• Syed Masood Hassan Akbari

Practical 2 Stability study of Nifedipine

INTRODUCTION

Nifedipine is considered as a prototype compound of the dihydropyridine class of calcium channel antagonists. Nifedipine is a selective arterial dilator, and also is used in the treatment of hypertension, angina and cardiovascular disorders. The drawback associated with nifedipine is that it can undergo photo degradation thus escalating in loss of pharmacological activity. This process involves the reduction of the aromatic nitro group to nitroso group or the oxidation of the dihydropyridine ring to a pyridine ring. Nifedipine synthesizers make use of light resistant coating to reduce their photo degradation. It has been found out that due to poor storage conditions the clinical efficacy of nifedipine can drop drastically. In order to fix this a selection of ingredients within the dosage form can be altered or enhanced in order to minimise photosensitivity. Ion-exchange beads are solid and suitably are high molecular weight polyelectrolytes that can easily exchange their mobile ions of equal charge with neighbouring medium reversibly. The ion-exchange bead can form a complex with nifedipine and its utility used to embrace drug in light natures. Instability of the drug product may lead to a reduction in the bioavailability of the drug. It is also highly important that the patients do receive their uniform does of the drug throughout the whole of the shelf life of the product.

This experiment goes through the photodecomposition kinetics of nifedipine at zero-order when the reaction commences. As the reaction proceeds further to 50% the kinetics of the reaction changes to first-order.

Figure 1: Represents nifedipine degradation at the two different wavelengths shown above.

This experiment consisted of 3 main components:

• Determining the stability of nifedipine using HPLC at various time intervals.
• Estimating the half-life for the disappearing drug.
• Examining the relative stability of the drug as powder and in solution.

EXPERIMENTAL

For HPLC

20mg of nifedipine was weighed out and placed in a 100ml volumetric flask using the mobile phase to top up the flask. The resulting solutions colouration was noted and transferred to a beaker and covered with paraffin. A small insertion was made to take up 2ml of the solution into an injection using a 0.2µm filter. The time was noted and the sample was loaded and run immediately and the beaker was covered with paraffin and placed on a ledge where sunlight was present and after every 15 min intervals the sample was injected into the apparatus and it was run. After each run was carried out the peak area for each of the runs were recorded.

For stability in solid form

100mg of the solid sample of nifedipine was weighed out in a weighing boat and spread out evenly and placed on a window ledge with proper Irish sunlight for a duration of 2 hours. The appearance was noted before and after the time lapsed. The solid sample was then homogenised and 20mg was taken and placed in a 100ml volumetric flask and made up to the mark with the mobile phase. Straight away 2ml solution was taken and placed on a 0.2µm filter and was ready for injection. The peak area of the solid sample was recorded in the report.

RESULTS & CONCLUSIONS

Determination of the stability of nifedipine solid and liquid samples with sunlight.

Table 1: Shows the nifedipine run time with 15 minute intervals, the solid form and a UV sample.

The illustrations below show the HPLC peaks at various time intervals with their retention times, peak area and heights.

Figure 2: Depicts nifedipine at T₀.

Figure 3: Depicts nifedipine at T₂₀.

Figure 4: Depicts nifedipine at T₃₅.

Figure 5: Depicts nifedipine at T₅₀.

Figure 6: Depicts nifedipine at T₆₅.

Figure 7: Depicts nifedipine at T₈₀.

Figure 8: Depicts nifedipine at T₉₅.

Figure 9: Represents the peak area plotted against time of nifedipine degradation taken at 15 minute intervals.

After carrying out the experiment it can be said looking at the data that the reaction kinetics are of zero order at least from T₀-T₈₀. At T₉₅ it is seen that the peak gets a bit broad it can show that a new peak is starting to form, it the experiment had continued for at least 45 minutes more then it could have been the start of the first order reaction kinetics. Therefore it can be deduced that according to the data attained the reaction kinetics are still at zero order during data acquisition.

From the graph using the equation: y = a + bx

b = slope = -12046.1 +/- 841.833

The half-life of nifedipine was found using the equation:

As we were dealing with zero order kinetics so, r = k

Using, t_1/2 = In(2)/k

t_1/2 = 0.693/0.988

t_1/2 = 0.701

According to the data gathered it is seen that the solid sample of nifedipine resembles to the first T₀ liquid sample. By this observation a person can say that the solid state sample did not show any signs of degradation after the 2 hour window. The colour of the liquid samples were changing from yellow at T₀ to faint yellow at T₈₀. On the other hand for the solid sample from the light yellow powder after the 2 hour window a layer of dark yellow powder was seen and the powder was mixed around and it was seen the underlying solid particles still retained the same colour of light yellow. According to the UV data at T₀ and Æ›_max of 334 it generated an absorbance of 0.604. Whereas for the T₁₀₀ with a Æ›_max of 279 generated an absorbance of 1.064.

QUESTIONS

1. Comment on the relative photostability of solid and solution nifedipine in daylight.

The solid sample of nifedipine after being expose to 2 hours of Irish sunlight visual observation showed a darker colour change from light yellow to a darker but when the powder was mixed the underlying layer was of the same light colouration as it started with. And after the 2 hours window the HPLC result showed no degradation at all. This could be due to the practical being carried out in doors and the lack of the photons getting to the solid sample.

On the other hand in the liquid sample things were not much better from a yellow solution starting at T₀ – T₈₀ there was no colour change but after performing the next HPLC the colour had gone faint yellow. If the apparatus would have been given 45 minute more the results attained would be of first order kinetics. As it stands it is zero order kinetics.

2. What type of reaction is the nifedipine decomposition?

Reduction

3. What other decomposition reactions do you think nifedipine might undergo?

Oxidation

4. What are the implications of your observations for the proper handling and storage of nifedipine?

Manufacturers of nifedipine products use light resistant coating or packing to minimize their photodegradation. Long term exposure to sunlight or artificial light may also occur if nifedipine formulations are inappropriately stored by patients. Poor storage conditions may potentially decrease clinical efficacy of nifedipine.

5. What validation do you think might be important in the type of analysis you performed today?

“Validation of an analytical method is the process that establishes by laboratory studies, that the performance characteristics of the method meet the requirements for the intended analytical applications.”

The typical analytical characteristics used in method validation vary but the relevant one for this experiment is stability, system suitability and system sensitivity.

“Prepared sample/ standard stability is defined as the ability of the analyte to remain stable in the diluent at the test concentration specified in the analytical method.”

System suitability testing is an integral part of many analytical procedures. The tests are based on the concept that the samples to be analysed constitute an integral system that can be evaluated as such. System suitability is determined by various methods. Any one or a combination of the following may be written into the method as necessary for using the method for routine post validation:

Resolution ensures that closely eluting compounds are resolved from each other and establishes the resolving power of the system. It contains as close as is possible to a complete sample matrix.

Finally capacity factor is a measure of the time the solute spends in the stationary phase relative to the time it spends in the mobile phase.