Small-Volume Dissolution Testing of Pediatric Mini-Tablets Using Miniaturized USP Apparatuses: Regulatory and Methodological Considerations

Abstract

Mini-tablets (MTs) have gained increasing importance as age-appropriate dosage forms, particularly for pediatric patients, due to their ease of swallowing and flexible dosing capabilities. Despite their growing use, regulatory guidance and standardized quality control (QC) methods specifically adapted to MTs, especially extended-release (ER) formulations, are still lacking. This study investigates the suitability of two miniaturized dissolution apparatuses for evaluating ER MTs: a Mini-Paddle system, which is a geometrically downscaled version of the compendial Paddle (USP 2) apparatus, and a small-volume Reciprocating-Holder (USP 7) apparatus. Two commercially available pediatric ER MT drug products containing melatonin and sodium valproate were tested individually and in deliberately manipulated batches to assess the methods’ sensitivity and discriminatory power. The Mini-Paddle apparatus reliably quantified drug release from melatonin MTs but encountered limitations with low-dose sodium valproate MTs, requiring the pooling of multiple units, which masked individual variability. Conversely, the small-volume Reciprocating-Holder enabled sensitive, single-unit testing for both products, effectively detecting batch-to-batch differences despite very low drug loads. Neither apparatus currently holds official compendial status, emphasizing the urgent need for regulatory recognition and standardized methodologies. This study highlights the critical need for sensitive, miniaturized dissolution methods tailored to the unique challenges of MTs to ensure consistent quality, safety, and efficacy, particularly in vulnerable populations. Future work should focus on harmonizing and validating miniaturized dissolution systems to support QC and regulatory acceptance of MT formulations.

Introduction

The development of medicinal products is governed by the fundamental principles of ensuring quality, efficacy, and safety. In this context, quality control (QC) methods are indispensable throughout pharmaceutical development and manufacturing. Their importance is reflected in the broad inclusion of standardized test procedures in international regulatory guidelines and pharmacopeial monographs. For solid oral dosage forms, particularly tablets and capsules, dissolution testing has become a cornerstone technique for assessing dissolution or drug release characteristics of immediate-release (IR), delayed-release (DR), and extended-release (ER) formulations and current official methods are well established for drug products targeted at the adult population. However, in recent years, there has been increasing focus on age- and patient-appropriate dosage forms [1,2,3,4]. Particularly, mini-tablets (MTs), defined as small tablets typically 1 to 4 mm in diameter [5,6,7,8], have gained prominence for the pediatric population due to their favorable swallowability and flexibility in dose adjustment [7, 9, 10]. By administering a tailored number of units, clinicians can achieve individualized dosing across various pediatric age groups without the need to manipulate standard tablets, a practice that may compromise dose accuracy, drug stability, and patient acceptability [11,12,13]. Moreover, MTs are also being explored for use in other populations with swallowing difficulties, such as the elderly or patients with dysphagia, highlighting their broader potential beyond pediatrics.

Despite offering significant potential for patient-centric drug delivery, MTs remain insufficiently defined within current regulatory frameworks. Neither the European Medicines Agency (EMA) nor the United States Food and Drug Administration (FDA) has yet issued formal guidance recognizing MTs as a distinct solid oral dosage form with specific quality attributes. Consequently, MTs are generally assessed using compendial methods designed for conventional tablets, which may not adequately reflect their unique manufacturing, analytical, and performance-related challenges. Given the absence of regulatory guidance on MT-specific dissolution testing, there is a critical need for research to inform both scientific practice and regulatory frameworks in this area.

In pediatric use, for example, a single MT may constitute the lowest administrable dose, making high unit-to-unit dose uniformity and consistent release performance critical. However, conventional in vitro dissolution methods often lack the sensitivity and discriminatory power required to detect variability at the individual-unit level. Standard apparatuses, such as Apparatus 1 (Basket) and Apparatus 2 (Paddle), operate with relatively large media volumes (typically 500–1000 mL) [14,15,16], rendering them suboptimal for low-dose MT formulations. Under these conditions, drug concentrations in the medium may fall below detection limits unless multiple units are pooled, masking unit-level variability and precluding individual assessment. The Flow-Through Cell (Apparatus 4) presents a potential alternative. When operated in closed-loop mode, it may enable testing of single units by maintaining drug concentrations above the analytical threshold. However, this mode utilizes a fixed media composition, limiting the simulation of dynamic gastrointestinal conditions, which may be an important factor in evaluating extended-release (ER) MTs.

The lack of appropriate dissolution methods for QC of MTs directly conflicts with the principles outlined in International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use Considerations (ICH) guidelines Q8(R2) [17] and Q6A [18], which advocate for formulation-specific specifications and control strategies based on a product’s critical quality attributes (CQAs). For MTs, the use of current compendial dissolution methods is associated with several limitations, with the primary drawback being their limited sensitivity to detect variability at the individual unit level. Given that dissolution or drug release performance is a critical CQA with direct implications for bioavailability, therapeutic efficacy, and patient safety, these methodological shortcomings pose significant scientific and regulatory challenges. To enable robust product development and ensure regulatory compliance, there is a critical need for miniaturized, sensitive, and reproducible in vitro dissolution methods capable of reliably characterizing individual MTs. Such methods must enable the use of small media volumes and support precise, physiologically relevant characterization of dissolution at the individual unit level. Bridging this methodological gap is critical to ensuring product quality, batch uniformity, and facilitating the broader clinical adoption of MTs. To meet these technical and regulatory needs, novel yet pharmacopeia-aligned dissolution platforms must be explored.

Building on these requirements, to achieve the highest possible degree of alignment with compendial frameworks, two miniaturized apparatuses present promising options for addressing the limitations of conventional dissolution methods. The first is a Mini-Paddle system that constitutes a geometrically downscaled version of the compendial Apparatus 2, as previously described by Klein [19, 20]. This setup offers the advantage of aligning closely with existing pharmacopeial principles, despite no formally recognized Mini-Paddle configuration is currently described in either the United States Pharmacopeia (USP) [14] or the European Pharmacopoeia (Ph. Eur.) [15]. The second option is the small-volume version of the Reciprocating-Holder apparatus, a scaled-down version of USP Apparatus 7 [21]. Originally developed for evaluating non-disintegrating dosage forms such as drug-eluting stents and transdermal systems, this platform offers significant potential for application to small oral dosage forms. It enables dissolution testing in media volumes as low as 5–10 mL, with precise control over temperature, agitation, and sampling. Moreover, it supports sequential media replacement, facilitating the simulation of physiological pH gradients under controlled conditions. Although not yet formally included in any pharmacopeial monograph, the small-volume version of the Reciprocating-Holder apparatus has demonstrated high reproducibility and analytical precision in previous studies [22, 23] and provides a scientifically robust and flexible platform for both method development and QC applications.

The lack of suitable, analytically sensitive dissolution methods for all types of MTs presents a significant challenge, particularly for ER MTs, where the in vitro dissolution or drug release profile serves as a critical predictor of the in vivo plasma concentration profile. Given the potential impact of impaired quality on the therapeutic performance, efficacy, safety, and reliability of ER MTs, compared to IR and DR MTs, the in vitro dissolution or drug release profile remains the most reliable tool for assessing these factors. Consequently, quality-indicating dissolution methods are essential not only for standard QC but also for formulation development. Due to their prolonged release characteristics, ER MTs provide an ideal platform for evaluating the discriminatory power of dissolution testing in terms of both the rate and extent of drug release. Furthermore, the limited availability of pediatric ER MTs underscores the urgent need for robust and reliable dissolution methods, which are crucial for streamlining the development of high-quality, safe, and effective ER MTs. This study aimed to evaluate the suitability of two miniaturized USP apparatuses, the Mini-Paddle apparatus representing a geometrically downscaled version of the compendial Paddle apparatus [19, 24] a small-volume Reciprocating-Holder apparatus, for developing dissolution test methods for ER MT formulations.

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Materials

Slenyto® 5 mg prolonged-release tablets containing the active ingredient melatonin (# 10413 C, 20,295 and 30163 A, RAD Neurim Pharmaceuticals EEC SARL, Paris, France), and Orfiril® long 300 mg capsules, containing sodium valproate as active ingredient (# 14,006,222, Desitin Arzneimittel, Hamburg, Germany), were obtained via the local hospital pharmacy. Melatonin reference material (# 22003650002) was purchased from Caesar & Lorentz GmbH (Hilden, Germany), and sodium valproate reference material (# MKBS5723V) was purchased from Sigma-Aldrich (Steinheim, Germany). All other chemicals and solvents for media preparation and analysis were of analytical or gradient grade and purchased commercially.

Broocks, S., Gebhardt, M. & Klein, S. Small-Volume Dissolution Testing of Pediatric Mini-Tablets Using Miniaturized USP Apparatuses: Regulatory and Methodological Considerations. AAPS PharmSciTech 27, 46 (2026). https://doi.org/10.1208/s12249-025-03252-y

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