Patrick Cieslik

The formation of discrete metal–fluoride bonds represents a promising strategy for the late-stage introduction of 18F-radioactivity to create targeted positron-emitting cancer imaging agents. This approach typically involves the coordination of [18F]fluoride to metal ion complexes that retain a vacant and accessible coordination site. While trivalent metal ions such as Al3+, Ga3+, and Sc3+ have shown promise due to their high fluoride affinity, experimental exploration of metal–fluoride bonding remains limited in scope with respect to metal ions used. According to standard tables of thermodynamic data on bond enthalpies, zirconium(IV) ions exhibit one of the highest reported affinities for fluoride, making Zr(IV) complexes promising candidates for developing new 18F-labeled radiotracers featuring [18F]ZrF bonds. Here, we investigated both the macroscopic (nonradioactive) and radiochemical behavior of the Zr(EDTA) complex with fluoride (or [18F]fluoride) ions. A combination of experimental and computational methods including electronic absorption spectroscopy, 19F NMR studies, isothermal titration calorimetry, density functional theory calculations, and 18F-radiochemical experiments confirmed that the Zr–F bond does form with the Zr(EDTA) complex in aqueous conditions. However, the [ZrF(EDTA)(OH2)]− complex is unstable with respect to fluoride exchange with the solvent. 18F-radiolabeling control reactions using metal-ion-free EDTA or RESCA derivatives revealed highly unusual behavior in which fluoride ions formed a tight ion pair with the chelating unit.

²²⁵Ac is a key α-emitter for targeted alpha therapy. Among available chelators, Macropa currently provides some of the most stable ²²⁵Ac complexes, yet the limited stability of [²²⁵Ac]Ac–Macropa indicates a further optimization potential. Here, we report the design and evaluation of a new Ac³⁺ chelator, coined Megapa, and assessed its suitability to produce stably labeled ²²⁵Ac-based radiopharmaceuticals using radiochemical and computational methods. Unexpectedly, Megapa showed poorer radiolabeling performance than Macropa, showing reduced ²²⁵Ac incorporation (80.2 ± 2.9% RCC vs quantitative labeling) across multiple conditions tested. In addition, [²²⁵Ac]Ac–Megapa displayed lower kinetic inertness than [²²⁵Ac]Ac–Macropa, with lower stability in human serum (45.8% intact after 7 days vs no detectable degradation) and substantially higher ²²⁵Ac release in La³⁺ challenge experiments (64.3% vs 0.7%). Thermodynamic stability studies supported these results, indicating a lower thermodynamic stability of La–Megapa compared to La–Macropa (log K_LaL of 10.53 vs 13.90). To rationalize these findings, quantum chemical calculations were performed on the Ac³⁺ and La³⁺ complexes of Megapa and Macropa. The computed low-energy structures were closely analogous for both chelators, indicating that the differing radiochemical behavior is unlikely to arise from intrinsic metal–ligand bonding. Instead, solvation effects and solution-phase molecular interactions are the most probable contributors to the poorer performance of Megapa.

Two-photon luminescent bioprobes have emerged as powerful tools for in cellulo and in vivo imaging.

In the area of nuclear medicine diagnosis and therapy of tumors, peptides play a central role as tumor-homing compounds, enabling high affinity and specific accumulation of the agents, thereby laying the foundation for successful imaging and therapy. Due to their favorable chemical and biological properties, peptides play an essential role as radionuclide carriers. For decades, therapy in nuclear medicine was based on the use of β– emitter-labeled peptides as receptor-specific therapeutics. This paradigm is beginning to change in the direction of α emitter-labeled analogs, as these demonstrate a considerably higher therapeutic efficiency. However, their application also poses significant challenges that need to be addressed in the future to unleash the full potential of this form of therapy. This perspective discusses the advantages of α-emitter-carrying radiopeptides for the targeted α therapy (TAT) of tumors and the remaining challenges that need to be addressed by future research.

Radiolabeled monoclonal antibodies (mAbs) form a major branch of nuclear medicine and are used in the development of tracers for both diagnostic imaging and molecularly targeted radio­(immuno)­therapy (RIT). Since treatment options for many types of late-stage cancers are limited and these diseases become refractory to classic chemotherapy, new tools are required to improve patient outcomes. The high tumor uptake and specificity of mAbs, coupled with increased therapeutic range of energetic β^–-emitting radionuclides, offers a potential solution to overcome traditional problems associated with poor tissue penetration of antibody-drug conjugates, chemotherapeutic resistance, and off-target accumulation, which can lead to adverse responses. The challenge is to develop efficient and reliable chemical methods that provide simultaneous selectivity and high stabilization of the radiometal via complexation chemistry, with rapid access to new bioconjugate bonds on protein that avoid the loss of bioactivity. Here, we designed a new octadentate bispidine-chelating system, functionalized with a light-responsive tetrazole unit, and demonstrated the chemoselective derivatization of sulfhydryl groups introduced on the protein surface. High radiolabeling and bioconjugation yields of ¹⁶¹Tb-onartuzumaban engineered mAb fragment targeting the human hepatocyte growth-factor receptor (c-MET; a characteristic biomarker found in clinical samples of several diseases, including gastric adenocarcinomas)were obtained under ambient conditions after 5 min of light-induced coupling. Comprehensive biochemical and animal experiments including cellular binding assays, noninvasive γ-ray imaging, biodistribution studies, and pharmacokinetic measurements established the viability of using ¹⁶¹Tb-onartuzumab to target c-MET expression in vivo. Subsequent RIT studies in MKN-45 xenograft models demonstrated that the ¹⁶¹Tb-onartuzumab radiotracer formed by photoradiosynthesis permitted low-dose therapy studies that led to efficient targeting and treatment of tumor models. Collectively, the new complexation and chemoselective photoconjugation chemistries overcome some of the limitations in traditional labeling approaches. Photoradiosynthesis represents an excellent platform for building future antibody-based radiotracers for applications in diagnostic and therapeutic medicine.

The rigidity and well-defined size and shape of the bispidine scaffold, as well as the ease of preparing 4- to 10-dentate ligands are the main features for its very rich coordination chemistry.

Radioimmunotherapy (RIT) is a promising alternative to conventional treatment options. Here, we present experimental work on the synthesis, radiochemistry, and in vivo performance of a lanthanoid‐selective nonadentate bispidine ligand suitable for ¹⁷⁷Lu³⁺ ion complexation. The ligand (bisp,1) was derivatised with a photoactivatable aryl azide (ArN₃) group as a bioconjugation handle for light‐induced labelling of proteins. Quantitative radiosynthesis of [¹⁷⁷Lu]Lu‐1⁺ was accomplished in 10 minutes at 40 °C. Subsequent incubation of [¹⁷⁷Lu]Lu‐1⁺ with trastuzumab, followed by irradiation with light at 365 nm for 15 min, at room temperature and pH 8.0–8.3, gave the radiolabelled mAb, [¹⁷⁷Lu]Lu‐1‐azepin‐trastuzumab ([¹⁷⁷Lu]Lu‐1‐mAb) in a decay‐corrected radiochemical yield of 14 %, and radiochemical purity (RCP)>90 %. Stability studies and cellular binding assays in vitro using the SK‐OV‐3 human ovarian cancer cells confirmed that [¹⁷⁷Lu]Lu‐1‐mAb remained biological active and displayed specific binding to HER2/neu. Experiments in immunocompromised female athymic nude mice bearing subcutaneous xenograft models of SK‐OV‐3 tumours revealed significantly higher tumour uptake in the normal group compared with the control block group (29.8±11.4 %ID g⁻¹ vs. 14.8±6.1 %ID g⁻¹, respectively; P‐value=0.037). The data indicate that bispidine‐based ligand systems are suitable starting points for constructing novel, high‐denticity chelators for specific complexation of larger radiotheranostic metal ion nuclides.

Octadentate and specifically nonadentate ligands with a bispidine scaffold (3,7-diazabicyclo[3.3.1]nonane) are known to be efficiently coordinated to a range of metal ions of interest in radiopharmaceutical chemistry and lead to exceedingly stable and inert complexes. Nonadentate bispidine L² (with a tridentate bipyridine acetate appended to N3 and a picolinate at N7) has been shown before to be an ideal chelator for ¹¹¹In³⁺, ¹⁷⁷Lu³⁺, and ²²⁵Ac³⁺, nuclides of interest for diagnosis and therapy, and a proof-of-principle study with an SSTR2-specific octreotate has shown potential for theranostic applications. We now have extended these studies in two directions. First, we present ligand derivative L³, in which the bipyridine acetate is substituted with terpyridine, a softer donor for metal ions with a preference for more covalency. L³ did not fulfill the hopes because complexation is much less efficient. While for Bi³⁺ and Pb²⁺ the ligand is an excellent chelator with properties similar to those of L², Lu³⁺ and La³⁺ show very slow and inefficient complexation with L³ in contrast to L², and ²²⁵Ac³⁺ is not fully coordinated, even at an increased temperature (92% radiochemical yield at 80 °C, 60 min, [L³] = 10^-4 M). These observations have led to a hypothesis for the complexation pathway that is in line with all of the experimental data and supported by a preliminary density functional theory analysis, which is important for the design of further optimized bispidine chelators. Second, the coordination chemistry of L² has been extended to Bi³⁺, La³⁺, and Pb²⁺, including solid state and solution structural work, complex stabilities, radiolabeling, and radiostability studies. All complexes of this ligand (La³⁺, Ac³⁺, Lu³⁺, Bi³⁺, In³⁺, and Pb²⁺), including nuclides for targeted α therapy (TAT), single-photon emission computed tomography, and positron emission tomography, are formed efficiently under physiological conditions, i.e., suitable for the labeling of delicate biological vectors such as antibodies, and the complexes are very stable and inert. Importantly, for TAT with ²²⁵Ac, the daughter nuclides ²¹³Bi and ²⁰⁹Pb also form stable complexes, and this is important for reducing damage to healthy tissue.

As an essential metal ion and an efficient relaxation agent, Mn²⁺ holds a great promise to replace Gd³⁺ in magnetic resonance imaging (MRI) contrast agent applications, if its stable and inert complexation can be achieved. Toward this goal, four pyridine and one carboxylate pendants have been introduced in coordinating positions on the bispidine platform to yield ligand L³. Thanks to its rigid and preorganized structure and perfect size match for Mn²⁺, L³ provides remarkably high thermodynamic stability (log K_MnL = 19.47), selectivity over the major biological competitor Zn²⁺ (log(K_MnL/K_ZnL) = 4.4), and kinetic inertness. Solid-state X-ray data show that [MnL³(MeOH)](OTf)₂ has an unusual eight-coordinate structure with a coordinated solvent molecule, in contrast to the six-coordinate structure of [ZnL³](OTf), underlining that the coordination cavity is perfectly adapted for Mn²⁺, while it is too large for Zn²⁺. In aqueous solution, ¹⁷O NMR data evidence one inner sphere water and dissociatively activated water exchange (k_ex²⁹⁸ = 13.5 × 10⁷ s^-1) for MnL³. Its water proton relaxivity (r₁ = 4.44 mM^-1 s^-1 at 25 °C, 20 MHz) is about 30% higher than values for typical monohydrated Mn²⁺ complexes, which is related to its larger molecular size; its relaxation efficiency is similar to that of clinically used Gd³⁺-based agents. In vivo MRI experiments realized in control mice at 0.02 mmol/kg injected dose indicate good signal enhancement in the kidneys and fast renal clearance. Taken together, MnL³ is the first chelate that combines such excellent stability, selectivity, inertness and relaxation properties, all of primary importance for MRI use.

We report a nonadentate bispidine (3,7-diazabicyclo[3.3.1]nonane) that unveils the potential to bind theranostically relevant radionuclides, including indium-111, lutetium-177, and actinium-225 under mild labeling conditions. This radiopharmaceutical candidate allows the simultaneous application of imaging and treatment (radionuclide theranostics) without changing the type of the bioconjugate; that is, it allows the strong binding to an imaging and a therapeutic radionuclide by the same chelator. Since sophisticated coordination chemistry is required to achieve high thermodynamic and kinetic stability (inertness), it is not surprising that only a few chelators have been reported that are able to strongly bind several radionuclides to a satisfactory extent. Bispidine-derived ligands have proven to be ideal for di- and trivalent metal ions with generally fast complexation kinetics and high in vitro and in vivo stabilities. The presented (radio)complexes are formed under mild conditions (pH 6, 177Lu- and ²²⁵Ac-labeled conjugates were investigated, considering their binding to two different SSTR2-positive cell lines, including the human pancreatic carcinoid tumor (BON-SSTR2+) and the murine pheochromocytoma cell line (MPC). The biodistribution and accumulation pattern in MPC tumor-bearing mice was also evaluated. The Lu^III and Ac^III complexes studied show how ligand structures can be optimized in general by extending the denticity and varying the donor set in order to allow for fast complex formation and medically relevant inertness.