Facile synthesis of chlorin bioconjugates by a series of click reactions

A toolbox for enzymatic modification of nucleic acids with photosensitizers for photodynamic therapy

Photodynamic therapy (PDT) is an approved cancer treatment modality. Despite its high efficiency, PDT is limited in terms of specificity and by the poor solubility of the rather lipophilic photosensitizers (PSs). In order to alleviate these limitations, PSs can be conjugated to oligonucleotides. However, most conjugation methods often involve complex organic synthesis and result in the appendage of single modifications at the 3'/5' termini of oligonucleotides. Here, we have investigated the possibility of bioconjugating a range of known PSs by polymerase-mediated synthesis. We have prepared a range of modified nucleoside triphosphates by different conjugation methods and investigated the substrate tolerance of these nucleotides for template-dependent and -independent DNA polymerases. This method represents a mild and versatile approach for the conjugation of single or multiple PSs onto oligonucleotides and can be useful to further improve the efficiency of the PDT treatment.

Synthesis of new representatives of A3B-type carboranylporphyrins based on meso-tetra(pentafluorophenyl)porphyrin transformations

A carboranylporphyrin of A3B-type bearing a single pentafluorophenyl ring was prepared through the regioselective nucleophilic aromatic substitution reaction of the p-fluorine atoms in 5,10,15,20-tetrakis(pentafluorophenyl)porphyrin with 9-mercapto-m-carborane. The reaction of this porphyrin with sodium azide led to the selective substitution of the p-fluorine atom in the pentafluorophenyl substituent with an azide functionality which upon reduction with SnCl2 resulted in the formation of the corresponding porphyrin with an amino group. Pentafluorophenyl-substituted A3B-porphyrins were studied and transformed to thiol and amino-substituted compounds allowing for the preparation of porphyrins with different reactive groups such as hydroxy and amino derivatives capable for further functionalization and conjugation of these porphyrins to other substrates. In addition, conjugates containing maleimide or biotin entities in the structure of carborane A3B-porphyrin were also synthesized based on the amino-substituted A3B-porphyrin. The structures of the prepared carboranylporphyrins were determined by UV-vis, IR, 1H, 19F, 11B NMR spectroscopic data and MALDI mass spectrometry.

NaV1.7 targeted fluorescence imaging agents for nerve identification during intraoperative procedures

Surgeries and trauma result in traumatic and iatrogenic nerve damage that can result in a debilitating condition that approximately affects 189 million individuals worldwide. The risk of nerve injury during oncologic surgery is increased due to tumors displacing normal nerve location, blood turbidity, and past surgical procedures, which complicate even an experienced surgeon's ability to precisely locate vital nerves. Unfortunately, there is a glaring absence of contrast agents to assist surgeons in safeguarding vital nerves. To address this unmet clinical need, we leveraged the abundant expression of the voltage-gated sodium channel 1.7 (NaV1.7) as an intraoperative marker to access peripheral nerves in vivo, and visualized nerves for surgical guidance using a fluorescently-tagged version of a potent NaV1.7-targeted peptide, Tsp1a, derived from a Peruvian tarantula. We characterized the expression of NaV1.7 in sensory and motor peripheral nerves across mouse, primate, and human specimens and demonstrated universal expression. We synthesized and characterized a total of 10 fluorescently labeled Tsp1a-peptide conjugates to delineate nerves. We tested the ability of these peptide-conjugates to specifically accumulate in mouse nerves with a high signal-to-noise ratio in vivo. Using the best-performing candidate, Tsp1a-IR800, we performed thyroidectomies in non-human primates and demonstrated successful demarcation of the recurrent laryngeal and vagus nerves, which are commonly subjected to irreversible damage. The ability of Tsp1a to enhance nerve contrast during surgery provides opportunities to minimize nerve damage and revolutionize standards of care across various surgical specialties.

Development and Validation of Nerve-Targeted Bacteriochlorin Sensors

We report an innovative approach to producing bacteriochlorins (bacs) via formal cycloaddition by subjecting a porphyrin to a trimolecular reaction. Bacs are near-infrared probes with the intrinsic ability to serve in multimodal imaging. However, despite their ability to fluoresce and chelate metal ions, existing bacs have thus offered limited ability to label biomolecules for target specificity or have lacked chemical purity, limiting their use in bio-imaging. In this work, bacs allowed a precise and controlled appending of clickable linkers, lending the porphyrinoids substantially more chemical stability, clickability, and solubility, rendering them more suitable for preclinical investigation. Our bac probes enable the targeted use of biomolecules in fluorescence imaging and Cerenkov luminescence for guided intraoperative imaging. Bacs' capacity for chelation provides opportunities for use in non-invasive positron emission tomography/computed tomography. Herein, we report the labeling of bacs with Hs1a, a (NaV1.7)-sodium-channel-binding peptide derived from the Chinese tarantula Cyriopagopus schmidti to yield Bac-Hs1a and radiolabeled Hs1a, which shuttles our bac sensor(s) to mouse nerves. In vivo, the bac sensor allowed us to observe high signal-to-background ratios in the nerves of animals injected with fluorescent Bac-Hs1a and radiolabeled Hs1a in all imaging modes. This study demonstrates that Bac-Hs1a and [64Cu]Cu-Bac-Hs1a accumulate in peripheral nerves, providing contrast and utility in the preclinical space. For the chemistry and bio-imaging fields, this study represents an exciting starting point for the modular manipulation of bacs, their development and use as probes for diagnosis, and their deployment as formidable multiplex nerve-imaging agents for use in routine imaging experiments.

Two birds one stone: β-fluoropyrrolyl-cysteine SNAr chemistry enabling functional porphyrin bioconjugation

Bioconjugation, a synthetic tool that endows small molecules with biocompatibility and target specificity through covalent attachment of a biomolecule, holds promise for next-generation diagnosis or therapy. Besides the establishment of chemical bonding, such chemical modification concurrently allows alteration of the physicochemical properties of small molecules, but this has been paid less attention in designing novel bioconjugates. Here, we report a "two birds one stone" methodology for irreversible porphyrin bioconjugation based on β-fluoropyrrolyl-cysteine S_NAr chemistry, in which the β-fluorine of porphyrin is selectively replaced by a cysteine in either peptides or proteins to generate novel β-peptidyl/proteic porphyrins. Notably, due to the distinct electronic nature between fluorine and sulfur, such replacement makes the Q band red-shift to the near-infrared region (NIR, >700 nm). This facilitates intersystem crossing (ISC) to enhance the triplet population and thus singlet oxygen production. This new methodology features water tolerance, a fast reaction time (15 min), good chemo-selectivity, and broad substrate scope, including various peptides and proteins under mild conditions. To demonstrate its potential, we applied porphyrin β-bioconjugates in several scenarios, including (1) cytosolic delivery of functional proteins, (2) metabolic glycan labeling, (3) caspase-3 detection, and (4) tumor-targeting phototheranostics.

Lanthanide porphyrinoids as molecular theranostics

In recent years, lanthanide (Ln) porphyrinoids have received increasing attention as theranostics. Broadly speaking, the term 'theranostics' refers to agents designed to allow both disease diagnosis and therapeutic intervention. This Review summarises the history and the 'state-of-the-art' development of Ln porphyrinoids as theranostic agents. The emphasis is on the progress made within the past decade. Applications of Ln porphyrinoids in near-infrared (NIR, 650-1700 nm) fluorescence imaging (FL), magnetic resonance imaging (MRI), radiotherapy, and chemotherapy will be discussed. The use of Ln porphyrinoids as photo-activated agents ('phototheranostics') will also be highlighted in the context of three promising strategies for regulation of porphyrinic triplet energy dissipation pathways, namely: regioisomeric effects, metal regulation, and the use of expanded porphyrinoids. The goal of this Review is to showcase some of the ongoing efforts being made to optimise Ln porphyrinoids as theranostics and as phototheranostics, in order to provide a platform for understanding likely future developments in the area, including those associated with structure-based innovations, functional improvements, and emerging biological activation strategies.

Applications of Fluorous Porphyrinoids: An Update†

Porphyrins and related macrocycles have been studied broadly for their applications in medicine and materials because of their tunable physicochemical, optoelectronic and magnetic properties. In this review article, we focused on the applications of fluorinated porphyrinoids and their supramolecular systems and summarized the reports published on these chromophores in the past 5-6 years. The commercially available fluorinated porphyrinoids: meso-perfluorophenylporphyrin (TPPF₂₀ ) perfluorophthalocyanine (PcF₁₆ ) and meso-perfluorophenylcorrole (CorF₁₅ ) have increased photo and oxidative stability due to the presence of fluoro groups. Because of their tunable properties and robustness toward oxidative damage these porphyrinoid-based chromophores continue to gain attention of researchers developing advanced functional materials for applications such as sensors, photonic devices, component for solar cells, biomedical imaging, theranostics and catalysts.

Leveraging synthetic chlorins for bio-imaging applications

Synthetic chlorins are not only fluorescent, the modulation of the tetrapyrrole system can also chelate metal ions. Conjugation of linkers at their pyrrolidines allows for conjugation to bio-molecules to create target specificity. By altering these chemo-photophysical properties, this work facilitates the use of chlorins in fluorescent imaging and positron emission tomography (PET).

Bimodal Imaging of Mouse Peripheral Nerves with Chlorin Tracers

Almost 17 million Americans have a history of cancer, a number expected to reach over 22 million by 2030. Cancer patients often undergo chemotherapy in the form of antineoplastic agents such as cis-platin and paclitaxel. Though effective, these agents can induce debilitating side effects; the most common neurotoxic effect, chemotherapy-induced peripheral neuropathy (CIPN), can endure long after treatment ends. Despite the widespread and chronic nature of the dysfunction, no tools exist to quantitatively measure chemotherapy-induced peripheral neuropathy. Such a tool would not only benefit patients but their stratification could also save significant financial and social costs associated with neuropathic pain. In our first step toward addressing this unmet clinical need, we explored a novel dual approach to localize peripheral nerves: Cerenkov luminescence imaging (CLI) and fluorescence imaging (FI). Our approach revolves around the targeting and imaging of voltage-gated sodium channel subtype Na_V1.7, highly expressed in peripheral nerves from both harvested human and mouse tissues. For the first time, we show that Hsp1a, a radiolabeled Na_V1.7-selective peptide isolated from Homoeomma spec. Peru, can serve as a targeted vector for delivering a radioactive sensor to the peripheral nervous system. In situ, we observe high signal-to-noise ratios in the sciatic nerves of animals injected with fluorescently labeled Hsp1a and radiolabeled Hsp1a. Moreover, confocal microscopy on fresh nerve tissue shows the same high ratios of fluorescence, corroborating our in vivo results. This study indicates that fluorescently labeled and radiolabeled Hsp1a tracers could be used to identify and demarcate nerves in a clinical setting.

One-Pot Synthesis of Four Chlorin Derivatives by a Divergent Ylide

Chlorins have unique photophysical properties that are exploited in diverse biological and materials applications. De novo chlorin synthesis with specific exocyclic motifs can be challenging and many are not stable to photobleaching and/or oxidation. A facile approach to a stable synthetic chlorin with a fused N-methyl pyrrolidine uses cyclo addition of a sarcosine-based azomethine ylide on 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)-porphyrin (TPPF₂₀) is reported, but this approach has limitations. We report the synthesis of stable chlorin scaffolds starting with TPPF₂₀ using a new glycine-based N-(hydroxymethyl)- N-methelenemethanideaminium ylide. Careful control of the 1,3-dipolar cycloaddition reaction allows a divergent use of the glycine derived ylide to yield four new chlorins, including the fused NH-pyrrolidine, two dimers, and the same N-methyl chlorin product from the sarcosine ylide reaction. The mechanism begins with the formation of a bis(hydroxymethyl)glycine, which then dehydrates and decarboxylates to form the active N-(hydroxymethyl)- N-methelenemethanideaminium ylide, which then reacts with TPPF₂₀ to form a key N-(hydroxymethyl)-17,18-pyrrolidinyl-chlorin intermediate. Deformylation of this intermediate affords the (17,18-pyrrolidinyl)-chlorin, whereas a Cannizzaro-type reaction promotes a hydride attack to an imine chlorin cation to yield the N-methyl chlorin. The exocyclic NH-pyrrolidine provides a unique mode of attaching chiral moieties that avoids formation of diasteromers at the bridgehead carbons.

Chlorophyll-Inspired Red-Region Fluorophores: Building Block Synthesis and Studies in Aqueous Media

Fluorophores that absorb and emit in the red spectral region (600-700 nm) are of great interest in photochemistry and photomedicine. Eight new target chlorins (and 19 new chlorins altogether)-analogues of chlorophyll-of different polarities have been designed and synthesized for various applications; seven of the chlorins are equipped with a bioconjugatable tether. Hydrophobic or amphiphilic chlorins in a non-polar organic solvent (toluene), polar organic solvent (DMF), and aqueous or aqueous micellar media show a sharp emission band in the red region and modest fluorescence quantum yield (Φ_f = 0.2-0.3). A Poisson analysis implies most micelles are empty and few contain >1 chlorin. Water-soluble chlorins each bearing three PEG (oligoethyleneglycol) groups exhibit narrow emission bands (full-width-at-half maximum <25 nm). The lifetime of the lowest singlet excited state and the corresponding yields and rate constants for depopulation pathways (fluorescence, intersystem crossing, internal conversion) are generally little affected by the PEG groups or dissolution in aqueous or organic media. A set of chlorin-avidin conjugates revealed a 2-fold increase in Φ_f with increased average chlorin/avidin ratio (2.3-12). In summary, the chlorins of various polarities described herein are well suited as red-emitting fluorophores for applications in aqueous or organic media.