Naringenin protects hepato-renal tissues against antituberculosis drugs induced toxic manifestations by modulating interleukin-6, insulin like growth factor-1, biochemical and ultra-structural integrity.

BACKGROUND: The antituberculosis drugs (ATDs), isoniazid, rifampicin, pyrazinamide and ethambutol prompt extreme hepatic and renal damage during treatment of tuberculosis. The present study aimed to investigate protective potential of naringenin against ATDs induced hepato-renal injury. METHODS: Rats were administered with ATDs (pyrazinamide; 210, ethambutol; 170, isoniazid; 85, rifampicin; 65 mg/kg b.wt) orally for 8 weeks (3 days/week) followed by naringenin at three different doses (10, 20 and 40 mg/kg b.wt) conjointly for 8 weeks (3 days/week alternately to ATDs administration) and silymarin (50 mg/kg b.wt) as positive control. RESULTS: Exposure to ATDs caused significant increase in interleukin-6 (IL-6), triglycerides, cholesterol, bilirubin whereas depletion in insulin like growth factor-1 (IGF-1), albumin and glucose in serum. Endogenous antioxidant enzymes glutathione reductase (GR), glutathione peroxidase (GPx) and glucose-6-phosphate-dehydrogenase (G-6-PDH) were diminished in liver and kidney tissues with parallel increase in triglycerides, cholesterol, microsomal LPO and aniline hydroxylase (CYP2E1 enzyme). Ultra-structural observations of liver and kidney showed marked deviation in plasma membranes of various cellular and sub-cellular organelles after 8 weeks of exposure to ATDs. CONCLUSIONS: Conjoint treatment of naringenin counteracted ATDs induced toxic manifestations by regulating IL-6, IGF-1, CYP2E1, biochemical and ultra-structural integrity in a dose dependent manner. Naringenin has excellent potential to protect ATDs induced hepato-renal injury by altering oxidative stress, modulation of antioxidant enzymes, serum cytokines and ultra-structural changes.

The role of the farnesoid X receptor in quadruple anti-tuberculosis drug-induced liver injury.

Anti-tuberculosis drugs-induced liver injury may be associated with the hepatic farnesoid X receptor (FXR). However, the relationship between isoniazid, rifampicin, pyrazinamide and ethambutol (HRZE) coadministration-induced liver injury and FXR has not been clarified. The purpose of this study was to clarify the role of FXR in HRZE-induced liver injury. To measure indices of liver injury, blood samples were collected from clinical tuberculosis patients who had taken HRZE for approximately two months; in these patients serum total bile acids were increased, while other hepatic biochemical indexes showed no significant changes. When Wistar rats were orally administered isoniazid (30 or 60 mg/kg) + rifampicin (45 or 90 mg/kg) + pyrazinamide (150 or 300 mg/kg) + ethambutol (75 or 150 mg/kg) in combination for 15 days, the expression and function of FXR was up-regulated, and hepatic bile acids were decreased. However, following 30 days of HRZE treatment the expression and function of FXR was down-regulated and bile acids accumulated in the liver, suggestive of hepatotoxicity. Treatment of HepaRG cells with HRZE lead to time- and dose- dependent cytotoxicity, with the expression of FXR up-regulated in early stage, but down-regulated with prolonged HRZE treatment, consistent with the results of animal experiments. In summary, HRZE may upregulate FXR with short-term administration, but more prolonged treatment appears to suppress FXR function, resulting in hepatic bile acid accumulation.

Exploration of the Plausible Mechanism of Ethambutol Induced Ocular Toxicity by Using Proteomics Informed Physiologically Based Pharmacokinetic (PBPK) Modeling.

PURPOSE: Ethambutol (EMB) is a first-line anti-tubercular drug that is known to cause optic neuropathy. The exact mechanism of its eye toxicity is unknown; however, proposition is metal chelating effect of both EMB and its metabolite 2,2'-(ethylenediamino)-dibutyric acid (EDBA). The latter is formed by sequential metabolism of EMB by alcohol dehydrogenases (ADHs) and aldehyde dehydrogenases (ALDHs). The purpose of this study was to predict the levels of drug and EDBA in the eye using physiologically based pharmacokinetic (PBPK) modeling. METHODS: The PBPK model of EMB was developed using GastroPlus. The intrinsic hepatic clearance of ALDH, calculated by the model, was scaled down using proteomics data to estimate the rate of formation of EDBA in the eye. Additionally, the comparative permeability of EMB and EDBA was assessed by employing in silico and in vitro approaches. The rate of formation of EDBA in the eye and permeability data were then incorporated in a compartmental model to predict the ocular levels of EMB and EDBA. RESULTS: The simulation results of compartmental model highlighted that there was an on-site formation of EDBA upon metabolism of EMB. Furthermore, in silico and in vitro studies revealed that EDBA possessed much lower permeability than EMB. These observations meant that once EDBA was formed in the eye, it was not permeated out and hence achieved higher ocular concentration. CONCLUSION: The on-site formation of EDBA in the eye, its higher local concentration due to lower ocular clearance and its pre-known characteristic to chelate metal species better explains the ocular toxicity shown by EMB.

Neuropatía óptica por etambutol en paciente con esclerosis múltiple / Optic neuropathy by ethambutol in a patient with multiple sclerosis

Presentamos el caso clínico de una paciente que desarrolló una neuropatía óptica tóxica por etambutol en el contexto de una reactivación tuberculosa y que, además, presentaba como antecedentes personales una esclerosis múltiple. El objetivo es destacar la importancia de realizar un buen diagnóstico diferencial de este efecto adverso y de conocer sus manifestaciones y características clínicas, campimétricas y tomográficas principales. Además, dado que la reversibilidad del daño sigue siendo discutida en la bibliografía, es esencial el diagnóstico precoz para lo que es importante informar al paciente de los posibles síntomas, así como llevar a cabo una exploración oftalmológica y test cromáticos antes de comenzar el tratamiento para valorar si hay progresión.

We present the clinical case of a patient who developed a toxic optic neuropathy due to ethambutol in the context of a tuberculosis reactivation and who also had a personal history of multiple sclerosis. The objective is to highlight the importance of making a good differential diagnosis of this adverse effect and of knowing its main clinical, campimetric and tomographic manifestations and characteristics. Furthermore, since the reversibility of damage is still discussed in the literature, early diagnosis is essential. For this purpose, it is important to inform the patient of the possible symptoms and to carry out an ophthalmological examination and colour tests before starting treatment to assess whether there is progression.

Prospective study to evaluate incidence and indicators for early detection of ethambutol toxicity.

AIMS: To evaluate incidence of toxic optic neuropathy in patients receiving ethambutol (EMB) for 6 months and to identify its early indicators. METHODS: We included 50 patients on anti-tubercular therapy (ATT) including EMB (HRE regimen) based on total body weight for 6 months. Best-corrected visual acuity (ETDRS), colour vision (Ishihara pseudo-isochromatic plates), contrast sensitivity (Pelli-Robson chart), Humphrey visual field analysis (HVF 30-2 SITA FAST), pattern visual evoked response (VER) and spectral-domain optical coherence tomography (SDOCT) for ganglion cell inner plexiform layer (GCIPL) and retinal nerve fibre layer (RNFL) analysis were assessed at baseline and at 2, 4 and 6 months after starting ATT. RESULTS: Mean age of the patients was 36.5±14.7 years with male:female ratio of 2.5:1. Mean daily dosage of EMB was 17.5±1.3 mg/kg/day. No significant change was observed in visual acuity, contrast sensitivity, color vision and mean or pattern SD on HVF at 6 months. Significant increase in VER latency of >2 SD (>125 ms) was observed in 46% eyes on follow-up indicating subclinical toxicity. Significant loss of mean RNFL (from 100.79±16.05 µm to 89.96±13.79 µm) and GCIPL thickness (from 83.1±5.60 µm to 79.85±6.45 µm) was observed at 6 months (p=0.001 for both). Patients with subclinical toxicity had significantly greater damage in temporal RNFL quadrant, supero-nasal and infero-nasal GCIPL sectors compared with others. CONCLUSION: The incidence of clinical EMB optic neuropathy was <2%, though subclinical damage in the form of increase in VER latency, and decrease in RNFL and GCIPL on OCT was seen in 46% eyes.

Isolated testicular tuberculosis with ethambutol cutaneous toxicity: A combination of two rare entities.

Tuberculosis (TB) is an infection that can affect any organ, affecting mainly the lungs. Isolated testicular TB is very rare. Six months of a multiple drug scheme is the mainstay of TB treatment. Adverse reaction to anti-TB chemotherapy is frequent and affects the course of the therapy, leading sometimes to discontinuation of drugs. Ethambutol optic nerve toxicity is frequent. However, severe cutaneous and anaphylactic reactions associated to ethambutol are very rare. We present the case of an immunocompetent patient presenting with isolated testicular TB that exhibited a severe cutaneous and anaphylactic reaction to ethambutol during the consolidation treatment phase. This led to exhaustive etiologic study and treatment modification.

Case Report: Long-term Structural and Functional Effects of Ethambutol Optic Neuropathy.

SIGNIFICANCE: This case report demonstrates reduction in the retinal nerve fiber layer (RNFL) thickness and an abnormal electroretinogram after toxic optic neuropathy from ethambutol, more than 1 year after improvements in visual acuity (VA) and visual fields (VFs) were seen. Although many studies have described complications of ethambutol, continuing reduction in RNFL thickness 2 years after discontinuation has not been described elsewhere. PURPOSE: It is well known that ethambutol can cause optic nerve toxicity, visual impairment, and VF loss. Visual acuity can be regained after stopping the drug; however, the amount and time frame are variable. There are few data on long-term follow-up of these cases to direct clinicians how to proceed once VA has stabilized. Here we present a case with 2 years of follow-up for a patient with ethambutol toxicity, showing the condition change even after VA becomes normal. CASE REPORT: A 61-year-old man presented shortly after discontinuing ethambutol for Mycobacterium avium complex. Visual acuity values were 20/70 in the right eye and 20/125 in the left eye with cecocentral VF scotomas. Optical coherence tomography showed normal RNFL. Visual-evoked potentials were significantly reduced and delayed. Over the course of 2 years, the patient became asymptomatic as VA and VF returned to normal and visual-evoked potential improved. However, the optical coherence tomography RNFL was reduced from each visit to the next, and the electroretinogram showed decreased scotopic and photopic amplitudes. CONCLUSIONS: Signs of ethambutol toxicity may remain or worsen years after discontinuation, even in the absence of patient symptoms and with normal VA and VF.

Cytochemical Evaluation of the Toxic Effects of Combined Antituberculosis Substances on Metabolic State of Blood Lymphocytes.

Combined antituberculosis substances induced a dose-dependent changes in activity of dehydrogenases and hydrolases in rat lymphocytes. The main toxic effect of the substances was related to inhibition of mitochondrial dehydrogenases (succinate dehydrogenase and α-glycerol phosphate dehydrogenase) usually followed by suppression of activity of hydrolytic enzymes (acid phosphatase and non-specific esterase). Opposite changes in lactate dehydrogenase activity reflected specific features of intoxication.

Exposure to first line anti-tuberculosis drugs in prepubertal age reduces the quality and functional competence of spermatozoa and oocytes in Swiss albino mice.

Prepubertal Swiss albino mice of both sex were administered with first-line anti-tuberculosis drugs (ATDs) viz; rifampicin, isoniazid, pyrazinamide, streptomycin and ethambutol intraperitoneally, for 4 weeks. Two weeks after the completion of treatment, male mice were sacrificed to collect caudal spermatozoa and female mice were superovulated with pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG) to collect metaphase II (MII) oocytes from oviduct. Administration of ATDs not only decreased the count, motility and, nuclear maturity and also, increased the head abnormalities, mitochondrial damage and DNA damage in epididymal spermatozoa. Reduction in number of ovulated oocytes, increased degeneration rate and altered distribution pattern of cytoplasmic organelles was observed in oocytes of female mice. Presence of ATDs in in vitro maturation (IVM) medium increased abnormalities in meiotic resulted in abnormal spindle organization (except ethambutol) without affecting nuclear maturation. In conclusion, the result of this study indicates that ATDs have considerable adverse effects on the functional competence of male and female gametes, however, with varied degree of toxicity.

Diagnostic value of ganglion cell-inner plexiform layer for early detection of ethambutol-induced optic neuropathy.

AIM: To evaluate the diagnostic value of macular ganglion cell-inner plexiform layer (mGCIPL) thickness versus peripapillary retinal nerve fibre layer (pRNFL) thickness for the early detection of ethambutol-induced optic neuropathy (EON). METHODS: Twenty-eight eyes of 15 patients in the EON group and 100 eyes of 53 healthy subjects in the control group were included. All patients with EON demonstrated the onset of visual symptoms within 3 weeks. Diagnostic power for pRNFL and mGCIPL thicknesses measured by Cirrus spectral-domain optical coherence tomography was assessed by area under the receiver operating characteristic (AUROC) curves and sensitivity. RESULTS: All of the mGCIPL thickness measurements were thinner in the EON group than in the control group in early EON (p<0.001). All of pRNFL thicknesses except inferior RNFL showed AUROC curves above 0.5, and all of the mGCIPL thicknesses showed AUROC curves above 0.5. The AUROC of the average mGCIPL (0.812) thickness was significantly greater than that of the average pRNFL (0.507) thickness (p<0.001). Of all the mGCIPL-related parameters considered, the minimum thickness showed the greatest AUROC value (0.863). The average mGCIPL thickness showed a weak correlation with visual field pattern standard deviations (r2=0.158, p<0.001). CONCLUSIONS: In challenging cases of EON, the mGCIPL thickness has better diagnostic performance in detecting early-onset EON as compared with using pRNFL thickness. Among the early detection ability of mGCIPL thickness, minimum GCIPL thickness has high diagnostic ability.

Mitochondrial Impairment in Antibiotic Induced Toxic Optic Neuropathies.

There is growing evidence for the role of mitochondrial dysfunction in the toxic optic neuropathies. Due to the structural similarities between antibiotic targets and mitochondrial machinery, several antibiotics known to cause optic neuropathy have deleterious effects on mitochondrial function. We review the literature on the prevalence, clinical manifestations, and management of antibiotic induced toxic optic neuropathies. The effect of these antibiotics on mitochondrial function in regard to the optic nerve is discussed.

Repeated administration of ethambutol in therapeutic dose causes testes alteration and spermatogenesis disruption in Wistar rats.

Ethambutol (EMB) is conventionally used to treat tuberculosis and atypical Mycobacterium infections in combination with other antimycobacterial drugs. Eventually, EMB testicular toxicity has not been explored extensively yet. The aim of the study is to evaluate testicular toxicity of EMB. We explored the impact of EMB on male rats' fertility, testosterone level and germ cells state, testicular pro- and anti-oxidant status and DNA damage, as well as identified EMB effects on cytochrome P-450 2E1 (CYP2E1) both with computer simulation and in vivo. We demonstrated that EMB administration to male rats decreased in epididymal sperm count (19%) and fertility index (53%). These events were accompanied by reduction in serum testosterone content (1.6 times) and appearance of spermatogenic epithelium damages. It was also found in testes the intensification of lipid peroxidation, decrease in reduced glutathione content and changes in DNA fragmentation. Additionally, computer simulation showed direct interaction of EMB with CYP2E1 active site and heme. On the top of this, we demonstrated that level of testicular CYP2E1 messenger RNA in EMB-treated rats was increased 8.7 folds and p-nitrophenol hydroxylase activity in testes rose three folds. As this shows, EMB-caused CYP2E1 induction, oxidative stress, and apoptosis in the testes contribute to inhibition of steroidogenesis enzymes and spermatogenesis disruption.

Ameliorative Effects of Kolaviron, a Biflavonoid Fraction from Garcinia Kola Seed, on Hepato-renal Toxicity of Anti-tuberculosis Drugs in Wistar Rats.

BACKGROUND: Tuberculosis (TB) is an infectious disease of international health priority. The combination of anti-TB drugs (4-Tabs)- isoniazid (INH), rifampicin (RIF), pyrazinamide (PZA) and ethambutol (ETB) are effective in the management of the disease, however, their toxic effect is a major concern. PURPOSE: The study was designed to evaluate the toxicity of anti-TB drugs in male Wistar rats and possible ameliorative effects of kolaviron (KV), a biflavonoid from Garcinia kola seeds. METHODS: Twenty-eight rats were assigned into four groups; Group 1 (Control) received corn oil, Group 2 (4-Tabs) received therapeutic doses of INH (5 mg/kg), RIF (10 mg/kg), PZA (15 mg/kg) and ETB (15 mg/kg) in combination, Group 3 (4-Tabs + KV) received INH, RIF, PZA, ETB and KV (200 mg/kg) and Group 4 (KV) received KV (200 mg/kg) by oral gavage three times per week for 8 consecutive weeks. RESULTS: Administration of 4-Tabs caused oxidative stress resulting in significant (p = 0.031, 0.027) increase in malondialdehyde levels in the liver and kidney of rats by 101% and 34%, respectively. Also, 4-Tabs caused significant (p = 0.023-0.035) elevation of serum alanine and aspartate aminotransferases by 41% and 48%, creatinine by 252% and total bilirubin by 89%, respectively. In contrast, hepatic and renal antioxidant indices- reduced glutathione, glutathione peroxidase, glutathione-s-transferase and superoxide dismutase were significantly (p = 0.028-0.039) decreased in 4-Tabs-treated rats. Co-administration of KV with 4-Tabs significantly restored the antioxidant parameters and biochemical indices to near normal. CONCLUSION: These findings suggest that anti-TB drugs elicit oxidative damage in liver and kidney of rats while KV protects against the adverse effects via antioxidative mechanism.

Evaluation of the Protective Effect of NMDA/Non-NMDA Receptor Antagonists Against Ethambutol Induced Retinal Toxicity Using ERG in Wistar Rats.

To study the protective effect of NMDA and non-NMDA receptor antagonists against ethambutol (EMB) induced retinal toxicity in Wistar rats using flash electroretinogram (ERG). Rats were randomized into four groups: Group-1 received vehicle. Group-2 received oral EMB (200 mg/kg/day). Group-3 and 4 were fed with oral EMB along with memantine (MEM) (1 mg/kg, ip) and trimetazidine (TMZ) (3mg/kg, ip) respectively. All treatments were continued up to 28 days. ERG was recorded at 0 and 21st day using green and white lights. Ethambutol and 2, 2' ethylene diimino dibutyric acid (EDBA) levels were quantified in rat body fluids and tissues using LC-MS/MS. A higher rate of rat mortality was observed between 21st and 28th day, 21st day considered for ERG recording among groups. Ethambutol did not cause any significant change in 'a'-wave amplitude of rat ERG but caused a predictable decrease in 'b'-wave amplitude of the rat ERG on the 21st day. Memantine treatment showed a significant (P=0.029) protection against the fall of 'b'-wave amplitude on 21st day. Interestingly, we found that plasma levels of EMB in memantine treated rats were significantly reduced when compared to the positive control group. Memantine reversed the effects of EMB on 'b'-wave of rat ERG suggests its protective role. We suggest MEM may be considered as a possible preventive treatment modality for EMB induced vision toxicity warranting further clinical investigations.

Fabrication and Validation of Gyro-Dot-Optomotor Response Device using Gold Fish to Study Drugs Affecting Visual Perception.

An appropriate model to predict the effect of xenobiotics on the vision perception in neuropsychoharmacological studies is of great importance in drug development and toxicity studies. The present study valuated the effect of CNS stimulant, depressant and therapeutic agents known to have ocular toxicity on ptomotor response (OMR) using goldfish in a newly developed device. A digital light processing aided gyrating poly-chromatic dotted pattern-OMR (Gyro-dot-OMR) analyzer was developed and standardized for this study in our laboratory. Goldfishes were exposed to varying concentrations of caffeine and pentobarbitone sodium to evaluate the effect of CNS stimulation and depression on OMR in white light. Ethambutol induced ocular toxicity was evaluated by intravitreal injection into both eyes of goldfishes. They were subjected for polychromatic Gyro-dot-OMR in both clock and anticlockwise directions. At the low concentration (5, 10 and 20 ng/mL) caffeine exposed animals showed significant (p<0.05) stimulant effect and the EC(50) of caffeine in goldfish was found to be 4.806 ng/mL. In contrast, pentbbarbitone sodium treated fishes showed significant (p<0.05) depressant effect with increasing the concentration. Ethambutol toxicity was reflected by the color iscrimination in the Gyro-dot-OMR pattern. For the first time, this model proved the possibility of running Irwin profile test on goldfish using Gyro-dot-OMR. This model successfully predicted ethambutol induced toxicity with poor discrimination of red-green color. This model can be used for predicting toxicity of drugs affecting vision perception.

Niosomal encapsulation of ethambutol hydrochloride for increasing its efficacy and safety.

CONTEXT: Tuberculosis (TB) is a worldwide health concern. In 2011, about 8.7 million new cases developed TB and 1.4 million people died from it. OBJECTIVE: Enhancement of ethambutol hydrochloride activity and safety in treatment of TB through niosomal encapsulation. MATERIALS AND METHODS: Niosomes were prepared by the thin-film hydration method. They were characterized, investigated for in vitro release, lung disposition and in vivo biological evaluation. RESULTS: Entrapment efficiency of ethambutol hydrochloride ranged from 12.20% to 25.81%. Zeta potential values inferred stability of neutral and negatively charged formulations. In vitro release was biphasic. Lung targeting was increased by niosomal encapsulation. Biological evaluation revealed superiority of niosomal ethambutol hydrochloride over the free drug. DISCUSSION: Neutral and negatively charged niosomal vesicles are dispersed homogenously unlike positively charged vesicles. Niosomal encapsulation results in controlled drug release. Niosomal formulations targeted more drugs to mice lungs for a prolonged period of time resulting in: decreased root-specific lung weight, bacterial counts in lung homogenates and optimizing pathological effect on guinea pigs lungs, livers and spleens. CONCLUSION: Encapsulation of ethambutol hydrochloride in niosomal formulations for the treatment of TB provides higher efficacy and safety compared with the free drug.

Assesment the role of oxidative stress and efficacy of caffeic acid phenethyl ester (CAPE) on neurotoxicity induced by isoniazid and ethambutol in a rat model.

OBJECTIVE: The aim of this study were to investigate a role of oxidative stress and the therapeutic efficacy of caffeic acid phenethyl ester (CAPE) in the pathogenesis of neurotoxicity induced by isoniazid and etambutol in a rat model. MATERIALS AND METHODS: Male Sprague-Dawley rats were randomly divided into eight experimental groups: control, INH, ETM, INH+ETM, INH+CAPE, ETM+CAPE, INH+ETM+CAPE, and CAPE treatment group, with ten animals in each group. INH and ETM doses were given orally within tap water for 30 days. CAPE was administered into relevant groups intraperitoneally for 30 days. Brain tissue and sciatic nerve were removed for biochemical and histopathological investigation. RESULTS: In the INH, ETM, and INH+ETM groups, malondialdehyde (MDA) and total oxidant status (TOS) levels were significantly higher than those of the control group (p < 0.05). Also, in these groups, brain total antioxidant capacity (TAC) levels, and superoxide dismutase (SOD) and PON-1 activities were decreased compared with the control group (p < 0.05). By a CAPE supplement within INH and ETM groups, there was a significant decrease in MDA and TOS (p < 0.05). In addition to a significant increase in TAC levels, and SOD and PON-1 activities both in brain and sciatic nerve tissues (p < 0.05). CONCLUSIONS: CAPE may protect against INH- and ETM-induced neurotoxicity in rat brain and sciatic nerve.

Therapeutic potential of thymoquinone against anti-tuberculosis drugs induced liver damage.

Hepatotoxicity is the most serious adverse effect related to tuberculosis treatment which interrupts the successful completion of tuberculosis treatment. The purpose of this study was to assess therapeutic effect of thymoquinone (TQ) against anti-tuberculosis drugs (ATD) induced liver damage. Rats were treated with ATD for 8 weeks (3 days/week) as given for the treatment of TB. This was followed by therapy of TQ for 8 weeks (3 days/week). Administration of combined ATD induced hepatotoxicity was evident from a significant elevation in the AST, ALT, ALP, bilirubin, albumin, cholesterol, urea, uric acid, creatinine, LPO and decreased activities of enzymes. These altered variables were significantly reversed toward control after treatment with TQ. Histological studies also supported biochemical findings. Results of this study strongly indicated protective effect of TQ and thus, can be expected as promising protective agent in maintenance of normal hepatic function during treatment with ATD.

Retinal function and morphology in monkeys with ethambutol-induced optic neuropathy.

PURPOSE: Ethambutol-induced optic neuropathy is a well recognized adverse ocular event. However, abnormalities of the retina in this optic neuropathy are not fully understood. Therefore, the purpose of the present study was to investigate both functional and morphological alterations of the retina induced by ethambutol in monkeys. METHODS: Ethambutol was orally administered to three cynomolgus monkeys, initially at 400 mg/kg/day followed by 800 mg/kg/day, for a maximum of 39 weeks. Full-field electroretinograms (ERGs) were recorded at intervals of approximately one month. The protocol included standard ERG responses to white flashes obtained under dark-adapted conditions (rod, combined rod-cone, oscillatory potentials) or with a white background (single-flash cone, 30 Hz flicker). In addition, we measured the ERG elicited with red flashes under blue background light (single-flash cone response [R/B]). All the ethambutol-treated monkeys were euthanized, and the retinae and various other nervous system tissues were examined histopathologically. RESULTS: No obvious changes were observed in the standard full-field ERGs. On the other hand, selective attenuation of the photopic negative response (PhNR) of the single-flash cone response (R/B) was observed in two out of three ethambutol-treated monkeys at week 22 or 28. Histopathology of these two monkeys revealed single cell necrosis of the retinal ganglion cells (RGCs), decreased RGCs in the parafovea and increased microglial cells in the nerve fiber layer in the retina, in addition to demyelination and glial reaction in the optic nerve, chiasm and tracts. CONCLUSIONS: The attenuated PhNR and histopathology of the retina indicated that RGCs were markedly damaged, both functionally and morphologically in monkeys with ethambutol-induced optic neuropathy. These results implied that RGCs are predominantly affected in the retina of patients with ethambutol-induced optic neuropathy.