AU8542591A

AU8542591A - Metal-radionuclide complex comprising isonitrile ligands

Info

Publication number: AU8542591A
Application number: AU85425/91A
Authority: AU
Inventors: Alfons M. Verbruggen
Original assignee: Mallinckrodt Medical Inc
Current assignee: Mallinckrodt Inc
Priority date: 1990-06-18
Filing date: 1991-06-14
Publication date: 1992-01-07
Also published as: EP0548245A1; CA2085364A1; JPH05508162A; EP0548245A4; WO1991019516A1

Description

Me al-radionuclide complex comprising isonitrile ligands

The invention relates to a metal-radionuclide complex comprising isonitrile ligands. The invention further relates to a radiopharmaceutical composition comprising said complex and to the use of said composition. The invention also relates to a kit for preparing a radiophar¬ maceutical composition, said kit comprising an isonitrile ligand. Frequently used diagnostic compositions comprise radionuclide-labelled compounds, which are used for diagnostic examination, e.g. into deviations in shape and function of internal organs and into the presence and location of pathological processes in the body. For this purpose, a composition in which the radioactive compound is present is administered to the patient, for example, in the form of an injectable liquid. By means of a suitable detector, e.g. a gamma camera, images can be obtained by recording the emitted radiation of, for example, the organ or the pathological process in which the radioactive compound has been incorporated or is involved.

The above radioactive compounds or agents have one characteristic in common in that they are administered in rather low dosages to achieve the desired purpose, viz. to enable a diagnostic examination. Ideally a radiodiagnostic agent should be taken up only in the target organ or tissue. Accumulation of these agents in other places of the body than in the target organ or tissue may have a serious impact on the examination of the target organ or tissue due to a decreased contrast between target organ and environ¬ mental tissue. In addition, when using radionuclide- labelled compounds, accumuITtion of radioactivity i-n other organs and tissues than the organ or tissue to be examined constitutes an extra radiation burden for these other organs and tissues which may adversely influence their health and proper functioning. It will be evident from the above explanation that in particular the "target organ specificity" is of utmost importance for the above compounds or agents to be used in diagnostic compositions. By the term "target organ specificity" is to be understood the selective presence of the compound in question in the target organ or tissue (i.e. compared to other organs or tissues) during a predetermined well-defined period of time. This latter requirement means that the compound is carried along to and accumulated in the target organ or tissue sufficiently fast and that its residence time in said organ or tissue is sufficiently long to allow a diagnostic examination.

Metal-radionuclide complexes comprising isonitrile ligands have been described in European patent specificati¬ on 0107734 and can be used in particular as tracer agents in nuclear cardiology. Holman et al . (J . Nucl. Med. 25. 1984, 1350), however, have demonstrated that such com¬ plexes, in particular technetium-99m complexes, comprising isonitrile ligands, such as tert . -butylisonitrile ligands, leave much to be desired as to the above-defined "target organ specificity" . In practice rather high concentrations of radioactivity are found in the lungs and in the liver when applied in human beings. The high early concentration of radionuclide in the lungs has required that imaging of the heart be delayed to allow the lung activity to clear before useful myocardial images can be obtained. In addition, the high concentration of radionuclide in the liver has made the detection of perfusion defects in the apical region of the myocardium more difficult. In connection with the above, metal-radionuclide complexes comprising ether-substituted isonitriles as ligands have been investigated; such complexes are disclosed in European patent application 0233368. These complexes show improved imaging characteristics as compared with the above previously known complexes and therefore can be used successfully as myocardial imaging agents. Hexakis(2-methoxyisobutylisonitrile) [ ^^^mTc] technetium (I) (99m_Tc_ MIBI) is the preferred complex for myocardial imaging, as will be obvious from the fact that this product is already in a commercial development stage; see e.g. Drugs of the Future 1A, 1989, 1169-1175. Also this product, however, is still unsatisfactory with regard to its "target organ specificity". After human application, this product shows a rather long retention time in the lungs and moreover a rather high activity in the liver.

It is the object of the present invention to provide a metal-radionuclide complex comprising isonitrile ligands, in particular intended for myocardial imaging, in which complex a good heart uptake is attended with a reduced lung and liver activity. A good uptake means an uptake which, after administration of the complex to a human, enables a clear visualization of the heart, without disturbing interference of radioactivity taken up in surrounding tissues.^'

This object can be achieved by a metal-radionuclide complex having the general formula

where in M is a me tal - radionuc l ide s e lec ted from radio is o topes of Tc, Ru, Fe, Cr, Mn, Cu , Pb , Ga , As and

In; ^ is a straight or branched alkylene group having

2-4 carbon atoms; R and R3 each independently represents a methyl group or an ethyl group; R4 is a hydrogen atom or a methyl group; x is an integer from 1 to 8 ; and n indicates the charge of the complex and is 0

(neutral) or a positive or negative integer; with the proviso, that the total number of carbon atoms in R^ plus R2 plus R3 plus R is from 4 to 6.

As a matter of fact it has appreared that the desired "target organ specificity" can best be realized if the symbols in formula I have the above-defined meanings, so including the restriction mentioned in the proviso. Only in that case an optimum heart uptake is accompanied with a decreased uptake in the surrounding tissues.

Suitable metal-radionuclides to be used for the above complex according to the invention are, for example, Tc-99m, Cu-62, Cu-64, Cu-67, Pb-203, Ga-67, Ga-68, As-72, In-Ill, In-113m, Ru-97, Fe-52, Mn-52m and Cr-51. On account of both the radiation characteristics and the complex- forming behaviour, Tc-99m is the preferred radionuclide. Suitable alkylene groups for R^ are: ethylene, trimethy- lene, tetramethylene , propylene, 2-methylethylene , etc.

Particularly effective is a complex of the formula

so a complex wherein Tc-99m as the central radio-nuclide is complexed with six 3 - isocyanopropionaldehyde dimethylace- tal molecules as ligands.

The present invention also relates to a method of preparing a metal-radionuclide complex as defined above, by bringing a salt or chelate of a suitable metal-radionu¬ clide into a complex-forming reaction with an isonitrile ligand or with a metal adduct of this isonitrile, said isonitrile having the general formula

wherein the symbols have the meanings given hereinbefore, if desired in the presence of a reducing agent. By a metal adduct of the above-defined isonitrile is to be understood an adduct of a metal selected from the group consisting of Zn, Ga, Cd, In, Sn, Hg, Tl, Pb , Bi, Cu, Mo, Pd, Co, Ni , Cr, Ag and Rh, having a suitable anion such as BF4, PFg , CIO , I, Br, Cl or CF3COO. Said method of preparing the desired complex, as described for the preparation of complexes comprising related isonitrile ligands in the recently published Int.pat. appln. WO 89/02433, can generally be carried out in a simple manner, preferably in a substanti¬ ally aqueous medium at a substantially neutral pH (5-8). For the complex formation the desired metal is offered to the ligand or the metal adduct of this ligand in the form of a salt or in the form of a chelate wherein the metal is bound to relatively weak chelators, such as pyrophosphate , a phosphonate or polyphosphonate , a polyphosphate , an oxinate, a carboxylate , a hydroxycarboxylate , an aminocar- boxylate, an enolate or a mixture thereof. In using a metal chelate as starting material for the complex formation, the desired complex is formed via the principle of ligand exchange. In case a metal adduct of the ligand instead of the ligand per se is used for the complex formation, the complex is also formed by ligand exchange.

As suitable reducing agents for the above reaction can be used Sn(II) compounds, dithionites , sodium borohy- dride, and the like. Such reducing agents are required in case the metal-radionuclide is present in the form of a higher-charged state than the desired charge n. of the complex. The invention further relates to a radiopharma- ceutical composition which comprises, in addition to a pharmaceutically acceptable liquid carrier material, a metal-radionuclide complex as defined above. Such a radiopharmaceutical composition is intended for diagnostic application. A radiodiagnostic composition comprises in addition to a pharmaceutically acceptable liquid carrier medium a complex of a radionuclide as defined hereinbefore. If desired the composition so obtained can be brought into a form more suitable- for intravenous or subcutaneous application, e.g. by a purification or by adding a pharmaceutically acceptable liquid carrier material. For intravenous or subcutaneous application the solution should of course be in a sterile condition.

For performing a radiodiagnostic examination the composition, as described above, if desired after dilution with a pharmaceutically acceptable liquid, preferably a physiological saline solution, can be administered to a warm-blooded living being in a quantity sufficient for detection by means of external imaging, viz. from 10 to 2000 MBq, preferably from 100 to 1200 MBq, per 70 kg of body weight. Thereupon the being is subjected to external imaging to detect accumulated radioactivity and thus to determine the location thereof in the body of the being. In connection with the often poor shelf life of the radiolabelled compound and/or the short half-life of the metal radionuclide used it is frequently impossible to put the ready- for-use composition at the disposal of the user. In such cases the user will carry out the labeling reaction with the radionclide in the clinical hospital or laborato¬ ry. For this purpose the various reaction ingredients are then offered to the user in the form of a so-called "kit" formulation. It will be obvious that the manipulations necessary to perform the desired reaction should be as simple as possible to enable the user to prepare from the kit the radioactive labelled composition by using the facilities that are at his disposal. Because the radio- pharmaceutical composition according to the present invention can be prepared in a simple and easy manner, this preparation process can be carried out very well by the user. Therefore the invention also relates to a kit for preparing a radiopharmaceutical composition, as described above, comprising (i) an isonitrile ligand of the general formula III, shown above, wherein the symbols have the meanings given hereinbefore, or a metal adduct thereof as defined hereinbefore, to which, if desired, an inert pharmaceutically acceptable carrier and/or formulating agents and/or auxiliary substances is /are added, (ii) a solution of a salt or chelate of a metal radionuclide, and

(iii) instructions for use with a prescription for reacting the ingredients present in the kit, i.e. for bringing (ii) in a complex-forming reaction with (i) as described hereinbefore. As mentioned ..bove, for said complex- forming reaction the desired radionuclide may be offered to the ligand or its adduct in the form of a chelate, bound to a relatively weak chelator, such as a pyrophosphate , a polyphosphate , a phosphonate or polyphosphonate , an oxinate, a carboxylate , a hydroxycarbox late , an aminocar- boxylate, an enolate or a mixture thereof, said reaction being carried out under moderate conditions. Examples of suitable chelators for the radionuclide are 8 -hydroxyquin- oline or derivatives thereof; dicarboxylic acids, polycar- boxylic acids or hydroxycarboxylic acids, such as oxalic acid, malonic acid, succinic acid, maleic acid, phtalic acid, malic acid, lactic acid, tartaric acid, citric acid, asorbic acid, salicylic acid or derivatives of these acids; pyrophosphates ; phosphonates or polyphosphonates such as methylene diphosphonate , hydroxyethylene diphosphonate or hydroxymethylene diphosphonate; or enolates, for example tropolone or enolates with a beta-diketone , such as furoylacetone , thenoylacetone , benzoylacetone , dibenzoylme- thane, or derivatives of these diketones. In particular are of interest 8 -hydroxyquinoline , citric acid, tartaric acid, ascorbic acid, glucoheptonic acid or derivative thereof as chelators, because it has appeared that a chelate of a radionuclide, for example indium-Ill or lead- 203, can easily be brought in a complexing reaction with the ligand as defined hereinbefore when using suitable conditions, preferably a buffered aqueous solution and a substantially physiological pH. Then the desired radionu¬ clide complex can be formed by ligand exchange in a high yield and purity. A buffered aqueous indium- 111- tropolonate solution suitable for this purpose is described in European patent application no. 131327. The kit to be supplied to the user may also comprise the ingredient defined sub (i) above together with instructions for use, whereas the solution of the metal radionuclide, defined sub (ii) above, which solution has a limited shelf life, may be put to the disposal of the user separately.

In a different, equally extremely favourable embodi- ment the kit according to the invention comprises (i) an isonitrile ligand of the general formula III, shown above, wherein the symbols have the meanings given hereinbefore, or a metal adduct thereof as defined hereinbefore, to which, if desired, an inert pharmaceutically acceptable carrier and/or formulating agents and/or auxiliary substances is/are added, (ii) a reducing agent and, if desired, a chelator, and (iii) instructions for use with a prescription for bringing technetium- 99m in the form of a pertechnetate solution in a complex-forming reaction with ingredients (i) and (ii) as described hereinbefore. The composition should comprise a reducing agent to reduce the pertechnetate, for example a dithionite or stannous ions. Such a kit is intended for preparing a pharmaceutical composition labelled with Tc-99m. The pertechnetate can be obtained by the user very simply from a molybdenum- technetium generator. Examples of suitable chelators are described hereinbefore. If desired, the ingredients defined sub (i) and (ii) above may be combined, provided they are compatible with each other.

The ingredient of both above kits mentioned sub (i) may be supplied as a solution, for example in the form of an physiological saline solution, or in some buffer solution. In case the ingredient of the above kits mentioned sub (i) is or comprises a metal adduct of an isonitrile ligand, said ingredient is preferably in a dry condition, e.g. in a lyophilized condition. If desired, the above-mentioned ingredients may be stabilized in a usual way with suitable stabilizers such as ascorbic acid, gentisic acid or salts of these acids, or may be provided with other auxiliaries such as fillers.

Preferably the above kits comprise as the ligand for the metal-radionuclide 3- isocyanopropionaldehyde dimethyla- cetal or a metal adduct thereof. It has appeared, that th i s ligand is extremely suitable for complexing metal-radionu- clides, in particular technetium-99m. 3- Isocyanopropional¬ dehyde dimethylacetal is a new compound. Therefore the invention finally relates to this compound per se . This compound can be prepared by using preparation methods known per se for related compounds, viz. by successive formyla- tion and dehydratation of the corresponding amino compound. Said formylation can be carried out, for example, with phosgene, diphosgene or a mixture of formic acid and acetic anhydride or, very conveniently, with chloroform. A metal adduct of this isonitrile can easily be prepared by dissolving a suitable salt of this metal in a polar organic solvent, adding the isonitrile, stirring and then isolating the metal adduct e.g. by precipitation; this reaction is carried out in an inert atmosphere.

The invention will now be described in more detail with reference to the ensuing specific examples.

Example I

Preparation of 3-isocyanopropionaldehyde dimethylacetal a) Preparation of 3-aminopropionaldehyde dimethylacetal

In an reaction bomb are mixed 13.9 g (0.1 ol) of 3- chloropropionaldehyde dimethylacetal, 320 ml of anhydrous methanol and 150 ml of liquid ammonia. The bomb is closed and heated during 5 hours at 135*C. After cooling in an ice-bath the solvents are evaporated at reduced pressure. 20 ml of NaOH 2N is added and the mixture is extracted three times with 200 ml diethylether . The organic layer is dried over anhydrous sodium sulphate and then evaporated at reduced pressure. 4.1 g of the desired amino compound remains as a yellowish oil. TLC on silicagel with acetone- methanol (9:1) reveals the presence of one spot. (b) Preparation of 3 - isocyanopropionaldehyde dimethylace¬ tal

4.1 g (34.4 mmol) 3-aminopropionaldehyde dimethylace¬ tal and 35 mg benzyltriethylammoniumchloride are dissolved in a mixture of 5.7 ml dichloromethane and 4 g chloroform. This mixture is added dropwise to a solution of 5.31 g NaOH in 5 ml water at 40^βC. The reaction mixture is kept at 45"C to 50^βC for 1 hour. After addition of 10 ml water and 10 ml dichloromethane the two layers are separated. The organic layer is washed with water, saline solution and water and then dried over anhydrous sodium sulphate. After evaporati¬ on at reduced pressure the residue is purified by adsorpti¬ on chromatography using silicagel as the stationary phase. The mobile phase is a gradient mixture (100% hexane to hexane-ether 9:1). TLC on silicagel with hexane-ether (9:1) is used to monitor the presence of the acetal in the eluate. The fractions which contain the main compound are combined and evaporated. The title compound is so obtained in a yield of 1.2 g as a yellow oil. Identification by means of NMR: 1H-NMR(CDC1₃ ; THS ) : f~1 . 8 - 2.1(m,2H) , ^3.3 (s, 6H),^ 3.4-3.6 (t, 2H), ^ 4.5 (t, 1H) . Example II Preparation of 3- isocyanopropionaldehyde diethylacetal

In a corresponding manner as described in Example 1(a) 3-aminopropionaldehyde diethylacetal is prepared.

4 g 3 - minopropionaldehyde diethylacetal thus obtained is mixed with 200 ml dichloromethane. 250 ml of a 50% m/m potassium hydroxide solution and 500 mg benzyltriethylammo- nium chloride are added. The solution turns yellow and 100 ml water is added. After cooling to 0°C in an ice-bath 4.85 g chloroform and 80 ml dichloromethane are added in the course of 15 min. The solution is then stirred for 1 hour at 0^βC. After separation of the layers the organic layer is washed twice with water and dried over anhydrous sodium sulphate. The organic solvent is evaporated at 15"C under reduced pressure and purified by adsorption chromatography

(vide supra-Example I/b) . Finally 1.45 g of the title

13 compound is obtained as a yellowish oil. NMR ( C-CDCI3,

TMS) :<$^* 15.2, 33.5, 34.2, 61.3 and 100.7. Example III

Preparation of , "^mTc (I)] -hexakis (3- isocyanopropionaldehy¬ de dimethylacetal) (Tc-IPADA) Labelling of the acetal, prepared according to

Example I, with technetium-99m is performed by heating in a boiling water bath the ligand in a solution of ethanol (1 mg/1 ml) for approx. 10 min with 2 ml pertechnetate in a saline solution from a Mo-Tc-generator (370-740 MBq Tc-99m) in the presence of dithionite (5 mg/ 0.25 ml water) as reducing agent. The Tc-99m complex obtained is analyzed for radiochemical purity on a silicagel column, viz. ITLC-SG (trademark), eluted with 20% saline solution or with acetone. The radiochemical purity of the title complex is 97%.

Paper electrophoresis (300 V, 30 min) in a methanol - 0.025 M phosphate buffer pH 7.4 mixture (70/30 v/v) is used to elucidate the cationic charge: 1+.

Example IV

Biological experiments

The labelled compound, prepared according to Example III, is tested in mice by intravenous injection of a 148 kBq solution in saline into five mice, followed by determining the distribution of the radioactivity over the various organs by sacrificing the animals at fixed time intervals after injection, isolating the various organs and then counting their radioactivity. The compound is compared with [^99mTc(I)]-hexakis(2-methoxyisobutylisonitrile)(Tc-MIBI) . The table below shows the activity in the heart, the lungs, the liver and the intestines, 10 and 30 min after injecti¬ on. The activity is calculated as the percentage of the injected dose (% I.D).

Table

¹ tested compound orpan after .mm , Tc-IPADA Tc-MIBI heart 10 1.07 1.01 30 0.99 0.95 lungs 10 0.35 0.43 30 0.22 0.52 liver 10 21.25 24.00 30 19.67 20.78 intestines 10 19.08 20.05 30 26.05 19.47

The above results show, that the new complex of the invention viz. Tc-IPADA, shows a substantially equal heart uptake as the known Tc-MIBI. The liver and lung activity of Tc-IPADA, however, is considerably smaller than that of the known complex. Moreover, the clearance from the liver to the intestines is considerably faster when the complex of the invention is used, as appears from the figures of the intestines in the above table.

The new complex Tc-IPADA is injected in a baboon to investigate the efficacy in a primate. The heart of the baboon can be clearly visualized, indicating that the compound is a promising tool in heart scintigraphy.

Claims

1. A metal-radionuclide complex comprising isonitrile ligands, said complex having the general formula

wherein M is a metal-radionuclide selected from radioiso¬ topes of Tc, Ru, Fe , Cr, Mn, Cu, Pb , Ga, As and In; R^ is a straight or branched alkylene group having

2-4 carbon atoms; R2 and R3 each independently represents a methyl group or an ethyl group; R4 is a hydrogen atom or a methyl group; x is an integer from 1 to 8 ; and n indicates the charge of the complex and is 0 (neutral) or a positive or negative integer; with the proviso, that the total number of carbon atoms in R^ plus R2 plus R3 plus R4 is from 4 to 6.

2. A complex as claimed in claim 1, having the formula

l*)

3. A method of preparing a metal-radionuclide complex as claimed in claim 1, characterized in that a salt or chelate of the metal-radionuclide M, as defined in claim 1, is brought into a complex-forming reaction with an isonitrile ligand or with a metal adduct of this isonitri- le, said isonitrile having the general formula °K

C ≡ Λ — P -. _C

^■ ø * (m)

R,

wherein the symbols have the meanings given in claim 1 , if desired in the presence of a reducing agent.

4. A radiopharmaceutical composition which comprises, in addition to a pharmaceutically acceptable liquid carrier material, a metal-radionuclide complex, characterized in that the composition comprises a complex as claimed in claim 1 or 2.

5. A method of performing a radiodiagnostic examina¬ tion, characterized in that a composition as claimed in claim 5, if desired after dilution with a pharmaceutically acceptable liquid, is administered to a warm-blooded living being in a quantity from 10 to 2000 MBq, preferably from 100 to 1200 MBq, per 70 kg of body weight, after which the radiation emitted by said being is recorded.

6. A kit for preparing a radiopharmaceutical composi¬ tion, comprising, (i) an isonitrile ligand of the general formula III, shown in claim 3, wherein the symbols have the meanings given in claim 1, or a metal adduct thereof, to which, if desired, an inert pharmaceutically acceptable carrier and/or formulating agents and/or auxiliary substances is/are added (ii) a solution of a salt or chelate of a metal-radionuclide, and (iii) instructions for use with a prescription for reacting the ingredients present in the kit.

7. A kit for preparing a radiopharmaceutical composi¬ tion, comprising, (i) an isonitrile ligand of the general formula III, shown in claim 3, wherein the symbols have the meanings given in claim 1, or a metal adduct thereof, to which, if desired, an inert pharmaceutically acceptable carrier and/or formulating agents and/or auxiliary substances is/are added, and (II) instructions for use with a prescription for reacting the ingredient sub (i) with a solution of a salt or chelate of a metal radionuclide.

8. A kit for preparing a radiopharmaceutical composi¬ tion, comprising, (i) an isonitrile ligand of the general formula III, shown in claim 3, wherein the symbols have the meanings given in claim 1, or a metal adduct thereof, to which, if desired, an inert pharmaceutically acceptable carrier and/or formulating agents and/or auxiliary substances is/are added, (ii) a reducing agent and, if desired, a chelator, ingredients (i) and (ii) optionally being combined, and (iii) instructions for use with a prescription for reacting the ingredients of the kit with technetium-99m in the form of a pertechnetate solution.

9. A kit as claimed in claim 6, 7 or 8, comprising as the ligand 3-isocyanopropionaldehyde dimethylacetal.

10. 3-Isocyanopropionaldehyde dimethylacetal.