Processes of drug absorption Flashcards
how is an aqueous solution absorbed after administration?
drugs must be in solution to be absorbed in the body - this usually requres the process of dissolution. however an aqueous solution already contains the drug in solution so it is the fastest route of administration to get a solution of drug in the GI fluids ready for absorption.
what can affect the absorption of an oral aqueous solution?
you may get some precipitation leading to a suspension of fine particles in GI fluids that must then dissolve before absorption can occur.
how is an aquoes suspension absorbed?
when you take an oral aquoesu suspension you get a suspension of fine particles in the GI fluids - this must then dissolve to form a solution of drug in GI fluids to then be absorbed.
how is an immediate release dosage form absorbed?
the immediate release dosage form will form aggregate granules, then a suspension of fine particles in the GI fluids then a solution of drug in GI fluidswhich can then be absorbed.
what is a solution?
a molecular dispersion formed by 2 or more components which form a 1 phase homogenous system - this can be applied to solid liquid and gaseous state.
whats a solvent?
a solvent is the component that determines the phase of the solution - it usually constitutes the largest proportion of the system.
what are solutes?
other components of the solution that are dispersed as molecules or ions throughout the solvent.
whats a saturated solution?
a solution is saturated when the solute is in equilibrium with the solid phase.
what is solubility?
the maximum mass/volume of solute that will dissolve in a given mass or volume of a solvent at a particular temperature. it is the concentration of solute in a saturated solution at a certain temperature.
whats a supersaturated solution?
a supersaturated solution contains more of the dissolved solute than it would normally contain at a definite temperature. for example salts like sodium thiosulphate dissolve in large amounts at an elevated temperature, upon cooling they fail to crystallize from the solution hence they will be at a higher concentration than usual.
whats molarity?
the number of moles of solute in 1L of solvent
whats molality?
the number of moles of solvent in 1kg solvent.
name some other liquid systems that are not solutions
disperse systems with two phases (emulsions and suspensions)
colloidal dispersions (2 phases)
solutions of macromolecular material like polymer material
association colloids like micelles
microemulsions: 1 phase that is isotropic hence its both homogenous and heterogeneous.
what is dissolution?
the process by which the drug particles dissolve - it must dissolve in the fluid at the site of absorption in order for absorption to occur.
describe the dissolution process
the drug molecules in the surface layer dissolve to form a saturated solution around the particle called the diffusion layer. dissolved drug molecules then pass throughout the dissolving fluid to contact absorbing mucosaand are absorbed. there is continued replenishement of the diffusing layer by further drug dissolution so absorption continues until all the drug is absorbed/dissolved.
The rate of dissolution is determined by the slowest steps, what are the steps involved and which one is rate limiting?
step 1 - release of solute molecules from the solid phase at the interface.
step 2 - solution in contact with the solid becomes saturated (Cs)
step 3 - solute molecules migrate through the diffusion layer - from the solid-liquid interface to the bulk of the solution (C)
the interfacial steps 1 and 2 are virtually instantaneous - the rate of dissolution is therefore determined by the diffusion of the solute through the static boundary layer - stp 3 .
diffusion follows ficks law - what is this?
J = (DxKxAx^C)/h
what can you see from ficks law?
the rate of mass transfer through a diffusion layer is directly proportional to the area for migration and the concentration difference whilst its inversely proportional to the thickness of the diffusion layer.
whats the noyes whitney equation for drug dissolution?
dm/dt = k A (Cs-C)
dm/dt is the mass of solute passed int solution per unit of time
how can dissolution rate be increased?
increasing the surface area of the drug (by reducing particle size), increasing the drug solubility in the diffusing layer and increasing K.
whats the dissolution rate constant?
k = D/Vh
how can dissolution rate be raised?
by increasing k - this would involve drug diffusion coefficient and the diffusion layer thickness.
drug bioavailability relies on both absorption and dissolution. how may these affect the bioavailability?
if a drug has high dissolution rates or is delivered already in solution, the rate of absorption relies on its ability to transverse the absorbing membrane.
Where as if drug dissolution is slow, dissolution may be the rate limiting step of absorption.
what factors affect the dissolution rate?
- surface area of the undissolved solid which is influenced by size of particles, porosity, dispersibility and surfactants.
- solubility is dissolution medium affected by the nature of the solute and solvent and pH
- concentration in the bulk solutionaffceted by the volume of dissolution, absorption and GI fluid available
- diffusion coefficient of the solute in the solvent - affected by the viscosity of the solvent and presence of food.
- thickness of boundary layer - affected by stirring and GI motility.