Issue link: https://www.e-digitaleditions.com/i/1067339
20 January 2019 Tablets & Capsules Figure 2 Proposed API degradation pathway Degradant 1 Degradant 2 H N H O H CO 2 N N N O O Photo 1: Capsules containing 60 milligrams of API showed signs of crosslinking after one-month storage at accelerated conditions (40°C/75 percent RH). dant 1, degradant 2, carbon dioxide, and formaldehyde. The proposed degradation pathway, as shown in Figure 2, would explain the crosslinking of the capsule shells due to the formation of an aldehyde, which has been shown to cause crosslinking in gelatin capsules [4]. To demonstrate that the proposed degradation path- way is accurate, the scientists needed to observe the sus- pected degradation products in stressed capsules. Figure 3 shows the amount of degradants 1 and 2 pres- ent in capsules stored at accelerated conditions for three months. The data was compiled from assay and impuri- ties testing, with the structures of degradants 1 and 2 being confirmed by comparison to marker preparations for each degradant. Figure 3 also shows the correlation between the degra- dant levels observed and the dissolution rate for the stressed capsules. As these impurity peaks increased, the percent dissolved in dissolution analysis decreased. From the impurity data, the scientists further suspected the presence of formaldehyde. To confirm the presence of formaldehyde after stressing, the scientists added approximately 10 milli- grams of API to a GC headspace vial and crimped immediately. The vial was placed in an oven at 40°C for one month. The stressed sample and an unstressed control were prepared via the following derivatization procedure inside the headspace vials at a sample con- centration of 0.5 mg/mL. Derivatization procedure: Scientists added 1.0 milliliter of acetonitrile to the vial then prepared a 2,4-dinitro- phenylhydrazine stock solution by adding 124 milli- grams to a 100-milliliter volumetric flask, adding 50 milliliters of acetonitrile to dissolve, and diluting to volume with acetonitrile. They then added 1.0 millili- ter of the 2,4-dinitrophenylhydrazine stock solution and 0.2N hydrochloric acid to the GC headspace vial, Investigation The scientists designed an experiment to determine if interactions between the API and excipients were causing the suspected crosslinking and reductions in dissolution release rate. The experiment consisted of filling one sample of gel- atin capsules with 60 milligrams of API and another sam- ple with 60 milligrams of a placebo blend. Once filled, these capsules were stored at accelerated conditions (40°C/75 percent RH) for one month. Empty capsule shells were also stored as a control. The scientists then analyzed the stressed capsules per the dissolution proce- dure previously described. After 45 minutes, both the empty and placebo-filled capsule shells were fully dissolved. The capsules filled with API were not dissolved at 30 minutes and exhibited signs of crosslinking, as shown in Photo1. The experi- ment showed that an API-gelatin interaction was the root cause for the crosslinking. Literature review The scientists initiated a literature review to further understand their observations. They inspected the chemical structure of the API used in the formulation and found a carbamate functional group to be present. The carbamate is comprised of a N-[(Acyloxy)]methyl group, which has the general formula RR'N-CHR''-O- COR''' where R'' = H or methyl and COR''' is the acyl group. This carbamate group is subject to degradation across the ester bridge followed by the chemical breakdown of the N-(hydroxymethyl) intermediate. The literature review also revealed that formaldehyde is liberated when R'' = H [3]. Inspection of this API chemical structure shows that R'' is a hydrogen atom. The scientists suspected that the API was generating the following hydrolytic degradation products: degra-