Trifluoromethyl analogs of X-ray contrast media for magnetic resonance imaging

Methods and compositions are disclosed for enhancing .sup.19 F magnetic resonance imaging which utilize trifluoromethyl derivatives of iodinated X-ray contrast media. Typical magnetic resonance contrast media within the scope of the present invention include bis(trifluoromethyl)benzene derivatives, tris(trifluoromethyl)benzene derivatives, tetrakis(trifluoromethyl)benzene derivatives, and other related trifluoromethylated benzene derivatives.

BACKGROUND OF THE INVENTION 
This invention relates to compositions for improving magnetic resonance 
imaging ("MRI"), magnetic resonance spectroscopy ("MRS"), and magnetic 
resonance spectroscopy imaging ("MRSI"). More particularly, the present 
invention relates to low concentration fluorine-19 imaging agents. 
The technique of MRI encompasses the detection of certain atomic nuclei 
(those possessing magnetic dipole moments) utilizing magnetic fields and 
radio-frequency radiation. It is similar in some respects to X-ray 
computed tomography ("CT") in providing a cross-sectional display of the 
body organ anatomy with excellent resolution of soft tissue detail. The 
technique of MRI is advantageously non-invasive as it avoids the use of 
ionizing radiation. 
The hydrogen atom, having a nucleus consisting of a single unpaired proton, 
has the strongest magnetic dipole moment of any nucleus. Since hydrogen 
occurs in both water and lipids, it is abundant in the human body. 
Therefore, MRI is most commonly used to produce images based upon the 
distribution density of protons and/or the relaxation times of protons in 
organs and tissues. Other nuclei having a net magnetic dipole moment also 
exhibit a nuclear magnetic resonance phenomenon which may be used in MRI, 
MRS, and MRSI applications. Such nuclei include carbon-13 (six protons and 
seven neutrons), fluorine-19 (9 protons and 10 neutrons), sodium-23 (11 
protons and 12 neutrons), and phosphorus-31 (15 protons and 16 neutrons). 
While the phenomenon of MRI was discovered in 1945, it is only relatively 
recently that it has found application as a means of mapping the internal 
structure of the body as a result of the original suggestion of Lauterbur 
(Nature, 242, 190-191 (1973)). The fundamental lack of any known hazard 
associated with the level of the magnetic and radio-frequency fields that 
are employed renders it possible to make repeated scans on vulnerable 
individuals. Additionally, any scan plane can readily be selected, 
including transverse, coronal, and sagittal sections. 
In an MRI experiment, the nuclei under study in a sample (e.g. protons, 
.sup.19 F etc.) are irradiated with the appropriate radio-frequency (RF) 
energy in a controlled gradient magnetic field. These nuclei, as they 
relax, subsequently emit RF energy at a sharp resonance frequency. The 
resonance frequency of the nuclei depends on the applied magnetic field. 
According to known principles, nuclei with appropriate spin when placed in 
an applied magnetic field (B, expressed generally in units of gauss or 
Tesla (10.sup.4 gauss)) align in the direction of the field. In the case 
of protons, these nuclei precess at a frequency, F, of 42.6 MHz at a field 
strength of 1 Tesla. At this frequency, an RF pulse of radiation will 
excite the nuclei and can be considered to tip the net magnetization out 
of the field direction, the extent of this rotation being determined by 
the pulse, duration and energy. After the RF pulse, the nuclei "relax" or 
return to equilibrium with the magnetic field, emitting radiation at the 
resonant frequency. The decay of the emitted radiation is characterized by 
two relaxation times, T.sub.1 and T.sub.2. T.sub.1 is the spin-lattice 
relaxation time or longitudinal relaxation time, that is, the time taken 
by the nuclei to return to equilibrium along the direction of the 
externally applied magnetic field. T.sub.2 is the spin-spin relaxation 
time associated with the dephasing of the initially coherent precession of 
individual proton spins. These relaxation times have been established for 
various fluids, organs, and tissues in different species of mammals. 
For protons and other suitable nuclei, the relaxation times T.sub.1 and 
T.sub.2 are influenced by the environment of the nuclei (e.g., viscosity, 
temperature, and the like). These two relaxation phenomena are essentially 
mechanisms whereby the initially imparted radio-frequency energy is 
dissipated to the surrounding environment. The rate of this energy loss or 
relaxation can be influenced by certain molecules or other nuclei which 
are paramagnetic. Chemical compounds incorporating these paramagnetic 
molecules or nuclei may substantially alter the T.sub.1 and T.sub.2 values 
for nearby nuclei having a magnetic dipole moment. The extent of the 
paramagnetic effect of the given chemical compound is a function of the 
environment within which it finds itself. 
In MRI, scanning planes and sliced thicknesses can be selected. This 
selection permits high quality transverse, coronal and sagittal images to 
be obtained directly. The absence of any moving parts in MRI equipment 
promotes a high reliability. It is believed that MRI has a greater 
potential than CT for the selective examination of tissue characteristics. 
The reason for this being that in CT, X-ray attenuation and coefficients 
alone determine image contrast, whereas at least four separate variables 
(T.sub.1, T.sub.2, proton density, and flow) may contribute to the MRI 
signal. For example, it has been shown (Damadian, Science, 171, 1151 
(1971)) that the values of the T.sub.1 and T.sub.2 relaxation in tissues 
are generally longer by about a factor of two (2) in excised specimens of 
neoplastic tissue compared with the host tissue. 
By reason of its sensitivity to subtle physiochemical differences between 
organs and/or tissues, it is believed that MRI may be capable of 
differentiating different tissue types and in detecting diseases which 
induce physicochemical changes that may not be detected by X-ray or CT 
which are only sensitive to differences in the electron density of tissue. 
In some cases, the concentration of nuclei to be measured is not 
sufficiently high to produce a detectable MR signal. For instance, since 
.sup.19 F is present in the body in very low concentration, a fluorine 
source must be administered to a subject to obtain a measurable .sup.19 F 
MR signal. Signal sensitivity is improved by administering higher 
concentrations of fluorine or by coupling the fluorine to a suitable 
"probe" which will concentrate in the body tissues of interest. High 
fluorine concentration must be balanced against increased tissue toxicity. 
It is also currently believed that a fluorine agent should preferably 
contain magnetically equivalent fluorine atoms in order to obtain a clear, 
strong signal. 
From the foregoing, it would be a significant advancement in the art to 
provide fluorine MRI agents for enhancing images of body organs and 
tissues which may be administered in relatively low concentrations, yet 
provide a clear, strong signal. 
Such MRI agent are disclosed and claimed herein. 
SUMMARY OF THE INVENTION 
The present invention provides methods and compositions for improved 
magnetic resonance imaging and spectroscopy, including fluorine-19 MRI 
agents. The MRI agents are derived from the class iodinated X-ray contrast 
media ("XRCM"). Over the years, a number of triiodinated benzene 
derivatives have been developed and brought to market as XRCM. The XRCM 
that have been brought successfully to market have had very low toxicity 
because of the large doses required for X-ray imaging. 
Since the doses required for proton magnetic resonance imaging are 
considerably lower than XRCM doses, magnetic resonance contrast media 
("MRCM") which are structurally similar to XRCM should result in a product 
having a high safety index (the ratio of toxic dose to imaging dose). Even 
if the MRCM is .sup.19 F based, the doses should be less than that for 
XRCM such that the resulting .sup.19 F MRCM has a high safety index. 
The present invention takes advantage of the low toxicity of triiodinated 
benzyl XRCM by replacing the iodine with trifluoromethyl ("CF.sub.3 ") 
groups or groups containing CF.sub.3. Typical CF.sub.3 analogs of XRCM 
within the scope of the present invention include 
bis(trifluoromethyl)benzene derivatives, tris(trifluoromethyl)benzene 
derivatives, tetrakis(trifluoromethyl)benzene derivatives, and other 
related trifluoromethylated benzene derivatives. 
Both iodine and CF.sub.3 are similar in size. Therefore, CF.sub.3 
replacement of iodine does not introduce steric effects that would affect 
chemical and biological stability. Moreover, the CF.sub.3 groups are 
chemically and biologically inert like iodine. The CF.sub.3 substituted 
MRCM within the scope of the present invention may be prepared such that 
all the fluorines are substantially chemically equivalent to avoid imaging 
problems associated with non-equivalent nuclei. 
Preparation of CF.sub.3 substituted MRCM with 2-4 CF.sub.3 groups would 
have 6-12 fluorines per molecule, thereby improving the efficacy of the 
molecule and lowering the imaging dose and raising the safety index. 
Also disclosed are diagnostic compositions and methods of performing MR 
diagnostic procedures which involve administering to a warm-blooded animal 
a diagnostically effective amount of the above-described fluorine 
substituted MRCM compositions and then exposing the warm-blooded animal to 
a MR procedure.

The following examples are offered to further illustrate the present 
invention. These examples are intended to be purely exemplary and should 
not be viewed as a limitation on any claimed embodiment. 
Example 1 
Synthesis of 
N-(2,3-dihydroxypropyl)-3,5-bis(trifluoromethyl)-benzenecarboxamide 
N-(2,3-dihydroxypropyl)-3,5-bis(trifluoromethyl)benzenecarboxamide, a 
bis(trifluoromethyl)benzene derivative, is prepared by dissolving 4.2 g 
(50 mmol) sodium bicarbonate and 4.6 g (50 mmol) 3-amino-1,2-propanediol 
in 50 mL of water. A solution of 3,5-bis(trifluoromethyl) benzoyl chloride 
(13.8 g, 50 mmol) in 50 mL of toluene is added. The heterogeneous mixture 
is stirred for 18 hours at room temperature. The mixture is poured into a 
separatory funnel. The aqueous layer is separated, washed with ether and 
evaporated. The residue is purified by C.sub.18 chromatography to give the 
amide, N-(2,3-dihydroxypropyl)-3,5-bis(trifluoromethyl)benzenecarboxamide. 
The chemical reaction is shown below: 
##STR5## 
Example 2 
Synthesis of 
N,N'-bis(2,3-dihydroxypropyl)-5-[(hydroxyacetyl)-(2-hydroxyethyl)-amino]-2 
,4,6-tris(trifluoromethyl)-1,3-benzenedicarboxamide 
N,N'-bis(2,3-dihydroxypropyl)-5-[(hydroxyacetyl)-(2-hydroxyethyl)-amino]-2, 
4,6-tris(trifluoromethyl)-1,3benzenedicarboxamide, a 
tris(trifluoromethyl)benzene derivative, is prepared as follows: a mixture 
of 
N,N'-bis(2,3-dihydroxypropyl)-5[(hydroxyacetyl)-(2-hydroxyethyl)-amino]-2, 
4,6-triiodo-1,3-benzenedicarboxamide (20 g, 25 mmol), sodium 
trifluoroacetate (61.2 g, 450 mmol), and copper(I) iodide (42.8 g, 225 
mmol) in 500 mL of N,N-dimethylacetamide is refluxed under argon for six 
hours. The solvent is evaporated. The product is isolated from the crude 
residue by C.sub.18 chromatography. The chemical reaction is shown below: 
##STR6## 
Example 3 
Synthesis of 3,5-Bis(acetylamino)-2,4-6-tris(trifluoromethyl) 
benzenecarboxylic acid, meglumine salt 
3,5-Bis(acetylamino)-2,4-6-tris(trifluoromethyl) benzenecarboxylic acid, a 
tris(trifluoromethyl)benzene derivative, is prepared according to the 
procedure of Example 2, except that 3,5-Bis(acetylamino)-2,4-6-triiodo 
benzenecarboxylic acid is used instead of 
N,N'-bis(2,3-dihydroxypropyl)-5[(hydroxyacetyl)-(2-hydroxyethyl)-amino]-2, 
4,6-triiodo-1,3-benzenedicarboxamide. The chemical reaction is shown below: 
##STR7## 
Example 4 
Synthesis of 2,3,5,6-tetrakis(trifluoromethyl)-1,4-benzenedicarboxylic 
acid, dimeglumine salt 
2,3,5,6-tetrakis(trifluoromethyl)-1,4-benzenedicarboxylic acid is prepared 
by charging a one liter stainless-steel autoclave with 
1,2,4,5benzenetetracarboxylic acid (51 g, 200mmol) then cooled in liquid 
nitrogen. Hydrogen fluoride (100 g, 5.0 mol) and sulfur tetrafluoride (173 
g, 1.6 mol) are added. The autoclave is sealed and heated at 150.degree. 
C. for six hours. The gases are vented and the contents are poured onto 
ice. The mixture is transferred to a separatory funnel and extracted into 
ether. The ether layers are washed with dilute sodium hydroxide, dried 
over magnesium sulfate, filtered and evaporated to leave crude product. 
Recrystallization is used to give pure 
1,2,4,5-tetrakis(trifluoromethyl)benzene. The chemical reaction is shown 
below: 
##STR8## 
A solution of n-butyl lithium (6.4 g, 100 mmol) in hexanes is added at room 
temperature to a solution of 1,2,4,5-tetrakis(trifluoromethyl)benzene 
(16.9 g, 50 mmol) 200 mL of anhydrous ether under argon. After one hour 
the reaction mixture is poured onto dry ice. The mixture is taken up into 
water, washed with ether and acidified to pH 2. The product is extracted 
into ether, washed with water and brine, dried over magnesium sulfate, 
filtered and evaporated. The crude product is recrystallized. The 
dimeglumine salt is prepared by adding two equivalents of 
N-methyl-D-Glucamine in appropriate solvent. 
##STR9## 
From the foregoing, it will be appreciated that the present invention 
provides fluorine MRI agents for enhancing images of body organs and 
tissues which may be administered in relatively low concentrations, yet 
provide a clear, strong signal. 
The invention may be embodied in other specific forms without departing 
from its spirit or essential characteristics. The described embodiments 
are to be considered in all respects only as illustrative and not 
restrictive. The scope of the invention is, therefore, indicated by the 
appended claims rather than by the foregoing description. All changes 
which come within the meaning and range of equivalency of the claims are 
to be embraced within their scope.