A new pyclen-3,9-diacetate derivative ligand (H₂3,9-OPC2A) was synthesized possessing an etheric O-atom opposite to the pyridine ring, to improve the dissociation kinetics of its Mn(II) complex (pyclen = 3,6,9,15-tetraazabicyclo(9.3.1)pentadeca-1(15),11,13-triene). The new ligand is less basic than the N-containing analogue (H₂3,9-PC2A) due to the non-protonable O-atom. In spite of its lower basicity, the conditional stability of the [Mn(3,9-OPC2A)] (pMn = −log(Mn(II)), c_L = c_Mn(II) = 0.01 mM. pH = 7.4) remains unaffected (pMn = 8.69), compared to the [Mn(3,9-PC2A)] (pMn = 8.64). The [Mn(3,9-OPC2A)] possesses one water molecule, having a lower exchange rate with bulk solvents (k_ex²⁹⁸ = 5.3 ± 0.4 × 10⁷ s⁻¹) than [Mn(3,9-PC2A)] (k_ex²⁹⁸ = 1.26 × 10⁸ s⁻¹). These mild differences are rationalized by density-functional theory (DFT) calculations. The acid assisted dissociation of [Mn(3,9-OPC2A)] is considerably slower (k₁ = 2.81 ± 0.07 M⁻¹ s⁻¹) than that of the complexes of diacetates or bisamides of various 12-membered macrocycles and the parent H₂3,9-PC2A. The [Mn(3,9-OPC2A)] is inert in rat/human serum as confirmed by ⁵²Mn labeling (nM range), as well as by relaxometry (mM range). However, a 600-fold excess of EDTA (pH = 7.4) or a mixture of essential metal ions, propagated some transchelation/transmetalation in 7 days. The H₂3,9-OPC2A is labeled efficiently with ⁵²Mn at elevated temperatures, yet at 37 °C the parent H₂3,9-PC2A performs slightly better. Ultimately, the H₂3,9-OPC2A shows advantageous features for further ligand designs for bifunctional chelators.

Hypoxia promotes angiogenesis, which is crucial for tumor growth, and induces malignant progression and increases the therapeutic resistance. Positron emission tomography (PET) enables the detection of the hypoxic regions in tumors using 2-nitroimidazole-based radiopharmaceuticals. We describe here a physicochemical study of the Sc(DO3AM-NI) complex, which indicates: (a) relatively slow formation of the Sc(DO3AM-NI) chelate in acidic solution; (b) lower thermodynamic stability than the reference Sc(DOTA); (c) however, it is substantially more inert and consequently can be regarded as an excellent Sc-binder system. In addition, we report a comparison of ⁴⁴Sc-labeled DO3AM-NI with its known ⁶⁸Ga-labeled analog as a hypoxia PET probe. The in vivo and ex vivo biodistributions of ⁴⁴Sc- and ⁶⁸Ga-labeled DO3AM-NI in healthy and KB tumor-bearing SCID mice were examined 90 and 240 min after intravenous injection. No significant difference was found between the accumulation of ⁴⁴Sc- and ⁶⁸Ga-labeled DO3AM-NI in KB tumors. However, a significantly higher accumulation of [⁶⁸Ga]Ga(DO3AM-NI) was found in liver, spleen, kidney, intestine, lung, heart and brain than for [⁴⁴Sc]Sc(DO3AM-NI), leading to a lower tumor/background ratio. The tumor-to-muscle (T/M) ratio of [⁴⁴Sc]Sc(DO3AM-NI) was approximately 10–15-fold higher than that of [⁶⁸Ga]Ga(DO3AM-NI) at all time points. Thus, [⁴⁴Sc]Sc(DO3AM-NI) allows the visualization of KB tumors with higher resolution, making it a promising hypoxia-specific PET radiotracer.

Rigid pyclen derivative ligands bearing hypoxia‐sensitive nitroimidazole (NI) pendants, PC2A‐Ph‐NI (hexadentate) and PC2AM‐NI (heptadentate), were synthesized, radiolabeled with ⁴⁴ Sc(III), and evaluated under preclinical circumstances. Physicochemical characterization was carried out using the structurally related model ligand, PC2AM ^nBu , by multinuclear NMR, UV‐Vis spectroscopy, and pH‐potentiometry. Sc(III) ion was found to form a stable complex with PC2AM ^nBu (log K _ScL = 19.53(4); pSc = 19.32), in a reasonably fast reaction, with outstanding inertness ( t _1/2 = 529 hours in 1 M HClO ₄ ). The [Sc(PC2AM ^nBu )] ⁺ complex can coordinate fluoride (log K _ScLF = 2.5(2)), and the ternary complex is remarkably inert against fluoride exchange. Based on these results, the PC2A‐mono(amide) platform seems to be a promising Sc(III)‐binder, paving the way for the labeling of NI‐containing ligands with ⁴⁴ Sc(III). Followed by successful radiolabeling with the positron‐emitting ⁴⁴ Sc isotope, labeling yields of 92% and over 99% were evidenced for [ ⁴⁴ Sc][Sc(PC2AM‐NI)] ⁺ and [ ⁴⁴ Sc][Sc(PC2A‐Ph‐NI)] ⁺ ; respectively, at 95 °C and 15 minutes; moreover, after purification, the radiochemical purity exceeded 99%. Both radiocomplexes remained stable in rat blood serum for at least 4 hours and exhibited high resistance toward transmetalation and transchelation processes. MicroPET imaging with preclinical models of B16‐F10 mouse melanoma was conducted to assess the tumor targeting capability and the in vivo biodistribution pattern of [ ⁴⁴ Sc][Sc(PC2AM‐NI)] and [ ⁴⁴ Sc][Sc(PC2A‐Ph‐NI)] ⁺ radiopharmaceuticals. Given the higher tumor uptakes of [ ⁴⁴ Sc][Sc(PC2A‐Ph‐NI)] ⁺ with decreasing off‐target activity and related better tumor/noise ratios, we may conclude that the diagnostic potential of the PC2A‐Ph‐NI derivative outperforms that of the [ ⁴⁴ Sc][Sc(PC2AM‐NI)] ⁺ .

⁶⁴Cu is considered to be one of the most promising radioisotopes in radiotheranostics (combining therapeutics with diagnostics) because its positron emission is suitable for PET imaging while the combination of its β⁻ and Auger electrons delivers a therapeutic effect. In this work, we present a thorough coordination chemical characterization of two Cu(II) complexes formed from the cross-bridged pentaazamacrocyclic CB-15aneN₅ and its bifunctional derivative. The Cu(II) chelates possess high thermodynamic stability, fast complexation and moderate inertness under harsh conditions (t_1/2 = 2.3 h, 5 M HCl, 50 °C). EPR measurements and DFT calculations were carried out for characterizing their structure in solution. Furthermore, the radiolabeling experiments performed with ⁶¹Cu(II) confirmed the fast complexation even in the μM concentration range at pH = 6.0. On the basis of these results, the CB-15aneN₅ ligand family serves as an excellent platform for Cu(II) complexation in radiotheranostics.