Delivering therapeutic agents to the airways maximizes their concentration in lung tissue, decreasing systemic exposure or facilitating systemic absorption as desired. Many formulations exist for the treatment of respiratory illnesses however, no controlled release inhalation formulation exists to-date. This review is an update of the current advances in controlled release inhalation formulations and evaluation. The major successful particle engineering strategies are discussed along with potential in vitro and in vivo methodologies required for their characterisation. Controlled release formulation has many challenges to overcome, specific to this kind of medicament for inhalation. With small particle size and thus an increase in surface area, it becomes more difficult to achieve an effective controlled release profile. In addition, the physiology of the lung and its impact on resident particles need to be considered. An important issue when developing controlled release inhalation formulation is the toxic, inflammatory and accumulation effects of the release modifying agents used. These effects will need to be scrutinized in much greater detail in order to bring these formulations to the market. Currently, strategies for controlling the release of inhalation therapy include molecular dispersions (liposomal-based systems), solid lipid microparticles, coating or encapsulating drug particles in a lipid outer shield, solid biodegradable (synthetic and natural excipient-based matrices), conjugates and viscous semisolid vehicles. However, the availability of standardized pharmacopoeia methodologies to test the in vitro release rates or in vivo methodologies to evaluate deposition, pharmacokinetics and clearance of controlled release systems are not available. These methodologies are presented and discussed in this review.
Keywords: Controlled release, pulmonary inhalation, in vitro release, in vivo, biodegradable polymers, particle engineering, diffusion
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