H4octox: Versatile Bimodal Octadentate Acyclic Chelating Ligand for Medicinal Inorganic Chemistry

Deciphering the Diversified Metabolic Behavior of Hydroxyalkyl Ferrocidiphenols as Anticancer Complexes

Ferrocidiphenols possessing appropriate substituents in the aliphatic chain have very promising anticancer properties, but a systematic approach to deciphering their diversified metabolic behavior has so far been lacking. Herein, we show that a series of novel ferrocidiphenols bearing different hydroxyalkyl substituents exhibit strong anticancer activity as revealed in a range of in vitro and in vivo experiments. Moreover, they display diversified oxidative transformation profiles very distinct from those of previous complexes, shown by the use of chemical and enzymatic methods and in cellulo and in vivo metabolism studies. In view of this phenomenon, unprecedented chemo-evolutionary sequences that connect all the ferrocidiphenol-related intermediates and analogues have been established. In addition, a comprehensive density functional theory (DFT) study has been performed to decipher the metabolic diversification profiles of these complexes and demonstrate the delicate modulation of carbenium ions by the ferrocenyl moiety, via either α- or β-positional participation.

Rearmed Bifunctional Chelating Ligand for 225Ac/155Tb Precision-Guided Theranostic Radiopharmaceuticals─H4noneunpaX

Superior bifunctional chelating ligands, which can sequester both α-emitting radionuclides (225Ac, 213Bi) and their diagnostic companions (155Tb, 111In), remain a formidable challenge to translating targeted alpha therapy, with complementary diagnostic imaging, to the clinic. H4noneupaX, a chelating ligand with an unusual diametrically opposed arrangement of pendant donor groups, has been developed to this end. H4noneunpaX preferentially complexes Ln3+ and An3+ ions, forming thermodynamically stable (pLa = 17.8, pLu = 21.3) and kinetically inert complexes─single isomeric species by nuclear magnetic resonance and density functional theory. Metal binding versatility demonstrated in radiolabeling [111In]In3+, [155Tb]Tb3+, [177Lu]Lu3+, and [225Ac]Ac3+ achieved high molar activities under mild conditions. Efficient, scalable synthesis enabled in vivo evaluation of bifunctional H4noneunpaX conjugated to two octreotate peptides targeting neuroendocrine tumors. Single photon emission computed tomography/CT and biodistribution studies of 155Tb-radiotracers in AR42J tumor-bearing mice showed excellent image contrast, good tumor uptake, and high in vivo stability. H4noneunpaX shows significant potential for theranostic applications involving 225Ac/155Tb or 177Lu/155Tb.

Synthesis of a polyaminocarboxylate-based aluminum complex and its structural studies using 1H{13C}-HMBC NMR and a Karplus-type function

The HBED chelator is used to stabilize small and hard metal ions such as Fe³⁺, Ti⁴⁺, Ga³⁺ and Al³⁺ in both medicine and industry. While the coordination of hexadentate HBED^4- is known in the case of Fe³⁺, Ti⁴⁺ and Ga³⁺, it is unknown in the case of the small Al³⁺ ion since its corresponding complex has never been fully characterized. Thus, in this work the coordination pattern in a newly synthesized aluminum HBED-based complex ([Al-HBED-NN]^-Na⁺) was determined using 2D NMR in conjunction with DFT calculations.

Azo-Stilbene and Pyridine-Amine Hybrid Multifunctional Molecules to Target Metal-Mediated Neurotoxicity and Amyloid-β Aggregation in Alzheimer's Disease

Neurodegenerative diseases such as Alzheimer's disease (AD) are associated with progressive neuronal cell death, and they are commonly correlated with aberrant protein misfolding and aggregation of Aβ peptides. Transition metal ions (Cu, Fe, and Zn) have been shown to promote aggregation and oxidative stress through formation of Aβ-metal complexes. In this context, integrating molecular scaffolds rationally is used here to generate multifunctional molecules as modulators for metal-induced abnormalities. This work encompasses two azo-stilbene (AS)-derived compounds (AS-HL1 and AS-HL2), the rationale behind the design, their synthesis, characterization, and metal chelation ability [Cu(II) and Zn(II)]. The molecular frameworks of the designed compounds consist of stilbene as an Aβ-interacting moiety, whereas N,N,O and N,N,N,O donor atoms are linked to generate the metal chelation moiety. Furthermore, we went on exploring their multifunctionality with respect to (w.r.t.) (i) their metal chelating capacities and (ii) their utility to modulate the aggregation pathways of both metal-free and metal-bound amyloid-β, (iii) scavenge free radicals, and (iv) inhibit the activity of acetylcholinesterase and (v) cytotoxicity. Moreover, the compounds were able to sequester Cu²⁺ from the Aβ-Cu complex as studied by the UV-visible spectroscopic assay. Molecular docking studies were also performed with Aβ and acetylcholinesterase enzyme. Overall, the studies presented here qualify these molecules as promising candidates for further investigation in the quest for finding a treatment for Alzheimer's disease.

Bioimaging guided pharmaceutical evaluations of nanomedicines for clinical translations

Nanomedicines (NMs) have emerged as an efficient approach for developing novel treatment strategies against a variety of diseases. Over the past few decades, NM formulations have received great attention, and a large number of studies have been performed in this field. Despite this, only about 60 nano-formulations have received industrial acceptance and are currently available for clinical use. Their in vivo pharmaceutical behavior is considered one of the main challenges and hurdles for the effective clinical translation of NMs, because it is difficult to monitor the pharmaceutic fate of NMs in the biological environment using conventional pharmaceutical evaluations. In this context, non-invasive imaging modalities offer attractive solutions, providing the direct monitoring and quantification of the pharmacokinetic and pharmacodynamic behavior of labeled NMs in a real-time manner. Imaging evaluations have great potential for revealing the relationship between the physicochemical properties of NMs and their pharmaceutical profiles in living subjects. In this review, we introduced imaging techniques that can be used for in vivo NM evaluations. We also provided an overview of various studies on the influence of key parameters on the in vivo pharmaceutical behavior of NMs that had been visualized in a non-invasive and real-time manner.

The Chemical Scaffold of Theranostic Radiopharmaceuticals: Radionuclide, Bifunctional Chelator, and Pharmacokinetics Modifying Linker

Therapeutic radiopharmaceuticals have been researched extensively in the last decade as a result of the growing research interest in personalized medicine to improve diagnostic accuracy and intensify intensive therapy while limiting side effects. Radiometal-based drugs are of substantial interest because of their greater versatility for clinical translation compared to non-metal radionuclides. This paper comprehensively discusses various components commonly used as chemical scaffolds to build radiopharmaceutical agents, i.e., radionuclides, pharmacokinetic-modifying linkers, and chelators, whose characteristics are explained and can be used as a guide for the researcher.

Dual-Labelling Strategies for Nuclear and Fluorescence Molecular Imaging: Current Status and Future Perspectives

Molecular imaging offers the possibility to investigate biological and biochemical processes non-invasively and to obtain information on both anatomy and dysfunctions. Based on the data obtained, a fundamental understanding of various disease processes can be derived and treatment strategies can be planned. In this context, methods that combine several modalities in one probe are increasingly being used. Due to the comparably high sensitivity and provided complementary information, the combination of nuclear and optical probes has taken on a special significance. In this review article, dual-labelled systems for bimodal nuclear and optical imaging based on both modular ligands and nanomaterials are discussed. Particular attention is paid to radiometal-labelled molecules for single-photon emission computed tomography (SPECT) and positron emission tomography (PET) and metal complexes combined with fluorescent dyes for optical imaging. The clinical potential of such probes, especially for fluorescence-guided surgery, is assessed.

Inorganic radiopharmaceutical chemistry of oxine

8-Hydroxyquinoline (8-HQ, oxine) is a small, monoprotic, bicyclic aromatic compound and its relative donor group orientation imparts impressive bidentate metal chelating abilities that have been exploited in a vast array of applications over decades. 8-HQ and its derivatives have been explored in medicinal applications including anti-neurodegeneration, anticancer properties, and antimicrobial activities. One long established use of 8-HQ in medicinal inorganic chemistry is the coordination of radioactive isotopes of metal ions in nuclear medicine. The metal-oxine complex with the single photon emission computed tomography (SPECT) imaging isotope [¹¹¹In]In³⁺ was developed in the 1970s and 1980s to radiolabel leukocytes for inflammation and infection imaging. The [¹¹¹In][In(oxine)₃] complex functions as an ionophore: a moderately stable lipophilic complex to enter cells; however, inside the cell environment [¹¹¹In]In³⁺ undergoes exchange and remains localized. As new developments have progressed towards radiopharmaceuticals capable of both imaging and therapy (theranostics), 8-HQ has been re-explored in recent years to investigate its potential to chelate larger radiometal ions with longer half-lives and different indications. Further, metal-oxine complexes have been used to study liposomes and other nanomaterials by tracking these nanomedicines in vivo. Expanding 8-HQ to multidentate ligands for highly thermodynamically stable and kinetically inert complexes has increased the possibilities of this small molecule in nuclear medicine. This article outlines the historic use of metal-oxine complexes in inorganic radiopharmaceutical chemistry, with a focus on recent advances highlighting the possibilities of developing higher denticity, targeted bifunctional chelators with 8-HQ.

H2pyhox - Octadentate Bis(pyridyloxine)

A new versatile chelating ligand for intermediate size and softness radiometals [⁶⁴Cu]Cu²⁺ and [¹¹¹In]In³⁺, H₂pyhox, was synthesized by introducing pyridine as a new donor moiety to complement 8-hydroxyquinoline on an ethylenediamine backbone. The combination of pyridine and oxine as donor sets was explored through structural analysis, and crystals of the three metal complexes with Cu²⁺, La³⁺, and In³⁺ demonstrate how the ligand adapts to accommodate metal ions of different sizes and charge. Exhaustive in-batch UV solution studies characterized the protonation constants of the free ligand as well as the formation constants of the metal complexes with Cu²⁺, In³⁺, and La³⁺. Preliminary concentration-dependent radiolabeling studies with [¹¹¹In]In³⁺ and [⁶⁴Cu]Cu²⁺ show the robustness of H₂pyhox to successfully coordinate both radiometals under mild conditions (<15 min, room temperature, pH 6). H₂pyhox is the first oxinate ligand to successfully radiolabel [²²⁵Ac]Ac³⁺, albeit only at high concentrations (0.1-1 mM) with gentle heating to 37 °C. Whole serum, protein, and ligand challenge assays further demonstrate the kinetic inertness of the [¹¹¹In]In³⁺ and [⁶⁴Cu]Cu²⁺ radiometal-ligand complexes, confirming H₂pyhox to be a promising versatile radiopharmaceutical chelator.

Metal ion size profoundly affects H3glyox chelate chemistry

The bisoxine hexadentate chelating ligand, H₃glyox was investigated for its affinity for Mn²⁺, Cu²⁺ and Lu³⁺ ions; all three metal ions are relevant with applications in nuclear medicine and medicinal inorganic chemistry. The aqueous coordination chemistry and thermodynamic stability of all three metal complexes were thoroughly investigated by detailed DFT structure calculations and stability constant determination, by employing UV in-batch spectrophotometric titrations, giving pM values (pM = −log[Mⁿ⁺]_free when [Mⁿ⁺] = 1 μM, [L] = 10 μM at pH 7.4 and 25 °C) – pCu (25.2) > pLu (18.1) > pMn (12.0). DFT calculated structures revealed different geometries and coordination preferences of the three metal ions; notable was an inner sphere water molecule in the Mn²⁺ complex. H₃glyox labels [^52gMn]Mn²⁺, [⁶⁴Cu]Cu²⁺ and [¹⁷⁷Lu]Lu³⁺ at ambient conditions with apparent molar activities of 40 MBq μmol⁻¹, 500 MBq μmol⁻¹ and 25 GBq μmol⁻¹, respectively. Collectively, these initial investigations provide insight into the effects of metal ion size and charge on the chelation with the hexadentate H₃glyox and indicate that further investigations of the Mn²⁺–H₃glyox complex in ^52g/55Mn-based bimodal imaging might be worthwhile.

The bisoxine hexadentate chelating ligand, H₃glyox was investigated for its affinity for Mn²⁺, Cu²⁺ and Lu³⁺ ions; all three metal ions are relevant with applications in nuclear medicine and medicinal inorganic chemistry.

High denticity oxinate-linear-backbone chelating ligand for diagnostic radiometal ions [111In]In3+ and [89Zr]Zr4+

A potentially decadentate oxinate-containing ligand was synthesized and assessed through solution thermodynamics studies, concentration dependent radiolabeling and serum stability assays with [^nat/111In]In³⁺ and [^nat/89Zr]Zr⁴⁺.

Monosized Polymeric Microspheres Designed for Passive Lung Targeting: Biodistribution and Pharmacokinetics after Intravenous Administration

Local as well as systemic therapy is often used to treat bacterial lung infections. Delivery of antibiotics to the vascular side of infected lung tissue using lung-targeting microspheres (MS) is a good alternative to conventional administration routes, allowing for localized high levels of antibiotics. This delivery route can also complement inhaled antibiotic therapy, especially in the case of compromised lung function. We prepared and characterized monodisperse poly(lactic-co-glycolic acid) (PLGA) MS loaded with levofloxacin using a flow-focusing glass microfluidic chip. In vitro characterization showed that the encapsulated LVX displayed a biphasic controlled release during 5 days and preserved its antibacterial activity. The MS degradation was investigated in vitro by cross-sectioning the MS using a focused ion beam scanning electron microscope and in vivo by histological examination of lung tissue from mice intravenously administered with the MS. The MS showed changes in the surface morphology and internal matrix, whereas the degradation in vivo was 3 times faster than that in vitro. No effect on the viability of endothelial and lung epithelial cells or hemolytic activity was observed. To evaluate the pharmacokinetics and biodistribution of the MS, complete quantitative imaging of the ¹¹¹indium-labeled PLGA MS was performed in vivo with single-photon emission computed tomography imaging over 10 days. The PLGA MS distributed homogeneously in the lung capillaries. Overall, intravenous administration of 12 μm PLGA MS is suitable for passive lung targeting and pulmonary therapy.

tBu4octapa-alkyl-NHS for metalloradiopeptide preparation

The peptide is an important class of biological targeting molecule; herein, a new bifunctional octadentate non-macrocyclic H4octapa, tBu4octapa-alkyl-NHS, which is compatible with solid-phase peptide synthesis and thus useful for radiopeptide preparation, has been synthesized. To preserve denticity, the alkyl-N-hydroxylsuccinimide linker was covalently attached to the methylene-carbon on one of the acetate arms, yielding a chiral carbon center. According to density-functional theory (DFT) calculations using [Lu(octapa-alkyl-benzyl-ester)]- as a simulation model, the chirality has minimal effects on the complex geometry; regardless of the S-/R-stereochemistry, DFT calculations revealed two possible geometric isomers, distorted bicapped trigonal antiprism (DBTA) and distorted square antiprism (DSA), due to the asymmetry in the chelator. To evaluate the biological behavior of the new bifunctionalization, two well-studied PSMA (prostate-specific membrane antigen)-targeting peptidomimetics of varying hydrophobicity were chosen as proof-of-principle targeting vector molecules. Radiolabeling both bioconjugates with lutetium-177 was highly efficient at room temperature in 15 min at micromolar chelator concentration pH = 7. Both the in vitro serum challenge and the lanthanum(iii) challenge studies revealed complex lability, and notably, progressive bone accumulation was only observed with the more hydrophobic linker (i.e. H4octapa-alkyl-PSMA617). This in vivo result informs potential alterations exerted by the linker on the complex geometry and stability, with an appropriate biological targeting vector adopted for such evaluations.

Rapid Thermodynamically Stable Complex Formation of [nat/111In]In3+, [nat/90Y]Y3+, and [nat/177Lu]Lu3+ with H6dappa

A phosphinate-bearing picolinic acid-based chelating ligand (H₆dappa) was synthesized and characterized to assess its potential as a bifunctional chelator (BFC) for inorganic radiopharmaceuticals. Nuclear magnetic resonance (NMR) spectroscopy was employed to investigate the chelator coordination chemistry with a variety of nonradioactive trivalent metal ions (In³⁺, Lu³⁺, Y³⁺, Sc³⁺, La³⁺, Bi³⁺). Density functional theory (DFT) calculations explored the coordination environments of aforementioned metal complexes. The thermodynamic stability of H₆dappa with four metal ions (In³⁺, Lu³⁺, Y³⁺, Sc³⁺) was deeply investigated via potentiometric and spectrophotometric (UV-vis) titrations, employing a combination of acidic in-batch, joint potentiometric/spectrophotometric, and ligand-ligand competition titrations; high stability constants and pM values were calculated for all four metal complexes. Radiolabeling conditions for three clinically relevant radiometal ions were optimized ([¹¹¹In]In³⁺, [¹⁷⁷Lu]Lu³⁺, [⁹⁰Y]Y³⁺), and the serum stability of [¹¹¹In][In(dappa)]^3- was studied. Through concentration-, time-, temperature-, and pH-dependent labeling experiments, it was determined that H₆dappa radiolabels most effectively at near-physiological pH for all radiometal ions. Furthermore, very rapid radiolabeling at ambient temperature was observed, as maximal radiolabeling was achieved in less than 1 min. Molar activities of 29.8 GBq/μmol and 28.2 GBq/μmol were achieved for [¹¹¹In]In³⁺ and [¹⁷⁷Lu]Lu³⁺, respectively. For H₆dappa, high thermodynamic stability did not correlate with kinetic inertness-lability was observed in serum stability studies, suggesting that its metal complexes might not be suitable as a BFC in radiopharmaceuticals.

[nat/44Sc(pypa)]-: Thermodynamic Stability, Radiolabeling, and Biodistribution of a Prostate-Specific-Membrane-Antigen-Targeting Conjugate

⁴⁴Sc is an attractive positron-emitting radionuclide for PET imaging; herein, a new complex of the Sc³⁺ ion with nonmacrocyclic chelator H₄pypa was synthesized and characterized with high-resolution electrospray-ionization mass spectrometry (HR-ESI-MS), as well as different nuclear magnetic resonance (NMR) spectroscopic techniques (¹H, ¹³C, ¹H-¹³C HSQC, ¹H-¹³C HMBC, COSY, and NOESY). In aqueous solution (pH = 7), [Sc(pypa)]^- presented two isomeric forms, the structures of which were predicted using density functional theory (DFT) calculation with a small energy difference of 22.4 kJ/mol, explaining their coexistence. [Sc(pypa)]^- was found to have superior thermodynamic stability (pM = 27.1) compared to [Sc(AAZTA)]^- (24.7) and [Sc(DOTA)]^- (23.9). In radiolabeling, [⁴⁴Sc][Sc(pypa)]^- formed efficiently at RT in 15 min over a range of pH (2-5.5), resulting in a complex that is highly stable (>99%) in mouse serum over at least six half-lives of scandium-44. Similar labeling efficiency was observed with the PSMA (prostate-specific membrane antigen)-targeting H₄pypa-C7-PSMA617 at pH = 5.5 (RT, 15 min), confirming negligible disturbance from the bifunctionalization on scandium-44 scavenging. Moreover, the kinetic inertness of the radiocomplex was proved in vivo. Surprisingly, the molar activity was found to have profound influence on the pharmacokinetics of the radiotracers where lower molar activity drastically reduced the background accumulations, particularly, kidney, and thus, yielded a much higher tumor-to-background contrast.

Functionally Versatile and Highly Stable Chelator for 111In and 177Lu: Proof-of-Principle Prostate-Specific Membrane Antigen Targeting

Here, we present the synthesis and characterization of a new potentially nonadentate chelator H₄pypa and its bifunctional analogue ^tBu₄pypa-C7-NHS conjugated to prostate-specific membrane antigen (PSMA)-targeting peptidomimetic (Glu-urea-Lys). H₄pypa is very functionally versatile and biologically stable. Compared to the conventional chelators (e.g., DOTA, DTPA), H₄pypa has outstanding affinities for both ¹¹¹In (EC, t_1/2 ≈ 2.8 days) and ¹⁷⁷Lu (β^-,γ, t_1/2 ≈ 6.64 days). Its radiolabeled complexes were achieved at >98% radiochemical yield, RT within 10 min, at a ligand concentration as low as 10^-6 M, with excellent stability in human serum over at least 5-7 days (<1% transchelation). The thermodynamic stabilities of the [M(pypa)]^- complexes (M³⁺ = In³⁺, Lu³⁺, La³⁺) were dependent on the ionic radii, where the smaller In³⁺ has the highest pM value (30.5), followed by Lu³⁺ (22.6) and La³⁺ (19.9). All pM values are remarkably higher than those with DOTA, DTPA, H₄octapa, H₄octox, and H₄neunpa. Moreover, the facile and versatile bifunctionalization enabled by the p-OH group in the central pyridyl bridge of the pypa scaffold (compound 14) allows incorporation of a variety of linkers for bioconjugation through easy nucleophilic substitution. In this work, an alkyl linker was selected to couple H₄pypa to a PSMA-targeting pharmacophore, proving that the bioconjugation sacrifices neither the tumor-targeting nor the chelation properties. The biodistribution profiles of ¹¹¹In- and ¹⁷⁷Lu-labeled tracers are different, but promising, with the ¹⁷⁷Lu analogue particularly outstanding.