drug factors affecting or influencing the rate of dissolution and salt formation, solubility in the diffusion layer, pH partition hypothesis. partition coefficient and lipid solubility. Henderson-Hasselbalch Equation. Limitations of the pH-partition hypothesis. Molecular size & hydrogen bonding.
Week 3 notes: Bioavailability: Drug and Dosage form factors - part 1
Learning outcomes:
1. Understand the concepts of:
- Dissolution
- Solubility
- Sink conditions
2. Discuss the parameters of the Noyes-Whitney Equation in terms of
physiological and drug factors affecting dissolution rate.
3. Discuss drug factors affecting absorption in terms of drug form:
- - Salts
- - Crystal form
4. Discuss factors affecting drug concentration in the GI fluids.
5. • Discuss drug absorption in terms of drug dissociation & lipid solubility:
- - Henderson-Hasselbalch Equation, calculations & limitations
- - Partition coefficient indication.
Drug factors incl:
● Solubility
● Dissolution rate
● Affected by pH and environment
● Surface area
● Lipid solubility
● Drug dissociation
Nevertheless, the physicochemical properties of the drug that will influence its
passage into solution and transfer across membranes include its dissolution rate, pKa,
lipid solubility, chemical stability and complexation potential.
Drug Dissolution:
● Solid drugs must dissolve before absorption.
● The Noyes–Whitney equation (proposed in 1897) describes the rate of diffusion
of solute through boundary layers around a dissolving spherical particle.
1
, ● When dissolution is diffusion-controlled and involves no chemical reaction:
𝐷𝐴( 𝐶𝑆 − 𝐶)
………………………………………………….1.
𝑑𝑚
𝑑𝑡
= ℎ
● Variables:
: Rate of dissolution of the drug particles
𝑑𝑚
○ 𝑑𝑡
○ D: Diffusion coefficient of the drug in gastrointestinal fluids
○ A: Effective surface area of the drug particles in contact with fluids
○ h: Thickness of the diffusion layer around each drug particle
○ Cs: Saturation solubility of the drug in the diffusion layer
○ C: Concentration of the drug in gastrointestinal fluids.
- Despite limitations, the equation illustrates how physicochemical and
physiological factors influence the dissolution rate in the gastrointestinal tract.
- Key influencing factors are summarised in Table 20.1
Physiological factors affecting dissolution rate of a drug
2
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