These data are produced by LC/MS/MS analysis of plasma and tumor samples collected in mouse tumor treatment studies utilizing T-Dxd (Enhertu) is a novel antibody-drug-conjugate (ADC) targeting HER2. After development of fit-for-purpose assays on Shimadzu 20A series/Applied Biosystems-SCIEX LC/MS/MS study samples were assayed against calibration standard samples and peak area integration and quantification was performed by Analyst 1.6.2.