Determination of rifampicin and its main metabolites in human plasma by high-performance liquid chromatography

Fast and Simple LC-MS/MS Method for Rifampicin Quantification in Human Plasma

A simple, fast, and cost-effective LC-MS/MS method for quantification of rifampicin in human plasma was developed and fully validated. The plasma samples containing rifampicin and isotopically labelled internal standard rifampicin D8, were cleaned up using a Captiva ND Lipids filtration plate. Chromatographic separation was achieved on an 1290 Infinity liquid chromatograph coupled to 6460 Triple Quadrupole operated in positive mode on a core-shell Kinetex C18 column (50 × 2.1 mm, 2.6 μm) by gradient elution using 0.1% formic acid in water and acetonitrile as a mobile phase. The proposed method is the fastest method published by now, both in terms of sample preparation (approximately one minute per sample) and chromatographic analysis (total run time 2.4 min). Another key benefit is the outstanding sensitivity and wide analytical range (5-40000 μg/L) with good linearity, accuracy, and precision. The method showed almost complete recovery (92%) and absence of any significant matrix effect as demonstrated by uniform responses from QC samples prepared in blood plasma from 6 volunteers (RSD <5%). The proposed method was successfully applied to rifampicin quantification in 340 patients' plasma samples, thus demonstrating its suitability for both therapeutic drug monitoring and pharmacokinetic analysis.

Validation of a simple HPLC-UV method for rifampicin determination in plasma: Application to the study of rifampicin arteriovenous concentration gradient

In clinical practice, rifampicin exposure is estimated from its concentration in venous blood samples. In this study, we hypothesized that differences in rifampicin concentration may exist between arterial and venous plasma. An HPLC-UV method for determining rifampicin concentration in plasma using rifapentine as an internal standard was validated. The method, which requires a simple protein precipitation procedure as sample preparation, was performed to compare venous and arterial plasma kinetics after a single therapeutic dose of rifampicin (8.6 mg/kg i.v, infused over 30 min) in baboons (n=3). The method was linear from 0.1 to 40 μg mL(-1) and all validation parameters fulfilled the international requirements. In baboons, rifampicin concentration in arterial plasma was higher than in venous plasma. Arterial Cmax was 2.1±0.2 fold higher than venous Cmax. The area under the curve (AUC) from 0 to 120 min was ∼80% higher in arterial plasma, indicating a significant arteriovenous concentration gradient in early rifampicin pharmacokinetics. Arterial and venous plasma concentrations obtained 6h after rifampicin injection were not different. An important arteriovenous equilibration delay for rifampicin pharmacokinetics is reported. Determination in venous plasma concentrations may considerably underestimate rifampicin exposure to organs during the distribution phase.

Antibacterial activity of rifamycins for M. smegmatis with comparison of oxidation and binding to tear lipocalin

A mutant of Mycobacterium smegmatis is a potential class I model substitute for Mycobacterium tuberculosis. Because not all of the rifamycins have been tested in this organism, we determined bactericidal profiles for the 6 major rifamycin derivatives. The profiles closely mirrored those established for M. tuberculosis. Rifalazil was confirmed to be the most potent rifamycin. Because the tuberculous granuloma presents a harshly oxidizing environment we explored the effects of oxidation on rifamycins. Mass spectrometry confirmed that three of the six major rifamycins showed autoxidation in the presence of trace metals. Oxidation could be monitored by distinctive changes including isosbestic points in the ultraviolet-visible spectrum. Oxidation of rifamycins abrogated anti-mycobacterial activity in M. smegmatis. Protection from autoxidation was conferred by binding susceptible rifamycins to tear lipocalin, a promiscuous lipophilic protein. Rifalazil was not susceptible to autoxidation but was insoluble in aqueous solution. Solubility was enhanced when complexed to tear lipocalin and was accompanied by a spectral red shift. The positive solvatochromism was consistent with robust molecular interaction and binding. Other rifamycins also formed a complex with lipocalin, albeit to a lesser extent. Protection from oxidation and enhancement of solubility with protein binding may have implications for delivery of select rifamycin derivatives.

Development and validation of liquid chromatography-mass spectrometry method for the estimation of rifampicin in plasma

A selective, rapid and sensitive liquid chromatography-mass spectrometry method was developed for the quantitative estimation of rifampicin in plasma. With phenacetin as internal standard, sample pretreatment involved a one-step extraction with ethyl acetate from plasma. The sample was analyzed using methanol: 2mM ammonium acetate: 80:20 v/v as mobile phase. Chromatographic separation was achieved on a BDS Hypersil Gold C(18) column which was followed by detection with mass spectrometry. Linear calibration curves were obtained in the concentration range of 5.021-1008.315 ng/ml. The inter- and intra-day accuracy values were below 15% at all quality control levels. Percent recoveries for rifampicin at high, middle and low quality control samples was obtained 55.15, 48.65 and 49.62%, respectively and for internal standard was 60.22%. Rifampicin was found stable through all validation parameters. Developed method was found to be simple, precise, accurate and rapid for estimation of rifampicin in plasma. Thus, the method can be employed for routine pharmacokinetic and bioequivalence studies.

Quantification of rifampicin in human plasma and cerebrospinal fluid by a highly sensitive and rapid liquid chromatographic-tandem mass spectrometric method

A highly sensitive and rapid liquid chromatography tandem mass spectrometry (LC-MS/MS) method has been developed to measure the levels of the antitubercular drug rifampicin (RIF) in human plasma and cerebrospinal fluid (CSF). The analyte and internal standard (IS) were isolated from plasma and CSF by a simple organic solvent based precipitation of proteins followed by centrifugation. Detection was carried out by electrospray positive ionization mass spectrometry in the multiple-reaction monitoring (MRM) mode. The assay was linear in the concentration range 25-6400 ng/mL with intra- and inter-day precision of <7% and <8%, respectively. The validated method was applied to the study of RIF pharmacokinetics in human CSF and plasma over 25 h period after a 10 mg/kg oral dose.

Pharmacokinetics and 48-Week Efficacy of Nevirapine: 400 mg Versus 600 mg per day in HIV–Tuberculosis Coinfection Receiving Rifampicin

Background

We aim here to determine the appropriate dose of nevirapine (NVP) in Thai HIV–tuberculosis (TB)-coinfected patients receiving rifampicin.

Methods

Thirty-two HIV-infected adults with CD4+ T-cell counts <200 cells/mm3 and active TB, receiving rifampicin for 2–6 weeks were randomized to receive either NVP 400 mg (NVP400) or 600 mg (NVP600) per day plus two nucleoside reverse transcriptase inhibitors; a 2-week NVP lead-in was performed at 200 mg once daily (OD) and 200 mg twice daily, respectively. Plasma NVP levels were determined at weeks 2, 4 and 12. Twelve-hour pharmacokinetics (PK) were obtained ( n=20) at week 4.

Results

Baseline body weight was comparable. There were more patients with NVP plasma concentration at 12 h (C12) <3.1 mg/l at week 2 in NVP400 than in NVP600 (79% versus 19%, respectively; P=0.002). However, the proportions were comparable at weeks 4 and 12. From week 4, 12 h PK studies showed that NVP400 had lower median NVP area under the plasma concentration-0–12 h (AUC0–12 h), maximum concentration in plasma (Cmax) and C12 than NVP600 ( P<0.05). Four patients in NVP600 developed NVP hypersensitivity. At week 48, the median CD4+ T-cell count rise and proportion with viral load <50 copies/ml (intention-to-treat analysis 56% versus 50% and as-treated analysis 75% versus 89%) were comparable.

Conclusions

In rifampicin-treated patients, 200 mg NVP OD lead-in led to a significant short-term suboptimal NVP C12 level, while NVP 400 mg lead-in then 600 mg/day was associated with a high rate of NVP hypersensitivity. Forty-eight week efficacy was comparable. Thus, NVP 600 mg/day in rifampicin-treated patients is not recommended.

Microanalysis of amoxicillin, flucloxacillin, and rifampicin in neonatal plasma

Simple and rapid reversed-phase high-performance liquid chromatographic assays with ultraviolet detection have been developed and validated for the determination of amoxicillin, flucloxacillin and rifampicin in neonatal plasma. Plasma samples were either precipitated with perchloric acid (amoxicillin) or methanol (rifampicin) or extracted with methylene chloride (flucloxacillin). Precision coefficients of variation and inaccuracy were less than 15% for all three assays. Only small sample volumes (20-40 microL) were required, making the assays suitable for therapeutic drug monitoring and pharmacokinetic studies in preterm and term neonates. The assays have successfully been applied to analysis of amoxicillin, flucloxacillin and rifampicin in previously published pharmacokinetic studies in neonates.

Screening of rifamycin producing marine sponge bacteria by LC-MS-MS

HPLC-MS-MS has been used for the identification and characterisation of rifamycin B and rifamycin SV in various strains of the marine sponge-derived bacterium Salinispora. Gradient elution using acetonitrile/water/ammonium acetate was used to separate the rifamycins from the matrix and negative ion-electrospray mass spectrometry was used for detection and confirmation. The presence of rifamycin in bacterial extracts was confirmed by matching retention times, parent ion spectra and the fragmentated parent ion spectra of the standard compounds and the bacterial extracts. All strains of the marine sponge bacterium Salinispora tested were found to contain rifamycin thus an alternate actinobacterial source of rifamycin was established.

High-performance liquid-chromatographic determination of rifampicin in plasma and tissues

A HPLC-UV method has been developed for assaying rifampicin in plasma and liver. The assay involved a liquid-liquid extraction procedure with dichloromethane-pentane (1:1). An Ultrabase-C18 column and a simple mobile phase consisting of a water (pH 2.27)-acetonitrile (40:60, v/v) mixture were used. The flow-rate was 1 ml/min and the effluent was monitored at 333 nm. Results from the HPLC analyses showed that the assay method is linear in the ranges 0.1-1 and 1-50 microg/ml for plasma, and 0.6-40 microg/g for liver. Intra- and inter-day R.S.D. were below 15% for all the sample types. Recoveries averaged 83 and 95% for plasma and liver, respectively. The method is being successfully applied to determine rifampicin in plasma and liver samples taken during pharmacokinetic studies in rats.

Managing antituberculosis drug therapy by therapeutic drug monitoring of rifampicin and isoniazid

BACKGROUND

Current therapeutic regimens with rifampicin and isoniazid have proven successful in treating tuberculosis, however, toxicity, therapeutic failure, relapse and multiple drug resistance are serious concerns. Optimizing drug dose using therapeutic drug monitoring (TDM) may be a better approach than administering therapy as a standard dose.

AIMS

To establish and evaluate a TDM service to optimize rifampicin and isoniazid therapy.

METHODS

A TDM service for rifampicin and isoniazid was established in November 1998. Drug concentration data were collected, with relevant information to interpret the results. The reason for the request, information on concomitant drug administration and a questionnaire to assess clinical response to the drug results were also obtained.

RESULTS

Ninety patient episodes were accepted for study. The rifampicin plasma concentrations showed significant scatter, with 46% of the rifampicin concentrations below the normal range and 2% above the normal range. Similarly, 48% of isoniazid concentrations were below the lower target of the normal range and 29% were above the upper normal limit. There was a greater proportion of isoniazid concentrations above the normal range in female patients.

CONCLUSION

Significant pharmacokinetic variability was observed for rifampicin and isoniazid in the patient population studied. Further, a substantial number of plasma concentrations fell outside the suggested normal range for both drugs. Isoniazid plasma concentrations were significantly higher in female patients compared with male patients. Despite these abnormal results, the dose of rifampicin and isoniazid was altered in only 17% of patients, however, many patients received follow-up education because of the drug result. The service was considered valuable by 83% of respondents to the questionnaire. While TDM of rifampicin and isoniazid is a valuable tool to optimize the dose of these drugs in some patients, there is an urgent need for concentration-effect studies and possibly education on the principles and practice of TDM for these drugs.

Correlation of "in vitro" release and "in vivo" absorption characteristics of rifampicin from ethylcellulose coated nonpareil beads

The purpose of this study was to investigate the possibility to develop different levels of correlation between in vitro dissolution parameters and in vivo pharmacokinetic parameters for three rifampicin formulations. A level A correlation of in vitro release and in vivo absorption could be obtained for individual plasma level data by means of the Wagner and Nelson method. Linear correlation could be obtained when percent dose released in vitro was plotted vs percent dose absorbed in vivo with correlation coefficients between 0.954,0.983 and 0.997 for the formulations studied. A second level correlation between mean in vitro dissolution time (MDT) and mean in vivo residence time (MRT) was performed with a correlation coefficient of 0.536,0.420 and 0.335. Finally, it was also possible to establish a good in vitro-in vivo correlation when the T(50%hrs) (time taken to release 50% of rifampicin) in vitro and C(max),T(max) or AUC in vivo were compared.

Simultaneous determination of rifampicin, isoniazid and pyrazinamide in tablet preparations by multivariate spectrophotometric calibration

The use of multivariate spectrophotometric calibration is presented for the simultaneous determination of the active components of tablets used in the treatment of pulmonary tuberculosis. The resolution of ternary mixtures of rifampicin, isoniazid and pyrazinamide has been accomplished by using partial least squares (PLS-1) regression analysis. Although the components show an important degree of spectral overlap, they have been simultaneously determined with high accuracy and precision, rapidly and with no need of nonaqueous solvents for dissolving the samples. No interference has been observed from the tablet excipients. A comparison is presented with the related multivariate method of classical least squares (CLS) analysis, which is shown to yield less reliable results due to the severe spectral overlap among the studied compounds. This is highlighted in the case of isoniazid, due to the small absorbances measured for this component.

Determination of rifampicin and its main metabolite in plasma and urine in presence of pyrazinamide and isoniazid by HPLC method

A reversed phase HPLC method is described for the simultaneous estimation of rifampicin and its major metabolite desacetyl rifampicin, in the presence of isoniazid and pyrazinamide, in human plasma and urine. The assay involves simple liquid extraction of drug, metabolite and internal standard (rifapentine) from biological specimens and their subsequent separation on a C18 reversed phase column and single wavelength UV detection. In plasma as well as in urine samples, all the three compounds of interest eluted within 17 min. Using methanol-sodium phosphate buffer (pH 5.2; 0.01 M) (65:35, v/v) as mobile phase under isocratic conditions, it was established that isoniazid, pyrazinamide and ascorbic acid (added to prevent oxidative degradation of analytes) did not interfere with the analyte peaks. Recoveries (extraction efficiency) for drug were greater than 90% in both plasma and urine, whereas for metabolite the values were found to be 79 and 86% in plasma and urine, respectively. The plasma and urine methods were precise (total coefficient of variation ranged from 5 to 23%) and accurate (-7 to 5% of the nominal values) for both the analytes. Individual variance components, their estimates and their contribution to the total variance were also determined. Using the same method, unknown samples supplied by WHO were assayed and good correlations were obtained between the found and intended values. The method developed proved to be suitable for simultaneous estimation of rifampicin and desacetyl rifampicin in plasma and urine samples.

Effects of rifampin on tacrolimus pharmacokinetics in healthy volunteers

Tacrolimus is a marketed immunosuppressant used in liver and kidney transplantation. It is subject to extensive metabolism by CYP3A4 and is a substrate for P-glycoprotein-mediated transport. A pharmacokinetic interaction with rifampin, an antituberculosis agent and potent inducer of CYP3A4 and P-glycoprotein, and tacrolimus was evaluated in six healthy male volunteers. Tacrolimus was administered at doses of 0.1 mg/kg orally and 0.025 mg/kg/4 hours intravenously. The pharmacokinetics of tacrolimus were obtained from serial blood samples collected over 96 hours, after single oral and intravenous administration prior to and during an 18-day concomitant rifampin dosing phase. Coadministration of rifampin significantly increased tacrolimus clearance (36.0 +/- 8.1 ml/hr/kg vs. 52.8 +/- 9.6 ml/hr/kg; p = 0.03) and decreased tacrolimus bioavailability (14.4% +/- 5.7% vs. 7.0% +/- 2.7%; p = 0.03). Rifampin appears to induce both intestinal and hepatic metabolism of tacrolimus, most likely through induction of CYP3A and P-glycoprotein in the liver and small bowel.

Chromatography as an analytical tool for selected antibiotic classes: a reappraisal addressed to pharmacokinetic applications

The first antibiotic discovered, penicillin, appeared on the market just after the Second World War. Intensive research in subsequent years led to the discovery and development of cephalosporins, aminoglycosides, tetracyclines and rifamycin. The chemotherapeutic quinolones and the more recently discovered fluoroquinolones have added promising new therapeutic weapons to fight the microbial challenge. The major role pharmacokinetics has played in developing these compounds should be highlighted. Plasma concentration-time profiles and the therapeutic activity evoked by these compounds allow the therapeutic window, doses and dose turnovers to be appropriately defined as well as possible dose adjustment to be made in renal failure. The pharmacokinetics of antimicrobial agents were initially explored by using microbiological methods, but these lack specificity. The HPLC technique with UV, fluorometric, electrochemical and, in some cases, mass spectrometry detection has satisfactory solved the problem of antimicrobial agent assay for pharmacokinetic, bioavailability and bioequivalence purposes alike. Indeed, in these studies, plasma concentrations of the given analyte must be followed up for a period > or = 3 times the half-life, which calls for specific sensitive assays. In the review, the authors have described the analytical methods employed in the pharmacokinetics of antibiotics, including some chemotherapeutic agents which are used in medical practice as alternatives to antibiotics. The pharmacokinetic characteristics of each class of drugs are also briefly described, and some historical and chemical notes on the various classes are given.

High-performance liquid chromatography assay of rifapentine in human serum

A high-performance liquid chromatographic method for the determination of rifapentine in human serum was developed. The method utilized a Spherisorb C18 column, ultraviolet detection (336 nm), rifampin as internal standard and a calibration curve (C = 7.010 As/Ain +/- 0.156, r = 0.999) with reproducibility studies which yield a coefficient of variation (C.V.) of intra-day and inter-day assays lower than 10%. The average recovery of rifapentine from serum in the concentration range of 0.5 to 30 micrograms/ml was 92.93 +/- 9.704%.

P-glycoprotein: a major determinant of rifampicin-inducible expression of cytochrome P4503A in mice and humans

The P-glycoprotein (Pgp) efflux pump can influence the hepatocellular concentration of xenobiotics that are modulators and substrates of cytochrome P4503A (CYP3A). We tested the hypothesis that Pgp is a determinant of drug-inducible expression of CYP3A. The magnitude of CYP3A induction by rifampicin was compared in the human parental colon carcinoma cell line LS 180/WT (wild type) and in two derivative clones overexpressing the human multidrug resistance gene MDR1 (also designated PGY1) because of either drug selection (LS 180/ADR) or transfection with MDRI cDNA (LS 180/MDR). In both MDR1 cDNA-overexpressing clones, rifampicin induction of CYP3A mRNA and protein was decreased and required greater rifampicin concentrations compared with parental cells. The role of Pgp in regulation of CYP3A expression in vivo was analyzed in mice carrying a targeted disruption of the mdr1a mouse gene. Oral treatment with increasing doses of rifampicin resulted in elevated drug levels in the livers of mdr1a (-/-) mice compared with mdr1a (+/+) mice at all doses. Consistent with the enhanced accumulation of rifampicin in mdr1a (-/-) mice, lower doses of rifampicin were required for induction of CYP3A proteins, and the magnitude of CYP3A induction was greater at all doses of rifampicin in mdr1a (-/-) mice compared with mdr1a (+/+) mice. We conclude that Pgp-mediated transport is a critical element influencing the CYP3A inductive response.

Determination of rifampin in human plasma by high-performance liquid chromatography with ultraviolet detection

A simple, specific and sensitive high-performance liquid chromatographic (HPLC) method was developed for the determination of rifampin in human plasma. Rifampin and sulindac (internal standard) are extracted from human plasma using a C2 Bond Elut extraction column. A 100-microliter volume of 0.1 M HCl is added to the plasma before extraction to increase the retention of the compounds on the extraction column. Methanol (1 ml) is used to elute the compounds and 0.5 ml of 3 mg/ml ascorbic acid in water is added to the final eluate to reduce the oxidation of rifampin. Separation is achieved by reversed-phase chromatography on a Zorbax Rx C8 column with a mobile phase composed of 0.05 M potassium dihydrogen phosphate-acetonitrile (55:45, v/v). Detection is by ultraviolet absorbance at 340 nm. The retention times of rifampin and internal standard are approximately 4.4 and 7.8 min, respectively. The assay is linear in concentration ranges of 50 to 35 000 ng/ml. The quantitation limit is 50 ng/ml. Both intra-day and inter-day accuracy and precision data showed good reproducibility.

Determination of rifabutin in human plasma by high-performance liquid chromatography with ultraviolet detection

A simple, specific and sensitive high-performance liquid chromatographic (HPLC) method was developed for the determination of rifabutin in human plasma. Rifabutin and sulindac (internal standard) are extracted from human plasma using a C8 Bond Elut extraction column. Methanol (1 ml) is used to elute the compounds. The methanol is dried down under nitrogen and reconstituted in 250 microliters of mobile phase. Separation is achieved by HPLC on a Zorbax Rx C8 column with a mobile phase composed of 0.05 M potassium dihydrogen phosphate and 0.05 M sodium acetate at pH 4.0-acetonitrile (53:47, v/v). Detection is by ultraviolet absorbance at 275 nm. The retention times of rifabutin and internal standard were approximately 10.8 and 6.9 min, respectively. The assay is linear over the concentration range of 5-600 ng/ml. The quantitation limit was 5 ng/ml. Both intra-day and inter-day accuracy and precision data showed good reproducibility.

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.

Automated high-performance liquid chromatographic method for the determination of rifampicin in plasma

Due to the unstable nature of rifampicin, a rapid automated high-performance liquid chromatographic method had to be developed for the analysis of a large number of plasma samples generated during a bioavailability trial. Extraction and injection of the samples were automatically done by a sample preparation system using C2, 100 mg Bond Elut extraction columns. The extracts were chromatographed on a 4-microns reversed-phase C18 column with a citrate buffer and acetonitrile as mobile phase. The analytes were detected at 342 nm. Calibration curves were linear to at least 20 micrograms/ml and the limit of quantification was 0.16 micrograms/ml.

A sensitive method for quantitation of rifabutin and its desacetyl metabolite in human biological fluids by high-performance liquid chromatography (HPLC)

Sensitive HPLC-UV methodology has been developed and validated for quantitating rifabutin, an antimycobacterial, and its 25-desacetyl metabolite, LM-565, in human plasma and urine. The HPLC separation for both plasma and urine samples was performed on an ODS, 5-microns, reverse-phase column (25 cm x 4.6-cm ID) using a mobile phase of acetonitrile/0.05 M potassium phosphate, pH 4.2, with triethylamine, (38:61.5:0.5, v/v), at a flow rate of 1.0 ml/min. The separation eluate was monitored by absorbance at 275 nm. Plasma samples (1 ml) were spiked with an internal standard (medazepam), buffered at pH 7.4 and extracted with 80:20 (v/v) hexane:ethyl acetate, and then back extracted with acidified water (0.05 M H3PO4). Linearity was established between 5.0-800 and 2.5-400 ng/ml for rifabutin and LM-565, respectively. Intraday imprecision for rifabutin and LM-565 plasma quality controls prepared at 7.3 and 3.2 ng/ml, respectively, was less than 15% relative standard deviation (RSD). Absolute recovery for parent drug and metabolite, from plasma, was greater than 90% throughout the respective dynamic ranges and greater than 70% for medazepam. Urine samples (1 ml) were acidified with 50 microliters of 3.6 M H2SO4 and diluted with 0.1 M ammonium acetate. Linearity was established between 100 and 5000 ng/ml for both rifabutin and LM-565. Intraday imprecision for a urine control at 200 ng/ml was less than or equal to 12% RSD for either component. The method is currently being used to support Phase I kinetics program for rifabutin in prophylaxis of MAC infection of AIDS patients. Application of this method to a bioavailability assessment is presented.