Introduction

"Pharmaceutical inhalers" have been used to treat respiratory diseases for centuries. Early therapies included the use of vapors from aromatic plants, balsams, myhrr, and sulfur. However, around the turn of the 19th century, with the advent of liquid nebulizers, these early treatments developed into legitimate pharmaceutical therapies. In the 1920 s adrenaline was introduced as a nebulizer solution, in 1925 nebulized porcine insulin was used in experimental studies in diabetes, and in 1945 pulmonary delivery of the recently discovered penicillin was investigated [1]. By the mid 1950s, steroids had been introduced for the treatment of asthma and nebulizers were enjoying widespread use. In 1956 the pressured metered-dose inhaler (pMDI) was introduced [2], over the past 5 decades, helped by the advances in molecule design and drug discovery, the pMDI has risen to become the main stay of asthma treatment around the globe. However, both the nebulizer and the pMDI have limitations, in terms of both the dose they can deliver and the ease with which they can be used. Thus, in the late 1950s the dry powder inhaler (DPI) was added to the arsenal of aerosol delivery techniques. By the early 1990s these three aerosol delivery modalities had become established technologies and had been used widely across a range of diseases and with a host of molecules.

However, at the start of the 1990s it was obvious to those involved in pulmonary drug delivery that is was going to be an exciting, if not exhausting, decade. With a few notable exceptions, mainly driven by technical limitations, the previous four decades of aerosol therapy had been dominated by chlorofluorocarbon-(CFC)-propelled metered-dose inhalers, and by the late 1980s it was becoming apparent that CFCs were an environmental threat that the global community was no longer prepared to tolerate. In 1991, the work started by Mollina and Rowlands [3] in the early 1970s finally led to the Montreal Protocols [4] and a planned withdrawal of CFCs. This event forced the pharmaceutical industry to look for alternatives to CFCs, and it was responsible, in large part, for the succeeding years of exploration and invention in the pulmonary drug delivery arena.

At about the same time, the emerging biotechnology industry was leading a revolution in protein therapeutics. The large macromolecules produced by recombinant technology had attendant formulation and delivery challenges. Most had to be delivered by either subcutaneous or intravenous injection, and there was, and to a large extent still is, a tremendous need for an alternative, noninvasive delivery route. The realization that the pulmonary epithelium may be one of the most effective noninvasive routes of delivery for macromolecules brought about interest in pulmonary delivery from a new group within the industry. However, the then-current platforms, developed for small-molecule topical therapy, lacked both the efficiency and the reproducibility needed to deliver this new class of molecules to the lung. It was apparent that new technology was needed. The stage was therefore set for a decade of exploration, discovery, and invention, with formulation and device technologies being driven by the need to replace CFCs and the desire to improve efficiency and reproducibility (Fig. 1).

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