ABSTRACT
I. INTRODUCTION One of the primary objectives in the design of novel drug delivery systems is controlled delivery of the pharmacological agent to its site of action at a therapeutically optimal rate and dose regimen (1). This site-specific or targeted delivery combined with delivery at an optimal rate would not only improve the efficacy of a drug but would also reduce the possibility of unwanted toxic side effects of the drug, thus improving the therapeutic index (2). Among the most promising systems to achieve this goal are colloidal drug delivery systems (see the Preface of this book). Colloidal drug delivery systems include the drug carrier systems liposomes, niosomes, nanoparticles, and microemulsions. (Ointments, which also can be conceived of as colloidal drug delivery systems, are mainly used topically and therefore are very different in their way of action from the other four mentioned colloidal systems.)
Liposomes, niosomes, nanoparticles, and microemulsions are very similar in their size, shape, and mode of administration, and for this reason they may be used alternatively. Nevertheless, these systems have a number of different advantages and disadvantages. The great advantage of liposomes,
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for instance, is that their main components are lecithin and, in most cases, cholesterol. Similar lecithins as well as cholesterol are materials that also exist in the body in major amounts, and consequently a good bioacceptability may be expected. Nanoparticles, on the other hand, possess a better stability. This property, for instance, may be very important for many modes of targeting. As a consequence of the mentioned advantages and disadvantages, the most suitable system will have to be chosen depending on the drug and the therapeutic goal to be reached. As will be shown later, the same drug, e.g., mitoxantrone, can be more effective against one type of tumor, entrapped in liposomes, whereas the same drug bound to nanoparticles may be more efficient against another type of tumor.
