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Updated: Feb 4, 2026

Covalent Fragment Screening Using the Quantitative Irreversible Tethering Assay
Published on: February 28, 2025
An Internally Controlled Quantitative Target Occupancy Assay for Covalent Inhibitors
Rasmus Hansen1, Sarah J Firdaus2, Shuangwei Li2
1Wellspring Biosciences, Inc., 3033 Science Park Rd., Ste. 220, San Diego, CA, 92121, USA. rasmus@wellspringbiosciences.com.
Abstract:
Assessing target occupancy is critical for establishing proof-of-mechanism for novel inhibitors and to determine whether robust target inhibition can be achieved at tolerated doses. This is challenging in the clinic using conventional methods due to the need for untreated controls. We describe a new mass spectrometry approach to quantitatively assess target occupancy for covalent inhibitors that does not require untreated controls, and apply the method to the KRASG12C inhibitor ARS-1620.
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