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This chapter introduces the routes of administration of therapeutic and diagnostic compounds to the body, the considerations in designing drug delivery systems, and some examples of the innovations that have contributed new controlled delivery products. In addition to the oral route, the most common nonparenteral routes of delivery are topical/transdermal, nasal, pulmonary, ocular, rectal, and vaginal. The parenteral route enables precise dosing for drugs that have a narrow therapeutic index, complete dosing for those with poor oral bioavailability to maintain optimal therapeutic concentrations, ready access in patients with swallowing difficulties, or an unconscious patient, and immediate blood levels of drugs in an emergency. The most common routes used for systemic delivery of a wide range of molecules including biopharmaceuticals, are subcutaneous route, intravenous, and intramuscular route. The chapter provides an overview of the formulation challenges and opportunities offered by the oral route.

It is critical to develop an effective understanding of the interaction between the drug, delivery system and skin in order to predict and assess skin penetration and permeation. Experimental models for the assessment of topical and transdermal delivery systems must permit evaluation of these complex interactions. Whilst in the past, animal models were commonly used, recent regulatory guidelines, based on 3R principles (refinement, reduction, replacement), encourage the rational use of animals. Alternative methods have been proposed for use in the development of topical and transdermal delivery systems which are often used in combination. We will review the current state of the art in alternative methods for topical and transdermal delivery systems development, including technologies that can assist in the characterization of skin penetration/permeation studies.