Webinar Highlights: Complete Solutions for Dissolution Testing – Monograph Methods for Tablets and Capsules

Dissolution testing is an integral part of the drug development lifecycle. The goals of dissolution testing change as the drug moves through the different stages of development, and the effectiveness of different drug dosage forms relies on the drug dissolving in a predictable manner prior to absorption into the systemic circulation. Optimization of the drug's bioavailability over time is also necessary; deviations from expected dissolution profiles mean that treatment could be either ineffective or dangerous to the patient.

In this informative webinar, Dr Patrik Appelblad, Application Team Manager, Instrumental Analytics, Merck KGaA, describes dissolution testing of small molecule drugs, following current USP 37 - NF32 monographs, and demonstrates the important role of filtration in relation to dissolution testing. Read on for solutions to improve separation methods and sample preparation, and discussion of the possible changes that can be made to existing monograph methods. If you missed the webinar, you can watch it on demand here.

1. When you compare the different membranes, such as PVDF and LCR etc, the value you give is higher than 100. Can you please explain the meaning of the value you present?

Yes, you can have results higher than 100% for several reasons. Firstly, in most cases the acceptance criteria for the assay of the drug is a range that has an upper limit higher than 100%. For instance, the range for the assay test in the USP-NF monograph for dextroamphetamine sulfate tablets is not less than 93% and not more than 107% of the labeled amount of dextroamphetamine sulfate. Secondly, the acceptance criteria for dosage uniformity (see USP-NF general chapter <905> Uniformity of Dosage Units) is in the range from 85% to 115% of the label claim for tablets. Dissolution results higher than 100% should be less frequent because they depend on the probability of testing a tablet with a dose higher than 100% of the label claim.

If you frequently have dissolution results higher than 100%, this may be an indication of interference in your dissolution system. You need to verify that substances are not leaching from the filter and interfering with the measurements. Other sources of interference could be carryover in automatic sampling systems and inappropriate cleaning procedures. (Answer by Margaret Marques and Will Brown, USP.)

For most of the tablets under study, as per USP guidance, the tablet can have between 90 – 110% of the labeled amount in the tablet. When you do the dissolution test, you assume that the labeled quantity is what is present in the tablet and that number is used for calculation of drug dissolution; but if that is higher than the label quantity, the actual percentage dissolution will be higher.

2. Could you please advise the impact of water in dissolution testing?

De-aeration of the dissolution medium is a key concern here, as presence of air / oxygen dissolved in dissolution medium affects dissolution profile. One example is the prednisone tablet, which is a USP calibrator tablet used for calibrating dissolution baths. The presence of oxygen in dissolution medium can affect rate of its dissolution. Vacuum filtration is one of the well known methods of de-aeration of dissolution medium. The quality of water used for preparing dissolution medium is equally important. For high volume users, a Milli-Q system with on-demand water delivery makes sense, whereas for low volume users, bottled water makes sense.

3. Do we have a list of APIs binding properties to membranes?

Binding of an API to a membrane depends on the physicochemical properties of the API, as well as the medium in which the API is present. Hence it is very difficult to provide a list of APIs and their binding properties to membrane filters. Typically, Millex LCR (Hydrophilic PTFE) and Millex HV have lowest binding to various classes of analytes, and hence should be the filter of choice if there is concern about API binding. Nylon membrane usually shows strong API binding. So if the protocol calls for use of Nylon membrane following dissolution, it’s a good practice to flush the syringe filter with sample prior to collecting the sample for analysis. This significantly reduces API binding and provides quantitative recovery.

4. What are equivalency certificates? What information do they have and how can we get them?

Most dissolution methods are written in a way allowing for substitution of one syringe filter by another from a competitor. Usually the membrane type and pore size are written in the method, but not the vendor. We can provide equivalency certificates to help you re-validate your method with our most comparable filter.

5. This is a question about polymorphism - we know that various solid forms can influence bioavailability of the API. Does dissolution testing demonstrate that polymorphism is not a concern for the finished product dosage?

Typically, polymorph studies on the drug would happen during formulation development. There, dissolution testing, along with other bioavailability studies (in-vivo, small animals etc.), should help understand the impact of different polymorphs of the drug on the bioavailability of the drug. Once the final formulation is selected, it should only contain a single polymorph with stability that is acceptable and does not change during the shelf-life period of the formulation.

6. Are losses from filtration concentration dependent? Do you get the same results at 1 mg/mL as at 0.01 mg/mL?

Losses from filtration are concentration dependent. When a drug binds to a filter, it usually happens by non-specific / non-covalent interactions and the number of binding sites in a filter are limited, so at high concentration these sites will get saturated much sooner, thereby showing much lower drug loss (% wise) than at a lower drug concentration. The amount bound should still be the same in both cases.

7. Are chromolith columns compatible with mineral acids, in particular 0.1M HCl, when used as dissolution media?

Typical silica based columns are stable between pH 2 - 8, and Chromolith columns are stable between pH 2-7.5 - 0.1M HCl has a pH about 1, but since the amount of sample injected is very small (5 - 10 ul) this should not impact column stability as much. If there is a concern with stability, the sample can be diluted 1:10 to raise the pH.

8. What is the chance of retaining the drug on the filter during filtration if the drug particle is more than 0.45 microns?

Typically the pore size of the filter (as determined by bubble point) provides measure of largest pore in the filter, so a 0.45 um rated filter should be able to quantitatively retain particles larger than 0.45 um.

9. Is there a library of HPLC or LC-MS chromatograms, which can help in the identification of drugs?

Since LC-MS conditions differ between different instruments, which leads to differences in fragmentation patterns, different vendors have different libraries of LC-MS spectra to help determine structure of their compound. There are no general compound libraries similar to GC-MS.

For more information about HPLC, see our HPLC Buying Guide, or see Merck's compendium of chromatology notes.