To exclude kinetic instability or degradation by proteolysis, the tracer was incubated in PBS buffer or human plasma at 37 °C for a period of two hours (Figure ). Samples were taken at 9 time points (0 -120 min) and analysed by radio-HPLC. Both experiments displayed one single peak at ∼6.2 min for the parent fraction, which remained the same for every single timepoint.