Evaluation of a nitroxyl fatty acid as liver contrast agent for magnetic resonance imaging

Electron paramagnetic resonance: a powerful tool to support magnetic resonance imaging research

The purpose of this paper is to describe some of the areas where electron paramagnetic resonance (EPR) has provided unique information to MRI developments. The field of application mainly encompasses the EPR characterization of MRI paramagnetic contrast agents (gadolinium and manganese chelates, nitroxides) and superparamagnetic agents (iron oxide particles). The combined use of MRI and EPR has also been used to qualify or disqualify sources of contrast in MRI. Illustrative examples are presented with attempts to qualify oxygen sensitive contrast (i.e. T1 - and T2 *-based methods), redox status or melanin content in tissues. Other areas are likely to benefit from the combined EPR/MRI approach, namely cell tracking studies. Finally, the combination of EPR and MRI studies on the same models provides invaluable data regarding tissue oxygenation, hemodynamics and energetics. Our description will be illustrative rather than exhaustive to give to the readers a flavour of 'what EPR can do for MRI'.

The evaluation of new and isotopically labeled isoindoline nitroxides and an azaphenalene nitroxide for EPR oximetry

Isoindoline nitroxides are potentially useful probes for viable biological systems, exhibiting low cytotoxicity, moderate rates of biological reduction and favorable Electron Paramagnetic Resonance (EPR) characteristics. We have evaluated the anionic (5-carboxy-1,1,3,3-tetramethylisoindolin-2-yloxyl; CTMIO), cationic (5-(N,N,N-trimethylammonio)-1,1,3,3-tetramethylisoindolin-2-yloxyl iodide, QATMIO) and neutral (1,1,3,3-tetramethylisoindolin-2-yloxyl; TMIO) nitroxides and their isotopically labeled analogs ((2)H(12)- and/or (2)H(12)-(15)N-labeled) as potential EPR oximetry probes. An active ester analogue of CTMIO, designed to localize intracellularly, and the azaphenalene nitroxide 1,1,3,3-tetramethyl-2,3-dihydro-2-azaphenalen-2-yloxyl (TMAO) were also studied. While the EPR spectra of the unlabeled nitroxides exhibit high sensitivity to O(2) concentration, deuteration resulted in a loss of superhyperfine features and a subsequent reduction in O(2) sensitivity. Labeling the nitroxides with (15)N increased the signal intensity and this may be useful in decreasing the detection limits for in vivo measurements. The active ester nitroxide showed approximately 6% intracellular localization and low cytotoxicity. The EPR spectra of TMAO nitroxide indicated an increased rigidity in the nitroxide ring, due to dibenzo-annulation.

Nano-emulsions of fluorinated trityl radicals as sensors for EPR oximetry

This article reports the development and evaluation of two nano-emulsions (F45T-03/HFB and F15T-03/PFOB) containing fluorinated trityl radicals dissolved in perfluorocarbons. Preparation with a high-pressure homogenizer conferred sub-micronic size to both nano-emulsions. In vitro and in vivo EPR spectroscopy showed that the nano-emulsions had much greater oxygen sensitivity than the hydrophilic trityl, CT-03. In vivo experiments in rodents confirmed the ability of the nano-emulsions to follow the changes in oxygen concentration after induced ischemia. Histological evaluation of the tissue injected with the nano-emulsions revealed some acute toxicity for the F45T-03/HFB nano-emulsion but none for the F15T-03/PFOB nano-emulsion. These new formulations should be considered for further EPR oximetry experiments in pathophysiological situations where subtle changes in tissue oxygenation are expected.

Assessment of liver phagocytic activity using EPR spectrometry and imaging

The aim of the present study was to evaluate the usefulness of electron paramagnetic resonance (EPR) spectroscopy and imaging in assessing the phagocytic activity of the liver after administration of India ink. We conducted experiments on livers from control rodents and from rodents in which the Kupffer cell population had been depleted by pretreatment with gadolinium chloride. The EPR signal intensity recorded in liver homogenates was about two times lower in GdCl(3) treated rats than in control rats. EPR imaging carried out on precision-cut liver slices indicated a good correlation between the depletion of Kupffer cells and the EPR signal intensity.

Bioreversible derivatives of phenol. 1. The role of human serum albumin as related to the stability and binding properties of carbonate esters with fatty acid-like structures in aqueous solution and biological media

With the overall objective of assessing the potential of utilizing plasma protein binding interactions in combination with the prodrug approach for improving the pharmacokinetics of drug substances, a series of model carbonate ester prodrugs of phenol, encompassing derivatives with fatty acid-like structures, were characterized in vitro. Stability of the derivatives was studied in aqueous solution, human serum albumin solution, human plasma, and rat liver homogenate at 37 degrees C. Stability of the derivatives in aqueous solution varied widely, with half-lives ranging from 31 to 1.7 x 10(4) min at pH 7.4 and 37 degrees C. The carbonate esters were subject to catalysis by plasma esterases except for the t-butyl and acetic acid derivatives, which were stabilized in both human plasma and human serum albumin solutions relative to buffer. In most cases, however, hydrolysis was accelerated in the presence of human serum albumin indicating that the derivatives interacted with the protein, a finding which was confirmed using the p-nitrophenyl acetate kinetic assay. Different human serum albumin binding properties of the phenol model prodrugs with fatty acid-like structure and neutral carbonate esters were observed. In the context of utilizing plasma protein binding in combination with the prodrug approach for optimizing drug pharmacokinetics, the esterase-like properties of human serum albumin towards the carbonate esters potentially allowing the protein to act as a catalyst of parent compound regenerations is interesting.

Albumin-bound MRI contrast agents: the dilemma of the rotational correlation time

Human serum albumin (HSA) binds numerous molecules, among which are suitably designed MRI contrast agents. The rotational tumbling of the protein is thus one of the parameters likely to affect the in vivo relaxivity of these agents. Literature unveils discrepancies about the value of the rotational correlation time (tau(R)) of HSA. In the present work, the tau(R) of this protein has been determined by studying the deuterium relaxation rate of small molecules known for their strong binding to HSA (warfarin and 4-hydroxycoumarin). Values of approx. 20-22 ns are obtained at 310 K in a 4% HSA solution and are in good agreement with the theoretical predictions.

Rationale and applications of lipids as prodrug carriers

Lipidic prodrugs, also called drug-lipid conjugates, have the drug covalently bound to a lipid moiety, such as a fatty acid, a diglyceride or a phosphoglyceride. Drug-lipid conjugates have been prepared in order to take advantage of the metabolic pathways of lipid biochemistry, allowing organs to be targeted or delivery problems to be overcome. Endogenous proteins taking up fatty acids from the blood stream can be targeted to deliver the drug to the heart or liver. For glycerides, the major advantage is the modification of the pharmacokinetic behavior of the drug. In this case, one or two fatty acids of a triglyceride are replaced by a carboxylic drug. Lipid conjugates exhibit some physico-chemical and absorption characteristics similar to those of natural lipids. Non-steroidal, anti-inflammatory drugs such as acetylsalicylic acid, indomethacin, naproxen and ibuprofen were linked covalently to glycerides to reduce their ulcerogenicity. Mimicking the absorption process of dietary fats, lipid conjugates have also been used to target the lymphatic route (e.g., L-Dopa, melphalan, chlorambucil and GABA). Based on their lipophilicity and resemblance to lipids in biological membranes, lipid conjugates of phenytoin were prepared to increase intestinal absorption, whereas glycerides or modified glycerides of L-Dopa, glycine, GABA, thiorphan and N-benzyloxycarbonylglycine were designed to promote brain penetration. In phospholipid conjugates, antiviral and antineoplasic nucleosides were attached to the phosphate moiety. After presenting the biochemical pathways of lipids, the review discusses the advantages and drawbacks of lipidic prodrugs, keeping in mind the potential pharmacological activity of the fatty acid itself.

Checking the integrity of albumin after pasteurisation: a fluorescent assay for quantitating monomeric albumin using anthroyloxy-fatty acids analogs

This study presents a new assay based on the binding of anthroyloxy-stearates to albumin and the change in the fluorescence intensity when these fluorescent probes are bound to the protein. This assay permits the quantification of the albumin monomers present in a mixture of monomers and polymers, and allows to check the integrity of albumin after pasteurisation.

The design of spin probes for electron magnetic resonance spectroscopy and imaging

The problems that have to be addressed in designing suitable spin probes are manifold. In this article it will be shown that some the properties the probes need to possess are (i) a simple EMR spectrum (in some types of imaging or oximetry a single line is desirable), (ii) sharp lines with no unresolved splittings, to provide maximum sensitivity to detection, (iii) chemical, metabolic and thermal stability, (iv) easy possible synthesis to permit substitution with isotopes, prosthetic groups or groups to control the partition coefficients in lipid/water systems, and (v) electronic anisotropy, if rotational correlation times are to be determined, for example for the measurement of local viscosity.

In vivo EPR: an effective new tool for studying pathophysiology, physiology and pharmacology

The development of spectrometers working at lower frequencies with improved resonators now permits the routine use of non-invasive EPR spectroscopy in vivo. The capabilities of EPR spectra to reflect environmental conditions, combined with the use of paramagnetic materials as selective non-toxic labels, has led to increasingly widespread and productive applications of the technique to complex problems involving physiology, pharmacology and pathophysiology. Some of the especially promising applications in which EPR techniques uniquely appear to provide valuable information are illustrated, including the measurement of oxygen and oxygen gradients, monitoring of the metabolism of xenobiotics, monitoring pharmacokinetics of drugs, measurement of perfusion, measurement of pH, recognition and labeling of receptors, and characterization of drug releasing systems.

Targeting of drug to the hepatocytes by fatty acids. Influence of the carrier (albumin or galactosylated albumin) on the fate of the fatty acids and their analogs

PURPOSE

The aim of this study was to evaluate the potential of fatty acids as shuttles to deliver xenobiotic inside the hepatocytes as well as to study the mechanism of incorporation into isolated hepatocytes when bound to native albumin or galactosylated albumin. Theoretically, they can enter into the hepatocytes after recognition of the Fatty Acid Binding Protein (FABPPM), or remain bound to galactosylated proteins and enter into these cells by a process known as receptor mediated endocytosis after selective recognition of the asialoglycoprotein receptor (ASGPR).

METHODS

We synthesized a 3H-benzoyl adduct of lauric acid (BLA) (benzoyl adduct chosen to mimic any low molecular weight drug or contrast agent), and compared the behavior of BLA with oleic acid for their binding properties to carrier-proteins and the uptake mechanism by isolated hepatocytes.

RESULTS

No significant difference was found in the binding properties of BLA for albumin and galactosylated albumin. The incorporation into the hepatocytes was found essentially depending on the FABPPM transport system whenever BLA was bound to albumin or to galactosylated albumin in the incubation medium: indeed, the transport was inhibited by phloretin (inhibitor of sodium dependent transport), increased when the free part of BLA was higher, and BLA was recovered in the cytosolic fraction of the hepatocytes.

CONCLUSIONS

This study showed the convenience in using fatty acids as drug carriers possessing tropism for the hepatocytes.

Use of nitroxides for assessing perfusion, oxygenation, and viability of tissues: in vivo EPR and MRI studies

Relative perfusion, pO2, and bioreduction were measured simultaneously in vivo in tissues in mice by following changes in the intensity and shape of the EPR spectra of nitroxides injected directly into the tissues, using low frequency (1.1 GHz) localized EPR spectroscopy. Using normal and blood flow restricted gastrocnemius muscles it was shown that the decrease of the EPR signals of the nitroxides in tissues was due principally to perfusion, which redistributed the nitroxides. Changes in pO2 were reflected by changes of the linewidth; only a perdeuterated nitroxide with a narrow line was an adequate indicator for this parameter. This technique was applied experimental murine tumors (MTG-B and RIF-1) to determine the perfusion and pO2 in these relatively hypoxic model tumor systems. Using the paramagnetic properties of the nitroxides to enhance T1-weighted MR images, heterogeneity in perfusion in individual tumors was demonstrated

The uptake of Mn-DPDP by hepatocytes is not mediated by the facilitated transport of pyridoxine

Manganese-dipyridoxal diphosphate (Mn-DPDP) is a liver-selective contrast agent selectively taken up by the hepatocytes. Because of the analogy of structure with pyridoxine (vitamin B6), it was previously suggested that this compound can be selectively taken up by the facilitated transport of vitamers B6. To understand the uptake mechanism, an in vivo binding study was performed based on a competition between 54Mn-DPDP and pyridoxine on the one hand, and Mn-DPDP and [3H]pyridoxine on the other. We found that the [3H]pyridoxine levels in the liver were not significantly different 5 min after intravenous administration of several doses of Mn-DPDP (5 nmol/kg to 50 mumol/kg): 5.0 +/- 0.3% of the injected dose/g tissue. The content of 54Mn (administered as 54Mn-DPDP) in the liver was not affected by a saturation dose of pyridoxine (1 mmol/kg) and was found to be constant (+/- 10% of the injected dose/g tissue) for 60 min. These experiments showed that the uptake of Mn-DPDP is not mediated by the transporter of pyridoxine.

The effects of pasteurisation on albumin: an EPR binding assay for polymeric albumin

The ability of a nitroxyl fatty acid (NFA) to bind specifically to albumin is abolished when, in the absence of stabilizers, a 4% solution of this protein is heated above a critical temperature of 60 degrees C. This treatment leads to the formation of "albumin polymers" as classically evidenced by GPC. Since the bound fraction is evidenced in EPR spectroscopy by a large anisotropic component, the presence of this anisotropy can be used in the assessment of the quality of the pharmaceutical preparations of albumin, which are usually pasteurized in order to inactivate viruses. Moreover, in sharp contrast with the behavior of albumin dispersions, lyophilised albumin subjected to heat treatment at 70 degrees C for 24 h left the protein untouched regarding its NFA binding and GPC profile.

Simultaneous 280 MHz EPR imaging of rat organs during nitroxide free radical clearance

A radio frequency (RF) (280 MHz) electron paramagnetic resonance (EPR) spectroscopy and imaging apparatus has been used to localize a pyrrolidine nitroxide free radical in the rat abdomen and thorax. The nitroxide 2,2.5.5,-tetramethylpyrrolidine-1-oxyl-3- carboxylic acid (PCA) had a whole body monoexponential decay with half-life of 13.3 +/- 0.7 (n = 4), 19.4 +/- 0.2 (n = 3), and 23 +/- 2 (n = 6) min for 1, 2, and 3 mmol/kg PCA, respectively. Up to seven one-dimensional longitudinal projections were collected on six rats in the presence of a 8 mT/m field gradient. With an injection dose of 3 mmol/kg, PCA half-lives were 19 +/- 1, 17 +/- 2, and 22 +/- 2 min (n = 6) in the lower abdomen, in the liver, and in the thorax, respectively. Thorax half-life was significantly longer than liver half-life. Sequential two-dimensional images of PCA distribution in a plane longitudinal to the rat body were obtained from eight spectra in the presence of a gradient of 12 mT/m (acquisition time 5 min; spatial resolution 8 mm). After 7 min, the nitroxide was detectable in the left side of the thorax area, but it was mostly localized in the liver. PCA was more uniformly distributed in the image collected after 17 min.

Spin labelled arabinogalactan as MRI contrast agent

In this study, we report the synthesis and the evaluation as MRI contrast agent of arabinogalactan/pyrrolidinoxyl radicals (PCA) covalent adduct (SLAG:Spin Labelled ArabinoGalactan). Arabinogalactan was used as targeting device, as it is recognized by the asialoglycoprotein receptor specific to the hepatocytes. The higher relaxivity R1 in water of SLAG, compared with small hydrophilic nitroxyl radicals, was explained by the molecular dynamics study using EPR spectroscopy that showed some immobilization of the radical into the polysaccharide. A binding study on isolated hepatocytes revealed that SLAG still recognizes the asialoglycoprotein receptor. MR imaging was performed using spin-echo T1 weighted images on mice to compare the contrast effect obtained with SLAG and PCA after IV injection (1 mmol/kg free radical). The percent signal enhancement observed in the liver 5 min after IV injection was 40 +/- 3% and 13 +/- 5% for SLAG and PCA, respectively. The signal was also dramatically increased in the renal cortex. This latter effect as well as the prolonged duration of the contrast (+/- 3 h), indicates at least a partial nonselective biodistribution; the high concentration needed to obtain a contrast effect could account for the saturation of the asialoglycoprotein receptor and hence for the apparent nonselective biodistribution.