Correlation between molar activity, injection mass and uptake of the PARP targeting radiotracer [18F]olaparib in mouse models of glioma

PURPOSE

Radiopharmaceuticals targeting poly(ADP-ribose) polymerase (PARP) have emerged as promising agents for cancer diagnosis and therapy. PARP enzymes are expressed in both cancerous and normal tissue. Hence, the injected mass, molar activity and potential pharmacological effects are important considerations for the use of radiolabelled PARP inhibitors for diagnostic and radionuclide therapeutic applications. Here, we performed a systematic evaluation by varying the molar activity of [18F]olaparib and the injected mass of [TotalF]olaparib to investigate the effects on tumour and normal tissue uptake in two subcutaneous human glioblastoma xenograft models.

METHODS

[18F]Olaparib uptake was evaluated in the human glioblastoma models: in vitro on U251MG and U87MG cell lines, and in vivo on tumour xenograft-bearing mice, after administration of [TotalF]olaparib (varying injected mass: 0.04-8.0 µg, and molar activity: 1-320 GBq/μmol).

RESULTS

Selective uptake of [18F]olaparib was demonstrated in both models. Tumour uptake was found to be dependent on the injected mass of [TotalF]olaparib (µg) but not the molar activity. An injected mass of 1 μg resulted in the highest tumour uptake (up to 6.9 ± 1.3%ID/g), independent of the molar activity. In comparison, both the lower and higher injected masses of [TotalF]olaparib resulted in lower relative tumour uptake (%ID/g; P < 0.05). Ex vivo analysis of U87MG xenograft sections showed that the heterogeneity in [18F]olaparib intratumoural uptake correlated with PARP1 expression. Substantial upregulation of PARP1-3 expression was observed after administration of [TotalF]olaparib (> 0.5 µg).

CONCLUSION

Our findings show that the injected mass of [TotalF]olaparib has significant effects on tumour uptake. Moderate injected masses of PARP inhibitor-derived radiopharmaceuticals may lead to improved relative tumour uptake and tumour-to-background ratio for cancer diagnosis and radionuclide therapy.

[18F]AZD2461, an Insight on Difference in PARP Binding Profiles for DNA Damage Response PET Imaging

BACKGROUND

Poly (ADP-ribose) polymerase (PARP) inhibitors are extensively studied and used as anti-cancer drugs, as single agents or in combination with other therapies. Most radiotracers developed to date have been chosen on the basis of strong PARP1-3 affinity. Herein, we propose to study AZD2461, a PARP inhibitor with lower affinity towards PARP3, and to investigate its potential for PARP targeting in vivo.

METHODS

Using the Cu-mediated 18F-fluorodeboronation of a carefully designed radiolabelling precursor, we accessed the 18F-labelled isotopologue of the PARP inhibitor AZD2461. Cell uptake of [18F]AZD2461 in vitro was assessed in a range of pancreatic cell lines (PSN-1, PANC-1, CFPAC-1 and AsPC-1) to assess PARP expression and in vivo in xenograft-bearing mice. Blocking experiments were performed with both olaparib and AZD2461.

RESULTS

[18F]AZD2461 was efficiently radiolabelled via both manual and automated procedures (9 % ± 3 % and 3 % ± 1 % activity yields non-decay corrected). [18F]AZD2461 was taken up in vivo in PARP1-expressing tumours, and the highest uptake was observed for PSN-1 cells (7.34 ± 1.16 %ID/g). In vitro blocking experiments showed a lesser ability of olaparib to reduce [18F]AZD2461 binding, indicating a difference in selectivity between olaparib and AZD2461.

CONCLUSION

Taken together, we show the importance of screening the PARP selectivity profile of radiolabelled PARP inhibitors for use as PET imaging agents.

Tumor Imaging Using Radiolabeled Matrix Metalloproteinase-Activated Anthrax Proteins

Increased activity of matrix metalloproteinases (MMPs) is associated with worse prognosis in different cancer types. The wild-type protective antigen (PA-WT) of the binary anthrax lethal toxin was modified to form a pore in cell membranes only when cleaved by MMPs (to form PA-L1). Anthrax lethal factor (LF) is then able to translocate through these pores. Here, we used a 111In-radiolabeled form of LF with the PA/LF system for noninvasive in vivo imaging of MMP activity in tumor tissue by SPECT. Methods: MMP-mediated activation of PA-L1 was correlated to anthrax receptor expression and MMP activity in a panel of cancer cells (HT1080, MDA-MB-231, B8484, and MCF7). Uptake of 111In-radiolabeled PA-L1, 111In-PA-WTK563C, or 111In-LFE687A (a catalytically inactive LF mutant) in tumor and normal tissues was measured using SPECT/CT imaging in vivo. Results: Activation of PA-L1 in vitro correlated with anthrax receptor expression and MMP activity (HT1080 > MDA-MB-231 > B8484 > MCF7). PA-L1-mediated delivery of 111In-LFE687A was demonstrated and was corroborated using confocal microscopy with fluorescently labeled LFE687A Uptake was blocked by the broad-spectrum MMP inhibitor GM6001. In vivo imaging showed selective accumulation of 111In-PA-L1 in MDA-MB-231 tumor xenografts (5.7 ± 0.9 percentage injected dose [%ID]/g) at 3 h after intravenous administration. 111In-LFE687A was selectively delivered to MMP-positive MDA-MB-231 tumor tissue by MMP-activatable PA-L1 (5.98 ± 0.62 %ID/g) but not by furin-cleavable PA-WT (1.05 ± 0.21 %ID/g) or a noncleavable PA variant control, PA-U7 (2.74 ± 0.24 %ID/g). Conclusion: Taken together, our results indicate that radiolabeled forms of mutated anthrax lethal toxin hold promise for noninvasive imaging of MMP activity in tumor tissue.

Induction of a Four-Way Junction Structure in the DNA Palindromic Hexanucleotide 5'-d(CGTACG)-3' by a Mononuclear Platinum Complex

Four-way junctions (4WJs) are supramolecular DNA assemblies comprising four interacting DNA strands that in biology are involved in DNA-damage repair. In this study, a new mononuclear platinum(II) complex 1 was prepared that is capable of driving the crystallization of the DNA oligomer 5'-d(CGTACG)-3' specifically into a 4WJ-like motif. In the crystal structure of the 1-CGTACG adduct, the distorted-square-planar platinum complex binds to the core of the 4WJ-like motif through π-π stacking and hydrogen bonding, without forming any platinum-nitrogen coordination bonds. Our observations suggest that the specific molecular properties of the metal complex are crucially responsible for triggering the selective assembly of this peculiar DNA superstructure.

PET Imaging of PARP Expression Using 18F-Olaparib

Poly(ADP-ribose) polymerase (PARP) inhibitors are increasingly being studied as cancer drugs, as single agents, or as a part of combination therapies. Imaging of PARP using a radiolabeled inhibitor has been proposed for patient selection, outcome prediction, dose optimization, genotoxic therapy evaluation, and target engagement imaging of novel PARP-targeting agents. Methods: Here, via the copper-mediated 18F-radiofluorination of aryl boronic esters, we accessed, for the first time (to our knowledge), the 18F-radiolabeled isotopolog of the Food and Drug Administration-approved PARP inhibitor olaparib. The use of the 18F-labeled equivalent of olaparib allows direct prediction of the distribution of olaparib, given its exact structural likeness to the native, nonradiolabeled drug. Results:18F-olaparib was taken up selectively in vitro in PARP-1-expressing cells. Irradiation increased PARP-1 expression and 18F-olaparib uptake in a radiation-dose-dependent fashion. PET imaging in mice showed specific uptake of 18F-olaparib in tumors expressing PARP-1 (3.2% ± 0.36% of the injected dose per gram of tissue in PSN-1 xenografts), correlating linearly with PARP-1 expression. Two hours after irradiation of the tumor (10 Gy), uptake of 18F-olaparib increased by 70% (P = 0.025). Conclusion: Taken together, we show that 18F-olaparib has great potential for noninvasive tumor imaging and monitoring of radiation damage.

Photochemical Resolution of a Thermally Inert Cyclometalated Ru(phbpy)(N-N)(Sulfoxide)+ Complex

In this work a photosubstitution strategy is presented that can be used for the isolation of chiral organometallic complexes. A series of five cyclometalated complexes Ru(phbpy)(N−N)(DMSO-κS)](PF₆) ([1]PF₆-[5]PF₆) were synthesized and characterized, where Hphbpy = 6′-phenyl-2,2′-bipyridyl, and N–N = bpy (2,2′-bipyridine), phen (1,10-phenanthroline), dpq (pyrazino[2,3-f][1,10]phenanthroline), dppz (dipyrido[3,2-a:2′,3′-c]phenazine, or dppn (benzo[i]dipyrido[3,2-a,2′,3′-c]phenazine), respectively. Due to the asymmetry of the cyclometalated phbpy^– ligand, the corresponding [Ru(phbpy)(N–N)(DMSO-κS)]⁺complexes are chiral. The exceptional thermal inertness of the Ru–S bond made chiral resolution of these complexes by thermal ligand exchange impossible. However, photosubstitution by visible light irradiation in acetonitrile was possible for three of the five complexes ([1]PF₆-[3]PF₆). Further thermal coordination of the chiral sulfoxide (R)-methyl p-tolylsulfoxide to the photoproduct [Ru(phbpy)(phen)(NCMe)]PF₆, followed by reverse phase HPLC, led to the separation and characterization of the two diastereoisomers of [Ru(phbpy)(phen)(MeSO(C₇H₇))]PF₆, thus providing a new photochemical approach toward the synthesis of chiral cyclometalated ruthenium(II) complexes. Full photochemical, electrochemical, and frontier orbital characterization of the cyclometalated complexes [1]PF₆-[5]PF₆ was performed to explain why [4]PF₆ and [5]PF₆ are photochemically inert while [1]PF₆-[3]PF₆ perform selective photosubstitution.

Selective Preparation of a Heteroleptic Cyclometallated Ruthenium Complex Capable of Undergoing Photosubstitution of a Bidentate Ligand

Cyclometallated ruthenium complexes typically exhibit red‐shifted absorption bands and lower photolability compared to their polypyridyl analogues. They also have lower symmetry, which sometimes makes their synthesis challenging. In this work, the coordination of four N,S bidentate ligands, 3‐(methylthio)propylamine (mtpa), 2‐(methylthio)ethylamine (mtea), 2‐(methylthio)ethyl‐2‐pyridine (mtep), and 2‐(methylthio)methylpyridine (mtmp), to the cyclometallated precursor [Ru(bpy)(phpy)(CH₃CN)₂]⁺ (bpy=2,2′‐bipyridine, Hphpy=2‐phenylpyridine) has been investigated, furnishing the corresponding heteroleptic complexes [Ru(bpy)(phpy)(N,S)]PF₆ ([2]PF₆–[5]PF₆, respectively). The stereoselectivity of the synthesis strongly depended on the size of the ring formed by the Ru‐coordinated N,S ligand, with [2]PF₆ and [4]PF₆ being formed stereoselectively, but [3]PF₆ and [5]PF₆ being obtained as mixtures of inseparable isomers. The exact stereochemistry of the air‐stable complex [4]PF₆ was established by a combination of DFT, 2D NMR, and single‐crystal X‐ray crystallographic studies. Finally, [4]PF₆ was found to be photosubstitutionally active under irradiation with green light in acetonitrile, which makes it the first cyclometallated ruthenium complex capable of undergoing selective photosubstitution of a bidentate ligand.

A Red-Light-Activated Ruthenium-Caged NAMPT Inhibitor Remains Phototoxic in Hypoxic Cancer Cells

We describe two water-soluble ruthenium complexes, [1]Cl₂ and [2]Cl₂ , that photodissociate to release a cytotoxic nicotinamide phosphoribosyltransferase (NAMPT) inhibitor with a low dose (21 J cm^-2 ) of red light in an oxygen-independent manner. Using a specific NAMPT activity assay, up to an 18-fold increase in inhibition potency was measured upon red-light activation of [2]Cl₂ , while [1]Cl₂ was thermally unstable. For the first time, the dark and red-light-induced cytotoxicity of these photocaged compounds could be tested under hypoxia (1 % O₂ ). In skin (A431) and lung (A549) cancer cells, a 3- to 4-fold increase in cytotoxicity was found upon red-light irradiation for [2]Cl₂ , whether the cells were cultured and irradiated with 1 % or 21 % O₂ . These results demonstrate the potential of photoactivated chemotherapy for hypoxic cancer cells, in which classical photodynamic therapy, which relies on oxygen activation, is poorly efficient.

Frontier orbitals of photosubstitutionally active ruthenium complexes: an experimental study of the spectator ligands' electronic properties influence on photoreactivity

The synthesis and characterization of [Ru(tpy)(R₂bpy)(L)](X)_n complexes (tpy = 2,2':6',2''-terpyridine, R₂bpy = 4,4'-dimethyl-2,2'-bipyridine (dmbpy), or 4,4'-bis(trifluoromethyl)-2,2'-bipyridine (tfmbpy), X = Cl^- or PF₆^-, and n = 1 or 2) are described. The dmbpy and tfmbpy bidentate ligands allow for investigating the effects of electron-donating and electron-withdrawing ligands, respectively, on the frontier orbital energetics as well as the photoreactivity of these ruthenium polypyridyl complexes for five prototypical monodentate ligands L = Cl^-, H₂O, CH₃CN, 2-(methylthio)ethanol (Hmte), or pyridine. According to spectroscopic and electrochemical studies, the dmbpy analogues displayed a singlet metal-to-ligand charge transfer (¹MLCT) transition at higher energy than the tfmbpy analogues. The shift of the ¹MLCT to higher energy results from the lowest unoccupied molecular orbital (LUMO) for the dmbpy analogues being tpy-based, whereas for the tfmbpy analogues orbital inversion occurs resulting in a tfmbpy-based LUMO. The energy level of the highest occupied molecular orbital (HOMO) was considerably affected by the nature of the monodentate ligand. Visible light irradiation of the complexes demonstrated that the tfmbpy analogue increased the rate and quantum yields of photosubstitution reactions, compared to the dmbpy analogue, suggesting that the electron-withdrawing substituents allowed better thermal accessibility of the triplet metal-centered (³MC) state from the photochemically generated triplet metal-to-ligand charge transfer (³MLCT) excited state. A correlation between the photolability of the monodentate ligands and the electrochemical reversibility of the metal-based oxidation is also reported.

Co-Registered Molecular Logic Gate with a CO-Releasing Molecule Triggered by Light and Peroxide

Co-registered molecular logic gates combine two different inputs and outputs, such as light and matter. We introduce a biocompatible CO-releasing molecule (CORM, A) as Mn(I) tricarbonyl complex with the ligand 5-(dimethylamino)-N, N-bis(pyridin-2-ylmethyl) naphthalene-1-sulfonamide (L). CO release is chaperoned by turn-on fluorescence and can be triggered by light (405 nm) as well as with hydrogen peroxide in aqueous phosphate buffer. Complex A behaves as a logic "OR" gate via co-registering the inputs of irradiation (light) and peroxide (matter) into the concomitant outputs fluorescence (light) and CO (matter). Cell viability assays confirm the low toxicity of A toward different human cell lines. The CORM has been used to track the inclusion of A into cancer cells.

Exploring the activity of a polyazine bridged Ru(ii)–Pt(ii) supramolecule in F98 rat malignant glioma cells

[(Ph₂phen)₂Ru(dpp)PtCl₂]Cl₂exhibits multiple light-dependent cytotoxicity pathways that preferentially target DNA, offering promise for the development of novel photodynamic therapy agents.

d- Versus l-Glucose Conjugation: Mitochondrial Targeting of a Light-Activated Dual-Mode-of-Action Ruthenium-Based Anticancer Prodrug

Light-activated ruthenium polypyridyl anticancer prodrugs often suffer from poor water solubility, poor selectivity, and/or ill-defined intracellular targets. Coordination of the d- or l-glucose thioether ligand 3 (2-(2-(2-(methylthio)ethoxy)ethoxy)ethyl-β-glucopyranoside) to the highly lipophilic ruthenium complex [Ru(tpy)(dppn)(H2 O)]2+ ([1]2+ ; dppn=benzo[i]dipyrido-[3,2-a:2',3'-c]phenazine, tpy=2,2':6',2''-terpyridine) solved all these problems at once. The two enantiomers of [Ru(tpy)(dppn)(3)][PF6 ]2 , [d-2][PF6 ]2 and [l-2][PF6 ]2 , were soluble in water, which allowed the influence of the chirality of the glucose moiety on uptake, toxicity, and intracellular localization of the prodrug to be probed without changing any other physicochemical properties. Both compounds showed mild, but different, cytotoxicity in A549 (human lung carcinoma) and MCF-7 (human breast adenocarcinoma) cancer cells in the dark, whereas following low doses of visible light irradiation (3.1 J cm-2 at λ = 454 nm), a similar, but high cytotoxicity (EC50 < 1 μm), was observed. Irrespective of the chirality, both slightly emissive Ru complexes were found in the mitochondria, and two modes of action may contribute to light-induced cell death: 1) the glucose thioether ligand is photosubstituted by water, thus [1]2+ , which interacts with DNA at an exceptionally high 400:1 base pair/Ru ratio, is released; 2) both [1]2+ and [2]2+ produce massive amounts of singlet oxygen, which leads to very efficient photodynamic DNA cleavage.

Imaging Upconverting Polymersomes in Cancer Cells: Biocompatible Antioxidants Brighten Triplet-Triplet Annihilation Upconversion

Light upconversion is a very powerful tool in bioimaging as it can eliminate autofluorescence, increase imaging contrast, reduce irradiation damage, and increase excitation penetration depth in vivo. In particular, triplet-triplet annihilation upconverting (TTA-UC) nanoparticles and liposomes offer high upconversion efficiency at low excitation power. However, TTA-UC is quenched in air by oxygen, which also leads to the formation of toxic singlet oxygen. In this work, polyisobutylene-monomethyl polyethylene glycol block copolymers are synthesized and used for preparing polymersomes that upconvert red light into blue light in absence of oxygen. In addition, it is demonstrated that biocompatible antioxidants such as l-ascorbate, glutathionate, l-histidine, sulfite, trolox, or even opti-MEM medium, can be used to protect the TTA-UC process in these polymersomes resulting in red-to-blue upconversion under aerobic conditions. Most importantly, this approach is also functional in living cells. When A549 lung carcinoma cells are treated with TTA-UC polymersomes in the presence of 5 × 10^-3 m ascorbate and glutathionate, upconversion in the living cells is one order of magnitude brighter than that observed without antioxidants. These results propose a simple chemical solution to the issue of oxygen sensitivity of TTA-UC, which is of paramount importance for the technological advancement of this technique in biology.

Chemical Swarming: Depending on Concentration, an Amphiphilic Ruthenium Polypyridyl Complex Induces Cell Death via Two Different Mechanisms

The crystal structure and in vitro cytotoxicity of the amphiphilic ruthenium complex [3](PF6 )2 are reported. Complex [3](PF6 )2 contains a Ru-S bond that is stable in the dark in cell-growing medium, but is photosensitive. Upon blue-light irradiation, complex [3](PF6 )2 releases the cholesterol-thioether ligand 2 and an aqua ruthenium complex [1](PF6 )2 . Although ligand 2 and complex [1](PF6 )2 are by themselves not cytotoxic, complex [3](PF6 )2 was unexpectedly found to be as cytotoxic as cisplatin in the dark, that is, with micromolar effective concentrations (EC50 ), against six human cancer cell lines (A375, A431, A549, MCF-7, MDA-MB-231, and U87MG). Blue-light irradiation (λ=450 nm, 6.3 J cm(-2) ) had little influence on the cytotoxicity of [3](PF6 )2 after 6 h of incubation time, but it increased the cytotoxicity of the complex by a factor 2 after longer (24 h) incubation. Exploring the unexpected biological activity of [3](PF6 )2 in the dark elucidated an as-yet unknown bifaceted mode of action that depended on concentration, and thus, on the aggregation state of the compound. At low concentration, it acts as a monomer, inserts into the membrane, and can deliver [1](2+) inside the cell upon blue-light activation. At higher concentrations (>3-5 μm), complex [3](PF6 )2 forms supramolecular aggregates that induce non-apoptotic cell death by permeabilizing cell membranes and extracting lipids and membrane proteins.

An in vitro cell irradiation protocol for testing photopharmaceuticals and the effect of blue, green, and red light on human cancer cell lines

Traditionally, ultraviolet light (100-400 nm) is considered an exogenous carcinogen while visible light (400-780 nm) is deemed harmless. In this work, a LED irradiation system for in vitro photocytotoxicity testing is described. The LED irradiation system was developed for testing photopharmaceutical drugs, but was used here to determine the basal level response of human cancer cell lines to visible light of different wavelengths, without any photo(chemo)therapeutic. The effects of blue (455 nm, 10.5 mW cm(-2)), green (520 nm, 20.9 mW cm(-2)), and red light (630 nm, 34.4 mW cm(-2)) irradiation was measured for A375 (human malignant melanoma), A431 (human epidermoid carcinoma), A549 (human lung carcinoma), MCF7 (human mammary gland adenocarcinoma), MDA-MB-231 (human mammary gland adenocarcinoma), and U-87 MG (human glioblastoma-grade IV) cell lines. In response to a blue light dose of 19 J cm(-2), three cell lines exhibited a minimal (20%, MDA-MB-231) to moderate (30%, A549 and 60%, A375) reduction in cell viability, compared to dark controls. The other cell lines were not affected. Effective blue light doses that produce a therapeutic response in 50% of the cell population (ED50) compared to dark conditions were found to be 10.9 and 30.5 J cm(-2) for A375 and A549 cells, respectively. No adverse effects were observed in any of the six cell lines irradiated with a 19 J cm(-2) dose of 520 nm (green) or 630 nm (red) light. The results demonstrate that blue light irradiation can have an effect on the viability of certain human cancer cell types and controls should be used in photopharmaceutical testing, which uses high-energy (blue or violet) visible light activation.

Green light-induced apoptosis in cancer cells by a tetrapyridyl ruthenium prodrug offering two trans coordination sites

In this work, two new photopharmacological ruthenium prodrugs are described that can be activated by green light. Cell death occurs via apoptosis; it is not a consequence of singlet oxygen generation, but of light-induced photosubstitution reactions.

In this work, two new photopharmacological ruthenium prodrugs are described that can be activated by green light. They are based on the tetrapyridyl biqbpy ligand (6,6′-bis[N-(isoquinolyl)-1-amino]-2,2′-bipyridine), which coordinates to the basal plane of the metal centre and leaves two trans coordination sites for the binding of monodentate sulphur ligands. Due to the distortion of the coordination sphere these trans ligands are photosubstituted by water upon green light irradiation. In vitro cytotoxicity data on A431 and A549 cancer cell lines shows an up to 22-fold increase in cytotoxicity after green light irradiation (520 nm, 75 J cm^–2), compared to the dark control. Optical microscopy cell imaging and flow cytometry indicate that the cancer cells die via apoptosis. Meanwhile, very low singlet oxygen quantum yields (∼1–2%) and cell-free DNA binding studies conclude that light-induced cell death is not caused by a photodynamic effect, but instead by the changes induced in the coordination sphere of the metal by light, which modifies how the metal complexes bind to biomolecules.

Effectivity of contact lens disinfection systems against Acanthamoeba culbertsoni

Thirteen commercially available contact lens solutions were tested for their ability to kill the cysts of Acanthamoeba culbertsoni. Miraflow, which contains 20% isopropyl alcohol, was the most effective at killing the cyst (94%), followed by solutions containing thimerosal (89%). The rigid gas permeable lens solutions in general were more effective than soft lens solutions. None of the solutions tested were completely cidal, but our data do suggest a combination of a good daily cleaner and disinfecting solution may be effective in reducing acanthamoeba exposure. These findings should provide guidelines for the practitioner in selecting the best disinfection system for the contact lens patient.