Development of APH003─a Highly Potent, Selective, and Orally Bioavailable IRAK4 PROTAC Degrader for the Treatment of Inflammatory Diseases

Interleukin-1 receptor-associated kinase 4 (IRAK4) represents an attractive therapeutic target in inflammatory and oncological indications due to its pivotal role as a signaling mediator downstream of TLR and IL-1R. We previously reported a series of spirocyclic IRAK4 degraders and identified APH02174 as a selective, orally bioavailable degrader. However, its hERG inhibition, hepatocyte stability, and variable cross-species PK inspired further structural optimization. The optimization gave eight PROTACs ( 5 – 10, 14, and APH003 ) which exhibited high IRAK4 degradation ability (DC 50 ≤ 2 nM, DC 90 ≤ 10 nM, and D max ≥ 85%) in vitro . APH003 exhibited favorable PK parameters, low hERG inhibition, improved hepatocyte stability, and pronounced anti-inflammatory efficacy in animal models. Dose range-finding studies in rat and dog models supported its safety profile. Based on these improvements in safety, cross-species PK, efficacy, and preliminary toxicology, APH003 was selected as a preclinical candidate for GLP toxicological studies.