Mechanism of Action
Fexofenadine hydrochloride, the major active metabolite of terfenadine, is an antihistamine with selective peripheral H1-receptor antagonist activity. Fexofenadine hydrochloride inhibited antigen-induced bronchospasm in sensitized guinea pigs and histamine release from peritoneal mast cells in rats. In laboratory animals, no anticholinergic or alpha1-adrenergic-receptor blocking effects were observed. Moreover, no sedative or other central nervous system effects were observed. Radiolabeled tissue distribution studies in rats indicated that fexofenadine does not cross the blood-brain barrier.
Pseudoephedrine hydrochloride is an orally active sympathomimetic amine and exerts a decongestant action on the nasal mucosa. Pseudoephedrine hydrochloride is recognized as an effective agent for the relief of nasal congestion due to allergic rhinitis. Pseudoephedrine produces peripheral effects similar to those of ephedrine and central effects similar to, but less intense than, amphetamines. It has the potential for excitatory side effects.
The pharmacokinetics of fexofenadine hydrochloride in subjects with seasonal allergic rhinitis were similar to those in healthy volunteers.
Fexofenadine hydrochloride and pseudoephedrine hydrochloride administered as ALLEGRA-D 24 HOUR tablets are absorbed at a similar rate and are equally available under single-dose and steady-state conditions as the separate administration of the components. Coadministration of fexofenadine and pseudoephedrine does not significantly affect the bioavailability of either component. The administration of ALLEGRA-D 24 HOUR tablets 30 minutes or 1.5 hour after a high-fat meal decreased the bioavailability of fexofenadine by approximately 50% (AUC 42% and Cmax 54%). Pseudoephedrine pharmacokinetics were unaffected when coadministered with a high-fat meal. Therefore, ALLEGRA-D 24 HOUR should be taken on an empty stomach with water (see DOSAGE AND ADMINISTRATION).
A pharmacokinetic study following single and multiple oral doses over 7 days of ALLEGRA-D 24 HOUR in 66 healthy volunteers showed that fexofenadine, the immediate release component of ALLEGRA-D 24 HOUR, was rapidly absorbed with mean maximum plasma concentrations of 634 ng/mL and 674 ng/mL after single and multiple doses, respectively. The median time to maximum concentration of fexofenadine was 1.8–2.0 hours post-dose. In the same study, the mean maximum plasma concentrations of pseudoephedrine, the extended-release component of ALLEGRA-D 24 HOUR, were 394 ng/mL and 495 ng/mL after single and multiple doses, respectively, with median time to maximum concentration of 12 hours post-dose. Pseudoephedrine concentrations at the end of the dosing interval (mean: 172 ng/mL) at steady state were equivalent to those observed from a comparator pseudoephedrine hydrochloride 240 mg tablet.
Fexofenadine hydrochloride is 60% to 70% bound to plasma proteins, primarily albumin and α1-acid glycoprotein. The protein binding of pseudoephedrine in humans is not known. Pseudoephedrine hydrochloride is extensively distributed into extravascular sites (apparent volume of distribution between 2.6 and 3.5 L/kg).
Approximately 5% of the total dose of fexofenadine hydrochloride and less than 1% of the total oral dose of pseudoephedrine hydrochloride were eliminated by hepatic metabolism.
The mean terminal elimination half-life of fexofenadine was 14.6 hours following administration of ALLEGRA-D 24 HOUR tablets in healthy volunteers, which is consistent with observations from separate administration. Human mass balance studies documented a recovery of approximately 80% and 11% of the [14C]-fexofenadine hydrochloride dose in the feces and urine, respectively. Because the absolute bioavailability of fexofenadine hydrochloride has not been established, it is unknown if the fecal component is primarily unabsorbed drug or the result of biliary excretion. The mean terminal half-life of pseudoephedrine was 7 hours following single-dose administration of ALLEGRA-D 24 HOUR tablets.
Pseudoephedrine has been shown to have a mean elimination half-life of 4–6 hours which is dependent on urine pH. The elimination half-life is decreased at urine pH lower than 6 and may be increased at urine pH higher than 8.
Pharmacokinetics in special populations (for renal, hepatic impairment, and age), obtained after a single dose of 80 mg fexofenadine hydrochloride, were compared to those from healthy volunteers in a separate study of similar design.
Effect of Age
In older subjects (≥65 years old), peak plasma levels of fexofenadine were 99% greater than those observed in younger subjects (<65 years old). Mean fexofenadine elimination half-lives were similar to those observed in younger subjects.
In subjects with mild (creatinine clearance 41–80 mL/min) to severe (creatinine clearance 11–40 mL/min) renal impairment, peak plasma levels of fexofenadine were 8% and 111% greater, respectively, and mean elimination half-lives were 59% and 72% longer, respectively, than observed in healthy volunteers. Peak plasma levels in subjects on dialysis (creatinine clearance ≤10 mL/min) were 82% greater and half-life was 31% longer than observed in healthy volunteers. No data are available on the pharmacokinetics of pseudoephedrine in renally impaired subjects. However, most of the oral dose of pseudoephedrine hydrochloride (43–96%) is excreted unchanged in the urine. A decrease in renal function is, therefore, likely to decrease the clearance of pseudoephedrine significantly, thus prolonging the half-life and resulting in accumulation. (See PRECAUTIONS and DOSAGE AND ADMINISTRATION.)
The pharmacokinetics of fexofenadine hydrochloride in subjects with hepatic disease did not differ substantially from that observed in healthy volunteers. The effect on pseudoephedrine pharmacokinetics is unknown.
Effect of Gender
Across several trials, no clinically significant gender-related differences were observed in the pharmacokinetics of fexofenadine hydrochloride.
Wheal and Flare
Human histamine skin wheal and flare studies following single and twice daily doses of 20 mg and 40 mg fexofenadine hydrochloride demonstrated that the drug exhibits an antihistamine effect by 1 hour, achieves maximum effect at 2–3 hours, and an effect is still seen at 12 hours. There was no evidence of tolerance to these effects after 28 days of dosing. The clinical significance of these observations is unknown.
Effects on QTc
In dogs (30 mg/kg orally twice daily for 5 days) and rabbits (10 mg/kg intravenously over 1 hour), fexofenadine hydrochloride did not prolong QTc at plasma concentrations that were at least 7 and 15 times, respectively, the therapeutic plasma concentrations in man (based on a 180 mg once daily fexofenadine hydrochloride dose when administered as ALLEGRA-D 24 HOUR). No effect was observed on calcium channel current, delayed K+ channel current, or action potential duration in guinea pig myocytes, Na+ current in rat neonatal myocytes, or on the delayed rectifier K+ channel cloned from human heart at concentrations up to 1 × 10−5 M of fexofenadine. This concentration was at least 8 times the therapeutic plasma concentration in man (based on a 180 mg once daily fexofenadine hydrochloride dose).
No statistically significant increase in mean QTc interval compared to placebo was observed in 714 subjects with seasonal allergic rhinitis given fexofenadine hydrochloride capsules in doses of 60 mg to 240 mg twice daily for 2 weeks or in 40 healthy volunteers given fexofenadine hydrochloride as an oral solution at doses up to 400 mg twice daily for 6 days.
A 1-year study designed to evaluate safety and tolerability of 240 mg of fexofenadine hydrochloride (n=240) compared to placebo (n=237) in healthy volunteers, did not reveal a statistically significant increase in the mean QTc interval for the fexofenadine hydrochloride treated group when evaluated pretreatment and after 1, 2, 3, 6, 9, and 12 months of treatment.
Administration of the 60 mg fexofenadine hydrochloride/120 mg pseudoephedrine hydrochloride combination tablet for approximately 2 weeks to 213 subjects with seasonal allergic rhinitis demonstrated no statistically significant increase in the mean QTc interval compared to fexofenadine hydrochloride administered alone (60 mg twice daily, n=215), or compared to pseudoephedrine hydrochloride (120 mg twice daily, n=215) administered alone.
Clinical efficacy and safety studies were not conducted with ALLEGRA-D 24 HOUR Extended-Release Tablets. The effectiveness of ALLEGRA-D 24 HOUR for the treatment of seasonal allergic rhinitis is based on an extrapolation of the demonstrated efficacy of ALLEGRA 180 mg and the nasal decongestant properties of pseudoephedrine hydrochloride.
In one 2-week, multicenter, randomized, double-blind clinical trial in subjects 12 to 65 years of age with seasonal allergic rhinitis (n=863), fexofenadine hydrochloride 180 mg once daily significantly reduced total symptom scores (the sum of the individual scores for sneezing, rhinorrhea, itchy nose/palate/throat, itchy/watery/red eyes) compared to placebo. Although the number of subjects in some of the subgroups was small, there were no significant differences in the effect of fexofenadine hydrochloride across subgroups of subjects defined by gender, age, and race.