Gelatin Capsule Shell Cross-Linking - Pharmaceutical Technology

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Gelatin Capsule Shell Cross-Linking
The authors develop a pratical approach to avoid unwanted interactions between pepsin and SLS in dissolution Tier II tests.

Pharmaceutical Technology
Volume 35, Issue 5, pp. 62-68

Results and discussion

Figure 1: Dissolution profiles for capsules stored at 40 C and 75% relative humidity for 1 and 3 months.
Table I and Figure 1 provide the dissolution results and profiles of capsules stored at 40 C and 75% RH for one and three months, respectively. Testing was performed using the Tier I method. Results should conform to a Q value of 70% at 45 min. Comparing the two sets of data, it is clear that capsules stored for three months had significant variation. Four out of twelve capsules had release of 73.0%, 66.2%, 47.7%, and 53.1%, respectively. The results did not meet Stage I or Stage II criteria.

Table II: Dissolution result comparison of different capsule samples.
The observation of reluctant capsule shell rupture was a good indication that dissolution failure was most likely caused by cross-linked capsule shell rather than drug performance. To further confirm this theory, an investigation in which capsules were switched and subjected to dissolution testing using Tier I was performed. Dissolution data for switched capsules are provided in Table II; data for fresh drug blend in fresh capsule shells are included for comparison. As expected, the capsules with fresh drug blend in aged capsule shells had individual low results and significantly high variation at every time point. The capsules with either old or fresh blend in fresh capsules shells both had satisfactory results. The study results proved that the aged capsule shells, rather than product-quality change, caused the original dissolution failure.

Table III: Dissolution results of coaddition of pepsin and SLS in the medium.
For hard gelatin capsules that do not conform to dissolution specification, USP <711> suggests that the test is repeated with the addition of purified pepsin that results in an activity of 750,000 units or less per 1000 mL to the medium that has a pH of less than 6.8 (5). Therefore, another six capsules from the original three-month 40 C and 75% RH storage were tested using the initial Tier II method with pre-mixed medium containing 900 mL of 0.01 N HCl, 1.0% SLS and 750,000 units/L purified pepsin in each vessel. The medium was freshly prepared. The results are provided in Table III. On visual observation, the capsule disintegrated slowly. Some capsules appeared to be gelling with blend trapped inside during the test until a high paddle speed of 250 rpm at "infinity" mechanically ruptured them. The dissolution was slow; the results did not conform to a Q value of 70% at 45 min and displayed high standard deviations. In this case, the presence of SLS may have deactivated pepsin as reported.

Table IV: Dissolution results of stepwise addition of pepsin and SLS in the medium.
To remove the effect of SLS on capsule shell disintegration, Medium #1 was prepared consisting of 0.01 N HCL with 750,000 units/L pepsin without the addition of SLS. Tier II dissolution was performed with 600 mL of Medium #1/vessel. Two minutes into the run, all six capsules were observed to be fully disintegrated. At 5 min, 300 mL of prewarmed Medium # 2, consisting of 0.01 N HCL with 3% SLS, was transferred into each running vessel without disturbing the dissolution run. The final composition of the resulting total medium was 0.01 N HCL with 1% SLS and 500,000 units/L pepsin. The dissolution results and profiles are provided in Table IV and Figure 2, respectively. Satisfactory results were obtained, with tight standard deviations.

Figure 2: Dissolution profile comparison of coaddition and stepwise addition of pepsin and sodium lauryl sulfate (SLS).
The results indicated that stepwise addition of pepsin and SLS enabled both agents to take effect individually and sequentially in the dissolution medium. Pepsin digested the cross-linked capsule shells at the beginning, whereas the addition of SLS afterwards increased drug solubility and wettability. Therefore, the addition of SLS to the dissolution medium need not be discouraged when developing dissolution methods for capsule formulations. SLS is commonly included as a wetting agent inside the capsule formulation; this practice should not be affected by the results of this study, because SLS deactivation of pepsin was observed outside of the capsule in the dissolution medium before dissolution took place. By taking a stepwise addition approach, once the cross-linked capsule shell ruptures and dissolution starts, SLS inside the formulation will work as expected.

The 5-min time delay between the addition of pepsin and SLS was further confirmed to be sufficient using more severely stressed capsules. The Tier II method was fully validated for linearity, specificity, accuracy, repeatability, intermediate precision, and stability of standard and sample solutions.


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