The invention relates to a technetium chelate, as well as to a method of preparing said chelate, and to a chelating agent to be used therefor. The invention also relates to a kit for preparing a radiopharmaceutical preparation comprising said chelate, and to the use of such a preparation for diagnostic examination.

Radionuclide-labelled compounds are used for diagnostic examination, e.g. of deviations in shape and function of internal organs and of the presence and location of pathological processes in the body. For this purpose, a preparation in which the radioactive compound is present is administered to the patient, for example, in the form of an injectable liquid. By means of suitable detectors, 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. Compounds generally used for examining the renal function are radio-iodinated Hippuran® and Tc99m-diethylene triamine pentaacetic acid (DTPA) , which will be discussed hereinafter.

In addition to glomerular filtration, an active tubular secretion takes place in the kidneys. The functioning of the kidneys is determined for a considerable extent by the functioning of the kidney tubules. In an adult person ap¬ proximately 125 ml of blood plasma is purified by glomerular filtration per minute. It is then said: the plasma clearance is 125 ml per minute. The total clearance which can be effected by the kidneys is from 600 to 700 ml of plasma per minute. It appears from the clearance of 125 ml of blood plasma per minute which is found for the above-mentioned

chelate of DTPA that said chelate is eliminated entirely or substantially entirely by glomerular filtration and is hence less suitable for examining the renal function.

An example of a radio-iodinated Hippuran compound generally used for examining the renal function is iodine-131-Hippuran which, as is generally known, is secreted actively tubularly and is hence very suitable for examining the renal function as regards organ specificity.

There is a great need for a suitable preparation for examining the renal function which is permanently available, in particular for kidney transplantation patients, victims of accidents and patients after large vascular operations.

The above-mentioned iodine-131-Hippuran would be excellently suitable for these applications, also due to its good availability. However, like all iodine-131-compounds, iodine-131-Hippuran constitutes a severe radiation burden for the patient. Therefore, this iodine-131-compound can be administered to the patient only in a restricted dose, as a result of which the resulting information is insufficient to obtain statistically reliable images of the renal function by means of a gamma camera.

Another radio-iodinated Hippuran compound which is much used for examining the renal function is iodine-123-Hippuran which is excellently suitable as regards the organ specificity and the restricted radiation burden. However, iodine-123-containing preparations have a restricted availability due to the short half-life, namely 13.3 hours, and the production of iodine-123 which necessarily has to be carried out in a cyclotron.

Technetium-99m complexes which do have a tubular secretion

which is comparable to that of radio-iodinated Hippuran are known from European Patent Application 173424. This application discloses inter alia the preparation of Tc99m-mercaptoacetylglycylglycylglycine (Tc99m-MAG3) , which complex is secreted by the kidneys selectively and slightly faster than radio-iodinated Hippuran. The same holds for related technetium complexes, disclosed in European Patent Application 250013 and Internat. patent application PCT/US92/03894, which, as for instance Tc99m-MAGAG and Tc99m-mercaptoisobutyryltriglycine, show significantly better secretion characteristics than Tc99m-MAG3.

However, the plasma clearance of the above-mentioned technetium-99m complexes in primates is generally still open to improvement and it would be an advantage, if the radiation burden for non-target organs would still further be reduced. It is therefore the objective of the invention to provide a radiodiagnostic agent for determining the renal function which exhibits a high plasma clearance in primates combined with a decreased radiation burden for non-target organs, and which can simply be produced in a clinic or hospital from easily available starting material.

This objective can be achieved by a technetium-99m chelate, which, according to the present invention, is characterized by the general formula

wherein:

Y is a thio function or a group of the formula

^13

N—C-Z

\

Rl4

Z is a carboxy group, a (C^C^)alkoxycarbonyl group, an aminocarbonyl group, an aminosulphonyl group or a carboxymethylaminocarbonyl group;

Tc represents technetium-99m;

R _x , R ₅ and R ₁₃ are each independently hydrogen atoms or methyl groups;

R ₂ , R ₆ , R ₉ and R ₁₄ are each independently hydrogen atoms or (C ₁ -C ₄ )alkyl groups, which alkyl groups are optionally substituted with amino, hydroxy, mercapto, halo, carboxy or aminocarbonyl; R ₃ has the meaning of Z, as defined above, and R ₄ is a hydrogen atom or a methyl group, or

R ₃ and R ₄ together constitute an oxo function;

R ₇ and R ₈ are each independently hydrogen atoms or methyl groups, or R ₇ and R ₈ together constitute an oxo function;

R ₁₀ has the meaning of Z, as defined above, or is a hydrogen atom or a methyl group; and

R _n and R ₁₂ are each independently hydrogen atoms or methyl groups, or R _u and R ₁₂ together constitute an oxo function; as well as water-soluble salts of this compound.

Water-soluble salts include alkalimetal salts, such as sodium salts, and ammonium salts.

Within the scope of the present invention are considered technetium chelates which can be defined more in particular by the general formula

wherein :

Z, Tc, R ₂ , R ₂ , R ₅ , R ₆ and R ₉ have the above meanings; and R ₄ ' , R ₇ ' , R ₈ ' , R _X1 ' and R ₁₂ ' are each independently hydrogen atoms or methyl groups; as well as water-soluble salts of this compound.

To be preferred are compounds within the above formula I structure, which can be characterized by the general formula

wherein :

Z, Tc, R _α , R ₂ , R ₅ , R ₆ , R ₉ , R ₁₃ and R ₁₄ have the above meanings; and

R ₁₀ ' is a hydrogen atom or a methyl group; as well as water-soluble salts of this compound.

Suitable examples of these last-mentioned new compounds are compounds of the above general formula III, wherein Z = COOH

and the R-symbols are at least all but two hydrogen, the remaining R-symbol(s) being optionally substituted methyl. Examples of formula III compounds are in more detail: compound

(4) Tc99 H H H H CH, CH, H H COOH m

(5) Tc99 H H H H H H CH, CH ₃ COOH m

(6) Tc99 H H H H CH, H H H COOH m

(7) Tc99 H H H H CH ₂ 0 H H H COOH m H

(β; Tc99 H H H CH ₃ H H H H COOH m

(9) Tc99 H CH, H H H H H H COOH m

In the above exemplified compounds (6) to (9) an asymmetrical carbon atom is present, as a result of which diastereoisomerism will occur. The biological properties of different diastereoisomeric Tc99m-compounds may differ. It will be evident, that the present invention is not restricted to the racemates but extends to the separate different diastereoisomers.

The new compounds of the invention can be prepared in a manner known per se for the preparation of related compounds. So the new compounds of formula I can be prepared by reacting technetium-99m in the form of a pertechnetate solution, in the presence of a reducing agent, with a chelating agent of the general formula

wherein:

Y and R _x to R ₁₂ have the meanings given hereinbefore; and A and B are each independently hydrogen atoms or suitable protecting groups; after deprotection, if A and/or B are protecting groups.

Examples of suitable protecting groups are acyl groups and acylaminoalkyl groups, such as acetyl, benzoyl, substituted benzoyl (e.g. p-methoxybenzoyl) , acetylaminomethyl, trifluoroacetyl, hydroxyacetyl and carboxyacetyl.

The chelating agents of the general formula V, mentioned hereinafter and to be used for the Tc99m-complexes of the above general formula III, can conveniently be prepared as described in detail in the specific Examples. The chelating agents to be used for the Tc99m-complexes of the above general formula II can conveniently be prepared according to the following reaction scheme, wherein, for convenience, a carboxylic group is inserted for the symbol Z and hydrogen atoms are inserted for the R-substituents.

Scheme: Synthesis of ethylene cysteine serine

Tos- Tosyl or p-toluenesulfαnyl Trit- Trityl σr triphenylmethyl

More in particular, the preferred compounds of the above formula III can be prepared in a suitable manner by reacting technetium-99m in the form of a pertechnetate solution, in the presence of a reducing agent, with a tripeptide compound of the general formula

wherein:

Z, R _x , R ₂ , R ₅ , R ₆ , R ₉ , R ₁₀ ' , R ₁₃ , R ₁₄ and A have the meanings given hereinbefore, after deprotection, if A is a protecting group.

The symbol A in the above formula V is preferably a hydrogen atom. It is a considerable advantage that the hydroxy group in the above formula V compound does not need to be protected to allow riskless storage and manipulation of this compound. It is a special merit of the present invention, that this latter reaction proceeds smoothly in the absence of a transfer ligand. It is a generally accepted fact in this art that a hydroxy group is considerably less suitable as a metal-chelating group than a mercapto group. Therefore it is quite a surprise that the above-mentioned reactions are able to produce the desired Tc99m chelates so easily and in a so high yield.

Preferably this reaction is performed in the presence of Sn(II) as a reducing agent, in the absence of a transfer

ligand, and in an at least substantially aqueous solvent system having a pH of at least 10, so in a very simple procedure. Under such conditions the reaction proceeds smoothly already at ambient temperature.

The invention also relates to new chelating agents, which may be used to prepare the above-mentioned Tc99m-compounds. These new chelating agents have the above general formula IV, wherein the symbols have the above meanings. The new chelating agents are very stable upon storage and can be prepared in a manner known per se for the preparation of related compounds. A method of preparation is illustrated above. In a preferred embodiment, these chelating agents can be represented by the general formula V. In the tripeptide compound of the general formula V the symbol A is preferably a hydrogen atom. As mentioned hereinbefore, the free hydroxy group in the above formula V compound does not need to be protected to allow riskless storage and intended use of this compound.

The chelating agents according to the invention are usually processed to compositions suitable for diagnostic purposes, in particular for determining the renal function. When the composition is to be used for the preparation of a Tc99m-containing radiopharmaceutical preparation, starting from Tc99m-pertechnetate, the composition should comprise a reducing agent, preferably stannous-ions. Such a composition with a suitable reducing agent can also be prepared in a sterile manner in a lyophilized form. Further the invention relates to a kit suitable for preparing a radiophar¬ maceutical preparation, comprising in an optionally dry condition a chelating agent as defined above, and a reducing agent, whether or not in a dry condition, and instructions for use with a prescription for the reaction of said composition with technetium-99m in the form of a pertechne-

tate solution. Preferably such a kit comprises a Sn(II) salt as the reducing agent, in addition a basic substance, and further separately a neutralizing agent in the form of an acid or a buffering substance.

The invention finally relates to a method of performing a radiodiagnostic examination by administering said composition to a living being in a quantity from 0.1 tot 30 mCi, preferably from 0.5 to 10 mCi, per 70 kg of body weight and by then recording the radioactive radiation emitted by the living being.

The invention will now be described in greater detail with reference to the ensuing specific Examples.

EXAMPLE I

Synthesis of O-benzoylglvcoloyltriσlvcine

To a solution of 3.6 g (20 mmol) O-benzoylglycolic acid, prepared according to the method of Ringshaw et al. (J. Chem. Soc. 1964, 1959-1962), and 2.3 g (20 mmol) N- hydroxysuccinimide in 40 ml dichloromethane is added a solution of 4.1 g (20 mmol) dicyclohexylcarbodiimide in 10 ml dichloromethane. The reaction mixture is stirred for 2 hours at room temp. Then the reaction mixture is filtered and the precipitate is washed with dichloromethane (3 x 15 ml) . The filtrate and wash fractions are combined and evaporated to drynesε, yielding an oily substance that crystallizes upon the addition of 50 ml hexane. The yield of O-benzoylglycolic acid N-hydroxysuccinimide ester is 5.5 g (99.6%). 4.2 g (15 mmol) of this product is, after dissolution in 60 ml acetonitrile, added to a solution of 2.8 g (15 mmol) triglycine in 20 ml saturated sodium bicarbonate solution. After stirring for 18 hours at room temperature, the acetonitrile is removed by evaporation and the water layer is acidified to pH 2.4 with IN hydrochloric

acid to yield the title compound as a white precipitate. The product is filtered off and washed with cold water; yield after drying 4.8 g (90.3%); m.p. 190-195°C (decomp.) . The product can be recrystallized from water:acetone (15:85) . ^** H-NMR (DMSO) : 3.7-3.9 (6H, d, 3 x NH-CH ₂ -CO) ; 4.8 (2H, s, 0- CH ₂ -C0) ; 7.5-8.1 (5H, m, Ar) ; 8.1-8.5 (3H, 3 x t, 3 x NH) . In a corresponding manner O-benzoylglycoloylglycyl-D- alanylglycine is prepared. ^** H-NMR (DMSO) : 1.2 (d, CH ₃ , 3H) , 3.7-3.9 (m, 2 x NH-CH ₂ -C0, 4H) , 4.2-4.5 (q, NH-CJ_(CH ₃ ) -CO, IH) , 4.8 (s, 0-CH ₂ -CO, 2H) , 7.5-8.1 (m, Ar, 5H) , 8.2-8.4 (m, 2 x C0-NH-CH ₂ , CO-NH-CH, 3H) .

Also in a corresponding manner O-benzoylglycoloyl-D- alanylglycylglycine is prepared. ""H-NMR (DMSO) : 1.3 (d, CH ₃ , 3H) , 3.7-3.8 (m, 2 x NH-CH ₂ -CO, 4H) , 4.1-4.5 (q, NH-CH(CH ₃ )- CO, IH) , 4.8 (s, 0-CH ₂ -CO, 2H) , 7.5-8.1 (m, Ar, 5H) , 8.3-8.6 (m, 2 x CO-NH-CH.,, CO-NH-CH, 3H) . Further in a corresponding manner O-benzoyl hydroxyacetyl-D-alanyldiglycine is prepared; identification by NMR.

EXAMPLE II

Synthesis of σlvcoloyltriσlvcine

1.75 g (5 mmol) O-benzoylglycoloyltriglycine is dissolved in 6 ml IN aqueous NaOH. The mixture is stirred for 15 hrs at room temperature and then acidified to pH 2. The precipitate is filtered off and the filtrate is evaporated under reduced pressure. The residue is extracted three times with 20 ml ethanol. Finally the residual powder is dried under vacuum over phosphorus pentoxide to yield the title compound.

EXAMPLE III

Synthesis of 2-O-benzoyl hvdroxypropionyltriαlvcine To a solution of 0.1 mol (10.41 g) methyl lactate in 40 ml pyridine is added at 0°C 0.12 mol (16.88 g, 14 ml) benzoyl

chloride. The reaction mixture is stirred for 15 hrs at room temperature. Pyridine is removed by evaporation under reduced pressure, and 200 ml diethylether is added. The solution is washed successively with 100 ml water, 100 ml 10% sodium bicarbonate solution, 100 ml water and 100 ml 0.3 N hydrochloric acid, and then dried over anhydrous sodium sulphate. After evaporation of the solvent, 18.7 g (90%) of O-benzoyl methyl lactate is obtained as a yellow oil. This product (50 mmol; 10.4 g) is selectively hydrolyzed with a mixture of potassium hydroxide and ethanol according to the above-mentioned method of Ringshaw et al . , converted to the N-hydroxysuccinimide ester and coupled with triglycine, in analogy with the synthesis of O-benzoylglycoloyltriglycine, described hereinbefore. --H-NMR (DMSO) : 1.4-1.6 (d, CH ₃ , 3H) , 3.7-3.9 (m, 3 x NH-CH ₂ - CO, 6H) , 5.2-5.4 (q, O-CH(CH ₃ ) -CO, IH) , 7.4-8.1 ( , Ar, 5H) , 8.1-8.2 (m, CH ₂ -NH-CO, 2H) , 8.4 (t, CO-NH-CH.-COOH, IH) .

EXAMPLE IV Synthesis of the Tc99m complex of hvdroxyacetyltriσlvcine [Tc99m-glycoloyltriglycine, Tc99m-HAG3 ; compound no. (1)] .

Method a

In a 10-ml labelling vial 1 mg O-benzoylglycoloyltriglycine is dissolved in 0.5 ml 0.1 N aqueous NaOH. After 20 min are added consecutively 100 μg SnCl ₂ .2H ₂ 0, dissolved in 25 μl 0.05 N hydrochloric acid, and 10-100 mCi Tc99m in the form of a sodium pertechnetate solution (generator eluate) in 2-5 ml saline solution. After approx. 15 sec the pH of the solution is adjusted to pH 7-8 by the addition of 0.5 M phosphate buffer pH 5. The product is analysed by TLC and HPLC; preparative HPLC can be used to purify the title compound.

In a corresponding manner the Tc99m complex of

hydroxyacetylglycyl-D-alanylglycine [Tc99m-glycoloylglycyl- D-alanylglycine, Tc99m-D-HAGAG; compound no. (6)] is prepared from the above O-benzoylglycoloylglycyl-D- alanylglycine. The product is in the form of two diastereoisomers, which are separatedly isolated by high- pressure liquid chromatography (HPLC) , as described for related diastereoisomers in the above-cited EP-A-250013. In this manner diastereoisomers A (first) and B can be eluted sequentially.

Further in a corresponding manner the Tc99m complex of hydroxyacetyl-D-alanyldiglycine [Tc99m-HAAG2 ; compound no. (8)] is prepared from the above O-benzoyl hydroxyacetyl-D- alanyldiglycine.

Also in a corresponding manner the Tc99m complex of hydroxypropionyl-triglycine [Tc99m-HPG3; compound no. (9)] is prepared from the above 2-O-benzoyl hydroxypropionyltriglycine.

Method b

Glycoloyltriglycine (1 mg) is dissolved in 1 ml 0.1 M phosphate buffer pH 12 in a 10 ml labelling vial. To this solution are added consecutively 100 μg SnCl ₂ .2H ₂ 0, dissolved in 25 μl 0.05 N hydrochloric acid, and 10-100 mCi Tc99m in the form of a pertechnetate solution (generator eluate) in 2-5 ml saline solution. Before use the pH of the solution is adjusted to pH 7-8 by the addition of 0.5 M phosphate buffer pH 5.

Method c (preparation method from a kit)

A kit is prepared by lyophilizing a solution of 1 mg glycoloyltriglycine and 100 μg SnCl ₂ .2H ₂ 0 in 1 ml 0.1 M phosphate buffer pH 12 in a vial. To the lyophilized residue in the vial is added 10-100 mCi Tc99m in the form of a

pertechnetate solution (generator eluate) in 2-5 ml saline solution. Before use the pH of the solution is adjusted to pH 7-8 by the addition of 0.5 M phosphate buffer pH 5.

EXAMPLE V

Investigations in a primate

Tc99m hydroxyacetyltriglycine, obtained according to Example

IV and purified by HPLC, is administered intravenously in a quantity of 37 MBq to a male baboon (12 kg) , sedated with Ketamine® and sodium pentobarbital. 1.85 MBq of 1131- Hippuran® is used as an internal biological standard. Scintigraphic images of the lower abdomen are obtained during 30 minutes by means of a gamma camera provided with a diverging collimator. The data are computer collected to construct time-activity curves (renogra s) of the kidneys. 2 ml blood samples are taken 2, 4, 6, 8, 10, 15, 20, 30, 45 and 60 minutes after injection. After centrifuging 500 μl plasma from each sample is dispensed in a counting tube and the activity of Tc99m and 1131 is counted with a Nal (Tl) scintillation detector, connected to an analyser and counter. The results are corrected for background activity, 1131-cross-over in the Tc99m channel and physical decay during measuring. From the results the lh-plasma clearance of the Tc99m compound is calculated. Corresponding experiments are carried out using Tc99m-hydroxyacetylglycyl- D-alanylglycine (diastereoisomer A) and Tc99m-MAG3, respectively.

The renograms obtained with Tc99m-MAG3 and with the Tc99m complexes according to the invention, viz. Tc99m- hydroxyacetyltriglycine (Tc99m-HAG3) and Tc99m- hydroxyacetylglycyl-D-alanylglycine (diastereoisomer A: Tc99m-D-HAGAG-A) are comparable. On the other hand, the lh- plasma clearance of the latter compounds is 113.8% and

119.7%, respectively, with respect to the lh-plasma clearance of the known Tc99m-MAG3.

The protein binding is 63.3% and 74.4% for Tc99m-HAG3 and Tc99m-D-HAGAG-A, respectively, with respect to the protein binding of Tc99m-MAG3.

From the scintigraphic images it appears that the compounds of the invention have a significantly lower liver-uptake than Tc99m-MAG3.

S-.W1PLE V?

Biodistribution studies in a human volunteer

In a manner comparable with that of Example V the biodistribution of Tc99m-hydroxyacetyltriglycine in comparison with the known Tc99m-MAG3 is determined in a human volunteer. The lh-plasma clearance of this compound is 108.3% with respect to the lh-plasma clearance of Tc99m- MAG3.