Evaluation of the effect of the addition of an albumin-binding motif on 177Lu-DOTATATE

1145 Objectives: 177Lu-DOTATATE is commonly used for peptide receptor radionuclide therapy of neuroendocrine tumours (NET). The purpose of this study was to evaluate whether prolonging the residence time of 177Lu-DOTATATE by the addition of an albumin binder motif could increase tumour accumulation and tumour to kidney ratios for improved therapeutic efficacy. Methods: DOTATATE and derivatives comprising an albumin-binder motif (GluAB-DOTATATE and AspAB-DOTATATE) were prepared using Fmoc-based solid-phase synthesis. 177Lulabeling was conducted in 0.1 M NaOAc buffer (pH 4.5) at 100°C for 15 min followed by HPLC purification. The binding affinities of natLu-labeled peptides were determined by competition binding assay using CHO-K1 membranes overexpressing human SSTR2 against 125I-Tyr11-somatostatin-14. SPECT imaging and biodistribution studies (1h, 4h, 24h, 72h, 120h) were performed in male immunodeficient mice bearing AR42J pancreatic tumour xenografts. Statistics and graphs were computed using Prism 7 or R 3.4.0. Results: GluAB-DOTATATE and AspAB-DOTATATE were synthesized in 19% and 14% yields, respectively. natLu-GluAB-DOTATATE and natLu-AspAB-DOTATATE were obtained in 87% and 50% yields, respectively. The Ki values of natLu-DOTATATE, natLu-GluAB-DOTATATE, and natLu-AspAB-DOTATATE were 3.84±0.30, 8.95±3.40, and 8.72±3.35 nM, respectively. The radiolabeled compounds were obtained in non-decay-corrected isolated yields of ≥ 40% and in > 96% radiochemical purity. Their average specific activities were in the range of 315-440 GBq/μmol. 177Lu-AspAB-DOTATATE had the highest blood activity (%ID/g) at 1h, 4h, and 24h (14.96±2.68, 5.99±0.84 and 0.57±0.12, respectively), which was significant against 177Lu-DOTATATE (0.32±0.03, 0.11±0.05 and 0.03±0.01, respectively) and 177Lu-GluAB-DOTATATE (8.08±2.05, 2.05±0.83 and 0.10±0.04, respectively) (p 60% at 1h post-injection. Conclusion: Both albumin binders increased blood residence time compared with 177Lu-DOTATATE. Sustained increased tumour uptake compared with 177Lu-DOTATATE was only significant for 177Lu-AspAB-DOTATATE, but its higher kidney uptake diminished the therapeutic index, making it less suitable for treatment. Interestingly, the addition of an albumin binder decreased pancreas uptake significantly.