One dated study evaluated the necessity of strict packaging restrictions for stabilized nitroglycerine tablets. Baseline tests assessed weight variation, content uniformity, average potency, and friability for two molded and two compressed sublingual tablet brands. Stability tests included simulated patient use over a month, exposure to air for 100 days, foil-based strip packaging, placement in medication cups for seven days, and repackaging in common prescription containers. All tablets maintained potency in their original containers during normal use. Under stress conditions, one brand demonstrated superior stability, retaining potency during prolonged exposure, repackaging, and alternative storage methods. Unfortunately, as the full article was not available, only the abstract was utilized for analysis. [1]
A 2018 open-label study assessed the stability of sublingual nitroglycerin (SL NTG) tablets in simulated real-world conditions. The authors explained that the common recommendation of refilling SL NTG every 3-6 months was based on particularly old data. Improved modern tablet formulations and manufacturing processes may improve its stability and potency. Commercially available SL NTG 0.4 mg tablets in 25-count and 100-count bottles were made to endure five different “real-life” storage conditions: (1) 25 ̊C/60% relative humidity (RH), (2) 40 ̊C/75% RH, (3) ambient (laboratory shelf), (4) “pocket”, and (5) “purse” conditions. One-hundred-count bottles placed in the pocket and purse were opened on average once per week while 25-count bottles were opened once per month for several seconds in an effort to replicate real-life use. Bottles stored at ambient, 25 ̊C/60%, and 40 ̊C/75% conditions were only opened on testing days. At 0, 1, 2, 3, 6, 12, 18, 24, 30 months, and at the expiration date stated on the bottles, three bottles from each storage condition were assayed by high-performance liquid chromatography to measure drug content with 90% potency being set as the lowest acceptable level. Results indicated that the potency of SL NTG is highly dependent on environmental conditions (e.g., storage temperature, and humidity) as well as physical conditions (e.g., movement and collision of tablets). [2, 3]
Tablets in either of the package sizes were found to maintain potency for more than two years when carried in a purse. In a pant pocket, the potency of 25-count bottles was maintained for two years, but the potency of 100-count bottles lasted only for 12 months. The increased body heat and physical collisions of the tablets with each other were suspected to cause a decrease in potency. After a period of 12 months, it was noted that the physical stability of the tablets became more fragile with increased loss of their sharp, flat edges and more loose powder deposited in the bottle. Tablets stored at 25 ̊C/60% RH were found to be the most stable, with drug potency meeting standards through the expiration date regardless of the package size. For tablets stored at 40 ̊C/75% RH, potency declined much more rapidly, with potency decreasing below 90% within two to six months. [3]
A commentary and follow-up study of the Nawarskas et al. study states that early published studies suggest SL NTG tablets rapidly lose potency over time (especially at higher temperatures); thus, it is recommended to replace the bottles every 6 months once opening. However, this recommendation does not account for various storage conditions. In addition to the results found by Nawarskas et al., a small study (N= 16) added that SL NTG degraded more rapidly when carried in pants pockets compared to in a purse or backpack. Participants were given devices programmed to log temperatures (at 5-minute intervals over one hour) and instructed to keep the devices in their pants pocket and carrying bag/purse; each participant was tested for three different days to eliminate fluctuations or bias. Temperatures in the pants pocket were around 9˚C higher than temperatures in both the purse/carry bag and ambient conditions. Moreover, compared to ambient conditions, a small increase in vapor pressure was discovered in the SL NTG bottles that were stored in bags or purses, and the pressure was doubled when stored in a pant pocket, which may be indicative of greater volatility due to the exposure to higher vapor pressure. Consistent with the findings from Nawarskas et al. study, this small-scale study also suggested that SL NTG should be carried by patients in a purse or carrying bag rather than in a pants pocket. If this is not possible, then SL NTG should be stored in a manner that minimizes close body contact, such as placing in a jacket pocket or a loose shirt pocket. [4]