BODIPY: A Highly Versatile Platform for the Design of Bimodal Imaging Probes

Tailored hyaluronic acid-based nanogels as theranostic boron delivery systems for boron neutron cancer therapy

Boron-rich nanocarriers possess great potential for advanced boron neutron capture therapy (BNCT) as an effective radiation treatment for invasive malignant tumors. If additionally, they can be imaged in a non-invasive and real-time manner allowing the assessment of local boron concentration, they could serve for dose calculation and image-guided BNCT to enhance tumor treatment efficacy. To meet this challenge, this study describes the design of a theranostic nanogel, enriched in ¹⁰B and fluorescent dye, to achieve selective imaging, and sufficient accumulation of boron at the tumor site. The boron-rich and fluorescent nanogels can be easily obtained via temperature triggered-assembly of hyaluronic acid (HA) modified with a thermoresponsive terpolymer. The latter was specifically designed to enable the efficient encapsulation of the fluorescent dye – an aza‑boron-dipyrromethene (aza-BODIPY) – linked to ¹⁰B-enriched sodium borocaptate (BSH), in addition to induce nanogel formation below room temperature, and to enable their core-crosslinking by hydrazone bond formation. The HA nanogel considerably concentrates aza-BODIPY-BSH into the hydrophobic nanodomains made of the terpolymer chains. Here, we present the detailed synthesis of the HA-terpolymer conjugate, nanogel formation, and characterization in terms of size, morphology, and stability upon storage, as well as the biological behavior of the boron nanocarrier using real-time fluorescence imaging in cells and in vivo. This work suggested the potential of the theranostic HA nanogel as a boron delivery system for the implementation of BNCT in brain cancer and sarcoma.

Development of a multi faceted platform containing a tetrazine, fluorophore and chelator: synthesis, characterization, radiolabeling, and immuno-SPECT imaging

BACKGROUND

Combining optical (fluorescence) imaging with nuclear imaging has the potential to offer a powerful tool in personal health care, where nuclear imaging offers in vivo functional whole-body visualization, and the fluorescence modality may be used for image-guided tumor resection. Varying chemical strategies have been exploited to fuse both modalities into one molecular entity. When radiometals are employed in nuclear imaging, a chelator is typically inserted into the molecule to facilitate radiolabeling; the availability of the chelator further expands the potential use of these platforms for targeted radionuclide therapy if a therapeutic radiometal is employed. Herein, a novel mixed modality scaffold which contains a tetrazine (Tz)--for biomolecule conjugation, fluorophore-for optical imaging, and chelator-for radiometal incorporation, in one construct is presented. The novel platform was characterized for its fluorescence properties, radiolabeled with single-photon emission computed tomography (SPECT) isotope indium-111 (¹¹¹In³⁺) and therapeutic alpha emitter actinium-225 (²²⁵Ac³⁺). Both radiolabels were conjugated in vitro to trans-cyclooctene (TCO)-modified trastuzumab; biodistribution and immuno-SPECT imaging of the former conjugate was assessed.

RESULTS

Key to the success of the platform synthesis was incorporation of a 4,4'-dicyano-BODIPY fluorophore. The route gives access to an advanced intermediate where final chelator-incorporated compounds can be easily accessed in one step prior to radiolabeling or biomolecule conjugation. The DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) conjugate was prepared, displayed good fluorescence properties, and was successfully radiolabeled with ¹¹¹In & ²²⁵Ac in high radiochemical yield. Both complexes were then separately conjugated in vitro to TCO modified trastuzumab through an inverse electron demand Diels-Alder (IEDDA) reaction with the Tz. Pilot small animal in vivo immuno-SPECT imaging with [¹¹¹In]In-DO3A-BODIPY-Tz-TCO-trastuzumab was also conducted and exhibited high tumor uptake (21.2 ± 5.6%ID/g 6 days post-injection) with low uptake in non-target tissues.

CONCLUSIONS

The novel platform shows promise as a multi-modal probe for theranostic applications. In particular, access to an advanced synthetic intermediate where tailored chelators can be incorporated in the last step of synthesis expands the potential use of the scaffold to other radiometals. Future studies including validation of ex vivo fluorescence imaging and exploiting the pre-targeting approach available through the IEDDA reaction are warranted.

Aza-BODIPY: A New Vector for Enhanced Theranostic Boron Neutron Capture Therapy Applications

Boron neutron capture therapy (BNCT) is a radiotherapeutic modality based on the nuclear capture of slow neutrons by stable ¹⁰B atoms followed by charged particle emission that inducing extensive damage on a very localized level (<10 μm). To be efficient, a sufficient amount of ¹⁰B should accumulate in the tumor area while being almost cleared from the normal surroundings. A water-soluble aza-boron-dipyrromethene dyes (BODIPY) fluorophore was reported to strongly accumulate in the tumor area with high and BNCT compatible Tumor/Healthy Tissue ratios. The clinically used ¹⁰B-BSH (sodium borocaptate) was coupled to the water-soluble aza-BODIPY platform for enhanced ¹⁰B-BSH tumor vectorization. We demonstrated a strong uptake of the compound in tumor cells and determined its biodistribution in mice-bearing tumors. A model of chorioallantoic membrane-bearing glioblastoma xenograft was developed to evidence the BNCT potential of such compound, by subjecting it to slow neutrons. We demonstrated the tumor accumulation of the compound in real-time using optical imaging and ex vivo using elemental imaging based on laser-induced breakdown spectroscopy. The tumor growth was significantly reduced as compared to BNCT with ¹⁰B-BSH. Altogether, the fluorescent aza-BODIPY/¹⁰B-BSH compound is able to vectorize and image the ¹⁰B-BSH in the tumor area, increasing its theranostic potential for efficient approach of BNCT.

Site-Specific 89Zr- and 111In-Radiolabeling and In Vivo Evaluation of Glycan-free Antibodies by Azide-Alkyne Cycloaddition with a Non-natural Amino Acid

Antibody-drug conjugates (ADCs) are a class of targeted therapeutics consisting of a monoclonal antibody coupled to a cytotoxic payload. Various bioconjugation methods for producing site-specific ADCs have been reported recently, in efforts to improve immunoreactivity and pharmacokinetics and minimize batch variance-potential issues associated with first-generation ADCs prepared via stochastic peptide coupling of lysines or reduced cysteines. Recently, cell-free protein synthesis of antibodies incorporating para-azidomethyl phenylalanine (pAMF) at specific locations within the protein sequence has emerged as a means to generate antibody-drug conjugates with strictly defined drug-antibody-ratio, leading to ADCs with markedly improved stability, activity, and specificity. The incorporation of pAMF enables the conjugation of payloads functionalized for strain-promoted azide-alkyne cycloaddition. Here, we introduce two dibenzylcyclooctyne-functionalized bifunctional chelators that enable the incorporation of radioisotopes for positron emission tomography with ⁸⁹Zr (t_1/2 = 78.4 h, β⁺ = 395 keV (22%), γ = 897 keV) or single photon emission computed tomography with ¹¹¹In (t_1/2 = 67.3 h, γ = 171 keV (91%), 245 keV (94%)) under physiologically compatible conditions. We show that the corresponding radiolabeled conjugates with site-specifically functionalized antibodies targeting HER2 are amenable to targeted molecular imaging of HER2+ expressing tumor xenografts in mice and exhibit a favorable biodistribution profile in comparison with conventional, glycosylated antibody conjugates generated by stochastic bioconjugation.

BODIPYS and aza-BODIPY derivatives as promising fluorophores for in vivo molecular imaging and theranostic applications

Since their discovery in 1968, the BODIPYs dyes (4,4-difluoro-4-bora-3a, 4a diaza-s-indacene) have found an exponentially increasing number of applications in a large variety of scientific fields. In particular, studies reporting bioapplications of BODIPYs have increased dramatically. However, most of the time, only in vitro investigations have been reported. The in vivo potential of BODIPYs and aza-BODIPYs is more recent, but considering the number of in vivo studies with BODIPY and aza-BODIPY which have been reported in the last five years, we can now affirm that this family of fluorophores can be considered important as cyanine dyes for future in vivo and even clinical applications. This review aims to present representative examples of recent in vivo applications of BODIPYs or aza-BODIPYs, and to highlight the potential of these dyes for optical molecular imaging.

Functionalization of theranostic AGuIX® nanoparticles for PET/MRI/optical imaging

A novel trifunctional imaging probe containing a chelator of radiometal for PET, a NIR heptamethine cyanine dye, and a bioconjugatable handle, has been grafted onto AGuIX® nanoparticles via a Michael addition reaction. The resulting functionalized nanoparticles have been fully characterized, radiolabelled with ⁶⁴Cu, and evaluated in a mice TSA tumor model using multimodal (PET/MRI/optical) imaging.

The controlled dual functionalization of AGuIX® nanoparticles afforded trimodal imaging (PET/MRI/fluorescence) theranostic agents.

Nuclear and Optical Bimodal Imaging Probes Using Sequential Assembly: A Perspective

New, targeted imaging tracers enable improved diagnosis, staging, and planning of treatment of disease and represent an important step toward personalized medicine applications. The combination of radioisotopes for nuclear imaging with fluorophores for fluorescence imaging provides the possibility to noninvasively assess disease burden in a patient using positron emission tomography/single-photon emission computed tomography, followed by fluorescence imaging-assisted surgical intervention in close succession. Probes enabling imaging with both modalities pose a design, synthesis, and pharmacokinetics challenge. In this study, the authors strive to summarize recent efforts toward optimized, discrete, bimodal probes as well as a perspective on future directions of this burgeoning subfield of targeted imaging probe development.

Firefly Luciferin-Inspired Biocompatible Chemistry for Protein Labeling and In Vivo Imaging

Biocompatible reactions have emerged as versatile tools to build various molecular imaging probes that hold great promise for the detection of biological processes in vitro and/or in vivo. In this Minireview, we describe the recent advances in the development of a firefly luciferin-inspired biocompatible reaction between cyanobenzothiazole (CBT) and cysteine (Cys), and highlight its versatility to label proteins and build multimodality molecular imaging probes. The review starts from the general introduction of biocompatible reactions, which is followed by briefly describing the development of the firefly luciferin-inspired biocompatible chemistry. We then discuss its applications for the specific protein labeling and for the development of multimodality imaging probes (fluorescence, bioluminescence, MRI, PET, photoacoustic, etc.) that enable high sensitivity and spatial resolution imaging of redox environment, furin and caspase-3/7 activity in living cells and mice. Finally, we offer the conclusions and our perspective on the various and potential applications of this reaction. We hope that this review will contribute to the research of biocompatible reactions for their versatile applications in protein labeling and molecular imaging.

Solvent-Sensitive Emitting Urea-Bridged bis-BODIPYs: Ready Access by a One-Pot Tandem Staudinger/Aza-Wittig Ureation

Herein we describe the synthesis, and computationally aided photophysical characterization of a new set of urea-bridged bis-BODIPY derivatives. These new dyads are efficiently obtained by a one-pot tandem Staudinger/aza-Wittig ureation protocol, from easily accessible meso-phenyl ortho-azidomethyl BODIPYs. These symmetric bis-BODIPYs outstand by a high absorption probability and excellent fluorescence and laser emission in less polar media. Nevertheless, this emission ability decreases in more polar media, which is ascribed to a light-induced charge-transfer from the urea spacer to the dipyrrin core, a process that can be modulated by appropriate changes in the substitution pattern of the BODIPY core. Furthermore, this ureation protocol can also be employed for the direct conjugation of our BODIPY-azides to amine-containing compounds, thus providing access to fluorescent non-symmetric ureas.

Synthesis of BODIPY-Peptide Conjugates for Fluorescence Labeling of EGFR Overexpressing Cells

Regioselective functionalization of 2,3,5,6,8-pentachloro-BODIPY 1 produced unsymmetric BODIPY 5, bearing an isothiocyanate group suitable for conjugation, in only four steps. The X-ray structure of 5 reveals a nearly planar BODIPY core with aryl dihedral angles in the range 47.4-62.9°. Conjugation of 5 to two EGFR-targeting pegylated peptides, 3PEG-LARLLT (6) and 3PEG-GYHWYGYTPQNVI (7), under mild conditions (30 min at room temperature), afforded BODIPY conjugates 8 and 9 in 50-80% isolated yields. These conjugates showed red-shifted absorption and emission spectra compared with 5, in the near-IR region, and were evaluated as potential fluorescence imaging agents for EGFR overexpressing cells. SPR and docking investigations suggested that conjugate 8 bearing the LARLLT sequence binds to EGFR more effectively than 9 bearing the GYHWYGYTPQNVI peptide, in part due to the lower solubility of 9, and its tendency for aggregation at concentrations above 10 μM. Studies in human carcinoma HEp2 cells overexpressing EGFR demonstrated low dark and photo cytotoxicities for BODIPY 5 and the two peptide conjugates, and remarkably high cellular uptake for both conjugates 8 and 9, up to 90-fold compared with BODIPY 5 after 1 h. Fluorescence imaging studies in HEp2 cells revealed subcellular localization of the BODIPY-peptide conjugates mainly in the Golgi apparatus and the cell lysosomes. The low cytotoxicity of the new conjugates and their remarkably high uptake into EGFR overexpressing cells renders them promising imaging agents for cancers overexpressing EGFR.

Near-Infrared-Emitting BODIPY-trisDOTA(111) In as a Monomolecular Multifunctional Imaging Probe: From Synthesis to In Vivo Investigations

A new generation of monomolecular imaging probes (MOMIP) based on a distyryl-BODIPY (BODIPY=boron-dipyrromethene) coupled with three DOTA macrocycles has been prepared (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The MOMIP presents good fluorescence properties and is very stable in serum. The bimodal probe was conjugated to trastuzumab, and an optical in vivo study showed high accumulation of the imaging agent at the tumor site. (111) In radiometallation of the bioconjugate was performed in high radiochemical yield, highlighting the potential of this new BODIPY-chelators derivative as a bimodal imaging probe.

Circularly Polarized Luminescence from Helically Chiral N,N,O,O-Boron-Chelated Dipyrromethenes

Helically chiral N,N,O,O-boron chelated dipyrromethenes showed solution-phase circularly polarized luminescence (CPL) in the red region of the visible spectrum (λem (max) from 621 to 663 nm). The parent dipyrromethene is desymmetrised through O chelation of boron by the 3,5-ortho-phenolic substituents, inducing a helical chirality in the fluorophore. The combination of high luminescence dissymmetry factors (|glum | up to 4.7 ×10(-3) ) and fluorescence quantum yields (ΦF up to 0.73) gave exceptionally efficient circularly polarized red emission from these simple small organic fluorophores, enabling future application in CPL-based bioimaging.

BODIPY atropisomer interconversion, face discrimination, and superstructure appending

Atropisomerism and atropisomer interconversion in the BODIPY series are presented. It was used to synthesize a picket-fence-like BODIPY and to examine the BODIPY face discrimination. Pickets are aimed at preventing π-stacking of the BODIPY platform.

Bioorthogonal Fluorophore Linked DFO-Technology Enabling Facile Chelator Quantification and Multimodal Imaging of Antibodies

Herein we describe the development and application of a bioorthogonal fluorogenic chelate linker that can be used for facile creation of labeled imaging agents. The chelate linker is based on the trans-cyclooctene(TCO)-tetrazine(Tz) chemistry platform and incorporates deferoxamine (DFO) as a (89)Zr PET tracer and a BODIPY fluorophore for multimodal imaging. The rapid (<3 min) ligation between mAb-TCO and Tz-BODIPY-DFO chelator is monitored using fluorescence and allows for determination of labeling completion. Utilizing BODIPY as the linker between mAb and DFO facilitates in chelator quantification using spectrophotometry, allowing for an alternative to traditional methods (mass and isotope dilution assay). Radiolabeling with (89)Zr to form (89)Zr-DFO-BODIPY-trastuzumab was found to be quantitative after incubation at room temperature for 1 h (1.5 mCi/mg specific activity). The cell binding assay using HER2+ (BT474) and HER2- (BT20) cell lines showed significant binding to (89)Zr-DFO-BODIPY-trastuzumab (6.45 ± 1.87% in BT474 versus 1.47 ± 0.39% in BT20). In vivo PET imaging of mice bearing BT20 or BT474 xenografts with (89)Zr-DFO-BODIPY-trastuzumab showed high tumor conspicuity, and biodistribution confirmed excellent, specific probe uptake of 237.3 ± 14.5% ID/g in BT474 xenografts compared to low, nonspecific probe uptake in BT20 xenografts (16.4 ± 5.6% ID/g) 96 h p.i. . Ex vivo fluorescence (465ex/520em) of selected tissues confirmed superb target localization and persistence of the fluorescence of (89)Zr-DFO-BODIPY-trastuzumab. The described platform is universally adaptable for simple antibody labeling.