Zero-order degradation

Effect of pH on partitioning Drug stability Zero order degradation First order degradation Stereochemistry of drugs Geometrical isomerism... 

Assuming that the degradation decreases linearly with time (zero-order degradation), it can be represented by the following model, which is analogous to Equation (1) ... 

FIGURE 5 Arrhenius plot for zero-order degradation. 

The first case with abimdant moisture was initially modeled by Leeson and Mattocks by assuming that the solid particle was surrounded by a layer of moisture sufficiently mobile to permit dissolution of the drug substance. If the layer remains saturated with drug, this approach yields an apparent zero-order degradation process given... 

A zero-order degradation rate model can be represented as follows ... 

When the drug has a substantial absorption in the UV and visible regions (maximum absorbance > 2), solutions of the compound will absorb most of the incident irradiation (see Chapter 3). The drug acts as a filter and prevents transmission of photons through the formulation. The rate limiting factor is then the intensity of the incident irradiation, and the photochemical degradation rate will follow zero-order degradation kinetics. Photochemical reactions can take place at the surface of the product the decomposition proceeds at a constant rate independent of the concentration of the reactant (Connors, 1990) ... 

Again, replacing km in Eq. (2.81) with the shelf life of a pharmaceutical, tm (time required for 10% degradation), yields Eq. (2.82). Similarly, Eq. (2.83) is obtained for zero-order degradation kinetics. 

Figure 192. Time-drug content curve with 95% confidence intervals. A zero-order degradation of 2%/year was...

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