The United States Pharmacopeia (USP) is developing a new general chapter, <1003> Resuspendability and Redispersability, to standardize the evaluation of these characteristics in pharmaceutical suspensions. This initiative addresses critical quality attributes in suspension formulations to ensure consistent dosing and therapeutic efficacy. The proposal is scheduled for publication in Pharmacopeial Forum 51(4) on July 1, 2025, with final comments due September 30, 2025.

Here, I discuss why resuspendability matters, its challenges, and how this new chapter can improve the quality of pharmaceutical suspensions and facilitate compliance with regulatory standards.

Why resuspendability matters

Pharmaceutical suspensions are biphasic systems in which solid particles are dispersed within a liquid medium. Over time, these particles may agglomerate and settle, leading to sedimentation. The ease with which this sediment can be re-suspended and re-dispersed upon agitation — known as resuspendability and redispersability — is vital for maintaining dose uniformity. Poor resuspendability and redispersability may result in inconsistent dosing, potentially compromising patient safety and treatment outcomes.

Challenges in formulating a good suspension

Developing a suspension with optimal resuspendability and redispersability involves addressing several formulation challenges:

Particle size distribution

Smaller, uniformly sized particles tend to remain suspended longer and are easier to resuspend. However, achieving this uniformity requires precise manufacturing processes, and variations can affect sedimentation behavior.

Suspending agents

Appropriate suspending agents can increase the viscosity of the medium, reducing sedimentation rates. However, excessive viscosity may hinder pourability and reduce patient acceptability.

Wettings agents

Used to improve the dispersion of hydrophobic drug particles within the liquid medium, wetting agents are crucial to prevent particle agglomeration, which can impair redispersability.

Ionic strength

The ionic strength of the suspension can influence particle interactions. Proper electrolyte balance is necessary to prevent flocculation or deflocculation, both of which can negatively impact resuspendability.

How resuspendability and redispersability are tested

Assessing resuspendability and redispersability is a multifaceted process that combines both qualitative and quantitative methods:

Visual inspection

This simple method involves observing the sediment’s behavior upon agitation. While it offers a basic assessment, it lacks precision and reproducibility.

Settled sediment to total suspension volume ratio

This quantitative metric measures the ratio of settled sediment volume to the total suspension volume over time, offering insights into sediment characteristics.

Rheological measurements

These assessments evaluate the flow properties of the suspension, offering insight into its structural integrity and resuspension behavior under different shear conditions.

Implications of USP’s new chapter

The introduction of USP’s general chapter <1003> will provide standardized methodologies for assessing resuspendability and redispersability across various suspension types and administration routes. These new guidelines may help:

Enhance consistency

Clear evaluation guidelines will support manufacturers in producing suspensions with reliable resuspendability profiles, ensuring consistent therapeutic outcomes.

Facilitate regulatory compliance

Standardized assessment frameworks can simplify the review process for regulatory agencies, potentially expediting product approvals.

Looking ahead

The forthcoming USP chapter on resuspendability and redispersability represents a significant advancement in the quality assessment of pharmaceutical suspensions. It’s anticipated to aid compliance with international guidelines such as ICH Q6A “Specifications: Test Procedures and Acceptance Criteria for New Drug Substances and New Drug Products: Chemical Substances,” as well as existing USP chapters such as USP <1> “Injections and Implanted Drug Products (Parenterals) — Product Quality Tests” and USP <2> “Oral Drug Products — Product Quality Tests With Respect to Resuspendability and Redispersability.”

By addressing formulation challenges and standardizing evaluation methods, this initiative aims to ensure that suspensions deliver consistent, safe, and effective therapeutic outcomes. Formulators and manufacturers should proactively adapt to these guidelines, fostering innovation and maintaining high-quality standards in suspension-based drug products.

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