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Class III – low permeability, high solubility . Example: cimetidine; The absorption is limited by the permeation rate but the drug is solvated very fast. If the formulation does not change the permeability or gastro-intestinal duration time, then class I criteria can be applied. Class IV – low permeability, low solubility . Example: bifonazole
Esterification is also used to control solubility. For example, stearate and estolate esters of drugs have decreased solubility in gastric fluid. Later, esterases in the gastrointestinal tract (GIT) wall and blood hydrolyze these esters to release the parent drug. Coatings on a tablet or pellet may act as barriers to reducing the dissolution rate.
The drug has very low solubility and sometimes can crystallize in the kidneys, due to its first pK a of around 10. [clarification needed] This is a very painful experience, so patients are told to take the medication with copious amounts of water.
The solubility of a specific solute in a specific solvent is generally expressed as the concentration of a saturated solution of the two. [1] Any of the several ways of expressing concentration of solutions can be used, such as the mass, volume, or amount in moles of the solute for a specific mass, volume, or mole amount of the solvent or of the solution.
An example for a simple case (mono-compartmental) would be to administer D=8 mg/kg to a human. A human has a blood volume of around V b l o o d = {\displaystyle V_{blood}=} 0.08 L/kg . [ 7 ] This gives a C 0 = {\displaystyle C_{0}=} 100 μg/mL if the drug stays in the blood stream only, and thus its volume of distribution is the same as V b l o ...
Where X m is the mole fraction solubility of the solute, X 1 and X 2 denote the mole fraction solubility in neat cosolvent and water. While this model is only correlative in nature, further analysis allows for the creation of a predictive element. Simplifying the above equation to: logX m = logX 2 + σ • ƒ 1
Conversely, hydrophilic drugs (low octanol/water partition coefficients) are found primarily in aqueous regions such as blood serum. [ 4 ] If one of the solvents is a gas and the other a liquid, a gas/liquid partition coefficient can be determined.
Solubility in water can be estimated from the number of hydrogen bond donors vs. alkyl sidechains in the molecule. Low water solubility translates to slow absorption and action. Too many hydrogen bond donors, on the other hand, lead to low fat solubility, so that the drug cannot penetrate the cell membrane to reach the inside of the cell.