GlyT1 inhibitor reduces oscillatory potentials of the electroretinogram in rats.

CONTEXT: Selective inhibitors of glycine transporter type 1 (GlyT1) increase synaptic glycine concentrations and are being developed to treat cognitive and negative symptoms of schizophrenia. However, increases in systemic glycine levels have been associated with visual disturbances and electroretinogram (ERG) alternations. OBJECTIVE: To determine whether the selective GlyT1 inhibitor PF-03463275 causes changes in ERG responses in albino rats. MATERIALS AND METHODS: Male Sprague-Dawley rats were administered PF-03463275 subcutaneously at 1, 3 and 10 mg/kg 1 h prior to ERG acquisition. Scotopic and photopic luminance responses, photopic adaptometry and flicker responses were measured. Plasma and vitreous samples were obtained at necropsy for determination of PF-03463275 concentrations. RESULTS: A dose-dependent reduction (up to ∼70%) in the amplitude of the scotopic ERG oscillatory potentials (OPs) was observed following PF-03463275 administration. The amplitude of the OPs was also negatively correlated to the concentration of PF-03463275 in the vitreous humor (r = -0.64, p < 0.0001). With the exception of a small increase in scotopic ERG a-wave amplitude and latency no effects were observed on other ERG parameters tested. CONCLUSIONS: We conclude that inhibition of the GlyT1 transporter in the retina causes ERG changes which may underlie recent reports of visual disturbance with GlyT1 inhibitors in clinical trials.

Experiments in the EpiDerm 3D Skin In Vitro Model and Minipigs In Vivo Indicate Comparatively Lower In Vivo Skin Sensitivity of Topically Applied Aneugenic Compounds.

Risk management of in vitro aneugens for topically applied compounds is not clearly defined because there is no validated methodology to accurately measure compound concentration in proliferating stratum basale keratinocytes of the skin. Here, we experimentally tested several known aneugens in the EpiDerm reconstructed human skin in vitro micronucleus assay and compared the results to flow cytometric mechanistic biomarkers (phospho-H3; MPM2, DNA content). We then evaluated similar biomarkers (Ki-67, nuclear area) using immunohistochemistry in skin sections of minipigs following topical exposure the potent aneugens, colchicine, and hesperadin. Data from the EpiDerm model showed positive micronucleus responses for all aneugens tested following topical or direct media dosing with similar sensitivity when adjusted for applied dose. Quantitative benchmark dose-response analysis exhibited increases in the mitotic index biomarkers phospho-H3 and MPM2 for tubulin binders and polyploidy for aurora kinase inhibitors are at least as sensitive as the micronucleus endpoint. By comparison, the aneugens tested did not induce histopathological changes, increases in Ki-67 immunolabeling or nuclear area in skin sections from the in vivo minipig study at doses in significant excess of those eliciting a response in vitro. Results indicate the EpiDerm in vitro micronucleus assay is suitable for the hazard identification of aneugens. The lack of response in the minipig studies indicates that the barrier function of the minipig skin, which is comparable to human skin, protects from the effects of aneugens in vivo. These results provide a basis for conducting additional studies in the future to further refine this understanding.

Crizotinib reduces the rate of dark adaptation in the rat retina independent of ALK inhibition.

Crizotinib (Xalkori) is a tyrosine kinase inhibitor of both anaplastic lymphoma kinase (ALK) and mesenchymal-epithelial transition factor (c-Met). Though not predicted from standard nonclinical toxicological evaluation, visual disturbance became a frequently observed adverse event in humans. To understand the possible mechanism of this vision effect, an in vivo electroretinogram (ERG) study was conducted to assess retinal functional changes following oral administration of crizotinib. Immunohistochemical (IHC) staining of ALK and c-Met in the neural retinas of human, non-human primate, dog, rat, and mouse was used to aid in the animal model selection. ALK IHC staining was identified predominantly in the ganglion cell and inner nuclear layers of most species evaluated, in the inner plexiform layer in human and rodent, and in the nerve fiber layer in human and rat only. There was no apparent staining of any layer of the neural retina for c-Met in any of the species evaluated. ERG measurements identified a significant reduction in b-wave amplitude during the initial phase of dark adaptation in the crizotinib-treated rats. ERGs were also taken following oral administration of PF-06463922 (an ALK-selective inhibitor), for an understanding of potential kinase involvement. ERG effects were not observed in PF-06463922-treated animals when comparable exposures in the vitreous humor were achieved. Collectively, our results suggest that the ERG b-wave amplitude decreases during dark adaption following crizotinib administration may be related to signaling changes within the retina in rats, likely independent of ALK inhibition.