Using skin biopsies to measure target occupancy of anti-fibrotic molecules: assay development and application for zampilimab in a primate model of chronic kidney disease and in healthy human volunteers

Abstract Background Assessing target engagement (TE) and target occupancy (TO) of novel antifibrotic molecules is challenging, as the target organs are inaccessible. In clinical trials, this often requires biopsies of internal organs, which increases both risk and discomfort for participants. Zampilimab (UCB7858) is a humanized monoclonal antibody that specifically inhibits the extracellular activity of transglutaminase 2 (TG2). Blocking TG2 activity reduces fibrosis in experimental models of chronic kidney disease. Here, a low-risk skin ‘biopsy-on-biopsy’ approach has been developed as a surrogate to assess TO of zampilimab in the kidney, ahead of further investigation of zampilimab in clinical studies. Methods A dual-antibody competitive immunofluorescence assay was developed to assess TO of TG2 with zampilimab. A cynomolgus monkey unilateral ureteral obstruction model was used to assess zampilimab TO in the kidney and compare this with TE measured by an in situ TG activity assay. Data were compared with TO in dermal wounds in the same animals. A human skin ‘biopsy-on-biopsy’ dermal wound approach was developed to induce fibrosis-relevant pathways. Skin sections from healthy volunteers were incubated ex vivo with increasing doses of zampilimab to assess TO. Results Zampilimab TO in cynomolgus monkey kidney and skin were positively correlated using our immunofluorescence assay, with an inverse correlation between skin TO and kidney TE using our in situ TG activity assay. In the human dermal wound model, maximal TG2 staining was observed on days 4–6 post initial dermal wound (biopsy). TO measurements increased dose-dependently with zampilimab application. Conclusions Zampilimab inhibited TG2 in cynomolgus monkey kidney and skin. Skin is an accessible surrogate tissue to assess kidney TO and predict TE, and our ex vivo model of skin biopsy has potential for application in the development of other antifibrotic therapeutics. A phase I first-in-human study of zampilimab in healthy volunteers (NCT02879877; 26/08/2016) will provide further proof of concept.