MnPcS4: a new MRI contrast enhancing agent for tumor localisation in mice

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.

CASSCF-based explicit ligand field models clarify the ground state electronic structures of transition metal phthalocyanines (MPc; M = Mn, Fe, Co, Ni, Cu, Zn)

Multireference electronic structure methods are used to assign ground state electronic configurations for a series of metallophthalocyanines. Ligand orbital occupancies remain constant across the period and are consistent with a formal 2– charge on the ligand. The d electron configurations of some metallophthalocyanines are straightforward and can be unambiguously assigned, (dxy)2(dxz,dyz)2,2( [Formula: see text])2([Formula: see text])n, with n = 2, 1, 0, respectively, for ZnPc, CuPc, and NiPc. Controversies over ground state electronic structure assignments for other metallophthalocyanines arise due to multiple complicating factors: accidental near-degeneracies, environmental effects, and different ligand field models used in interpreting experimental spectra. We demonstrate that explicit ligand field models provide more reliable and consistent interpretations of experimental data than implicit, parameterized alternatives. On this basis, we assign gas-phase electronic ground states for MnPc, (dxy)2(dxz,dyz)1,1([Formula: see text])1 and CoPc, (dxy)2(dxz,dyz)2,2([Formula: see text])1, and show that the ground state of FePc cannot be resolved to a single state, with two near-degenerate states that are likely spin-orbit coupled: (dxy)2(dxz,dyz)1,1( [Formula: see text])2 and (dxy)2(dxz,dyz)2,1([Formula: see text])1. Remaining differences between computational predictions and experimental observations are small and may be ascribed primarily to environmental effects but are also partly due to incomplete modelling of electron correlation.

Recent applications of phthalocyanines and naphthalocyanines for imaging and therapy

With high extinction coefficients and long absorption wavelengths in the near infrared region, phthalocyanines (Pcs) and naphthalocyanines (Ncs) are well-suited for optical imaging and phototherapies in biological tissues. Pcs and Ncs have been used in a range of theranostic applications. Peripheral and axial substituents can be introduced to Pcs and Ncs for chemical modification. Seamless metal chelation of Pcs or Ncs can expand their possibilities as medical therapeutic and imaging agents. Nanoparticulate approaches enable unique ways to deliver Pcs and Ncs to target tissues and improve their solubility, biocompatibility, biodistribution and stability. Herein, we highlight some recent Pc or Nc nanoscale systems for theranostic applications. WIREs Nanomed Nanobiotechnol 2017, 9:e1420. doi: 10.1002/wnan.1420 For further resources related to this article, please visit the WIREs website.

Gadolinium complexes of chlorin derivatives applicable for MRI contrast agents and PDT

One of the ultimate goals of contrast agent (CA) research in magnetic resonance imaging (MRI) is to identify tumor-seeking materials. Thus, cancer diagnosis by Photodynamic Therapy (PDT) using photosensitizer will be one of the best tools for cancer research. By linking paramagnetic gadolinium [ Gd(III) ] ion to tumor selective chlorin-based photosensitizers, the cancer cell can be easily detected with high sensitivity and cured by appropriate laser irradiation simultaneously. The synthesis of monomer and dimer chlorin derivatives, such as pyropheophorbide-a and purpurin systems, conjugated with Gd(III) diethylenetriaminepentaacetic acid (DTPA) by means of diethylene-triaminepentaacetic dianhydride (caDTPA) for the evaluation as CAs for MRI was described in the present study.

Current issues and perspectives in small rodent magnetic resonance imaging using clinical MRI scanners

Small rodents such as mice and rats are frequently used in animal experiments for several reasons. In the past, animal experiments were frequently associated with invasive methods and groups of animals had to be killed to perform longitudinal studies. Today's modern imaging techniques such as magnetic resonance imaging (MRI) allow non-invasive longitudinal monitoring of multiple parameters. Although only a few institutions have access to dedicated small animal MR scanners, most institutions carrying out animal experiments have access to clinical MR scanners. Technological advances and the increasing field strength of clinical scanners make MRI a broadly available and viable technique in preclinical in vivo research. This review provides an overview of current concepts, limitations, and recent studies dealing with small animal imaging using clinical MR scanners.

Necrosis Avid Contrast Agents

Two categories of necrosis-avid contrast agents (NACAs), namely porphyrin- and nonporphyrin-based complexes, have thus far been discovered as necrosis-targeting markers for noninvasive magnetic resonance imaging (MRI) identification of acute myocardial infarction, assessment of tissue or organ viability, and therapeutic evaluation after interventional therapies. In addition to necrosis labeling, other less-specific functions, such as first-pass perfusion, blood pool contrast effect, hepatobiliary contrast enhancement (CE), adrenal and spleen CE, and renal functional imaging, also are demonstrated with NACAs. Despite various investigations with a collection of clues in favor of certain hypotheses, the mechanisms of such a unique targetability for NACAs still remain to be elucidated. However, a few things have become clear that porphyrin-like structures are not necessary for necrosis avidity and the albumin binding is not the supposed driving force but only a parallel nonspecific feature shared by both NACAs and non-NACA substances. Although the research and development of NACAs still remain in preclinical stage at a relatively small scale, their significance rests upon striking enhancement effects, which may warrant their eventual versatile clinical applications. The present review article is intended to summarize the cumulated facts about the evolving research on this topic, to demonstrate experimental observations for better understanding of the mechanisms, to trigger broader public interests and more intensive research activities, and to advocate, toward both academics and industries, further promotion of preclinical and clinical development of this unique and promising class of contrast agents.

Challenges in small animal noninvasive imaging

The current status and challenges of small animal non-invasive imaging is briefly reviewed. The advantages of non-invasive studies on living animals versus post-mortem studies are evaluated. An argument is advanced that even in post-mortem situations, non-invasive imaging may play an important role in efficiently characterizing small animal phenotypes as well as pathology. Issues of data interpretation under anesthetized conditions in live animal studies are also reviewed. The five imaging technologies covered include CT, PET, ultrasound, MRI and optical imaging. The structural and physiological information content of these different modalities is reviewed along with the ability of these techniques to scale down for use in small mammals such as mice and rats. In general, it was found that most of these technologies scale favorably to the study of small mammals, generally providing more physiological information than when used on the larger human scale. This suggests that these types of small mammal imaging capabilities will play a very significant role in the full utilization of these important animal models in biomedical research.

Synthesis and estrogen receptor binding affinity of a porphyrin-estradiol conjugate for targeted photodynamic therapy of cancer

A tetraphenylporphyrin-C11-beta-estradiol conjugate has been synthesized. Competitive binding assay of the conjugate with estrogen receptor (ER)-ligand-binding domain showed that the conjugate binds specifically to the protein with high affinity. Potential use of this conjugate to selectively deliver cytotoxic porphyrins to ER-positive cells in various carcinomas is discussed.

Enhancing the relaxivity of paramagnetic coordination complexes through the optimization of the molecular electrostatic potential

The low relaxivity of paramagnetic coordination complexes limits their use as contrast agents in magnetic resonance imaging (MRI). To address this problem, we study the relationship between the molecular structure of these complexes and their relaxivity. While others have investigated the vibrational modes as molecular determinants of the electronic spin relaxation time, we focus on the analysis of the molecular electrostatic potential (MEP) of the paramagnetic coordination complex. Electrostatic forces dominate the interaction between the coordination complex and water. Hence, in addition to steric forces, the molecular electrostatic potential should be a determinant of the lifetime of the water-metal link (tm), the internuclear distance between the water hydrogens and the metal (R), and the number of water molecules attached to the metal in the inner and outer spheres of coordination. We compute the molecular electrostatic potential for a series of model metalloporphyrins because their physical and biologic properties are well known, and they are putative magnetic resonance imaging contrast agents with affinity to neoplastic tissue. Replacing the sulfonato groups in MnTPPS4 with carboxylate groups in the ortho position of the phenyl rings attached to the meso carbons results in an electrostatic focusing field that should reduce R and increase tm. Similar substitutions involving polar groups, including one modeled after a well-known picket-fence porphyrin, are not strong enough to generate a focusing field. Instead, these polar groups should modulate the water-metal interactions through steric interactions. Molecular dynamic simulations show a large outer sphere of coordination around the paramagnet that extends almost three times the distance of the inner sphere of coordination.

Magnetic resonance imaging-histomorphologic correlation studies on paramagnetic metalloporphyrins in rat models of necrosis

RATIONALE AND OBJECTIVES

The authors intended to confirm previous findings that paramagnetic porphyrins are avid only for intratumoral nonviable tissues, but not for viable tumor cells, and to test the hypothesis that necrosis, regardless of location and origin, can be visualized by metalloporphyrin enhanced magnetic resonance imaging (MRI).

METHODS

Intravenous administrations of gadolinium mesoporphyrin (Gd-MP), manganese tetraphenylporphyrin (Mn-TPP), manganese methylpyrroporphyrin-gadopentetate dimeglumine complex (Mn-MPP-Gd) and manganese tetra(4-sulfonatophenyl)porphyrin (MnTPPS4) at 0.05 mmol/kg were compared with those of gadopentetate dimeglumine (Gd-DTPA) at 0.1 mmol/kg in 38 rats with cholestatic liver necrosis, alcohol- and laser-induced coagulation necrosis in liver, and skeletal muscle, reperfused hepatic infarction, and segmental renal infarction. T1-weighted spin echo MRI (TR/TE = 300/15 mseconds) was acquired before and as long as 48 hours after injection, matched with histologic findings, and correlated with Gd/ Mn tissue content measurements.

RESULTS

Both Gd-DTPA and the four metalloporphyrins initially caused a similar nonspecific negative contrast enhancement in the necrosis. However, a strong and persisting positive enhancement (necrosis-to-normal contrast ratio ranging from 1.5 to 2.0) developed only with metalloporphyrins in all types of necrosis. In liver and kidney, Gd and Mn concentrations at 24 hours were comparable in necrotic and normal tissues. In muscle, the concentrations were more than eight times higher in necrotic than in normal tissue.

CONCLUSIONS

The implied affinity of metalloporphyrins for necrosis with presumably increased relaxivity suggests a possible mode of targetability for MRI contrast media that may elicit novel applications.

Toxicity of subacute intravenous manganese chloride administration in beagle dogs

Manganese (Mn), a naturally occurring essential trace element, is currently being used as a metal complex for pharmaceutical and magnetic resonance imaging agents. Despite its popularity in these practices, minimal attention has focused on possible toxicity of released free Mn ions, which could occur if these agents decomplexed. There is especially limited information available regarding acute toxicity of Mn in dogs. In this study, we performed an in-depth evaluation of acute toxicologic potential of manganese chloride (MnCl2) when administered as a 4-hr/day intravenous infusion to male beagle dogs. The dose of MnCl2 used (16 mg/kg/day) was equivalent to approximately 3-5 times the daily dose of Mn typically administered in some of the Mn-complexed agents. All routine toxicologic endpoints were evaluated, including cardiovascular parameters. This dosing regimen resulted in the death or moribund sacrifice of all the animals within 4 days of initiation of treatment. Clinical evidence of toxicity included loss of appetite, reduction in blood pressure with reflex tachycardia, and a marked increase in liver enzymes, beginning with the first dose and increasing in severity with successive doses. Gross and histopathologic evaluations confirmed severe hepatotoxicity, which was characterized by massive hepatocellular necrosis, periportal hemorrhages, and mild biliary epithelial hyperplasia. These results indicate that acute treatment of beagle dogs with MnCl2 causes severe hepatotoxicity and hypotension with reflex tachycardia and suggest that dogs are very sensitive to toxic effects of Mn.