FR2612400A1 - Microcapsules containing a radioactive and/or paramagnetic label in chelate form, and their use in the field of medical imaging - Google Patents

Abstract

Microcapsules which can be used in medical imaging, consisting of a wall made of biodegradable polymeric material enclosing an aqueous solution of an active substance, wherein the aforementioned active substance is a complexing or chelating agent or a paramagnetic and/or radioactive agent present in the form of a complex or chelate, and wherein the aforementioned particles can range from 0.5 to 5 micrometres approximately in size; and their use.

Description

 The present invention relates to new microcapsules which can be used in the field of medical imaging.

 It is known that microcapsules are particles having dimensions ranging from a micrometer to a few hundred micrometers, constituted by a solid spherical wall surrounding a liquid or a solid.

A distinction is made between microcapsules and microspheres, which are spherical particles made up of a continuous network of solid material in which the substance to be encapsulated is dispersed.

 The microcapsules are used in therapy with the aim of releasing active substances in a controlled manner. One of the advantages of using microcapsules is that the choice of their dimensions determines the preferred place of action of the active substance that they contain. Thus very small particles (called nanoparticles or millimicrospheres because they have dimensions not exceeding a few hundred nanometers) remain in the general circulation where they can gradually release a drug capable of acting generally.

The particles having dimensions of the order of a few tens of micrometers can be administered into a vessel irrigating a diseased organ and will be temporarily blocked by the capillary vessels of said organate (so-called microembolization technique), where they will gradually release, in situ, the active substance. As for microcapsules with twist dimensions of a few micrometers, which are not blocked by the capillaries, they are generally retained briefly in the pulmonary capillaries, then pass back into the general circulation where they are captured by the cells of the reticulo system. -endothelial and can therefore act in the organs containing these cells, in particular the liver, spleen and bone marrow, and finally in the kidneys where they are finally eliminated.

 We also know that medical imaging techniques aim to explore internal anatomical structures and to give an image of them, with the aim of distinguishing healthy tissues or organs from diseased tissues or organs. Among these techniques, scintigraphy and nuclear magnetic resonance imaging have so far received only limited applications, due to the technical problems raised by their use.

 It is known that scintigraphy consists in carrying out a selective fixation of radioactive molecules (tracers) in a particular organ, so as to obtain, by detection of the emitted radiations, an image of the organ where this fixation was carried out. The radiation emitted by the tracers is collected by a scintillation camera. An analog calculator identifies the brightness of each scintillation according to its coordinates and the photographic impression of each light point gives an image of the emitting source.

 The phenomenon of nuclear magnetic resonance (R.M.N.) results from the interaction of an external magnetic field with the magnetic moments of atomic nuclei. This phenomenon has been studied in particular on hydrogen nuclei, and more particularly on the hydrogen nuclei of water molecules. Magnetic fields of variable frequency are applied to a sample, in the form of pulses, and the system of magnetic moments is observed during its return to the normal state, after the pulse source has been switched off. This phenomenon of return to the initial state by a so-called relaxation time, corresponding to the time taken by the nuclear system to yield its excess energy to the environment (relaxation time T1, called spin-network). The excess energy of a nucleus can also be transferred to another nucleus of lower energy. This phenomenon is characterized by a so-called spin-spin relaxation time (T2). These relaxation times are characteristic of the water molecule in a given physical state. In particular, for the H20 molecule, the relaxation times of the two hydrogens depend on the mobility of the molecule: for example, for the water molecule bound to a protein, the relaxation times will be different from those observed for the molecule d open water. These phenomena can be taken advantage of in the study of biological tissues, which we know contain a high proportion of water, and we have been able to demonstrate different relaxation times between healthy tissues and pathological tissues, in particular tumors. By subjecting biological samples to a non-uniform magnetic field (gradient), a spatial distribution of the resonant frequencies is obtained, which is a function of the spatial distribution and the physical state of the water molecules present in the sample. Each area of the sample is characterized by different signals having a specific frequency and characteristic relaxation times, with an intensity depending on the water concentration. By comparison and appropriate transformation of the signals obtained in several directions of the space delimiting the plane or the volume of the studied sample, one can translate the signals of R.M.N. of this sample in pictures.

 It is also known that certain chemical substances, in particular paramagnetic and ferromagnetic substances, can influence the relaxation times of the hydrogen nuclei of the water contained in the tissues. These substances are therefore capable of playing the role of real contrast agents in magnetic resonance imaging (MRI), by accentuating the discrimination between healthy tissue and pathological tissue, provided of course that they can be concentrated in a sufficiently selective manner in a specific organ.

 The main problem in the two medical imaging techniques mentioned above is therefore to obtain selective fixation of the radioactive and / or paramagnetic marker.

 We have already described the use of albumin microspheres, which can be labeled with radioactive technetium, which make it possible to perform pulmonary scintigraphies. However, these microspheres have a life and stability incompatible with lasting labeling. In addition, their diameter is responsible for embolization in the capillaries. In addition, labeling with technetium must be carried out after the manufacture of the microcapsules, which complicates the scintigraphy operation by complex prior manipulations.

 In the field of I.R.M., gadolinium salts have hitherto been used in the form of complexes making it possible to reduce the toxicity.

However, these complexes are quickly stored and eliminated by the kidneys, so that in practice, only these organs can be correctly visualized by I.R.M. using such agents.

 The present invention relates to new microcapsules intended to be used in medical imaging (scintigraphy or MRI) either for diagnostic purposes or for research purposes, in particular for the study of the biodistribution of said particles and of the radioactive marker and / or paramagnetic they contain. These capsules make it possible to transport a paramagnetic and / or radioactive marker agent under a force making it possible to attenuate the toxicity, to increase the plasma lifespan, or to improve the specificity with respect to particular target organs.

 A more specific subject of the invention is microcapsules which can be used in medical imaging, constituted by a wall made of biodegradable polymeric material enveloping an aqueous solution of an active substance, characterized by the fact that said active substance is a complexing or chelating agent, or a paramagnetic and / or radioactive agent present in the form of a complex or chelate, and that said particles have dimensions which can range from 0.5 to 5 micrometers approximately.

 When the microcapsules of the invention contain a complexing or chelating agent, their use in medical imaging techniques includes a prior step of bringing the microcapsules into contact with an aqueous solution of a salt of the paramagnetic metal agent and / or radioactive that we want to encapsulate, so as to form the desired complex or chelate.

 The polymers constituting the wall are of course biocompatible polymers. Among the biocompatible and biodegradable polymers, gold may in particular include poly (lactic acid), poly (glycolic acid), copolymers of lactic and glycolic acids, poly (piperazine terephthalamide), and polycyanoacrylates.

 Obtaining microcapsules based on such polymers can be carried out for example using coacervation or interfacial polymerization techniques, the principle of which is known per se.

 The paramagnetic substances present or to be incorporated in the microcapsules of the invention in the form of complexes or chelates are for example manganese, copper, iron, cobalt and rare earths, in particular gadolinium and dysprosium, as well as radioactive isotopes of these elements. These radioactive isotopes (for example Fe 59, Co 57, Gd 153) are useful, for a preliminary research purpose, to study in particular the biodistribution in animals by scintigraphy of the microcapsules in each particular case, while the non-radioactive paramagnetic elements will be used to obtain magnetic resonance images.

 Among the radioactive substances, other than those mentioned above, present in the microcapsules which can be used in scintigraphy for diagnostic purposes, mention will be made, for example, of the radioactive isotopes of technetium (Tc 99m), of indium (In 111, In 113m) , gold (Au 198), ruthenium (Ru 103), cobalt (Co 57), chromium (Cr 51), etc ...

 The quantities of paramagnetic and / or radioactive elements depend on each element studied, on the particular complexing or chelating system studied, on the desired effect and on the retention possibilities of each type of microcapsule. These amounts can be determined in each case by simple routine experiments.

 As for the quantity of complexing or chelating agent, it is of course directly proportional to the quantity of paramagnetic and / or radioactive elements encapsulated or to be encapsulated.

 The complexing or chelating agents present or used in the microcapsules of the invention can be any organic molecule compatible with parenteral administration and capable of giving complexes or chelates with metals.

 Agents capable of giving coordination complexes with metals are in particular molecules of which at least one atom has a free doublet (nitrogen of tertiary amines, oxygen of ethers or ketones, etc.). The chelating agents are capable of exchanging hydrogen bonds with the metals and are generally characterized by the presence of at least two functional groups chosen from the acid or ester carboxylic, amine, ketone or thioketone, enolisable, phenol, alcohol, thiol, etc ...

Among the chelators which can be used in the microcapsules of the present invention, mention may be made in particular of the following known compounds: ethylenediaminetetraacetic acid (EDTA); diethylenetriamine pentacetic acid (DTPA); triethylene tetramine hexacetic acid; the compounds of the following formulas

NH-CO-CH2-N (C2-CO2H) 2 or bis (carboxymethyl) amino-2
N- (2,6-dimethylphenyl) acetamide
The following complexing agents can also be used, which serve as new compounds
- diethylenetriamine tetracetic acid (DETAT) of formula
(HO2C-CH2) 2N- (CH2) 2-NH- (CH2) 2 N (CE2C02H) 2
- the compound of formula

 - bis (carboxymethyl) amino-1 chloro-2 ethane of formula Cl- (CH2) 2-N (CH2-CO2H) 2.

 The preparation of these compounds is described below in the experimental part.

et les dérivés polyméthacryliques, à base de motifs de formule

dans laquelle Z représente un groupement -C0-N(CH2C02H)2 ou - C02 - CH2 - CHOM - CH2 - N(CH2CO2H)2. De tels polymères sont décrits dans la littérature.Among the complexing or chelating agents which can be used, mention will also be made of macromolecular complexing or chelating agents, which have the advantage of less diffusion through the microcapsules, and of less toxicity. Among the known macromolecular chelators, mention may in particular be made of products of the albumin-DTPA type, polyvinylamine derivatives based on units of formula

and polymethacrylic derivatives, based on units of formula

in which Z represents a group -C0-N (CH2C02H) 2 or - C02 - CH2 - CHOM - CH2 - N (CH2CO2H) 2. Such polymers are described in the literature.

dans laquelle R représente -H ou -CH3, et X représente un groupement de formule

dans laquelle
a = O ou 1
p est un nombre pouvant varier de 1 à 4, Y1 et Y2 représentent alors -H, et Z2 représente alors -C02H
ou bien p est un nombre égal à 1, Y2 représente -E1, Y1 représente -CE2SH et Z2 représente alors -CO2E
ou bien p est un nombre égal à 3 ou 4, Y1 et Y2 représentent -H, et
Z2 représente le groupement -CH(NH2)-C02H;
ou bien a = 1, p = O et Y2 et Z2 représentent alors un groupement -C0 ;;
ainsi que les esters, les sels et les complexes métalliques formés avec ces polymères.It is also possible to use the chelating polymers containing units of formula I

in which R represents -H or -CH3, and X represents a group of formula

in which
a = O or 1
p is a number which can vary from 1 to 4, Y1 and Y2 then represent -H, and Z2 then represents -C02H
or p is a number equal to 1, Y2 represents -E1, Y1 represents -CE2SH and Z2 then represents -CO2E
or p is a number equal to 3 or 4, Y1 and Y2 represent -H, and
Z2 represents the group -CH (NH2) -C02H;
or else a = 1, p = O and Y2 and Z2 then represent a group -C0 ;;
as well as the esters, salts and metal complexes formed with these polymers.

 These chelating polymers, which can be either homopolymers or copolymers, are new products. They can be prepared for example by the action of acryloyl or methacryloyl chloride, or vinyl or propenyl chloroformate, on glycine or other omega-amino acid, lysine, ornithine or cysteine, then polymerization or copolymerization of the product obtained.

 They can also be prepared for some by the action of polyacrylamide or polymethacrylamide on chloracetic acid or the like, and for others by the action of glycine or other omega-amino acid, on polychloride of acryloyl or methacryloyl .

 The copolymers are in particular those which contain, next to the units of formula I, other units derived from ethylenically unsaturated monomers making it possible to confer on the polymers properties of solubility in water: they are for example units derived from N-vinylpyrrolidone, N-methacryloyl D-glucosamine, acrylic or methacrylic acid, acrylamide or methacrylamide, etc.

 These polymers and their preparation are described in the patent application filed by the same applicants on the same day as the present application under the nc and entitled "New chelating polymers, their preparation and their application".

 As indicated above, the microcapsules of the invention can be used in the field of medical imaging and more particularly in scintigraphy and in I.R.M.

 To this end, the microcapsules of the invention are administered intravenously or orally, in suspension in an appropriate vehicle (physiological serum).

 The amounts injected depend of course on the marker used, the organ explored and the patient's body weight. These amounts can be determined in each case by routine experiments, for example on an animal model.

 In the case of scintigraphy, quantities corresponding to radioactivity which can range for example from 200 pCi to 20 mCi are generally administered.

 In the case of I.R.M., amounts are administered which may range, for example, from G, 1 to 0.5 mmol of marker metal per kg of body weight.

 We will now describe more particularly, by way of illustration, an embodiment of microcapsules based on alkyl polycyanoacrylate and in particular alkyl polycyanoacrylates, the alkyl group of which contains from 2 to 6 carbon atoms.

 It is known that these polycyanoacrylates have long been used as biodegradable adhesive tissue in surgery, and their absence of toxicity has been demonstrated by numerous studies.

The principle of the preparation of cyanoacrylate microcapsules by interfacial polymerization is also known and has been described for example by DA WOODS et al, International Journal of
Pharmaceutics, 8, 35-43 (1981). These authors have prepared such microcapsules for the purpose of encapsulating proteins and in particular enzymes. They obtained microcapsules with dimensions ranging from 3C to 400 micrometers. Such capsules are unusable for administration into the bloodstream since they would inevitably cause embolizations by capillary blockage.

 We have now discovered that it is possible to obtain, by the choice of particular operating conditions, alkyl polycyanoacrylate microcapsules having dimensions which can range from 0.5 to 5 micrometers, with a small variation in the size distribution. particles, which allows the administration of these microcapsules in the bloodstream, without risk of capillary blockage.

 The alkyl cyanoacrylates polymerize by an anionic polymerization process, that is to say that the initiator of the reaction is an anion or more generally a nucleophilic reagent.

The polymerization scheme is as follows

 In the previous scheme, A represents the nucleophilic initiating reagent, and R 'represents an alkyl group having 2 to 6 carbon atoms.

 As can be seen in the diagram above, the nucleophilic initiator is chemically incorporated into the polymer, and this phenomenon can be put c.

profit in the realization of microcapsules, as will be indicated below.

 To make polycyanoacrylate microcapsules, one operates according to the known method of interfacial polymerization which consists in preparing, with a basic aqueous solution and an organic phase, an emulsion of the water-in-oil type whose dispersed droplets have the required dimensions , then add the monomer in solution in the organic phase. The basic conditions favor the polymerization, and under these conditions the monomer will polymerize at the interface of the microdroplets of water and the organic phase, this polymerization leading to the construction of the wall of the microcapsules in which the microdroplets of water will be imprisoned.

To prepare the polycyanoacrylate microcapsules which can be used according to the invention, the process is carried out according to the fact that
- that an aqueous solution of a chelating or complexing agent, or of a complex or chelate of a radioactive and / or paramagnetic marker is prepared
- that a solution of a surfactant in an organic solvent immiscible with water is prepared, said surfactant being capable of promoting the production of emulsions of the water-in-oil type
- pour the aqueous solution into the organic solution
- that a water-in-oil emulsion is produced with an agitation system having a sufficient speed to obtain microdroplets having the desired dimensions
- that a solution of the monomer in an organic solvent immiscible with water and miscible in the organic phase of the emulsion is added to said emulsion
- that agitation is continued, if desired, for a time not exceeding 3 minutes
- that, if desired, the organic phase is diluted by adding a compatible organic solvent, so as to stop the polymerization
- And let sediment then wash the microcapsules and store them in an appropriate liquid medium.

In particular embodiments, this process can also have the following characteristics, taken individually or in combination
- The solvent is a mixture of cyclohexane and chloroform in proportions which can range, for example, from 2: 1 to 6: 1
- The aqueous phase has a pH of 9 to 12, this pH can be obtained for example by adding a suitable amount of sodium carbonate;
the aqueous phase represents from 5 to 30% and preferably from 5 to 20%, by volume, of the initial organic phase
the surfactant present in the initial organic phase is a lipophilic type surfactant such as, for example, Span 85 (Trioleate
Sorbitant marketed by Fluka); for example, from 2 to 8X and preferably from 3 to 7% (weight / volume) of surfactant are added in the initial organic phase; it has been found that the particle dimensions and the size distribution pass through a minimum when the concentration of surfactant is increased; for example in the case of Span 85, the microcapsules of minimum size are obtained with a concentration of 4%
- Agitation is carried out with a homogenizing device having a rotation speed ranging for example from 15,000 to 45,000 revolutions / minute; the higher the speed, the smaller the particle diameter and the narrower the size distribution;
the particles are washed in an aqueous ethanol solution containing a hydrophilic type detergent, so as to disperse the residual organic phase in water; the last washes are carried out using physiological saline.

 The microcapsules can be stored in the cold in physiological saline.

 Of course, all the manipulations during the preparation must be done under rigorous aseptic conditions.

In a particular embodiment, a protein is incorporated into the starting aqueous phase which is capable of playing the role of nucleophilic reagent, of initiating the polymerization of cyanoacrylates, and of thus being incorporated into the wall of the microcapsules. The incorporation of a protein into the wall of the microcapsules has several advantages
- it strengthens the solidity of the wall and reduces the speed of diffusion of the marker outside the capsule
- it allows secondary labeling of the capsules by labeling the protein with, for example, radioactive iodine or technetium according to the usual techniques
- in addition, it is likely to favor in certain cases, the transport of microcapsules towards a privileged target organ.

 Among the proteins which it is possible to incorporate into the microcapsules, mention will be made in particular of gelatin, serum] bovine or human bumine, hemoglobin, fibrinogen, gamma globulins, including monoclonal antibodies, etc.

 The protein concentration in the encapsulated aqueous phase is for example 10 9 to 10 mole / liter.

 The following examples illustrate the invention without, however, limiting it.

 EXAMPLE 1 DTPA-gadolinium complex in isobutyl polycyanoacrylate microcapsules containing albumin.

 On the one hand, a solution of 4% Span 85 in 40m1 of a 4: 1 cyclohexane-chloroform mixture is prepared, and on the other hand an aqueous solution containing 0.2tut sodium carbonate containing 0.57 (weight / volume) of bovine serum albumin and 0.2M of a DTPA-gadolinium complex.

 4 ml of this aqueous solution are poured into the organic detergent solution and 11 emulsions are carried out with a Virtis 45 homogenizer (VIRTIS CY, New York) rotating at a speed of 25,000 revolutions / minute.

 A solution of 0.49 g of isobutyl cyanoacrylate in 20 cm 3 of a cyclohexane-chloroform mixture (4: 1) is slowly added, stirring constantly for 3 minutes.

 Cyclohexane is then added to obtain a total volume of 100ml.

 After decantation of the microcapsules, the maximum amount of solvent is removed by suction and then 20 ml of an aqueous solution of Tween 20 (polyoxyethylenated sorbitan monolaurate - Merck) is added at 30%. After 5 minutes with moderate stirring, 40 ml of water and 20 ml of ethanol are added. The microcapsules are separated by centrifugation (at a speed of 1000-4000 revolutions / minute) at OOC. The microcapsules are then washed 3 times with a physiological saline solution and then stored in the minimum of sterilized physiological serum.

 The number-average diameter of the microcapsules, measured by particle size by Coulter effect, is approximately 2.5 μm.

Similarly, the following microcapsules were prepared
- isobutyl polycyanoacrylate + gelatin ~: for this we replace the serum albumin solution with 40ml of an aqueous solution containing 5g of gelatin. In this case, it is advantageous to add 5 ml of 37% formalin after the addition of cyclohexane, and then leave to stand for 12 hours at 4 ° C. in order to promote the hardening of the gelatin.

Microcapsules having the following characteristics were also prepared in an analogous manner.
- DTPA-gadolinium complex in isobutyl polycyanoacrylate microcapsules;
- chelating-gadolinium polymer complex in isobutyl polycyanoacrylate microcapsules containing fibrinogen labeled with iodine 125
- DTPA-Indium complex in isobutyl polycyanoacrylate microcapsules containing 0.5X bovine or human serum albumin
- albumin-DTPA-In 111 complex (prepared according to the
HNATOWICH et al., Science, 1983, vol. 220, pp. 613-615) in microcapsules of isobutyl polycyanoacrylate containing 0.5% bovine or human serum albumin
- DTPA-Indium complex in isobutyl polycyanoacrylate microcapsules containing 5% bovine or human serum albumin labeled with iodine 125.

 We also prepared microcapsules of isobutyl albumin polycyanoacrylate labeled with technetium 99m. This marking is carried out after manufacture and washing of the microcapsules, in a solution containing microcapsules and sodium pertechnetate, in the presence of stannous chloride, under nitrogen atmosphere, according to a method known per se. This technique allows labeling with Tc 99m of the microcapasules at the level of the albumin they contain.

 Study of the biodistribution of microcapsules in rats.

This study was carried out using various types of microcapsules
- isobutyl polycyanoacrylate + 5% (weight / volume) of albumin labeled with iodine 125;
- isobutyl polycyanoacrylate + 5% albumin labeled with technetium 99m
- isobutyl polycyanoacrylate + 0.5Z albumin labeled with Tc 99m;
- isobutyl + 5, albumin polycyanoacrylate, labeled with the albumin-DTPA-Indium 111 complex.

 The microcapsules, suspended in physiological saline, were administered to rats, at a rate of 1 ml per kg of body weight, the volume ratio capsules / (total volume) being approximately 1/4. The amounts administered correspond, depending on the case, to values of 10 pCi to 2 mCi per rat.

 The proportion of radioactivity uptake by the various organs was evaluated by two methods: on the one hand by dissection, organ weighing and counting of radioactivity, and on the other hand by scintigraphy in the areas of interest (case of Tc 99m).

The results obtained with the different types of microcapsules studied are comparable and are summarized in the following table
Time after Z of radioactivity capture by
injection
(minutes) lung liver spleen kidney blood
10 7 60 2 2 29
30 6 67 4 1 22
40 5 69 4 1 21
Preparation of diethylenetriamine tetracetic acid
0.8 g of iminodiacetic acid is dissolved in 60 ml of ethanol containing 36 ml of 0.5N sodium hydroxide. 0.54 g of bis (2-chloroethyl) amine hydrochloride is then added. It is brought to and maintained at reflux for 6 hours.

 The product obtained is isolated by evaporation and drying.

 The structure of the product is confirmed by the spectrum of R.M.N. h.

A complex of this chelating compound was prepared with technetium 99m, by careful reduction of sodium pertechnetate, under a nitrogen atmosphere, with stannous chloride, in the presence of said chelating agent.

 It has been verified by chromatography that the majority of the radioactivity introduced is found after formation of the complex, on the chelating structure.

 A ferric complex of diethylene triamine tetracetic acid (DETAT) was also prepared. For this, a 0.1 μM solution of Fe Gl3 in ethanol is prepared. To 50 ml of this solution, 50 ml of an 0.1M aqueous solution of DETAT is added. The pH is adjusted to 7 by adding a dilute ammonia solution. The mixture is left in a water bath for 3 hours and then cooled.

The precipitated ferric complex is collected by filtration, washed with ethanol and dried under reduced pressure.

Preparation of a chelating compound of formula

 Analogously to that described in the previous example, the iminodiacetic acid is made to act on p = [bis (2-chloroethyl) amino3-phenyl-4-butyric acid (chlorambucil) in the presence of sodium hydroxide in ethanol. At the end of the reaction, the product having the formula indicated above precipitates by acidification (pH 3.5) of the reaction medium.

 The R.M.N. It confirms the structure of the product.

 A complex was prepared using this chelating compound with Tc 99m and with iron (III).

 It was verified that the complexes formed were stable.

Preparation of bis (carboxymethyl) amino-1 chloro-2 ethane
This product is obtained by reaction, for 2 hours at reflux, of 2 moles of chloroacetic acid on 1 mole of chloroethylamine hydrochloride, in ethanol containing 6 moles of sodium hydroxide.

 Complexes of this compound have been prepared with iron (III).

Claims (15)

CLAIMS S  1. Microcapsules usable in medical imaging, constituted by a wall of biodegradable polymeric material enveloping an aqueous solution of an active substance, characterized by the fact that said active substance is a complexing or chelating agent or a paramagnetic and / or radioactive agent present under the form of a complex or chelate, and that said particles have dimensions which can range from 0.5 to 5 micrometers approximately.  2. Microcapsules according to claim 1, characterized in that the said polymeric material is chosen from poly (lactic acid), poly (glycolic acid), copolymers of lactic and glycolic acids, poly (piperazine terephthalamide), polymethyl methacrylate, and polycyanoacrylates.  3. Microcapsules according to any one of the preceding claims, characterized in that the said paramagnetic substance present in the form of complexes or chelates is chosen from manganese, copper, iron, cobalt and rare earths, in particular gadolinium and dysprosium, as well as the radioactive isotopes of these elements.  4. Microcapsules according to any one of claims 1 and 2, characterized in that the said radioactive substance is chosen from the radioactive isotopes of technetium (Tc 99m), of indium (In 111, In 113m), gold (Au 198), ruthenium (Ru 103), cobalt (Co 57), chromium (Cr 51), gadolinium (Gd 153), etc ...  5. Microcapsules according to any one of the preceding claims, characterized in that the said chelate is obtained with a chelating agent containing at least two acid or ester groups of carboxylic acid, amine, ketone or thioloketone, phenol, alcohol, thiol, etc. ..  6. Microcapsules according to any one of the preceding claims, characterized in that the said chelating agent is diethylene triamine tetracetic acid, or bis (carboxymethyl) amino-1-chloro-2 ethane.  7. Microcapsules according to any one of the preceding claims, characterized in that the said chelating agent is a compound of formula

 8. Microcapsules according to any one of claims 1 to 5, characterized in that the said complex or chelate is formed with a macromolecular complexing or chelating agent.  9. Microcapsules according to any one of the preceding claims, characterized in that the polymeric material of their wall is an alkyl polycyanoacrylate.  10. Microcapsules according to claim 9, characterized in that their wall contains, in addition to said polymeric material, at least one protein.  11. Microcapsules according to claim 10, characterized in that the said protein is chosen from gelatin, bovine or human serum albumin, hemoglobin, fibrinogen, gamma globulin, monoclonal antibodies, etc.  12. Microcapsules according to any one of claims 10 and 11, characterized in that the proportion of protein in the encapsulated aqueous phase is from 10-o to 10-2 mole / liter.  13. A method of preparing microcapsules as defined in any of the preceding claims, characterized in that said capsules are prepared according to known techniques of coacervation or interfacial polymerization.  14. Process for the preparation of microcapsules as defined in any one of claims 9 to 12, characterized in that  - that an aqueous solution of a chelating or complexing agent or a complex or chelate of a radioactive and / or paramagnetic marker is prepared  - that a solution of a surfactant in an organic solvent immiscible with water is prepared, said surfactant being capable of promoting the production of emulsions of the water-in-oil type  - pour the aqueous solution into the organic solution  - that a water-in-oil emulsion is produced with a stirring system having a sufficient speed to obtain microdroplets having the desired dimensions  - That a solution of the monomer in an organic solvent immiscible with water and miscible in the organic phase of the emulsion is added to said emulsion;  - that agitation is continued, if desired, for a time not exceeding 3 minutes; ;  - that, if desired, the organic phase is diluted by adding a compatible organic solvent, so as to stop the polymerization  - And let sediment then wash and isolate said microcapsules and store them in an appropriate liquid medium.  15. Use of microcapsules as defined in any one of claims 1 to 12, in the field of medical imaging by scintigraphy or by magnetic resonance.