Deciphering the Tumor Uptake of Heterobivalent (SST₂/Albumin) [⁶⁴Cu]Cu-NODAGA-cLAB-TATEs

Radioligands with albumin-binding moieties exhibit a great potential for the treatment of tumor diseases owing to the general finding of an increased integral tumor uptake compared to radioligands without such moieties. However, the reasons for this pharmacokinetic behavior are less explored. Herein, we focused on identifying potential mechanisms for our previously developed heterobivalent (SST₂/albumin) <b>[</b>^<b>64</b><b>Cu]Cu-NODAGA-cLAB-TATEs</b>. For this purpose, we designed two novel derivatives that show either negligible binding to albumin or lack the SST₂-targeting capability. Based on the <i>in vivo</i> results, we hypothesize that binding of the albumin-bound radioligand to SST₂ in addition to that of the free radioligand causes the increased tumor uptake. This is supported by saturation binding analyses in the presence of albumin and compartment modeling considerations. Overall, the results of this study provide a first tentative explanation for the phenomenon of increased tumor uptake for albumin-binding radioligands, which may support the prospective design of such radioligands.