High-performance liquid chromatography (HPLC) coupled to mass spectrometry (MS) using an electrospray ionization (ESI) interface provides a sensitive method for the quantitative analysis of peptide drugs in complex biological matrixes. ESI HPLC-MS was applied to the analysis of a pentapeptide drug (IRI-514) in rabbit and human plasma. Prior to analysis, the plasma samples were prepared using protein precipitation followed by solid-phase extraction. The lower limit of quantitation using selected ion monitoring was determined to be 2 ng/mL, when 8 mL of human plasma spiked with 1-40 ng/mL was extracted. Rabbit plasma (1 mL) samples spiked with 10-40,000 ng of authentic drug/mL gave a linear response when a deuterated peptide analog was employed as an internal standard. A commercial ESI interface was modified to permit higher flow rates (10-20 microL/min) to enter the mass spectrometer source. The revised interface provided a 10-fold increase in sensitivities and permitted the use of standard HPLC columns (2.0-mm i.d.) and HPLC instrumentation. ESI HPLC-MS analysis was automated to provide unattended, precise, and sensitive detection of small peptides in both human and rabbit plasma. Using this methodology, a toxicokinetic study of intravenously administered IRI-514 at three dose levels indicated that the area under the curve values were dose proportional.

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The relative bioavailability and pharmacokinetics of felbamate (FBM) after a single oral dose and after 10 once-daily oral doses of 60 mg/kg were investigated in adult and pediatric dogs of both sexes. The pediatric and adult dogs were aged 4-6 weeks and 1-2 years, respectively. Analysis of variance (ANOVA) was performed on the bioavailability parameters among all groups and between the first and last doses. No sex-related differences in bioavailability and pharmacokinetic parameters were observed. The bioavailability of FBM in pediatric dogs was significantly less as compared with that in adult dogs. Rapid overall elimination of the drug in pediatric dogs appears to be responsible for the lower bioavailability. The bioavailability of FBM after the last dose was also significantly lower than after the first dose for both age groups. No major differences in the rate constant of FBM absorption (ka) and volume of distribution at steady state (VSS) were observed between the two age groups. As with other clinically useful antiepileptic drugs (AEDs), higher doses of FBM may be required in pediatric populations to achieve optimum drug levels, assuming that age-related changes in FBM disposition will also be confirmed in humans.

The correlation between the antiasthma activity of azelastine and the concentrations of azelastine and its major metabolite, desmethylazelastine, in the blood and lung were investigated in guinea pigs. Blood and lung tissue samples collected at 15 min after aeroallergen (ovalbumin, 0.5 mg/ml, 30 s, 15 psi) challenge, i.e., 2-1/4 h after oral administration of the drug, were analyzed for azelastine and its desmethyl metabolite by specific HPLC assay. Azelastine afforded protection against immediate allergic responses (IAR, characterized by gasping and bronchospasmic convulsions). Based on the reduction of the severity of IAR, the ID50 was 0.04 mg/kg. It also delayed the onset of IAR and prevented mortalities. A positive correlation (r2 = 0.944) between the antiallergic activity and the sum concentrations of azelastine and desmethylazelastine in the lung was observed. Preferential uptake of azelastine and desmethylazelastine by the lung was demonstrated by the mean lung/blood ratio of about 22 and 146, respectively. The selective uptake of azelastine and its active metabolite by the lungs could contribute to its antiasthmatic/antiallergic activity in guinea pigs.

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