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Clinical and Molecular Analysis of Macrolide Resistance in Mycobacterium avium Complex Lung Disease

Departments of Medicine, Microbiology, Thoracic Surgery, and Occupational Health Sciences, University of Texas Health Center, Tyler; and Department of Pathology, Baylor College of Medicine, Houston, Texas

Correspondence and requests for reprints should be addressed to David E. Griffith, M.D., The University of Texas Health Center, Department of Medicine, 11937 U.S. Hwy 271, Tyler, TX 75708. E-mail: david.griffith{at}uthct.edu

	ABSTRACT

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Rationale: The clinical features and outcome of macrolide-resistant Mycobacterium avium complex (MAC) lung disease are not known.

Objectives: Characterize patients, treatment, and isolates in macrolide-resistant MAC lung disease.

Methods: Retrospective chart review, susceptibility testing, molecular fingerprinting, and DNA sequence analyses of resistant MAC isolates.

Measurements and Main Results: We identified 51 patients over a 15-yr period with clarithromycin-resistant MAC (minimum inhibitory concentration (MIC) 32 µg/ml) lung disease at a single referral center. Twenty-four (47%) patients had nodular disease with bronchiectasis and 27 (53%) had upper lobe cavitary disease. Most patients (77%) had M. intracellulare. Sequencing of the 23S r-RNA gene showed 49 of 51 isolates (96%) with the expected mutation in adenine 2058 or 2059. Risk factors for resistance included macrolide monotherapy or combination with a quinolone only (39/51 or 76%). Macrolide resistance developed in 12 of 303 (4.0%) patients started on the American Thoracic Society–recommended two companion drugs, with no risk difference in clarithromycin versus azithromycin and daily versus intermittent therapy. Sputum conversion with macrolide-resistant MAC occurred in 11 of 14 (79%) patients who received more than 6 mo of injectable aminoglycoside therapy and lung resection, compared with 2 of 37 (5%) who did not. The 1-yr mortality in patients who remained culture positive was 34% (13/38) compared with 0% (0/13) of patients who became culture negative (converted).

Conclusions: Macrolide resistance rarely occurs in patients also receiving ethambutol and a rifamycin. Macrolide-resistant MAC lung disease requires aggressive drug and surgical therapy for cure.

Key Words: aminoglycosides • sequencing mutations • surgery

Macrolides have become the mainstay of treatment regimens and prophylaxis for Mycobacterium avium complex (MAC). A major therapeutic dilemma faced in these settings is how best to prevent the development of macrolide resistance, and how to treat it when it occurs.

We have been involved in single-center macrolide clinical drug trials of MAC lung disease since the availability of clarithromycin in 1991 for clinical use. Patients have been enrolled in single- or multidrug regimens that included an initial macrolide (clarithromycin or azithromycin) alone, or a macrolide combined with ethambutol and a rifamycin (rifabutin or rifampin) (1–6). The objectives of the current study are to assess the effectiveness of these latter two drugs in preventing the development of macrolide resistance, to determine reasons for the development of such resistance, to look at the results of treatment of resistant disease, and to characterize the MAC strains associated with this resistance. We reviewed the development of macrolide resistance in all patients enrolled in these clinical treatment trials as well as patients referred for treatment failure whose isolate was subsequently found to be macrolide resistant. Partial results of this study have been previously reported in an abstract (7).

	METHODS

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Patient Population
Approval for this study was obtained from the University of Texas Health Center at Tyler (UTHCT) Human Subjects Institutional Review Board. Susceptibility results of the Mycobacteria/Nocardia Research Laboratory at UTHCT were screened for clarithromycin resistance among patients with MAC treated at UTHCT between 1991 and 2005. All patients with resistant isolates had multiple positive sputum specimens that grew MAC, abnormal chest radiographs, and consistent clinical findings as defined by the most recent American Thoracic Society statement on nontuberculous mycobacteria (8). None was known to be seropositive for HIV or at high risk for HIV infection, and all had MAC isolates with clarithromycin minimum inhibitory concentration (MIC) of 32 µg/ml or greater (9, 10). Patients with cystic fibrosis were excluded.

Approximately 50% of patients with macrolide-resistant MAC were enrolled in one or more noncomparative, open, prospective treatment trials that involved either clarithromycin or azithromycin (1–6). Patients in the trials signed an informed consent approved by the UTHCT institutional review board. Patients received their medications either daily or three times weekly. After completion of these trials, all subsequently encountered patients with initial macrolide-susceptible MAC lung disease were treated either daily or with the three-times-weekly regimen with a three-drug regimen using the drug dosages used in the clinical trials but without being part of a treatment study. The remaining 24 patients with macrolide-resistant MAC were referred to one of the authors because of treatment failure and/or their MAC isolate had become macrolide resistant.

Development of Macrolide Resistance
Patient histories were reviewed for risk factors that may have contributed to the development of macrolide resistance. These included macrolide monotherapy, macrolide plus only a quinolone, and a macrolide combination regimen other than a quinolone alone that did not include ethambutol (e.g., a macrolide plus rifampin or rifabutin). These were considered as potential risk factors if given for 1 mo or more. Noncompliance was considered to be present if the patient did not take his/her medicines as instructed (11).

Patient Treatment, Safety Tests, and Outcome
The details of the medical and surgical therapy of patients after their isolates were found to be macrolide resistant were reviewed. All patients were treated at UTHCT by one of the authors. No written treatment protocol was followed, but each case was discussed among the authors. Surgical resection for cure or debulking of the patients' MAC lung disease was considered in each patient. The chest surgeons involved in these cases had more than 30 yr experience with mycobacterial surgery, including MAC lung disease (12). Surgical results for four of these patients have been published previously (12).

Drug therapy used for patients with macrolide-resistant MAC, especially over the past 8 to 10 yr, included discontinuation of the macrolide, and initiation of therapy with ethambutol 25 mg/kg daily, rifabutin 300 to 450 mg daily, and streptomycin 5 to 10 mg/kg (up to 1,000 mg daily), or amikacin 5 to 7 mg/kg (up to 500 mg once daily). In patients with normal serum creatinine and blood urea nitrogen and an age younger than 60 yr, the injectables were usually given five times per week for the first 3 mo, then decreased to three times weekly for as long as could be tolerated. Patients between the ages of 60 and 70 were dosed three times weekly, and those older than 70 were dosed only twice weekly, again for as long as could be tolerated. With the exception of two patients, injectables were given intramuscularly. Aminoglycoside serum levels were not routinely obtained, in part because of the difficulty in obtaining streptomycin serum levels. Patients were treated until they were culture negative for 12 mo on therapy.

Aminoglycoside-related ototoxicity was monitored clinically by questioning for tinnitus and subjective changes in hearing at baseline and on each clinic visit. Audiograms were done if such changes were reported. Aminoglycosides were continued for the duration of therapy, if possible, in patients who had conversion of sputum to acid-fast bacilli (AFB) culture-negative status. Drug dosages or frequency were not changed for the development of tinnitus, but were changed or the aminoglycoside discontinued on an individual basis after review with the patient when subjective hearing loss was confirmed by audiogram. Other safety tests, including monitoring of renal and hepatic function, and blood counts were performed routinely. Patients filled out a symptom questionnaire at the time of each visit. Ethambutol-related toxicity was monitored with eye examinations, including the Snellen eye chart and the Ishihara color book, performed at baseline at the time of each visit, and whenever new eye symptoms (blurriness, change in vision) occurred (13).

Cultures
Sputum samples for AFB analysis were obtained on a monthly basis when possible while on therapy and at least every 2 mo off therapy for the first 12 mo. Details of sputum processing and culture methods have previously been published (1, 3, 14).

Sputum conversion was defined as a minimum of three consecutive negative cultures over a minimum time of 3 mo. Cure was defined as negative sputum cultures for the infecting MAC strain for 12 mo or longer off drug therapy. Relapse isolates were compared with initial strains using pulsed-field gel electrophoresis (PFGE) (14–16).

Isolates
Isolates were submitted to the Mycobacteria/Nocardia Research Laboratory at UTHCT. Organisms from a swab of an isolation plate were placed in tryptic soy broth with 15% glycerol and frozen at –70°C until needed. Organisms were identified as MAC based on a commercial DNA/RNA probe (AccuProbe; GenProbe, Inc., San Diego, CA). Macrolide-resistant MAC isolates were also identified to species using restriction fragment length polymorphism (RFLP) analysis of the hsp65 gene (see PCR RESTRICTION ENZYME ANALYSIS OF THE hsp65 GENE) (17, 18).

Susceptibility Testing
Susceptibility testing to clarithromycin was performed by broth microdilution in Mueller Hinton broth supplemented with oleic acid, albumin, and dextrose using current Clinical Laboratory Standards Institute guidelines (10). Isolates with MICs of 32 µg/ml or greater were considered resistant (10). MICs for azithromycin were not performed, with clarithromycin used as the class drug for the macrolides (19, 20).

PFGE
PFGE was performed on macrolide-susceptible and -resistant isolates from the same patient (where available) using previously described techniques (14–16). Definitions of isolates as the same (indistinguishable), probably related, possibly related, or unrelated (different or unique) were the same as in a previous MAC study (14). These were modified from the criteria of Tenover and colleagues used to define bacterial outbreaks (21).

23S r-RNA Partial Gene Sequencing
Genomic DNA was isolated; polymerase chain reaction (PCR) was then performed on a DNA fragment of approximately 445 bp in the 23S r-RNA that spanned position 2058 and 2059. PCR product purification was performed by the enzymatic ExonucleaseI/Shrimp Alkaline Phosphatase method. All sequencing reactions had excess dyes removed by size exclusion spin columns (Millipore, Billerica, MA) and were electrophoresed on an ABI 377 DNA sequencer according to Applied Biosystems (Big Dye Terminator; Applied Biosystems, Foster City, CA) standard sequencing protocols. Sequencing was performed by Amplicon Express (Pullman, WA).

PCR Restriction Enzyme Analysis of the hsp65 Gene
A 441-bp portion of the hsp65 gene (Telenti fragment) was amplified by PCR as previously described (17, 18), then digested with BstEII or HaeIII. PCR restriction enzyme analysis (PRA) patterns were compared with the previous profiles characterized by Smole and colleagues (18) for M. avium, M. intracellulare, and undefined MAC taxa.

Statistical Analysis
Group data are expressed as means ± SD. Comparison of outcomes between patient treatment groups was done with Fisher's exact test. Analysis of the effect of clinical variables on treatment response was done with the t test for equality of means after evaluation of the data with Levene's test for equality of variances. Two-tailed p values were used for all t tests. Significance of all comparisons was determined with a p value of less than 0.05.

	RESULTS

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Patient Population
Characteristics of all 51 patients with macrolide-resistant MAC isolates are shown in Table 1. Of the 27 patients with upper lobe cavitary disease, the majority were males (78%), most were former heavy smokers, one-third abused alcohol, and their mean age at the onset of MAC disease was 58 yr. These are typical features of patients with MAC with upper lobe cavitary disease (22–24). Of the 24 patients with nodular disease, more than 90% were women, the majority (71%) had never smoked, and none abused alcohol. Their mean age at the onset of MAC disease was 65 yr. These features are typical of patients with nodular type MAC lung disease (25–28).

Resistance among patients in the drug trials was identified in two treatment groups (see Table 2). Group 1 included 59 patients who received 4 mo of initial macrolide monotherapy, with subsequent addition of ethambutol and a rifamycin (1). All isolates of MAC were susceptible when treatment was initiated, but 12 isolates (20.3%) became resistant over the subsequent months of therapy.

An additional 24 patients were referred for treatment due to previous treatment failure or because of known macrolide resistance, and their isolates were confirmed to be macrolide resistant.

An additional three patients placed on an initial three-drug regimen but not part of treatment trials (two patients) or an IFN- randomized trial (one patient) also had MAC isolates that became macrolide resistant.

Initial macrolide monotherapy, a macrolide plus a quinolone, or either of these occurring as a result of discontinuation of ethambutol appeared to be the major reasons for development of resistance in most patients. These reasons changed with time. In the first 5 yr (1991–1995), all resistance (17/17 cases) was related to macrolide monotherapy. Over the subsequent 10 yr (1996–2005), this became less common (only 11/34, or 33%), with the use of a macrolide plus a quinolone (essentially monotherapy) becoming increasingly important (11/34, or 33%). Overall, 39 of 51 (76%) isolates from patients with known histories became resistant after one of these two therapeutic approaches. Only 9 of 51 isolates (18%) from patients with available histories from 1991 to 2005 developed macrolide resistance when started on standard three-drug therapy.

Patient Treatment, Safety Tests, and Outcome
The 51 patients were divided into four groups on the basis of their therapy for macrolide-resistant MAC (see Table 3). Of the 14 patients from group 1 who had lung resections and prolonged aminoglycoside therapy (mean, 12.0 ± 4.1 mo; range, 7–19 mo) combined with daily high-dose ethambutol and rifabutin, sputum samples from 11 patients (79%) have converted to culture negative. Nine of these patients have completed therapy, one is still on therapy, and one died of respiratory complications. These latter two patients have had more than 6 mo of negative sputum cultures. Of the patients included as treatment failures, two died of medical complications within 6 wk of their lung surgery, so the microbiological success of the surgery could not be assessed. The third patient considered a failure was lost to follow-up after surgery in another institution. Thus, of the patients who survived their perioperative period with adequate follow-up, 11 of 11 (100%) are culture negative to date. No patient who met the case definition of sputum conversion in this group has relapsed on or off therapy.

Of the remaining 37 patients, 2 received surgery alone without prolonged ( 6 mo) aminoglycoside therapy (group 2), 8 received prolonged aminoglycoside therapy without surgical resection (group 3), and 27 patients received neither of these treatment modalities (group 4). Only two patients (5.4%) from these three groups converted their sputum specimens to negative (see Table 3). Of these 37 patients, 22 (59%) are known to have died of this disease, with a mean survival of 23.8 ± 25.5 mo (range, 2–81 mo). Of the two patients from groups 3 and 4 who converted their sputum to negative, one has relapsed and the other has been lost to follow-up.

Of the patients who failed therapy (totals, groups 2–4), the 1-yr mortality from diagnosis of their MAC resistance was 13 of 38 (34%) and the 2-yr mortality was 17 of 38 (45%). Eight of these patients survived 5 yr after the diagnosis of macrolide resistance; 11 are still living but have survived less than 5 yr (follow-up, 16–58 mo).

Of the patients whose sputum converted to negative, the 1- and 2-yr mortality was 0 of 13 (0%), with the caveat that some patients in both groups have been monitored less than 2 yr. Two of these patients survived 5 yr after the diagnosis of macrolide resistance; 10 are still living but have survived less than 5 yr (follow-up, 18–54 mo). These data must be interpreted in the context that patients with good lung reserve were surgical candidates, and those with respiratory failure and/or poor respiratory reserve at the time of diagnosis of their drug resistance were not surgical candidates.

Drug Toxicity
Drug-related adverse events were common in patients treated for macrolide-resistant MAC lung disease. Most patients received multidrug regimens that included rifabutin, ethambutol, and a parenteral agent, most often streptomycin. These drug toxicity data are pertinent only for drug regimens begun after patients were recognized as having macrolide-resistant MAC lung disease. Significant drug toxicity or adverse events were defined as symptoms or adverse events requiring either dosage change or discontinuation of a drug. The incidence of significant adverse events and the most common adverse events with each drug were as follows: rifabutin, 18 of 44 (41%) patients had gastrointestinal symptoms or polymyalgia/polyarthralgia syndrome; ethambutol, 4 of 40 (10%) patients experienced ocular toxicity; ethionamide, 2 of 7 (29%) patients had gastrointestinal symptoms; streptomycin, 10 of 24 (42%) patients experienced ototoxcity, vestibular toxicity, or hypersensitivity; amikacin, 3 of 11 (27%) patients experienced ototoxicity or vestibular toxicity; ciprofloxacin, 1 of 4 (25%) patients had gastrointestinal symptoms; clofazimine, 2 of 4 (50%) patients had gastrointestinal symptoms; linezolid, 3 of 4 (75%) patients had peripheral neuropathy or anemia; 8-methoxy-fluoroquinolones, 0 of 6; and rifampin, 0 of 15 (most patients receiving rifampin were switched from rifabutin). No patients experienced significant hepatotoxicity with any of the agents used. These figures likely isolates represent an underestimation of the true incidence of significant drug-related adverse events because not all patients were monitored at UTHCT for the duration of macrolide-resistant MAC lung disease therapy.

Isolates
All isolates were identified as MAC using the commercial DNA/RNA probe (AccuProbe; GenProbe Inc., San Diego, CA). Among the 51 patients identified with macrolide-resistant MAC, all had resistant isolates available for study and 31 had pretreatment or early-treatment macrolide-susceptible isolates available for study. Six of the paired isolates were part of an earlier study that helped define the molecular basis of macrolide resistance in M. intracellulare (29).

Susceptibility Testing
All 51 patients had one and most had two or more treatment failure isolates with MICs to clarithromycin of 32 µg/ml or greater. There were 32 patients with pretreatment isolates, with 31 clarithromycin-susceptible isolates having MICs of 8 µg/ml or less (see Table 4).

23S r-RNA Sequencing
A total of 30 preresistant isolates underwent sequencing of the 23S rRNA gene. All had a wild-type adenine at position 2058 and 2059 (Escherichia coli numbering) of the 23S r-RNA gene. Of the 51 macrolide-resistant isolates that were sequenced, 49 (96%) had the expected point mutation of the adenine at position 2058 or 2059 (29–34). Two isolates had two mutations. At position 2058, three had an AT, 11 had an AC, and 12 had an AG. At position 2059, one had an AT, seven had an AC, and 17 had an AG. Overall, there were 26 mutations involving the adenine at position 2058 and 25 mutations involving the adenine at position 2059.

PRA of the hsp65 Gene
A total of 53 macrolide-resistant MAC isolates from 51 patients underwent hsp65 gene PRA. Forty-one isolates (77%) gave a profile of M. intracellulare, all of which matched the Min v1 profile of Smole and colleagues (18). An additional 12 macrolide-resistant isolates (23%) gave a PRA profile of M. avium, of which eight matched the profile of Mav1 and four matched the profile of Mav2 (18). There were no PRA profiles that matched the described patterns of non–M. avium, non–M. intracellulare MAC species (18). Random isolates (14/53, or 26%) underwent species confirmation using the species-specific DNA/RNA probe (AccuProbe) with 100% concordance with the PRA results. M. avium strains were more common among isolates from nodular disease than among isolates from cavitary disease (9/26, or 35%, vs. 3/27, or 11%; p = 0.04).

	DISCUSSION

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The optimal regimen for treatment of macrolide-susceptible MAC lung disease has not been established, although the key role of the macrolide is generally well accepted (1, 3, 8, 35–37). Treatment of disseminated MAC with clarithromycin monotherapy in patients with AIDS and in mice is followed within 2 to 6 mo by the emergence of macrolide resistance (32, 38, 39). Ethambutol has been advocated as the preferred second drug to be used with a macrolide, but animal studies suggest this combination only delays the onset of macrolide resistance but does not prevent it (40). The macrolide clinical trials of MAC lung disease generally used three drugs, with rifampin or rifabutin being given in addition to ethambutol and a macrolide (1–6). A comparison of the value of this three-drug regimen (daily clarithromycin, ethambutol, and rifabutin) versus a two-drug regimen (daily clarithromycin plus ethambutol) was performed by Gordin and coworkers in patients with disseminated MAC (41). The addition of rifabutin did not increase efficacy but did decrease the development of macrolide resistance from 6 of 42 (14%) in the two-drug group to 1 of 44 (2%) in the three-drug group with rifabutin (p = 0.055) among patients with a bacteriologic response at 16 wk of therapy. There was no difference in the incidence of resistance among the nonresponders, however. Long-term follow-up beyond Week 16 was not reported, so the overall resistance rates at the end of therapy, such as were reported in the current study, were not available (41).

In a second randomized, open-label, phase 3, 48-wk clinical trial of treatment of disseminated MAC by Benson and colleagues (42), patients were randomized to receive clarithromycin plus ethambutol, clarithromycin plus rifabutin, or clarithromycin plus both agents. Relapse rates (associated with macrolide resistance) were as follows: 24% in the clarithromycin plus rifabutin group; 7% in the clarithromycin plus ethambutol group; and 6% (p = 0.027 when compared with the clarithromycin plus rifabutin group) among patients receiving clarithromycin, ethambutol, and rifabutin.

The current study of patients on three-drug macrolide regimens that included ethambutol and rifampin or rifabutin given daily or three times weekly supports the benefit of ethambutol and a rifamycin in protecting against macrolide resistance, with a resistance rate of only 4% among 303 patients started on the three-drug regimen, which included both responders and nonresponders and some patients unable to tolerate a companion drug (usually the rifamycin). Of the 48 patients with available drug histories who had macrolide-resistant MAC identified in the current study, only 20% had been started on the same three-drug regimen. A recently reported study of individuals treated with the three-times-weekly three-drug regimen for MAC lung disease reported that 6 of 91 (6.6%) patients became resistant (43). This was based on intent to treat, and other potential risk factors were not detailed. This resistance rate is very comparable to the currently reported rate. The benefit of the rifamycin in this regimen (for MAC lung disease) has not been studied. The concern that the rifamycin does more harm than good because it lowers serum levels of clarithromycin (44) is an important issue yet to be answered, especially for rifampin.

Previous studies of M. avium isolates from patients with AIDS (30, 31) and M. intracellulare isolates from patients with chronic pulmonary disease (29) have demonstrated mutations in the 23S r-RNA gene at positions 2058 and 2059 as the major mechanism of acquired resistance with both clinical isolates and laboratory-derived resistant strains. Approximately 90% of mutations identified from patient isolates involved guanine or cytosine substitutions at position 2058.

The current study had a much higher incidence of mutations at adenine 2059, with approximately 50% of mutations involving position 2058 and 50% involving 2059. However, this is the largest study to date involving MAC lung disease, and most isolates were M. intracellulare rather than M. avium. Adenine 2059 mutations were seen in 33% of in vitro–selected laboratory mutants resistant to clarithromycin in the study by Meier and colleagues (29), but only rarely among clinical patients before the current study. Studies in a macrophage model have shown equivalent virulence for MAC macrolide mutants 2058 AC, 2058 AT, and 2059 AC compared with their isogenic wild-strain parents (45).

Smole and colleagues performed PRA of the 441-bp segment of the hsp65 gene using restriction enzymes BstEII and HaeIII in 278 MAC isolates, including 126 blood isolates from patients with AIDS, and 59 isolates from sputum of HIV-seronegative patients with chronic pulmonary disease (18). (No clinical details of these patients with associated lung disease were provided, however.) They identified only three PRA patterns with M. avium and two patterns with M. intracellulare. Among these 59 isolates, 35 isolates (59%) belonged to M. avium PRA types, only 13 isolates (22%) belonged to M. intracellulare types, and the remaining 11 isolates (19%) were MAC non–M. avium, non–M. intracellulare types. Twenty-four of 35 (69%) M. avium isolates belonged to Mav v2, and 12 of 13 (92%) of M. intracellulare isolates belonged to Min v1. The current study, although a selected patient population, but one with definite lung disease, produced very different results. The majority of the 53 studied strains (77%) were M. intracellulare and only 23% were M. avium. These latter percentages are similar to those of prior studies in our patient population involving macrolide-susceptible disease (14, 16) and other studies of patients with established chronic lung disease (46). The hsp65 PRA type that was present in all strains of M. intracellulare (Min v1) was the same as the Smole and colleagues' study (18), but the predominant M. avium type in that study (Mav v2) was outnumbered 2:1 by Mav v1 in the current study. These features are important as we try to establish the molecular features of the pathogenic human lung strains among the large number of environmental, animal, and human strains that comprise the MAC family.

This is the first study to address the epidemiology, treatment, and outcome of patients with macrolide-resistant MAC lung disease. The overall clinical outcome in these patients is poor, with most patients who remain culture positive dying of respiratory failure.

There are multiple important similarities between macrolide-resistant MAC lung disease and multidrug-resistant tuberculosis (MDR-TB). Both disease processes are inexorably progressive without appropriate therapy and the therapeutic options are limited for both, probably more so for the patient with macrolide-resistant MAC because of the lack of reliably effective oral drugs other than macrolides. It is also apparent that successful therapy for both involves surgery. In a recent review of treatment and outcome of more than 200 patients with MDR-TB, the only intervention that was statistically significantly correlated with improved clinical outcome in all patient groups was surgical resection (47). No medication combination or strategy was consistently associated with favorable clinical response in every patient group (46). In this study of patients with macrolide-resistant MAC, the situation is further complicated in that the best treatment response and outcomes were in patients with both surgical intervention and parenteral aminoglycoside (amikacin or streptomycin) administration. The treatment response was much less favorable for those patients who could not undergo surgery for any reason (e.g., extensive disease, lack of respiratory reserve, lack of consent) or who did not receive parenteral medications for any reason (e.g., poor overall condition, drug toxicity, lack of consent). Although based on a small number of patients, the results of this study suggest that the treatment strategy with the best chance of clinical improvement or cure involves both the use of surgery and administration of parenteral aminoglycosides (amikacin or streptomycin).

The issue of optimal choice and number of companion oral drugs (ethambutol/rifamycin) was not addressed. However, these and other oral agents are clearly not effective without surgery and parenteral aminoglycosides. Conversely, it is not likely, even with surgery, that a parenteral agent would be adequate therapy without companion oral agents. Our recommendation would be daily high-dose ethambutol (25 mg/kg) and daily rifabutin (300–600 mg). The potential for other oral agents such as the 8-methoxy-fluoroquinolones (moxifloxacin or gatifloxacin) is not known. Another important question that this study did not address is the role of in vitro susceptibility testing for managing these patients medically. Previous studies have failed to show a correlation between in vitro susceptibility results for MAC and in vivo treatment response, with the notable exception of the macrolides. Clearly, an important priority for macrolide-resistant MAC disease is the introduction of oral agents with improved activity against MAC. It also should be a priority that patients with macrolide-resistant MAC are managed by physicians who have experience treating such patients given the low response rate and high drug toxicity even in the hands of physicians experienced in the management of MAC lung disease.

	FOOTNOTES

 
Supported by institutional funding at the University of Texas Health Science Center at Tyber (UTHCT).

Originally Published in Press as DOI: 10.1164/rccm.200603-450OC on July 20, 2006

Conflict of Interest Statement: None of the authors has a financial relationship with a commercial entity that has an interest in the subject of this manuscript.

Received in original form March 30, 2006; accepted in final form July 17, 2006

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