The Pharmaceutics and Compounding Laboratory
Capsules

Measuring Drug Release

This sample experiment was conducted in water and showed that after 2.5 hours, release was still continuing from the controlled release capsule.

Conventional Capsule   Controlled Release Capsule
Clock Time Run Time Absorb. Conc. Cum. Amt. Released   Clock Time Run Time Absorb. Conc. Cum. Amt. Released
2:40 0 0.000 0.0 0.0   2:35 0 0.000 0.0 0.0
2:55 15 0.005 0.0010 0.10   2:50 15 0.012 0.001 0.15
3:10 30 0.016 0.0017 0.17   3:05 30 0.031 0.003 0.27
3:25 45 0.031 0.0027 0.27   3:20 45 0.059 0.004 0.44
3:40 60 0.048 0.0037 0.37   3:35 60 0.089 0.006 0.63
3:55 75 0.065 0.0048 0.48   3:50 75 0.123 0.008 0.84
4:10 90 0.592 0.037 3.75   4:05 90 0.160 0.011 1.07
4:25 105 0.609 0.039 3.85   4:20 105 0.194 0.013 1.28
4:40 120 0.610 0.039 3.86   4:35 120 0.240 0.016 1.56
4:55 135 0.614 0.039 3.88   4:50 135 0.287 0.019 1.85
5:10 150 0.612 0.039 3.87   5:05 150 0.302 0.019 1.95

 

The same experiment was repeated in 0.01N HCl.

Conventional Capsule   Controlled Release Capsule
Clock Time Run Time Absorb. Conc. Cum. Amt. Released   Clock Time Run Time Absorb. Conc. Cum. Amt. Released
2:20 0 0.000 0.0 0.0   2:15 0 0.000 0.0 0.0
2:35 15 0.301 0.014 1.38   2:30 15 0.030 0.00044 0.044
2:50 30 0.808 0.039 3.87   2:45 30 0.176 0.0076 0.76
3:05 45 0.815 0.039 3.90   3:00 45 0.368 0.017 1.71
3:20 60 0.822 0.039 3.94   3:15 60 0.516 0.024 2.48
3:35 75 0.800 0.038 3.83   3:30 75 0.637 0.030 3.03
3:50 90 0.800 0.038 3.83   3:45 90 0.740 0.035 3.54
4:05 105 0.804 0.039 3.85   4:00 105 0.814 0.039 3.90

 

The "rate of release" is determined by plotting the Cumulative Amount Released versus some function of time. For matrix diffusion controlled release, adaptations of the Higuchi equation are used; time is expressed as the square root of time and has units of minutes1/2. A linear trendline is fit through the points that occur after a lag time or before any asymptotic values are reached. The release of salicylic acid in 0.01N HCl is shown in the plot below. The conventional capsule apparently released all of its contents by 30 minutes, since after that time the amount released remained constant. The release rate was 1.55 mg/minutes1/2 which is almost three times as fast as seen with the controlled release capsule. But more importantly, the controlled release capsule continuously releases drug for hours, where the conventional capsule released all the drug within 30 minutes.