[²¹²Pb]Pb-eSOMA-01:A Promising Radioligand for Targeted Alpha Therapy of Neuroendocrine Tumors

Peptide receptor radionuclide therapy (PRRT) has been applied to the treatment of neuroendocrine tumors (NETs) for over two decades. However, improvement is still needed, and targeted alpha therapy (TAT) with alpha emitters such as lead-212 (212Pb) represents a promising avenue. A series of ligands based on octreotate was developed. Lead-203 was used as an imaging surrogate for the selection of the best candidate for the studies with lead-212. 203/212Pb radiolabeling and in vitro assays were carried out, followed by SPECT/CT imaging and ex vivo biodistribution in NCI-H69 tumor-bearing mice. High radiochemical yields (≥99%) and purity (≥96%) were obtained for all ligands. [203Pb]Pb-eSOMA-01 and [203Pb]Pb-eSOMA-02 showed high stability in PBS and mouse serum up to 24 h, whereas [203Pb]Pb-eSOMA-03 was unstable in those conditions. All compounds exhibited a nanomolar affinity (2.5–3.1 nM) for SSTR2. SPECT/CT images revealed high tumor uptake at 1, 4, and 24 h post-injection of [203Pb]Pb-eSOMA-01/02. Ex vivo biodistribution studies confirmed that the highest uptake in tumors was observed with [212Pb]Pb-eSOMA-01. [212Pb]Pb-eESOMA-01 displayed the highest absorbed dose in the tumor (35.49 Gy/MBq) and the lowest absorbed dose in the kidneys (121.73 Gy/MBq) among the three tested radioligands. [212Pb]Pb-eSOMA-01 is a promising candidate for targeted alpha therapy of NETs. Further investigations are required to confirm its potential.</p

Orthogonal synthesis of a versatile building block for dual functionalization of targeting vectors

Dual functionalization of targeting vectors, such as peptides and antibodies, is still synthetically challenging despite the increasing demand for such molecules serving multiple purposes (i.e., optical and nuclear imaging). Our strategy was to synthesize a versatile building block via the orthogonal incorporation of chemical entities (e.g., radionuclide chelator, fluorescent dye, cytotoxic drugs, click handle, and albumin binder) in order to prepare various dual functionalized biovectors. The functional groups were introduced on the building block using straightforward chemical reactions. Thus, an azidolysine and a biogenic lysine were installed into the building block to allow the coupling of the second functional group and the regioselective conjugation to the biovector via the strain-promoted azide–alkyne cycloaddition, while the first functional group was inserted during the solid-phase peptide synthesis. To extend the applicability of the building block to large biomolecules, such as antibodies, a DBCO-maleimide linker was clicked to the azidolysine to present a maleimide group that could react with the exposed sulfhydryl groups of the cysteine residues. To exemplify the possibilities offered by the building block, we synthesized two dual-functionalized compounds containing a 2,2′,2″′,2‴-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl) tetraacetic acid chelator and an albumin binder (4a) to extend the blood half-life of radiolabeled biovectors or a click handle (4b) to enable the late-stage click reaction; 4a and 4b were conjugated to a model cyclic peptide bearing a short thiolated linker at the N-terminal position, in a single step via the thiol–maleimide Michael addition. Both dual-functionalized peptides, 9a and 9b, were obtained rapidly in high chemical purity (>95%) and labeled with [111In]InCl3. Both radiopeptides showed good stability in mouse serum and PBS buffer

Development of 203Pb-labeled SSTR2 radioligands for neuroendocrine tumors (NETs) imaging

Objectives : Development of theranostic pairs is generally guided by the biodistribution and pharmacokinetic properties of the imaging agent. Indeed, conducting such studies with beta or alpha emitters, such as lead-212, can be challenging. Therefore, it is often preferable to use a surrogate to facilitate detection of radioactivity in tissues, follow distribution of the radioactive drug over time by non-invasive imaging, and ease radiation safety. In this context, lead203, a gamma emitter (t1/2 =52 h), is perfectly adapted to study the distribution of 212Pb-labeled drugs, as it provides chemically identical companion diagnostics. Our objective was therefore to design and pre-clinically evaluate two novel 203Pb-labeled SSTR2 ligands for theranostic application of NETs. Methods : Two octreotate derivatives, containing a 4-(aminomethyl)cyclohexane-1-carbonyl (Amcha) or a 4-amino-1-carboxymethyl-piperidinyl (Pip) linker, were synthesized by SPPS. In vitro competitive binding assays with [111In]In-DOTATATE were performed using CHO-K1 cell membranes over-expressing human SSTR2. Both ligands were labeled with 203Pb in sodium acetate buffer for 20 minutes at room temperature and analyzed by iTLC and radioHPLC. LogD7.4 and stability in serum were also determined. SPECT/CT imaging was performed at 1, 4 and 24 h post-injection in H69 tumorbearing mice, followed by ex-vivo biodistribution. Results : DO3AM-Amcha-TATE and DO3AM-Pip-TATE were successfully synthesized with a yield and purity of 10% and 99%, respectively. Both ligands showed low nanomolar binding affinity for SSTR2 and hydrophilic behavior. DO3AM-Amcha-TATE and DO3AM-PipTATE were labeled with 203Pb with a radiochemical yield and purity exceeding 98%. Stability studies revealed that our 203Pb-labeled octreotate derivatives were stable in serum for 24 h. Image analysis of the two radioligands showed high uptake at 1 h in the SSTR2-positive tumors (4.0 ± 0.57 and 3.3 ± 1.08%ID/mL for [203Pb]Pb-DO3AMAmcha-TATE and [203Pb]Pb-DO3AM-PIP-TATE, respectively), as well as in the kidneys ([203Pb]Pb-DO3AM-Amcha-TATE: 10.6 ± 1.7%ID/mL and [203Pb]Pb-DO3AM-Pip-TATE: 12.8 ± 1.80%ID/mL), but also at 24 h p.i. (tumors: 2.4 ± 0.27%ID/mL and 2.2 ± 0.64%ID/mL; kidneys: 3.6 ± 1.2%ID/mL and 6.6 ± 1.17%ID/mL for [203Pb]Pb-DO3AM-Amcha-TATE and [203Pb]Pb-DO3AM-Pip-TATE, respectively). The tumor-to-kidney ratio increased over time for [203Pb]Pb-DO3AM-Amcha-TATE (0.4 at 1 h to 0.7 at 24 h p.i.), while it remained constant for the [203Pb] Pb-DO3AM-Pip-TATE (0.26 at 1 h to 0.34 at 24 h p.i.). Ex-vivo biodistribution confirmed that administration of [203Pb]Pb-DO3AMPip-TATE resulted in a higher uptake in the pancreas and bones than [203Pb]Pb-DO3AM-Amcha-TATE. Conclusions : Our results indicated that these two ligands are promising candidates for imaging of SSTR2-positive tumors. Further investigations will be conducted with the 212Pb-labeled analogs to determine their potential as theranostic agents