Page 109 - Interactive Theoritical Notes of Bioharmaceutics and pharamcokinetics.docx compressed

PharmD clinical pharmacy program Level 3, Semester 2 Biopharmaceutics & Pharmacokinetics (PT608(

o The faster-dissolving anhydrous form of ampicillin was absorbed to a
greater extent than was the slower-dissolving trihydrate form.
Example 2: Mercaptopurine
o The anhydrous form dissolves more rapidly than the hydrous one. On the
other hand, have high solubility and
dissolution rate compared to the anhydrous form of the drug

o The amorphous form usually dissolves more rapidly than the corresponding

crystalline form(s).

o The rapidly dissolving amorphous

form was readily absorbed following oral administration of an aqueous

suspension. However, the slower-dissolving crystalline form was not absorbed.

o A mixture of amorphous and crystalline forms provides

an initial fast absorption followed by sustained effect. The amorphous form is

rapidly dissolved while the crystalline form is slowly dissolved and absorbed.

(c) Solubility in the diffusion layer, C

➢ The dissolution rate of a drug under sink conditions, according to the Noyes-

Whitney equation, is directly proportional to its intrinsic solubility in the

diffusion layer surrounding each dissolving drug particle, Cs.

➢ In the case of drugs that are weak electrolytes, their aqueous solubilities are

dependent on pH:

❖ The solubility of weakly acidic drugs increases with pH, and so as a drug moves

down the GIT from the stomach to the intestine, its solubility will increase

❖ Conversely, the solubility of weak bases decreases with increasing pH, i.e. as the

drug moves down the GIT

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