For immediate-release capsule or tablet formulations, Apparatus 1 (baskets) at 50–100 rpm or Apparatus 2 (paddles) at 50 or 75 rpm are used most commonly.
Other agitation speeds are acceptable with appropriate justification. Rates outside 25–150 rpm for both the paddle and the basket are usually not appropriate because of mixing inconsistencies that can be generated by stirring too slow or too fast.
Agitation rates between 25 and 50 rpm are generally acceptable for suspensions. For dosage forms that exhibit coning (mounding) under the paddle at 50 rpm, the coning can be reduced by increasing the paddle speed to 75 rpm, thus reducing the artifact and improving the data. If justified, 100 rpm may be used with Apparatus 2, especially for extended-release products.
Decreasing or increasing the apparatus rotation speed may be justified if to achieve an in-vitro–in-vivo correlation (IVIVC) the resulting profiles better reflect in vivo performance, or if the method results in better discrimination without adversely affecting method variability.
Apparatus 3 (reciprocating cylinder) can be used at dip rates ranging from 5 to 30 dips/min. The hydrodynamics are influenced by the cylinder's reciprocating motion and the resulting movement of the sample in the medium. The reciprocating motion of the cylinder and screen may cause foaming if the medium contains surfactants. The addition of an anti-foaming agent such as simethicone or n-octanol may be useful for avoiding foaming from surfactants.
Apparatus 4 (flow-through cell) can be used at flow rates up to 50 mL/min with pump speeds as low as 2 mL/min. Agitation in Apparatus 4 is not only related to the pump speed but can also be affected by cell diameter. At a set flow rate, as measured by volume, the 12-mm cell will develop a greater linear fluid velocity than is achieved in the 22.6mm cell.
The addition of glass beads in the entry cone of the flow-through cell has been said to produce a laminar flow; this is in contrast to the turbulent flow said to occur in the cell with no glass beads. Independent of the flow-through cell diameters, the fluid flow is expected to be laminar either with the 1-mm glass beads in the inlet cone (packed column) or without glass beads (open column) at compendial flow rates (≤16 mL/min) (12).
The placement of the sample in the flow-through cell will influence the flow patterns that occur and thus should be a consideration in the attempt to reduce the variability of the results.
Selection of the agitation rate and other study design elements for modified-release dosage forms should be similar to that for immediate-release products. These elements should conform to the requirements and specifications given in 711 when the apparatus has been calibrated appropriately.
Read also:
- USP Dissolution Apparatus | Types and Priniciples
- What is Intrinsic Dissolution Rate and Why It is Important?
Resource Person: Prakash Amate
