Filter adsorption studies are determined whether the drug is adsorbed onto a filter during dissolution sample filtration. Filter adsorption can lead to underestimation of dissolution results.

If filter adsorption occurs, you need to saturate the filter with sufficient sample so that further sample filtrate is no longer significantly affected by drug adsorption.

As with any dissolution medium, the filter adsorption sample should be prepared at a concentration representative of the expected drug concentration at the first nominal dissolution sampling time point. For example, this may correspond to approximately 10% or 20% drug release, depending on the expected early dissolution profile.

The filter adsorption study may then be typically performed using the recommended procedure shown below with at least 3 filters.

Dilute the unfiltered and filtrate samples and analyse by HPLC. 

Determine the percentage of drug adsorbed onto the filters for EACH medium used and establish the total dissolution sampling volume required at each time point. The total sampling volume should include both the filter saturation/discard volume and the sample filtrate volume required for analysis.

The percentage of drug adsorbed may be calculated using:

% Adsorbed = [(Average AUC of unfiltered sample − Average AUC of filtered sample) / Average AUC of unfiltered sample] × 100

The filtration fraction that shows acceptable filter saturation/recovery, low % adsorbed, and acceptable %RSD can be used to define the minimum discard volume required to saturate the filter before collecting the dissolution sample for analysis.

For example, if acceptable saturation is achieved after filtering and discarding 4 mL, then the total dissolution sampling volume should include:

4 mL discard volume + 1 mL required analytical sample volume = 5 mL total sampling volume

If acceptable filter saturation/recovery is not achieved, a larger discard volume may be required before collecting the sample for HPLC analysis.