A Theranostic Small-Molecule Prodrug Conjugate for Neuroendocrine Prostate Cancer

After androgen deprivation therapy, a significant number of prostate cancer cases progress with a therapy-resistant neuroendocrine phenotype (NEPC). This represents a challenge for diagnosis and treatment. Based on our previously reported design of theranostic small-molecule prodrug conjugates (T-SMPDCs), herein we report a T-SMPDC tailored for targeted positron emission tomography (PET) imaging and chemotherapy of NEPC. The T-SMPDC is built upon a triazine core (TZ) to present three functionalities: (1) a chelating moiety (DOTA: 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) for PET imaging when labeled with 68Ga (t1/2 = 68 min) or other relevant radiometals; (2) an octreotide (Octr) that targets the somatostatin receptor 2 (SSTR2), which is overexpressed in the innervated tumor microenvironment (TME); and (3) fingolimod, FTY720-an antagonist of sphingosine kinase 1 that is an intracellular enzyme upregulated in NEPC. Polyethylene glycol (PEG) chains were incorporated via conventional conjugation methods or a click chemistry reaction forming a 1,4-disubstituted 1,2,3-triazole (Trz) linkage for the optimization of in vivo kinetics as necessary. The T-SMPDC, DOTA-PEG3-TZ(PEG4-Octr)-PEG2-Trz-PEG3-Val-Cit-pABOC-FTY720 (PEGn: PEG with n repeating ethyleneoxy units (n = 2, 3, or 4); Val: valine; Cit: citrulline; pABOC: p-amino-benzyloxycarbonyl), showed selective SSTR2 binding and mediated internalization of the molecule in SSTR2 high cells. Release of FTY720 was observed when the T-SMPDC was exposed to cathepsin B, and the released FTY720 exerted cytotoxicity in cells. In vivo PET imaging showed significantly higher accumulation (2.1 ± 0.3 %ID/g; p = 0.02) of [68Ga]Ga-DOTA-PEG3-TZ(PEG4-Octr)-PEG2-Trz-PEG3-Val-Cit-pABOC-FTY720 in SSTR2high prostate cancer xenografts than in the SSTR2low xenografts (1.5 ± 0.4 %ID/g) at 13 min post-injection (p.i.) with a rapid excretion through the kidneys. Taken together, these proof-of-concept results validate the design concept of the T-SMPDC, which may hold a great potential for targeted diagnosis and therapy of NEPC.

Abstract 2478: Development of a novel HIF2a PET tracer: From proof of concept to a clinical trial

Normally induced by hypoxia, hypoxia-inducible factor 2 alpha (HIF2a) is arguably the most important driver of kidney cancer. HIF2a is constitutively activated following von Hippel-Lindau (VHL) gene inactivation, which is the signature event of the most common type of kidney cancer, clear cell renal cell carcinoma (ccRCC). HIF2a functions as a heterodimeric transcription factor in partnership with the constitutive HIF1b subunit and regulates a program of gene expression that promotes cell proliferation, stemness, and angiogenesis. While as a transcription factor HIF2a had escaped drug targeting, structural studies revealed an unusual cavity, which became the foundation for the development of small molecule inhibitors such as PT2385 (a first-in-class drug), or the related PT2399 tool compound and the recently FDA-approved PT2977 (also called belzutifan). PT drugs bind a small pocket in the PAS-B domain of HIF2a inducing a conformational change that triggers dissociation from its obligatory partner HIF1b. PT drugs are highly specific - they do not bind the close paralog HIF1a and do not induce changes in gene expression in cells devoid of HIF2a. Using an extensive library of patient-derived xenografts (PDXs), we previously showed that PT drugs have activity against 50% of ccRCCs implanted in mice, and similar observations were made in the clinic. Perhaps unsurprisingly, sensitive tumors showed higher HIF2a levels. Here, we leverage the specificity of PT2385 to develop a HIF2a tracer for positron emission tomography (PET). By substituting a native fluorine atom for 18F, we generated [18F]PT2385. [18F]PT2385 was able to discriminate HIF2a-expressing ccRCCs from tumors that did not express HIF2a in mice simultaneously implanted with both. These data set the foundation for an investigator new drug (IND) approval from the FDA, and a clinical trial that is currently accruing patients (NCT04989959). [18F]PT2385 PET may have applications in identifying kidney cancer patients most likely to respond to HIF2a-targeted therapies, the identification of other tumors relying on HIF2a, and beyond oncology. Reporting on a hypoxia sensor, a HIF2a radiotracer may be a useful ischemia probe. In summary, we report the development of a novel radiotracer with extensive potential applications currently being evaluated in humans.

Citation Format: Sashi Debnath, Christina Stevens, Olivia Brandenburg, Justin Sovich, Paulina Gonzalez, Qian (Janie) Qin, Sydney Haldeman, Vanina Toffessi Tcheuyap, Alana Christie, Pawan Thapa, Ning Zhou, Aditi Mulgaonkar, Guiyang Hao, Jeffrey Miyata, Deyssy Carrillo, Jeffrey Cadeddu, Payal Kapur, Jon Anderson, Ivan Pedrosa, Marianna Dakanali, Orhan Oz, Xiankai Sun, James Brugarolas. Development of a novel HIF2a PET tracer: From proof of concept to a clinical trial [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2478.

Abstract 1234: PD-L1 PET: A potential biomarker of checkpoint inhibitor sensitivity in renal cancer

Immune checkpoint inhibitors (ICIs) have revolutionized cancer therapy. Program death-ligand 1 (PD-L1) and its corresponding receptor (PD-1) are the target of most FDA-approved ICIs. Perhaps unsurprisingly, PD-L1 levels are predictive of response to targeting drugs in lung and other cancers. However, PD-L1 is not predictive in other tumor types including renal cell carcinoma (RCC). PD-L1 is typically measured on tumor biopsies or archival tissues and we hypothesized that the lack of PD-L1 predictive power is due to tumor heterogeneity and tumor evolution. To test this notion, we embarked upon the generation of a PD-L1 probe that would enable real time assessment of PD-L1 across sites of disease using molecular imaging. We leveraged atezolizumab, a highly specific PD-L1 therapeutic antibody with a mutant Fc. Atezolizumab (ATZ) was labeled with 89Zr after conjugation with a bifunctional chelator and evaluated by positron emission tomography (PET). In extensive preclinical studies using multiple patient-derived xenografts (PDXs) with variable PD-L1 levels, we show that 89Zr-ATZ PET is able to distinguish RCC with variable PD-L1 expression. These data set the foundation for an investigator new drug (IND) approval from the FDA and an ongoing clinical trial in RCC patients (NCT04006522). Consistent with our hypothesis, 89Zr-ATZ PET shows substantial PD-L1 heterogeneity not only across but also within patients at different sites of disease. Furthermore, preliminary data suggest that PD-L1 levels may be predictive of response to ICI. By assessing PD-L1 expression in real time, 89Zr-ATZ PET may enable: (1) the identification of patients most likely to respond to PD-L1/PD-1 targeted therapies; (2) tailored management of the disease across different sites; (3) the evaluation of interventions that modulate PD-L1 levels; (4) insights into toxicities; and (5) probing resistance.

Citation Format: Aditi Mulgaonkar, Layton Woolford, Roy Elias, Kien Nham, Bing Guan, Guiyang Hao, Christina Stevens, Vanina Toffessi Tcheuyap, Sydney Haldeman, Jeffrey Miyata, Deyssy Carrillo, Qian (Janie) Qin, Nirmish Singla, Isaac Bowman, Jeffrey Cadeddu, Vitaly Margulis, Alana Christie, Payal Kapur, Ivan Pedrosa, Marianna Dakanali, Orhan Oz, Xiankai Sun, James Brugarolas. PD-L1 PET: A potential biomarker of checkpoint inhibitor sensitivity in renal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1234.

A renal cell carcinoma tumorgraft platform to advance precision medicine

Renal cell carcinoma (RCC) encompasses a heterogenous group of tumors, but representative preclinical models are lacking. We previously showed that patient-derived tumorgraft (TG) models recapitulate the biology and treatment responsiveness. Through systematic orthotopic implantation of tumor samples from 926 ethnically diverse individuals into non-obese diabetic (NOD)/severe combined immunodeficiency (SCID) mice, we generate a resource comprising 172 independently derived, stably engrafted TG lines from 148 individuals. TG lines are characterized histologically and genomically (whole-exome [n = 97] and RNA [n = 102] sequencing). The platform features a variety of histological and oncogenotypes, including TCGA clades further corroborated through orthogonal metabolomic analyses. We illustrate how it enables a deeper understanding of RCC biology; enables the development of tissue- and imaging-based molecular probes; and supports advances in drug development.

An exploratory study of 89Zr-DFO-Atezolizumab ImmunoPET/CT in patients with locally advanced or metastatic renal cell carcinoma

TPS759

Background: Immune checkpoint inhibitors (ICIs) have changed the treatment landscape for renal cell carcinoma (RCC). Despite this, responses to single-agent anti-PD-1 occur in only 25% of patients. Biomarkers such as PD-L1 expression and tumor infiltrating lymphocyte density may predict response, but require invasive biopsies that fail to fully interrogate the complex tumor microenvironment throughout the patient. Efforts are underway to identify who will benefit from ICIs. Among these are the use of radiolabeled antibodies against PD-L1. We propose to combine the specificity of anti-PD-L1 antibodies with the sensitivity, resolution, and quantification offered by positron emission tomography to develop immunoPET (iPET). iPET will allow real-time monitoring of tumor and its microenvironment in-vivo, and may correlate with response and toxicity with ICIs. Methods: The anti-PD-L1 antibody Atezolizumab is conjugated to Zirconium-89 via desferrioxamine (DFO). 89Zr-DFO-Atezolizumab will be administered to two cohorts of approximately 20 patients. Cohort 1 have localized RCC prior to undergoing surgery, and cohort 2 have metastatic RCC prior to treatment with an ICI. The main exclusion criteria are existing use of an ICI or comorbidities that would preclude treatment with an ICI. All subjects will receive an IV injection of 89Zr-DFO-Atezolizumab followed 7 days (+/- 1 day) by a PET/CT scan. Subjects in cohort 2 will receive treatment with ICI per clinician discretion. Subjects will receive a repeat scan if disease recurrence is documented (cohort 1) or should progression or toxicity occur after treatment with an ICI (cohort 2). Biopsy of disease recurrence (cohort 1) will be standard, and encouraged for sites of disease progression on ICI (cohort 2). Co-primary endpoints are an exploratory analysis of 89Zr-DFO-Atezolizumab uptake on PET/CT with PD-L1 expression assessed by IHC in patients undergoing surgery (cohort 1) and to evaluate uptake across metastatic sites and explore the relationship with response or toxicity with ICI therapy (cohort 2). An investigational new drug approval has been granted for 89Zr-DFO-Atezolizumab (IND 143266) and accrual commenced 1 October 2019. Clinical trial information: NCT04006522.

A-ring oxygenation modulates the chemistry and bioactivity of caged Garcinia xanthones

Natural products of the caged Garcinia xanthones (CGX) family are characterized by a unique chemical structure, potent bioactivities and promising pharmacological profiles. We have developed a Claisen/Diels-Alder reaction cascade that, in combination with a Pd(0)-catalyzed reverse prenylation, provides rapid and efficient access to the CGX pharmacophore, represented by the structure of cluvenone. To further explore this pharmacophore, we have synthesized various A-ring oxygenated analogues of cluvenone and have evaluated their bioactivities in terms of growth inhibition, mitochondrial fragmentation, induction of mitochondrial-dependent cell death and Hsp90 client inhibition. We found that installation of an oxygen functionality at various positions of the A-ring influences significantly both the site-selectivity of the Claisen/Diels-Alder reaction and the bioactivity of these compounds, due to remote electronic effects.

Aminonaphthalene 2-cyanoacrylate (ANCA) probes fluorescently discriminate between amyloid-β and prion plaques in brain

A major challenge for diagnosing and monitoring the progression of amyloid-based diseases is the capability to distinguish between amyloid deposits that are associated with related, but distinctly different, diseases. Here, we demonstrate that aminonaphthalenyl 2-cyanoacrylate-based probes can fluorescently discriminate between different types of amyloid deposits in brain. The discriminating capability of these molecular rotors is due to the stabilization of the ground versus excited states of these probes as a function of the polarity of their microenvironment (i.e., within the binding pocket on the amyloid). This property makes it possible, for the first time, to estimate the inherent static relative permittivity (ε(0)) of the binding pocket of each amyloid within tissue. The capability to selectively follow the deposition of specific amyloids in tissue may provide important information for therapeutic development that is not readily accessible from currently available technology.

Self-calibrating viscosity probes: design and subcellular localization

We describe the design, synthesis and fluorescence profiles of new self-calibrating viscosity dyes in which a coumarin (reference fluorophore) has been covalently linked with a molecular rotor (viscosity sensor). Characterization of their fluorescence properties was made with separate excitation of the units and through resonance energy transfer from the reference to the sensor dye. We have modified the linker and the substitution of the rotor in order to change the hydrophilicity of these probes thereby altering their subcellular localization. For instance, hydrophilic dye 12 shows a homogeneous distribution inside the cell and represents a suitable probe for viscosity measurements in the cytoplasm.

Subcellular localization and activity of gambogic acid

The natural product gambogic acid (GA) has shown significant potential as an anticancer agent as it is able to induce apoptosis in multiple tumor cell lines, including multidrug-resistant cell lines, as well as displaying antitumor activity in animal models. Despite the fact that GA has entered phase I clinical trials, the primary cellular target and mode of action of this compound remain unclear, although many proteins have been shown to be affected by it. By thorough analysis of several cellular organelles, at both the morphological and functional levels, we demonstrate that the primary effect of GA is at the mitochondria. We found that GA induces mitochondrial damage within minutes of incubation at low-micromolar concentrations. Moreover, a fluorescent derivative of GA was able to localize specifically to the mitochondria and was displaced from these organelles after competition with unlabeled GA. These findings indicate that GA directly targets the mitochondria to induce the intrinsic pathway of apoptosis, and thus represents a new member of the mitocans.

Intrinsic and extrinsic temperature-dependency of viscosity-sensitive fluorescent molecular rotors

Molecular rotors are a group of environment-sensitive fluorescent probes whose quantum yield depends on the ability to form twisted intramolecular charge-transfer (TICT) states. TICT formation is dominantly governed by the solvent's microviscosity, but polarity and the ability of the solvent to form hydrogen bonds play an additional role. The relationship between quantum yield ϕ(F) and viscosity η is widely accepted as a power-law, ϕ(F) = C · η(x). In this study, we isolated the direct influence of the temperature on the TICT formation rate by examining several molecular rotors in protic and aprotic solvents over a range of temperatures. Each solvent's viscosity was determined as a function of temperature and used in the above power-law to determine how the proportionality constant C varies with temperature. We found that the power-law relationship fully explains the variations of the measured steady-state intensity by temperature-induced variations of the solvent viscosity, and C can be assumed to be temperature-independent. The exponent x, however, was found to be significantly higher in aprotic solvents than in protic solvents. We conclude that the ability of the solvent to form hydrogen bonds has a major influence on the relationship between viscosity and quantum yield. To use molecular rotors for the quantitative determination of viscosity or microviscosity, the exponent x needs to be determined for each dye-solvent combination.

Intramolecular Cyclization Strategies Toward the Synthesis of Zoanthamine Alkaloids

Stabilized 2-amino-1,3-dienes can participate in intramolecular Diels-Alder (IMDA) reactions with pendant dienophiles. We found that these dienes can be readily prepared via standard palladium-mediated coupling reactions and have comparable reactivity to 2-oxodienes. Application of these substrates to the synthesis of tetracyclic model systems representing the ABCE motif of the zoanthamines is presented.

Enantioselective synthesis of the ABC ring motif of norzoanthamine based on asymmetric Robinson annulation reactions

An enantioselective strategy for the synthesis of tetracyclic motif 5, representing the northern fragment of norzoanthamine, is presented. Key to the strategy is the use of two asymmetric Robinson annulation reactions that produce the tricyclic ABC ring system with excellent stereoselectivity. Further functionalization at the periphery of the C ring produces compound 5 containing six contiguous stereocenters of the natural product.

ANCA: A Family of Fluorescent Probes that Bind and Stain Amyloid Plaques in Human Tissue

A new family of fluorescent markers containing an Amino Naphthalenyl-2-Cyano-Acrylate (ANCA) motif has been synthesized and evaluated for its capability to associate with aggregated β-amyloid (Aβ) peptides. These fluorescent probes contain a nitrogen donor group that is connected via a naphthalene unit to an electron acceptor motif containing Water Solubilizing Groups (WSG). Chemical modifications were introduced to explore their effect on the capability of the ANCA-based probes to fluorescently label aggregated Aβ peptides. All synthesized probes bind to aggregated Aβ fibrils with low micromolar affinity and fluorescently stain amyloid deposits in human brain tissue from patients with Alzheimer's disease. We found that structural modifications of the WSG site do not affect considerably the binding affinity. However, changes of the nitrogen donor group alter significantly the binding affinity of these probes. Also, increasing the hydrophilicity of the donor group leads to improved contrast between the Aβ deposits and the surrounding tissue in histological staining experiments.

Synthesis and evaluation of self-calibrating ratiometric viscosity sensors

We describe the design, synthesis and fluorescent profile of a family of self-calibrating dyes that provide ratiometric measurements of fluid viscosity. The design is based on covalently linking a primary fluorophore (reference) that displays a viscosity-independent fluorescence emission with a secondary fluorophore (sensor) that exhibits a viscosity-sensitive fluorescence emission. Characterization of fluorescent properties was made with separate excitation of the units and through Resonance Energy Transfer from the reference to the sensor dye. The chemical structures of both fluorophores and the linker length have been evaluated in order to optimize the overall brightness and sensitivity of the viscosity measurements. We also present an application of such ratiometric dyes for the detection of membrane viscosity changes in a liposome model.

Detection of liposome membrane viscosity perturbations with ratiometric molecular rotors

Molecular rotors are a form of fluorescent intramolecular charge-transfer complexes that can undergo intramolecular twisting motion upon photoexcitation. Twisted-state formation leads to non-radiative relaxation that competes with fluorescence emission. In bulk solutions, these molecules exhibit a viscosity-dependent quantum yield. On the molecular scale, the fluorescence emission is a function of the local free volume, which in turn is related to the local micro-viscosity. Membrane viscosity, and the inverse; fluidity, are characteristic terms used to describe the ease of movement withing the membrane. Often, changes in membrane viscosity govern intracellular processes and are indicative of a disease state. Molecular rotors have been used to investigate viscosity changes in liposomes and cells, but accuracy is affected by local concentration gradients and sample optical properties. We have developed self-calibrating ratiometric molecular rotors to overcome this challenge and integrated the new molecules into a DLPC liposome model exposed to the membrane-fluidizing agent propanol. We show that the ratiometric emission intensity linearly decreases with the propanol exposure and that the ratiometric intensity is widely independent of the total liposome concentration. Conversely, dye concentration inside liposomes influences the sensitivity of the system. We suggest that the new self-calibrating dyes can be used for real-time viscosity sensing in liposome systems with the advantages of lifetime measurements, but with low-cost steady-state instrumentation.

Molecular rotors: Synthesis and evaluation as viscosity sensors

It has been shown that compounds containing the p-N,N,-dialkylaminobenzylidene cyanoacetate motif can serve as fluorescent non-mechanical viscosity sensors. These compounds, referred to as molecular rotors, belong to a class of fluorescent probes that are known to form twisted intramolecular charge-transfer complexes in the excited state. In this study we present the synthesis and spectroscopic characterization of these compounds as viscosity sensors. The effects of the molecular structure and electronic density of these rotors to the emission wavelength, fluorescence intensity and viscosity sensitivity are discussed.

Rational design of amyloid binding agents based on the molecular rotor motif

Alzheimer’s disease (AD) is characterized by a progressive loss of cognitive function and constitutes the most common and fatal neurodegenerative disorder.[1 ] Genetic and clinical evidence supports the hypothesis that accumulation of amyloid deposits in the brain plays an important role in the pathology of the disease. This event is associated with perturbations of biological functions in the surrounding tissue leading to neuronal cell death, thus contributing to the disease process. The deposits are comprised primarily of amyloid (Aβ) peptides, a 39–43 amino acid sequence that self aggregates into a fibrillar β-pleated sheet motif. While the exact three-dimensional structure of the aggregated Aβ peptides is not known, a model structure that sustains the property of aggregation has been proposed.[2 ] This creates opportunities for in vivo imaging of amyloid deposits that can not only help evaluate the time course and evolution of the disease, but can also allow the timely monitoring of therapeutic treatments.[3 ]

A short biomimetic approach to the fully functionalized bicyclic framework of type A acylphloroglucinols

A biomimetic approach toward type A polyprenylated acylphloroglucinols (PPAPs) is described. The method is based on a C-alkylation-cation cyclization reaction sequence, leading to a convenient buildup of molecular complexity, employing the simple and readily available deoxycohumulone and an appropriately functionalized hydroxy halide. Thus, a versatile construction of the fully functionalized bicyclic framework of type A PPAPs (5) was achieved.

Anti-inflammatory and antioxidant activity of coumarins designed as potential fluorescent zinc sensors

A series of coumarin analogs, designed and synthesised as potential fluorescent zinc probes were evaluated for their biological activity as anti-inflammatory and antioxidant agents. The effect of the synthesised compounds on inflammation, using the carrageenin-induced rat paw oedema model, was studied. In general, the compounds were found to be potent anti-inflammatory agents (26.5-64%). Compound 5 was found to interact significantly with 1,1-diphenyl-2-picryl-hydrazyl stable free radical (DPPH) whereas the remainder were inactive in this assay. The compounds inhibit in general the soybean lipoxygenase and scavenge superoxide anion radicals. The anti-inflammatory activity seems to be connected with their reducing activity. Their RM values were determined as an expression of their lipophilicity. Theoretical calculations of their lipophilicity as clog P were performed indicating that only a poor relationship exists between their lipophilicity and anti-inflammatory activity.

pH-Specific Aqueous Synthetic Chemistry in the Binary Cadmium(II)−Citrate System. Gaining Insight into Cadmium(II)−Citrate Speciation with Relevance to Cadmium Toxicity

The involvement of Cd(II) in toxic manifestations and pathological aberrations in lower and higher organisms entails interactions with low and high molecular mass biological targets. To understand the relevant chemistry in aqueous media, we have launched pH-dependent synthetic efforts targeting Cd(II) with the physiological ligand citric acid. Reactions of Cd(II) with citric acid upon the addition of NaOH at pH 2.5 and pyridine at pH 3 and the addition of ammonia at pH approximately 7 led to the new complexes [Cd3(C6H5O7)2(H2O)5] x H2O (1) and (NH4)[Cd(C6H5O7)(H2O)] x H2O (2), respectively. Complexes 1 and 2 were characterized by elemental analysis, spectroscopy (FT-IR and NMR), and X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P2(1)/n, with a = 18.035(6) A, b = 10.279(4) A, c = 12.565(4) A, beta = 109.02(1) degrees, V = 2202(2) A3, and Z = 4. Complex 2 crystallizes in the monoclinic space group P2(1), with a = 9.686(4) A, b = 8.484(4) A, c = 7.035(3) A, beta = 110.28(1) degrees, V = 542.3(4) A3, and Z = 2. Complex 1 is a trinuclear assembly with the citrate ligand securing a stable metallacyclic ring around one Cd(II), with the terminal carboxylates spanning into the coordination sphere of two nearby Cd(II) ions. Complex 2 contains mononuclear units of Cd(II) bound by citrate in an overall coordination number of 8. In both 1 and 2, the participating citrates exhibit three different modes of coordination, thus projecting a distinct yet variable aqueous structural chemistry of Cd(II) with physiological substrates. The pH-dependent chemistry and its apparent structural diversity validate past solution speciation studies, projecting the existence of mononuclear species such as the one in the anion of 2. The spectroscopic and structural properties of 2 emphasize the significance of the information emerging from synthetic studies that otherwise would not have been revealed through conventional solution studies, while concurrently shedding light onto the linkage of the requisite chemistry with the potential biological toxicity of Cd(II).

A new dinuclear Ti(IV)-peroxo-citrate complex from aqueous solutions. Synthetic, structural, and spectroscopic studies in relevance to aqueous titanium(IV)-peroxo-citrate speciation

The wide use of titanium in applied materials has prompted pertinent studies targeting the requisite chemistry of that metal's biological interactions. In order to understand such interactions as well as the requisite titanium aqueous speciation, we launched investigations on the synthesis and spectroscopic and structural characterization of Ti(IV) species with the physiological citric acid. Aqueous reactions of TiCl(4) with citric acid in the presence of H(2)O(2) and neutralizing ammonia afforded expediently the red crystalline material (NH(4))(4)[Ti(2)(O(2))(2)(C(6)H(4)O(7))(2)].2H(2)O (1). Complex 1 was further characterized by UV-vis, FT-IR, FT- and laser-Raman, NMR, and finally by X-ray crystallography. Compound 1 crystallizes in the monoclinic space group P2(1)/n, with a = 10.360(4) A, b = 10.226(4) A, c = 11.478(6) A, beta = 107.99(2) degrees, V = 1156.6(9) A(3), and Z = 2. The X-ray structure of 1 reveals a dinuclear anionic complex containing a Ti(IV)(2)O(2) core. In that central unit, two fully deprotonated citrate ligands are coordinated to the metal ions through their carboxylate moieties in a monodentate fashion. The central alkoxides serve as bridges to the two titanium ions. Also attached to the Ti(IV)(2)O(2) core are two peroxo ligands each bound in a side-on fashion to the respective metal ions. NH(4)(+) ions neutralize the 4- charge of the anion in 1, further contributing to the stability of the derived lattice through H-bond formation. The structural similarities and differences with congener vanadium(V)-peroxo-citrate complexes may point out potential implications in the chemistry of titanium with physiological ligands, when the former is present in a biologically relevant medium.

Synthesis and spectroscopic and structural studies of a new cadmium(II)-citrate aqueous complex. Potential relevance to cadmium(II)-citrate speciation and links to cadmium toxicity

The presence of cadmium in the environment undoubtedly contributes to an increased risk of exposure and ultimate toxic influence on humans. In an effort to comprehend the chemical and biological interactions of Cd(II) with physiological ligands, like citric acid, we explored the requisite aqueous chemistry, which afforded the first aqueous Cd(II)-citrate complex [Cd(C(6)H(6)O(7))(H(2)O)](n)() (1). Compound 1 was characterized by elemental analysis, and spectroscopically by FT-IR and (113)Cd MAS NMR. Compound 1 crystallizes in the orthorhombic space group P2(1)2(1)2(1), with a = 6.166(2) A, b = 10.508(3) A, c = 13.599(5) A, V = 881.2(5) A(3), and Z = 4. The X-ray structure of 1 reveals the presence of octahedral Cd(II) ions bound to citrate ligands in a molecular crystal lattice. Citrate acts as a tridentate binder promoting coordination to one Cd(II) through the central alcoholic moiety, one terminal carboxylate group, and the central carboxylate group. In addition, the central carboxylate binds to three Cd(II) ions. Specifically, one of the oxygens of the central carboxylate serves as a bridge to two neighboring Cd(II) ions, while the other oxygen binds to a third Cd(II). A bound water molecule completes the coordination requirements of Cd(II). (113)Cd MAS NMR studies project the spectroscopic signature of the nature of the coordination environment around Cd(II) in 1, thus corroborating the X-ray findings. Collectively, the data at hand are in line with past solution studies. The latter predict that other similar low molecular mass Cd(II)-citrate complexes may exist in the acidic pH region, thus influencing the uptake of cadmium by living (micro)organisms, their ability to metabolize organic substrates, and possibly Cd(II) toxicity.

Synthesis, pH-dependent structural characterization, and solution behavior of aqueous aluminum and gallium citrate complexes

Reactions of Al(III) and Ga(III) with citric acid in aqueous solutions, yielded the complexes (NH(4))(5)[M(C(6)H(4)O(7))(2)].2H(2)O (M(III) = Al (1), Ga (2)) at alkaline pH, and the complexes (Cat)(4)[M(C(6)H(5)O(7))(C(6)H(4)O(7))].nH(2)O (M(III) = Al (3), Ga (4), Cat. = NH(4)(+), n = 3; M(III) = Al (5), Ga (6), Cat. = K(+), n = 4) at acidic pH. All compounds were characterized by spectroscopic (FT-IR, (1)H, (13)C, and (27)Al NMR, (13)C-MAS NMR) and X-ray techniques. Complex 1 crystallizes in space group P1, with a = 9.638(5) A, b = 9.715(5) A, c = 7.237(4) A, alpha = 90.96(1) degrees, beta = 105.72(1) degrees, gamma = 119.74(1) degrees, V = 557.1(3) A(3), and Z = 1. Complex 2 crystallizes in space group P1, with a = 9.659(6) A, b = 9.762(7) A, c = 7.258(5) A, alpha = 90.95(2) degrees, beta = 105.86(2) degrees, gamma = 119.28(1) degrees, V = 564.9(7) A(3), and Z = 1. Complex 3 crystallizes in space group I2/a, with a = 19.347(3) A, b = 9.857(1) A, c = 23.412(4) A, beta = 100.549(5) degrees, V = 4389(1) A(3), and Z = 8. Complex 4 crystallizes in space group I2/a, with a = 19.275(1) A, b = 9.9697(6) A, c = 23.476(1) A, beta = 100.694(2) degrees, V = 4432.8(5) A(3), and Z = 8. Complex 5 crystallizes in space group P1, with a = 7.316(1) A, b = 9.454(2) A, c = 9.569(2) A, alpha = 64.218(4) degrees, beta = 69.872(3) degrees, gamma = 69.985(4) degrees, V = 544.9(2) A(3), and Z = 1. Complex 6 crystallizes in space group P1, with a = 7.3242(2) A, b = 9.4363(5) A, c = 9.6435(5) A, alpha = 63.751(2) degrees, beta = 70.091(2) degrees, gamma = 69.941(2) degrees, V = 547.22(4) A(3), and Z = 1. The crystal structures of 1-6 reveal mononuclear octahedral complexes of Al(III) (or Ga(III)) bound to two citrates. Solution NMR, on both 4- and 5- species, reveals rapid intramolecular exchange of the bound and unbound terminal carboxylates. Upon dissolution in water, the complexes, through a complicated reaction cascade, transform to oligonuclear 1:1 species that, in agreement with previous studies, represent the thermodynamically stable state in solution. The data provide, for the first time, structural details of low MW, mononuclear complexes of Al(III) (or Ga(III)) with citrate that are dictated, among other factors, by pH. The properties of 1-6 may provide clues relevant to their biological association with humans.

Manganese citrate chemistry: syntheses, spectroscopic studies, and structural characterizations of novel mononuclear, water-soluble manganese citrate complexes

The first two mononuclear manganese citrate complexes, (NH4)4[MnII(C6H5O7)2] (1) and (NH4)5[MnIII(C6H4O7)2].2H2O (2) were synthesized in aqueous solutions near physiological pH values. They were isolated in their pure crystalline forms and characterized by elemental analyses and spectroscopic techniques, including UV/visible, electron paramagnetic resonance, Fourier transformed infrared, and magnetic susceptibility measurements. Compound 1 crystallizes in the monoclinic space group P2(1)/c, with a = 8.777(1) A, b = 13.656(3) A, c = 9.162(2) A, beta = 113.62(2) degrees, V = 1006.2(6) A3, and Z = 2. Compound 2 crystallizes in the triclinic space group P1, with a = 9.606(3) A, b = 9.914(3) A, c = 7.247(3) A, alpha = 91.05(1) degrees, beta = 105.60(1) degrees, gamma = 119.16(1) degrees, V = 571.3(3) A3, and Z = 1. The X-ray crystal structures of 1 and 2 revealed that, in both cases, the manganese ion is six-coordinate and is bound by two citrate ligands in a distorted octahedral fashion. In the case of complex 1, the citrate ion binds to Mn2+ as a triply deprotonated ligand, retaining the central carbon hydroxyl hydrogen, whereas, in the case of compound 2, the citrate ligand coordinates to Mn3+ as a fully deprotonated entity. Compound 2 contains water molecules of crystallization in the unit cell which, through extensive hydrogen-bonding interactions, bestow considerable stability upon the Mn(3+)-citrate assembly. There are significant contributions to the stabilities of the assembled lattices in 1 and 2 arising from the ammonium counterions neutralizing the high anionic charges of the complexes. The EPR spectra attest to the presence of paramagnetic Mn2+ and Mn3+ species in the solid state. Corroborative evidence is obtained from the magnetic susceptibility measurements in the range 5-300 K. Complexes 1 and 2 present clear cases of mononuclear manganese citrate species relevant to manganese speciation in biological media and potentially related to the beneficial as well as toxic effects of manganese on humans.

Synthesis, spectroscopic, and structural characterization of the first aqueous cobalt(II)-citrate complex: toward a potentially bioavailable form of cobalt in biologically relevant fluids

Citric acid represents a class of carboxylic acids present in biological fluids and playing key roles in biochemical processes in bacteria and humans. Its ability to promote diverse coordination chemistries in aqueous media, in the presence of metal ions known to act as trace elements in human metabolism, earmarks its involvement in a number of physiological functions. Cobalt is known to be a central element of metabolically important biomolecules, such as B12, and therefore its biospeciation in biological fluids constitutes a theme worthy of chemical and biological perusal. In an effort to unravel the aqueous chemistry of cobalt in the presence of a physiologically relevant ligand, citrate, the first aqueous, soluble, mononuclear complex has been synthesized and isolated from reaction mixtures containing Co(II) and citrate in a 1:2 molar ratio at pH approximately 8. The crystalline compound (NH4)4[Co(C6H5O7)2] (1) has been characterized spectroscopically (UV/vis, EPR) and crystallographically. Its X-ray structure consists of a distorted octahedral anion with two citrate ligands fulfilling the coordination requirements of the Co(II) ion. The magnetic susceptibility measurements of 1 in the range from 6 to 295 K are consistent with a high-spin complex containing Co(II) with a ground state S=3/2. Corroborating this result is the EPR spectrum of 1, which shows a signal consistent with the presence of a Co(II) system. The spectroscopic and structural properties of the complex signify its potential biological relevance and participation in speciation patterns arising under conditions consistent with those employed for its synthesis and isolation.