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Week 4: Drug Dissolution Lecture
Terms in this set (19)
Drug in the body, including in the gastrointestinal tract (GIT), is considered to be in
an aqueous environment.
Body has 55-75 percent water. If you evaporaate a lot of water that could reach to almost
50 percent loss of weight
The oral solid drug product containing the drug generally disintegrates into small particles and then releases drug in the solution so that it can the be absorbed. Talk about slow-release/extended release.
Some exceptions: some tablets like slow-release tablets which serve as an way of extending the release of the drug. You want it to slowly release the drug so it will slowly disintegrate prior to its release of the drug.
Drug needs to be released from
dosage forms into the aqueous environment before it can be adsorbed.
Dissolution Process: 2 steps.
(This probably what happens immediately after the drug gets released from the dosage form - btw don't overthink it too much because professor said that drug release is synonymous with drug dissolution (which makes sense because one will invenitably lead to the other (even if the molecule is not super polar)
1. Drug dissolves at the surface of particles to form a layer of saturated solution (AKA, stagnant layer)
2. Drug diffuse from the stagnant layer to the bulk of the solvent (i.e. the release medium in vitro, and GI fluid in vivo)
dC/dt = (define and state formula)
= rate of drug dissolution
•D = diffusion coefficient (cm2/s)
•A = surface area of the particles
•CS = concentration of drug in the stagnant layer
•C = concentration of drug in the bulk solvent (or release medium)
•h = thickness of the stagnant layer.
In summary we can modify actually increase various factors that increase drug solubility: surface area, temperature, and speed of stirring: to increase solubility. These are the important factors.
If you increase the concnetration of the drug in the stagnant layer (Cs), this will increase/decrease solubility
You will increase the differentce between this and C (Cs-C), proprotional to drug dissolution rate; this will increase solubility.
bigger surface area (effect on solubility)
bigger amount of surface particles getting exposed to solvation hence better dissolution.
The smaller the particle (effect on dissolution)
the bigger the surface area hence better dissolution.
Increasing temperature will decrease/increase solubility. Why or how?
It will increase DIFFUSION coefficient: (in other words it will increase the kinetic energy of the molecules and make go more readily able to DIFFUSE from the stagnant layer to the solvent layer)
Define Diffusion Coefficient
it is a constnat for the molecule in the solvent (relates to how fast the molecules of that particular drug will move away and be released); will increase with temperature
Effect of increasing stirring on solubility?
Affects the staganant layer, when you increase the speed of the stirring, the thickness of the staganant layer will decrease and better release to the bulk solvent (because the drug particle will travel a smaller distance): better solubility.
How to calculate the y values of a slope if the line is straight/ curvy
Straight line you can pick any two points because it is a linear relationship for a curvy line picking any two points will give you different answers therefore choose points that are on a straight line.
Remember: dissolution of the drug (whether it is polar or non-polar) is an important step before it gets absorbed into the blood. The IVIVC is the correlation between the in-vitro drug dissolution and its in-vivo concentration. It will be a positive relationship or slope will be positive depending on the class of the drug.
Properties of Class 3 drugs in terms of permeability, solubility, and the IVIVC correlation
Has high solubility and low permeability, drug absorption is the rate limiting step and will not necessarily correlate with the rate of drug dissolution because permeability is a major factor hence no ivivc.
Properties of Class 2 drug
Drug that is low ins olublity but high in permeability, makes sense that a correlation is present because permeation across the membrane will occur readily as long as we've solubilized the drug. But as soon as we've for instance increased the rate of solubilization of the drug, then the rate of absorption will increase too so there is a correlation.
IVIVC is mantained but NOT at high concentration of the drug if the drug needs a transporter to be absorbed (because the transporters are saturable and hence the in-vivo plasma concnetration of the drug will plateau
Properties of Class 1 drug
So this class has high solubility and high permeability. since solubility is important for permeability, then there will be a correlation and as we increase the rate of drug molecules solublilized we will increase the rate of drug molecules absorbed.
Properties of class 4 drug
Low solubility and low permeability, low amount will be solubilized but also low permeability which is a big factor that will be independent of dissolution and hence no correlation.
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