Structural Investigations, Cellular Imaging, and Radiolabeling of Neutral, Polycationic, and Polyanionic Functional Metalloporphyrin Conjugates

Abstract

Over the past decade, porphyrin derivatives have emerged as invaluable synthetic building blocks and theranostic kits for the delivery of cellular fluorescence imaging and photodynamic therapy. Tetraphenylporphyrin (TPP), its metal complexes, and related derivatives have been investigated for their use as dyes in histology and as components of multimodal imaging probes. The photophysical properties of porphyrin–metal complexes featuring radiometals have been a focus of our attention for the realization of fluorescence imaging probes coupled with radioimaging capabilities and therapeutic potential having “true” theranostic promise. We report hereby on the synthesis, radiochemistry, structural investigations, and preliminary in vitro and in vivo uptake studies on a range of functionalized porphyrin-based derivatives. In pursuit of developing new porphyrin-based probes for multimodality imaging applications, we report new functionalized neutral, polycationic, and polyanionic porphyrins incorporating nitroimidazole and sulfonamide moieties, which were used as targeting groups to improve the notoriously poor pharmacokinetics of porphyrin tags. The resulting functional metalloporphyrin species were stable under serum challenges and the nitroimidazole and sulfonamide derivatives remained fluorescent, allowing in vitro confocal studies and visualization of the lysosomal uptake in a gallium­(III) sulfonamide derivative. The molecular structures of selected porphyrin derivatives were determined by single crystal X-ray diffraction using synchrotron radiation. We also investigated the nature of the emission/excitation behavior of model functional porphyrins using in silico approaches such as TD DFT in simple solvation models. The conjugation of porphyrins with the [7-13] and [7-14] fragments of bombesin was also achieved, to provide targeting of the gastrin releasing peptide receptor (GRPR). Depending on the metal, probe conjugates of relevance for single photon emission computed tomography (SPECT) or positron emission tomography (PET) probes have been designed and tested hereby, using TPP and related functional free base porphyrins as the bifunctional chelator synthetic scaffold and 111In­[In] or 68Ga­[Ga], respectively, as the central metal ions. Interestingly, for simple porphyrin conjugates good radiochemical incorporation was obtained for both radiometals, but the presence of peptides significantly diminished the radio-incorporation yields. Although the gallium-68 radiochemistry of the bombesin conjugates did not show radiochemical incorporation suitable for in vivo studies, likely because the presence of the peptide changed the behavior of the TPP-NH2 synthon taken alone, the optical imaging assays indicated that the conjugated peptide tags do mediate uptake of the porphyrin units into cells.

Nano-Theranostics for the Sensing, Imaging and Therapy of Prostate Cancers

“…In prostate cancer, their expression rates are in the range between 63–100% ( Beer et al, 2012 ) and moreover, they upregulate promoters of angiogenesis, essential for the metastasis of the cancerous cells ( Elshafae et al, 2016 ). In recent years, GRPRs have increasingly received a lot of interest for being targeted to treat and image prostate cancer ( Ciaffaglione et al, 2021 ). These receptors have shown a high affinity to two natural peptides: GRP consists of 27 amino acids, found in mammals, which is homologous to the amphibian bombesin (BBN) peptide from the species Bombina ( Anastasi et al, 1971 ; Spindel, 1986 ; Lee et al, 1994 ), a 14 amino acids chain ( Schroeder et al, 2011 ; Beer et al, 2012 ).…”

Porphyrins as Chelating Agents for Molecular Imaging in Nuclear Medicine

“…An extended study of neutral, polycationic, and polyanionic metalloporphyrins labeled with 68 Ga showed extremal flexibility for modifications of the porphyrin cores [ 84 ]. The addition of nitroimidazole or sulfonamide groups, which were used as vectors, improved the pharmacokinetics of porphyrin tags and the stability in serum.…”

Synthesis and Applications of Porphyrin-Biomacromolecule Conjugates

“…Hence, a likely cause for the poor uptake many be due to low solubility of the compound ( inter alia , as a result of the long aliphatic linker). ( Ciaffaglione et al, 2021 ).…”

“…The reaction between amines on peptides and activated carboxylic acids on porphyrin derivatives have been extensively investigated ( Tomé et al, 2004 ; Gabriel et al, 2007 ; Giuntini et al, 2011 ; Shi et al, 2011 ; Dosselli et al, 2013 ; Orosz et al, 2013 ; Li et al, 2015 ; Jiang et al, 2016 ; Chen et al, 2018 ; Liu et al, 2018 ; Ciaffaglione et al, 2021 ). For example, in 2011, Mező et al conjugated one and two tetrapeptides to tri-cationic meso -tri (4-N-methylpyridyl)-mono- (4-carboxyphenyl) porphyrin (TMPCP) and bi-cationic meso -5,10-bis(4-N-methylpyridyl)-15,20-di- (4-carboxyphenyl)porphyrin (BMPCP) respectively.…”