阅读说明：本技术 由含钍-228及其子体的水溶液生产铅-212的方法 (Method for producing lead-212 from aqueous solution containing thorium-228 and daughter thereof ) 是由 雷米·迪罗 J·托格 于 2019-09-17 设计创作，主要内容包括：本发明涉及一种用于由含钍-228及其子体的水溶液来生产具有x射线极高纯度的铅-212的方法。应用：制备基于铅-212的放射性药物,可用于核医学,特别是用于治疗癌症的靶向α放射疗法中。(The invention relates to a method for producing lead-212 with extremely high x-ray purity from an aqueous solution containing thorium-228 and its daughter. The application comprises the following steps: preparation of lead-212 based radiopharmaceuticals useful in nuclear medicine, in particular in targeted alpha radiotherapy for the treatment of cancer.)

1. A process for the production of lead-212 from an acidic aqueous solution a1 containing thorium-228 and its daughter, said process comprising the following successive steps:

-purifying the lead-212 present in the aqueous solution a1, the purification comprising the steps of:

a) providing a first chromatography column (10) containing a first stationary phase (20) at a first pH value at which thorium-228 and its daughters are at pH₁And a second pH value of pH₂Selectively retain lead relative to thorium and radium when in an acidic aqueous solution, wherein the pH is₂Greater than pH₁；

b) Loading the aqueous solution A1 into the first chromatography column (10), the aqueous solution A1 having a pH of₁And pH₂A pH of between;

c) at pH with pH₁And pH₂At least one wash of the first stationary phase (20) with an acidic aqueous solution a2 in between;

d) with pH greater than pH₂Eluting the lead-212 from the first stationary phase (20), thereby obtaining an aqueous solution A4 containing lead-212; then the

-purifying the lead-212 present in the aqueous solution a4, the purification comprising the steps of:

e) loading the aqueous solution A4 into a second chromatography column (40) containing a second stationary phase (50) at a first pH value pH at which the thorium-228 and its daughters are at pH₃And a second pH value of pH₄Selectively retain lead relative to thorium and radium when in an acidic aqueous solution, wherein the pH is₄Greater than pH₃；

f) At pH with pH₃And an acidic aqueous solution a5 between pH to perform at least one wash of the second stationary phase (50); and

g) with pH greater than pH₄A6 elutes lead-212 from the second stationary phase (50).

2. The method of claim 1, wherein the pH of the aqueous solution a2 is greater than the pH of the aqueous solution a 1.

3. The process according to claim 1 or claim 2, wherein step c) comprises a first washing of the first stationary phase (20) with the aqueous solution a2 and a pH at pH₁And pH₂A second wash of the first stationary phase (20) with an aqueous solution a2' at a pH between but greater than the aqueous solution a 2.

4. The method of any one of claims 1 to 3, wherein the aqueous solution A3 and aqueous solution A6 are aqueous solutions of a reagent comprising complexed lead.

5. The method of any one of claims 1 to 4, wherein the first chromatography column (10) has a first end and a second end opposite to each other, the aqueous solution A1 and the aqueous solution A2 are circulated in the first chromatography column from the first end to the second end, and the aqueous solution A3 is circulated in the first chromatography column from the second end to the first end.

6. The method according to any one of claims 1 to 5, further comprising, between step d) and step e), pairingThe aqueous solution A4 was acidified to bring the pH of the aqueous solution A4 to pH₃And pH₄A value in between.

7. The process according to claim 6, wherein the acidification of the aqueous solution A4 comprises at least one washing of the first stationary phase (20) with an acidic aqueous solution A7 and the addition of all or part of the aqueous solution resulting from the washing to the aqueous solution A4.

8. The process of any one of claims 1 to 7, wherein the pH of the aqueous solution A5 is greater than the pH of the aqueous solution A4.

9. The process according to any one of claims 1 to 8, wherein step f) comprises a first washing of the second stationary phase (50) with the aqueous solution A5 and with a pH at pH₃And pH₄A second wash of the second stationary phase (50) with an aqueous solution a 5' at a pH between but greater than the aqueous solution a 5.

10. The method of any one of claims 1 to 9, wherein the second chromatography column (40) has a first end and a second end opposite to each other, the aqueous solution a4 and the aqueous solution a5 are circulated in the second chromatography column from the first end to the second end, and the aqueous solution a6 is circulated in the second chromatography column from the second end to the first end.

11. The process according to any one of claims 1 to 10, wherein the first stationary phase and/or the second stationary phase (20, 50) is made of a material comprising a solid carrier impregnated with a solution comprising a crown ether in a water-immiscible organic diluent, preferably an alcohol having 8 or more carbon atoms.

12. The method of claim 11, wherein the crown ether is dicyclohexyl-18-crown-6 or dibenzo-18-crown ether-6 wherein cyclohexyl or benzyl is substituted by one or more linear or branched C₁To C₁₂Alkyl substitution.

13. The process according to claim 12, wherein the crown ether is 4,4'(5') -di-tert-butylcyclohexylo-18-crown-6 and the organic diluent is isodecanol.

14. The process of any one of claims 1 to 13, wherein the aqueous solution a1 and aqueous solution a4 comprise from 1 to 2mol/L nitric acid.

15. The process of any one of claims 1 to 14, wherein the aqueous solution a2 comprises at least 0.01mol/L nitric acid and less than 1mol/L nitric acid.

16. The method according to claim 15, wherein the aqueous solution a2 comprises at least 0.5 and less than 1mol/L nitric acid, and step c) comprises a first wash of the first stationary phase (20) with the aqueous solution a2, and a second wash of the first stationary phase (20) with an aqueous solution a2' comprising at least equal to 0.1 and less than 0.5mol/L nitric acid.

17. The process of any one of claims 1 to 16, wherein the aqueous solution a5 comprises at least 0.1mol/L nitric acid and less than 1mol/L nitric acid.

18. The method of claim 17 wherein the aqueous solution a5 comprises at least 0.5 and less than 1mol/L nitric acid and step f) comprises a first wash of the second stationary phase (50) with the aqueous solution a5 and a second wash of the second stationary phase (50) with an aqueous solution a 5' comprising at least 0.01 and less than 0.5mol/L nitric acid.

19. The process according to any one of claims 1 to 18, wherein said aqueous solutions A3 and a6 are aqueous solutions of ammonium or sodium salts of citric, oxalic or acetic acid having a pH at least equal to 5 and at most equal to 9.

20. Process according to any one of claims 1 to 19, wherein thorium-228 and its subvolumes are in radioactive equilibrium in said aqueous solution a 1.

21. The method according to any one of claims 1 to 20, further comprising, before step a), a step of removing lead-208 present in said aqueous solution a1, which step comprises circulating said aqueous solution a1 in a chromatography column containing a stationary phase that selectively retains lead when thorium-228 and its daughters are in an acidic aqueous phase, and collecting the aqueous solution a1 that has flowed through said chromatography column.

Technical Field

The present invention relates to the field of producing radioisotopes, also known as radioisotopes.

More specifically, the invention relates to a process which enables the production of lead-212 with very high radioactive purity from an aqueous solution containing thorium-228 and its daughters (also called daughter products).

Thus, the method can be applied for the preparation of lead-212 based radiopharmaceuticals suitable for use in nuclear medicine, in particular targeted alpha radiotherapy for the treatment of cancer.

Background

Lead-212, a rare radioisotope of lead, has been the subject of intense research for many years, particularly for the treatment of cancer by targeted alpha radiation therapy (also known as targeted alpha therapy), particularly for the treatment of pancreatic, ovarian, colon, breast and prostate cancers.

Lead-212 is also one of the radioisotopes that has proven to be of interest for medical imaging, particularly in studies by single photon emission computed tomography combined with computed tomography.

In both cases, the use of lead-212 means that it is injected into the patient in the form of a radiopharmaceutical, i.e. a product that is bound, typically by a chelator, to a molecule, such as an antibody, that is capable of highly specific targeting to the cells to be destroyed (in the case of targeted alpha therapy) or to be observed (in the case of medical imaging).

For this purpose, the lead-212 must meet extremely stringent quality requirements, and in particular in terms of radioactive purity, which ideally should be at least equal to 99.95%.

In this respect, the term radioactive purity of a radioisotope (such as lead-212) means: the radioisotope has a purity with respect to the radioisotope obtained by radioactive disintegration and with respect to other radioisotopes that are not part of its radioactive isotope disintegration chain, rather than a purity with respect to the radioisotope of the radioisotope produced by its own radioactive disintegration (i.e. its daughter).

As shown in fig. 1, which represents the radioactive disintegration or decay chain of thorium-232, lead-212 belongs to the daughter product of the radioactive family of thorium-232. Lead-212 is also a daughter product of thorium-228 and radium-224 incorporated in the chain between thorium-232 and lead-212.

In order to produce medical grade lead-212, i.e. lead-212 satisfying the above-mentioned radioactive purity requirements, international PCT applications WO 2013/174949 and WO 2017/093069 (hereinafter referred to as references [1] and [2]) propose the following method comprising:

-producing lead-212 by radioactive disintegration of radium-224 in a generator containing a solid medium on which radium-224 is retained;

-extracting lead-212 from the generator in the form of a strongly acidic aqueous solution; and

-purification of lead-212 by liquid chromatography.

The chromatography is carried out in the following manner: loading a strongly acidic aqueous solution onto a chromatography column, wherein the chromatography column comprises a stationary phase that retains lead under strong acidity but does not retain radioactive impurities that are readily present in the aqueous solution; the stationary phase is then washed with an aqueous solution, also strongly acidic, to remove the radioactive impurities from the column; and then eluting the lead-212 with an aqueous solution having a pH between 5 and 9, such that the lead-212 can be obtained in a purified form.

Production of lead-212 by the methods described in references [1] and [2] requires prior production of radium-224.

The generator is typically a liquid chromatography column containing a solid stationary phase that selectively retains the parent radioisotope and is periodically washed with a liquid phase, thereby enabling elution of the daughter radioisotope formed by radioactive decay of the parent radioisotope in the column.

Considering that the stationary phase of the generator must have an affinity with respect to a specific metal element, it should be understood that the stationary phase composition materials for the stationary phases of the radium-224 and lead-212 generators, respectively, are not only different from each other, but also different from the stationary phase materials for purifying lead-212 in references [1] and [2 ].

However, to simplify the production of lead-212 and for the sake of reducing its cost, it is desirable to be able to reduce the number of different generators and the number of stationary phase materials currently used for such production.

Furthermore, in theory, such individual generators are possible so long as the amount of parent radioisotope immobilized on the stationary phase of the generator is sufficient to ensure production of daughter radioisotope in satisfactory yield.

However, in practice, it has been found that prolonged retention of the parent radioisotope on the stationary phase can lead to progressive degradation of the stationary phase by radiation decomposition, which can progressively affect its ability to retain the parent radioisotope, leading to leakage of the radioisotope, the necessity of regular maintenance of the generator to prevent such leakage, and premature failure of the generator (despite such maintenance). Again, however, this premature failure can result in increased production costs for the lead-212.

In view of the above, the inventors set themselves the object of providing a process which enables the production of lead-212 having a radioactive purity comparable to that of the lead-212 obtained in references [1] and [2] at a significantly lower cost (in particular by reducing maintenance in the production).

Disclosure of Invention

It is precisely an object of the present invention to propose a method which meets these requirements.

The method comprises the following steps:

-purifying the lead-212 present in the aqueous solution a1, the purification comprising the following steps:

a) providing a first chromatography column containing a first stationary phase at a first pH at which the thorium-228 and its daughters are at pH values₁And a second pH value of pH₂Selectively retain lead relative to thorium and radium when in an acidic aqueous solution, wherein the pH is₂Greater than pH₁；

b) Loading an aqueous solution A1 into a first chromatographic column, the aqueous solution A1 having a pH of₁And pH₂A pH of between;

c) at pH with pH₁And pH₂The first stationary phase is washed at least once by the acidic aqueous solution A2;

d) with pH greater than pH₂The aqueous solution A3 of (a) elutes lead-212 from the first stationary phase, thereby obtaining an aqueous solution A4 containing lead-212; then the

-purifying the lead-212 present in the aqueous solution a4, the purification comprising the following steps:

e) the aqueous solution a4 was loaded into a second chromatography column containing a second stationary phase at a first pH value pH at which thorium-228 and its daughters were at pH₃And a second pH value of pH₄Selectively retain lead relative to thorium and radium when in an acidic aqueous solution, wherein the pH is₄Greater than pH₃；

f) At pH with pH₃To pH₄The second stationary phase is washed at least once by the aqueous solution A5 in between; and

g) with pH greater than pH₄The aqueous solution a6 elutes lead-212 from the second stationary phase.

Thus, an aqueous solution containing very high radioactive purity of lead-212 can be obtained using only two stationary phases and without using a radium-224 generator or a lead-212 generator.

The term "daughter" of thorium-228, hereinabove and hereinbelow, refers to all of the radioactive isotopes incorporated downstream from thorium-228 in the deciduate chain of thorium-232 shown in FIG. 1 (i.e., radium-224, radon-220, polonium-216, lead-212, bismuth-212, thallium-208 and polonium-212), as well as lead-208, where lead-208 is a stable isotope of lead, located at the terminus of the chain.

Moreover, the expressions "from … … to … …", "range from … … to … …" and "between … … and … …" are equivalent and are meant to include the boundary values.

According to the invention, the first stationary phase and the second stationary phase may be made of the same or different materials, which may be chosen when these materials are associated with a material having a certain acidity (defined herein as being intermediate in pH for the first stationary phase)₁To pH₂For the second stationary phase at pH₃And pH₄In between) and in which thorium-228 andthe aqueous acidic solution of the daughter was able to retain lead but not all of the material of thorium and radium when contacted. When the first stationary phase and the second stationary phase are composed of the same material, pH₁Is equal to pH₃And pH of₂Is equal to pH₄。

Such materials may include inert, inorganic (such as silica or alumina particles or silica gels), organic (such as polymers) or inorganic-organic solid supports functionalized by grafting or impregnation of organic molecules that retain lead ions (Pb) by ion exchange, extraction, molecular recognition or any other mechanism²⁺) But does not retain thorium and radium ions.

Furthermore, the acid present in aqueous solutions a1, a2, and a5 may be any acid, strong acid, or weak acid, which is preferably the same in aqueous solutions a1, a2, and a 5.

As mentioned above, step c) comprises at least one washing of the first stationary phase with aqueous acidic solution a 2.

This washing is essentially intended to remove from the first chromatography column, in particular from the interstitial volume of the first stationary phase, thorium-228 and its daughter other than lead (in particular radium 224) which can remain in the first chromatography column in step b). Therefore, the washing can be performed with the aqueous solution a2 having the same pH as that of the aqueous solution a 1.

However, advantageously, the washing can also be used to prepare the first stationary phase for contact with the aqueous solution A3, which, as mentioned above, has a pH value of the aqueous solution A3 above the upper limit of the pH range (referred to as pH value)₂) And, therefore, the acidity is less than the lower limit of the range of acidity used in materials that would be retained in the first stationary phase when lead is present in an acidic aqueous solution.

In this case, the aqueous solution A2 is at pH₁To pH₂And at the same time, a pH greater than that of aqueous solution a 1.

Preferably, step c) comprises two successive washes of the first stationary phase, each wash having the function of removing thorium-228 and its daughter, other than lead, capable of remaining in the first chromatographic column, from the first chromatographic column, and of preparing the first stationary phase for contact with the aqueous solution a 3.

In this case, step c) comprises using the pH at pH₁And pH₂The first stationary phase (20) is subjected to a first wash with aqueous phase a2 at a pH between and greater than aqueous phase a1, and with a pH at pH₁And pH₂A first wash of the second stationary phase (20) with aqueous solution a2' at a pH between and greater than aqueous solution a 2.

According to the invention, the aqueous solution A3 used in step d) is advantageously an aqueous solution comprising a reagent that complexes lead.

The complexing agent may be chosen in particular from citrate, oxalate or acetate ions. It may also consist of glycine or an aminopolycarboxylic acid such as ethylenediaminetetraacetic acid (or EDTA), or salts thereof (such as sodium salts). Preferably, the solution is an aqueous solution of ammonium or sodium salts of citric, oxalic or acetic acid having a pH between 5 and 9, and even better a pH between 5 and 7.

Considering that like any chromatography column, the first chromatography column has a first end and a second end opposite to each other, the aqueous solutions a1 and a2 are circulated in the column from its first end to its second end, whereas the aqueous solution A3 is preferably circulated in the opposite direction, i.e. from the second end of the column to the first end of the column.

At a pH of the aqueous solution A4 above the pH₄In the case of (b), then the process advantageously further comprises, between step d) and step e), acidifying the aqueous solution A4 to bring its pH to a value at pH₃And pH₄A value in between. It is clear that when the material of the second stationary phase is the same as the material of the first stationary phase and when the pH of the aqueous solution a4 is greater than pH₂The aqueous solution A4 was acidified in order to bring its pH to pH₁And pH₂A value in between.

This acidification can be carried out by simply adding an acid to the aqueous solution a 4.

However, within the scope of the present invention, it is preferred to acidify the aqueous solution a4 by subjecting the first stationary phase to at least one wash with an acidic aqueous solution a7 and adding all or part of the aqueous solution from this wash to the aqueous solution a 4.

Thus, the volume of the aqueous solution A7 and its acid concentration are advantageously selected so that the addition of the aqueous solution A7 in whole or in part to the aqueous solution A4 makes it possible to bring the pH of the aqueous solution A4 to pH at pH by the aqueous solution A7 itself₃And pH₄Between or at pH₁And pH₂To a value in between (when the material of the second stationary phase is the same as the material of the first stationary phase) and even better to a pH value equal or substantially equal to the pH value of the aqueous solution a 1.

The aqueous solution a7 is preferably circulated in the first chromatography column from the second end of the column to the first end thereof.

Step f) comprises at least one wash of the second stationary phase with aqueous acidic solution a5, as described above.

Here, it is also preferred that the washing is also capable of providing for contacting the second stationary phase with the aqueous solution a6, in addition to removing thorium-228 and its traces of daughter other than lead, which could remain in the second chromatography column in step e) above, from the second chromatography column, in particular from the interstitial volume of the second stationary phase. Like aqueous solution A3, the pH of this aqueous solution a6 is greater than the upper limit of this pH range and, therefore, the acidity is less than the lower limit of the range of acidity used in the material for the first stationary phase when lead is present in the acidic aqueous solution.

Thus, although the washing of step f) can be carried out with aqueous solution A5 having the same pH as that of aqueous solution A4, it is preferred to use a pH at which the pH is at pH₃And pH₄And at the same time, solution a5, which is greater than the pH of aqueous solution a 4.

Like step c), step f) preferably comprises two successive washes of the second stationary phase, i.e. at pH₃And pH₄A first wash with aqueous phase A5 at a pH between and greater than aqueous phase A4 and with a pH at pH₃And pH₄The second washing was carried out with an acidic aqueous solution a 5' between and above the pH of aqueous solution a 5.

The aqueous solution a6 used in step g) is advantageously an aqueous solution comprising a reagent complexing lead.

As mentioned above, the complexing agent may be selected from citrate, oxalate or acetate ions, glycine or an aminopolycarboxylic acid (such as EDTA), and salts of these acids.

Here, it is also preferred that the solution is an aqueous solution of ammonium or sodium salts of citric, oxalic or acetic acid having a pH between 5 and 9, particularly preferably a pH between 5 and 7.

Furthermore, the aqueous solution a6 is preferably circulated in the second chromatography column in a direction opposite to the direction in which the acidic aqueous solutions a4 and a5 are circulated.

In a preferred embodiment of the invention, the first stationary phase and the second stationary phase are made of a stationary phase material that retains lead by extraction, and more particularly of a material comprising a solid support impregnated with a solution containing a crown ether, in particular dicyclohexyl-18-crown-6 or dibenzo-18-crown-6, as extractant in a water-immiscible organic diluent, in which the cyclohexyl or benzyl group is substituted by one or more linear or branched C₁To C₁₂Alkyl substituted, the organic diluent typically being a long chain hydrocarbon alcohol, i.e. C₈Or longer chain hydrocarbon alcohols.

More preferably, the stationary phase material used comprises 4,4'(5') -di-tert-butylcyclohexylo-18-crown-6 as extractant in isodecanol, preferably in solution.

Stationary phase materials of this type are available in particular in the form of particles which can be packed in bottles, but also in the form of ready-to-use chromatography columns or cartridges under the trademark Pb Resin (Pb Resin) from triskam International and Eichrom Technologies, inc.

Such stationary phase materials are capable of retaining lead in an acidic medium (e.g., nitric acid or hydrochloric acid) at a pH typically less than or equal to 2.

However, within the scope of the present invention, it is preferred that the aqueous solution A1 used in step b) is a solution containing nitric acid, advantageously nitric acid with a molar concentration of 1 to 2mol/L, and even better nitric acid with a molar concentration of 2 mol/L.

It is also preferred that the other acidic aqueous solution used in the process also comprises nitric acid.

In this case:

-if step c) comprises only one washing, the aqueous solution a2 has a nitric acid concentration preferably at least equal to 0.01mol/L, but less than 1 mol/L; while

-if step c) comprises two washes, then:

the aqueous solution A2 has a nitric acid concentration preferably at least equal to 0.5mol/L, but less than 1mol/L, with a concentration of 0.5mol/L being particularly preferred, and

the aqueous solution a2' has a nitric acid concentration preferably at least equal to 0.01mol/L, but less than 0.5mol/L, with a concentration of 0.1mol/L being particularly preferred.

Preferably, the aqueous phase A4 used in step f) comprises, like the aqueous phase A1, from 1 to 2mol/L, even better 2mol/L, of nitric acid.

And:

-if step f) comprises only one washing, the aqueous solution a5 has a nitric acid concentration preferably at least equal to 0.01mol/L, but less than 1 mol/L; while

-if step f) comprises two washes, then:

the aqueous solution A5 has a nitric acid concentration preferably at least equal to 0.5mol/L, but less than 1mol/L, with a concentration of 0.5mol/L being particularly preferred, and

the aqueous solution a 5' has a nitric acid concentration preferably at least equal to 0.01mol/L, but less than 0.5mol/L, with a concentration of 0.1mol/L being particularly preferred.

As regards the aqueous solutions A3 and a6, they preferably consist of aqueous solutions of ammonium or sodium salts of citric, oxalic or acetic acid, having a pH at least equal to 5 so that the citrate, oxalate or acetate ions exert their complexing power with respect to the lead-212 and thus facilitate the elution of the lead-212 from the first and second stationary phases, respectively, and a pH at most equal to 9, with a particularly preferred pH between 5 and 7.

Advantageously, thorium-228 and its daughter are in radioactive equilibrium in aqueous solution a 1.

In the case where it is sought to optimize the specific radioactivity of the lead-212 to be produced, it is possible to provide, before step a), a step aimed at removing the lead-208, in which the lead-208 is located at the end of the deciduate chain of thorium-232 and tends to accumulate in the aqueous solution a1 in the form of a stable lead isotope.

In this case, this step comprises passing an aqueous solution a1 through a chromatography column containing a stationary phase which selectively retains lead when the thorium-228 and its daughter are in an acidic aqueous solution, and collecting the aqueous solution a1 which has passed through the chromatography column.

Step a) can then be performed by simply allowing the lead-212 to reform through the decay of the radium-224 present in aqueous solution A1, which requires waiting about 48 hours before performing step a).

According to the present invention, the stationary phase used to remove lead-208 from aqueous phase a1 may be comprised of the same or different material as the first stationary phase and the second stationary phase. However, in order to reduce the number of different stationary phase materials used, it is preferable that the stationary phase used for removing the lead 208 from the aqueous phase a1 be composed of the same material as the material forming the first stationary phase and the second stationary phase.

In addition to the advantages already mentioned, the process also includes the advantage of being able to be carried out without using hydrochloric acid, which is the acid present in the acidic aqueous solution used in the examples of references [1] and [2 ]. In fact, it is advantageous to avoid the use of hydrochloric acid, since hydrochloric acid is a highly corrosive acid, which in the long term can damage the equipment used to carry out the production of lead-212.

Further features and advantages of the method according to the invention will appear from reading the following supplementary description relating to a preferred embodiment of the method.

It is clear that this preferred embodiment is given only by way of illustration of the subject of the invention and does not represent in any way a limitation of this subject.

Drawings

FIG. 1, shows the radioactive disintegration chain of thorium-232 as described previously.

Fig. 2 schematically shows the different steps of a preferred embodiment of the method according to the invention.

Detailed Description

With reference to fig. 2, the different steps of a preferred embodiment of the method according to the invention are schematically shown (reference numerals 1 to 7).

In this embodiment, the method comprises the steps of:

1. loading an aqueous nitric acid solution a1, comprising thorium-228 and its daughter preferably in a radioactive equilibrium state, into a first column (reference numeral 10) whose stationary phase (reference numeral 20) consists of Pb resin particles, to immobilize the lead-212 contained in the solution on the stationary phase 20;

2. two successive washes of the stationary phase 20 with two aqueous solutions of nitric acid a2 and a2' of decreasing concentration are carried out to remove from the column 10 the radioisotopes other than lead-212 that could remain in the column 10 in the previous step, while preparing for contacting the stationary phase 20 with an aqueous solution having a higher pH in the next step;

3. eluting lead-212 from the stationary phase 20 by means of an aqueous ammonium acetate solution A3 and collecting the eluate or aqueous solution a4 containing lead-212 in a container such as a beaker, flask or similar (reference numeral 30);

4. washing the stationary phase 20 with an aqueous nitric acid solution a7 and collecting the solution resulting from the washing in a vessel 30 to acidify the eluent present in the vessel;

5. loading the eluent/wash mixture or acidified aqueous solution a4 present in vessel 30 into a second chromatographic column (reference numeral 40) whose stationary phase (reference numeral 20) consists of Pb resin particles to immobilize lead-212 contained in the mixture on the stationary phase (reference numeral 50);

6. two successive washes of the stationary phase 50 with two aqueous solutions of nitric acid a5 and a 5' of decreasing concentration are carried out to remove from the column 40 the traces of radioisotopes other than lead-212 that could remain in the column 40 in the previous step, while preparing for contacting the stationary phase 50 with an aqueous solution having a higher pH in the next step; and

7. the lead-212 is eluted from the stationary phase 50 by means of an aqueous ammonium acetate solution a6 and the eluate containing the lead-212 is collected in a container (reference numeral 60).

All these steps, detailed hereinafter, are carried out at ambient temperature, i.e. at a temperature of from 20 ℃ to 25 ℃.

Furthermore, all solutions used are preferably Optima^TMStage or use of Optima^TMA grade or "trace metal grade" reagent.

*Step 1：

The chromatography column 10 is a column having a bed volume (or BV), for example in the range of 0.104mL to 1.25mL, and packed with Pb resin particles (size between 50 μm to 100 μm), for example in a proportion of particles of 42mg to 500mg, depending on the BV of the column.

At an acidity of the nitric acid of between 0.01mol/L and 10mol/L, the Pb resin retains any isotopic form of lead (A)²¹²Pb and²⁰⁸pb) but does not retain thorium-228 or its daughter other than lead, and in particular does not retain radium in any isotopic form.

However, its affinity for lead is optimal at nitric acid concentrations of 1mol/L to 2 mol/L.

The loading of the column is carried out by circulating an aqueous solution a1 in several BVs of the column 10, the aqueous solution a1 comprising:

thorium-228 and its daughters, preferably in radioactive equilibrium, in particular radium-224 produced by radioactive disintegration of thorium-228 and lead-212 produced by radioactive disintegration of radium-224; and

nitric acid, preferably at a concentration of between 1 and 2mol/L, and even better still of 2mol/L, as described above, in order to obtain an optimal retention of the lead 212 on the stationary phase 20.

The aqueous solution A1 is circulated in the column 10 at a flow rate of preferably 0.5BV/min to 2 BV/min.

For BV from 0.104mL to 1.25mL, the volume of aqueous solution A1 used is advantageously between 80mL and 400mL, while the activity of thorium-228 in this solution is between 2mCi and 11 mCi.

*Step 2：

As mentioned above, this step comprises subjecting the stationary phase 20 to two successive washes, each wash having the following functions:

removing from the column 10, in particular from the interstitial volume of the stationary phase 20, the radioisotopes other than lead-212, in particular thorium-228 and radium-224, which could remain in the column 10 in step 1 above, and

provision is made for contacting the stationary phase 20 with an aqueous ammonium salt solution which is used in step 3 thereafter for eluting the lead-212 and thus facilitates elution.

Thus, the aqueous solutions a2 and a2 'used for these washes were two nitric acid solutions, in which the acidity of the aqueous solution a2 used for the first wash was lower than that of the aqueous solution a1 used for immobilizing lead-212 on the stationary phase in the above step 1, and the acidity of the aqueous solution a2' used for the second wash was again lower than that of the aqueous solution a 2.

However, the acidity of the aqueous solutions a2 and a2' must be selected so that elution of lead-212 can be avoided at this stage of the process.

Thus:

the aqueous solution A2 preferably contains at least 0.5mol/L but less than 1mol/L of nitric acid, and even better 0.5mol/L of nitric acid, while

The aqueous solution A2' preferably contains at least 0.01mol/L but less than 0.5mol/L nitric acid, and even better 0.1mol/L nitric acid.

The amount of BV used is, for example, 20BV for aqueous solution a2 and 10BV for aqueous solution a 2'.

As to the flow rates of the aqueous solutions A2 and A2' through the column 10, for example, 1.6BV/min for both solutions.

*Step 3：

The lead-212 was eluted from the stationary phase 20 by passing several BV of an aqueous ammonium acetate solution a3 through the column 10 in the opposite direction to the direction in which the loading of step 1 above and the washing of step 2 above were performed.

The pH of the aqueous solution A3 is at least equal to 5 to enable acetate ions to exert their complexing power with respect to lead-212, but it is preferably at most equal to 7 to avoid having to use a large excess of BV of the aqueous solution a7 to acidify the eluate in step 4 thereafter.

Ideally, 25BV of an aqueous solution A3 containing 0.4mol/L ammonium acetate at pH 6.5 was used, the aqueous solution A3 was circulated through the column 10 at a rate of 1.6 BV/min.

*Step 4：

The washing of the stationary phase 20 provided in this step is used not only to wash the column 10 but also to acidify the eluate or aqueous solution a4 collected in the previous step to carry out the subsequent step 5.

Thus, the washing is performed by: several BV of aqueous nitric acid a7 are made to circulate in the column 10, also in the opposite direction to the direction in which the loading of step 1 and the washing of step 2 above are carried out, and the solution resulting from this washing is collected in the container 30 in which the eluent collected in step 3 above is present in the container 30. This collection is advantageously carried out with stirring, so that the solution resulting from the washing is mixed with the eluent present when it comes out of the column 10.

The number of BV's of the aqueous solution a7 used for the washing and the nitric acid concentration of the solution are advantageously selected so as to obtain an eluent/washing solution mixture or an acidified aqueous solution a4, or in other words an acidified aqueous solution a4 containing 1 to 2mol/L of nitric acid, in which the nitric acid concentration is such as to optimally fix the lead-212 contained in the mixture on the stationary phase 50 used in step 5 hereafter.

Typically, an aqueous solution A7 containing 2 to 4mol/L nitric acid is used at 10 to 20 BV.

Thus, for example, for an eluate obtained with 25BV of an aqueous solution A3 containing 0.4mol/L ammonium acetate, an aqueous solution A7 containing 3mol/L nitric acid of 15BV was found to be very suitable.

The flow rate of the aqueous solution A7 through the column 10 was, for example, 1.6 BV/min.

If necessary, the nitric acid concentration of the acidic solution A4 may be adjusted by adding nitric acid if the concentration is to be adjusted upward, or the nitric acid concentration of the acidified solution A4 may be adjusted by adding ultrapure water (at 25 ℃ C., resistivity: 18.2 M.OMEGA.cm) if the concentration is to be adjusted downward, before proceeding to the next step.

*Step 5：

The lead-212 present in the acidified aqueous solution a4 has not yet reached the radioactive purity standard required for medical use.

Thus, step 5 is a second loading of the acidified aqueous solution a4 into a chromatography column packed with Pb resin particles to further purify the lead-212, in particular with respect to traces of thorium-228 and radium-224 still present in the acidified aqueous solution a 4.

The chromatographic column 40 used for this purpose may be exactly the same column as the column 10, having the same bed volume and the same mass of Pb resin particles.

However, as shown in fig. 2, it is preferable to use a column 40 having a size significantly smaller than that of the column 10 to purify the lead in conformity with the concentration of the lead 212.

Thus, in particular, the column 40 having a mass of BV and Pb resin particles 4 to 7 times smaller than that of the column 10 can be used.

The acidified aqueous solution a4 was loaded onto the stationary phase 50 by flowing through the column 40 at a flow rate of preferably 0.5BV/min to 2 BV/min.

*Step 6：

The two washes provided in this step have the same function as the wash provided in step 2 above, namely:

removing from the column 40, in particular from the interstitial volume of the stationary phase 50, the radioisotopes other than lead-212, in particular thorium-228 and radium-224, which can remain in the column 40 in step 5 above, and

in preparation for contacting the stationary phase 50 with an aqueous ammonium acetate solution a6, which aqueous ammonium acetate solution a6 is used in step 7 thereafter for elution of lead-212 and thus facilitates elution.

Therefore, it is preferable to perform these two washes with aqueous nitric acid solutions a5 and a5 'having the same concentrations as the aqueous solutions a2 and a2' used in the above-described step 2, respectively, and under conditions similar to those described for this step 2.

*Step 7：

Like the elution of the lead 212 of the stationary phase 20, the lead-212 is eluted from the stationary phase 50 by circulating in the column 40 an aqueous ammonium acetate solution a6 of several BV with a pH at least equal to 5, and even better between 5 and 7, such as a solution containing 0.4mol/L ammonium acetate (pH 6.5) at a flow rate of, for example, 2 BV/min.

Preferably, the elution fraction collected during this elution is maintained according to the medical application for which the lead-212 is intended.

Thus, only the elution fraction most rich in lead-212 can be maintained to obtain an aqueous solution with a high concentration of lead-212, although all elution fraction names can also be maintained but at the expense of diluting the lead-212.

The method according to the invention is implemented according to the above described embodiment using the following steps:

-step 1: the column 10 had a BV equal to 0.63mL and contained 255mg of Pb resin particles, and 275BV of an aqueous solution A1 containing 6.1mCi of thorium-228 and 2mol/L nitric acid was circulated therein at a flow rate of 1 mL/min;

-step 2: 20BV of an aqueous solution A2 containing 0.5mol/L nitric acid was circulated at a flow rate of 1mL/min, then 10BV of an aqueous solution A2' containing 0.1mol/L nitric acid was circulated at a flow rate of 1 mL/min;

-step 3: 25BV of an aqueous solution A3, pH equal to 6.5 and containing 0.4mol/L ammonium acetate, was circulated at a flow rate of 1 mL/min;

-step 4: 15BV of an aqueous solution A7 containing 3mol/L nitric acid was passed through at a flow rate of 1 mL/min;

-step 5: the column 40 has a BV equal to 0.104mL and contains 52mg of Pb resin particles and the aqueous acidic solution a4 obtained at the end of step 4 circulates in the column at a flow rate of 0.2mL/min, this acidified aqueous solution a4 corresponding to an aqueous solution of 220BV for the column 40, considering the BV of the column 40;

-step 6: 20BV of an aqueous solution A5 containing 0.5mol/L nitric acid was circulated at a flow rate of 1mL/min, then 10BV of an aqueous solution A5' containing 0.1mol/L nitric acid was circulated at a flow rate of 0.2 mL/min;

-step 7: 40BV of an aqueous solution A6, pH equal to 6.5 and containing 0.4mol/L ammonium acetate, was passed through at a flow rate of 0.2 mL/min.

An aqueous solution containing lead-212 is thus obtained, in which the radioactive purity of lead-212 is at least equal to 99.99% with respect to thorium-228 and at least equal to 99.95% with respect to radium-224, i.e. a radioactive purity comparable to that obtained in references [1] and [2 ].

The activity of lead-212, thorium-228 and radium-224 was measured by gamma spectroscopy and the radioactive purity was determined using the following formula:

the purity of lead 212 relative to thorium-228 [, ]²¹²Pb Activity/(²¹²Pb activity +²²⁸Thorium activity)]×100；

The lead 212 has a purity of [2], [ alpha ] with respect to the radium-224 ]²¹²Pb Activity/(²¹²Pb activity +²²⁴Radium activity)]×100。

Reference to the literature

[1]WO-A-2013/174949

[2]WO-A-2017/093069