Microspheres And Nanospheres

Improvements in the techniques and ability to incorporate proteins and peptides, better targeting approaches, and new materials have been developed since the early 1990s, and thus many microsphere-based products are under clinical development for delivery via most routes of administration [22,95,96]. One of their chief advantages is that the duration of drug release can be extended to a greater degree than virtually any other formulation option, although this benefit may be curtailed in the lungs. Other issues surrounding micropshere use have included stability, loading efficiency, and a method of production that has traditionally involved organic solvents [97,98]. There are, however, newer technologies (e.g., Promaxx®—Epic Therapeutics, www. epictherapeutics.com) where processing is conducted entirely in an aqueous environment using water-soluble polymers. Unfortunately, with pulmonary delivery come the added concerns of what materials can be used and whether micropsheres of a size suitable for pulmonary delivery (1-5 mm) can readily be prepared. These general issues have, to date, limited the pulmonary use of microspheres, although those formed using albumin [99,100], lipid [101], or PLGA [102] have been delivered to the lungs, demonstrating the feasibility of using the dosage form.

Can they be used practically in the lungs? Albumin would an ideal biodegradable encapsulating agent, but, to be practical, a low-cost recombinant version of the protein is a prerequisite. Using synthetic spray-dried micropsheres in combination with drugs that would otherwise be cleared rapidly seems logical, especially where the duration of therapy would be short-term rather than of a chronic nature. A nontherapeutic example might be their use as diagnostic contrast agents [103]. This would reduce safety concerns about the accumulation of polymeric material or, say, acidity arising from the biodegradation of commonly employed PLA and PLGA compositions [104,105]. Nevertheless, as long as simpler formulation options exist, the use of a microsphere preparation is unlikely to be developed for pulmonary use in the near term.

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