Is generic rifaximin still a poorly absorbed antibiotic? A comparison of branded and generic formulations in healthy volunteers.

Rifaximin is an antibiotic, locally acting in the gastrointestinal tract, which may exist in different crystal as well as amorphous forms. The branded rifaximin formulation contains the polymorph rifaximin-α, whose systemic bioavailability is very limited. This study was performed to compare the pharmacokinetics of this formulation with that of a generic product, whose composition in terms of solid state forms of the active pharmaceutical ingredient was found to be different. Two tablets (2×200mg) of branded and generic formulations were given to 24 healthy volunteers of either sex, according to a single-blind, randomized, two-treatment, single-dose, two-period, cross-over design. Plasma and urinary samples were collected at preset times (for 24h or 48h, respectively) after dosing, and assayed for rifaximin concentrations by high-performance liquid chromatography-mass spectrometry. Rifaximin plasma and urine concentration-time profiles showed relevant differences when generic and branded rifaximin were compared. Most pharmacokinetic parameters were significantly higher after administration of generic rifaximin than after rifaximin-α. In particular, the differences for Cmax, AUC and cumulative urinary excretion between the generic formulation and the branded product ranged from 165% to 345%. The few adverse events recorded were not serious and not related to study medications. The results of the present investigation demonstrate different systemic bioavailability of generic and branded formulations of rifaximin. As a consequence, the therapeutic results obtained with rifaximin-α should not be translated sic et simpliciter to the generic formulations of rifaximin, which do not claim containing only rifaximin-α and will display significantly higher systemic absorption in both health and disease.

Adsorptive stripping voltammetry of antibiotics rifamycin SV and rifampicin at renewable pencil electrodes.

Adsorptive stripping voltammetry of antibiotics of rifamycin SV (RSV) and rifampicin (RIF) was investigated by cyclic voltammetry and differential pulse voltammetry using a renewable pencil graphite electrode (PGE). The nature of the oxidation process of RSV and RIF taking place at the PGE was characterized. The results show that the determination of highly sensitive oxidation peak current is the basis of a simple, accurate and rapid method for quantification of RSV and RIF in bulk forms, pharmaceutical formulations and biological fluids by differential pulse adsorptive stripping voltammetry (DPASV). Factors influencing the trace measurement of RSV and RIF at PGE are assessed. The limits of detection for the determination of RSV and RIF in bulk forms are 6.0 × 10(-8) mol/L and 1.3 × 10(-8) mol/L, respectively. Moreover, the proposed procedure was successfully applied to assay both RSV and RIF in pharmaceutical formulations and in biological fluids. The capability of the proposed procedure for simultaneous assay of antibiotics RSV-isoniazid and RIF-isoniazid was achieved. The statistical analysis and calibration curve data for trace determination of RSV and RIF are reported.

Determination of rifaximin in rat serum by ionic liquid based dispersive liquid-liquid microextraction combined with RP-HPLC.

An efficient and environmental friendly ionic liquid based dispersive liquid-liquid microextraction procedure was optimized for determination of rifaximin in rat serum by reverse phase high-performance liquid chromatography. The effect of ionic liquids, dispersive solvents, extractant/disperser ratio, and salt concentrations on sample recovery and enrichment factors were studied. Among the five ionic liquids studied in the present investigation, 1-butyl-3-methylimidazolium hexafluorophosphate was found to be most effective for extraction of rifaximin. The recovery was found to be more than 98% using 1-butyl-3-methylimidazolium hexafluorophosphate and methanol as extraction and dispersive solvents, at an extractant/disperser ratio of 0.43. The recovery was further enhanced to 99.5% by the addition of 5.0% NaCl solution. A threefold enhancement in detection limit was achieved when compared to protein precipitation. The ionic liquid containing the extracted rifaximin was directly injected into HPLC system. The linear relationship was observed in the range of 0.03-10.0 µg/mL with the correlation coefficient (r(2)) 0.9998. Limits of detection and quantification were found to be 0.01 and 0.03 µg/mL, respectively. The relative standard deviation was 2.5%. The method was validated and applied to study pharmacokinetics of rifaxmin in rat serum.

Systemic absorption of rifamycin SV MMX administered as modified-release tablets in healthy volunteers.

The new oral 200-mg rifamycin SV MMX modified-release tablets, designed to deliver rifamycin SV directly into the colonic lumen, offer considerable advantages over the existing immediate-release antidiarrheic formulations. In two pharmacokinetics studies of healthy volunteers, the absorption, urinary excretion, and fecal elimination of rifamycin SV after single- and multiple-dose regimens of the new formulation were investigated. Concentrations in plasma of >2 ng/ml were infrequently and randomly quantifiable after single and multiple oral doses. The systemic exposure to rifamycin SV after single and multiple oral doses of MMX tablets under fasting and fed conditions or following a four-times-a-day (q.i.d.) or a twice-a-day (b.i.d.) regimen could be considered negligible. With both oral regimens, the drug was confirmed to be very poorly absorbable systemically. The amount of systemically absorbed antibiotic excreted by the renal route is far lower than 0.01% of the administered dose after both the single- and multiple-dose regimens. The absolute bioavailability, calculated as the mean percent ratio between total urinary excretion amounts (ΣXu) after a single intravenous injection and after a single oral dose under fasting conditions, was 0.0410±0.0617. The total elimination of the unchanged rifamycin SV with feces was 87% of the administered oral dose. No significant effect of rifamycin SV on vital signs, electrocardiograms, or laboratory parameters was observed.

Rapid determination of rifaximin on dried blood spots by LC-ESI-MS.

The use of blood spot collection cards is a simple way to obtain specimens for therapeutic drug monitoring, assessing adherence to medications and preventing toxicity in a clinical setting. A high-throughput liquid chromatography-electrospray ionization mass spectrometric (LC-ESI-MS) method for determination of rifaximin on dried blood spots (DBS) was developed and validated. It involves solvent extraction of a punch of DBS followed by reversed-phase LC on a monolithic column consisting of a silica rod with bimodal pore structure and detection by ESI-MS. Rifampicin was used as an internal standard (IS). The run time was within 5.0 min with a very low back-pressure at a flow rate of 0.5 mL/min. The assay was linear from 0.1 to 10 ng/mL. The mean recovery was 98.42%. The developed method is very simple, rapid and useful for clinical applications.

On-line 2D-LC-ESI/MS/MS determination of rifaximin in rat serum.

A highly sensitive and selective on-line two-dimensional reversed-phase liquid chromatography/electrospray ionization-tandem mass spectrometry (2D-LC-ESI/MS/MS) method was developed and validated to determine rifaximin in rat serum by direct injection. The 2D-LC-ESI/MS/MS system consisted of a restricted access media column for trapping proteins as the first dimension and a Waters C(18 )column as second dimension using 0.1% aqueous acetic acid:acetonitrile as mobile phase in a gradient elution mode. Rifampacin was used as an internal standard. The linear dynamic range was 0.5-10 ng/mL (r(2) > 0.998). Acceptable precision and accuracy were obtained over the calibration range. The assay was successfully used in analysis of rat serum to support pharmacokinetic studies.

Rapid determination of rifaximin in rat serum and urine by direct injection on to a shielded hydrophobic stationary phase by HPLC.

A simple and rapid reversed-phase HPLC method for determination of rifaximin in rat serum and urine was developed. Separation of rifaximin from biological matrix was achieved by direct injection of rat serum and urine onto a restricted-access medium, Supelco LC-Hisep, a shielded hydrophobic stationary phase, using acetonitrile:water:acetic acid (18:82:0.1 v/v/v) as a mobile phase. The linear range was 0.10-20 microg/mL (r(2 )> 0.999, n = 6), intraday and interday variation was <6.10%. The limits of detection and quantification were 0.03 (signal-to-noise ratio >3) and 0.10 microg/mL (signal-to-noise ratio >10), respectively. The method was successfully applied to pharmacokinetic studies of rifaximin after an oral administration to rats.

Effect of food on the pharmacokinetics of rifalazil, a novel antibacterial, in healthy male volunteers.

Rifalazil is a new antibiotic structurally related to rifampin but devoid of the metabolic liabilities typically associated with the rifamycin class of antibiotics. A randomized, 3-way crossover study in healthy male volunteers (n = 12) investigated the safety and pharmacokinetics of a single 25-mg oral rifalazil dose administered under a standard breakfast containing fat as 30% of calories, a high-fat breakfast containing fat as 60% of calories, and an overnight fast of 10 hours with a 21- to 28-day washout between doses. Systemic exposure to rifalazil based on Cmax, AUC(0-Tlast), and AUC(0-infinity) was increased progressively as the fat content of the test breakfast was increased from 30% to 60% compared with fasting. The confidence intervals for both fat-containing breakfasts are outside the limits of 80% to 125% allowed for food effect bioequivalence based on Cmax, AUC(0-Tlast), and AUC(0-infinity). This food effect may be a result of increased fractional absorption with increasing dietary fat content. Another striking finding was the large reduction of the pharmacokinetic intersubject variability after rifalazil administration with food. Rifalazil was safe and well tolerated under fed and fasted conditions.

Sensitive quantification of rifaximin in human plasma by liquid chromatography-tandem mass spectrometry.

A liquid chromatography-mass spectrometry method (LC-MS/MS) for the quantitative determination of rifaximin in human plasma was developed and validated. In the developed procedure, metoprolol was added to human plasma as an internal standard (IS) and acetonitrile was used to precipitate the plasma proteins before LC-MS/MS analysis. Chromatographic separation was obtained on a RESTEK Pinnacle C18 column (50 mm x 2.1mm, 5 microm) with a mobile phase consisted of ammonium acetate solution (15 mM, pH 4.32) as buffer A and methanol as mobile phase B. Quantification was performed in positive mode using multiple reaction monitoring (MRM) of the transitions m/z 786.1-->754.1 for rifaximin and m/z 268.3-->116.1 for the IS. The assay has been validated over the concentration range of 0.5-10 ng/ml (r=0.9992) based on the analysis of 0.2 ml of plasma. The assay accuracy was between 98.2% and 109%. The within-day and between-day precision was better than 3.9% and 8.9% at three concentration levels. The freeze-thaw stability was also investigated and it was found that both rifaximin and the IS were quite stable. This method provides a rapid, sensitive, specific and robust tool for the quantitative determination of rifaximin in human plasma, which is especially useful for the pharmacokinetic study of rifaximin.

Determination of rifalazil, a potent antibacterial agent, in human plasma by liquid-liquid extraction and LC-MS/MS.

A sensitive assay for determination of rifalazil (also known as ABI-1648 and KRM-1648) in human plasma is described. The analytical method utilizes liquid-liquid extraction of plasma with methyl tert-butyl ether, followed by reversed-phase liquid chromatography with a C18 column and a mobile phase gradient utilizing 0.1% formic acid in water and acetonitrile, respectively. Electrospray mass spectrometry in the positive ion mode with selected reaction monitoring of rifalazil and an isotope labeled internal standard, 13C4-rifalazil (ABI-9901) was used for selective and sensitive detection. The calibration range was 0.050-50 ng/mL plasma using 200 microL plasma sample volume. The absolute extraction recovery of rifalazil from K2-EDTA plasma, evaluated at three concentration levels, was 88.6-97.3%, and the recovery for the internal standard was 96.8%. A study of plasma matrix effects showed a peak area response at 90-99% compared to neat solutions for both rifalazil and the internal standard. Stability evaluation of rifalazil in plasma, whole blood and methanol showed that the analyte stability was adequate when stored under study conditions. The precision, as evaluated in three validation batches, was consistent for fortified plasma quality control (QC) samples at four concentration levels, with < or =6% R.S.D. except for at the lowest quality control level where it was 10.7% R.S.D. The accuracy for QC samples (difference between found and nominal concentration) ranged from -2.3% to 5.1%. Similar precision and accuracy values were obtained over 6 months of routine application of this method. It was concluded that the performance improved markedly during routine operation by replacing a closely related structural analog internal standard with the stable isotope internal standard.

Absence of effect of oral rifaximin on the pharmacokinetics of ethinyl estradiol/norgestimate in healthy females.

BACKGROUND: Rifaximin is an oral rifampin analog, and its activity is targeted within the gastrointestinal tract. Some analogs induce the cytochrome P450 family of oxidative enzymes. Ethinyl estradiol (EE), commonly found in oral contraceptives (OCs), is a known CYP3A4 substrate. OBJECTIVE: To determine the potential effect of rifaximin on EE and norgestimate pharmacokinetics. METHODS: In an open-label, crossover study, healthy females received a single dose of OC (EE 0.07 mg/norgestimate 0.50 mg). Following a 1 week washout period, individuals received rifaximin 200 mg every 8 hours for 3 days, with a single dose of OC administered with the ninth rifaximin dose. During both treatment periods, blood samples were collected periodically for up to 96 hours after each OC dose. Plasma concentration-time profiles and pharmacokinetic parameters were characterized for EE and 2 major metabolites of norgestimate, norgestrel (NG) and 17-deacetyl norgestimate (17-DNGM). A drug-drug interaction was confirmed if the 90% CI for the 2 treatment period comparison was outside the 80-125% limit. RESULTS: Twenty-six of 28 women completed the study. No differences in pharmacokinetic parameters were observed for EE, NG, or 17-DNGM when a single dose of the OC was administered alone or with rifaximin. In addition, the 90% CI for the bioavailability contrasts (OC alone vs OC with rifaximin) for the maximum plasma concentration, area under the plasma concentration-time curve from zero to the last measurable plasma concentration or to infinity for EE, NG, and 17-DNGM all ranged from 86-118%. These intervals were within the predefined range for equivalence; therefore, no interaction was observed between OC and rifaximin. Rifaximin was well tolerated. CONCLUSIONS: Administration of a 3 day dosing regimen of oral rifaximin was well tolerated and did not alter the pharmacokinetics of a commonly used combination OC containing EE and norgestimate.

Determination of rifalazil in dog plasma by liquid-liquid extraction and LC-MS/MS: quality assessment by incurred sample analysis.

Incurred dog plasma samples were utilized for method quality assessment in this study, where a sensitive LC-MS/MS method was used for determination of the antibacterial agent rifalazil (KRM-1648 or ABI-1648). Reproducibility was estimated by repeated analysis of samples from a pharmacokinetic study, where 23 out of 864 study samples were reassayed during the course of the study. Precision for same-day duplicates was %R.S.D. 3.1 (concentration range 0.7-149 ng/ml), and over the whole study %R.S.D. 11.0 (concentration range 0.5-52 ng/ml). Moreover, standard addition experiments with incurred samples (concentration range 0.30-45 ng/ml) are described, where the recovery of spiked rifalazil amount was measured as a surrogate parameter for accuracy. The mean recovery of the added rifalazil amount was 103% (%R.S.D. 26.2). It was concluded that the described method is robust and reproducible for incurred samples. Liquid-liquid extraction was used for isolation of rifalazil and an isotope labeled internal standard from plasma. A manual procedure, based on an 8 x 12 array format, was used for sample extraction. Extracts were analyzed by reversed-phase liquid chromatography using octylsilica column with gradient elution (1mM ammonium acetate+0.01% (v/v) acetic aid in water and methanol). Mass spectrometric detection was made with positive ion electrospray ionization and LC-MS/MS analysis in a triple-quadrupole mass spectrometer. The lower limit of quantification was 50 pg/ml. MS characteristics of rifalazil are presented. In particular, two different sets of ionization and selected reaction monitoring (SRM) conditions, where in-source fragmentation was used for precursor ion formation in one of the sets, were compared. The good correlation found between the two sets of results for authentic sample extracts indicated that either condition could be used for quantification of rifalazil in dog plasma.

Determination of the rifamycin antibiotics rifabutin, rifampin, rifapentine and their major metabolites in human plasma via simultaneous extraction coupled with LC/MS/MS.

A novel assay using high pressure liquid chromatography (HPLC) coupled to mass spectrometer (MS) detection was developed and validated for the rifamycin anti-tuberculosis antibiotics rifampicin (RIF), rifabutin (RBT), rifapentine (RPT) and their active desacetyl metabolites (dRIF, dRBT and dRPT, respectively) in human plasma. The assay uses 50 µL of human plasma with a quick and simple protein-precipitation extraction to achieve a dynamic range of 75-30,000 ng/mL for RIF, RBT and RPT and 37.5-15,000 ng/mL for dRIF, dRBT and dRPT, respectively. The average %CV and %deviation were less than 20% at the lower limit of quantitation and less than 15% over the range of the curve. The method was fully validated according to FDA criteria for bioanalytical assays and has successfully been used to support three large international tuberculosis trials.

Effect of Mycobacterium avium infection on the influx, accumulation, and efflux of KRM-1648 by human macrophages.

KRM-1648 is a new benzoxazinorifamycin with activity in vitro and in vivo against organisms of the Mycobacterium avium complex. We investigated the ability of 14C-KRM-1648 to concentrate within human monocyte-derived macrophages in vitro. KRM-1648 is rapidly taken up by uninfected macrophages, with 90% of the initial concentration added to the monolayer found within macrophages by 1 h and approximately 80% at 2 h. Comparable results were obtained in assays using macrophages that have been infected with an AIDS-related strain of M. avium for 24 h. In contrast, macrophages infected with M. avium for 3 days, showed an impaired ability to concentrate KRM-1648, primarily because of a significant efflux of the antibiotic (intracellular concentration of 86% of the available drug was present within macrophages at 1 h vs. 47% at 2 h). Daily administrations of KRM-1648 to a macrophage monolayer for 3 consecutive days resulted in significant accumulation of the drug within phagocytic cells. Although the efflux was greater in M. avium-infected macrophages than in uninfected cells, consecutive administration of KRM-1648 led to a total intracellular accumulation of drug that exceeded the initial level and appeared to continue to accumulate. The ability of KRM-1648 to rapidly accumulate in human macrophages, including M. avium-infected cells, may explain, in part, the improved therapeutic effectiveness in animal models against M. avium and M. tuberculosis.

Physiological disposition of a series of rifamycins in rat: a comparative study.

The disposition of four C3-substituted piperazinyl rifamycins was studied in the rat following the intravenous administration of 5 mg/kg of the 14C-labelled antibiotics. Considerable quantitative differences in the pharmacokinetics of these antibiotics were shown in blood levels, tissue distributions and body clearances. Feces were largely the major route of elimination for the parent drug and metabolites. The results suggest that the liver compartimentalization, regulating the biliary excretion, is to be the kinetic parameter affecting the pharmacokinetic behaviour of this class of antibiotics.

Studies of binding C3-substitute rifamycins to human and bovine serum albumin.

The interactions of a series of C3-substituted rifamycins with human and bovine serum albumins were studied in order to find possible correlations between the degree of binding and the structural features of the various molecules. The results obtained indicate some of the physicochemical properties and, therefore, of the structural requirements which appear to determine or influence the bonding mechanisms of this series of rifamycins. Two types of interaction were found to exist, ionic and hydrophobic types. The findings suggest that the inhibition by protein of the antibacterial activities of these antibiotics depends on the type of bonding mechanism rather than the degree of binding.

Impaired plasma clearance of nicotinic acid and rifamycin-SV in Gilbert's syndrome: evidence of a functional heterogeneity.

Patients with Gilbert's syndrome (GS) have impaired clearance by the liver of some organic anions. We looked for possible differences in hepatic clearance of nicotinic acid (NA) and rifamycin-SV (R-SV) among GS patients, and examined the effect produced by these anions on the plasma levels of unconjugated bilirubin (UCB). Two subgroups of GS patients, GS1 and GS2, were differentiated according to their ability to handle R-SV and NA. Compared with a control group, the alteration of the half-life both of NA and R-SV was less marked in GS1 than in GS2. UCB plasma concentration after NA and R-SV loading was more greatly increased in GS2 than in GS1 patients. In addition, a striking correlation was found in all subjects studied between UCB and the half-life of NA and R-SV. These related alterations of plasma UCB and plasma half-life or organic anions suggests a common defect of hepatic uptake. It is hypothesized that this defect is located at the level of a hepatic plasma membrane carrier.

Plasma clearance of nicotinic acid and rifamycin-SV, and their interaction in Gilbert's syndrome: application of a compartmental model.

The bicompartmental kinetics of nicotinic acid (NA) and rifamycin-SV (R-SV)--2 organic anions that probably share a common hepatic uptake mechanism--were studied in 7 cases of Gilbert's syndrome (GS) and in 7 healthy controls matched for sex and age. In GS the NA and R-SV uptake constants (K21) were significantly decreased. In GS patients, simultaneous loads of NA and R-SV, the latter at increasing doses, produced: 1) a progressive lowering only of R-SV K21; and 2) an increase in R-SV hepatic plasma reflux (K12). Changes in biliary excretion ( Kee ) and hepatocellular pool (Ke) of both NA and R-SV probably depend on the rates of uptake and reflux constants of the two anions. The study of the parameters of compartmental kinetics of NA and R-SV confirms that the two organic anions, which have different metabolic routes and/or a different affinity for intracellular carriers, share common uptake mechanisms.

Pharmacokinetic study of rifaximin after oral administration in healthy volunteers.

Eighteen healthy male volunteers, with a mean age of 24 yrs (range 18-40), underwent an open pharmacokinetics study, aimed at detecting rifaximin concentration in blood and urine after a single oral administration of 400 mg of the antibiotic. Administration took place after a 9 hours' fast and was followed by a breakfast after 2 hours and a lunch after 5 hours. Blood samples were collected before rifaximin administration and 1, 2, 4, 8, 12, 24 and 48 hours after dosing. Urine samples were collected immediately before dosing (reference sample) and then at the end of the following intervals of time: 0-6 h, 6-12 h, 12-24 h, 24-48 h. During the whole study period, the local and general tolerance to rifaximin administration was checked. Rifaximin concentration was assessed by reversed phase high performance liquid chromatography with electrochemical detection. In almost every plasma sample, rifaximin concentration was undetectable (lower than the detection limit of the analytical method, i.e. 2 ng/ml). In urine, very small amounts of the unchanged molecule (< 0.01% of the administered dose) were found in the period 0-48 hours. These results confirm the negligible absorption by the intestinal tract of a single oral dose of rifaximin (400 mg). Local and general tolerance of the administered drug was very good.

[Anti-Mycobacterium avium complex activities of KRM-1648, clarithromycin and levofloxacin in 7HSF medium at peak or average blood concentrations after their oral administration of clinical dosages].

We evaluated the anti-Mycobacterium avium complex (MAC) activities of KRM-1648 (KRM), clarithromycin (CAM) and levofloxacin (LVFX) in 7HSF medium at concentrations in human blood after their oral administration of clinical dosages. MAC organisms were inoculated into tubes containing the medium with or without the addition of test drugs at either 1/10 Cmax, Cmax or C0-8h (average concentrations during the first 8 h) in the blood. KRM at Cmax or C0-8h showed strong bactericidal activity against MAC, CAM showed weak or moderate degree of bactericidal activity at Cmax and C0-8h, while LVFX added at Cmax or C0-8h showed only a weak bacteriostatic effect against M. avium, but did not show any effect against M. intracellulare.