Effectiveness of once-weekly rifapentine and moxifloxacin regimens against Mycobacterium tuberculosis in mice

Pharmacokinetic/Pharmacodynamic Background and Methods and Scientific Evidence Base for Dosing of Second-line Tuberculosis Drugs

A World Health Organization workshop systematically examined the evidence base for dosing second-line tuberculosis drugs, identifying knowledge gaps. To fill these in, pharmacokinetics/pharmacodynamics, Monte Carlo experiments, and artificial intelligence algorithms were used in hollow-fiber model studies and clinical data analyses.

Effect of the alkyl group in the piperazine N-substitution on the therapeutic action of rifamycins: A drug-membrane interaction study

In this work, we studied the effects of the N-alkyl group (methyl, cyclopentyl) in the piperazine ring of, respectively, rifampicin (RIF) and rifapentine (RPT) to correlate this substitution with their differential pharmacokinetic properties and overall clinical performance. Since this group is their only structural change, and given that they share the same pharmacological target, differences in their therapeutic behavior may respond to this asset, particularly in their interaction with lipid membranes across the organism. In this study, surface pressure-area isotherms, as well as spectroscopic and microscopic techniques of characterization of phospholipid monolayers at the air/water interface were used to gain insight into drug-membrane interactions. Differences in the affinity for lipid membranes for both drugs, given by the vibration frequency of characteristic chemical groups in the lipid, as well as by reflectivity and mean molecular area of the monolayer, seem to be due to the N-alkyl substituent and can contribute to provide a molecular explanation as why they pose different choices in the chemotherapy against the deadliest infectious disease, tuberculosis.

Comparing efficacies of moxifloxacin, levofloxacin and gatifloxacin in tuberculosis granulomas using a multi-scale systems pharmacology approach

Granulomas are complex lung lesions that are the hallmark of tuberculosis (TB). Understanding antibiotic dynamics within lung granulomas will be vital to improving and shortening the long course of TB treatment. Three fluoroquinolones (FQs) are commonly prescribed as part of multi-drug resistant TB therapy: moxifloxacin (MXF), levofloxacin (LVX) or gatifloxacin (GFX). To date, insufficient data are available to support selection of one FQ over another, or to show that these drugs are clinically equivalent. To predict the efficacy of MXF, LVX and GFX at a single granuloma level, we integrate computational modeling with experimental datasets into a single mechanistic framework, GranSim. GranSim is a hybrid agent-based computational model that simulates granuloma formation and function, FQ plasma and tissue pharmacokinetics and pharmacodynamics and is based on extensive in vitro and in vivo data. We treat in silico granulomas with recommended daily doses of each FQ and compare efficacy by multiple metrics: bacterial load, sterilization rates, early bactericidal activity and efficacy under non-compliance and treatment interruption. GranSim reproduces in vivo plasma pharmacokinetics, spatial and temporal tissue pharmacokinetics and in vitro pharmacodynamics of these FQs. We predict that MXF kills intracellular bacteria more quickly than LVX and GFX due in part to a higher cellular accumulation ratio. We also show that all three FQs struggle to sterilize non-replicating bacteria residing in caseum. This is due to modest drug concentrations inside caseum and high inhibitory concentrations for this bacterial subpopulation. MXF and LVX have higher granuloma sterilization rates compared to GFX; and MXF performs better in a simulated non-compliance or treatment interruption scenario. We conclude that MXF has a small but potentially clinically significant advantage over LVX, as well as LVX over GFX. We illustrate how a systems pharmacology approach combining experimental and computational methods can guide antibiotic selection for TB.

Tuberculosis (TB) is caused by infection with the bacterium Mycobacterium tuberculosis (Mtb) and kills 1.5 million people each year. TB requires at least 6 months of treatment with up to four drugs, and is characterized by formation of granulomas in patient lungs. Granulomas are spherical collections of host cells and bacteria. Fluoroquinolones (FQs) are a class of drug that could help shorten TB treatment. Three FQs that are used to treat TB are: moxifloxacin (MXF), levofloxacin (LVX) or gatifloxacin (GFX). To date, it is unclear if one FQ is better than the others at treating TB, in part because little is known about how these drugs distribute and work inside the lung granulomas. We use computer simulations of Mtb infection and FQ treatment within granulomas to predict which FQ is better and why. Our computer model is calibrated to multiple experimental data sets. We compare the three FQs by multiple metrics, and predict that MXF is better than LVX and GFX because it kills bacteria more quickly, and it works better when patients miss doses. However, all three FQs are unable to kill a part of the bacterial population living in the center of granulomas. Our results can now inform future experimental studies.

A Review of Moxifloxacin for the Treatment of Drug-Susceptible Tuberculosis

Moxifloxacin, an 8-methoxy quinolone, is an important drug in the treatment of multidrug-resistant tuberculosis and is being investigated in novel drug regimens with pretomanid, bedaquiline, and pyrazinamide, or rifapentine, for the treatment of drug-susceptible tuberculosis. Early results of these studies are promising. Although current evidence does not support the use of moxifloxacin in treatment-shortening regimens for drug-susceptible tuberculosis, it may be recommended in patients unable to tolerate standard first-line drug regimens or for isoniazid monoresistance. Evidence suggests that the standard 400-mg dose of moxifloxacin used in the treatment of tuberculosis may be suboptimal in some patients, leading to worse tuberculosis treatment outcomes and emergence of drug resistance. Furthermore, a drug interaction with the rifamycins results in up to 31% reduced plasma concentrations of moxifloxacin when these are combined for treatment of drug-susceptible tuberculosis, although the clinical relevance of this interaction is unclear. Moxifloxacin exhibits extensive interindividual pharmacokinetic variability. Higher doses of moxifloxacin may be needed to achieve drug exposures required for improved clinical outcomes. Further study is, however, needed to determine the safety of proposed higher doses and clinically validated targets for drug exposure to moxifloxacin associated with improved tuberculosis treatment outcomes. We discuss in this review the evidence for the use of moxifloxacin in drug-susceptible tuberculosis and explore the role of moxifloxacin pharmacokinetics, pharmacodynamics, and drug interactions with rifamycins, on tuberculosis treatment outcomes when used in first-line tuberculosis drug regimens.

Insights into the pharmacokinetic properties of antitubercular drugs

INTRODUCTION

The furiously advancing cases of multidrug-resistant tuberculosis (TB) along with the recent emergence of total drug resistant TB and TB-AIDS comorbidity present an increased threat to global public health. Knowledge of pharmacokinetic properties helps in selecting an appropriate anti-TB dosage regimen to achieve optimal results in patients.

AREAS COVERED

This article provides a brief compilation of the information available regarding published pharmacokinetic data for anti-TB drugs and may act as a single window for investigators/medical practitioners in this field. The information regarding absorption, tissue distribution, elimination and pharmacokinetic interactions of the first- and second-line anti-TB drugs and candidate drugs under clinical trials is discussed.

EXPERT OPINION

Pharmacokinetic properties such as poor absorption, too short biological half-life, extensive first-pass metabolism, drug-food and drug-drug related interactions are not attractive for prospective anti-TB drugs and significantly contribute to treatment failure and further resistance. The long duration, monotonous and multidrug treatment plan leads to poor patient compliance and resulted in a greater occurrence of anti-TB drug resistance worldwide. Few new agents, which are in development phase, are considering the aspect of shortening duration of the treatment regimen and provide a boost in therapy that is sorely needed.

Effects of Fluroquinolones in Newly Diagnosed, Sputum-Positive Tuberculosis Therapy: A Systematic Review and Network Meta-Analysis

BACKGROUND

Tuberculosis is a major public health problem especially in developing countries, the comparative efficacy and safety of fluroquinolones (FQs) for adult patients with newly diagnosed, sputum-positive tuberculosis remains controversial. We aimed to investigate the benefits and risks of FQs-containing (addition/substitution) regimens in this population.

METHODS

A network meta-analysis was performed to compare FQs (C: ciprofloxacin; O: ofloxacin; Lo: levofloxacin; M: moxifloxacin; G: gatifloxacin) addition/substitution regimen with standard HRZE regimen (ie isoniazid, rifampicin, pyrazinamide and ethambutol) in newly diagnosed, sputum-positive tuberculosis. Medline, Embase and Cochrane Central Register of Controlled Trials were systematically searched, randomized trials with duration longer than 8 weeks were included. The primary outcome was week-8 sputum negativity, and secondary outcomes included treatment failure, serious adverse events and death from all cause.

RESULTS

Twelve studies comprising 6465 participants were included in the network meta-analysis. Löwenstein-Jensen culture method showed that HRZEM (OR 4.96, 95% CI 2.83-8.67), MRZE (OR 1.48, 95% CI 1.19-1.84) and HRZM (OR 1.32, 95% CI 1.08-1.62) had more sputum conversion than HRZE by the eighth week, whereas HRC (OR 0.39, 95% CI 0.19-0.77) and HRZO (OR 0.47, 95% CI 0.24-0.92) were worse than HRZE. Moxifloxacin-containing regimens showed more conversion than HRZE by liquid method at the end of two months. But by the end of treatment, FQs-containing regimens didn't show superiority than HRZE on treatment failure. There were no significant differences between any regimens on other outcomes like serious adverse events and all-cause death.

CONCLUSION

This comprehensive network meta-analysis showed that compared with HRZE, moxifloxacin-containing regimens could significantly increase sputum conversion by the eighth week for patients with newly diagnosed pulmonary tuberculosis while HRC and HRZO regimens were inferior. But all the FQs-containing regimens did not show superiority in other outcomes (such as treatment failure, serious adverse events and all-cause death). Thus, HRZE is still an effective regimen for this population. Although moxifloxacin-containing regimens have deomonstrated their potential, FQs-containing regimens should be used with great caution to avoid widespread FQs-resistance worldwide.

WITHDRAWN: Moxifloxacin is an effective and safe candidate agent for tuberculosis treatment: a meta-analysis

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High-dose rifapentine with moxifloxacin for pulmonary tuberculosis

BACKGROUND

Tuberculosis regimens that are shorter and simpler than the current 6-month daily regimen are needed.

METHODS

We randomly assigned patients with newly diagnosed, smear-positive, drug-sensitive tuberculosis to one of three regimens: a control regimen that included 2 months of ethambutol, isoniazid, rifampicin, and pyrazinamide administered daily followed by 4 months of daily isoniazid and rifampicin; a 4-month regimen in which the isoniazid in the control regimen was replaced by moxifloxacin administered daily for 2 months followed by moxifloxacin and 900 mg of rifapentine administered twice weekly for 2 months; or a 6-month regimen in which isoniazid was replaced by daily moxifloxacin for 2 months followed by one weekly dose of both moxifloxacin and 1200 mg of rifapentine for 4 months. Sputum specimens were examined on microscopy and after culture at regular intervals. The primary end point was a composite treatment failure and relapse, with noninferiority based on a margin of 6 percentage points and 90% confidence intervals.

RESULTS

We enrolled a total of 827 patients from South Africa, Zimbabwe, Botswana, and Zambia; 28% of patients were coinfected with the human immunodefiency virus. In the per-protocol analysis, the proportion of patients with an unfavorable response was 4.9% in the control group, 3.2% in the 6-month group (adjusted difference from control, -1.8 percentage points; 90% confidence interval [CI], -6.1 to 2.4), and 18.2% in the 4-month group (adjusted difference from control, 13.6 percentage points; 90% CI, 8.1 to 19.1). In the modified intention-to-treat analysis these proportions were 14.4% in the control group, 13.7% in the 6-month group (adjusted difference from control, 0.4 percentage points; 90% CI, -4.7 to 5.6), and 26.9% in the 4-month group (adjusted difference from control, 13.1 percentage points; 90% CI, 6.8 to 19.4).

CONCLUSIONS

The 6-month regimen that included weekly administration of high-dose rifapentine and moxifloxacin was as effective as the control regimen. The 4-month regimen was not noninferior to the control regimen. (Funded by the European and Developing Countries Clinical Trials Partnership and the Wellcome Trust; RIFAQUIN Current Controlled Trials number, ISRCTN44153044.).

Drug-resistant tuberculosis: controversies and challenges in pediatrics

Tuberculosis remains one of the top two causes of death caused by a single infectious disease worldwide, despite curative therapy. Children with tuberculosis are especially difficult to detect, since acid fast bacilli smears and cultures are usually negative and clinical signs are nonspecific or lacking. Multidrug-resistant tuberculosis, or tuberculosis resistant to at least isoniazid and rifampin, has emerged in most areas of the world over the past 20 years. Treatment of multidrug-resistant tuberculosis is more expensive and difficult. The second-line tuberculosis medications required for treatment are more toxic and less efficacious than standard treatment. These medications are not readily available in many areas of the world where drug resistance is most common. Fluoroquinolones are one of the most promising classes of second-line medications, but are not generally recommended for use in children. Ethambutol is recommended in the initial treatment of tuberculosis in children treated in areas where there is a risk of drug-resistant disease and the susceptibility of the source case is not known. Some experts have been hesitant to use ethambutol due to the risk of visual impairment associated with the drug and the difficulties in monitoring vision in young children. Pediatric drug formulations are not available for most antituberculosis medications, even the first-line tuberculosis drugs. Treatment of children exposed, infected or ill with multidrug-resistant tuberculosis is reviewed with special emphasis on second-line drugs, including recommended dosage, available formulations and necessary monitoring. While new cases of multidrug-resistant tuberculosis have decreased in most developed countries over the past 10 years, cases continue to increase in many developing countries and among immigrants from high-risk areas. Tuberculosis and multidrug-resistant tuberculosis are serious threats requiring worldwide strategies to control and treat. Better diagnostic tests, medications, public health strategies and vaccines will all be needed to eliminate tuberculosis.

Randomized clinical trial of thrice-weekly 4-month moxifloxacin or gatifloxacin containing regimens in the treatment of new sputum positive pulmonary tuberculosis patients

BACKGROUND

Shortening tuberculosis (TB) treatment duration is a research priority. This paper presents data from a prematurely terminated randomized clinical trial, of 4-month moxifloxacin or gatifloxacin regimens, in South India.

METHODS

Newly diagnosed, sputum-positive HIV-negative pulmonary TB patients were randomly allocated to receive gatifloxacin or moxifloxacin, along with isoniazid and rifampicin for 4 months with pyrazinamide for first 2 months (G or M) or isoniazid and rifampicin for 6 months with ethambutol and pyrazinamide for first 2 months (C). All regimens were administered thrice-weekly. Clinical and bacteriological assessments were done monthly during treatment and for 24 months post-treatment. The Data and Safety Monitoring Board recommended termination of the trial due to high TB recurrence rates in the G and M regimens.

RESULTS

Of 416 patients in intent-to-treat analysis, 6 (5%) of 124, 2 (2%) of 110 and 2 (2%) of 137 patients with drug-susceptible TB in the G, M and C arms respectively had unfavorable response at the end of treatment; during the next 24 months, 17 (15%) of 115, 11 (11%) of 104 and 8 (6%) of 132 patients respectively, had TB recurrence. Of 38 drug-resistant patients 1 of 8 and 3 of 26 in the G and C arms respectively had unfavourable response at the end of treatment; and TB recurrence occurred in 2 of 7 and 2 of 23 patients, respectively. The differences in TB recurrence rates between the G and C arms was statistically significant (p = 0.02). Gastro-intestinal symptoms occurred in 23%, 22% and 9% of patients in the G, M and C arms respectively, but most reactions were mild and manageable with symptomatic measures; 1% required regimen modification.

CONCLUSIONS

4-month thrice-weekly regimens of gatifloxacin or moxifloxacin with isoniazid, rifampicin and pyrazinamide, were inferior to standard 6-month treatment, in patients with newly diagnosed sputum positive pulmonary TB.

TRIAL REGISTRATION

Clinical Trials Registry of India CTRI/2012/10/003060.

Accumulating Evidence and Research Organization (AERO) model: a new tool for representing, analyzing, and planning a translational research program

BACKGROUND

Maximizing efficiency in drug development is important for drug developers, policymakers, and human subjects. Limited funds and the ethical imperative of risk minimization demand that researchers maximize the knowledge gained per patient-subject enrolled. Yet, despite a common perception that the current system of drug development is beset by inefficiencies, there remain few approaches for systematically representing, analyzing, and communicating the efficiency and coordination of the research enterprise. In this paper, we present the first steps toward developing such an approach: a graph-theoretic tool for representing the Accumulating Evidence and Research Organization (AERO) across a translational trajectory.

METHODS

This initial version of the AERO model focuses on elucidating two dimensions of robustness: (1) the consistency of results among studies with an identical or similar outcome metric; and (2) the concordance of results among studies with qualitatively different outcome metrics. The visual structure of the model is a directed acyclic graph, designed to capture these two dimensions of robustness and their relationship to three basic questions that underlie the planning of a translational research program: What is the accumulating state of total evidence? What has been the translational trajectory? What studies should be done next?

RESULTS

We demonstrate the utility of the AERO model with an application to a case study involving the antibacterial agent, moxifloxacin, for the treatment of drug-susceptible tuberculosis. We then consider some possible elaborations for the AERO model and propose a number of ways in which the tool could be used to enhance the planning, reporting, and analysis of clinical trials.

CONCLUSION

The AERO model provides an immediate visual representation of the number of studies done at any stage of research, depicting both the robustness of evidence and the relationship of each study to the larger translational trajectory. In so doing, it makes some of the invisible or inchoate properties of the research system explicit - helping to elucidate judgments about the accumulating state of evidence and supporting decision-making for future research.

New regimens for reducing the duration of the treatment of drug-susceptible pulmonary tuberculosis

Tuberculosis (TB) remains an important health problem worlwide. The structure necessary for delivering TB treatment and implementing the directly observed treatment accounts for more than two-thirds of its final cost. Furthemore, although with efficacy greater than 90%, the effectiveness of present treatment regimens ranges from 55-85%, depending on the setting, mainly due to poor adherence. Duration of treatment with the current first-line anti-TB drugs is a minimum of 6 months. Reducing the duration of the treatment from six to two months or less could result in significant increase of adherence to treatment and cost reduction. The aim of this review is to highlight potential new agents or new drug combinations that could reduce the time of treatment of drug-susceptible TB, currently under study or recently evaluated through clinical trials. We conducted a literature search in the English language for clinical studies as well as an electronic computer-assisted and manual search. The literature search was conducted on November 2010, using MEDLINE (2000-2010), EMBASE (2000-2010) and the National Institute of Health (NIH) Clinical Trials Register database (2000-2010). Most of the new agents identified as anti-TB drug candidates are still in the preclinical phases. Nitroimidazole-PA-824 and fluoroquinolones are evaluated while two first line drugs - rifampicin and rifapentine -are re-evaluated to optimize their efficacy in new ultra-short anti-TB regimens through phases II/III clinical studies. A summary of the studies are presented, with their potential to change recommendations for TB treatment in the near future.

New drugs for tuberculosis treatment

Available data on anti-tuberculosis drug research reveal different properties of the agents and provoke speculation about future directions. Higher doses of the rifamycins are promising and are currently being evaluated in regimens of shorter duration that the isoniazid plus rifampin-based, six-to-nine month-course therapy. Moxifloxacin and gatifloxacin might shorten tuberculosis treatment as well, possibly in combination with rifapentine, while SQ109 could enhance the activity of rifampin-containing regimens. On the other hand, co-administration of moxifloxacin and PA-824 could be active against latent tuberculosis, whereas linezolid, PA-824 and TMC207 are candidates for a rifampin-free regimen in multidrug-resistant and extensively-resistant tuberculosis. Unfortunately, shorter than existent treatment regimens based on the new agents discussed here are likely to take at least another decade to be fully developed and implemented in clinical practice.

Rifamycins--obstacles and opportunities

With nearly one-third of the global population infected by Mycobacterium tuberculosis, TB remains a major cause of death (1.7 million in 2006). TB is particularly severe in parts of Asia and Africa where it is often present in AIDS patients. Difficulties in treatment are exacerbated by the 6-9 month treatment times and numerous side effects. There is significant concern about the multi-drug-resistant (MDR) strains of TB (0.5 million MDR-TB cases worldwide in 2006). The rifamycins, long considered a mainstay of TB treatment, were a tremendous breakthrough when they were developed in the 1960's. While the rifamycins display many admirable qualities, they still have a number of shortfalls including: rapid selection of resistant mutants, hepatotoxicity, a flu-like syndrome (especially at higher doses), potent induction of cytochromes P450 (CYP) and inhibition of hepatic transporters. This review of the state-of-the-art regarding rifamycins suggests that it is quite possible to devise improved rifamycin analogs. Studies showing the potential of shortening the duration of treatment if higher doses could be tolerated, also suggest that more potent (or less toxic) rifamycin analogs might accomplish the same end. The improved activity against rifampin-resistant strains by some analogs promises that further work in this area, especially if the information from co-crystal structures with RNA polymerase is applied, should lead to even better analogs. The extensive drug-drug interactions seen with rifampin have already been somewhat ameliorated with rifabutin and rifalazil, and the use of a CYP-induction screening assay should serve to efficiently identify even better analogs. The toxicity due to the flu-like syndrome is an issue that needs effective resolution, particularly for analogs in the rifalazil class. It would be of interest to profile rifalazil and analogs in relation to rifampin, rifapentine, and rifabutin in a variety of screens, particularly those that might relate to hypersensitivity or immunomodulatory processes.

Substitution of moxifloxacin for isoniazid during intensive phase treatment of pulmonary tuberculosis

RATIONALE

Moxifloxacin has potent activity against Mycobacterium tuberculosis in vitro and in a mouse model of antituberculosis (TB) chemotherapy, but data regarding its activity in humans are limited.

OBJECTIVES

Our objective was to compare the antimicrobial activity and safety of moxifloxacin versus isoniazid during the first 8 weeks of combination therapy for pulmonary TB.

METHODS

Adults with sputum smear-positive pulmonary TB were randomly assigned to receive either moxifloxacin 400 mg plus isoniazid placebo, or isoniazid 300 mg plus moxifloxacin placebo, administered 5 days/week for 8 weeks, in addition to rifampin, pyrazinamide, and ethambutol. All doses were directly observed. Sputum was collected for culture every 2 weeks. The primary outcome was negative sputum culture at completion of 8 weeks of treatment.

MEASUREMENTS AND MAIN RESULTS

Of 433 participants enrolled, 328 were eligible for the primary efficacy analysis. Of these, 35 (11%) were HIV positive, 248 (76%) had cavitation on baseline chest radiograph, and 213 (65%) were enrolled at African sites. Negative cultures at Week 8 were observed in 90/164 (54.9%) participants in the isoniazid arm, and 99/164 (60.4%) in the moxifloxacin arm (P = 0.37). In multivariate analysis, cavitation and enrollment at an African site were associated with lower likelihood of Week-8 culture negativity. The proportion of participants who discontinued assigned treatment was 31/214 (14.5%) for the moxifloxacin group versus 22/205 (10.7%) for the isoniazid group (RR, 1.35; 95% CI, 0.81, 2.25).

CONCLUSIONS

Substitution of moxifloxacin for isoniazid resulted in a small but statistically nonsignificant increase in Week-8 culture negativity.

Moxifloxacin versus ethambutol in the initial treatment of tuberculosis: a double-blind, randomised, controlled phase II trial

BACKGROUND

New treatments are needed to shorten the time required to cure tuberculosis and to treat drug-resistant strains. The fluoroquinolone moxifloxacin is a promising new agent that might have additive activity to existing antituberculosis agents. We assessed the activity and safety of moxifloxacin in the initial stage of tuberculosis treatment.

METHODS

We undertook a phase II, double-blind, randomised controlled trial of a regimen that included moxifloxacin in adults with sputum smear-positive tuberculosis at one hospital in Rio de Janeiro, Brazil. 170 participants received isoniazid, rifampicin, and pyrazinamide at standard doses and were assigned by permuted block randomisation to receive either moxifloxacin (400 mg) with an ethambutol placebo (n=85) or ethambutol (15-20 mg/kg) plus moxifloxacin placebo (n=85) 5 days per week for 8 weeks. The primary endpoint was the proportion of patients whose sputum culture had converted to negative by week 8. Analysis was by modified intention to treat (ITT); patients whose baseline cultures were negative, contaminated, or contained drug-resistant Mycobacterium tuberculosis were excluded from the analysis. Additionally, all missing 8-week results were deemed treatment failures. This study is registered with ClinicalTrials.gov, number NCT00082173.

FINDINGS

74 patients assigned to the moxifloxacin group and 72 in the ethambutol group were included in the modified ITT population. 125 patients had 8-week data (moxifloxacin n=64, ethambutol n=61); the main reason for absence of data was culture contamination. At 8 weeks, culture conversion to negative had occurred in 59 (80%) of 74 patients in the moxifloxacin group compared with 45 (63%) of 72 in the ethambutol group (difference 17.2%, 95% CI 2.8-31.7; p=0.03). There were 16 adverse events (eight in each group) in 12 patients. Only one event was judged related to study drug (grade 3 cutaneous reaction in the ethambutol group).

INTERPRETATION

Moxifloxacin improved culture conversion in the initial phase of tuberculosis treatment. Trials to assess whether moxifloxacin can be used to shorten the duration of tuberculosis treatment are justified.

Tuberculosis pharmacotherapy: strategies to optimize patient care

INTRODUCTION

The treatment of tuberculosis (TB) is a mature discipline, with more than 60 years of clinical experience accrued across the globe. The requisite Multi-drug treatment of drug-susceptible TB, however, lasts 6 months and has never been optimized according to current standards. Multi-drug resistant TB and TB in individuals coinfected with HIV present additional treatment challenges.

OBJECTIVE

This article reviews the role that existing drugs and new compounds could have in shortening or improving treatment for TB. The key to treatment shortening seems to be sterilizing activity, or the ability of drugs to kill mycobacteria that persist after the initial days of multi-drug treatment.

RESULTS

Among existing anti-TB drugs, the rifamycins hold the greatest potential for shortening treatment and improving outcomes, in both HIV-infected and HIV-uninfected populations, without dramatic increases in toxicity. Clinical studies underway or being planned, are supported by in vitro , animal and human evidence of increased sterilizing activity--without significant increases in toxicity--at elevated daily doses. Fluoroquinolones also seem to have significant sterilizing activity. At present, at least two class members are being evaluated for treatment shortening with different combinations of first-line drugs. However, in light of apparent rapid selection for fluoroquinolone-resistant mutants, relative frequency of serious adverse events and a perceived need to 'reserve' fluoroquinolones for the treatment of drug-resistant TB, their exact role in TB treatment remains to be determined. Other possible improvements may come from inhaled delivery or split dosing (linezolid) of anti-TB drugs for which toxicity (ethionamide) or lack of absorption (aminoglycosides and polypeptides) precludes delivery of maximally effective, oral doses, once daily. New classes of drugs with novel mechanisms of action, nitroimidazopyrans and a diarylquinoline, among others, may soon provide opportunities for improving treatment of drug-resistant TB or shortening treatment of drug-susceptible TB.

CONCLUSION

More potential options for improved TB treatment currently exist than at any other time in the last 30 years. The challenge in TB pharmacotherapy is to devise well-tolerated, efficacious, short-duration regimens that can be used successfully against drug-resistant and drug-resistant TB in a heterogeneous population of patients.

A once-weekly R207910-containing regimen exceeds activity of the standard daily regimen in murine tuberculosis

RATIONALE

R207910 (TMC207 or J) is a member of the diarylquinolines, a new family of antituberculous drugs with high bactericidal activity when given daily in the murine model of tuberculosis. R207910 exhibits a long half-life and thus is a good candidate for once-weekly therapy of tuberculosis.

OBJECTIVES

To study the activity of once-weekly R207910 monotherapy and combinations of R207910 with other antituberculous agents (isoniazid, rifapentine, moxifloxacin, and pyrazinamide).

METHODS

The established infection model of murine tuberculosis was used. Colony counts were determined in the lungs.

MEASUREMENTS AND MAIN RESULTS

Eight weeks of monotherapy reduced the bacillary load by 3 to 4 log(10) for rifapentine and by 5 to 6 log(10) for R207910 (P < 0.05). The addition of rifapentine and isoniazid or moxifloxacin did not improve the bactericidal activity of R207910 monotherapy. In contrast, the triple combination of R207910 plus rifapentine plus pyrazinamide given once weekly for 2 months (i.e., a total of only eight administrations), was significantly (P < 0.05) more active than R207910 monotherapy or other R207910 combinations, and led to lung culture negativity in 9 of 10 mice, whereas all lungs were culture positive in the groups treated with other drug combinations. Moreover, R207910 plus rifapentine plus pyrazinamide given once weekly was more active than the current standard regimen of rifampin plus isoniazid plus pyrazinamide given five times per week.

CONCLUSIONS

The unprecedented activity of the triple combination of R207910 plus rifapentine plus pyrazinamide suggests that it may be feasible to develop a fully intermittent once-weekly regimen.

Impact of the interaction of R207910 with rifampin on the treatment of tuberculosis studied in the mouse model

New drugs are needed to shorten the duration of tuberculosis treatment. R207910, a diarylquinoline, is very active against Mycobacterium tuberculosis both in vitro and in mice. In healthy volunteers, the coadministration of R207910 and rifampin induced the increased metabolism of R207910, resulting in a 50% reduction in the level of R207910 exposure. We assessed the impact of reducing the dose of R207910 on its efficacy when R207910 was combined with a background regimen of isoniazid, rifampin, and pyrazinamide. Addition of 25 mg/kg of body weight or 12.5 mg/kg R207910 to the background regimen resulted in faster bacterial clearance and culture negativity. The difference in efficacy between the two doses was not statistically significant. The minimal bactericidal dose of R207910 when it was tested as part of the combination was identical to that when it was tested as monotherapy. Because of the drug-drug interaction in humans, the activity of R207910 in humans could be less than that expected from studies with mice. Our data from the mouse model demonstrate that R207910 has significant activity, even when its exposure is reduced by 50% and when it is added to a strong background regimen of isoniazid, rifampin, and pyrazinamide. In killing kinetic studies, the bactericidal effect of R207910 in mice was modest during the first week of treatment, but it increased in the following 3 weeks, while the bactericidal activity of isoniazid was limited to the first week of treatment.

Bactericidal and sterilizing activities of several orally administered combined regimens against Mycobacterium ulcerans in mice

Treatment with rifampin-clarithromycin or moxifloxacin-clarithromycin for 8 weeks displayed promising bactericidal activity against Mycobacterium ulcerans in mice; none of the mice treated with rifampin-clarithromycin relapsed, whereas 59% of those treated with moxifloxacin-clarithromycin relapsed after treatment was stopped. The bactericidal and sterilizing activities of the five-times-weekly (5/7) administration of 5 mg of rifapentine/kg of body weight, either alone or in combination, were virtually identical to those of the corresponding regimens with 10 mg of rifampin/kg of body weight; however, because of the long half-life of rifapentine, accumulation of the drug after 5/7 administration is a concern. The bactericidal activity of 20 mg/kg rifapentine in monotherapy or 20 mg/kg rifapentine in combination with 150 mg/kg streptomycin or 200 mg/kg moxifloxacin administered twice weekly was as effective as the corresponding regimens containing 10 mg/kg rifampin administered 5/7, suggesting that Buruli ulcer might be treated with intermittently administered rifapentine-containing combinations.

New Tuberculosis Therapeutics: A Growing Pipeline

Novel chemotherapeutic drugs are needed to improve tuberculosis (TB) control, especially in the developing world. Given the magnitude of the problem and the resources available in countries that have the highest burden of disease, the present standards of care for the treatment of drug-susceptible TB, drug-resistant TB, TB/human immunodeficiency virus (HIV) coinfection, and latent TB infection are all unsatisfactory. Because no truly novel compounds for the treatment of TB have been discovered in the past 40 years, the recent enhanced activity in the research and development of new TB drugs is extremely encouraging. Seven compounds are presently in clinical development specifically for the treatment of TB. Other known antibiotic compound families are being investigated preclinically, in an attempt to identify new antimicrobial drugs with specific antituberculous activity. In addition, novel targets have been identified and are the subject of efforts to validate their potential usefulness in the treatment of TB.

Rifapentine for the Treatment of Pulmonary Tuberculosis

Rifapentine is a recently approved antituberculosis drug that has not yet been widely used in clinical settings. Clinical data support intermittent use of rifapentine with isoniazid during the continuation phase of tuberculosis treatment. Patients with culture-positive, noncavitary, pulmonary tuberculosis whose sputum smear is negative for acid-fast bacilli at the end of the 2-month intensive treatment phase are eligible for rifapentine therapy. Rifapentine should not be used in human immunodeficiency virus-infected patients, given their increased risk of developing rifampin resistance with currently recommended dosages. Rifapentine is not currently recommended for children aged <12 years, pregnant or lactating women, or individuals with culture-negative or extrapulmonary tuberculosis. Rifapentine (600 mg) is administered once weekly with isoniazid (900 mg) during the continuation phase of treatment. This combination should only be given under direct observation. As with rifampin, drug-drug interactions are common, and regular patient monitoring is required. Ease of administration makes this regimen attractive both for tuberculosis-control programs and for patients.

Combinations of R207910 with drugs used to treat multidrug-resistant tuberculosis have the potential to shorten treatment duration

The objective of the present study was to identify the optimal R207910-containing regimen to administer to patients who cannot receive rifampin (RIF) and isoniazid (INH) because of multidrug-resistant tuberculosis (MDR-TB), concomitant use of antiretroviral drugs, or toxicity. Mice were infected intravenously with 5 x 10(6) CFU of the H37Rv strain and treated five times per week with R207910 alone or various combinations of R207910 with the second-line drugs amikacin (AMK), pyrazinamide (PZA), moxifloxacin (MXF), and ethionamide (ETH). All R207910-containing regimens were significantly more active than the non-R207910-containing regimens after 1 month of therapy. When given for 2 months, R207910 alone was more active than the WHO standard first-line regimen RIF-INH-PZA. When R207910 was combined with second-line drugs, the combinations were more active than the currently recommended regimen of MDR-TB AMK-ETH-MXF-PZA, and culture negativity of both the lungs and spleen was reached after 2 months of treatment in almost every case.

Moxifloxacin versus ethambutol in the first 2 months of treatment for pulmonary tuberculosis

RATIONALE

Moxifloxacin has promising preclinical activity against Mycobacterium tuberculosis, but has not been evaluated in multidrug treatment of tuberculosis in humans.

OBJECTIVE

To compare the impact of moxifloxacin versus ethambutol, both in combination with isoniazid, rifampin, and pyrazinamide, on sputum culture conversion at 2 mo as a measure of the potential sterilizing activity of alternate induction regimens.

METHODS

Adults with smear-positive pulmonary tuberculosis were randomized in a factorial design to receive moxifloxacin (400 mg) versus ethambutol given 5 d/wk versus 3 d/wk (after 2 wk of daily therapy). All doses were directly observed.

MEASUREMENTS

The primary endpoint was sputum culture status at 2 mo of treatment.

RESULTS

Of 336 patients enrolled, 277 (82%) were eligible for the efficacy analysis, 186 (67%) were male, 175 (63%) were enrolled at African sites, 206 (74%) had cavitation on chest radiograph, and 60 (22%) had HIV infection. Two-month cultures were negative in 71% of patients (99 of 139) treated with moxifloxacin versus 71% (98 of 138) treated with ethambutol (p = 0.97). Patients receiving moxifloxacin, however, more often had negative cultures after 4 wk of treatment. Patients treated with moxifloxacin more often reported nausea (22 vs. 9%, p = 0.002), but similar proportions completed study treatment (88 vs. 89%). Dosing frequency had little effect on 2-mo culture status or tolerability of therapy.

CONCLUSIONS

The addition of moxifloxacin to isoniazid, rifampin, and pyrazinamide did not affect 2-mo sputum culture status but did show increased activity at earlier time points.

Study of the interaction between rifapentine and isoniazid under acid conditions

A well-known problem of anti-tuberculosis fixed-dose combination (FDC) products containing rifampicin (R) and isoniazid (H) is the fall in bioavailability, in particular of R, when two or more drugs are present together. The same has been ascribed to hydrolysis of R to 3-formylrifamycin (3-RIF) under stomach acid conditions and reaction of the latter with H to form isonicotinyl hydrazone (HYD). The objective of present study was to explore whether the same reaction occurred when H was present along with rifapentine (Rp), a newer long acting rifamycin, which is structurally similar to R. Clinical trials are currently undergoing for co-administration of Rp with H in patients who had completed 2 months of standard chemotherapy. For the purpose, first a validated HPLC method was developed for the separation of Rp and H, and the same was used for the study of interaction between the two drugs. Like R, Rp was also found to convert to 3-RIF in acid conditions, which reacted further with H to form HYD. The pH-rate profile was also similar in shape to that established with the combination of R and H; maximum decomposition occurred at pH 2, where Rp loss was to an extent of approximately 30%, while corresponding decomposition of H was approximately 9%. These values were similar to those reported for the combination of R (approximately 33%) and H (approximately 10%). Hence, the study suggests that co-administration of Rp and H should be avoided, like in case of R and H, and the two drugs should not be formulated directly into a single dosage form.

Efficient intermittent rifapentine-moxifloxacin-containing short-course regimen for treatment of tuberculosis in mice

Long-half-life drugs raise the hope of once-a-week administration of antituberculous treatment. In a previous study with the murine model of tuberculosis, the most active intermittent regimen which contained rifapentine (RFP), isoniazid (INH), and moxifloxacin (MXF) given once a week during 5.5 months, preceded by 2 weeks of daily treatment with INH, rifampin (RIF), pyrazinamide (PZA), and MXF, was less active than the standard 6-month daily RIF-INH-PZA regimen. We evaluated with the same model similar regimens in which we increased the dosing of rifapentine from 10 to 15 mg/kg of body weight and of moxifloxacin from 100 to 400 mg/kg. Mice infected intravenously by 6.2 x10(6) CFU of Mycobacterium tuberculosis H37Rv were treated 2 weeks later when infection was established. After 6 months of treatment, all mice had negative lung culture. After 3 months of follow-up, no relapse occurred in the two groups that received moxifloxacin at 400 mg/kg, whatever the dosage of RFP, and in the group receiving the standard RIF-INH-PZA control regimen. In contrast, in the two groups receiving moxifloxacin at a lower dosage, the relapse rate was significantly higher (13% in mice receiving RFP at 15 mg/kg and 27% in those receiving RFP at 10 mg/kg). Finally, the fully intermittent once-a-week regimen (26 drug ingestions) of INH, RFP (15 mg/kg), and MXF (400 mg/kg) led to a relapse rate of 11%. In conclusion, when used at high dosage, rifapentine and moxifloxacin are very efficient when combined with isoniazid in a once-a-week treatment in mouse tuberculosis.

Rifapentine, moxifloxacin, or DNA vaccine improves treatment of latent tuberculosis in a mouse model

RATIONALE

Priorities for developing improved regimens for treatment of latent tuberculosis (TB) infection include (1) developing shorter and/or more intermittently administered regimens that are easier to supervise and (2) developing and evaluating regimens that are active against multidrug-resistant organisms.

OBJECTIVES AND METHODS

By using a previously validated murine model that involves immunizing mice with Mycobacterium bovis bacillus Calmette-Guérin to augment host immunity before infection with virulent Mycobacterium tuberculosis, we evaluated new treatment regimens including rifapentine and moxifloxacin, and assessed the potential of the Mycobacterium leprae heat shock protein-65 DNA vaccine to augment the activity of moxifloxacin.

MEASUREMENTS

Quantitative spleen colony-forming unit counts, and the proportion of mice with culture-positive relapse after treatment, were determined.

MAIN RESULTS

Three-month, once-weekly regimens of rifapentine combined with either isoniazid or moxifloxacin were as active as daily isoniazid for 6-9 mo. Six-month daily combinations of moxifloxacin with pyrazinamide, ethionamide, or ethambutol were more active than pyrazinamide plus ethambutol, a regimen recommended for latent TB infection after exposure to multidrug-resistant TB. The combination of moxifloxacin with the experimental nitroimidazopyran PA-824 was especially active. Finally, the heat shock protein-65 DNA vaccine had no effect on colony-forming unit counts when given alone, but augmented the bactericidal activity of moxifloxacin.

CONCLUSIONS

Together, these findings suggest that rifapentine, moxifloxacin, and, perhaps, therapeutic DNA vaccination have the potential to improve on the current treatment of latent TB infection.

Weekly moxifloxacin and rifapentine is more active than the denver regimen in murine tuberculosis

RATIONALE

Treatment of tuberculosis with an efficacious once-weekly regimen would be a significant achievement in improving patient adherence. Currently, the only recommended once-weekly continuation phase regimen of isoniazid plus rifapentine (10 mg/kg) is inferior to standard twice-weekly therapy with isoniazid plus rifampin and is, therefore, restricted to non-high-risk patients. The substitution of moxifloxacin, a new 8-methoxyfluoroquinolone, for isoniazid and an increase in the dose of rifapentine could augment the activity of once-weekly regimens.

METHODS

To test this hypothesis we evaluated the sterilizing activity of improved once-weekly rifapentine-based continuation phase regimens in a murine model that mimics the treatment of high-risk patients with tuberculosis. The bactericidal activity of standard daily therapy and standard intermittent therapy ("Denver" regimen) was also assessed to evaluate the effect of intermittent drug administration during the initial phase of therapy.

RESULTS

After 2 mo of treatment, lung colony-forming unit counts were 1 log(10) lower in mice treated with standard daily therapy than with the Denver regimen. During the continuation phase, the sterilizing activity of once-weekly moxifloxacin plus rifapentine (15 mg/kg) was significantly greater than that of the predominantly twice-weekly Denver regimen of isoniazid plus rifampin. No significant difference in sterilizing activity was detected between once-weekly isoniazid plus rifapentine (15 mg/kg) and the Denver regimen.

CONCLUSIONS

These results suggest that the efficacy of the once-weekly isoniazid plus rifapentine continuation phase regimen can be increased by substituting moxifloxacin for isoniazid and by increasing the dose of rifapentine to a clinically acceptable level of 15 mg/kg.

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.

Fluoroquinolone Susceptibility among Mycobacterium tuberculosis Isolates from the United States and Canada

BACKGROUND

There is increasing interest in the possible role of new fluoroquinolone antibiotics for treatment of tuberculosis, but widespread use of fluoroquinolones for treatment of other bacterial infections may select for resistant strains of Mycobacterium tuberculosis.

METHODS

We evaluated fluoroquinolone susceptibility using the proportion method (critical ciprofloxacin concentration for susceptibility testing, 2.0 mu g/mL) in isolates obtained from patients enrolled in Tuberculosis Trial Consortium clinical trials during the period of 1995-2001 and in a referral sample of isolates sent to the Centers for Disease Control and Prevention (Atlanta, GA) during the period of 1996-2000 for additional testing, often because of drug resistance.

RESULTS

Of the 1373 isolates from the clinical trials, 1324 (96%) were susceptible to isoniazid and rifampin; 2 (0.15%) of these isolates were also resistant to ciprofloxacin. Of the 1852 isolates from the referral sample, 603 (32.6%) were resistant to isoniazid and rifampin (i.e., multidrug resistant), 849 (45.7%) were resistant to >or=1 first-line drug but were not resistant to both isoniazid and rifampin, and 400 (21.6%) were susceptible to all first-line agents. Ciprofloxacin resistance was found in 33 (1.8%) of the referral-sample isolates. Most ciprofloxacin-resistant isolates (25 [75.8%]) were resistant to isoniazid and rifampin.

CONCLUSIONS

Despite widespread use of fluoroquinolones for treatment of common bacterial infections, resistance among clinical isolates of M. tuberculosis in the United States and Canada remains rare, occurring primarily among multidrug-resistant strains.

Prospects for new tuberculosis treatment in Africa

Health services in Africa are being overburdened by a continuous increase of cases of tuberculosis (TB), largely resulting from the large pool of infected individuals becoming co-infected with HIV. To help deal with the situation, TB treatment schedules need to be shorter and simpler, with minimal contact between the patient and the service provider required, if the problems of non-compliance and of ineffective service provision are to be overcome. Various drugs not marketed for use in the treatment of TB are currently under investigation for their potential roles in the simplification or shortening of treatment schedules. These mainly include the long-acting rifamycins and the fluoroquinolones. Furthermore, new drug development is focused on an understanding of the host-pathogen interaction leading to infection, latency and disease. Of these, latency is least understood. The use of molecular diversity and combinatorial chemistry, proteomics, and the use of the whole genome to discover drug targets are expected to produce new lead compounds for turning into drugs to treat active, latent and multi-drug-resistant TB more effectively in the foreseeable future.

M. tuberculosis persistence, latency, and drug tolerance

The success of Mycobacterium tuberculosis as a pathogen is largely attributable to its ability to persist in host tissues, where drugs that are rapidly bactericidal in vitro require prolonged administration to achieve comparable effects. Latency is a frequent outcome of untreated or incompletely treated M. tuberculosis infection, creating a long-standing reservoir of future disease and contagion. Although the interactions between the bacterium and its host that result in chronic or latent infection are still largely undefined, recent years have seen a resurgence of interest and research activity in this area. Here we review some of the classic studies that have led to our current understanding of M. tuberculosis persistence, and discuss the varied approaches that are now being brought to bear on this important problem.

Fluoroquinolone-containing third-line regimen against Mycobacterium tuberculosis in vivo

The objective of the present study was to compare the activities of a third-line regimen recommended by the World Health Organization (WHO) and two derivatives of that regimen with the activity of the standard combination of isoniazid, rifampin, and pyrazinamide as a positive control against Mycobacterium tuberculosis in a murine model. The WHO regimen combines ofloxacin (OFX), ethionamide, amikacin, and pyrazinamide; in the two derivatives of this regimen, OFX was replaced by levofloxacin (LVX) or moxifloxacin (MXF). The four drugs, a fluoroquinolone (either OFX, LVX, or MXF), ethionamide, pyrazinamide, and amikacin, were administered for the first 2 months (initial phase); and two drugs, a fluoroquinolone (either OFX, LVX, or MXF) and ethionamide, were administered for the following 10 months (continuation phase). After 6 months of treatment, only the spleens and lungs of mice treated with the standard regimen became culture negative. From 9 months onward, all of the organs of mice treated with the MXF-containing third-line regimen were culture negative. The majority of organs from mice treated with the OFX-containing regimen continued to be culture positive, and the mean CFU counts remained unchanged for as long as 12 months. The results for mice treated with the LVX-containing regimen fell between those for the groups receiving the MXF- and OFX-containing regimens. In conclusion, the activity of the OFX-containing third-line regimen against M. tuberculosis was rather weak in vivo, whereas when OFX was replaced by MXF, 9 months of treatment with a modified third-line regimen displayed bactericidal activity comparable to that of 6 months of treatment with the standard regimen in mice. The MXF-containing third-line regimen seems to be a powerful alternative for the treatment of tuberculosis (TB) when isoniazid and rifampin cannot be used, which is the main feature of multidrug-resistant TB.

In vitro postantibiotic effects of rifapentine, isoniazid, and moxifloxacin against Mycobacterium tuberculosis

Postantibiotic effects (PAEs) of rifapentine, isoniazid, and moxifloxacin against Mycobacterium tuberculosis ATCC 27294 were studied using a radiometric culture system. Rifapentine at 20 mg/liter gave the longest PAE (104 h) among the drugs used alone. The combinations of rifapentine plus isoniazid, rifapentine plus moxifloxacin, and isoniazid plus moxifloxacin gave PAEs of 136.5, 59.0, and 8.3 h, respectively.

Moxifloxacin-containing Regimen Greatly Reduces Time to Culture Conversion in Murine Tuberculosis

Tuberculosis continues to be a major cause of morbidity and mortality in the world. The expansion of tuberculosis control programs has been limited by the lengthy and cumbersome nature of current chemotherapeutic regimens. A new drug that improves the sterilizing activity of current regimens would reduce the duration of therapy without sacrificing efficacy, thereby enhancing treatment completion rates and preserving precious public health resources. The new 8-methoxyfluoroquinolone moxifloxacin has potent activity against both actively multiplying and nonactively multiplying tubercle bacilli. Using a murine model that is representative of chemotherapy for human tuberculosis, we show that the combination of moxifloxacin, rifampin, and pyrazinamide reduced the time needed to eradicate Mycobacterium tuberculosis from the lungs of infected mice by up to 2 months when compared with the standard regimen of isoniazid, rifampin, and pyrazinamide. The findings suggest that this regimen has the potential to substantially shorten the duration of therapy needed to cure human tuberculosis.

Fluoroquinolones, tuberculosis, and resistance

Although the fluoroquinolones are presently used to treat tuberculosis primarily in cases involving resistance or intolerance to first-line antituberculosis therapy, these drugs are potential first-line agents and are under study for this indication. However, there is concern about the development of fluoroquinolone resistance in Mycobacterium tuberculosis, particularly when administered as monotherapy or as the only active agent in a failing multidrug regimen. Treatment failures as well as relapses have been documented under such conditions. With increasing numbers of fluoroquinolone prescriptions and the expanded use of these broad-spectrum agents for many infections, the selective pressure of fluoroquinolone use results in the ready emergence of fluoroquinolone resistance in a diversity of organisms, including M tuberculosis. Among M tuberculosis, resistance is emerging and may herald a significant future threat to the long-term clinical utility of fluoroquinolones. Discussion and education regarding appropriate use are necessary to preserve the effectiveness of this antibiotic class against the hazard of growing resistance.