Effect of ethambutol on Mycobacteria

Prospective evaluation of ethambutol toxic optic neuropathy in patients of pulmonary tuberculosis

AIM

The aim of the study was to evaluate visual parameters for early detection of ethambutol toxicity to prevent irreversible optic nerve damage.

METHOD

This cohort study included 50 newly diagnosed cases of pulmonary tuberculosis, i.e. 100 eyes, in people aged 18-72 years, who received ethambutol as part of the antitubercular therapy. Before starting antitubercular treatment, at each monthly visit, ophthalmic examination was done that included best corrected visual acuity, optic disc evaluation, colour vision, contrast sensitivity, pupil cycle time and automated perimetry for a period of six months. Ethambutol was discontinued in patients who showed signs of visual impairment. These patients were observed for another three months for reversibility of toxicity.

RESULTS

Visual functions were normal in all patients at baseline. Out of 50 patients, 47 didn't show any signs of deterioration in visual functions even after six months of oral administration of ethambutol. However, 3 out of 50 patients, i.e. 6 eyes (6 %), showed signs of toxicity after 4-5 months - LogMAR visual acuity deteriorated from 0.00 to 1.08 (±0.40); mean contrast sensitivity deteriorated from 1.725 (±0.075) to 1.12 (±0.18); mean deviation in visual fields got impaired from -1.37 (±0.15) to -10.93 (±2.66); and colour vision got severely affected. All the affected patients were aged above 65.

CONCLUSION

In the present study, six percent of 50 patients showed ethambutol toxic optic neuropathy. It was concluded that patients should be comprehensively evaluated for visual parameters before starting the ethambutol treatment. They should be closely monitored by an ophthalmologist for visual functions every month to detect ethambutol toxicity at the early reversible stage to prevent irreversible damage to the optic nerve. Patients should be asked to report immediately if they detect any blurring of vision or colour vision abnormality.

The role of Technetium-99m-Ethambutol scintigraphy in the management of spinal tuberculosis

Spines is a common site of extrapulmonary Mycobacterium tuberculosis infection (MTI). Spine destruction due to MTI can mimic other etiologies. Treatment of choice for spinal tuberculosis (STB) is anti-TB drugs while surgery could be needed in other causes. The gold standard for STB diagnosis is histopathology examination from biopsy tissue. Technetium-99m-ethambutol (^99mTc-EMB) scintigraphy can be used to detect and localize of TB. The aim of this study was to evaluate the role of ^99mTc-EMB scintigraphy in STB management. Retrospective study was carried out from 2006 to 2014. Subject STB were patient STB with suspected of STB and underwent ^99mTc-EMB scintigraphy. The histopathologic result was used as gold standard. Whole body planar acquisition was taken at 1 and 3 h postinjection of 370 MBq. Single-photon emission computed tomography/computed tomography acquisition was performed on suspected area. ^99mTc-EMB image were analyzed by two nuclear medicine specialis. The 93 subject STB were included in this study. Histopathologic data were available in 40/93 subject STB. Positive and negative ^99mTc-EMB scintigraphy were 32 and 8 subject STB. ^99mTc-EMB scintigraphy result STB was concordance with the histopathologic finding in 37 subject STB. Sensitivity, specivicity, positive- and negative-predictive value, and accuracy of ^99mTc-EMB scintigraphy 90.91%, 71.43%, 93.75%, 62.5%, and 87.5%, respectively. This study showed that patient STB with suspected ^99mTc-EMB scintigraphy result could be directly treated with anti-TB. ^99mTc-EMB scintigraphy has significant role in the management of STB.

Diagnostic value of 99mTc-ethambutol scintigraphy in tuberculosis: compared to microbiological and histopathological tests

OBJECTIVE

Tuberculosis (TB) still remains the world's endemic infection. TB affects the lungs and any part of the body other than the lung. The diagnosis of TB has not changed much over the decades. Ethambutol is one of the first line treatments for TB. It can be labeled using ^99mTc. ^99mTc-ethambutol will be accumulated in the site of TB lesion and can be imaged using gamma camera. The aim of this study was to evaluate the diagnostic value of ^99mTc-ethambutol scintigraphy in detecting and localizing of TB.

METHODS

Retrospective cross-sectional study was done. Subjects were patients suspected of having TB infection. Whole body and SPECT-CT imaging at the suspected area was done 1 and 4 h after injection of 370-555 MBq ^99mTc-ethambutol. ^99mTc-ethambutol scintigraphy was analyzed visually. The results were compared with that of histopathological or microbiological tests. Statistical analysis was done to determine the sensitivity, specificity, PPV, NPV and accuracy.

RESULTS

One hundred and sixty-eight subjects were involved in this study. There were 110 men and 58 women with mean age of 34.52 ± 11.94 years. There were concordance results in 156 (92.86%) and discordant in 12 (7.14%) subjects between ^99mTc-ethambutol scintigraphy and histopathological or microbiological result. The sensitivity, specificity, PPV, NPV and accuracy of ^99mTc-ethambutol scintigraphy in the diagnosis of pulmonary TB were 93.9, 85.7, 93.9, 85.7 and 91.4%, respectively, for extra-pulmonary TB 95.5, 77.8, 97.9, 63.6, and 85.1%, respectively, and for total tuberculosis 94.9, 83.3, 96.3, 78.1 and 92.8%, respectively. There was no side effect observed in this study.

CONCLUSION

^99mTc-ethambutol scintigraphy is a useful diagnostic imaging technique to detect and localize intra- and extra-pulmonary TB. It is safe to be performed even in pediatric patient. Consuming ethambutol less than 2 weeks did not influence the result.

Incidence and clinical features of ethambutol-induced optic neuropathy in Korea

BACKGROUND

The incidence, clinical manifestations, and contributory factors in ethambutol-induced optic neuropathy (EON) have not been widely studied.

METHODS

A retrospective chart review of 857 patients who took ethambutol for treatment of tuberculosis identified 89 patients who complained of decreased vision after initiation of treatment and were therefore referred to the ophthalmology clinic of Seoul National University Hospital, Korea, between January 2003 and December 2005. Information was extracted in regard to systemic illnesses, duration and dose of ethambutol therapy, visual acuity, color vision, ophthalmoscopic examination, visual fields, and visual evoked potentials (VEPs).

RESULTS

EON was diagnosed in 13 (1.5%) patients during a follow-up period of 12.54 +/- 9.97 months. The average dose of ethambutol was 17.85 +/- 2.21 mg/kg/day, and the duration of therapy was 9.38 +/- 10.12 months. Ophthalmic findings included decreased visual acuity (65.4%), abnormal visual fields (65.4%), abnormal color vision (61.5%), optic disc pallor (38.5%), or increased latency on VEP tests (65.4%). Slightly less than one third of patients showed improvement in visual function after discontinuing ethambutol. The latency for recovery was 5.38 +/- 1.71 months. No patient with optic disc pallor at the time of diagnosis of EON showed visual function improvement. Renal dysfunction and the daily dose of ethambutol, but not the duration of treatment, contributed to EON.

CONCLUSIONS

Based on this study, the incidence of EON in Koreans is estimated to be <2%. However, visual function after discontinuation of ethambutol is reversible in only a minority of patients and does not occur if optic disc pallor is present. Renal dysfunction and daily dose of ethambutol, but not duration of ethambutol treatment, seem to be related to development of EON.

Ocular ethambutol toxicity

Ethambutol is an antimicrobial agent used frequently to treat tuberculosis. The most commonly recognized toxic effect of ethambutol is optic neuropathy, which generally is considered uncommon and reversible in medical literature. We describe a 43-year-old man who developed signs and symptoms of bilateral optic neuropathy during treatment with ethambutol. This case and a review of the literature show the severe and unpredictable nature of ethambutol toxicity and its potential for irreversible vision loss despite careful ophthalmologic monitoring.

Ethambutol in tuberculosis

Ethambutol gained rapid acceptance as a substitute for PAS in tuberculosis therapy because of improved patient tolerance and convenience of administration. Ocular toxicity was recognized early on but was thought to be a problem only at higher dosages. There have, however, been a number of reports of serious visual impairment on conventional dosage, with permanent blindness in some cases, and painfully slow recovery in others. The precise mechanism for the optic neuritis is not clear, and toxicity is difficult to predict in the individual patient. Ethambutol appears to contribute only marginally to modern short course regimens and should be replaced with a less toxic agent.

Ethambutol MICs and MBCs for Mycobacterium avium complex and Mycobacterium tuberculosis

We determined the MICs of ethambutol for both Mycobacterium avium and Mycobacterium tuberculosis strains by using broth dilution (7H12 broth, radiometric method) and agar dilution (7H11 agar) methods. We found the MICs to be much lower in liquid than in solid medium. The broth-determined MICs for susceptible M. tuberculosis and most of the M. avium strains were comparable to the levels in blood of patients, being lower than the peak levels. We propose that the MICs, determined radiometrically in in 7H12 broth, be considered as tentative criteria for susceptibility testing of M. avium isolates in future clinical trials. The use of these values instead of critical concentrations should also be considered as an alternative to the conventional susceptibility testing method in chemotherapy of tuberculosis. Ethambutol produced bactericidal effects against both M. tuberculosis and M. avium, and the MIC/MBC ratios were in the same range for both species when MICs and MBCs were tested in 7H12 broth by conventional sampling and plating.

Clinical pharmacology of antitubercular drugs

Isoniazid, rifampin, and ethambutol are the three major drugs used in the modern treatment of patients with tuberculosis. Data on these drugs in children have been derived primarily from their clinical use in pediatrics and extrapolation from experiences in adults. A number of questions remain concerning the clinical pharmacology and appropriate use of these drugs in children. Additional pediatric pharmacokinetic studies are necessary to confirm the current dosage recommendation and use of these agents in the pediatric patient.

Chemical characterization of ethambutol binding to Mycobacterium smegmatis

Recent studies showed that critical binding of ethambutol to Mycobacterium smegmatis was both inhibited and reversed by ions. This apparent ion-susceptible characteristic suggested that ethambutol is held at critical sites by electrostatic bonds. Dissociation constant, pH, and ionic strength studies were designed to further characterize ethambutol binding. M. smegmatis was grown in Sauton synthetic liquid medium (pH 7.4) under aerated conditions at 37 C. The pH or ionic strength of the medium was modified to meet the needs of particular experiments. Titration data revealed that ethambutol dihydrochloride has two apparent dissociation constants (pKa(1) = 6.35, pKa(2) = 9.35). Uptake experiments, in which pH was varied, showed that dihydrochloride and free base ethambutol were bound to a greater extent than the monohydrochloride. However, dihydrochloride and free base binding were not related to biological activity. Ethambutol exerted its maximal growth inhibitory effect at pH values near neutrality, where it exists primarily as the monohydrochloride and showed minimal binding. The increased ethambutol binding observed at pH 7.4 in media of lowered ionic strength was consistent with growth studies showing a reduction in the minimal inhibitory growth concentration in such media. However, nonspecific as well as critical binding was enhanced at low ionic strength. We conclude that binding of ethambutol by M. smegmatis involves a heterogeneous group of drug binding sites, only one of which is directly related to biological activity. Although nothing is known about the ethambutol target site itself, critical binding of the drug seems to require the single positively charged monohydrochloride form. Both hydrogen bonds and ionic linkages are probably involved.

Uptake and binding of 14C-ethambutol by tubercle bacilli and the relation of binding to growth inhibition

Studies were designed to characterize ethambutol uptake by Mycobacterium tuberculosis (H37Ra) and to relate uptake to the time-dependent, concentration-independent nature of growth inhibition by ethambutol. When cells grown aerated at 37 C in Sauton medium were exposed for 7 hr to 0.2, 0.5, 1.0, 2.5, and 5.0 mug of (14)C-ethambutol per ml, uptake increased with time and was a linear function of concentration. The process was inhibited at 22 C. Studies with chloramphenicol, sodium azide, and 2,4-dinitrophenol indicated that uptake is independent of requirements for protein synthesis and energy. The organism did not accumulate ethambutol against a concentration gradient. It can be concluded that ethambutol enters the cells in a passive manner. Kinetic studies of (14)C loss from tubercle bacilli pretreated with labeled drug suggested the existence of two ethambutol fractions within the cell: a highly variable labile pool and a second fraction that is small and quite firmly bound. Levels of cell-bound drug may be independent of total uptake, but this possibility was not established unequivocally. Definitive evidence showing identity in the concentrations of bound drug regardless of total uptake could explain the apparent discrepancy between concentration-dependent uptake and concentration-independent growth inhibition.

In vitro chemotherapeutic combinations against isoniazid-resistant Mycobacterium tuberculosis and Mycobacterium fortuitum

It is an acceptable medical practice to use second-line antimycobacterial drugs in combination with isoniazid in treatment of isoniazid-resistant tuberculosis. Recent investigations have demonstrated the importance of determining chemotherapeutic interaction in instances of multiple antibiotic use. We studied the inhibitory effect of combinations of isoniazid with ethambutol, rifampin, ethionamide, cycloserine, viomycin, and kanamycin against three isoniazid-resistant strains of Mycobacterium tuberculosis and three strains of M. fortuitum. The isobologram technique with drug concentrations of 0.4 to 100 mug/ml was used. With the exception of single instances in which kanamycin plus isoniazid (M. tuberculosis strain 9999) and ethionamide plus isoniazid (M. fortuitum strain 2080) seemed to have a synergistic effect, neither synergy nor antagonism was noted for any of the combinations. These studies show that the combined use of isoniazid and a second line antimycobacterial agent results in vitro in indifferent inhibitory activity.