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.

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.

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 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.

Identification and antimicrobial activity of urinary metabolites of a rifamycin derivative in dog.

1. Three metabolites of the antimicrobial agent 3'-hydroxy-5'-(4-isobutyl-1-piperazinyl)benzoxazinorifamycin (KRM-1648) were isolated from dog urine obtained after administration of a single oral dose. These metabolites of KRM-1648 were identified by mass spectrometry and 1H and 13C-nmr spectrometry. 2. Three metabolites of KRM-1648 were identified as 25-deacetyl KRM-1648, 30-hydroxy KRM-1648 and 25-deacetyl-30-hydroxy KRM-1648. 3. The antimicrobial activities of 25-deacetyl KRM-1648 were comparable with those of the parent compound, whereas 30-hydroxy KRM-1648 was equipotent and 2-8-fold less active than the parent compound against bacteria and mycobacteria, respectively.

High-performance liquid chromatographic determination of 3'-hydroxy-5'-(4-isobutyl-1-piperazinyl)benzoxazinorifamycin (KRM-1648) and its deacetyl metabolite in plasma, whole blood, urine and tissue samples in rats.

A reversed-phase high-performance liquid chromatographic method was developed for the determination of 3'-hydroxy-5'-(4-isobutyl-1-piperazinyl)benzoxazinorifamycin (KRM-1648, I), a new rifamycin derivative, and its 25-deacetyl metabolite (KRM-1671, II) in plasma, whole blood, tissues and urine from rats. I and II were coextracted with an internal standard from each sample matrix by solid-phase extraction (Bond Elut). Plasma and urine were directly loaded onto Bond Elut, while whole blood and tissues were homogenized and extracted with methanol or dichloromethane-chloroform prior to Bond Elut extraction. The extracts were chromatographed on Shim-pack CLC-ODS(M) using acetonitrile-0.02 M citrate buffer containing 0.1 M sodium perchlorate (2:1, v/v), and peaks were detected at 643 nm. The validation data showed that the assays for I and II in plasma, whole blood, tissues and urine were selective, accurate and reproducible.

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.

Absorption, disposition, and urinary metabolism of 14C-rifabutin in rats.

14C-rifabutin was given orally (25 mg/kg) and intravenously (i.v.) (10 mg/kg) to female Sprague-Dawley rats. Radioactivity was eliminated by both the renal and fecal routes, amounting to 44.49 and 43.39% of the dose, respectively, in urine and feces at 96 h after the oral dose and to 47.81 and 40.76% of the dose, respectively, in urine and feces after the i.v. dose. Differences between the two routes of administration were negligible. Tissue distribution of radioactivity after the oral dose was investigated by the combustion technique. At 2 h, the highest concentration of radioactivity was observed in the liver, followed by the lung, abdominal adipose tissue, and spleen, whereas at 72 h, the sequence was abdominal adipose tissue, liver, spleen, bone marrow, and lung. Brain levels of radioactivity were very low. The results of whole-body autoradiography after i.v. administration confirmed the above. Whole-body autoradiography of pregnant rats showed higher concentrations of radioactivity in the uterus than in the placenta and trace levels in the fetuses up to 8 h. Radioactivity was absent in the amniotic fluid. The urinary metabolism was studied by radio-high-pressure liquid chromatography. Rifabutin accounted for 7.4 and 7.2% of the dose in 0- to 48-h urine after oral and i.v. administration, respectively. Metabolites 31-OH rifabutin and 25-O-deacetyl rifabutin amounted to 4.3 and 1.6% of the dose, respectively, after oral administration and to 2.6 and 0.7% of the dose, respectively, after i.v. administration. The remaining urinary radioactivity was mainly due to polar compounds.

Absorption, disposition and preliminary metabolic pathway of 14C-rifabutin in animals and man.

14C-Rifabutin was given orally to rats, rabbits and monkeys at a dose of 25 mg/kg and to healthy volunteers at a dose of 270 mg. Radioactivity was eliminated by both the renal and faecal routes in all species, with a predominance of the renal route in man and monkeys (50.19% and 46.73% of the dose, respectively, in urine at 96 h), whereas in rats and rabbits a slight predominance of faecal excretion was observed (48.09% and 45.01% of the dose, respectively, at 96 h in faeces; 42.22% and 36.37% in urine). Radioactivity as expired 14CO2 was detected in the rat and accounted for less than 0.5% of the dose within 96 h. The drug was rapidly absorbed and peak plasma radioactivity levels were reached from 1 to 4 h after dosing. Rifabutin was the predominant compound circulating in plasma at the first sampling times, but significant levels of 31-OH rifabutin were detected up to 8-24 h in all species studied. 25-O-deacetyl rifabutin was detected only in rat and man. Polar metabolites were also present, particularly at the later sampling times. The urinary metabolism was studied by radio-HPLC. Rifabutin accounted for 8.5% and 4.6% of the dose in 0-24 h urine of rats and man respectively, whereas in rabbit and monkey urine only traces of this compound were detected. The main known metabolite in all animal species was 31-OH rifabutin; 25-O-deacetyl rifabutin was detected only in rat and man. The remaining urinary radioactivity was mainly due to polar compounds.

Urinary metabolites of rifabutin, a new antimycobacterial agent, in human volunteers.

1. Metabolites of the antimycobacterial agent 4-deoxo-3,4-[2-spiro-(N-isobutyl-4-piperidyl)]-(1H)-imidazo-(2,5-dihydro )- rifamycin S (rifabutin) were isolated from human urine after administration of a single oral dose of the drug. Some of these metabolites were identified by direct inlet mass spectrometry, 1H-n.m.r. spectrometry and, in two cases, by chromatographic comparison with reference compounds. 2. Unchanged drug, 25-O-deacetyl rifabutin and four other metabolites were identified in human urine. 25-O-Deacetyl rifabutin was the main urinary metabolite, other metabolites were characterized as oxidized, and oxidized-deacetylated derivatives. 3. Routes of metabolic transformation were: (a) deacetylation at position 25, (b) oxidation of methyl groups 31 or 32 or at the piperidine nitrogen, and (c) combination of these.

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.