Early MNU–induced retinal toxicity detected by electroretinography and visual acuity in funduscopically normal rats

Abstract

Abstract: : Purpose:Pharmaceutical Safety Science must be able to reliably identify and evaluate retinal toxicity in preclinical animal models. N–methyl–N–nitrosourea (MNU), a DNA alkylating agent, causes retinal degeneration in rats via apoptosis within a few days following a single 60 mg/kg dose. The purpose of this study was to compare routine indirect funduscopic and histopathologic endpoints of structural retinal change induced by MNU with functional measures of retinal status determined by flash electroretinography (ERG) and visual acuity measured by an opticokinetic (OKR) method. Methods:A single 60 mg/kg dose of MNU was injected intraperitoneally into 8 Sprague Dawley (SD; non–pigmented) and 8 Long Evans (LE; pigmented) rats. OKR was obtained from LE rats pre–study and 2 days post treatment. Funduscopic examinations and ERG responses were obtained from all rats pre–study and 3 days post treatment. Immediately following the 3–day ERGs, treated rats were euthanatized and the eyes collected for histopathologic assessment. Additionally, 4 SD and 3 LE control (vehicle–treated) rats were tested pre–study and at the end–of–study. Results:Three days following MNU administration all treated rats had retinal thinning associated with marked photoreceptor loss in the central retina and minimal sparing of the peripheral retina. These retinal changes are consistent with those reported in the literature (Nakajima et al, 1996). The amplitudes of both a–waves and b–waves of the ERGs were largely eliminated by this time indicating disruption of photoreceptor and interneuronal activity. Two days after MNU administration visual acuity thresholds were nearly doubled in most animals compared to pre–study values (mean of 39 minutes of arc increased to 72). Notably, funduscopic examinations revealed no apparent retinal changes in either LE or SD rats 3 days post treatment. Conclusions:Although funduscopic examinations are the primary method of assessing retinal toxicity in pharmaceutical safety studies, the findings described here with MNU suggest that toxicologically significant photoreceptor loss and retinal thinning may not be detected during the in–life portion of a safety study by routine funduscopic examination alone. Both ERG and OKR changes were more predictive of early MNU–induced structural changes observed histopathologically. Finally, our findings extend observations of ERG and structural changes that occur with MNU treatment to include changes in a measure of visual acuity.

Keywords: electroretinography: non–clinical • drug toxicity/drug effects • imaging/image analysis: non–clinical