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This article provides an overview of uniformity of delivered dose testing and labeling, and considers requirements from the USP, Ph. Eur., EMA and FDA. Defining the dosage strength for labeling of DPIs: Use, limitations and relevance of in vitro data Uniformity of delivered dose testing and labeling: An overview Jagdeep Shur, PhD a ; Robert Price, PhD a ; Tim Rouse, BSc b ; David Lewis, PhD b ; Dilraj Singh, PhD c ; Bradley Morrical, PhD c and Stephen Edge, PhD c a University of Bath b Chiesi, Ltd. c Novartis Pharma AG will be used throughout this article). Clearly, such descriptions and disparities in pharmaceutical tests that use the word "dose" within regulatory source docu- ments may be somewhat confusing. The development of inhaled products: Defining the dose During early phase pharmaceutical development, where the optimal efficacious dose or dosing regimen has not yet been defined, technical and clinical drug product batches (including different strengths) are used to elaborate the in vitro specifications for the DPI prod- uct, including the APSD and UDD. However, in con- trast to the APSD test, through all the stages of clinical development, the DPI product must comply with the applicable pharmacopoeial/regulatory prescribed limits of a test for UDD, unless both justified and authorized by competent authorities. The specifications for any APSD metrics and delivered dose of the product are only finalized after the health authority's review and approval of the marketing authorization dossier. Again, the key question regarding these two important critical quality attributes is which is more relevant for describing the dose, and thereby the efficacy, of the prod- uct, and which is therefore more appropriate for the labeling of an inhaled product such as a DPI. Indeed, it may appear somewhat surprising that, despite not being related to any APSD metric, limits for UDD are pre- scribed across several regulatory sources, whereas there are no limits/criteria for any more clinically relevant APSD metrics, except in the Chinese Pharmacopoeia Introduction Marketed dry powder inhaler (DPI) products utilize various formulation and device technologies to achieve consistent delivery of the efficacious dose of active phar- maceutical ingredient(s) (APIs) to the patient. However, regardless of the drug delivery platform, this dose must be of an appropriate quality and aerodynamic particle size distribution (APSD) to deposit in the target region of the lung and achieve the desired therapeutic effect. 1 e general and simplistic concept that the APSD of the API must be < 5 µm in order to reach the deep lung is a useful guide for the development and quality con- trol of both innovator and generic inhaled products. But it can be asked how this descriptor and concept relate to any established labeled dose or strength of an inhaled product, such as a DPI, and which methods were used for that determination. e terms used to describe the dose of DPI products appear in several pharmacopoeial tests for DPIs. For example, the European Pharmacopoeia (Ph. Eur.) not only describes methodologies to be used to determine the APSD and the metric fine particle dose (FPD), but also contains a test for the assessment of the uniformity of delivered dose (UDD), with acceptance limits. In contrast, even though the United States Pharmacopoeia (USP) describes a delivered dose uniformity test, albeit with no prescribed limits, it has, in contrast to Ph. Eur., no description of any APSD metric such as fine particle dose. 2 (Not all regions or countries' guidances/pharma- copoeias use the wording "uniformity of delivered dose" to represent the delivered dose test, but this phrase Inhalation February 2019 7