Determination of Particle Size of Active Pharmaceutical Ingredients in Dry Powder Inhaler Formulations

Abstract

Background/Objectives: Accurate determination of active pharmaceutical ingredient (API) particle size within dry powder inhaler (DPI) formulations is essential for ensuring effective pulmonary delivery but remains analytically challenging due to low API content and micronized particle size.

Methods: In this study, scanning electron microscopy (SEM) coupled with energy-dispersive X-ray microanalysis (EDX) was used to directly identify and calculate the API particle size within several different commercial DPI products fit for purpose under regulatory constraints. The method exploits unique elemental markers inherent to each API, enabling reliable discrimination from excipients without prior sample modification or API extraction.

Results: Large-area SEM–EDX mapping was used to localize API particles, followed by high-magnification imaging and confirmatory spot microanalysis. Particle sizes were manually measured for at least 50 API particles per formulation using image analysis software, and particle size distribution parameters were calculated from equivalent spherical diameters.

Conclusions: The methodology was successfully applied to Spiriva^®, Anoro^® Ellipta, and Relvar^® Ellipta inhalation powders, revealing micronized APIs with distinct morphological features and verifying systematic application across products. Cross-validation against laser diffraction measurements of pure APIs demonstrated statistical equivalence, confirming the robustness and analytical utility of the proposed method for particle size assessment in DPI formulations.

Introduction

The particle size and morphology of active pharmaceutical ingredients is a major concern in drug development, affecting absorption rate, bioavailability, manufacturability and the overall quality and performance of solid dosage forms [1,2,3,4,5,6,7]. In dry powder inhalers, particle size determination of APIs is particularly important owing to the stringent specifications required for pulmonary delivery. DPI formulations typically consist of an ordered mixture of micronized drug particles and larger lactose carrier particles, the latter being incorporated to enhance powder flowability. The patient’s inhalation through the device is used to disperse the powder and to ensure that some of the dose is carried into the lungs [8]. To reach the small airways of the deep lung, the API particles in DPIs need to have an aerodynamic diameter of 1–5 μm to avoid impaction and particle sedimentation in the upper respiratory tract [9]. Consequently, the evaluation of API particle size is essential throughout all stages of pharmaceutical development of DPI products.

Conventional particle size analysis methods, such as laser diffraction, sieve analysis, and direct imaging techniques, cannot be applied, at least alone, to solid dosage forms due to the concomitant presence of excipients [10]. To the best of our knowledge, the literature data addressing the determination of API particle size and morphology within final pharmaceutical products are limited. Some researchers have developed methodologies that exploit the differences in melting points between excipients and APIs in tablet formulations. Using hot-stage microscopy, they successfully measured the particle size of tadalafil and meloxicam within pharmaceutical products [11]. This approach, although original, exhibits some limitations since the API melting point should be higher than the excipients. Finally, degradation and/or decomposition of the components is highly possible, resulting in toxicity or discoloration, etc.

Several studies employing micro-Raman chemical imaging have also been reported [12,13,14,15]. However, this technique alone appears to be more suitable for identifying the so-called “domain” particle size, as the discrimination of frequently occurring agglomerated particles remains unresolved. This technique has demonstrated considerable promise in the field and has therefore been employed in combination with other analytical methods to enable the measurement of API particle size in solid dosage forms.

Our team has developed a methodology for the determination of bismuth oxide API particle size in bismuth oxide 120 mg film-coated (FC) tablets. The sample preparation involved partial dissolution of excipients in selected solvents to enable efficient extraction of the API from the granules. Subsequent identification of particle size and morphology within the tablets was achieved using micro-Raman mapping spectroscopy in combination with ImageJ software (V.1.8.0) [16]. Similarly, Alexios Tsiligiannis et al. employed a micro-Raman spectrometer equipped with ParticleFinder™ module of LabSpec Software version 6.6.6.2 to chemically identify fluticasone propionate particles in Dymista® nasal spray and to simultaneously assess their size distribution [17].

In recent years, a novel technique that has gained prominence for monitoring particle size and shape in solid dosage formulations is Morphologically Directed Raman Spectroscopy (MDRS®). MDRS integrates automated particle imaging with Raman microspectroscopy in a single platform, thereby enabling rapid, automated chemical and morphological characterization of individual components in multicomponent systems. According to the literature data, MDRS has been applied to determine the particle size of ibuprofen in a model formulation containing ibuprofen, starch, and lactose; to compare morphology and transparency properties of proteinaceous particles with reference standards; and to evaluate the particle size distribution of mometasone furoate monohydrate in five different nasal suspension drug products [18,19,20]. However, none of the aforementioned techniques have been applied to the determination of API particle size in DPIs, primarily due to the inherently low API concentration and the extremely small particle size characteristic of these formulations, since particulates must be ~2 µm or larger in size in order to be measured with MDRS [21].

The determination of API particle size in DPI formulations has previously been briefly reported [22,23]. Emily Landsperger et al. employed automated scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDX) microanalysis. API identification was based on the presence of bromine, the characteristic element of the API, enabling its differentiation from the excipients [22].

In that study, particle size distribution was expressed using the Davg morphological parameter (average diameter). The instrument calculates this parameter by drawing sixteen chords through the center of each particle, with Davg representing the mean chord length. The average diameter was determined under the assumption of spherical particle geometry. However, the study was primarily illustrative, aiming to demonstrate the proposed method’s capability, and did not address any analytical issues, equivalence to established particle sizing techniques, or applicability to multiple commercial DPI products under low API loading conditions.
In the present study, we sought to expand upon this earlier work by establishing a controlled manual SEM–EDX analytical workflow and evaluating its quantitative performance, robustness (through APIs containing different characteristic elements), and limitations for API particle size determination in multiple finished DPI products with an API content as low as 0.1% w/w. We successfully determined the particle size and morphology of umeclidinium bromide and vilanterol trifenatate in Anoro®, fluticasone furoate and vilanterol trifenatate in Relvar® and tiotropium bromide in Spiriva®. Particle size distribution was expressed in terms of d10, d50, and d90 values, analogous to the laser diffraction technique. The proposed approach can be implemented in pharmaceutical laboratories for bioequivalence studies with minor modifications.

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Materials

The monohydrate tiotropium bromide (Cipla limited, Mumbai, India), monohydrate α-lactose (DFE Pharma, Goch, Germany), magnesium stearate (Peter Greven Nederland C V, Netherlands), umeclidinium bromide (INKE Pharmaceutical, Barcelona, Spain), vilanterol trifenatate (Glaxosmithkline pharmaceuticals, Mumbai, India) fluticasone furoate (Sterling S.p.A., Deeside, Wales, UK) and glycopyrronium bromide (INKE Pharmaceutical, Barcelona, Spain) were kindly provided by ELPEN S.A. (Pikermi, Attica, Greece). The Spiriva® 18 microgram inhalation powder (Boehringer Ingelheim, Biberach, Germany), Anoro® 55 micrograms/22 micrograms inhalation powder (Glaxosmithkline pharmaceuticals, London, UK) and Relvar® 92 micrograms/22 micrograms inhalation powder (Glaxosmithkline pharmaceuticals, Dublin, Ireland) were purchased from a local pharmacy store.

Fertaki, S.; Orkoula, M.; Kontoyannis, C. Determination of Particle Size of Active Pharmaceutical Ingredients in Dry Powder Inhaler Formulations. Pharmaceuticals 2026, 19, 543. https://doi.org/10.3390/ph19040543