QUESTION 1
What is the rate (in micrograms per day) that a tablet will lose active ingredient, as it degrades?
- Since the decay is zero order, the amount lost between measurements will follow a linear decay (see Table 2)
- Only one line of the table needs to be analysed to work out the rate of decay, but multiple lines allow greater confidence that the answer is correct.
- 0.9 micrograms of X are lost per day
Table 2. Rate that a tablet will lose active ingredient as it degrades
| Time after manufacture (days) | Amount of drug lost (micrograms) | Amount lost per day (mcg/day) |
|---|---|---|
| 0 | (10 000 – 0) = 0 | N/A |
| 30 | (10 000 – 9973) = 27 | 27/30 = 0.9 |
| 90 | (10 000 – 9919) = 81 | 81/90 = 0.9 |
| 200 | (10 000 – 9820) = 180 | 180/200 = 0.9 |
QUESTION 2
Using the data in Table 1 (page 432), calculate the shelf life of these drug X tablets
- T90 for this product will occur when the tablet contains 90% of its original amount of X. 10 mg x 0.9 = 9 mg. Therefore, T90 will occur when (10 mg – 9 mg) = 1 mg of X has degraded. Since the rate of decay is 0.9 micrograms per day the time it will take for only 9 mg to remain per tablet is 1000 micrograms/0.9 micrograms per day = 1111 days, therefore the shelf life for these tablets is approximately 3 years.
QUESTION 3
If the drug X tablets have a Q10 value of 3 and they are stored in a hot room 10 degrees above the manufacturers rated storage temperature, how long will they last before they reach T90 and are unsuitable for use?
- Q10 of 3 means that the decomposition rate will triple if stored at 10 degrees above the intended storage temperature. If the original decomposition rate was 0.9 micrograms per day, then the new decomposition rate will be 3 x 0.9 = 2.7 micrograms per day.
- To reach T90 we still need to lose 1mg of active ingredient but now the time to reach this concentration will be 1000 micrograms / 2.7 micrograms per day = 370 days = approximately one year, which, as expected, is around one third of the original shelf life of 1111 days (3 years).