Pre-clinical and clinical studies of NTBC and other compounds as potential treatments for albinism

Brooks, Brian

Abstract

Our previously-published work has established that nitisinone (NTBC) can increase melanin pigmentation in a mouse model of OCA1B. 1. Clinical Protocol for Studying the Effect of Nitisinone (NTBC) Treatment in Human Subjects with OCA1B We have published the results of our open-label pilot study with 5 adult patients with OCA-1B (Adams D.R., et al., JCI Insight. 2019). After establishing baseline measurements of iris, skin, and hair pigmentation, the patients were treated over 12 months with 2 mg/d oral NTBC. Changes in pigmentation and visual function were evaluated at 3-month intervals. Iris melanin remained unchanged after NTBC treatment, however, hair and skin pigmentation appeared to have increased in some patients with OCA-1B. The iris transillumination grading scale used in this study proved robust, with potential for use in future clinical trials. Additional clinical work on albinism was published in collaboration with NEI investigators (Jung H. et al., JCI Insight. 2019) and research groups at the Medical College of Wisconsin (Lee D.J., et al., Invest Ophthalmol Vis Sci. 2018). In the former, we demonstrate a potentially novel application of fluorescence microscopy to detect subclinical changes to the RPE, a technical advance that has direct implications for improving our understanding of oculocutaneous albinism as well as late-onset retinal degeneration, and Bietti crystalline dystrophy. In the latter, we show that foveal Henle fiber layer and outer nuclear layer topography are significantly altered in albinism relative to normal controls. These measurements may help further stratify grading schemes used to assess foveal hypoplasia in patients with albinism. 2. Visual function in NTBC-treated OCA1B mice vs. controls Based on the success with increasing pigmentation in OCA1B adult mice and in pups treated in utero, we asked whether we could detect eye morphological, cellular and functional changes in the latter after weaning. Through collaborations with the NEI Visual Function Core and the Neurobiology, Neurodegeneration & Repair Laboratory, we have studied the effect of prenatal NTBC treatment on neural retina development, neural circuit development and spatial visual acuity. Preliminary data on a small cohort of mice indicated that prenatal treatment of OCA1B mice with NTBC results in measurable changes in electroretinogram (ERG) and direct coupled (dc)-ERG responses that returned within normal values in the treated mice. However, no change in photoreceptor cell number, accessory optic tract routing, nor visual behavior measured by optokinetic and optomotor response was observed. While photoreceptor function measured by ERG seems to be preserved in OCA1B mice, behavioral measures such as optomotor response seem to indicate that the mice are blind. Further studies in the OCA1B mice as well as in mouse strains in which pigmentation of the iris but not of the RPE is preserved (KitW-v mouse) are underway to test the hypothesis that iris pigmentation is a confounding factor in the interpretation of optomotor responses in OCA1B mice. 3. Effect of NTBC in mouse models of OCA3, OCA 2 and OCA4 Our recently published article (Onojafe I.F., et al., Invest Ophthalmol Vis Sci. 2019) describes how treatment of a mouse model of OCA3 with oral NTBC does not have a favorable clinical effect on melanin production and minimally affects the number of pigmented melanosomes in the iris stroma. We concluded that treatment of OCA3 patients with NTBC is unlikely to be therapeutic. We further hypothesized that NTBC might improve melanization in mouse models of OCA2 (melanocyte-specific transporter protein) and of OCA4 (the so-called underwhite allele of SLC45A2). Similar to the OCA3 mice, plasma tyrosine concentration was increased in NTBC-treated OCA2 and OCA4 mice with no overt toxic side-effects. Unlike the OCA3 mice, fur pigmentation was augmented in both mouse lines. Iris pigmentation was augmented in both OCA2 and OCA4 mouse lines, with a more predominant effect in OCA4 mice. Electron microscopy also confirmed a small increase in pigmentation of the choroid in treated OCA2 and OCA4 mice whereas, no effect was observed in the RPE in both lines. A manuscript describing these data will be submitted shortly for publication. 4. High-throughput drug screening to find compounds that regulate Tyr activity We used purified truncated Tyr protein (previously tested and validated to have equivalent enzymatic activity to full length protein) in a high-throughput drug screening. In collaboration with NCATS, we screened 34,000 compounds from the Genesis Drug Collection, the Natural Products Library, and the NCATS Pharmaceutical Collection. We identified new inhibitors (>100) and several activators of tyrosinase. After validation in a secondary enzymatic screen in vitro, and in vivo in zebrafish, we tested the top candidate compound on the OCA1B mouse model. At the lowest dose tested, the drug was well tolerated when administered i.p. for 30 days. However, no significant change was observed in hair, iris or fundus pigmentation. We are planning to test a higher dose as well as alternative routes of administration, checking for safety and efficacy.