Search

The in vitro susceptibilities of baseline Mycobacterium avium complex (MAC) blood isolates from 86 patients with AIDS who were treated with clarithromycin, ethambutol, and rifabutin were determined to examine whether these results predict bacteriologic response to treatment. No patient received prior prophylaxis with clarithromycin or azithromycin. Minimum inhibitory concentrations (MICs) of clarithromycin for all isolates were < or = 2 micrograms/mL. The median MIC of rifabutin was between 0.25 and 0.5 microgram/mL, and all isolates were susceptible to < or = 2 micrograms of rifabutin/mL. The median MIC of ethambutol was 4 micrograms/mL, and the MIC90 was 8 micrograms/mL. There was no correlation between ethambutol susceptibility and subsequent bacteriologic clearance. At all time points through week 12, bacteriologic clearance occurred more frequently in patients with isolates for which MICs of rifabutin were lower, but this difference was statistically significant only at week 2. Susceptibility testing for baseline MAC isolates from AIDS patients not previously treated with clarithromycin or azithromycin does not appear to be useful in guiding therapy.

We compared the in-vitro antimycobacterial activities of rifabutin and KRM-1648, two rifamycin derivatives, with that of rifampicin against 163 strains of Mycobacterium tuberculosis. We also evaluated the correlation between the level of resistance to rifampicin, rifabutin and KRM-1648 and genetic alterations in the rpoB gene. All 82 strains susceptible to rifampicin or resistant to rifampicin with MICs < or = 16 mg/L were susceptible to rifabutin and KRM-1648 with MICs < or = 1 mg/L. Seventy-six of 81 strains resistant to rifampicin with MICs > or = 32 mg/L were resistant to both rifabutin and KRM-1648, but with lower MICs than those of rifampicin. KRM-1648 showed more potent antimycobacterial activity than rifabutin against organisms with low MICs (< or = 1 mg/L), while rifabutin was more active than KRM-1648 against organisms with high MICs (> or = 2 mg/L). A total of 96 genetic alterations around the 69 bp core region of the rpoB gene were detected in 92 strains. Alterations at codons 515, 521 and 533 in the rpoB gene did not influence the susceptibility to rifampicin, rifabutin and KRM-1648. Point mutations at codons 516 and 529, deletion at codon 518 and insertion at codon 514 influenced the susceptibility to rifampicin but not that to rifabutin or KRM-1648. With the exception of one strain, all alterations at codon 513 and 531 correlated with resistance to the three test drugs. The resistant phenotype of strains with an alteration at codon 526 depended on the type of amino acid substitution. Our results suggest that analysis of genetic alterations in the rpoB gene might be useful not only for predicting rifampicin susceptibility, but also for deciding when to use rifabutin for treating tuberculosis. Further studies may be required to determine the usefulness of KRM-1648.

This open-label, randomized, three-way crossover study of 28 healthy premenopausal women was conducted to compare the impact of concomitant rifabutin and rifampicin on the safety, pharmacokinetics, and pharmacodynamics of the oral contraceptives ethinylestradiol and norethindrone (Ortho-Novum 1/35; Ortho Pharmaceutical, Raritan, NJ). Each participant received oral contraceptives daily for 21 days for the first control cycle, then was randomized to one of two sequences to receive oral contraceptives with concomitant rifampicin and rifabutin at equal doses of 300 mg/day for 10 days. Ethinylestradiol, norethindrone, follicle stimulating hormone (FSH), luteinizing hormone (LH), progesterone, rifampicin, and rifabutin (and metabolite) were measured in plasma over the same time frames in all three cycles. Safety was assessed from before the beginning to the end of each cycle. Twenty-two subjects completed all three cycles. Compared with the control cycle, rifabutin and rifampicin significantly altered the disposition of the oral contraceptive. Area under the concentration-time curve from 0 to 24 hours (AUC0-24) and maximum plasma concentration (Cmax) of ethinylestradiol decreased by 64% and 42%, respectively, after coadministration with rifampicin and by 35% and 20%, respectively, after coadministration with rifabutin. The AUC0-24 of norethindrone decreased by 60% and 20% after coadministration with rifampicin and rifabutin, respectively. Unlike progesterone levels, FSH and LH levels increased during coadministration with rifampicin and rifabutin. The incidence of spotting was significantly higher after coadministration with rifampicin (36.4%) and rifabutin (21.7%) than during the control cycle (3.7%). Although both rifampicin and rifabutin affected the pharmacokinetics of ethinylestradiol and norethindrone, the magnitude of this effect was more pronounced with rifampicin. Likewise, the fact that the highest incidence of spotting occurred with rifampicin was consistent with higher metabolic induction by rifampicin. Despite the fact that there was no change in progesterone levels, it is recommended that patients be advised to use additional contraceptive methods while receiving rifabutin or rifampicin with oral contraceptives to prevent inadvertent pregnancy.

A 14-day, randomized, open, phase I clinical trial was designed to examine possible pharmacokinetic interactions between rifabutin and two other antibiotics, azithromycin and clarithromycin, used in the treatment of Mycobacterium avium complex infections. Thirty healthy male and female volunteers were divided into five groups of six participants each: 18 received 300 mg/day of rifabutin, 12 in combination with therapeutic doses of either azithromycin or clarithromycin; the remaining 12 received azithromycin or clarithromycin alone. On day 10 the study was terminated because of adverse events, including severe neutropenia. Fourteen participants who received rifabutin developed neutropenia, including all 12 participants who received azithromycin or clarithromycin concomitantly. Analyses of serum revealed no apparent pharmacokinetic interaction between azithromycin and rifabutin. However, the mean concentrations of rifabutin and 25-O-desacetyl-rifabutin (an active metabolite) in participants who received clarithromycin and rifabutin concomitantly were more than 400% and 3,700%, respectively, of concentrations in those who received rifabutin alone. Physicians should be aware that recommended prophylactic doses of rifabutin may be associated with severe neutropenia within 2 weeks after initiation of therapy, and all patients receiving rifabutin, especially with clarithromycin, should be monitored carefully for neutropenia.

Heat shock and other proteotoxic stresses cause accumulation of nonnative proteins that trigger activation of heat shock protein (Hsp) genes. A chaperone/Hsp functioning as repressor of heat shock transcription factor (HSF) could make activation of hsp genes dependent on protein unfolding. In a novel in vitro system, in which human HSF1 can be activated by nonnative protein, heat, and geldanamycin, addition of Hsp90 inhibits activation. Reduction of the level of Hsp90 but not of Hsp/c70, Hop, Hip, p23, CyP40, or Hsp40 dramatically activates HSF1. In vivo, geldanamycin activates HSF1 under conditions in which it is an Hsp90-specific reagent. Hsp90-containing HSF1 complex is present in the unstressed cell and dissociates during stress. We conclude that Hsp90, by itself and/or associated with multichaperone complexes, is a major repressor of HSF1.

1. The effect of a new rifamycin derivative, rifalazil (KRM-1648), on liver microsomal enzyme induction was studied in rat and dog with repeated oral administration of the compound. Relative liver weight, cytochrome b5 and P450 contents, enzyme activities of NADPH-cytochrome c reductase, aniline hydroxylase, p-nitroanisole O-demethylase, aminopyrine N-demethylase, and erythromycin N-demethylase were measured. 2. In rat, rifalazil treatment at 300 mg/kg/day for 10 days increased cytochrome b5 content but it did not affect liver weight, P450 content or enzyme activities. In contrast, rifampicin and rifabutin increased relative liver weights, cytochrome contents and enzyme activities under similar conditions. 3. In dog, rifalazil did not affect any parameters at 30 or 300 mg/kg/day for 13 weeks. 4. These findings indicate that rifalazil is not an enzyme inducer in rat and dog. This property differs from other rifamycin derivatives such as rifampicin and rifabutin.

Setting: University of Texas Medical Branch, Clinical Microbiology Laboratory., Objective: To evaluate in vitro the activity of rifabutin plus each of four other antimicrobial agents: isoniazid, amoxicillin/clavulanate, sparfloxacin and clarithromycin, against five susceptible (S-MTB) and five multidrug-resistant (MDR-TB) clinical isolates of Mycobacterium tuberculosis., Design: The antimicrobial activity of each two-drug combination was assessed by macrodilution checkerboard technique using Bactec 460 and classified as synergistic, additive, or antagonistic, using the fractional inhibitory concentration index., Results: For S-MTB, rifabutin + amoxicillin/clavulanate, rifabutin + isoniazid, rifabutin + sparfloxacin and rifabutin + clarithromycin acted synergistically against four, three, one and one isolate, respectively; clarithromycin + rifabutin was additive for one isolate. None of the combinations were antagonistic against S-MTB. For MDR-TB, rifabutin + amoxicillin/clavulanate, rifabutin + isoniazid, rifabutin + sparfloxacin and rifabutin + clarithromycin acted synergistically against one, one, one and two isolates, respectively, and additively against two, one, one and two isolates, respectively. Rifabutin + amoxicillin/clavulanate, rifabutin + isoniazid, and rifabutin + clarithromycin were antagonistic against one, two and two MDR-TB isolates, respectively., Conclusion: Certain drug combinations acted synergistically against MDR-TB; however, the clinical predictive value of these in vitro experiments is unknown.

When tested by agar dilution on Mueller-Hinton agar supplemented with 5% horse blood, the ketolides HMR 3004 and HMR 3647 were slightly more active (MIC at which 90% of the isolates were inhibited [MIC90], 0.03 microg/ml) against Bordetella pertussis than azithromycin, clarithromycin, erythromycin A, and roxithromycin. Azithromycin (MIC90, 0.06 microg/ml) was the most active compound against B. parapertussis. Rifampin and rifapentine were considerably less active.

We investigated the in vitro antifungal activity of amphotericin B, alone and in combination with rifabutin, an inhibitor of bacterial RNA polymerase, against 26 clinical isolates of Aspergillus and 25 clinical isolates of Fusarium. Synergy or additivism between these drugs was demonstrated against all isolates tested. Amphotericin B MICs were reduced upon combination with rifabutin from a mean of 0.65 microg/ml to a mean of 0.16 microg/ml against Aspergillus, and from a mean of 0.97 microg/ml to a mean of 0.39 microLg/ml against Fusarium (P < 0.000001 for both). Similarly, the MICs of rifabutin were reduced upon combination with amphotericin B from a mean of >32 microg/ml to a mean of 1.1 microg/ml against both fungi (P < 0.000001 for both). These positive interactions were corroborated by a colony count study with two Fusarium isolates, for which treatment with the combination of subinhibitory concentrations of amphotericin B (at concentrations 2- and 4-fold less than the MIC) and rifabutin (at concentrations ranging from 4- to 64-fold less than the MIC) resulted in 3.2-log reductions in colony counts compared to those after treatment with either drug alone. Inhibition of RNA synthesis was shown to be the mechanism of antifungal activity. These results suggest that inhibition of fungal RNA synthesis might be a potential target for antifungal therapy.

The postantibiotic effects (PAEs) of antimycobacterial agents determined with a BACTEC TB-460 instrument (CO2 production) and by a traditional viable-count method against Mycobacterium avium complex (MAC) were not significantly different (P > 0.05). The longest PAEs following a 2-h exposure to 2x the MIC were induced by amikacin (10.3 h), rifampin (9.7 h), and rifabutin (9.5 h), while the shortest PAEs resulted from clofazimine (1.7 h) and ethambutol (1.1 h) exposure. CO2 generation is a valid and efficient means of determining in vitro PAEs against MAC.

In our laboratory, primary human hepatocytes are being investigated as an in vitro experimental system for the evaluation of pharmacokinetic drug-drug interactions. Our study here represents the first reported study that directly compares the cytochrome P450 isozyme 3A (CYP3A) induction potential of three antimicrobials derived from rifamycin B, namely, rifampin, rifapentine and rifabutin. Two endpoints of CYP3A activity, testosterone 6 beta-hydroxylation and midazolam 1-hydroxylation have been used. Results obtained with hepatocytes from four different human donors show consistently that rifampin and rifapentine are potent inducers of CYP3A, while a significantly lower induction potential is observed for rifabutin. The relative induction potency of the three antimicrobials (rifampin > rifapentine >> rifabutin) is consistent with the available human in vivo data. For CYP1A measured as ethoxyresorufin O-deethylase activity, CYP2C8/9 measured as tolbutamide 4-hydroxylation activity, CYP2D6 measured as dextromethorphan O-demethylation, and AZT glucuronidation, there is either no effect or, where induction is found to be statistically significant in these other endpoints, the maximum induction values are consistently < 100% of the control. Our results suggest that CYP3A is the major CYP induced by these rifamycin B derivatives. These studies illustrate the application of human hepatocytes in the evaluation of the structure-activity relationships in CYP induction for this class of chemicals and as an in vitro screen for drug-drug interaction potential via CYP induction.

Setting: The activity of KRM 1648 (KRM), a new benzoxazinorifamycin, and rifabutin (RBT), alone or in combination with clarithromycin (CLA), was evaluated against Mycobacterium avium complex (MAC) that multiplied in human alveolar macrophages (AM)., Design: AM were recovered by bronchoalveolar lavage, incubated in RPMI 1640 medium with 10% human AB serum, infected with four strains of MAC (of non-acquired immune deficiency syndrome [AIDS] origin), and then treated with each drug alone or in combination. After incubation for 7 days, colony forming units in each well were counted on 7H10 agar., Results: Although concentrations between 0.2 microgram/ml and 20 micrograms/ml of both rifamycins showed clear dose-dependent activities against all MAC strains tested, only 20 micrograms/ml of each drug had modest bactericidal effect. In combination with 2.0 micrograms/ml of CLA, however, 0.2 microgram/ml of both drugs caused a bactericidal response against two of the four MAC strains examined., Conclusion: According to this human alveolar macrophage model of MAC infection, KRM and RBT in combination with CLA was found to be a promising candidate against human pulmonary MAC infection, and deserves clinical evaluation.

Mycobacteria Growth Indicator Tube (MGIT) is a recently introduced rapid growth detection method which uses an oxygen quenched fluorescent indicator. The present study evaluated the ability of this new method to determine the drug susceptibility of Mycobacterium avium complex (MAC). Thirty strains recovered from patients with AIDS were tested for susceptibility to clarithromycin, rifabutin, ciprofloxacin, azithromycin and amikacin using MGIT. Results were compared to susceptibilities determined by the agar dilution method. The results obtained showed a 100% correlation between MGIT and the agar dilution method for rifabutin and clarithromycin. There was a 100% correlation between the two methods for azithromycin against 27 strains. MGIT was well correlated with the agar dilution method for detecting resistance to clarithromycin, rifabutin and azithromycin in 4 days, but the correlation was poor when susceptibilities to ciprofloxacin and amikacin were determined. This rapid method is non-radiometric, noninvasive and does not require any special instruments.

Setting: Mycobacterium avium complex (MAC) isolates grown in the presence of clarithromycin, ciprofloxacin, ethambutol or rifabutin were examined by electron microscopy for drug-induced ultrastructural changes., Objective: To further the understanding of the activity of clarithromycin, ciprofloxacin, ethambutol and rifabutin against MAC., Design: Four MAC isolates, 1 control and 3 patient, were cultured for 24 or 72 hours in liquid medium containing one of two concentrations of an antimicrobial agent prior to examination by electron microscopy., Results: One or more of the following ultrastructural changes was observed after exposure to the antimicrobial agent: disorganized nucleoid and cytoplasm, condensation of nucleoid, vacuolization, enlargement of the periplasmic space with or without vesicles, intracytoplasmic lipid-like inclusions, and cell lysis. Changes were seen after as little as 24 hours of exposure to the antimicrobial agent and were more evident with the higher of the two concentrations of the drug. Changes were present regardless of the minimum inhibitory concentration (MIC) of the antimicrobial agent against the isolates tested, but were more pronounced if the MIC was less than the concentration of drug tested., Conclusion: Consistent ultrastructural changes were observed following exposure to a given antimicrobial agent. No drug-specific changes were identified.

The activity of rifabutin (LM 427) against Mycobacterium leprae was evaluated in armadillos inoculated earlier with human-derived M. leprae. Rifabutin was administered daily at a dose of 6 mg/kg body weight/day. The effect of rifabutin on M. leprae harvested from armadillos was determined by measuring the intracellular levels of ATP (an indicator of metabolic activity) of M. leprae and also their ability to multiply in the mouse footpads and in vitro in DH medium. Within 2 weeks of initiating the treatment, ATP levels declined to 21% of the original (pre-treatment level) and these M. leprae failed to multiply in the footpads of mice as well as in the in vitro culture system. This suggests that rifabutin was able to kill all M. leprae within 2 weeks. After 8 weeks the treatment was terminated and results showed that M. leprae from the treated armadillos remained non-viable in the mouse footpad system as well as in the in vitro system, indicating bactericidal action of rifabutin. The results suggest that rifabutin can be a substitute for rifampin in the leprosy multi-drug therapy regimen.

Treatment of SKBr3 human breast carcinoma cells with the benzoquinoid ansamycin, geldanamycin, rapidly depletes p185c-erbB-2 protein-tyrosine kinase. Loss of p185c-erbB-2 is initiated by disruption of a heteromeric complex between p185c-erbB-2 and the 94-kDa glucose-regulated protein, GRP94, to which geldanamycin binds avidly. Here we report that within minutes of exposure to geldanamycin, mature p185c-erbB-2 becomes polyubiquitinated. Treatment of cells with the specific proteasome proteolytic inhibitor, lactacystin, blocked geldanamycin-induced degradation of p185c-erbB-2 and enhanced the accumulation of polyubiquitinated p185c-erbB-2. Following geldanamycin and lactacystin treatment, a higher molecular weight form of p185c-erbB-2, which likely represents ubiquitin-p185c-erbB-2 conjugates, was detected by anti-p185c-erbB-2 immunoblotting. Nascent p185c-erbB-2 synthesized in the presence of geldanamycin is incompletely glycosylated and remains sequestered in the endoplasmic reticulum. While this immature form of the protein is not ubiquitinated in the presence of geldanamycin, its marked, drug-induced instability is nonetheless antagonized by lactacystin. Thus, the rapid depletion of mature p185c-erbB-2 caused by geldanamycin and the marked, drug-stimulated decrease in half-life of the newly synthesized protein are both mediated by the proteasome, although only the former phenomenon involves polyubiquitination.

Ansamycin antibiotics (1-4) were isolated from the cultured broth of Streptomyces sp. USF-319 strain as a result of our screening for free radical scavengers. They inhibited the bactericidal effect of the Fenton reagent toward Bacillus subtilis by their radical scavenging activity. Some of them also showed inhibitory activity against lipid peroxidation and lipoxygenases.

The efficacy of rifabutin (RIFA) alone or in combination with atovaquone (ATO) was examined in vitro and in a murine model of acute toxoplasmosis. In vitro studies were performed with MRC5 fibroblast tissue cultures, with quantification of Toxoplasma growth by enzyme-linked immunosorbent assay. For in vivo studies, mice were acutely infected with 10(4) tachyzoites of the virulent RH strain and were then treated perorally for 10 days from day 1 or day 4 postinfection. The efficacy of each drug regimen was assessed by determination of survival rates and sequential titration of parasites in blood, brain, and lungs by a tissue culture method. In vitro, RIFA was inhibitory for Toxoplasma growth at concentrations between 0.5 and 20 micrograms/ml; the 50% inhibitory concentration was estimated to be 1.68 micrograms/ml. When RIFA and ATO were combined, synergistic effects were noted for RIFA at 20 micrograms/ml combined with ATO at 0.01 or 0.02 microgram/ml and RIFA at 1, 2, or 5 micrograms/ml combined with ATO at 0.02 microgram/ml. In vivo, administration of RIFA at 200 mg/kg of body weight per day from day 1 to day 10 resulted in a 100% protection during treatment, with clearance of parasites from the blood, brain, and lungs. After the cessation of therapy, relapses occurred in the brain and lungs; the mortality was 46% at the end of the experiment (day 30). Among the mice treated with RIFA at 200 mg/kg/day from day 4 to day 14, no death was recorded during the treatment period and a marked reduction in parasite burdens was observed in blood and tissues; however, relapses occurred and 10% of mice survived until day 30. Administration of RIFA at 200 mg/kg/day in combination with ATO at 100 mg/kg/day resulted in a marked prolongation of survival compared with that for mice that received ATO or RIFA alone. However, in mice receiving the combination, parasite burdens in blood and organs were similar to those in mice treated with RIFA alone. These results confirmed the activity of RIFA in the treatment of acute toxoplasmosis and the potential of the combination of RIFA-ATO since the two drugs act synergistically against Toxoplasma gondii.

Rifampin and rifabutin induce the metabolism of many drugs, which may result in subtherapeutic concentrations and failure of therapy. However, differences between rifabutin and rifampin in potency of induction, and the specific enzymes which are altered, are not clear. This study, involving 12 adult male volunteers, compared the effects of 14-day courses of rifampin and rifabutin on clearance of theophylline, a substrate for the hepatic microsomal enzyme CYP1A2. Subjects were given oral theophylline solution (5 mg/kg of body weight) on day 1 and then randomized to receive daily rifampin (300 mg) or rifabutin (300 mg) on days 3 to 16. Theophylline was readministered as described above on day 15. The first treatment sequence was followed by a 2-week washout period; subjects then received the alternative treatment. Theophylline concentrations were determined for 46 h after each dose, and pharmacokinetic parameters were determined. One subject developed flu-like symptoms while taking rifabutin and withdrew voluntarily. Results from the remaining 11 subjects are reported. Compared with the baseline, the mean area under the concentration-time curve (AUC) (+/- standard deviation) for theophylline declined significantly following rifampin treatment (from 140 +/- 37 to 100 +/- 24 micrograms . h/ml, P <0.001); there was no significant change following rifabutin treatment (136 +/- 48 to 128 +/- 45 micrograms.h/ml). Baseline theophylline AUCs before each treatment phase were not different. A comparison of equal doses of rifampin and rifabutin administered to healthy volunteers for 2 weeks indicates that induction of CYP1A2, as measured by theophylline clearance, is significantly less following rifabutin treatment than it is following rifampin treatment. However, the relative induction potency for other metabolic enzymes remains to be investigated.

In vitro antimicrobial activities of the benzoxazinorifamycin derivative KRM-1648 (KRM) against 50 strains of Mycobacterium tuberculosis isolated from patients with mainly intractable pulmonary tuberculosis were studied. MIC90 values of KRM, rifabutin (RBT) and rifampicin (RFP) for RFP-sensitive strains (27 strains; defined as those with MICRFP values of < 1.56 micrograms/ml) were 0.013, 0.1 and 0.4 micrograms/ml, respectively, when determined by the agar dilution method using 7H11 medium. MIC90 values of KRM, RBT, and RFP for RFP-resistant strains (23 strains; defined as those having MICRFP values of > or = 1.56 micrograms/ml) were 100, 12.5 and > 100 micrograms/ml, respectively. MICs of KRM against 50 clinical isolates of M. tuberculosis distributed over a much lower range than those of RFP. KRM showed more potent antimicrobial activity than RBT against the organisms with low MIC values (< or = 1.56 micrograms/ml), while it was not so active as RBT against the organisms with high MIC values (> or = 3.13 micrograms/ml). Cross-resistance between KRM and RFP or RBT was observed for M. tuberculosis.

Important species differences have been reported concerning the induction properties of rifampicin towards enzymes of the P-450 superfamily. Mice, rabbits and humans are far more responsive than rats and guinea pigs. In the present study a strong induction of cytochrome P-450 3A-dependent enzyme activities was observed in female rat liver microsomes after high dose treatment (> or = 250 mg/kg/day for 9 days) with rifampicin, resulting in an up to 30-fold enhanced hydroxylation rate of testosterone in the 2 beta-, 6 beta- and 15 beta-position in vitro. Other cytochrome P-450 isozyme-selective reactions were not, or only marginally, affected. A steep increase in cytochrome P-450 3A activity on a moderate elevation of the dose administered, together with the previously observed lack of efficient induction with doses below 200 mg/kg/day demonstrated that there is a threshold in enzyme induction by rifampicin. For rifabutin such a threshold was not apparent. Induction by rifabutin showed an isoenzyme-selectivity profile similar to that produced by rifampicin, but the maximally achievable induction of cytochrome P-450 3A by rifabutin was about two-fold lower compared with rifampicin. Rifampicin and rifabutin enhanced the glucuronidation of 1-naphthol, 4-hydroxybiphenyl and beta-estradiol by a factor of two to three. The potential implications of the enzyme induction by rifampicin derivatives in terms of possible drug-drug interactions are discussed.

New ansamycins designated hydroxymycotrienins A and B were isolated from culture broths of Bacillus sp. BMJ958-62F4. The two antibiotics inhibited more strongly the growth of human cervical cancer cell lines of human papilloma virus (HPV) positive than that of HPV negative cell lines. The structures, some biological and biochemical properties are reported.

Rifabutin is a spiro-piperidyl-rifamycin derived from rifamycin-S. It is structurally related to rifampin and shares many of its properties. Rifabutin has a broad spectrum of antimicrobial activity. It is considerably more active than rifampin in vitro against the Mycobacterium avium complex (MAC), Mycobacterium tuberculosis, and Mycobacterium leprae. It also is active against most atypical mycobacteria, including Mycobacterium kansasii, but Mycobacterium chelonae is relatively resistant. Rifabutin also is active against staphylococci, group A streptococci, Neisseria gonorrhoeae, Neisseria meningitidis, Haemophilus influenzae, Haemophilus ducreyi, Campylobacter jejuni, Helicobacter pylori, Chlamydia trachomatis, and Toxoplasma gondii. It has poor activity against Enterobacteriaceae and Pseudomonas species. This review focuses on the antimicrobial profile of rifabutin in relation to its pharmacological properties. Special emphasis is placed on its in vitro activity against MAC and other mycobacteria, its efficacy in cell culture and animal models, and its potential as a component of multidrug therapy for mycobacterial and other infectious diseases.

Rifabutin, a macrophage-penetrating lipophilic rifamycin, has a long half-life and activity in vitro against Mycobacterium avium complex (MAC). Preliminary studies of patients with AIDS and those with AIDS-related complex defined dose-limiting toxic effects (arthralgia/arthritis and uveitis) and suggested that rifabutin alone might reduce the incidence of MAC infections. Two double-blind, randomized, placebo-controlled trials involving 1,100 subjects confirmed the safety and efficacy of low doses of rifabutin; this therapy halved the rate of MAC bacteremia and significantly reduced symptoms associated with disseminated MAC infections. Subsequent experience with rifabutin as prophylaxis for and treatment of MAC infections has generally supported its safety profile. Emergence of drug-resistant MAC causing infection in patients receiving rifabutin therapy has not been reported. Interactions with azoles and macrolides increase blood levels of rifabutin and have led to the development of uveitis in certain patients. Studies of the cost-effectiveness of rifabutin prophylaxis for MAC infection suggest that it is comparable with other preventive interventions employed in clinical practice.

Several European clinical trials of rifabutin alone or in multidrug regimens for treatment of infections due to Mycobacterium avium complex (MAC) in patients with AIDS are discussed. A prospective open study in France assessed 50 patients treated with the combination rifabutin/ isoniazid/ethambutol/clofazimine. Treatment was well tolerated, and the rate of conversion of cultures (from positive to negative) was 70% at 6 months. A randomized, double-blind, placebo-controlled, prospective trial (the NTMT01 trial) assessed the role of rifabutin in treatment with a combination containing clofazimine, isoniazid, and ethambutol. The culture conversion rate was 74% for the rifabutin group and 53% for the placebo group at the last valid observation for evaluable patients. In the ongoing Curavium study, patients are receiving clarithromycin plus either clofazimine or both rifabutin and ethambutol. Treatment of MAC disease in patients with AIDS requires a combination of drugs, and available results indicate that rifabutin can be used safely in multidrug regimens.

Disseminated infection with Microbacterium avium complex (MAC) in patients with AIDS is currently treated with a combination of antimycobacterial agents in order to prevent the selection of resistant mutant strains. Although clinical and microbiological responses can generally be achieved within a few weeks, relapses are common and require modification of the combination regimen or identification of effective alternate therapies. In this study we investigated the activities of rifabutin 0.5 mg/L, sparfloxacin 1 mg/L, clarithromycin 4 mg/L, amikacin 16 mg/L and ethambutol 2 mg/L, alone and in combination, against nine strains of M. avium isolated from the blood of patients with AIDS in order to identify regimens with the greatest therapeutic potential. Macrophages derived from human monocytes were infected with M. avium and inoculated with a single drug or a combination of drugs; cfu counts were performed at 0, 4 and 7 days after infection. At day 4 and at day 7, the combination of rifabutin, clarithromycin, amikacin and sparfloxacin displayed the highest degree of activity. However, the activity did not differ significantly from that of the combination of rifabutin, clarithromycin and ethambutol. The results of this study confirm the activity of combinations including rifabutin and clarithromycin (+/- ethambutol) in human monocyte-derived macrophages and suggest potentially useful associations in incorporating sparfloxacin and amikacin.

Typing of the glycopeptidolipid antigens performed by thin layer chromatography on 59 Mycobacterium avium-intracellulare (MAC) strains isolated in Italy from AIDS patients showed that the most frequent types were 1, 4, 3, 8, and 21 (24, 19, 14, 14 and 8% of the strains, respectively). Among non-AIDS patients, types 1, 4 and 8 were also frequently found. The antimicrobial susceptibility tested in agar and/or liquid media to a panel of drugs indicated in clofazimine and rifabutin effective agents against both AIDS and non-AIDS strains. The data obtained show that MAC type distribution in Italy appears to be different from that reported for other countries. No major differences in drug susceptibility between AIDS and non-AIDS related strains were found.

The antimicrobial effects of T9 (3-(4-cinamyl-1-piperazinyl)iminomethyl rifamycin SV) alone and in combination with ofloxacin, against strains of M. leprae were evaluated, using an in vitro cell-free culture system. The minimum inhibitory concentrations (MICs) of T9 against rifampin-sensitive and rifampin-resistant strains of M. leprae were 0.1 microgram/ml and 0.4 microgram/ml, respectively. Furthermore, in common with rifabutin, but not with rifamycin, T9 demonstrated synergy with ofloxacin against both rifampin-sensitive rifampin-resistant strains of M. leprae. The results suggest that T9, in combination with ofloxacin as part of multidrug regimens, warrants further evaluation as treatment for patients with leprosy.

Clarithromycin and rifabutin are among the most promising drugs for the therapy of infections caused by Mycobacterium avium or other atypical mycobacteria. Since synergism of combined drugs is important in order to achieve strong antimycobacterial activity, the combined inhibitory effects of antibacterial agents should also be investigated when agents are evaluated for possible use in antimycobacterial drug therapy. In the present study we examined the antimycobacterial activity of clarithromycin, rifabutin, and their combination against 51 clinical isolates of the M. avium complex from patients with acquired immune deficiency syndrome (AIDS) with disseminated mycobacteriosis. A concentration-dependent inhibition was seen for each drug. The antibacterial effect was significantly more pronounced for the combined drugs than for the agents tested separately. Synergism, against up to 88% of the strains tested, was seen for the tested drugs combined at different concentrations. All 51 M. avium strains were susceptible to the combination of 4 mg/l clarithromycin and 2 mg/l rifabutin.

Objective: To review rifabutin-associated uveitis and discuss the mechanism and potential role of drug interactions with clarithromycin and fluconazole in contributing to this adverse event., Data Sources: A MEDLINE search (1991 through September 1994) of English-language literature using the main MeSH headings "rifabutin" and "uveitis" and the subheadings "adverse effects" and "chemically induced." Relevant articles also were selected from references of identified articles. Abstracts from recent medical conferences of infectious diseases, pharmacology, and HIV were screened for additional data., Study Selection and Data Extraction: All articles and abstracts reporting uveitis potentially related to rifabutin were considered for inclusion. Fifty-four cases were identified. Pertinent information from the case reports, as judged by the authors, was selected and synthesized for discussion., Data Synthesis: Rifabutin is being prescribed increasingly for the treatment and prophylaxis of Mycobacterium avium complex (MAC) infection in the HIV-infected population. Uveitis was initially thought to be a rare, dose-limited complication of rifabutin therapy. In an early dose-ranging tolerance study, uveitis was associated with daily doses of 1200 mg or more. Because this toxicity appeared to be dose-related, lower dosages (300-600 mg/d) of rifabutin were selected for study in subsequent clinical trials. More recent reports noting the association of uveitis with these lower dosages of rifabutin have raised concerns about the prevalence of this adverse event. In the 54 identified cases, patients presented with symptoms of unilateral or bilateral uveitis from 2 weeks to more than 7 months following initiation of rifabutin therapy. In all reported cases, patients were receiving concurrent therapy with clarithromycin and/or fluconazole, both of which have inhibitory effects on rifabutin metabolism. In most cases, uveitis resolved within 1-2 months following discontinuation of rifabutin with or without administration of topical corticosteroids., Conclusions: Rifabutin is prescribed frequently for the prophylaxis and treatment of MAC infection, especially in patients with HIV. Uveitis is a rare, dose-related toxicity of this therapy. The risk of rifabutin-associated uveitis may be increased in patients receiving concurrent therapy with clarithromycin or fluconazole because of drug interactions. Patients receiving therapy with combinations of any of these agents should be warned about signs and symptoms of uveitis and be monitored closely for the development of rifabutin toxicity. If uveitis develops, rifabutin therapy should be discontinued promptly.

We investigated the effects of rifabutin (300 mg daily administered for 7 or 14 days) on the pharmacokinetics of zidovudine in nine patients who were infected with human immunodeficiency virus (HIV). Serial blood and urine samples were collected over a 6-hour period on each day that the pharmacokinetics of zidovudine were studied. Pharmacokinetic parameters were determined for zidovudine and its glucuronide metabolite and compared with use of analysis of variance (ANOVA) appropriate for a repeated-measures design. Except for a statistically significant decrease (28%) in the terminal half-life of zidovudine from 1.5 to 1.1 hours (P = .005) after coadministration of both agents for 14 days, concurrent administration of rifabutin for 7 or 14 days had no statistically significant effects on zidovudine plasma and urine pharmacokinetic parameters (the difference among treatment means was < 25%). Treatment with rifabutin is unlikely to influence the effectiveness of treating HIV-infected patients with zidovudine because of any pharmacokinetic interaction between these drugs.

Rifabutin, a spiropiperidyl derivative of rifampicin, is approved for the prophylaxis of Mycobacterium avium infections in AIDS patients in the US, and for the treatment of M. avium infections, tuberculosis and multiple drug resistant tuberculosis in many countries. In the present study, rifabutin was compared with rifampicin for its activity against drug susceptible and multi-drug resistant tubercle bacilli by several in-vitro and macrophage studies. Rifabutin exhibited similar or greater in-vitro activity than rifampicin as judged by the minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC) and MBC/MIC ratios, as well as continuous exposure and post-antibiotic effect studies. Rifabutin has been shown to be active against some multiple drug resistant strains which were resistant to rifampicin. In macrophage studies with continuous exposure to the drug or when the drug had been removed after 24 h, rifabutin also demonstrated high activity which was better than RMP against intracellular tubercle bacilli. This long-acting intracellular anti-mycobacterial activity may explain, in part, the clinical efficacy of rifabutin.

Mycobacterium xenopi is an opportunistic pathogen, frequently isolated in various areas of Europe from pulmonary specimens, which may also cause infections in AIDS patients. We used the Bactec radiometric system (Becton Dickinson, USA) with a procedure expressly adapted to the particular growth characteristics of M. xenopi to determine the susceptibility patterns of 40 clinical isolates to six antimicrobial drugs. The majority of the strains were resistant to ethambutol and susceptible to amikacin, ciprofloxacin, rifampin, rifabutin and streptomycin.

The activities of four newly synthesized benzoxazinorifamycin derivatives, either alone or in combination with ofloxacin, against strains of Mycobacterium leprae were determined by assessing their effects on two biochemical parameters of metabolic activity which served as surrogate markers for growth in vitro. KRM-1648 and KRM-2312 were the most active agents tested against both a rifampicin-susceptible isolate (MICs of 0.05 and 0.1 mg/L respectively) and a rifampicin-resistant isolate (MICs of 0.2 and 0.3 mg/L respectively); both compounds were more active than either rifampicin or rifabutin. The activities of the two other derivatives, KRM-1657 and KRM-1668, against a rifampicin- susceptible strain (MICs of 0.3 mg/L) were similar to that of rifampicin, while the MIC of each of these agents for the rifampicin-resistant strain was 1.0mg/L. In common with rifabutin, both of the more active derivatives demonstrated synergy with ofloxacin against the rifampicin-susceptible isolates. The results of this study suggest that these compounds, in combination with ofloxacin as part of multidrug regimens, warrant further evaluation as treatment for patients with leprosy.

The in vitro activity of dirithromycin alone and in combination with clofazimin, ethambutol and rifabutin was tested against thirty strains of Mycobacterium avium isolated from patients. Extracellular activity of dirithromycin was assessed by determining MICs using the radiometric methodology in 7H12 broth at two pHs 6.8 and 7.4. The MICs obtained at pH 7.4 were 3 to 4 more dilutions lower than those obtained at pH 6.8. Activity of pairs of antibiotics was measured using the FIC indices. Dirithromycin-clofazimin combination demonstrated the most important additive effects and even produced synergic effect against 5 of 30 strains. Studies of intracellular bacteria showed that the most effective bactericidal combination was dirithromycin, clofazimin and ethambutol together.

Sixteen weeks of treatment with clarithromycin (CLARI) alone displayed significant bactericidal activity against Mycobacterium avium complex infection in beige mice. Only two combined regimens, CLARI combined with an initial 4 or 8 weeks of amikacin (AMIKA), displayed activity greater than that displayed by CLARI alone. Four other combined regimens, CLARI combined with ethambutol (EMB), rifabutin (RBT), or both EMB and RBT during the entire 16 weeks of treatment or with AMIKA administered in an initial 2-week course showed bactericidal activity not significantly greater than that of CLARI alone. After 16 weeks of treatment, CLARI-resistant mutants were isolated from the majority of mice that had been treated with CLARI alone, CLARI-RBT, CLARI-EMB, or CLARI-EMB-RBT, as was the case for untreated controls, but the frequencies of occurrence of mutants were significantly greater in the groups treated with these combinations or CLARI alone. On the other hand, no CLARI-resistant mutants were isolated from the mice that had been treated with the combination of CLARI plus an initial 4 or 8 weeks of AMIKA and were isolated from only a tiny proportion of mice that had been treated with CLARI plus an initial 2 weeks of AMIKA. Therefore, only treatment with CLARI combined with an initial 4 or 8 weeks of AMIKA but not combined with RBT or EMB or both, could enhance the activity of the drug treatment and prevent the selection of CLARI-resistant mutants.

The combined effect of clarithromycin and rifabutin against Mycobacterium avium multiplying either within human monocyte-derived macrophages or extracellularly in a liquid medium was additive: both MICs and MBCs were twofold lower for the combination than they were for each drug alone. Prolonged exposure for 4 weeks of M. avium-infected macrophages to combined concentrations that were only twofold greater than the MICs resulted in a 100-fold decrease in the number of viable bacteria, while in the drug-free controls a 100-fold or greater increase in comparison with the initial viable counts took place. Comparison of this effect with the results of the prolonged exposure to each drug alone suggested that under these experimental conditions rifabutin enhanced the antimicrobial activity of clarithromycin against intracellular bacteria. At the same time, inhibition of intracellular growth by a 2-h pulsed exposure of the infected macrophages to the combination of the two drugs was not different from the effect induced by clarithromycin alone. In conclusion, clarithromycin played the major role in the antimicrobial activity of the tested combination, while rifabutin may have enhanced this effect during a prolonged exposure of the intracellular bacteria to these two agents.

Animal studies have demonstrated that the mouse and rabbit are far more responsive to the inductive properties of rifamycin derivatives than the rat and guinea pig. The rat hepatic cytochrome P450 system seems to be resistant to the action of rifampicin unless very high doses are used. Mouse hepatic microsomal mixed-function oxidase activity is markedly increased by repeated dosing with rifampicin, whereas administration of rifabutin may be ineffective. In humans, both rifampicin and rifabutin are extensively metabolized and induce their own metabolism. The induced metabolic pathways remain essentially unknown. Under autoinduction conditions, the elimination half-life of rifampicin decreases, whereas that of rifabutin is not altered. Although the effects of repeated administration of rifampicin and rifabutin on the various forms of cytochrome P450 in humans have not been extensively examined, there is convincing evidence that the P4503A subfamily is induced by either drug, whereas the P4501A subfamily and P4502D6 do not appear to be affected by rifampicin. Limited reliable information is available concerning the induction of human glucuronyltransferase activities by rifampicin and rifabutin which, however, do not seem to influence zidovudine glucuronide formation in healthy subjects.

Even though the macrophage is the host cell for the intracellular bacterial parasite Mycobacterium avium, macrophages have undergone only limited evaluation as models for determining the capacities of antimycobacterial drugs to inhibit the growth of M. avium within this relevant intracellular environment. In the present study, we demonstrated that a panel of M. avium isolates could actively infect homogeneous monolayers of murine bone marrow-derived macrophages. A number of established and experimental antimycobacterial drugs were then added to these cultures at a range of concentrations, and their effects on the numbers of surviving bacilli were determined 8 days later. By plotting such numbers versus drug concentrations it was then possible to clearly distinguish between compounds with bactericidal activity (such as rifabutin and PD 125354) and those with bacteriostatic effects (such as clarithromycin), even though several of these compounds had very similar MICs. In addition, an estimate of the potential therapeutic efficiency of each drug could be made by determining the concentration needed to destroy an arbitrary percentage of the inoculum (in this case, the bactericidal concentration destroying 99% of the inoculum). Such values were considerably in excess of the MICs and may more realistically reflect the concentrations in serum required to effectively reduce the bacterial burden in vivo.

The activity of rifabutin against Toxoplasma gondii was investigated in vitro and in vivo. One set of in-vitro studies used either a low virulence or a high virulence T. gondii strain subcultured in drug-free conditions after exposure to various drug concentrations. No parasite growth was observed after subculture at 1 month after exposure to 6 mg/L of rifabutin for the low virulence and 12 mg/L for the high virulence strain. The IC50 of rifabutin was 26.5 mg/L in a different series of experiments using an enzyme-linked immunoassay) and the T. gondii high virulence strain. In similar experiments with clarithromycin, low virulence and high virulence parasites grew in drug-free subcultures following exposure to drug concentrations as high as 100 mg/L; the IC50 of clarithromycin exceeded 100 mg/L for the high virulence strain. In the acute toxoplasmosis murine model, mice received a 12 day treatment starting 5 days before infection with high virulence parasites. Survival was significantly improved compared with untreated controls in response to 50, 100 and 200 mg/kg/day rifabutin and 100 and 200 (but not 50) mg/kg/day clarithromycin, and with the combination of the two drugs at 25, 50 and 100 mg/kg/day. Survival was significantly improved when combination therapy was administered. The calculated ED50 values (mg/kg/day) were 160.5 for rifabutin, 119.4 for clarithromycin, and 114.8 for the combination. In the present experimental conditions rifabutin proved effective in vitro and in vivo against T. gondii and showed potentiation with clarithromycin in vivo.

Objective: To determine the effect of a delay in processing blood specimens, that may occur by mailing them to a specialized laboratory, on the quantitation of the number of mycobacteria., Design: Simulated blood specimens in sodium polyanetholium sulfonate (SPS)-containing vacutainers were spiked with Mycobacterium avium, and stored for 4 days after addition of clarithromycin or rifabutin, or remained drug-free. Daily samples were lysed with sodium desoxycholate, and inoculated onto 7H11 agar plates for the subsequent colony-forming units (CFU)/ml count. The isolation rates on two media, 7H11 agar and BACTEC 12B broth, were determined with 1212 specimens from AIDS patients., Results: The number of bacteria in blood specimens remained without substantial change during the period of storage, even in the presence of clarithromycin or rifabutin, especially after refrigeration for 2 or 24 h before storage. 7H11 agar and 12B broth were equally sensitive for isolation of M. avium., Conclusions: A 4-day delay in processing blood specimens in SPS vacutainers does not affect the results of quantitation of the number of mycobacteria. Combined use of 12B broth and 7H11 agar increased the overall recovery along with an opportunity to quantitate the number of CFU/ml.

Standard short-course regimens for tuberculosis are used worldwide with very few problems. Unfortunately, the emergence of multiple drug-resistant tuberculosis in many parts of the world is leading to a diversification of drug regimens and to the use of drugs that are more toxic per se and more likely to interact with others. In addition, the treatment of HIV/AIDS patients with tuberculosis or disease due to Mycobacterium avium-intracellulare complex (MAC) infection with new drugs and multidrug regimens has added to the problem of drug interactions, especially as such patients may often be receiving concomitant treatment for a range of bacterial, fungal and viral infections. In general, there are very few clinically significant interactions between the first-line antituberculosis drugs themselves, although problems of bioavailability, notably of rifampicin (rifampin), have been encountered in the manufacture of combination tablets. Of the first-line drugs used to treat tuberculosis, i.e. rifampicin, isoniazid and pyrazinamide, rifampicin is particularly likely to cause clinically significant drug interactions as it is a potent inducer of the cytochrome P450 enzyme group, which is involved in the metabolism of many drugs, in particular oral contraceptives, corticosteroids, oral anticoagulants and cyclosproin. The use of quinolones to treat multiple drug-resistant tuberculosis and AIDS-related MAC disease raises further problems of drug interactions as, in contrast to rifampicin, these drugs inhibit some cytochrome isoenzymes, leading to reduced metabolism of certain drugs.

The ever-increasing incidence of tuberculosis calls for the implementation of control measures, including new efficient, short-term preventive therapies to replace 6 to 12 months of isoniazid therapy. The efficacies of 12-week regimens of rifabutin or isoniazid given daily and the combination of the two drugs administered intermittently were evaluated in mice infected with Mycobacterium tuberculosis after vaccination with the bacillus Calmette-Guérin (BCG) to imitate some features of the natural infection in humans with a low number of persisting bacteria. Rifabutin at 10 mg/kg of body weight per day was highly effective as early as the eighth week of treatment: all spleens were sterilized and the number of bacteria was drastically reduced in the lungs (mean +/- standard deviation log CFU, 0.2 +/- 0.3, compared with 5.9 +/- 0.6 for untreated controls). No bacilli were found in the spleens or lungs of any of the animals treated for 12 weeks. The combination of rifabutin at 10 mg/kg plus isoniazid at 25 mg/kg twice weekly was almost as effective as rifabutin daily: after 8 weeks of treatment only two of six mice harbored a small number of mycobacteria in their spleens and lungs; at week 12, all spleens were sterilized and a total of eight colonies were isolated from the lungs of two of six mice. Daily isoniazid and once-weekly rifabutin plus isoniazid therapies were less effective. Colonies randomly isolated from the spleens and lungs of mice from different experimental groups were also tested for their susceptibilities to the two drugs. The three surviving colonies from rifabutin-treated mice and all colonies from those administered rifabutin plus isoniazid remained fully susceptible to either drug. In contrast, 2 (18%) of the 11 colonies randomly selected from isoniazid-treated mice became resistant to isoniazid (MIC, > 2 micrograms/ml), although they were still susceptible to rifabutin. Three months of treatment with rifabutin, either daily alone or twice a week combined with isoniazid, proved to be a valid candidate for tuberculosis preventive therapy.