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

1. nitisinone （NTBC）治疗OCA1B患者的临床方案研究

1. Clinical protocol for studying the effect of nitisinone (NTBC) treatment in human subjects with OCA1B Our previously-published work had established that nitisinone (NTBC) can increase melanin pigmentation in a mouse model of OCA1B. We performed an open-label pilot study with five 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. 2. Effect of NTBC in mouse models of OCA 2, OCA3 and OCA4 Treatment of a mouse model of OCA3 with oral NTBC does not have a favorable effect on melanin production and minimally affects the number of pigmented melanosomes in the iris stroma (Onojafe I.F., et al., Invest Ophthalmol Vis Sci. 2019). We conclude 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 OCA3 mice, plasma tyrosine concentration was increased in NTBC-treated OCA2 and OCA4 mice after a month of nitisinone treatment with no overt toxic side-effects. Unlike the OCA3 mice, fur pigmentation was augmented in both mouse lines. Iris pigmentation was augmented predominantly in OCA4 mice. Electron microscopy also confirmed a small increase in choroidal pigmentation only in treated OCA4 mice, whereas, no effect was observed in the RPE in both lines. In a manuscript in preparation, we suggest that NTBC treatment might be beneficial in partially restoring pigmentation in OCA4 but not OCA2. 3. High-throughput drug screening to identify 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 >100 new inhibitors and a few activators of tyrosinase. After validation in a secondary enzymatic screen in vitro and in zebrafish in vivo, we are testing two doses of 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. 4. In vitro disease-in-a-dish modeling of OCA We developed a disease-in-a-dish model of oculocutaneous albinism type 1A (OCA1A) and type 2 (OCA2) using induced pluripotent stem cell (iPSC) technology. OCA patient fibroblasts were reprogrammed to iPSCs and differentiated to retinal pigment epithelium (OCA-RPE) using a developmentally-guided differentiation protocol. Tissue morphology and physiology as well as expression and functionality of the visual cycle apparatus were comparable to control pigmented iPSC-derived RPE. Furthermore, pigmentation defects comprising immature melanosomes in the OCA-RPE cells in vitro faithfully replicated tissue characteristics in vivo.