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What does the tortuosity of the matrix indicate?
What does the tortuosity of the matrix indicate?
In the context of drug release, the simplified Higuchi model primarily suggests that...
In the context of drug release, the simplified Higuchi model primarily suggests that...
Which parameter in the Hixson-Crowell model is related to the surface-volume relation?
Which parameter in the Hixson-Crowell model is related to the surface-volume relation?
What type of relationship is used to describe the data obtained in drug dissolution studies according to the Higuchi model?
What type of relationship is used to describe the data obtained in drug dissolution studies according to the Higuchi model?
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What condition must be true for drug release to be described by the Higuchi equation?
What condition must be true for drug release to be described by the Higuchi equation?
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For what type of pharmaceutical dosage forms is the Hixson-Crowell model most applicable?
For what type of pharmaceutical dosage forms is the Hixson-Crowell model most applicable?
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What does the variable Q represent in the context of the Higuchi model?
What does the variable Q represent in the context of the Higuchi model?
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Which aspect of drug release does porosity of the matrix influence?
Which aspect of drug release does porosity of the matrix influence?
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What characterizes a zero-order release model in drug dissolution?
What characterizes a zero-order release model in drug dissolution?
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Which equation best describes the first-order kinetics for drug release?
Which equation best describes the first-order kinetics for drug release?
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Which of the following describes the Higuchi model of drug release?
Which of the following describes the Higuchi model of drug release?
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Controlled release systems primarily aim to achieve which of the following?
Controlled release systems primarily aim to achieve which of the following?
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In the context of cumulative drug release, what does plotting cumulative percentage of drug released against time typically demonstrate?
In the context of cumulative drug release, what does plotting cumulative percentage of drug released against time typically demonstrate?
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Which of the following dosage form modifications is NOT typically associated with controlled release systems?
Which of the following dosage form modifications is NOT typically associated with controlled release systems?
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What is the main difference between zero-order and first-order drug release kinetics?
What is the main difference between zero-order and first-order drug release kinetics?
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What is typically the role of the release constant in controlled drug delivery systems?
What is typically the role of the release constant in controlled drug delivery systems?
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What is represented by the slope of the straight line obtained from plotting log cumulative percentage of drug remaining against time?
What is represented by the slope of the straight line obtained from plotting log cumulative percentage of drug remaining against time?
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Which assumption is NOT a part of the Higuchi model for drug release?
Which assumption is NOT a part of the Higuchi model for drug release?
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In the context of controlled release systems, which factor does the Higuchi model specifically consider as negligible?
In the context of controlled release systems, which factor does the Higuchi model specifically consider as negligible?
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What does Q represent in the Higuchi model equation?
What does Q represent in the Higuchi model equation?
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Which of the following is the correct formula to plot the drug release kinetics in first-order kinetics?
Which of the following is the correct formula to plot the drug release kinetics in first-order kinetics?
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In the Higuchi model, which condition must be achieved for the model's relation to hold valid?
In the Higuchi model, which condition must be achieved for the model's relation to hold valid?
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What does 'Cs' represent in the Higuchi model equation?
What does 'Cs' represent in the Higuchi model equation?
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Which statement is true regarding pharmaceutical dosage forms when applying the Higuchi model?
Which statement is true regarding pharmaceutical dosage forms when applying the Higuchi model?
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Study Notes
Release Kinetic Modelling
- There are various oral controlled drug release formulations with varying physical properties influencing drug release.
- Release patterns are categorized into slow zero or first-order rates and those that provide initial rapid dose followed by slow zero or first-order release of the sustained component.
- There are several kinetic models to describe overall drug release from dosage forms.
Zero-Order Model
- Drug dissolution from dosage forms that do not disaggregate and release drug slowly can be represented by: Qt = K0t + Q0.
- Where, Qt is the amount of drug dissolved in time t, Q0 is the initial amount of drug in the solution (most times, Q0 = 0), and K0 is the zero-order release constant expressed in units of concentration/time.
- A graph plotted between time on the x-axis and cumulative percentage of drug release on the y-axis will give a straight line.
- This relationship describes the drug dissolution of several types of modified release pharmaceutical dosage forms, such as controlled release transdermal systems, matrix tablets with low soluble drugs in coated forms, and osmotic systems.
First-Order Model
- This model describes absorption and/or elimination of some drugs, although it is difficult to conceptualize on a theoretical basis.
- Drug release following first order kinetics can be expressed by: Ct = C0e-kt, where K is the first order rate constant expressed in units of time-1, C0 is the initial concentration of drug, k is the first order rate constant, and t is the time.
- A graph plotted between time on the x-axis and log cumulative percentage of drug remaining to be released on the y-axis will give a straight line.
- This relationship describes the drug dissolution in conventional pharmaceutical dosage forms like tablets, capsules.
Higuchi Model
- The first mathematical model to describe drug release from a matrix system was proposed by Higuchi in 1961.
- Initially conceived for planar systems, it was then extended to various geometries and porous systems.
- Based on the hypotheses that (i) initial drug concentration in the matrix is much higher than drug solubility, (ii) drug diffusion takes place in one dimension, (iii) drug particles are smaller than system thickness, (iv) matrix swelling and dissolution are negligible, (v) drug diffusivity is constant, and (vi) perfect sink conditions are always attained in the release environment.
- The model expression is given by: Q/A = √(DCst). Where Q is the amount of drug released in time t per unit area A, C is the drug initial concentration, Cs is the drug solubility in the matrix media, and D is the diffusivity of the drug molecules in the matrix substance.
- To study dissolution from a planar heterogeneous matrix system, where the drug concentration is lower than its solubility and release occurs through pores, the expression is given by: Q/A = √(DδτCst).
- Where D is the diffusion coefficient, δ is the porosity, τ is the tortuosity, and Q, A, Cs, and t have the meaning assigned above.
- Tortuisity is defined as the dimensions of radius and branching of the pores and canals in the matrix.
- The general simplified Higuchi model is: Q = KH√(t) where Q is cumulative amount of drug release at time t, KH is Higuchi constant, and t is time.
- The Higuchi Equation suggests that drug release occurs by diffusion.
- A graph plotted between cumulative percentage drug release and the square root of time will be a straight line.
- This relationship describes the drug dissolution from several types of modified release pharmaceutical dosage forms, such as transdermal systems and matrix tablets with water soluble drugs.
Hixson-Crowell Model
- Hixson and Crowell (1931) recognized that the particles’ regular area is proportional to the cube root of its volume.
- They derived the equation: Wt1/3 = W01/3 - κt.
- Where W0 is the initial amount of drug, Wt is the remaining amount at time t, and κ (kappa) is a constant incorporating the surface-volume relation.
- The equation describes drug release from systems where there is a change in surface area and diameter of particles or tablets.
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