Page 137 - Cellular Imaging in Regenerative Medicine, Cancer and Osteoarthritis

Albutate-1, a novel long-circulating radiotracer
So, according our results and the above mentioned literature, albumin binders attached to tracers can improve circulation time and increase tumor uptake. If the plasma protein-binding takes a long time to reverse, the blood circulation time is extended to a level that all other organs also receive a high radiation dose, reducing the therapeutic index dramatically. Future perspectives are to create a tracer with more optimal pharmacokinetics, applying adaptations in albumin binding strength, spacer length and lipophilicity.
4. Materials and Methods
4.1. Labeling of DOTA-TATE and Albutate-1
DOTA-TATE (BioSynthema, Saint Louis, USA) and Albutate-1 (piCHEM, Graz, Austria) were radiolabeled with [111In]InCl3 (Covidien, Zaltbommel, The Netherlands) as previously described [34, 35], or with [177Lu]LuCl3 (Lu-Mark, IDB Holland/AAA, Baarle Nassau, The Netherlands) [36]. For [111In]In-DOTA-TATE and [111In]In-Albutate-1 the molar activity was 20 MBq/nmol. For [177Lu]Lu- DOTA-TATE the molar activity was 60 MBq/nmol and for [177Lu]Lu-Albutate-1 the molar activity was 125 MBq/nmol. The radiochemical yield was >99% and radiochemical purity was > 90%.
4.2. Stability study
[111In]In-Albutate-1 was added (18 μL, 50 MBq/nmol) to 300 μL of mouse plasma and this mixture was incubated at 37 °C for 1 hour in an Eppendorf ThermoMixer® heating block (Eppendorf, Nijmegen, The Netherlands). After centrifugation (5 min, ~10.000 g), plasma stability of [111In]In-Albutate-1 was measured by HPLC at 1, 4 and 24 h. Stability of [111In]In-Albutate-1 in the labeling buffer was also determined by HPLC at 1 h, 1, 2 and 3 days.
4.3. Competition binding assay
A competition binding assay was performed in triplicate on SSTR2-positive H69 tumor cryosections (10 μm). The sections were incubated for 1 h with [111In]In- DOTA-TATE (100 μL, 10−9 M) in the presence of unlabeled Albutate-1 (range: 10−6 – 10-12 M). After incubation, excess of radiotracer was removed and the sections were exposed to super resolution phosphor screens for 3 days (PerkinElmer, Rotterdam, The Netherlands). Binding of [111In]In-DOTA-TATE
135
6