The heterobivalent (SSTR2/albumin) radioligand [67Cu]Cu-NODAGA-cLAB4-TATE enables efficient somatostatin receptor radionuclide theranostics

Abstract

Somatostatin type 2 receptor (SSTR2) radionuclide therapy using β^- particle-emitting radioligands has entered clinical practice for the treatment of neuroendocrine neoplasms (NENs). Despite the initial success of [¹⁷⁷Lu]Lu‑DOTA-TATE, theranostic SSTR2 radioligands require improved pharmacokinetics and enhanced compatibility with alternative radionuclides. Consequently, this study evaluates the pharmacokinetic effects of the albumin-binding domain cLAB4 on theranostic performance of copper‑67-labeled NODAGA-TATE variants in an SSTR2-positive mouse pheochromocytoma (MPC) model. Methods: Binding, uptake, and release of radioligands as well as growth-inhibiting effects were characterized in cells grown as monolayers and spheroids. Tissue pharmacokinetics, absorbed tumor doses, and projected human organ doses were determined from quantitative SPECT imaging in a subcutaneous tumor allograft mouse model. Treatment effects on tumor growth, leukocyte numbers, and renal albumin excretion were assessed. Results: Both copper‑64- and copper‑67-labeled versions of NODAGA-TATE and NODAGA-cLAB4‑TATE showed similar SSTR2 binding affinity, but faster release from tumor cells compared to the clinical reference [¹⁷⁷Lu]Lu‑DOTA-TATE. The bifunctional SSTR2/albumin-binding radioligand [⁶⁷Cu]Cu‑NODAGA-cLAB4‑TATE showed both an improved uptake and prolonged residence time in tumors resulting in equivalent treatment efficacy to [¹⁷⁷Lu]Lu‑DOTA-TATE. Absorbed doses were well tolerated in terms of leukocyte counts and kidney function. Conclusion: This preclinical study demonstrates therapeutic efficacy of [⁶⁷Cu]Cu‑NODAGA-cLAB4‑TATE in SSTR2-positive tumors. As an intrinsic radionuclide theranostic agent, the radioligand provides stable radiocopper complexes and high sensitivity in SPECT imaging for prospective determination and monitoring of therapeutic doses in vivo. Beyond that, copper‑64- and copper‑61-labeled versions offer possibilities for pre- and post-therapeutic PET. Therefore, NODAGA-cLAB4-TATE has the potential to advance clinical use of radiocopper in SSTR2-targeted cancer theranostics.

Keywords: SPECT imaging; albumin binder; allograft model; copper-67; dosimetry; pheochromocytoma.

Figure 1

Clinically approved TATE variants for theranostic application in the management of SSTR2-positive neuroendocrine neoplasms and experimental variants (this work) for theranostic use of radiocopper; pharmacokinetic properties of the diagnostic variants [⁶⁴Cu]Cu‑NODAGA-TATE and [⁶⁴Cu]Cu‑NODAGA‑cLAB4-TATE in tumor-bearing mice have been reported previously ; (cLAB4) 'clickable' lysine-derived albumin binder 4; (EMA) European Medicines Agency; (FDA) Food and Drug Administration of the United States; (GEP) gastroenteropancreatic; (NENs) neuroendocrine neoplasms.

Figure 2

Saturation binding, uptake, and release of lutetium‑177- and copper‑64-labeled TATE variants measured using intact MPC cell monolayers; (A) Total and non-specific binding at radioligand concentrations between 0.3-40 nmol/L (transfer-corrected); (B) Specific binding and effective receptor-mediated uptake at a radioligand concentration of 25 nmol/L; (C) Biphasic release from cells after incubation at a radioligand concentration of 25 nmol/L followed by radioligand removal; cells were incubated with radioligands for 1 hour at 37° C; non-specific binding was determined in presence of 1 µmol/L acetyl-TATE; (A_m) molar activity at incubation start; data presented as means ± standard error; significance of differences compared to [¹⁷⁷Lu]Lu‑DOTA-TATE *** p < 0.001.

Figure 3

Distribution, tissue-specific pharmacokinetics and cumulated activity of lutetium‑177- and copper‑67-labeled TATE variants in MPC allograft mice measured by quantitative SPECT imaging; animals received 50 MBq of the radioligands each prepared at different molar activities corresponding to molar amounts of 1.25 nmol (A_m = 40 MBq/nmol) and 2.5 nmol (A_m = 20 MBq/nmol) at treatment start; (A) Maximum-intensity projections of SPECT images at indicated time points and scaling; image series from treatments with radioligands at a molar activity of 40 MBq/nmol; (B-C) Time-courses of tissue-specific activity concentration after treatment; (D) Tissue-specific mean cumulated volume activity calculated from areas under curves; (E) Time-courses of excreted activity; data presented without decay correction as means ± standard error; significance of differences: * p < 0.05 ** p < 0.01, *** p < 0.001.

Figure 4

Treatment effects of lutetium‑177- and copper‑67-labeled TATE variants in MPC allograft mice; all animals received an initial activity dose of 50 MBq corresponding to molar amounts of 1.25 nmol (A_m = 20 MBq/nmol) or 2.5 nmol (A_m = 20 MBq/nmol) at treatment start; (A) Progression-free survival (PFS) defined as < 2-fold change in tumor volume compared to treatment start; censored data points along horizontal lines indicate animals withdrawn from follow-up before exceeding the PFS threshold; (B) Leukocyte counts in blood at indicated time points after treatment start; (C) Albumin/creatinine ratio (ACR) in urine estimating nephrotoxicity at indicated time points after treatment start, pooled data from treatments at different molar activities; (dotted line) lower threshold for microalbuminuria at ACR > 30 < 300 mg/g; data presented as means ± standard error; significance of differences: * p < 0.05, ** p < 0.01, *** p < 0.001.