M. Bialer et al., PHARMACOKINETIC ANALYSIS OF 2 NEW SUSTAINED-RELEASE PRODUCTS OF DILTIAZEM DESIGNED FOR TWICE-DAILY AND ONCE-DAILY TREATMENT, Biopharmaceutics & drug disposition, 15(1), 1994, pp. 45-52
The pharmacokinetics of two new sustained-release (SR) products of dil
tiazem, Dilapress 120 mg tablets and Dilapress 240 mg tablets, was ana
lysed and characterized in three different studies, in comparison to t
he following diltiazem SR formulations: Cardizem Retard, Cardizem SR,
and Cardizem CD. Dilapress 120, designated for twice-daily dosing, was
found to be bioequivalent to Cardizem SR and to Cardizem Retard with
mean (+/- SD) relative bioavailability values of 99 +/- 27% and 113 +/
- 38%, respectively. Dilapress 240, designed for once-a-day treatment,
was found to have a slower absorption rate than Cardizem SR and its e
xtent of absorption was 56 +/- 19% relative to that of Cardizem SR. Ho
wever, the bioavailability of Dilapress 240 relative to that of Cardiz
em CD was 118 +/- 46%, indicating that the bioavailability of Cardizem
CD relative to that of Cardizem SR was only 54 +/- 29%. Diltiazem is
partially available due to a saturable liver first-pass effect. A high
dose of Cardizem SR may partially escape this first-pass effect and,
thus, achieve a higher extent of absorption than a slower SR product.
Consequently, SR products of diltiazem designed for once-daily treatme
nt may not reach the saturation stage in the liver first-pass effect p
rocess that diltiazem is susceptible to. Consequently, a twice-daily S
R product of diltiazem cannot serve as a reference for extent of absor
ption assessments of a once-daily SR product.