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Rationale and objectives: To develop and partially characterize a new class of potential blood pool magnetic resonance (MR) contrast agents.Methods: Various copolymeric chelates of gadolinium-diethylenetriamine pentaacetic acid (Gd-DTPA) were prepared with differing molecular weights of polyethylene glycol (PEG) or polypropylene glycol (PPG) as linkers between the monomeric chelate units. Gadolinium content of the polymeric chelates was determined by atomic absorption spectra. Relaxivity of the polymeric chelates was measured at 1.5 Tesla and compared with Gadolinium-DTPA. MR angiography (MRA) was performed in rabbits comparing Gd-DTPA with Gd-copolymers.Results: The gadolinium content of the copolymeric chelates ranged from 2.95 to 22.2% on weight basis. The molecular weight of the PEG linkers in the copolymers ranged from about 150 to about 3400. Ther1 (1/T1, mM−1 s−1) for Gd-DTPA=4.1. Ther1 values for the different Gd-containing polymers ranged from 3.8 to 5.8, with the lowestr1 for the polymer prepared with the lowest-molecular-weight complex. The higher-molecular-weight complexes resulted in moderately higher relaxivity. MRA with Gd-copolymers, in rabbits, showed markedly greater vascular enhancement relative to an equivalent dose of Gd-DTPA. Vascular enhancement was much more sustained with the copolymeric agent and confined to vascular space; i.e. no appreciable background tissue enhancement—a reflection of distribution into extravascular fluid space—was observed.Conclusions: Relative to Gd-DTPA monomers, PEG-containing Gd-DTPA polymeric complexes provided moderate increases in relaxivity but markedly greater efficacy during in vivo MRA. In vitro relaxivity studies of Gd-copolymers showed only an approximately 50% increase inr1 relaxivity compared with Gd-DTPA. The PEG-containing complex's lack of rigidity may have diminished the effect of spin diffusion on relaxation, thereby accounting for this modest increase. The greater efficacy of Gd-copolymers during in vivo MRA may reflect compartmentalization within the vascular space and possibly enhanced relaxation of the macromolecular copolymers in the blood. Gd-copolymers are promising agents that merit additional study.

Liposomes are spheres composed of relatively non-toxic and biodegradable lipids which are useful for entrapping a variety of drugs, decreasing drug toxicity and targeting. For a number of years we have evaluated the use of liposomes as MR contrast agents. We have prepared and tested contrast agents entrapped within the internal aqueous space of liposomes as well as liposomes incorporating lipophilic contrast agents in the lipid bilayer. When chelates such as Gd-DTPA are entrapped within the internal aqueous space of lipid vesicles, delivery is primarily to the Kupffer cells and clearance is slow. Manganese ions entrapped within lipid vesicles cause more enhancement per micromole of paramagnetic ion than gadolinium. Lipophilic derivatives of manganese EDTA chelates when incorporated into liposomes confer the greatest hepatic enhancement per micromole of metal ion and have favorable clearance kinetics. An apparently hepatocyte specific liposomal MR contrast agent has been prepared based upon a lipop...

The core ionization energies of three strained lactams have been measured and compared with those of model lactams, amides, amines, and ketones. In general, the high values for N1s and the low values for O1s in planar amide (lactam) linkages compared to those in model amines and ketones are consistent with traditional resonance arguments. The N1s and O1s data for the distorted lactams 1,3-di-tert-butylaziridinone and 1-azabicyclo-[3.3.1]nonan-2-one are consistent with a reduced positive charge on nitrogen and a reduced negative charge on oxygen in accord with the classical resonance viewpoint

Liposomes are generally thought of as being useful for entrapping drugs within their internal aqueous space. When used with MR contrast agents, this has the drawback that water flux across the membrance bilayer is limiting to contrast enhancement. This can be partially overcome by making the liposomes very small, such that surface area is relatively great compared to internal volume, thereby facilitating water exchange. Alternatively the membranes can be designed to be permeable to water but this may render the vesicles unstable in serum. Another approach is to incorporate the contrast agents into the lipid bilayer. By designing novel complexes of manganese with the ligands incorporated onto dual acyl chains, we have achieved R1 and R2 values of over 20/mmol sec−1 or more than five times higher than Gd-DTPA. Hepatic metastasis detection is significantly improved in rats at doses of only 10 μmol/kg manganese. Membrane-bound manganese complexes function as highly effective liver imaging agents and merit further study for development as agents to undergo human clinical trials. © 1991 Academic Press, Inc.

An updated compendium of silatrane structural data is presented. The relationships between nonplanarity at silicon (δSi) andd(N-Si), the length of the dative bond, and nonplanarity at nitrogen ΔN andd(N-Si) are examined. Excluding a platinum-substituted derivative: the ΔSi relationship is strong and predicts the limiting length of a strong N → Si bond, while the ΔN relationship is weak. A good relationship between ΣσI for the four (non-N) exo- and endocyclic substituents attached to Si holds for silatranes, methyl, keto, and diketo derivatives as well as for 2-carbasilatranes. The relationship fails for strong π-donor exocyclic substituents, implicating resonance, and for benzo- and tribenzosilatranes, where steric effects dominate.

The published method for synthesis of the bridgehead lactam 6,6,7,7-tetramethyl-1-azabicyclo [2.2.2]octan-2-one yields polymers almost exclusively. Use of dilution techniques provides a 27% yield of this highly strained molecule. Base-induced methanolysis of this tetramethyl compound occurs exceedingly slowly compared to some other less crowded homologues. A buttressing effect is postulated to account for part of this effect.

Ultrasound is used as a primary diagnostic technique for the detection of deep venous thrombosis. The purpose of this study is to describe the development of a new thrombus-specific ultrasound contrast agent: The linear hexapeptide (lysine-glutamine-alanine-glycine-aspartate-valine) was synthesized and coupled to a lipid moiety. The targeted lipid was then incorporated into the lipid blend for the contrast agent Aerosomes (ImaRx, Tucson, AZ, USA). The lipid blend was used to entrap perfluorobutane microbubbles. The microbubbles were sized and studied in vitro for acoustic stability, binding to blood clot, and ultrasound enhancement in vitro of blood clot. The results showed the mean size of the specific ultrasound contrast agent (MRX-408) was about 2.0 μm. The microbubbles appeared as smooth spherical structures. Microscopy showed that the targeted bubbles bound to blood clot whereas control, nontargeted bubbles did not bind to blood clot. In vitro acoustic study showed similar stability of the microbubbles compared with control microbubbles. The targeted microbubbles enhanced blood clot in vitro whereas nontargeted microbubbles did not enhance clot. Thus this promising new thrombus-specific ultrasound contrast agent could potentially improve detection of thrombosis by ultrasound and might be useful for distinguishing between new and old thrombosis. In vivo studies are in progress.

RATIONALE AND OBJECTIVES The purpose of this study was to further develop and compare manganese-based liposomes prepared by two different approaches wherein a manganese ion was entrapped within the internal aqueous space of the vesicles or into the bilayer surface via membrane bound complexes. METHODS Small unilamellar liposomes (SUVs) were prepared entrapping manganese chloride. Alkylated complexes of manganese were prepared and also incorporated into SUVs. The two different manganese-based liposomes were compared for in-vitro relaxivity, stability, toxicity, and in-vivo imaging in rats with liver tumors. RESULTS Liposomes entrapping manganese had a concentration-dependent change in relaxivity that was maximal at a several-fold molar excess of phospholipid relative to manganese ion. Liposomes bearing membrane-bound complexes showed relaxivity inversely proportional to vesicle size. In-vivo imaging showed greater and more specific hepatic enhancement with manganese liposomes bearing alkylated complexes than those entrapping manganese ion. CONCLUSIONS Correlation effects likely explain the increased relaxivity of manganese entrapped in phospholipid vesicles. Greater efficacy, however, is afforded by liposomes bearing alkylated complexes.

An updated compendium of silatrane structural data is presented. The relationships between nonplanarity at silicon (δSi) andd(N-Si), the length of the dative bond, and nonplanarity at nitrogen ΔN andd(N-Si) are examined. Excluding a platinum-substituted derivative: the ΔSi relationship is strong and predicts the limiting length of a strong N → Si bond, while the ΔN relationship is weak. A good relationship between ΣσI for the four (non-N) exo- and endocyclic substituents attached to Si holds for silatranes, methyl, keto, and diketo derivatives as well as for 2-carbasilatranes. The relationship fails for strong π-donor exocyclic substituents, implicating resonance, and for benzo- and tribenzosilatranes, where steric effects dominate.

Compounds used as contrast agents in MRI are described. The compounds comprise a copolymer comprising at least two of a first monomer of the formula: X1-(CHR2CHR2-Y)n-(CHR2)m-CHR2CHR2-X2, wherein X and X2 are, independently, OH, NH2, NHR1, COOH, COOR1, SH, or Z, Y is O, NH, NR1, S, or CO, n is 0-10,000, m is 0 or 1, each Z is C20 substituted alkyl or cycloalkyl, and each R2 is, independently, H or OH, and at least one of a second monomer which is a polynitrilo chelating agent having at least two COOH, COOR1, or COZ groups, the first and second monomers bound to one another from a copolymer through an ester, amide, or carboxylic thioester linkage of at least one of OH, NH2, NHR1, COOH, COOR1, SH or Z groups of the first monomer, and at least one of the COOH, COOR1, or COZ groups of the second monomer. A third monomer which is a targeting agent or ligand having an OH, NH2, NHR1, COOH, COOR1, SH or COZ is optionally included. The third monomer forms an ester, amide, or carboxylic thioester linkage with the others. A paramagnetic ion chelated to the chelating agent may be added.

Our group has developed technology for stabilizing microbubbles with phospholipid coatings (Aerosomes®‐ImaRx Pharmaceutical Corp.). The first agent (MRX‐115) is based upon lipid‐coated microspheres filled with perfluoropropane gas and is in phase III clinical trials (October, 1997) for radiology and cardiology applications. Myocardial perfusion studies show the potential for the agent to detect ischemia in patients with myocardial infarcts. Targeting ligands have been covalently bound to lipids and incorporated into the stabilizing shells on the microbubbles. The first targeted agent planned for clinical trials is MRX‐408 (perfluorobutane gas, linear hexapeptide‐RGD analog). In vitro studies show enhanced visualization of thrombi, and in vivo studies in dogs with arterial and venous thrombi show selective enhancement, even in animals injected with heparin. In vitro studies have been performed to test sonothrombolysis comparing MRX‐115 to MRX‐408 with and without urokinase. The targeted contrast agent MRX‐408 increases the rate of clot dissolution with ultrasound. Targeted, tissue‐specific agents are feasible for ultrasound. A thrombus‐specific ultrasound contrast agent has the potential to improve clot characterization and detection as well as clot lysis.

Transfection experiments were performed in vitro in cell culture and in vivo in fish and mice using cationic liposomes with several different reporter genes. The effect of continuous‐wave 1‐MHz ultrasound on cell viability, cell permeability, and gene expression was assessed. RT‐PCR was used to monitor gene expression of cell repair genes in insonated cells. Acoustically active halocarbon‐containing liposomes were prepared for gene delivery. Ultrasound increased cell permeability and gene expression following liposomal transfection. RT‐PCR showed upregulation of repair of a number of cell repair genes after ultrasound exposure. The acoustically active gene delivery vehicles provided highly efficient transfection both with and without ultrasound in vitro and in vivo. Following IV administration of halocarbon‐containing liposomes/gene complexes, preferential gene expression was observed in the insonated tissue. Ultrasound has potentially useful applications for targeting and enhancing gene therapy.

During a comparison study of the fast atom bombardment (FAB) and electron impact mass spectra of 1-organyl-2,9,10-trioxa-6-aza-1-silatricyclo[4.3.3.01,6]dodecanes, an unusual ion peak at m/z 164 was noticed in the FAB spectra. Accurate mass measurement indicated that m/z 164 corresponds to protonated 1-[N,N-bis(2-hydroxyethyl)amino]-1-propanol (BHAP), which is produced by a ring-rupture reaction. Further experiment showed that the intensity of the [BHAP + H]+ ion peak increased markedly as the 7keV Ar0 bombardment proceeded, suggesting a FAB-induced reaction. The possible mechanism is also discussed.