The Impact of the Timing of Dosing on the Severity of UNC569-Induced Ultrastructural Changes in the Mouse Retina

Abstract

<jats:p> Mer proto-oncogene tyrosine kinase (MerTK), expressed in the retinal pigment epithelium (RPE), regulates the phagocytosis of shed photoreceptor outer segments. To investigate the influence of dosing time on MerTK inhibitor UNC569-induced retinal toxicity, UNC569 at 100 mg/kg was orally administered to male mice at 2 different Zeitgeber times (ZT5.5 or ZT22) for 28 days. Electron microscopy was conducted at ZT2 after the final dosing. Additionally, the visual cycle components (11-cis-retinal, all-trans-retinal, all-trans-retinol, and 11-cis-retinol), which play an important role in maintaining retinal homeostasis, were quantified by liquid chromatography/mass spectrometry/mass spectrometry. Under electron microscopic examination, the number of phagosomes and phagolysosomes in the RPE increased in both the ZT5.5 and ZT22 administered groups, while endoplasmic reticulum dilatation in the RPE and chromatin aggregation of photoreceptor nuclei were observed only in the ZT22 administered group. No change was observed in any of the visual cycle components. These results suggest that the timing of the dosing in relation to the physiological MerTK phosphorylation affected the severity of changes in the RPE, leading to the apoptosis of the photoreceptor cells. </jats:p>

Pharmacological inhibition of MERTK induces in vivo retinal degeneration: a multimodal imaging ocular safety assessment

“…We sampled eyes at 2-h post treatment which was approximately 4-h post light onset, which was rationalized as the most relevant sampling time in mouse given it covered the most likely window of peak phagocytosis. Given the association of time of dosing and target coverage with differential severity of retinal degeneration, characterized following treatment with UNC-569 (Sayama et al 2020 ), it will be important in the future to fully explore the kinetics of retinal target engagement with diverse chemotypes.…”

“…The counter-intuitive increase in phagosomes and phagolysosomes is tentatively attributed to increased phagocytosis and may result from time of sampling during the non-peak period of disc shedding. Furthermore, the synchronization of MERTK phosphorylation, phagocytosis and circadian rhythms were explored in a follow-up 28-day mouse study with UNC-569 (Sayama et al 2020 ). This showed that the severity of UNC-569 induced retinal toxicity was influenced by the time of dosing (dosing at 5.5-h and 22-h post light onset, Zeitgeber times (ZT))), which was associated with the physiological MERTK phosphorylation window (ca.…”

Blood retinal barrier and ocular pharmacokinetics: Considerations for the development of oncology drugs

“…Understanding the potential for toxicity early in discovery allows appropriate studies to be conducted. One such example is UNC-569, a MERTK inhibitor that exhibited ocular effects in mice (Sayama et al, 2018(Sayama et al, , 2020White, 2019). While these events may be assessed in animal models, UNC-569 demonstrates how ocular events can take many months/years to establish in animal models, and similarly MERTK mutations in human often take many years for functional effects to be observed (Chun et al, 2015).…”

“…All of these TKIs inhibit MERTK (either as the primary target or as an off-target "hit") with the exception of alectinib (' Alectinib NDA,' 2015;'Crizotinib NDA,' 2011;'Gilteritinib NDA,' 2018;Lin et al, 2016;Wang & Frye, 2014). With previously reported unbound plasma concentrations, MER in vitro potency (IC 50 ) and the observation of MERTK inhibition resulting in ocular events in the clinic (Table 1; 'Alectinib NDA,' 2015;Bauer, 2019;'Cabozantinib Label,' 2016;'Crizotinib NDA,' 2011;'Gilteritinib NDA,' 2018;Sayama et al, 2018Sayama et al, , 2020Yan et al, 2013), the relationship between unbound plasma exposure and ocular toxicity findings can be investigated. As shown in Table 1, unbound maximal plasma concentrations (C max,u ) were not sufficient to cover MER IC 50 values; however, the unbound plasma exposure at steady state (AUC) was sufficient to provide a margin above the TK IC 50 , suggesting the observed ocular toxicity findings could be attributed to the unbound AUC rather than C max,u .…”

Design, Synthesis, and Biological Evaluation of 2-Substituted Aniline Pyrimidine Derivatives as Potent Dual Mer/c-Met Inhibitors

“…Mer inhibitors are broadly classified into two types: aminopyrimidine pyrazole (pyrrole) and aminopyrimidine series ( Figure 1 ). The initial small-molecule Mer inhibitor UNC569 effectively inhibits Mer and downstream signaling pathways ERK and AKT but suffers from poor pharmacokinetic properties [ 6 , 7 ]. Subsequent developments, such as compounds UNC2225 and MRX-2843 derived from UNC569 [ 8 , 9 , 10 , 11 ], exhibit enhanced inhibition of Mer signal transduction, improved pharmacokinetic properties, and favorable drug-like characteristics.…”