Correlation of Dissolution-Dialysis Rates with Bioavailability of Nitrofurantoin Solid Dosage Forms

The correlation of in-vitro dissolution-dialysis rates of solid dosage forms with in-vivo bioavailability was investigated. Dissolution-dialysis measurements were made of 50mg and 100mg tablets and capsules of Nitrofurantoin commercial products. The samples used represented product lots whose bioavailability had been previously reported. The dissolution-dialysis medium used was pH 7.2 phosphate buffer. A cellulose dialysis membrane was used. A high degree of correlation was osbserved between apparent dialytic rate constant (K_app) of the drug and reported in-vivo bioavailability parameters for all 50mg tablets. But the 50mg capsule K_app value, measured under the same test conditions, was higher and did not correlate with the tablet data. However a value correlating with tablet data was obtained when the stirring speed was reduced from 100 RPM to 10 RPM. A satisfactory correlation was not obtained for the 100 mg dosage forms. This might be due to bladder drug saturation reported to occur at higher dose levels of Nitrofurantoin.