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

b-Complexation with dietary components and products
▪ Tetracycline forms non-absorbable complexes with calcium and iron.

c-Complexation with excipients

The presence of calcium (e.g. from the diluent dicalcium phosphate) in the dosage

form of tetracycline reduces its bioavailability via the formation of a poorly soluble

complex.

Other examples of complexes that reduce drug bioavailability:

Amphetamine and sodium carboxymethylcellulose

Phenobarbitone and polyethylene glycol 4000

2-Complexation for increasing drug solubility

Cyclodextrins are composed of glucopyranose units which form a ring of either six

(α-cyclodextrin), seven (β-cyclodextrin) or eight (γ-cyclodextrin) units.

The outer surface of the ring is hydrophilic and the inner cavity is hydrophobic

Lipophilic molecules can fit into the ring to form soluble inclusion complexes.

The ring of β-cyclodextrin is the correct size for the majority of drug molecules, and

normally one drug molecule will associate with one cyclodextrin molecule to form

reversible complexes.

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