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Original ArticleOpen Access
Bioavailability of Phenytoin Sodium Capsules Available in Thailand Part I : In vitro Study
Suthisisang C 
,
Payakachat N
,
Payakachat N Phenytoin is commonly used as an antiepileptic drug worldwide. The unique properties
of phenytoin such as poor water solubility and zero-order kinetics of its metabolism, together
with difference in pharmaceutical formulations can result in dramatic changes in bioavailability
of phenytoin capsule.
The innovator (Dilantin®, Parke Davis), three local brands (brand A, B and C) were
investigated for pharmaceutical characteristics including drug content, content uniformity, and
dissolution. All these tests were performed as described in the monograph of extended phenytoin
sodium capsules in USP XXII with slight modification in HPLC analysis. It was found that all
the products, except brand C, had drug content and content uniformity within the standard range.
Brand A and brand C failed to meet the USP XXII specification for the dissolution test. Per cent
dissolution of brand A was lower whereas per cent dissolution of brand C was much higher than
the standard value. The qualities of innovator and brand B were within the pharmacopoeial specification.
This result revealed that phenytoin capsules available in Thailand did not have homogeneous
pharmaceutical equivalence which may lead to difference in plasma phenytoin levels (see
part II : In vivo study). Thus, changing the brand of phenytoin in stable epileptic patients should
be performed with caution.
of phenytoin such as poor water solubility and zero-order kinetics of its metabolism, together
with difference in pharmaceutical formulations can result in dramatic changes in bioavailability
of phenytoin capsule.
The innovator (Dilantin®, Parke Davis), three local brands (brand A, B and C) were
investigated for pharmaceutical characteristics including drug content, content uniformity, and
dissolution. All these tests were performed as described in the monograph of extended phenytoin
sodium capsules in USP XXII with slight modification in HPLC analysis. It was found that all
the products, except brand C, had drug content and content uniformity within the standard range.
Brand A and brand C failed to meet the USP XXII specification for the dissolution test. Per cent
dissolution of brand A was lower whereas per cent dissolution of brand C was much higher than
the standard value. The qualities of innovator and brand B were within the pharmacopoeial specification.
This result revealed that phenytoin capsules available in Thailand did not have homogeneous
pharmaceutical equivalence which may lead to difference in plasma phenytoin levels (see
part II : In vivo study). Thus, changing the brand of phenytoin in stable epileptic patients should
be performed with caution.
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