Pharmacokinetic parameters
Elimination Half-life
Drugs having less half-life
(t1/2) only selected for CR-DDS. Half-life should be between 2 to 8
hours.
Absolute Bioavailability
More bioavailability suits CR-DDS. Drugs having 75%
bioavailability or more is optimal.
Apparent volume of distribution -
Must be higher than release rate.
Therapeutic Index - Should be high
Physicochemical parameters
Molecular size of the drug should be less than
1000 Daltons.
Ability of a drug to diffuse through membrane is called
diffusivity. For intermediate drug (molecular size 100-400 Daltons),
diffusivity co-efficient is 10 power -6 to 10 power -9 cm2/sec.
Aqueous solubility -
More than 0.1 mg/ml for pH 1 to pH 7.8
Apparent partition coefficient -
High
Drugs with high partition coefficient readily penetrate the
membrane but are unable to proceed further, while drugs with low partition
coefficient cannot penetrate the membrane.
Absorption mechanism -
Diffusion
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