Texaphyrin: From molecule to nanoparticle

Synthesis and Characterization of Thallium-Texaphyrin Nanoparticles and Their Assessment as Potential Delivery Systems for Auger Electron-Emitting 201Tl to Cancer Cells

Thallium-201 is an Auger electron-emitting radionuclide with significant potential for targeted molecular radiotherapy of cancer. It stands out among other Auger electron emitters by releasing approximately 37 Auger and Coster-Kronig electrons per decay, which is one of the highest numbers in its category. It has also a convenient half-life of 73 h, a stable daughter product, established production methods, and demonstrated high in vitro radiotoxicity. However, its full potential in targeted radiotherapy remains unexplored, primarily due to the lack of available efficient chelators for [201Tl]Tl+ or [201Tl]Tl3+. This study aims to assess texaphyrin for macrocycle chelation of [201Tl]Tl3+. Texaphyrins are known for effective binding of trivalent metals with similar ionic radii, such as indium and gadolinium. Optimization of [201Tl]Tl+ to [201Tl]Tl3+ oxidation and subsequent chelation with texaphyrin-lipid conjugate were assessed using thin-layer chromatography. The formation and stability of nonradioactive Tl-texaphyrin-lipid complexes were confirmed by UV-Vis spectroscopy and ultrahigh performance liquid chromatography-mass spectrometry. [201Tl]Tl/Tl-texaphyrin-lipid nanoparticles (nanotexaphyrins) were assembled by using a microfluidic system, and their morphology and stability were evaluated by using dynamic light scattering and transmission electron microscopy. The uptake of these nanotexaphyrins in lung cancer and ovarian cancer cells was evaluated using both radioactive and nonradioactive methods. The conversion of [201Tl]Tl+ to [201Tl]Tl3+ in 0.25 M HCl achieved an average yield of 91.8 ± 3.1%, while the highest radiolabeling yield of the texaphyrin-lipid with [201Tl]Tl3+ was 25.5 ± 4.5%. Tl-texaphyrin-lipid conjugates were stable at room temperature for at least 72 h. These conjugates were successfully assembled into homogeneous nanotexaphyrins with an average hydrodynamic diameter of 147.4 ± 1.4 nm. Throughout a 72 h period, no changes in size or polydispersity of the synthesized nanoparticles were observed. [201Tl]Tl-nanotexaphyrins were synthesized with an average radiochemical purity of 77.4 ± 10.3% and a yield of 5.1 ± 4.4%. The release of [201Tl]Tl+ from [201Tl]Tl-nanotexaphyrins in phosphate-buffered saline exhibited a time- and temperature-dependent pattern, with a faster release observed at 37 °C than at room temperature. Additionally, the uptake of Tl-nanotexaphyrins and [201Tl]Tl-nanotexaphyrins in cancer cells was similar to that of unbound Tl+ and [201Tl]Tl+. This is the first time that texaphyrins have been investigated as chelators for radiothallium. Although [201Tl]Tl-nanotexaphyrins were found to be thermodynamically and kinetically unstable, we successfully synthesized stable texaphyrin-lipid complexes with natTl3+. This opens up opportunities for further refinements in the nanotexaphyrin-lipid structure to enhance [201Tl]Tl3+ stability and prevent its reduction to a 1+ oxidation state. Future research should consider further modifications to the texaphyrin structure or using texaphyrins without the lipid component.

Advancements of Porphyrin-Derived Nanomaterials for Antibacterial Photodynamic Therapy and Biofilm Eradication

The threat posed by antibiotic-resistant bacteria and the challenge of biofilm formation has highlighted the inadequacies of conventional antibacterial therapies, leading to increased interest in antibacterial photodynamic therapy (aPDT) in recent years. This approach offers advantages such as minimal invasiveness, low systemic toxicity, and notable effectiveness against drug-resistant bacterial strains. Porphyrins and their derivatives, known for their high molar extinction coefficients and singlet oxygen quantum yields, have emerged as crucial photosensitizers in aPDT. However, their practical application is hindered by challenges such as poor water solubility and aggregation-induced quenching. To address these limitations, extensive research has focused on the development of porphyrin-based nanomaterials for aPDT, enhancing the efficacy of photodynamic sterilization and broadening the range of antimicrobial activity. This review provides an overview of various porphyrin-based nanomaterials utilized in aPDT and biofilm eradication in recent years, including porphyrin-loaded inorganic nanoparticles, porphyrin-based polymer assemblies, supramolecular assemblies, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs). Additionally, insights into the prospects of aPDT is offered, highlighting its potential for practical implementation.

Myoglobin-loaded gadolinium nanotexaphyrins for oxygen synergy and imaging-guided radiosensitization therapy

Gadolinium (Gd3+)-coordinated texaphyrin (Gd-Tex) is a promising radiosensitizer that entered clinical trials, but temporarily fails largely due to insufficient radiosensitization efficacy. Little attention has been given to using nanovesicles to improve its efficacy. Herein, Gd-Tex is transformed into building blocks "Gd-Tex-lipids" to self-assemble nanovesicles called Gd-nanotexaphyrins (Gd-NTs), realizing high density packing of Gd-Tex in a single nanovesicle and achieving high Gd-Tex accumulation in tumors. To elucidate the impact of O2 concentration on Gd-Tex radiosensitization, myoglobin (Mb) is loaded into Gd-NTs (Mb@Gd-NTs), resulting in efficient relief of tumor hypoxia and significant enhancement of Gd-Tex radiosensitization, eventually inducing the obvious long-term antitumor immune memory to inhibit tumor recurrence. In addition to Gd3+, the versatile Mb@Gd-NTs can also chelate 177Lu3+ (Mb@177Lu/Gd-NTs), enabling SPECT/MRI dual-modality imaging for accurately monitoring drug delivery in real-time. This "one-for-all" nanoplatform with the capability of chelating various trivalent metal ions exhibits broad clinical application prospects in imaging-guided radiosensitization therapy.

Radiobiological impact of gadolinium neutron capture from proton therapy and alternative neutron sources using TOPAS-nBio

PURPOSE

A multi-scale investigation of the biological properties of gadolinium neutron capture (GdNC) therapy with applications in particle therapy is conducted using the TOPAS Monte Carlo (MC) simulation code. The simulation results are used to quantify the amount of gadolinium dose enhancement produced as a result of the secondary neutron production from proton therapy scaled by measured data.

MATERIALS AND METHODS

MC modeling was performed using the radiobiology extension TOol for PArticle Simulation TOPAS-nBio MC simulation code to study the radiobiological effects produced from GdNC on a segment of DNA, a spherical cellular model, and from the modeling of previous experimental measurements. The average RBE values were calculated from two methods, microdosimetric kinematic (MK) and biological weighting r(y) within a 2 nm DNA segment for GdNC. The single-strand breaks (SSBs) and double-strand breaks (DSBs) were calculated from within the nucleus of a 20 µm diameter, spherical cell model. From a previous experimental proton therapy measurement using a spread-out Bragg peak (SOBP) of 4.5-9.5 cm and a delivered absorbed dose of 10.4 Gy, the amount of Gd neutron captures was calculated and used to quantify the amount of GdNC absolute dose from particle therapy.

RESULTS

The average RBE from microdosimetric kinematic and biological weighting was 1.35, and 1.70 for a 10% cell survival on HSG cell-line and weighting function data from early intestinal tolerance of mice. From a central isotropic GdNC source, the energy deposition is found to decrease from roughly 2.7 eV per capture down to approximately 0.01 eV per capture, a drop of two orders of magnitude within 50 nm. This result suggests that Gd needs to be close to the DNA (within 10-20 nm) in order for neutron capture to induce a significant dose enhancement due to the short-range electrons emitted after Gd neutron capture. Within a spherical cell model, the SSBs, and DSBs were determined to be 39 and 1.5 per neutron capture, respectively. From the total neutron captures produced from an experimental proton therapy measurement on a 3000 PPM Gd solution, an insignificant absolute Gd dose enhancement was quantified to be 5.4 × 10^-6  Gy per Gy of administered proton dose.

CONCLUSION

From this study and literature review, the production of secondary thermal neutrons from proton therapy is determined to be a limiting factor and unlikely to produce a clinically useful dose enhancement for secondary neutron capture therapy. Moreover, alternative neutron sources, such as, a compact deuterium-tritium (D-T) neutron generator, a "high yield" deuterium-deuterium (D-D) generator, or an industrial strength (100 mg) ²⁵² Cf source were investigated, with the ²⁵² Cf source the most likely to be capable of producing enough neutrons for 1 Gy of localized GdNC absolute dose within a reasonable treatment time.

Manganese(II) Texaphyrin: A Paramagnetic Photoacoustic Contrast Agent Activated by Near-IR Light

The NIR absorptivity of the metallotexaphyrin derivatives MMn, MGd, and MLu for photoacoustic (PA)-based imaging is explored in this study. All three complexes demonstrated excellent photostabilities; however, MMn provided the greatest PA signal intensities in both doubly distilled water and RAW 264.7 cells. In vivo experiments using a prostate tumor mouse model were performed. MMn displayed no adverse toxicity to major organs as inferred from hematoxylin and eosin (H&E) staining and cell blood count testing. MMn also allowed for PA-based imaging of tumors with excellent in vivo stability to provide 3D tumor diagnostic information. Based on the present findings and previous magnetic resonance imaging (MRI) studies, we believe MMn may have a role to play either as a stand-alone PA contrast agent or as a single molecule dual modal (PA and MR) imaging agent for tumor diagnosis.

Porphyrinoid Drug Conjugates

Drawing inspiration from nature today remains a time-honored means of discovering the therapies of tomorrow. Porphyrins, the so-called "pigments of life" have played a key role in this effort due to their diverse and unique properties. They have seen use in a number of medically relevant applications, including the development of so-called drug conjugates wherein functionalization with other entities is used to improve efficacy while minimizing dose limiting side effects. In this Perspective, we highlight opportunities associated with newer, completely synthetic analogs of porphyrins, commonly referred to as porphyrinoids, as the basis for preparing drug conjugates. Many of the resulting systems show improved medicinal or site-localizing properties. As befits a Perspective of this type, our efforts to develop cancer-targeting, platinum-containing conjugates based on texaphyrins (a class of so-called "expanded porphyrins") will receive particular emphasis; however, the promise inherent in this readily generalizable approach will also be illustrated briefly using two other common porphyrin analogs, namely the corroles (a "contracted porphyrin") and porphycene (an "isomeric porphyrin").