Identification of DKK3 as an Ocular Pharmacodynamic Biomarker for anti-HtrA1 in a Phase 1 study for Geographic Atrophy Secondary to Age-related macular degeneration

Abstract

Purpose : Genetic polymorphisms in ARMS2/HTRA1 locus are associated with increased risk of age-related macular degeneration (AMD) and disease progression. High Temperature Requirement A1 (HtrA1), a serine protease active in the retina, can degrade extracellular matrix proteins and proteins involved in the visual cycle. An inhibitory anti-HtrA1 Fab has been developed to test the therapeutic hypothesis that HtrA1 protease activity is involved in the progression of geographic atrophy secondary to AMD (GA). Here we describe the discovery of Dickkopf-related protein 3 (DKK3) as an in vivo HtrA1 substrate and demonstrate its’ clinical utility as an HtrA1-specific pharmacodynamic (PD) biomarker in a first-in-human Phase I study in patients with GA.

Methods : N-terminomic proteomic profiling in pre-clinical models was used to elucidate the in vivo repertoire of HtrA1 specific substrates and identify ocular substrates that can serve as PD biomarkers of HtrA1 activity. DKK3 was identified and validated as an HtrA1 substrate and tested as an exploratory PD biomarker of anti-HtrA1 Fab inhibition.

The Phase 1 study consisted of a single ascending dose (SAD) stage with doses of anti-HtrA1 Fab ranging from 1 mg to 20 mg (n=15) and a multiple-dose (MD) stage with the maximum tested dose of 20 mg administered every 4 weeks for 3 doses (n=13). Levels of cleaved DKK3 were monitored in aqueous humor of patients at baseline and at multiple timepoints following anti-HtrA1 Fab administration by western blot analysis.

Results : Western blot analysis of SAD patient aqueous humor revealed dose dependent inhibition of DKK3 cleavage by anti-HtrA1 Fab. Higher doses yielded longer duration of anti-HtrA1 inhibitory activity and at 20 mg dose, inhibition of DKK3 cleavage was sustained for > 8 weeks after a single ITV administration. In the MD stage, DKK3 cleavage remained inhibited throughout the treatment period and for >8 weeks post last dose.

Conclusions : Identification of HtrA1 substrates enabled the development of a clinical PD biomarker for evaluating anti-HtrA1 Fab activity. Analysis of cleaved DKK3 in patient aqueous humor provided evidence of anti-HtrA1 activity and information on duration of activity in Phase 1. The PD biomarker data supports evaluation of a q4w and q8w dosing interval in a phase 2 study to evaluate the potential efficacy of anti-HtrA1 Fab.

This is a 2020 ARVO Annual Meeting abstract.