Fast-release portion

FIGURE 40.15 Release profiles of nifedipine from double-layer tablets (25 mg as 5 mg nifedipine) consisting of nifedipine-HP-(3-CyD with 3% HCO-60 as fast-release portion (FRP) and nifedipine-HPC-M as slow-release portion (SRP) in various weight ratios (FRP SRP) in water at 37°C. O, 1 3 , 1 2 A, 1 1 A, 2 1 , 3 1. 

Clearly, if in equation 4.78 k is very fast and k2 is negligibly slow, the release of P is easily measured and related to the concentration of enzyme. However, in practice, k2 is generally not negligible, so that there is an initial burst of formation of P, followed by a progressive increase as the intermediate turns over. The mathematics of this situation was described previously (equations 4.37 to 4.46). It was shown that the overall release of products is linear with time after an initial transient. From equation 4.46 it can be seen that the linear portion extrapolates back to a burst, ir, given by... 

In Step 1, the hydrated metal ions lose one H2O molecule and form an intermediate complex with a surface site. The fast relaxation associated with Step 1 was ascribed to simultaneous adsorption/desorption of the metal ions on a major portion of the 7-AI2O3 surface sites. In the second step a metal ion-surface complex is formed that results in the release of a proton. This slow relaxation was attributed to the adsorption/desorption of metal ions on the remaining, multiple type sites of the 7-AI2O3 surface that comprise a small fraction of the total surface sites. Yasunaga and Ikeda (1986) characterized the first type of surface sites as strong sites and the multiple type sites as weak sites. Linearized rate equations relating reciprocal relaxation times to the intrinsic rate constants were developed and validated for the two-step reaction mechani.sm. A plot of the linearized equation for Step 2 (the faster... 

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