Selective radiolabelling with 68Ga under mild conditions: a route towards a porphyrin PET/PDT theranostic agent

Radiometal-Labeled Photoactivatable Pt(IV) Anticancer Complex for Theranostic Phototherapy

A novel photoactivatable Pt(IV) diazido anticancer agent, Pt-succ-DFO, bearing a pendant deferoxamine (DFO) siderophore for radiometal chelation, has been synthesized for the study of its in vivo behavior with radionuclide imaging. Pt-succ-DFO complexation of Fe(III) and Ga(III) ions yielded new heterobimetallic complexes that maintain the photoactivation properties and photocytotoxicity of the parent Pt complex in human cancer cell lines. Radiolabeled Pt-succ-DFO-68Ga (t1/2 = 68 min, positron emitter) was readily prepared under mild conditions and was stable in the dark upon incubation with human serum. PET imaging of Pt-succ-DFO-68Ga in healthy mice revealed a promising biodistribution profile with rapid renal excretion and limited organ accumulation, implying that little off-target uptake is expected for this class of agents. Overall, this research provides the first in vivo imaging study of the whole-body distribution of a photoactivatable Pt(IV) azido anticancer complex and illustrates the potential of radionuclide imaging as a tool for the preclinical development of novel light-activated agents.

Inorganic Chemistry of the Tripodal Picolinate Ligand with Gallium(III) and Radiolabeling with Gallium-68

We report here the improved synthesis of the tripodal picolinate chelator Tpaa, with an overall yield of 41% over five steps, in comparison to the previously reported 6% yield. Tpaa was investigated for its coordination chemistry with Ga(III) and radiolabeling properties with gallium-68 (68Ga). The obtained crystal structure for [Ga(Tpaa)] shows that the three picolinate arms coordinate to the Ga(III) ion, fully occupying the octahedral coordination geometry. This is supported by 1H NMR which shows that the three arms are symmetrical when coordinated to Ga(III). Assessment of the thermodynamic stability through potentiometry gives log KGa-Tpaa = 21.32, with a single species being produced across the range of pH 3.5-7.5. Tpaa achieved >99% radiochemical conversion with 68Ga under mild conditions ([Tpaa] = 6.6 μM, pH 7.4, 37 °C) with a molar activity of 3.1 GBq μmol-1. The resulting complex, [68Ga][Ga(Tpaa)], showed improved stability over the previously reported [68Ga][Ga(Dpaa)(H2O)] in a serum challenge, with 32% of [68Ga][Ga(Tpaa)] remaining intact after 30 min of incubation with fetal bovine serum.

Two cationic meso-thiophenium porphyrins and their zinc-complexes as anti-HIV-1 and antibacterial agents under non-photodynamic therapy (PDT) conditions

The anti-HIV-1 and antimicrobial activities of novel cationic meso-thiophenium porphyrins and their zinc-complex are reported under in vitro non-photodynamic (PDT) conditions. While all the cationic porphyrins led to the inhibition of de novo virus infection, the Zn(II)-complexes of T₂(OH)₂M (A₂B₂-type) and T(OH)₃M (AB₃-type) displayed potent inhibition of HIV-1 entry with T₂(OH)₂MZn displaying maximal anti-HIV activity. The Zinc complex of both the thiophenium porphyrins T₂(OH)₂M and T(OH)₃M also depicted antibacterial activities against Escherichia coli (ATCC 25922) and more prominently against Staphylococcus aureus (ATCC 25923). Again, the antibacterial activity was more potent for T₂(OH)₂MZn. Our study highlighted that the presence of two thiophenium groups at the meso-positions of the A₂B₂-type porphyrins along with zinc strongly enhanced anti-HIV and antimicrobial properties of these novel thiophenium porphyrins under non-PDT conditions.

Recent Strategies to Develop Innovative Photosensitizers for Enhanced Photodynamic Therapy

This review presents a robust strategy to design photosensitizers (PSs) for various species. Photodynamic therapy (PDT) is a photochemical-based treatment approach that involves the use of light combined with a light-activated chemical, referred to as a PS. Attractively, PDT is one of the alternatives to conventional cancer treatment due to its noninvasive nature, high cure rates, and low side effects. PSs play an important factor in photoinduced reactive oxygen species (ROS) generation. Although the concept of photosensitizer-based photodynamic therapy has been widely adopted for clinical trials and bioimaging, until now, to our surprise, there has been no relevant review article on rational designs of organic PSs for PDT. Furthermore, most of published review articles in PDT focused on nanomaterials and nanotechnology based on traditional PSs. Therefore, this review aimed at reporting recent strategies to develop innovative organic photosensitizers for enhanced photodynamic therapy, with each example described in detail instead of providing only a general overview, as is typically done in previous reviews of PDT, to provide intuitive, vivid, and specific insights to the readers.

Supramolecular Assembly of Hybrid Pt(II) Porphyrin/Tomatine Analogues with Different Nanostructures and Cytotoxic Activities

A simple strategy for synthesizing supramolecular hybrids was developed for the preparation of bioavailable nanohybrid photosensitizers by assembling visible-light-sensitive Pt(II) meso-tetrakis(4-carboxyphenyl)porphyrinporphyrin (PtTCPP)/tomatine analogues. The hybrids were self-assembled into nanofibrous or nanosheet structures approximately 3-5 nm thick and several micrometers wide. α-Tomatine generated a unique fibrous vesicle nanostructure based on intermolecular interactions, while dehydrotomatine generated nanosheet structures. Nanoassembly of these fibrous vesicles and sheets directly affected the properties of the light-responsive photosensitizer for tumor photodynamic therapy (PDT), depending on the nanostructure of the hybrid PtTCPP/tomatine analogues. The cytotoxicity of PtTCPP to cancer cells under photoirradiation was significantly enhanced by a tomatine assembly with a fibrous vesicle nanostructure, attributable to increased incorporation of the drug into cells.

A Coumarin-Porphyrin FRET Break-Apart Probe for Heme Oxygenase-1

Heme oxygenase-1 (HO-1) is a vital enzyme in humans that primarily regulates free heme concentrations. The overexpression of HO-1 is commonly associated with cardiovascular and neurodegenerative diseases including atherosclerosis and ischemic stroke. Currently, there are no known chemical probes to detect HO-1 activity, limiting its potential as an early diagnostic/prognostic marker in these serious diseases. Reported here are the design, synthesis, and photophysical and biological characterization of a coumarin-porphyrin FRET break-apart probe to detect HO-1 activity, Fe-L1. We designed Fe-L1 to "break-apart" upon HO-1-catalyzed porphyrin degradation, perturbing the efficient FRET mechanism from a coumarin donor to a porphyrin acceptor fluorophore. Analysis of HO-1 activity using Escherichia coli lysates overexpressing hHO-1 found that a 6-fold increase in emission intensity at 383 nm was observed following incubation with NADPH. The identities of the degradation products following catabolism were confirmed by MALDI-MS and LC-MS, showing that porphyrin catabolism was regioselective at the α-position. Finally, through the analysis of Fe-L2, we have shown that close structural analogues of heme are required to maintain HO-1 activity. It is anticipated that this work will act as a foundation to design and develop new probes for HO-1 activity in the future, moving toward applications of live fluorescent imaging.

Probing the Interactions of Porphyrins with Macromolecules Using NMR Spectroscopy Techniques

Porphyrinic compounds are widespread in nature and play key roles in biological processes such as oxygen transport in blood, enzymatic redox reactions or photosynthesis. In addition, both naturally derived as well as synthetic porphyrinic compounds are extensively explored for biomedical and technical applications such as photodynamic therapy (PDT) or photovoltaic systems, respectively. Their unique electronic structures and photophysical properties make this class of compounds so interesting for the multiple functions encountered. It is therefore not surprising that optical methods are typically the prevalent analytical tool applied in characterization and processes involving porphyrinic compounds. However, a wealth of complementary information can be obtained from NMR spectroscopic techniques. Based on the advantage of providing structural and dynamic information with atomic resolution simultaneously, NMR spectroscopy is a powerful method for studying molecular interactions between porphyrinic compounds and macromolecules. Such interactions are of special interest in medical applications of porphyrinic photosensitizers that are mostly combined with macromolecular carrier systems. The macromolecular surrounding typically stabilizes the encapsulated drug and may also modify its physical properties. Moreover, the interaction with macromolecular physiological components needs to be explored to understand and control mechanisms of action and therapeutic efficacy. This review focuses on such non-covalent interactions of porphyrinic drugs with synthetic polymers as well as with biomolecules such as phospholipids or proteins. A brief introduction into various NMR spectroscopic techniques is given including chemical shift perturbation methods, NOE enhancement spectroscopy, relaxation time measurements and diffusion-ordered spectroscopy. How these NMR tools are used to address porphyrin-macromolecule interactions with respect to their function in biomedical applications is the central point of the current review.

Metalloporphyrins in Medicine: From History to Recent Trends

The history of metalloporphyrins dates back more than 200 years ago. Metalloporphyrins are excellent catalysts, capable of forming supramolecular systems, participate in oxygen photosynthesis, transport, and used as contrast agents or superoxide dismutase mimetics. Today, metalloporphyrins represent complexes of conjugated π-electron system and metals from the entire periodic system. However, the effect of these compounds on living systems has not been fully understood, and researchers are exploring the properties of metalloporphyrins thereby extending their further application. This review provides an overview of the variety of metalloporphyrins that are currently used in different medicine fields and how metalloporphyrins became the subject of scientists' interest. Currently, metalloporphyrins utilization has expanded significantly, which gave us an opprotunuty to summarize recent progress in metalloporphyrins derivatives and prospects of their application in the treatment and diagnosis of different diseases.

Synthesis of a porphyrin with histidine-like chelate: an efficient path towards molecular PDT/SPECT theranostics

The goal of "personalised" medicine has seen a growing interest in the development of theranostic agents. Bifunctional, and targeted-trifunctional, theranostic water-soluble porphyrins with a histidine-like chelating group have been synthesised via copper-catalysed azide-alkyne cycloaddition (CuAAC) "click" chemistry in high yield and purity. They are capable of photodynamic treatment and [99mTc(CO)3]+ complexation for single-photon emission computed tomography (SPECT) imaging, with a radiochemical yield of >95%. The toxicity and phototoxicity were evaluated on HT-29 cells, DU145, and DU145-PSMA cell lines, with the targeted theranostic showing more potent phototoxicity towards DU145-PSMA expressing cells.

Expanding PET-applications in life sciences with positron-emitters beyond fluorine-18

Positron-emission-tomography (PET) has become an indispensable diagnostic tool in modern nuclear medicine. Its outstanding molecular imaging features allow repetitive studies on one individual and with high sensitivity, though no interference. Rather few positron-emitters with near favourable physical properties, i.e. carbon-11 and fluorine-18, furnished most studies in the beginning, preferably if covalently bound as isotopic label of small molecules. With the advancement of PET-devices the scope of in vivo research in life sciences and especially that of medical applications expanded, and other than "standard" PET-nuclides received increasing significance, like the radiometals copper-64 and gallium-68. Especially during the last decades, positron-emitters of other chemical elements have gotten into the focus of interest, concomitant with the technical advancements in imaging and radionuclide production. With known nuclear imaging properties and main production methods of emerging positron-emitters their usefulness for medical application is promising and even proven for several ones already. Unfortunate decay properties could be corrected for, and β⁺-emitters, especially with a longer half-life, provided new possibilities for application where slower processes are of importance. Further on, (bio)chemical features of positron-emitters of other elements, among there many metals, not only expanded the field of classical clinical investigations, but also opened up new fields of application. Appropriately labelled peptides, proteins and nanoparticles lend itself as newer probes for PET-imaging, e.g. in theragnostic or PET/MR hybrid imaging. Furthermore, the potential of non-destructive in-vivo imaging with positron-emission-tomography directs the view on further areas of life sciences. Thus, exploiting the excellent methodology for basic research on molecular biochemical functions and processes is increasingly encouraged as well in areas outside of health, such as plant and environmental sciences.

Green Layer-by-Layer Assembly of Porphyrin/G-Quadruplex-Based Near-Infrared Nanocomposite Photosensitizer with High Biocompatibility and Bioavailability

A simple and green layer-by-layer assembly strategy is developed for the preparation of a highly bioavailable nanocomposite photosensitizer by assembling near-infrared (NIR) light-sensitive porphyrin/G-quadruplex complexes on the surface of a highly biocompatible nanoparticle that is prepared via Zn²⁺-assisted coordination self-assembly of an amphiphilic amino acid. After being efficiently delivered to the target site and internalized into tumor cells via enhanced permeability and retention effect and interactions between aptamers and tumor markers, the as-prepared nanoassembly can be directly used as an NIR light-responsive photosensitizer for tumor photodynamic therapy (PDT) since the porphyrin/G-quadruplex complexes are exposed on the nanoassembly surface and kept in an active state. It can also disassemble under the synergistic stimuli of an acidic pH environment and overexpressed glutathione, leasing more efficient porphyrin/G-quadruplex composite photosensitizers while reducing the interference caused by glutathione-dependent ¹O₂ consumption. Since the nanoassembly can work no matter if it is disassembled or not, the compulsory requirement for in vivo photosensitizer release is eliminated, thus resulting in the great improvement of the bioavailability of the photosensitizer. The PDT applications of the nanoassembly were well demonstrated in both in vitro cell and in vivo animal experiments.

G-Quadruplex/Porphyrin Composite Photosensitizer: A Facile Way to Promote Absorption Redshift and Photodynamic Therapy Efficacy

Photosensitizer is one of the most important elements of photodynamic therapy (PDT). Herein, we reported a novel strategy to prepare a new series of composite photosensitizers. The composite photosensitizer was prepared by simply mixing DNA G-quadruplexes with a hydrophilic porphyrin (TMPipEOPP)⁴⁺·4I^-. Compared with the conventional porphyrin photosensitizers, the excitation wavelength of the composite one has been ∼50 nm redshifted (from 650 to 700 nm), which is beneficial to the penetration of the light. Moreover, the composite photosensitizer showed an about 7.4-fold increase of light absorption efficiency, thus greatly enhancing the singlet oxygen (¹O₂) generation capacity and PDT efficacy. What is more, the introduction of nucleic acids in the composite photosensitizer could also provide some extra charming properties, such as the targeted recognition ability conferred by aptamer and high capability to assemble with various drug carriers. We demonstrated that the composite photosensitizer could be easily assembled with MnO₂ nanosheet. The obtained nanodevice integrated the merits of a composite photosensitizer and MnO₂ nanosheet, thus showing strong near-infrared absorption, high ¹O₂ generation efficiency, avoidance of nonideal ¹O₂ consumption by glutathione, and in situ O₂ generation to relieve tumor hypoxia. This nanodevice showed greatly improved PDT efficacy both in vitro and in vivo, presenting a huge potential for applications in clinical therapy for tumors.

Aggregation-induced emission (AIE)-active molecules bearing singlet oxygen generation activities: the tunable singlet–triplet energy gap matters

Two-photon active photosensitizers showed relatively strong intersystem crossing facilitating ¹O₂ generation and cell apoptosis with near-infrared excitation.