New USP Stimuli Articles on Dissolution Testing

In the Pharmacopeial Forum, PF 52(2), two Stimuli articles addressing the mechanical performance qualification of USP Apparatus 3 (Reciprocating Cylinder) and USP Apparatus 4 (Flow-Through Cell) were published.

Although Stimuli articles are not official compendial requirements, they reflect USP’s current thinking. For GMP laboratories, they provide practical guidance on how to ensure equipment suitability in the absence of an official Performance Verification Test (PVT).

For both apparatus types, no active USP PVT material is currently available. USP therefore emphasizes a strengthened mechanical qualification strategy aligned with <711> Dissolution and <1058> Analytical Instrument Qualification.

Both documents are currently open for public comment and are available after registration in the Pharmacopeial Forum. Comments can be submitted until 31 May 2026.

USP Apparatus 3 (Reciprocating Cylinder)

The article titled "USP Guideline on Procedures for Mechanical Performance Qualification for USP Apparatus 3—Reciprocating Cylinder" provides detailed recommendations covering geometry, motion control, and temperature performance.

Key elements include strict levelness requirements of no more than 1° for the benchtop and no more than 0.5° for vessel plates, as well as verification of shaft and vessel verticality within 0.5° deviation. Reciprocation rate must remain within plus or minus 5 percent of the set value, typically assessed at different dip rates. Stroke distance is defined as 10.0 cm plus or minus 0.1 cm and requires documented measurement using calibrated tools. Vessel temperature must be maintained at 37.0 plus or minus 0.5 °C.

The guideline also emphasizes preventive maintenance of mechanical components such as belts, shafts, vessels, screens, and bath systems. Before each run, analysts must document checks confirming cleanliness, screen integrity, proper vessel positioning, stable evaporation caps, and correct temperature control.

In the absence of a PVT, these mechanical controls serve as the primary demonstration of apparatus suitability.

The article also mentions that "USP is seeking proposals for drug products that could be used in the PVT of USP Apparatus 3. These drug products should have a modified-release mechanism compatible with the functioning of USP Apparatus 3—for example, not disintegrating completely in the first row of the apparatus and exhibiting a release rate that allows for two or more sampling time points over a reasonable test length." 

USP Apparatus 4 (Flow-Through Cell)

The corresponding article, entitled "USP Guideline on Procedures for Mechanical Performance Qualification for USP Apparatus 4—Flow-Through Cell" focuses on mechanical integrity of the flow-through system and flow accuracy.

Levelness of both the benchtop and the cell support manifold must not exceed 1°, with verification recommended under filled bath conditions. Dimensional conformity of 12 mm and 22.6 mm cells must be confirmed using calibrated measuring devices. Where certificates of conformance are unavailable, laboratories are expected to document in-house measurements.

Critical operating parameters requiring periodic verification include flow rate accuracy within plus or minus 5 percent of nominal flow, temperature control at 37.0 plus or minus 0.5 °C, pump pulsation if applicable, bead dimensions, and sampling time accuracy within plus or minus 2 percent. Preventive maintenance must address seals, tubing, pumps, heater systems, and bath integrity.

Operational checks before each run must confirm absence of leaks, correct filter placement, proper bead configuration, and verified temperature and flow conditions.

As with the first article, the authors again mention that USP is seeking proposals for drug products that could be used in the PVT for USP Apparatus 4. Suggestions can be addressed to the contact address mentioned in the article.