Development potential of rifalazil

A Comparative Insight on the Newly Emerging Rifamycins: Rifametane, Rifalazil, TNP-2092 and TNP-2198

Rifamycins are considered a milestone for tuberculosis (TB) treatment because of their proficient sterilizing ability. Currently, available TB treatments are complicated and need a long duration, which ultimately leads to failure of patient compliance. Some new rifamycin derivatives, i.e., rifametane, TNP-2092 (rifamycin-quinolizinonehybrid), and TNP-2198 (rifamycin-nitromidazole hybrid) are under clinical trials, which are attempting to overcome the problems associated with TB treatment. The undertaken review is intended to compare the pharmacokinetics, pharmacodynamics and safety profiles of these rifamycins, including rifalazil, another derivative terminated in phase II trials, and already approved rifamycins. The emerging resistance of microbes is an imperative consideration associated with antibiotics. Resistance development potential of microbial strains against rifamycins and an overview of chemistry, as well as structure-activity relationship (SAR) of rifamycins, are briefly described. Moreover, issues associated with rifamycins are discussed as well. We expect that newly emerging rifamycins shall appear as potential tools for TB treatment in the near future.

Approaches to the management of uncomplicated genital Chlamydia trachomatis infections

Genital chlamydial infection remains a highly prevalent sexually transmitted infection in the USA. A multifaceted approach to the management of chlamydial infection is essential to ensure cure and prevention of reinfection. This article will review current approaches to the management of uncomplicated genital chlamydial infection, discussing: the pathogen; antimicrobials that are and are not recommended for therapy by the US Centers for Disease Control and Prevention; partner treatment; follow-up; antimicrobial resistance; and potential future therapies.

New advances in the treatment of Clostridium difficile infection (CDI)

Clostridium difficile infections (CDI) have increased in frequency throughout the world. In addition to an increase in frequency, recent CDI epidemics have been linked to a hypervirulent C. difficile strain resulting in greater severity of disease. Although most mild to moderate cases of CDI continue to respond to metronidazole or vancomycin, refractory and recurrent cases of CDI may require alternative therapies. This review provides a brief overview of CDI and summarizes studies involving alternative antibiotics, toxin binders, probiotics, and immunological therapies that can be considered for treatment of acute and recurrent CDI in severe and refractory situations.

New agents for Clostridium difficile-associated disease

BACKGROUND

Clostridia-derived diseases, in particular C. difficile-associated disease (CDAD), have been increasing in incidence, severity, and morbidity. The mainstay of treatment options has relied upon metronidazole and vancomycin, but these treatments routinely result in high relapse rates (20%) and, in the case of metronidazole, decreasing efficacy.

OBJECTIVE

Evaluate and compare the current clinical and preclinical therapies of CDAD.

METHODS

RESULTS/CONCLUSION

The new antibiotics in development and preclinical development reflect next-generation versions of older drugs or two new mechanism-of-action class drugs (OPT-80, REP3123). Based on the current preclinical and clinical data, the next-generation drugs impart only a subtle difference from the intrinsic weaknesses of their genre. In contrast, OPT-80 and REP3123 seem to be differentiated.

Rifalazil retains activity against rifampin-resistant mutants of Chlamydia pneumoniae

Rifampin-resistant mutants of the obligate intracellular pathogen Chlamydia pneumoniae were isolated and characterized, including strains that contained multiple mutations in the rpoB gene encoding the rifampin binding site. The highest MIC of rifampin against a mutant strain exceeded 100 microg/ml, whereas the highest MIC of rifalazil was 0.125 microg/ml. Derivatives of rifalazil (new chemical entities; NCEs) showed from 2 approximately 4 fold lower MICs, as well as 2 approximately 8 fold lower bactericidal concentrations against both wild type and mutant strains when compared with rifalazil. These results suggest that rifalazil and NCEs are appropriate therapeutic agents for the treatment of C. pneumoniae infections from the point of view of potency and resistance development.

Extractive biotransformation for production of metabolites of poorly soluble compounds: synthesis of 32-hydroxy-rifalazil

A novel reaction system was developed for the production of metabolites of poorly water-soluble parent compounds using mammalian liver microsomes. The system includes the selection and use of an appropriate hydrophobic polymeric resin as a reservoir for the hydrophobic parent compounds and its metabolites. The utility of the extractive biotransformation approach was shown for the production of a low-yielding, synthetically challenging 32-hydroxylated metabolite of the antibiotic rifalazil using mouse liver microsomes. To address the low solubility and reactivity of rifalazil in the predominantly aqueous microsomal catalytic system, a variety of strategies were tested for the enhanced delivery of hydrophobic substrates, including the addition of mild detergents, polyvinylpyrrolidone, glycerol, bovine serum albumin, and hydrophobic polymeric resins. The latter strategy was identified as the most suitable for the production of 32-hydroxy-rifalazil, resulting in up to 13-fold enhancement of the volumetric productivity compared with the standard aqueous system operating at the solubility limit of rifalazil. The production process was optimized for a wide range of reaction parameters; the most important for improving volumetric productivity included the type and amount of the polymeric resin, cofactor recycling system, concentrations of the biocatalyst and rifalazil, reaction temperature, and agitation rate. The optimized extractive biotransformation system was used to synthesize 32-hydroxy-rifalazil on a multimilligram scale.

Management of multidrug-resistant tuberculosis: Update 2007

Multidrug-resistant tuberculosis (MDR-TB) with bacillary resistance to at least isoniazid and rifampicin in vitro is a worldwide phenomenon. Hot spots of the disease are found scattered in different continents. Prevention of its development through good tuberculosis control programmes operating under the directly observed therapy, short-course (DOTS) strategy is of paramount importance. However, with established MDR-TB, treatment with alternative and specific chemotherapy is necessary to achieve a beneficial outcome. Such an approach on a programme basis is currently known as the 'DOTS-Plus' strategy. Second-line (reserve) drugs utilized in the treatment of MDR-TB are generally less potent and more toxic, perhaps with the notable exceptions of some fluoroquinolones and injectable agents. Surgery has a distinct adjunctive role for the management of MDR-TB in selected patients. The emergence of extensively drug-resistant tuberculosis (XDR-TB), that is, MDR-TB with additional bacillary resistance to the fluoroquinolones and injectables, has provided a very alarming challenge to global health, as the disease currently has a low cure rate and high mortality. In order to combat XDR-TB, strengthening of DOTS and DOTS-Plus programmes is mandatory, especially in the face of surging HIV infection. Furthermore, more attention needs to be focused on developing new drugs with potent bactericidal and sterilizing activities and low side-effects, and above all, drugs that are affordable for communities worldwide.

Preparation and in vitro anti-staphylococcal activity of novel 11-deoxy-11-hydroxyiminorifamycins

We report herein the preparation and anti-staphylococcal activity of a series of novel 11-deoxy-11-hydroxyiminorifamycins. Many of the compounds synthesized exhibit potent activity against wild-type Staphylococcus aureus with MICs equivalent to, or better than, rifamycin reference agents. In addition, some of the compounds retain potent activity against an intermediate rifamycin-resistant strain of Staphylococcus aureus. For instance, compound 5k exhibits an MIC of 0.12 microg/mL against an intermediate rifamycin-resistant strain, while the rifamycin reference agents, rifampin and rifalazil, exhibit MICs of 16 microg/mL and 2 microg/mL, respectively, against the same strain.

Rifalazil and Other Benzoxazinorifamycins in the Treatment of Chlamydia-Based Persistent Infections

Rifalazil is a benzoxazinorifamycin which inhibits bacterial DNA-dependent RNA polymerase. The benzoxazine ring endows benzoxazinorifamycins with unique physical and chemical characteristics which favor the use of rifalazil and derivatives in treating diseases caused by the obligate intracellular pathogens of the genus chlamydia. Minimal inhibitory concentrations of benzoxazinorifamycins against chlamydia are in the pg/mL range. These compounds have potential as monotherapeutic agents to treat chlamydia-associated disease because they retain activity against chlamydia strains resistant to currently approved rifamycins such as rifampin. A pivotal clinical trial with rifalazil has been initiated for the treatment of peripheral arterial disease. The rationale for this innovative use of rifalazil, including the association of C. pneumoniae in atherosclerotic plaque formation, as well as rifalazil's potency and efficacy against chlamydia in both preclinical and clinical studies, is discussed. Other benzoxazino derivatives may have utility as stand-alone topical antibacterials or combination antibacterials to treat serious Gram-positive infections. None of the benzoxazinorifamycins examined to date induce the cytochrome P450 3A4 enzyme. This is in contrast to currently approved rifamycins which are strong inducers of P450 enzymes, resulting in drug-drug interactions that limit the clinical utility of this drug class.

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

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

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

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

Efficacy of a novel rifamycin derivative, ABI-0043, against Staphylococcus aureus in an experimental model of foreign-body infection

We compared the efficacy of a novel rifamycin derivative, ABI-0043, with that of rifampin, alone and in combination with levofloxacin, against methicillin-susceptible Staphylococcus aureus ATCC 29213 in a guinea pig tissue-cage infection model. The MIC, logarithmic-growth-phase minimal bactericidal concentration, and stationary-growth-phase minimal bactericidal concentration of ABI-0043 were 0.001, 0.008, and 0.25 microg/ml, respectively; the corresponding concentrations of rifampin were 0.016, 0.8, and 3.6 microg/ml, respectively. After a single intraperitoneal dose of 12.5 mg/kg of body weight, the peak concentration in cage fluid was 1.13 micarog/ml of ABI-0043 and 0.98 microg/ml of rifampin. Five days after completion of treatment, levofloxacin administered alone (5 mg/kg/12 h) resulted in bacterial counts in cage fluid that were similar to those for untreated controls (>8.0 log(10) CFU/ml), whereas rifampin and ABI-0043 administered alone (12.5 mg/kg/12 h) decreased the mean titers of bacteria +/- standard deviations to 1.43 +/- 0.28 log(10) and 1.57 +/- 0.53 log(10) CFU/ml, respectively, in cage fluid. In combination with levofloxacin, both rifamycins cleared bacteria from the cage fluid. The cure rates of cage-associated infections with rifampin and ABI-0043 administered alone were 46% and 58%, respectively, and increased to 88% and 92% in combination with levofloxacin. Emergence of rifamycin resistance was observed in 42% of cages after ABI-0043 therapy and in 38% of cages after rifampin therapy; no emergence of resistance occurred with combination treatment with levofloxacin. In conclusion, ABI-0043 had cure rates comparable to that of rifampin. ABI-0043 in combination with a quinolone has the potential for treatment of implant-associated infections caused by susceptible strains of S. aureus, potentially without drug-drug interactions.

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

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

Progress in targeting bacterial transcription

The bacterial RNA polymerase (RNAP) is an essential enzyme that is responsible for making RNA from a DNA template and is targeted by several antibiotics. Rifampicin was the first of such antibiotics to be described and is one of the most efficient anti-tuberculosis drugs in use. In the past five years, structural studies of bacterial RNAP and the resolution of several complexes of drugs bound to RNAP subunits have revealed molecular details of the drug-binding sites and the mechanism of drug action. This knowledge opens avenues for the development of antibiotics. Here these drugs are reviewed, together with their mechanisms and their potential interest for therapeutic applications.

New rifabutin analogs: Synthesis and biological activity against Mycobacterium tuberculosis

The synthesis, structure, and biological evaluation of a series of novel rifamycin derivatives, Rifastures (RFA) with potent anti-tuberculosis activity are presented. Some of these derivatives showed higher in vitro activity than rifabutin and rifampicin against not only Mycobacterium tuberculosis strains but also against MAC and Mycobacterium kansasii.

Efficacy of novel rifamycin derivatives against rifamycin-sensitive and -resistant Staphylococcus aureus isolates in murine models of infection

Novel rifamycins (new chemical entities [NCEs]) having MICs of 0.002 to 0.03 microg/ml against Staphylococcus aureus and retaining some activity against rifampin-resistant mutants were tested for in vivo efficacy against susceptible and rifampin-resistant strains of S. aureus. Rifalazil and rifampin had a 50% effective dose (ED50) of 0.06 mg/kg of body weight when administered as a single intravenous (i.v.) dose in a murine septicemia model against a susceptible strain of S. aureus. The majority of NCEs showed efficacy at a lower i.v. dose (0.003 to 0.06 mg/kg). In addition, half of the NCEs tested for oral efficacy had ED50s in the range of 0.015 to 0.13 mg/kg, i.e., lower or equivalent to the oral ED50s of rifampin and rifalazil. NCEs were also tested in the septicemia model against a rifampin-resistant strain of S. aureus. Twenty-four of 169 NCEs were efficacious when administered as a single oral dose of 80 mg/kg. These NCEs were examined in the murine thigh infection model against a susceptible strain of S. aureus. Several NCEs dosed by intraperitoneal injection at 0.06 mg/kg caused a significant difference in bacterial titer compared with placebo-treated animals. No NCEs showed efficacy in the thigh model against a highly rifampin-resistant strain. However, several NCEs showed an effect when tested against a partially rifampin-resistant strain. The NCEs having a 25-hydroxyl moiety were more effective as a group than their 25-O-acetyl counterparts. These model systems defined candidate NCEs as components of potential combination therapies to treat systemic infections or as monotherapeutic agents for topical applications.

Development potential of rifalazil and other benzoxazinorifamycins

Rifalazil and other benzoxazinorifamycins (new chemical entities [NCEs]) are rifamycins that contain a distinct planar benzoxazine ring. Rifalazil has excellent antibacterial activity, high intracellular levels and high tissue penetration, which are attributes that favour its use in treating diseases caused by the obligate intracellular pathogens of the genus Chlamydia. Recent studies have shown that rifalazil has efficacy in the treatment of human sexually transmitted disease caused by Chlamydia trachomatis. The extraordinary potency of rifalazil and other NCEs, such as ABI-0043, extends to the related microorganism, C. pneumoniae, a respiratory pathogen that can disseminate and persist chronically in the vasculature, resulting in increased plaque formation in animal studies. A pivotal clinical trial with rifalazil has been initiated for the treatment of peripheral arterial disease. Other opportunities include gastric ulcer disease caused by Helicobacter pylori and antibiotic-associated colitis caused by infection with Clostridium difficile in the colon. The NCEs could prove to be valuable as follow-on compounds in these indications, as rifampin replacements in antibacterial combination therapy or as stand-alone topical antibacterials (e.g., to treat acne). Neither rifalazil nor NCEs appear to induce the cytochrome P450 3A4, an attribute of rifampin that can result in adverse events due to drug-drug interactions.

Enhanced Activity of Rifalazil in Combination with Levofloxacin, Linezolid, or Mupirocin against Staphylococcus aureus In Vitro

Rifalazil is a potent second-generation ansamycin that kills bacterial cells by inhibiting the beta subunit of RNA polymerase. Rifalazil has several improved properties compared with rifampicin, but retains rifampicin's propensity to develop resistant mutants at high frequency. To explore strategies to overcome resistance development, we studied the effects of rifalazil in combination with several different antibiotics in an in vitro time-kill model, against both log phase and stationary phase Staphylococcus aureus cells. Experiments were carried out at high initial cell density so that the frequency and proliferation of resistant mutants could be monitored. We found that each combination was advantageous in terms of enhanced killing and the suppression of mutants, compared with each drug used alone. None of the three combinations was effective against stationary phase cells.

In vitro activity of novel rifamycins against rifamycin-resistant Staphylococcus aureus

We describe novel rifamycin derivatives (new chemical entities [NCEs]) that retain significant activity against a comprehensive collection of Staphylococcus aureus strains that are resistant to rifamycins. This collection of resistant strains contains 21 of the 26 known single-amino-acid alterations in RpoB, the target of rifamycins. Some NCEs also demonstrated a lower frequency of resistance development than rifampin and rifalazil in S. aureus as measured in a resistance emergence test. When assayed for activity against the strongest rifamycin-resistant mutants, several NCEs had MICs of 2 microg/ml, in contrast to MICs of rifampin and rifalazil, which were 512 microg/ml for the same strains. The properties of these NCEs therefore demonstrate a significant improvement over those of earlier rifamycins, which have been limited primarily to combination therapy due to resistance development, and suggest a potential use of these NCEs for monotherapy in several clinical indications.

Rifalazil pretreatment of mammalian cell cultures prevents subsequent Chlamydia infection

Chlamydia species are widely disseminated obligate intracellular pathogens that primarily cause urogenital, ocular, and respiratory infections. In these studies, we show that exposing mammalian cells to antibacterial agents prior to Chlamydia inoculation protects the host cells against subsequent challenge by chlamydiae (the protective effect [PE]). Rifalazil exhibited a considerably stronger PE than did azithromycin, rifampin, doxycycline, and ofloxacin. Specifically, 0.002 microg/ml rifalazil incubated for 1 day with a monolayer of McCoy cells was sufficient to protect against a challenge 2 days later with Chlamydia trachomatis serovar D (UW-3). The PE was observed with five different mammalian cell lines and with a variety of C. trachomatis and Chlamydia pneumoniae isolates. The duration of the PE was 6 to 12 days for rifalazil (depending on the cell line), a maximum of 3 days for azithromycin, and less than a day for the other drugs tested. For rifalazil, the PE was shown to be mediated by inhibition of the chlamydial RNA polymerase since mutants with altered RNA polymerases had correspondingly altered PEs. These results suggest that rifalazil may be unique in its ability to prevent infection with obligate intracellular pathogens for a considerable time after treatment. This characteristic may be of particular public health value in reducing reinfection with chlamydiae.

In Vitro Time-kill Activities of Rifalazil, Alone and in Combination with Vancomycin, against Logarithmic and Stationary Cultures of Staphylococcus aureus

Rifalazil is a novel rifamycin that, like other members of this class, inhibits bacterial transcription by targeting the beta subunit of prokaryotic DNA-dependent RNA polymerase. To address the high-frequency resistance seen with rifamycins, we assessed the ability of rifalazil, alone and in combination with vancomycin, to both kill cells and to suppress the appearance of resistant mutants in log and stationary phase Staphylococcus aureus cultures, using high cell densities in an in vitro kill curve model. We found that (1) rifalazil alone killed log-phase cultures more rapidly than rifampicin, but both drugs quickly selected for resistant mutants, (2) co-treatment of log phase cultures with rifalazil and vancomycin increased bacterial killing by about 3-Log10 over either drug used alone and delayed the appearance of rifamycin-resistant mutants, (3) rifalazil and vancomycin in combination killed stationary phase cultures

New drug candidates and therapeutic targets for tuberculosis therapy

Despite advances in chemotherapy and the BCG (Bacillus Calmette-Guérin) vaccine, tuberculosis remains a significant infectious disease. Although it can be cured, the therapy takes at least 6-9 months, and the laborious and lengthy treatment brings with it dangers of noncompliance, significant toxicity and drug resistance. The increasing emergence of drug resistance and the problem of mycobacterial persistence highlight the need to develop novel TB drugs that are active against drug resistant bacteria but, more importantly, kill persistent bacteria and shorten the length of treatment. Recent new and exciting developments in tuberculosis drug discovery show good promise of a possible revolution in the chemotherapy of tuberculosis.

Interaction of rifalazil with oxidant-generating systems of human polymorphonuclear neutrophils

It is well acknowledged that ansamycins display immunosuppressive and anti-inflammatory properties in vitro and in vivo. Rifalazil, a new ansamycin derivative, has not been studied in the context of inflammation. In particular, there are no data on the possible interference of rifalazil with oxidant production by phagocytes. We have compared the antioxidant properties of rifalazil to those of rifampin, a drug well known in this context, by using cellular and acellular oxidant-generating systems. Oxidant production by polymorphonuclear neutrophils was measured in terms of cytochrome c reduction, lucigenin-amplified chemiluminescence (Lu-ACL), and the 2',7'-dichlorofluorescin diacetate H2 (DCFDA-H2) technique (intracellular oxidant production). Rifalazil impaired O2- production in a concentration-dependent manner, with 50% inhibitory concentrations (IC50) (concentrations which inhibit 50% of the response) of 5.4 (30 and 60 min of incubation) and 6.4 (30 min) mg/liter, respectively, for phorbol myristate acetate (PMA) and formyl-methionyl-leucyl-phenylalanine (fMLP) stimulation. In agreement with the published fMLP-like activity of rifampin, the inhibitory effect of rifampin was significantly greater for fMLP (IC50 of 5.6 mg/liter) than for PMA (IC50 of 58 mg/liter) stimulation. Alteration of intracellular oxidant production was also observed with IC50 values similar to those obtained by the cytochrome assay. In addition, rifalazil and rifampin (> or = 25 mg/liter) scavenged O2-, as demonstrated by the acellular (hypoxanthine-xanthine oxidase) system. Interference with light detection systems was evidenced for both drugs by Lu-ACL. The clinical relevance of the antioxidant effect of rifalazil demonstrated in vitro, in particular its potential anti-inflammatory activity, requires further investigation.

Activities of rifamycin derivatives against wild-type and rpoB mutants of Chlamydia trachomatis

Rifalazil, a semisynthetic rifamycin, was shown previously to have exceptional potency against Chlamydia trachomatis (MIC of 0.00025 microg/ml). We therefore tested 250 additional rifamycin derivatives and identified 12 with activities that are eightfold more potent than that of rifalazil. These compounds also showed exceptional activities against rifampin-resistant strains that carry missense mutations in the rpoB gene. The antimicrobial potency and intracellular penetration of these agents suggest their potential in treatment of chlamydial infections.

New Drugs for Tuberculosis: Current Status and Future Prospects

This article reviews two classes of compounds that have advanced into phase II and III clinical trials, long-acting rifamycins and fluoroquinolones, and a number of other drugs that have entered or may enter clinical development in the near future.

Rifampin-resistant RNA polymerase mutants of Chlamydia trachomatis remain susceptible to the ansamycin rifalazil

Stable, homotypic mutants of Chlamydia trachomatis for which MICs of rifampin and rifalazil are elevated were isolated by serial passage at sub-MIC concentrations of these compounds. An alternative approach, in which Chlamydia cells were incubated and subsequently passaged three times, all in the presence of the selective agent at concentrations above the MIC, appeared to be a more effective means of selecting for mutants. In every instance where an elevation in the MIC occurred, one or more mutations in the rpoB gene, encoding the rifampin binding site, were detected. With one exception, all rpoB mutants that contained a single mutation conferred lower levels of resistance than mutants containing multiple mutations. Some rpoB mutations conferred very high levels of resistance to rifampin, up to 512 mug/ml. In all cases, mutants remained susceptible to concentrations of rifalazil at or below 0.064 microg/ml. Thus, rifalazil, a compound that is extremely potent against Chlamydia wild-type cells (MIC of 0.00025 microg/ml), may also protect against the selection of mutants at physiologically achievable concentrations.

Emergence of resistance to rifampin and rifalazil in Chlamydophila pneumoniae and Chlamydia trachomatis

Although rifamycins have excellent activity against Chlamydophila pneumoniae and Chlamydia trachomatis in vitro, concerns about the possible development of resistance during therapy have discouraged their use for treatment of chlamydial infections. Rifalazil, a new semisynthetic rifamycin with a long half-life, is the most active antimicrobial against C. pneumoniae and C. trachomatis in vitro, indicating its potential for treatment of acute and chronic C. pneumoniae and C. trachomatis infections. We investigated the effect of serial passage of two C. pneumoniae isolates and two serotypes of C. trachomatis in subinhibitory concentrations of rifalazil and rifampin on the development of phenotypic and genotypic resistance. C. trachomatis developed resistance to both antimicrobials within six passages, with higher level resistance to rifampin (128 to 256 microg/ml) and lower level resistance to rifalazil (0.5 to 1 microg/ml). C. pneumoniae TW-183 developed only low-level resistance to rifampin (0.25 microg/ml) and rifalazil (0.016 microg/ml) after 12 passages. C. pneumoniae CWL-029 failed to develop resistance to either drug. Two unique mutations emerged in the rpoB gene of rifampin (L456I) and rifalazil (D461E)-resistant C. pneumoniae TW-183. A single mutation (H471Y) was detected in both rifampin- and rifalazil-resistant C. trachomatis UW-3/Cx/D, and a unique mutation (V136F) was found in rifalazil-resistant BU-434/L(2). No mutations were detected in the entire rpoB gene of rifampin-resistant BU-434/L(2). This is the first description of antibiotic resistance-associated mutations in C. pneumoniae and of rifampin resistance in C. trachomatis not associated with mutations in the rpoB gene.

New advances in antibiotic development and discovery

The development of new antibiotics is crucial to controlling current and future infectious diseases caused by antibiotic-resistant bacteria. Increased development costs, the difficulty in identifying new drug classes, unanticipated drug toxicities, the ease by which bacteria develop resistance to new antibiotics and the failure of many agents to address antibiotic resistance specifically, however, have all led to an overall decline in the number of antibiotics that are being introduced into clinical practice. Although there are few, if any, advances likely in the immediate future, there are agents in both clinical and preclinical development that can address some of the concerns of the infectious disease community. Many of these antibiotics will be tailored to specific infections caused by a relatively modest number of susceptible and resistant organisms.

Anti-inflammatory activity of ansamycins

Inflammation represents a complex biologic and biochemical process involving cells of the immune system and a plethora of biologic mediators in response to mechanical, chemical or infectious injuries. When mobilization of effector cells and molecules becomes excessive, the beneficial aspect of this response--to limit damage and promote healing, can be overriden, resulting in host-cell and tissue dysfunction. Based on the hypothesis that chronic infections underly some inflammatory diseases, antibacterial therapy has long been assessed in various inflammatory settings. Recently, the anti-inflammatory activity of some antibacterial agents has also been suspected. Of these duel-action drugs, ansamycins represent an interesting family. Although their therapeutic use is restricted to potentially infectious inflammatory diseases, many experimental data suggest that these drugs also possess direct inhibitory activity on some crucial proinflammatory effectors. To date, the potent antimycobacterial activity of the therapeutically useful ansamycins precludes their widespread use in inflammatory diseases. However, biosynthetic manipulation remains an attractive route for the generation of pharmacologically useful analogs.

Natural products to drugs: natural product derived compounds in clinical trials

Natural product and natural product-derived compounds that are being evaluated in clinical trials or in registration (current 31 December 2004) have been reviewed. Natural product derived drugs launched in the United States of America, Europe and Japan since 1998 and new natural product templates discovered since 1990 are discussed.

Rifalazil treats and prevents relapse of clostridium difficile-associated diarrhea in hamsters

Although vancomycin and metronidazole effectively treat Clostridium difficile-associated diarrhea and colitis (CDAD), their use is associated with a high incidence of relapsing C. difficile infection. Rifalazil is a new benzoxazinorifamycin that possesses activity against Mycobacterium tuberculosis and gram-positive bacteria. Here we compared rifalazil and vancomycin for effectiveness in preventing or treating clindamycin-induced cecitis in a hamster model of CDAD. Golden Syrian hamsters were injected subcutaneously with clindamycin phosphate (10 mg/kg), followed 24 h later by C. difficile gavage. Hamsters received by gavage for 5 days vehicle, vancomycin (50 mg/kg), or rifalazil (20 mg/kg) either simultaneously with (prophylactic protocol) or 24 h after C. difficile administration (treatment protocol). While all vehicle-administered animals became moribund within 48 h of C. difficile administration, no rifalazil- or vancomycin-treated animals in either protocol showed signs of morbidity after 7 days. Ceca of rifalazil-treated animals showed absence of epithelial cell damage, significantly reduced congestion and edema, and less, but not statistically significantly less, neutrophil infiltration compared to those of vehicle-treated animals. In contrast, vancomycin-treated animals demonstrated severe epithelial cell damage and mildly reduced congestion and edema. Moreover, hamsters relapsed and tested C. difficile toxin positive (by enzyme-linked immunosorbent assay) 10 to 15 days after discontinuation of vancomycin treatment. None of the rifalazil-treated hamsters showed signs of disease or presence of toxins in their feces 30 days after discontinuation of treatment. Our results indicate that once daily rifalazil may be superior to vancomycin for curative treatment of CDAD.

Recent developments in the treatment of tuberculosis

The history of chemotherapy of tuberculosis commenced in 1944 with the discovery of streptomycin. Currently, short-course chemotherapy comprising rifampicin, isoniazid, pyrazinamide and ethambutol/streptomycin administered under directly observed settings for 6 months (initially all four drugs followed by the former two drugs), constitutes the cornerstone treatment for pulmonary tuberculosis. Multi-drug resistant tuberculosis requires alternative chemotherapy, ideally in the form of individualised regimens, for management. To improve on the duration of chemotherapy for drug-susceptible tuberculosis and to achieve better treatment for multi-drug resistant tuberculosis as well as latent tuberculosis infection, there arises a genuine need for new drugs. The quest for new agents is, however, impeded by obstacles. Hopefully, tackling these through collaborative public-private partnerships on an international scale will lead to a fruitful outcome.