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In this work, we describe the complex formation and radiochemistry of the cyclen-based chelator DOTI-Me bearing four methylimidazole arms. Radiolabeling properties were evaluated for 52g Mn, 64 Cu, 68 Ga, 111 In, 161 Tb, and 177 Lu, and DOTI-Me showed distinct differences to the structurally related H 4 DOTA. While radiochemical conversions (RCCs) for 52g Mn and 111 In were comparable to those of H 4 DOTA, DOTI-Me was not suited for 68 Ga. Conversely, quantitative RCCs were achieved for 64 Cu at ambient temperature, while elevated temperatures were required for complexation with H 4 DOTA. For 161 Tb and 177 Lu, good but not quantitative RCCs were obtained with DOTI-Me. With the exemption of 68 Ga 3+ , radiolabeled complexes showed high stability in ligand challenge experiments and in human serum. X-ray analysis of the nonradioactive complexes revealed the formation of 8-coordinate Mn 2+ and In 3+ DOTI-Me complexes. Cu 2+ adopted a unique distorted square-pyramidal 2 + 3 with the neutral DOTI-Me ligand and a Jahn-Teller distorted 4 + 2 coordination geometry for the diprotonated H 2 DOTI-Me 2+ cation, respectively. For Zn 2+ , the complex with HDOTI-Me + showed a distorted 4 + 3 pentagonal bipyramidal geometry. Summarizing, the ligand DOTI-Me may be an interesting alternative to H 4 DOTA for 52g Mn, 64 Cu, 111 In, 161 Tb, and 177 Lu, covering diagnostic as well as therapeutic radionuclides. Further studies of targeted radiopharmaceuticals based on the DOTI-Me scaffold in combination with the set of radiometals presented herein are thus warranted.

Herein, we present the synthesis and coordination chemistry of copper(II) and zinc(II) complexes of two novel heterocyclic triazacyclononane (tacn)-based chelators (H NODThia and NODThia-AcNHEt ). The chelator H NODThia was further derivatized to obtain a novel PSMA-based bioconjugate ( NODThia-PSMA ) and a bifunctional photoactivatable azamacrocyclic analogue, NODThia-PEG 3 -ArN 3 , for the development of copper-64 radiopharmaceuticals. 64 Cu radiolabeling experiments were performed on the different metal-binding chelates, whereby quantitative radiochemical conversion (RCC) was obtained in less than 10 min at room temperature. The in vitro stability of NODThia-PSMA in human plasma was assessed by ligand-challenge and copper-exchange experiments. Next, we investigated the viability of the photoactivatable analog ( NODThia-PEG 3 -ArN 3 ) for the light-induced photoradiosynthesis of radiolabeled proteins. One-pot photoconjugation reactions to human serum albumin (HSA) as a model protein and the clinically relevant monoclonal antibody formulation MetMAb were performed. [ 64 Cu]Cu- 7 -azepin-HSA and [ 64 Cu]Cu- 7 -azepin-onartuzumab were prepared in less than 15 min by irradiation at 395 nm, with radiochemical purities (RCP) of >95% and radiochemical yields (RCYs) of 42.7 ± 5.3 and 49.6%, respectively. Together, the results obtained here open the way for the development of highly stable 64 Cu-radiopharmaceuticals by using aza -heterocyclic tacn-based chelators, and the method can easily be extended to the development of 67 Cu radiopharmaceuticals for future applications in molecularly targeted radio(immuno)therapy.

The gastrin-releasing peptide receptor (GRPr) is overexpressed in various cancer types including prostate and breast carcinomas, making it an attractive target for molecular imaging and therapy. In this work, we designed a novel GRPr antagonistic probe comprising metal chelator NODIA-Me. This 1,4,7-triazacyclononane-based chelator forms positively charged metal complexes due to its neutral methylimidazole arms. Because a positive charge at the N -terminus of GRPr conjugates is responsible for high receptor affinity as exemplified by the current gold standard DOTA-RM2, we investigated if a positively charged radiometal complex can be used as a pharmacokinetic modifier to also produce high-affinity GRPr conjugates. In this respect, the bioconjugate NODIA-Me-Ahx-JMV594 was prepared by a combination of solid-phase peptide synthesis and solution-based reactions in a 94% yield. Radiolabeling provided the 68 Ga-labeled conjugate in radiochemical yields of >95% and radiochemical purities of >98% with mean molar activities of A m ∼17 MBq nmol -1 . The competitive GRPr affinity of the metal-free and 69/71 Ga-labeled conjugate was determined to be IC 50 = 0.41 ± 0.06 and 1.45 ± 0.06 nM, respectively. The metal-free GRPr antagonist DOTA-RM2 and its corresponding 69/71 Ga complex had IC 50 values of 1.42 ± 0.07 and 0.98 ± 0.19 nM, respectively. Small-animal PET imaging of mice bearing GRPr(+) PC-3 tumors revealed high radioactivity accumulation in the tumors and in the pancreas as an organ with high levels of GRPr expression. These findings were corroborated by the corresponding ex vivo biodistribution data, in which the tumors and the pancreas exhibited the highest radioactivity accumulation. By coinjection of an excess of NODIA-Me-Ahx-JMV594, uptake in the tumors and GRPr(+) organs was significantly reduced, confirming specific receptor-mediated uptake. The estrogen receptor-positive tumor of a female breast cancer patient was clearly visualized by PET imaging using 68 Ga-labeled NODIA-Me-Ahx-JMV594. To summarize, the positive charge at the N -terminus of the conjugate induced by the Ga(NODIA-Me) complex resulted in high GRPr affinity comparable to that of the potent antagonist DOTA-RM2. The conjugate NODIA-Me-Ahx-JMV594 is a promising probe for imaging of GRPr tumors that warrants further evaluation in larger patient cohorts as well as in combination with other radiometals.

Three new bifunctional copper chelators based on the 1,4,7-triazacyclononane (tacn) platform have been synthesized and conjugated to peptides. The first one is constituted of the tacn with two methylpyridinyl and one methylthiazolyl carboxylic acid pendant arms, while, in the second and third ones, the macrocycle is functionalized by three methylpyridinyl groups, with an additional hexynoic acid chain on a carbon of one or two pyridine rings. These three bifunctional chelators have been conjugated to the antagonist DPhe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 peptide for targeting the gastrin-releasing peptide receptor, which is overexpressed in prostate cancer. The resulting monomeric bioconjugates have shown their efficiency to be radiolabeled with β+ emitter 64Cu, and the hydrophilicity and PC-3 cell internalization properties of these radiolabeled conjugates have been studied. PC-3 cell binding affinity of mono- and dimeric metal-free and natCu metallated conjugates have been evaluated by IC50 measurements. The results demonstrate the potential of these methylpyridinyl tacn derivatives for radiopharmaceutical applications., (© The Author(s) 2022. Published by Oxford University Press.)

Despite the promising results of prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) in metastatic castration-resistant prostate cancer (mCRPC), some patients show worsening disease during PSMA-RLT. We investigated the value of combined [ 18 F]FDG and [ 68 Ga]Ga-PSMA-11 PET imaging in this setting. In n = 29 mCRPC patients with worsening disease after a median of four cycles of [ 177 Lu]Lu-PSMA-617 RLT, combined [ 18 F]FDG and [ 68 Ga]Ga-PSMA-11 PET imaging was performed to detect [ 18 F]FDG-avid lesions with low or no PSMA expression (mismatch lesions). To evaluate prognostic implication of mismatch, survival analyses regarding presence, location, and [ 18 F]FDG PET-derived parameters such as SUV max , metabolic tumor volume (MTV m ), and total lesion glycolysis (TLG m ) of mismatch findings were performed. Seventeen patients (59%) showed at least one mismatch metastasis. From the time point of combined PET imaging, the median overall survival (OS) of patients with mismatch findings was significantly (p = 0.008) shorter than those without (3.3 vs. 6.1 mo). Patients with a high MTV m revealed a significantly ( p = 0.034) shorter OS of 2.6 mo than patients with low MTV m (5.3 mo). Furthermore, patients with hepatic mismatch showed a significantly ( p = 0.049) shorter OS than those without (2.9 vs. 5.3 mo). Difference in OS regarding SUV max and TLG m was not significant. In mCRPC patients with worsening disease during PSMA-RLT, combined [ 18 F]FDG and [ 68 Ga]Ga-PSMA-11 PET imaging is essential to identify mismatch findings, as these are associated with poor outcomes requiring a change in therapy management.

The use of 225 Ac in prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT), either as monotherapy or in combination with 177 Lu, is a promising therapy approach in patients with metastatic castration-resistant prostate carcinoma (mCRPC). In this study, we report the efficacy and safety of [ 225 Ac]Ac-PSMA-617 augmented [ 177 Lu]Lu-PSMA-617 RLT in 177 Lu-naive mCRPC patients ( n = 15) with poor prognosis (presence of visceral metastases, high total tumor burden with diffuse bone metastases or a short PSA doubling time of <2 months). Biochemical (by PSA serum value) and molecular imaging response (by [ 68 Ga]Ga-PSMA-11 PET/CT) was assessed after two cycles of [ 177 Lu]Lu-PSMA-617 RLT, with at least one [ 225 Ac]Ac-PSMA-617 augmentation. In addition, PSA-based progression-free survival (PSA-PFS), overall survival (OS) and toxicity (according to CTCAE) were analyzed. We observed a biochemical- and molecular imaging-based partial remission in 53.3% (8/15) and 66.7% (10/15) of patients, respectively. The median PSA-PFS and OS was 9.1 and 14.8 months, respectively. No serious acute adverse events were recorded. Two out of fifteen patients experienced grade 3 anemia. No other grade 3/4 toxicities were observed. RLT-related xerostomia (grade 1/2) was recorded in 2/15 patients. Our data showed a high clinical efficacy with a favorable side effects profile of [ 225 Ac]Ac-PSMA-617 augmented [ 177 Lu]Lu-PSMA-617 RLT in this highly challenging patient cohort.

In recent years, radiolabeled tracers targeting prostate-specific membrane antigen (PSMA) have had a tremendous impact on prostate cancer management. Here, we report on the formation of radioactive impurities formed during the clinical production of 177 Lu-labeled PSMA-617. We provide compelling evidence that these impurities are the result of a spontaneous, thermally mediated condensation reaction of the Glu-CO-Lys moiety resulting in the formation of three different five-membered ring systems. Density functional theory (DFT) calculations show that the condensation and cyclization of the Glu-CO-Lys moiety is thermodynamically spontaneous. In cell experiments, no affinity of these cyclized compounds toward PSMA was observed. HPLC analyses of urine samples from patient studies showed rapid renal excretion of these radioactive cyclized species. Radiolabeling conditions were identified that significantly reduced the formation of cyclized side products yielding 177 Lu-labeled PSMA-617 in high radiochemical yield and purity in concordance with current good manufacturing practice (cGMP) requirements.