Rifaximin clinical pharmacology

# Clinical Pharmacology

## Mechanism of Action
Rifaximin is an antibacterial drug .

## Pharmacokinetics

### Absorption
Systemic absorption of rifaximin (200 mg three times daily) was evaluated in 13 subjects challenged with shigellosis on Days 1 and 3 of a three-day course of treatment.  Rifaximin plasma concentrations and exposures were low and variable. There was no evidence of accumulation of rifaximin following repeated administration for 3 days (9 doses).  Peak plasma rifaximin concentrations after 3 and 9 consecutive doses ranged from 0.81 to 3.4 ng/mL on Day 1 and 0.68 to 2.26 ng/mL on Day 3.  Similarly, AUC0-last estimates were 6.95  5.15 ngh/mL on Day 1 and 7.83  4.94 ngh/mL on Day 3.  XIFAXAN is not suitable for treating systemic bacterial infections because of limited systemic exposure after oral administration .
After a single dose and multiple doses of rifaximin 550 mg in healthy subjects, the mean time to reach peak plasma concentrations was about an hour. The pharmacokinetic (PK) parameters were highly variable and the accumulation ratio based on AUC was 1.37.
The PK of rifaximin in patients with a history of HE was evaluated after administration of XIFAXAN, 550 mg two times a day. The PK parameters were associated with a high variability and mean rifaximin exposure (AUC) in patients with a history of HE (147 ngh/mL) was approximately 12-fold higher than that observed in healthy subjects following the same dosing regimen (12.3 ngh/mL).  When PK parameters were analyzed based on Child-Pugh Class A, B, and C, the mean AUC  was 10-, 13-, and 20-fold higher, respectively, compared to that in healthy subjects (Table 3).

## Food Effect in Healthy Subjects
A high-fat meal consumed 30 minutes prior to XIFAXAN dosing in healthy subjects delayed the mean time to peak plasma concentration from 0.75 to 1.5 hours and increased the systemic exposure (AUC) of rifaximin by 2-fold (Table 4).
XIFAXAN can be administered with or without food .

## Distribution
Rifaximin is moderately bound to human plasma proteins.  In vivo, the mean protein binding ratio was 67.5% in healthy subjects and 62% in patients with hepatic impairment when XIFAXAN 550 mg was administered.

## Metabolism and Excretion
In a mass balance study, after administration of 400 mg 14C-rifaximin orally to healthy volunteers, of the 96.94% total recovery, 96.62% of the administered radioactivity was recovered in feces almost exclusively as the unchanged drug and 0.32% was recovered in urine mostly as metabolites with 0.03% as the unchanged drug. Rifaximin accounted for 18% of radioactivity in plasma.  This suggests that the absorbed rifaximin undergoes metabolism with minimal renal excretion of the unchanged drug.  The enzymes responsible for metabolizing rifaximin are unknown.
In a separate study, rifaximin was detected in the bile after cholecystectomy in patients with intact gastrointestinal mucosa, suggesting biliary excretion of rifaximin.

## Specific Populations

### Hepatic Impairment
The systemic exposure of rifaximin was markedly elevated in patients with hepatic impairment compared to healthy subjects.  The mean AUC in patients with Child-Pugh Class C hepatic impairment was 2-fold higher than in patients with Child-Pugh Class A hepatic impairment (see Table 3), .

### Renal Impairment
The pharmacokinetics of rifaximin in patients with impaired renal function has not been studied.

## Drug Interactions
In vitro drug interaction studies have shown that rifaximin, at concentrations ranging from 2 to 200 ng/mL, did not inhibit human hepatic cytochrome P450 isoenzymes 1A2, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, and 3A4.
In an in vitro study, rifaximin was shown to induce CYP3A4 at the concentration of 0.2 M.
An in vitro study suggests that rifaximin is a substrate of P-glycoprotein.  In the presence of P-glycoprotein inhibitor verapamil, the efflux ratio of rifaximin was reduced greater than 50% in vitro.  The effect of P-glycoprotein inhibition on rifaximin was not evaluated in vivo.
The inhibitory effect of rifaximin on P-gp transporter was observed in an in vitro study.  The effect of rifaximin on P-gp transporter was not evaluated in vivo.

### Midazolam
The effect of rifaximin 200 mg administered orally every 8 hours for 3 days and for 7 days on the pharmacokinetics of a single dose of either midazolam 2 mg intravenous or midazolam 6 mg orally was evaluated in healthy subjects.  No significant difference was observed in the metrics of systemic exposure or elimination of intravenous or oral midazolam or its major metabolite, 1-hydroxymidazolam, between midazolam alone or together with rifaximin.  Therefore, rifaximin was not shown to significantly affect intestinal or hepatic CYP3A4 activity for the 200 mg three times a day dosing regimen.
After XIFAXAN 550 mg was administered three times a day for 7 days and 14 days to healthy subjects, the mean AUC of single midazolam 2 mg orally was 3.8% and 8.8% lower, respectively, than when midazolam was administered alone.  The mean Cmax of midazolam was also decreased by 4-5% when XIFAXAN was administered for 7-14 days prior to midazolam administration.  This degree of interaction is not considered clinically meaningful.
The effect of rifaximin on CYP3A4 in patients with impaired liver function who have elevated systemic exposure is not known.

### Oral Contraceptives Containing 0.07 mg Ethinyl Estradiol and 0.5 mg Norgestimate
The oral contraceptive study utilized an open-label, crossover design in 28 healthy female subjects to determine if rifaximin 200 mg orally administered three times a day for 3 days (the dosing regimen for travelers diarrhea) altered the pharmacokinetics of a single dose of an oral contraceptive containing 0.07 mg ethinyl estradiol and 0.5 mg norgestimate.  Results showed that the pharmacokinetics of single doses of ethinyl estradiol and norgestimate were not altered by rifaximin .
Effect of rifaximin on oral contraceptives was not studied for XIFAXAN 550 mg twice a day, the dosing regimen for hepatic encephalopathy.