investigate and characterize the physico-chemical properties of beclomethasone dipropionate (BDP) crystallized from tricholoromonofluoromethane (CFC-11). Physical interactions in a model pressurised metered dose inhaler (pMDI) system and changes in surface energy after size reduction (micronization) were determined. Although CFC-11 has largely been phased out of use in pMDIs due to its ozone depletion potential, the BDP CFC-11 clathrate is a stable entity and thus suitable as a model for our initial investigations. In addition, although propellant clathrates have been known for sometime, as far as the authors are aware, their surface energies and adhesive interactions have not been reported. The structure of the clathrate was investigated using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray powder diffraction (X-RPD). In addition, atomic force microscopy (AFM) was employed to determine the dispersive surface free energy (SE) and force of adhesion (Fadh) of the BDP CFC-11 clathrate with different pMDI components in a model propellant (decafluoropentane).The dispersive surface free energies for anhydrous BDP (micronized), the CFC-11 clathrate and ball-milled BDP CFC-11 clathrate are (47.5 ± 4.9) mJ m−2, (11.3 ± 4.1) mJ m−2 and (15.2 ± 1.3) mJ m−2 respectively. Force of adhesion results shows that BDP CFC-11 clathrates, even after being ball-milled for 2.5 h, have a lower Fadh compared to micronized anhydrous BDP with different pMDI components. This shows that the formation of the crystalline CFC-11 clathrate is advantageous when compared to the micronized anhydrous form, in terms of its surface energy and potential interactions within a suspension MDI formulation. In the wider context, this work has implications for the future development of HFA formulations with APIs which are prone to the formation of propellant clathrates.