Fig. 6. A constant amount of the radioligand was incubated with an increasing concentration of SK-OV-3 (HER2-positive) and MDA-MB-231 (HER2-negative) cells. The specific cell binding is expressed as a function of the number of cells. A binding curve (dashed line) is fitted using nonlinear regression, and the immunoreactive fraction corresponding to the extrapolation to infinite antigen excess is calculated using Graphpad Prism 7 software.³⁴ 81% of the immunoreactive fraction of [¹¹¹In]In-DTPA-trastuzumab for SK-OV-3 showed a conserved affinity for HER2 after DTPA regiospecific conjugation and radiolabeling with [¹¹¹In]indium. The immunoreactivity decreased to 15% with MDA-MB-231 (HER2-negative).