Selecting the right syringe filter
To achieve reliable dissolution results, selection of the right filter is important. There is not one single filter that works for all drugs because drugs possess varying binding properties to different filter materials. Drug adsorption to filters can lower the apparent dissolution rate, particularly at early sampling time points.
This is one of the reasons why the USP Chapter 1092 <The Dissolution Procedure> suggests carrying out a filter compatibility test on the filter or selection of filters you want to use by assessing adsorptive interference. This is typically conducted with a sample of medium which has a drug concentration within the concentration range of your dissolution test. This sample containing drug is filtered and analysed after serial passages and compared against the drug concentration in the unfiltered medium. In cases where drug adsorption is saturable, it should be possible to collect a sample that reflects the actual concentration.
When sampling multiple vessels, it is also important there is no clogging or little resistance during sampling so that samples can be filtered very rapidly and in close succession without delay and worry of the filter bursting. This is particularly important when the sample contains a high amount of insoluble drug and/or excipient.
Filter dimensions with 0.45 μm pore size and 25 mm diameter are generally appropriate for testing most immediate release drug products (including micronised drugs) in biorelevant media. Different filter dimensions and types are needed if nanosized particles are being evaluated.
Formulations with a high dose insoluble drug or high excipient level may need a pre-filter (such as the one stacked on glass microfibre filter) to prevent clogging and bursting. For most drugs, we find that PP, PTFE (hydrophilic), PVDF membrane filters with the above dimensions are excellent potential candidates because drug binding is usually very low and they filter biorelevant media easily.
Other filters such as PS and PES can be used but more often than not these adsorb more drug than glass microfibre with a prefilter, PTFE (hydrophilic), PVDF and PP filters. A nylon membrane can be used but is less suited for use with FEDGAS media due to higher resistance during filtration. We recommend avoiding PTFE (hydrophobic) filters which will not filter biorelevant media.
Glass microfibre with a prefilter, PTFE (hydrophilic), PVDF and PP filters are available to buy from us in packs of 60 and we also now sell a convenient Selection Pack of all 4 types for easy filter compatibility studies.