Dissolution sampling time points

Key operating parameters that may change (or be optimized) throughout a product s development and approval cycle are dissolution sampling time points and dissolution limits or specifications by which the dissolution results should be evaluated. The results generated from the dissolution test need to be evaluated and interpreted based on the intended purpose of the test. If the test is used for batch-to-batch control, the results should be evaluated in regard to the established limits or specification value. If the test is being utilized as a characterization test (i.e., biopharmaceutical evaluations, formulation development studies, etc.) the results are usually evaluated by profile comparisons. 

An f value between 50 and 100 suggests the two dissolution profiles are similar. Also, the 2 average difference at any dissolution sampling time point... 

Therefore, the development and validation of a scientifically sound dissolution method requires the selection of key method parameters that provide accurate, reproducible data that are appropriate for the intended application of the methodology. It is important to note that while more extensive dissolution methodologies may be required for bioequivalency evaluations or biowaivers (i.e., multiple media, more complex dissolution media additives, and multiple sampling time points), it is also essential for the simplified, routine quality control dissolution method to discriminate batch-to-batch differences that might affect the product s in vivo performance. 

Dissolution indicates the rate-limiting step for compound absorption when drugs are administered orally. The solubility of a pharmaceutical compound represents its maximum concentration in an aqueous buffer. Additional compound will not dissolve above this concentration. The solubility value is often heavily dependent upon pH and temperature and is typically measured at physiologically important pH levels and body temperature. The standards for dissolution testing are determined by the United States Pharmacopoeia (USP). Testing typically requires sampling of a solution at 15, 30, 45, and 60 min for immediate-release products. /./Pl.C is ideally suited for use in conjunction with USP apparatus types I or II and can rapidly analyze multiple time points or replicate samples. 

FIGURE I Typical fast HPLC chromatograms for a set of three dissolution samples at three time points. 

Case The required documentation must include the undertaking of the technical report and assessment of the results of the dissolution profile employing Pharmacopeial conditions and removing samples from the medium at appropriate time points until the plateau is reached. The dissolution profile obtained must be similar to the profile of the unaltered formulation. ... 

Linearity. Neat standard solutions are prepared to cover the nominal concentration of the sample. ICH Q2B [1] proposes an acceptable range of 20%. A range of 25 to 125% of nominal concentration is commonly used for the linearity determination. This range allows quantitation at the early dissolution time point. 

Different analytical tests for a given time point may be carried out on different days. What then is considered the analysis date for a sample Again, there is no universal rule, and the important thing is a justifiable SOP, consistency, and adherence to the SOP. The start date for the most critical test, usually impurities and/or assay but sometimes dissolution or release, is a logical point. 

An example for an automated stability test in plasma is described by Linget and du Vignaud (1999). Incubations are performed on a 215 Gilson liquid handler. Incubation was done at substrate concentrations of 50 pM on 96 deep well plates. Each incubation tube contained 375 pL of a 200 pM test compound solution (in 0.1 M Tris buffer with 3% BSA, added to assist dissolution of compounds with poor solubility) and 1125 pL of plasma. Samples are taken after incubation times of 0, 1, 2, 3, 4 and 5 min. At each of these time points an aliquot of the incubation mixture was transferred from the incubation tube into a well in a 96 deep well plate containing an equal volume of acetonitrile for quenching by protein precipitation followed by centrifugation of the plates. Supernatants were analyzed by HPLC for metabolic screening. 

HPLC methods are preferred if excipients would interfere, if nonspecific detection techniques (mainly in UV) would be used, or when multiple APIs (combination product) are present in a drug product. Since dissolution sample set analysis can be very long due to six samples per bath as well as multiple time points for prohle testing, fast run times are preferred to quickly determine the results. If a fast HPLC method for CU is available, then the identical HPLC method can be utilized for dissolution analysis. 

The flow rate of the dissolution medium through the cell must be specified for each product. The USP recommends a flow rate between 4 and 16ml/min with an allowance of 5%. Manual operation and sampling for this type of test can be tedious and the system can be automated to control the pump, heat exchanger and test procedure, and deliver samples to a fraction collector. The system can be programmed to switch between different media at predetermined time points to allow pH changes during the test. 

Assay Impurities/degradation products (low levels, 0.1%) Impurities/degradation products (higher levels, 0.5%) Dissolution (immediate release) Dissolution/drug release (controlled release, multipoint specification) +/— 2% absolute difference between labs, 2% RSD for each sample/analyst +/— 40% relative or 0.1% absolute difference between labs +/— 25% relative difference between labs, 10% RSD for each sample/analyst +/— 5% absolute difference between labs at the Q time point 5-10% absolute difference between labs at each specified time point... 

For modified release dosage forms or transdermal products, a dissolution profile is run during method transfer. Similar to immediate release products, the range of product strengths should be covered. The product specification will include three or more time points and acceptance criteria will need to be incorporated in the protocol for each. The criteria for each time point can be the same as described above (e.g., 5% difference between labs, meet product specifications) although dependent on previous data obtained on release/stability samples the criteria are often widened to 7-10%. It is particularly important for these dosage forms that the uniformity and expected variability in the results are taken into account as well as any special... 

Often, a paper tape printout is used to record the RPM, temperature, and speed at every sample pull time point. Temperature of the vessels is recorded before the test and after the test to ensure the proper temperature is maintained. The dissolution apparatus is set up and maintained based on manufacturers recommendations in a space free of vibration, and calibrated according to USP procedures. 

Observations are very important in dissolution testing. They should be made at the time the drug product is introduced and while sampling at every time point. Observations can vary from the normal from tablets starting to break up with excipients floating around the vessel, to samples completely broken up, to out of the ordinary observations like sample in vessel 5 not breaking up at all. These obser-... 

Since dissolution is a time-dependent process, the contact between the powder and water is important. The kinetics depends on the dissolution rate, which is itself influenced by the type of agitation and the surface of contact between the solute and water (dictated by particle size). Dissolution kinetics is usually inversely correlated with solubility, although there is considerable variation in this relationship. The approach to equilibrium is usually asymptotic, which means that the solute initially enters in solution quite rapidly, and as it approaches equilibrium, the rate of dissolution decreases. To check that the equilibrium has been reached, one must take samples at various time points up to a constant concentration is obtained. 

The Ph. Eur. specifies (for information only and so not compulsory) that about the determination of the dissolution of a capsule or tablet the following points should be recorded type of equipment composition, volume and temperature of the dissolution medium, rotation speed, sampling times, sample volume and sampling method, method of analysis, acceptance criteria. 

In another example from SSCI where the solubility of an API was altered through formation of a co-crystal is the case study on fluoxetine HCl. For cocrystals of fluoxetine HCl, the aqueous solubility (called powder dissolution in the paper) was measured at various time points up to 120 minutes and the solutions were analyzed by Samples were sieved to obtain a particle size range of 53-150 pm. The fluoxetine hydrochloride solubility was 11.4 mg mL. The benzoic add co-crystal solubility was lower at 5.6 mg mL and the fumaric co-crystal solubility was higher at 14.8 mg mL. The succinic acid had a peak solubility of 20.2 mg mL after about one minute, but then decreased to that of fluoxetine HCl based on the dissociation of the co-crystal to crystalline fluoxetine hydrochloride. This system exhibited higher and lower solubilities along with dissociation, making it a very interesting and complex set of co-crystal solubilities. 

The most common manner to study apparent solubility will be to conduct the shake flask method as described above, but instead of waiting for 24 h the sample will be assayed at earlier time points (1, 2, 4 h...) to generate a plot of apparent solubility versus time. This profile should be measured at multiple pH values to see how the apparent solubility will be affected by different pH values. Finally, the use of surfactants will typically be studied. This has some biorelevance, since there are a number of bile salts in the intestine that will have a surfactant effect. Also this will provide valuable technical information for solubilizing the API for sample preparation, dissolution testing, as well as utilizing surfactants in the drug product. 

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