*2.3. Stability, Serum Proteins Binding, and Lipophilicity Studies*

The long-term stability of the radiotracer (up to 120 h) was tested by TLC and RP-HPLC in various media, evaluating the resistance to degradation of the non-peptide-based backbone at different pHs (NaCl 0.9% solution pH 7 and 0.1 M 4-(2-hydroxyethyl)-1 piperazineethanesulfonic acid (HEPES) pH 4) and to enzymatic cleavage (for instance, to proteases in human serum and blood). The kinetic inertness of [111In]In–DOTA complexes were also evaluated in presence of a 0.1 M ethylenediaminetetraacetic acid (EDTA) solution or serum proteins as competitors. All these studies revealed a high stability of the radiotracer showing a percentage of intact compound generally higher than 95% after 120 h when incubated with NaCl 0.9%, HEPES and EDTA, and after 48 h when challenged with human serum or blood (HB). Results obtained are summarized in Table 2. Amount of radiotracer bound to serum proteins was computed in the samples incubated with HB after 8, 24 and 48 h. After treatment with a MeCN/H2O/TFA 50/45/5 *v*/*v*/*v* solution and centrifugation, the radioactivity in the precipitate was found noteworthy being from 12 to 20% of the total. Lipophilicity calculation gave a partition coefficient (LogP) of 0.45.

**Table 2.** Amount of intact [111In]In-IP-001 over time in different media (*<sup>n</sup>* = 3; mean <sup>±</sup> SD). The percentages were computed by means of radio-TLC (thin-layer chromatography) or RP-HPLC analyses. HEPES: 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid.

