Technetium-99m generator system

Abstract

A .sup.99 Mo/.sup.99m Tc generator system includes a sorbent column loaded with a composition containing .sup.99 Mo. The sorbent column has an effluent end in fluid communication with an anion-exchange column for concentrating .sup.99m Tc eluted from the sorbent column.A method of preparing a concentrated solution of .sup.99m Tc includes the general steps of:a. providing a sorbent column loaded with a composition containing .sup.99 Mo, the sorbent column having an effluent end in fluid communication with an anion-exchange column;b. eluting the sorbent column with a salt solution to elute .sup.99m Tc from the sorbent and to trap and concentrate the eluted .sup.99m Tc on the ion-exchange column; andc. eluting the concentrated .sup.99m Tc from the ion-exchange column with a solution comprising a reductive complexing agent.

Description

FIELD OF THE INVENTFON

The present invention relates to processes for the production of technetium-99m (.sup.99m Tc) via decay of molybdenum-99 (.sup.99 Mo), and more particularly to such processes which involve utilization of .sup.99 Mo produced via neutron capture of a .sup.98 Mo target.

BACKGROUND OF THE INVENIION

.sup.99m Tc is the principal radioisotope used in diagnostic nuclear medicine with an estimated 10million medical tests per year and $34 billion cost to patients annually. This radioisotope is produced from the decay of .sup.99 Mo which at the present time is produced from fission of uranium-235 (.sup.235 U) and is primarily available from Nordion Inc., Canada.

For the following three reasons there are distinct long-term advantages for the routine production of .sup.99 Mo via alterative routes.

1. The fission route produces very high levels of radioactive waste including several radioactive gases, thus requiring complex, dedicated processing and waste disposal facilities which are nearly prohibitively expensive to operate.

2. In the fission produced .sup.99 Mo, the target material is highly enriched .sup.235 U which is a weapons grade substance requiring extensive safeguards. As a part of the international nuclear non-proliferation movement led by United States, there is concern about availability of highly enriched .sup.235 U in the near future.

3. There are environmental issues which are an integral part of such processes. During chemical processing of irradiated .sup.235 U, there exists potential for catastrophic criticality accidents resulting in release of highly radioactive fission products.

Concerning the manufacture and use of .sup.99 Mo/.sup.99m Tc generators, the principal issue which differentiates fission produced .sup.99 Mo (Mo-F) from direct neutron capture produced .sup.99 Mo (Mo-N) is specific activity. The theoretical specific activity of Mo-F is 480 Ci/mg. Commercially available .sup.99 Mo/.sup.99m Tc generators require .sup.99 Mo having specific activity of 1-2 Ci/mg. Fission-produced .sup.99 Mo has specific activity of about 100 Ci/mg, whereas specific activity of neutron capture produced .sup.99 Mo, even that produced in the highest available neutron flux (the well known hydraulic tube facility of the High Flux Isotope Reactor located at the Oak Ridge National Laboratory, Oak Ridge, Tenn.), is in order of 50-100 mCi/mg--a factor of up to 40 times lower than required for conventional generator systems. For example, in one widely used generator, .sup.99 Mo is loaded onto a column of alumina and .sup.99m Tc is eluted from the column with normal saline. The required bolus volume for the quantitative elution of .sup.99m Tc obviously depends on the size of the column which in turn is inversely proportional to the specific activity of .sup.99 Mo. Thus the lower specific activity of neutron-capture-produced .sup.99 Mo requires a larger alumina column, and hence the specific volume of .sup.99m Tc (mCi /mL) is lower.

In order to provide .sup.99m Tc in the specific volumes which are attainable from a fission-produced .sup.99 Mo/.sup.99m Tc generator, methods of concentrating .sup.99m Tc have been suggested by S. Seifert, G. Wagner and A. Eckardt, "Highly Concentrated �.sup.99m Tc! Pertechnetate Solutions from (n,.gamma.).sup.99 Mo/.sup.99m Tc Generators for Nuclear Medicine Use", Appl Radiat. Isot., 45, 577 (1994). Such an approach for concentration of .sup.99m Tc is a two-step, rather complex process which is not readily amenable to automation. For example, in that method, .sup.99m Tc is first eluted from an alumina column with normal saline, followed by reduction of the pertechnetate by addition of SnCI.sub.2. The reduced .sup.99m Tc is then loaded on a second alumina column and is then oxidized by washing the column with a solution of ammonium hydroxide containing H.sub.2 O.sub.2. After evaporating the H.sub.2 O.sub.2 at elevated temperature, .sup.99m Tc is eluted with 2-3 mL of normal saline from the second alumina column. It is thus evident that a need exists for an inherently simple method which can be easily operated.

A two-stage tandem generator system was developed for concentration of carrier-free .sup.188 Re. See U.S. Pat. No. 5,186,913, issued Feb. 16, 1993 and U.S. Pat. No. 5,275,802, issued Jan. 4, 1994, both issued to F. F. Knapp, E. C. Lisic, S. Mirzadeh, and A. P. Callahan, and entitled, "Tungsten-188/Carrier free Re-188 Perrhenic Acid Generator System". In part of the process taught therein, ".sup.8 Re is eluted from an anion-exchange column with strong HNO.sub.3, followed by evaporation of HNO.sub.3 to near dryness for conversion of .sup.188 Re to the chloride form. The conversion step is not amenable to simple operation.

N. C. Schroeder, "Technetium Partitioning for Hanford Tank Waste Remediation System: Sorption of Technetium from DSS and DSSF-7 Waste Stimulants Using Rellex-HPQ Resin", LAUR-95-40, Jan. 6, 1995, primarily deals with the separation of long-lived .sup.99 Tc from legacy waste. .sup.99 Tc is adsorbed onto an anion exchange resin followed by reductive elution of .sup.99 Tc with a solution containing a complexing agent, ethylenediamine (EDA).

There is a need to provide a method of reconcentrating .sup.99m Tc produced by decay of .sup.99 Mo, thus providing an inherently simple method and a device that can be easily operated.

It should be understood that when isotopes are referred to herein by a simple term such as .sup.99 Tc, that a solution containing that isotope is usually what is inferred.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide new and improved processes for the production of .sup.99m Tc which do not involve the use of .sup.235 U.

It is another object of the present invention to provide new and improved processes for the production of .sup.99m Tc which do not involve nuclear fission of .sup.235 U.

It is also an object of the present invention to provide new and improved processes for the production of .sup.99m Tc which do not produce large amounts of radioactive waste.

It is a further object of the present invention to provide new and improved processes for the production of .sup.99m Tc which involve the production of .sup.99 Mo via a non-fission process.

It is yet another object of the present invention to provide new and improved processes for the production of .sup.99m Tc which minimize health and environmental risks.

It is yet a further object of the present invention to provide new and improved processes for the production of .sup.99m Tc which significantly increase availability thereof, while significantly reducing unit cost thereof

It is another object of the present invention to provide a method of reconcentrating .sup.99m Tc produced by decay of .sup.99 Mo.

It is another object of the present invention to provide .sup.99m Tc in highly concentrated form.

It is another object of the present invention to provide .sup.99m Tc produced by a non-fission route in specific volume (mCi/mL) comparable to that obtainable from the fission route.

It is a further object of the present invention to provide .sup.99m Tc which is suitable for subsequent radiopharmaceutical formulations, such as attachments of .sup.99m Tc to antibodies.

Further and other objects of the present invention will become apparent from the description contained herein.

SUMMARY OF THE INVENION

In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method of preparing a concentrated solution of .sup.99m Tc includes the general steps of:

a. providing a sorbent column loaded with a composition containing .sup.99 Mo, the sorbent column having an effluent end in fluid communication with an anion-exchange column;

b. eluting the sorbent column with a salt solution to elute .sup.99m Tc from the sorbent and to trap and concentrate the eluted .sup.99m Tc on the ion-exchange column; and

c. eluting the concentrated .sup.99m Tc from the ion-exchange column with a solution comprising a reductive complexing agent.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing:

FIG. 1 is an apparatus suitable for use in carrying out the present invention.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.

DETAILED DESCRIPTION OF THE INVENTION

Development of Preferred Method

On the basis of our experience with the development of the two-stage tandem generator system for preparation of carrier-free .sup.188 Re, we expected that carrier-free .sup.99m Tc (which chemically resembles Re) should be strongly retained in a small anion exchange column from a dilute solution of NH.sub.4 NO.sub.3 and then eluted with strong HNO.sub.3. In a typical study, .sup.99m Tc was eluted from a .sup.99 Mo-N generator with 10 mL of 0.1 M NH.sub.4 NO.sub.3 and loaded onto an anion exchange column (2.times.10 mm, Dowex AG W 1X8, 100-200 mesh, N0.sub.3 - form preequilibrated with the 0.1 M NH.sub.4 NO.sub.3). The column was then washed with 5 mL of 0.1 M NH.sub.4 NO.sub.3 and followed by 2 mL of H.sub.2 O. Subsequently, .sup.99m Tc was quantitatively stripped from the column with 3.times.1 mL of 6 M HNO.sub.3. Because of high volatility, solutions of .sup.99m Tc in nitric acid were deemed impractical for use in a hospital setting. Thus, another method was needed for quantitative elution of .sup.99m Tc from the anion-exchange column.

Subsequently, it was found that the retained .sup.99m Tc on such an anion-exchange column can be eluted with a weakly basic solution of stannous chloride (SnCl.sub.2) and ethylenediamine (EDA). Such a column is herein after referred to as a .sup.99m Tc concentrator column.

The following steps set forth a suitable method of carrying out the invention. It should be understood that parameters such as molarities, amounts, etc. are expressed as generalities and are not to be construed as critical to the operability of the invention.

Step 1. A typical sorbent (usually adsorbent) type generator column is charged with an adsorbent such as alumina, zirconia, zirconium molybdate, etc. and pre-equilibrated, usually with a salt solution such as 0.1 M NH.sub.4 NO.sub.3.

Step 2. Molybdenum is loaded on the column as MoO.sub.4.sup.2- or hydrated MoO.sub.3 at pH 3-4. The loading solution can be prepared, for example, by titrating about 1 mL of a 0.003 mg/L .sup.99 Mo stock solution (.about.3 mg) with 1 M HNO.sub.3.

Step 3. .sup.99m Tc is eluted from a .sup.99 Mo-N alumina generator with a salt solution such as 0.1 M NH.sub.4 NO.sub.3 or normal saline.

Step 4. The eluted solution is loaded onto a .sup.99m Tc concentrator column containing, for example 2.times.10 mm, Dowex AG W 1X8, 100-200 mesh, NO.sub.3 - form preequilibrated with 0.1 M NH.sub.4 NO.sub.3. Most commercially available anion exchange resins are suitable for this step, for example, Dowex AG 1, Dowex AG 2, Dowex AG 4, REILLEX HPQ, REILLEX 402, etc.

Step 5. The .sup.99m Tc concentrator column is washed with a small amount of 0.1 M NH.sub.4 NO.sub.3 followed by a small amount of H.sub.2 O.

Step 6. .sup.99m Tc is stripped from the column with 5 small portions of a reductive complexing agent such as EDA reagent, which is a mixture comprising, for example, about 0.004 parts of SnCl.sub.2 about 1 part of 10% EDA/H.sub.2 O, about 1 part of 0.1 M NaOH and about 10 parts of H.sub.2 O, purged with N.sub.2). Other suitable complexing agents include, but are not limited to molecules containing at least one of these functional groups: amines, amides, ketones, organic acids, organic bases, dithiol chelating groups.

Many salt solutions are suitable for use instead of the preferred 0.1 M NH.sub.4 NO.sub.3 described above. Other examples include solutions of NaNO.sub.3, NH.sub.4 CI, NaCl, etc.

Description of Preferred Generator System

A suitable generator system is shown schematically in FIG. 1, with arrows showing the flow of fluid into and out of system components. An alumina column 11, generally having a capacity of about 25-1000 mL, is attached via a three-way valve 13 to a smaller .sup.99m Tc concentrator column 15, which generally has a capacity of about 1-5 mL. A submicron filter (for example, 0.22 .mu.m) 17 is preferably located at the exit end of the alumina column for preventing adsorbent particles from contaminating the .sup.99m Tc concentrator column 15. A peristaltic or other type of pump 19 can be used to force fluid through the system.

In a preferred configuration, a first port 21 of the three-way valve 13 is attached to the effluent 31 of the alumina generator 11, and a common port 23 of the three-way valve 13 is attached to the inlet 33 of the .sup.99m Tc concentrator column 15. A second port 25 of the three-way valve 13 is used to introduce fluids directly into the .sup.99m Tc concentrator column 15 in order to perform functions such as elution of purified .sup.99m Tc solutions therefrom and regeneration and washing thereof.

The function of the three-way valve 13 can be achieved by a simple Y-connection, with backflow preventing means such as pinchcocks or check valves in the influent lines (not illustrated). Check valves would not require attention such as that needed to operate pinchcocks or a three-way valve. It is also evident to those skilled in the art that there are other possible configurations of various conventional components that are suitable for carrying out the present invention. The invention is not limited by the apparatus used to carry out the method.

Operation of Preferred Generator System

The following is a more specific description of a preferred embodiment of the present invention, to include how to carry out a preferred embodiment of the invention via the preferred generator system.

Prior to elution of .sup.99m Tc from the generator system, the three-way valve 13 is adjusted so that the second port 25 communicates with the common port 23. Through the second port 25, the .sup.99m Tc concentrator column 15 is preferably equilibrated, for example, with about 5 mL of 0.1 M NH.sub.4 NO.sub.3.

The .sup.99m Tc concentrator column 15 is preferably regenerated after each use thereof. For example, regeneration can be carried out by the following sequence: about 5 mL each of 0.1 M NaOH, H.sub.2 O, 0.1 M HNO.sub.3, H.sub.2 O, through the second port 25.

The three-way valve is adjusted so that the first port 21 communicates with the common port 23. The generator system is cluted with about 10 mL of 0.1 M NH.sub.4 NO.sub.3 or normal saline. The eluted .sup.99m Tc is trapped and concentrated in the .sup.99m Tc concentrator column 15.

The three-way valve 13 is adjusted so that the second port 25 communicates with the common port 23. The .sup.99m Tc concentrator column 15 is washed, through the second port 25, with about 2 mL of each 0.1 M NH.sub.4 NO.sub.3 and H.sub.2 O, The .sup.99m Tc is then cluted with 5 portions of about 0.5 mL of the EDA reagent to obtain about a ten-fold concentration thereof.

The method can be succesfully repeated using a single .sup.99 Mo-N charge 5 times or more, extended over 2 weeks (shelf-life of .sup.99 Mo-N).

EXAMPLE I

.sup.99m Tc was eluted from a .sup.99 Mo-N alumina generator with 10 mL of 0.1 M NH.sub.4 NO.sub.3. The eluted solution was loaded onto a .sup.99m Tc concentrator column (2.times.10 mm, Dowex AG W 1X8, 100-200 mesh, NO.sub.3 form preequilibrated with the 0.1 M NH.sub.4 NO.sub.3). After loadin& the column was washed with 2 mL of 0.1 M NH.sub.4 NO.sub.3 followed by 2 mL of H.sub.2 O. Subsequenty, .sup.99m Tc was stripped from the column with 5.times.0.5 mL of EDA reagent (a freshly prepared mixture of 4.0 mg of SnCl.sub.2, 1 mL of 10% EDA/H.sub.2 O, 1.0 mL of 0.1 M NaOH and 10 mL of H.sub.2 O, purged with N.sub.2). As shown in Table 1, greater than 75% of .sup.99m Tc is eluted in the first two-4.5 mL portions of EDA reagenL The specific volume of .sup.99m Tc was 0.35 mCi/mL after elution from alumina column, and 2.7 mCi/imL after concentration--a factor of 8 increase in concentration thereof.

TABLE 1______________________________________Typical data from operation of .sup.99 Mo/.sup.99m Tc generator system .sup.99m Tc activityStep Time Operation step/Sample mCi %______________________________________1 10:05 Elution of .sup.99m Tc from alumina column 3.50 100 (10 mL, 0.9% NaCl)2 10:12 .sup.99m Tc concentrator column load 0.350 10 wash 1, (2 mL, 0.1 M NH.sub.4 NO.sub.3) 0.010 0.03 wasb 2, (2 mL, H.sub.2 O) 0.006 0.0203 10:22 Strip 1, (0.5 mL of EDA reagent) 2.10 60.0 10.28 Strip 2, (0.5 mL of EDA reagent) 0.60 17.1 10.30 Strip 3, (0.5 mL of EDA reagent) 0.126 3.6 10.32 Strip 4, (0.5 mL of EDA reagent) 0.036 1.0 10:34 Strip 5, (0.5 mL of EDA reagent) 0.104 3.0Total steps 3.33 95.12 & 3______________________________________

Moreover, upon acidification of the .sup.99m Tc-EDA complex to a pH of about 4, the EDA ligand readily exchanges with other ligands such as citrate or gluconate with 92-94% complexation within 30 minutes (citrates and gluconates are typical ligands used in the process of .sup.99m Tc labeling of various tissue specific agents).

EXAMPLE II

.sup.99m Tc-EDA complex prepared in accordance with Example I was acidified to a pH of about 4 by addition of 500 .mu.L 0.5 of M citrate buffer (pH=4.2). The EDA ligand exchanged with the citrate ligand, with 92-94% complexation within 30 minutes.

EXAMPLE III

.sup.99m Tc-EDA complex prepared in accordance with Example I was acidified to a pH of about 4 by addition of 500 .mu.L 0.5 of M gluconate buffer (pH=4.2). The EDA ligand exchanged the gluconate ligand, with 92-94% complexation within 30 minutes.

Furthermore, the .sup.99m Tc-gluconate complex prepared via exchange with EDA can be used for direct labeling of an antibody with .sup.99m Tc with yields of greater than 80% within one hour.

EXAMPLE IV

.sup.99m Tc-gluconate complex prepared in accordance with Example III was used for direct labeling of an antibody with .sup.99m Tc with yields of greater than 80% within one hour.

Unique advantages of this invention include:

1. The neutron capture route utilized in the preferred embodiment of the present invention generates minimal waste and does not require a dedicated processing facility. The fission route, on the other hand, produces very high levels of radioactive waste including several radioactive gases, thus requiring dedicated processing facilities which are expensive to operate.

2. The target material for production of .sup.99m Tc by the neutron capture route is enriched .sup.98 Mo. This eliminates the necessity for the use of highly enriched .sup.235 U (the current target material) which is a weapons grade substance requiring extensive safeguards.

3. Enriched .sup.98 Mo is readily available, whereas availability of highly enriched .sup.235 U in near future is in doubt.

4. In relative terms, the neutron capture route is by far more environmentally friendly than the fission route, where the potential for a catastrophic accident exists resulting in release of fission products.

Moreover, the invention is useful for concentrating .sup.99m Tc product of fission-produced Mo-F. The useful shelf-life of Mo-F/.sup.99m Tc generator is about a week. After this period, the specific volume (mCi/mL) of .sup.99m Tc becomes too low for mrrost clinical use. The above described .sup.99m Tc concentrator is also suitable for use with fission Mo generator, therefore by attaching the .sup.99m Tc concentrator to the outlet of a conventional Mo-F/.sup.99m Tc generator, the shelf-life of such generators can be substantially extended.

While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the inventions defined by the appended claims.

Claims

1. A method of preparing a concentrated solution of .sup.99m Tc comprising the steps of:

a. providing a sorbent column loaded with a composition containing .sup.99 Mo, said sorbent column having an effluent end in fluid communication with an anion-exchange column;

b. eluting said sorbent column with a salt solution to elute a first eluate comprising .sup.99m Tc from said sorbent and to-trap said eluted .sup.99m Tc on said ion-exchange column;

c. eluting said .sup.99m Tc from said ion-exchange column with a solution comprising a reductive complexing agent to elute a second eluate comprising said .sup.99m Tc in the form of complexed. reduced .sup.99m Tc in a concentration at least 8 times greater than the concentration of said .sup.99m Tc in said first eluate: and,

d. acidifying said complexed. reduced .sup.99m Tc to produce a .sup.99m Tc product in a form which is suitable for subsequent radiopharmaceutical use.

2. A method of preparing a concentrated solution of .sup.99m Tc in accordance with claim 1 wherein said sorbent comprises at least one of alumina, zirconia, or zirconium molybdate.

3. A method of preparing a concentrated solution of .sup.99m Tc in accordance with claim 1 wherein said anion-exchange column contains an anion exchange resin comprising at least one of Dowex AG W 1X8, Dowex AG 1, Dowex AG 2, Dowex AG 4, REILLEX HPQ, or REILLEX 402.

4. A method of preparing a concentrated solution of .sup.99m Tc in accordance with claim 1 wherein said reductive complexing agent comprises EDA.