Patent Application
20200243210
The present invention relates generally to the field of radiopharmaceutical production. More particularly, it relates to systems, apparatus, and methods of producing gallium radioisotopes from solid zinc targets irradiated by an accelerated particle beam. It also relates to a gallium-68 composition produced by these methods.
Gallium-68 (Ga-68) is a positron emitting radioactive isotope of gallium that is desirable for medical use. Ga-68 possesses two desirable properties for medical use, a short half-life (t½: 68 min) and a high branching ratio for positron emission (β+%: 89%). Ga-68 tracers may be used for brain, heart, bone, lung or tumor imaging. Specifically, Ga-68 is useful for the production of radiolabeled compounds used as tracer molecules in positron emission tomography (PET) imaging techniques. It forms stable complexes with chelating agents, for example DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) and HBED-CC (N,N′-bis-[2-hydroxy-5-(carboxyethyl)benzyl]ethylenediamine-N,N′-diaceticacid). 68Ge/Ga-68 generators may deliver Ga-68, but this Ga-68 activity decreases over time due to the decay of the parent nuclide 68Ge (t½: 271 d). Potential breakthrough of Ge-68 with eluted gallium is an undesirable possible consequence of making Ga-68 using 68Ge/Ga-68 generators. Cyclotron production of Ga-68 provides a way to meet a large demand for Ga-68 while eliminating the possibility of 68Ge breakthrough during the production process.
The present invention is directed to a solid target assembly apparatus for making gallium isotopes, such as Ga-68. The assembly has a target backing portion and a Zn portion on top of it.
The present invention is also directed to method of making a solid target assembly apparatus. In an embodiment, this is done by preparing a quantity of Zn, depositing the Zn onto a substrate, heating the Zn until at least some of it begins to melt, and (actively or passively) allowing the Zn to cool off and solidify. In an embodiment, this is done by providing a metal disc with front and rear surfaces and some Zn, preparing the top surface of the disc, applying the Zn onto this surface to form the stacked target apparatus, and bonding the quantity of Zn to the surface of the disc (e.g. by applying heat to it).
The present invention is also directed to a solid target assembly apparatus made according to any of the methods discussed above.
The present invention is also directed to a method of producing Ga-68 by cyclotron by:
The present invention is also directed Ga-68 composition made according to the any of the methods discussed above.
The present invention is directed to a system, apparatus, and method for producing gallium radioisotopes (e.g. Ga-68) from a non-radioactive isotope of zinc (e.g. Zn-68) on particle accelerators and a Ga-68 composition produced by this method.
In an embodiment, Ga-68 is produced in a cyclotron via the 68Zn(p,n)Ga-68 reaction in a solid target. The parent compound, zinc, for example Zn-68, a naturally occurring stable isotope of zinc, is deposited on a substrate that is irradiated with a proton beam. After irradiation, the target is dissolved in a strong acid solution to obtain a solution that is then purified to obtain Ga-68.
The zinc portion 15 is on the front surface of the target backing 20. In an embodiment, the zinc may be impregnated in the target backing material, but not substantially within it. In an embodiment, the zinc material mostly contains zinc Zn-68 (at least 90%), a stable (non-radioactive) isotope of zinc, and also has traces of other zinc isotopes, such as Zn-64, Zn-66, Zn-67, and/or Zn-70 and other elements, such as Al, As, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Pb, Si, and/or Sn.
The target backing material may be made of chemically inert metals, such as the noble metals or the refractory metals, or any other material with a high thermal conductivity that is suitable for mechanical or other modification and bonds easily to zinc, such as silver, copper or aluminum. The backing material is of sufficient robustness to dissipate an exemplary proton beam current of at least approximately 10 μA and energy of approximately 15 MeV on a beam spot of approximately 10 mm diameter.
The present invention is also directed to method of making a solid target assembly apparatus.
In an embodiment, a method of making a solid target assembly apparatus comprises the steps of:
In an embodiment, bonding the Zn to the surface of the disc may be done by heating the zinc until it at least partially melts (e.g. heating it to at least 419.5° C. for up to 30 minutes) and then allowing it to cool to ambient. In an embodiment, bonding the Zn to the surface of the disc may be done by in an oxygen-free or low oxygen environment.
The target assembly is heated using any suitable heat source such as a hot plate, furnace, blow torch, induction heating, laser, arc melting, or a combination thereof.
In an embodiment, the target assembly discussed above may be made according to any of the methods discussed above.
First, the target backings 20 (a/k/a target backings) are made. They may be various sizes or shapes. In an embodiment, they are smooth, solid, planar discs with or without a recess.
Next, the bonding surfaces are prepared for bonding the target backing 20 with the zinc portion 15 to form the target assembly apparatus 10. Many metal bonding methods, such as soldering or diffusion bonding, require preparation of the metal surfaces of materials to be bonded. In an embodiment, the front surface 22 of the target backing 20 is prepared as a bonding surface for bonding to the zinc portion 15. Some example preparation techniques include, but are not limited to mechanically cleaning, degreasing, etching, roughening (e.g. using an abrasive such as sand paper), polishing, laser engraving, and/or mechanically indenting the surface. Adhesion may occur independent of the surface finish.
Also, many metals exposed to air become coated with an oxide layer, which may compromise bonding between the target backing 20 and the zinc portion 15. This oxide layer may be removed from the target backing 20 mechanically (e.g. via sanding) or chemically (e.g. via etching with chemicals) before bonding. Alternatively, plasma etching or other techniques may be applied. Oxide layers may also be removed during the bonding process by using corrosive fluxes.
As discussed above, in an embodiment, the target backing 20 of the apparatus 10 may contain silver, aluminum, or copper. Commercial aluminum may be naturally coated with a relatively thick oxide layer that protects the metal from further corrosion. In an embodiment where the target backing 20 contains aluminum, the front surface 22 of a backing 20 containing aluminum may be prepared by removing this oxide layer mechanically or chemically (e.g. using mineral acids or bases, such as alkali hydroxide or alkali carbonates). In this embodiment, if the aluminum backing is in air or any oxygen rich environment, the cleaned surface may then be rinsed and used for target assembly apparatus fabrication as soon as possible before re-oxidation occurs. Alternatively, the preparation and fabrication steps may be done in an oxygen free environment in order to avoid re-oxidation. In an embodiment, the bonding surface of the target backing 20 containing aluminum may be prepared and cleaned using an aqueous zincate solution containing 10% sodium hydroxide (w/w), 2% zinc oxide (w/w), and 0.2% sodium cyanide (w/w). In an embodiment, the zincate process may be applied at least twice, with acid etching and rinsing steps in between. An exemplary double zincate method would be: Cleaning and degreasing; sodium hydroxide etching; rinsing; etching with half-concentrated nitric acid; rinsing; zincate; rinse; etching with half-concentrated nitric acid; rinse; zincate; rinse. In an embodiment where the target backing 20 contains silver, the target backing 20 may not oxidize as rapidly as aluminum or other metals. Bonding surface preparation of a target backing 20 containing silver may be prepared by cleaning it mechanically (e.g. with and abrasive such as sandpaper) and/or chemically (e.g. removing a silver oxide layer with acid such as sulfuric acid). In an embodiment, the target backing 28 may also be made of copper.
Next, the zinc is deposited onto the prepared surface of the metal disc 20. The zinc may be in a variety of forms such as a solid disc, powder, compressed powder, compacted powder, a foil, shavings or granules that are loose or compacted into a thin pellet, or the like. The zinc is applied directly to the metal disc 20, for example in accordance with any of the application methods discussed below. In an embodiment, the zinc may be applied to the metal disc 20 by plasma spraying or a similar technique.
After this, heat is applied to one or both components in order to bond the components to one another. The zinc should be heated until it melts (i.e. to a temperature of at least its melting point) to achieve a strong bond between the two components. In an embodiment method using a powerful heat source, the zinc may be heated briefly (e.g. a few seconds). When heated in ambient air, heating should be stopped shortly after the zinc melts. In an embodiment where the target backing contains aluminum, the zinc should not be melted for more than approximately 30 minutes.
In an embodiment, the heat source is a hotplate, large industrial solder table, or a blow torch. The zinc is applied on the front surface of the metal disc 20 (either on the front surface 22 or within a recess). The zinc and metal disc assembly is then heated (e.g. placed on a hotplate, within the flame of a blow torch, in a furnace, using induction heating, laser, arc melting, a combination thereof, etc.) and heated to a predetermined temperature and/or for a predetermined period of time until the zinc melts. The assembly is removed from the heat and allowed to cool down (actively or passively) to ambient in order to allow the zinc to solidify.
In an embodiment, pressure is applied to the assembly during or immediately after heating to facilitate bonding between the components. For example, a weight made out of an inert material that does not bond to zinc (e.g. quartz) is placed on top of the zinc before heating. The small force caused by this additional weight aids in the bonding process.
Other heating sources and methods, such as metallurgical or brazing furnace, induction heating, or hot pressing, may be used.
In an embodiment, the bonding process is performed in an oxygen free environment (or substantially oxygen free environment), for example in an inert gas atmosphere or in a vacuum.
As this process is similar to soldering, the flowing of the zinc and its adhesion may be improved by using a flux material (e.g. a paste which contains e.g. a corrosive substance, some binder and other chemicals). In an embodiment, the process may include pre-coating the backing with a minute quantity of ammonium chloride before melting the zinc onto the backing. Ammonium chloride decomposes upon heating, liberating hydrochloric acid, which aids in the removal of oxide films on both the zinc and the backing, thus improving the diffusion bond. Unused flux may be removed after the soldering.
In an embodiment, the target assembly may be fabricated using a die casting process. Liquid zinc may be applied to the target backing 20 through a heated injection system (e.g. using a heated Pasteur pipette) directly onto the target backing (pre-heated or at ambient temperature). In an embodiment, the zinc may be laser melted onto the disc.
After fabricating the targets assembly apparatus 10, it is placed into a target station in a cyclotron and irradiated for a predetermined period of time. The assembly 10 is bombarded with a proton beam having a predetermined energy level and beam current. In an embodiment, a method of producing Ga-68 by cyclotron comprises the steps of:
In an exemplary embodiment, the target assembly 10 is irradiated with a proton beam having a current of up to 100 μA, beam energy of no more than 12.7 MeV, and a beam spot of approximately 10 mm diameter. In an embodiment, the apparatus 10 is irradiated for at least 5 minutes and no more than approximately hours.
In addition to producing the desired Ga-68 isotope, irradiation of the zinc target also produces other isotopes such as Ga-64, Ga-66, Ga-67, and Ga-70. These other radioisotopes decay over time (i.e. 2 minutes-3 days). After irradiation, the Ga-68 that forms in irradiated zinc target material must be separated chemically from the irradiated target.
A number of chemical separation procedures for gallium-zinc separations exist. Applying these protocols to an irradiated zinc target to isolate Ga-68 will result in the isolation of Ga-68 with unique isotope ratios over time after the end of bombardment.
In an embodiment, where the target backing is silver or another noble metal, a purification method based on ion exchange chromatography in strong hydrochloric acid to dissolve the zinc and perform a standard purification protocol may be used.
Silver does not dissolve remarkably in hydrochloric acid due to the formation of insoluble silver chloride on the surface of the silver backing, whereas zinc and radio-gallium are rapidly dissolved. The resulting solution may be processed immediately in an ion exchange separation.
In an embodiment, a variation of this method may be used in which thermal diffusion is used to help Ga-68 migrate to the surface of the zinc layer 15 on the target assembly 10, which is then etched with a small amount of a suitable acid to recover a large fraction of Ga-68 while minimizing the quantity of zinc that needs to be dissolved and then separated. Further purification of Ga-68 may be achieved by liquid-liquid extraction.
In an embodiment where the target backing 20 contains aluminum, hydrochloric acid may be used but it dissolves both zinc and aluminum. A high concentration of aluminum in the solution may affect the separation chemistry, thus leading to lower yield and/or lower purity or reactivity of the Ga-68 product. For example, dissolving a 200 mg zinc pellet on a 4.0 g aluminum target disc by immersion in 12N HCl resulted in a zinc chloride solution contained approximately 15 mg of aluminum.
In an embodiment, zinc may be dissolved from a target disc containing aluminum using acetic or nitric acid. In an embodiment of a zinc dissolution method using acetic acid, the dissolution may be expedited by adding a small quantity of an oxidizing agent, such as hydrogen peroxide, and/or by applying heat. The resulting acetate solution may be evaporated and taken up in hydrochloric acid for subsequent standard ion exchange separation. Alternatively, purification may be achieved via cation exchange in ammonia containing solution. The dissolution of zinc in acetic acid proceeds rather slowly (e.g. >20 minutes for a 200 mg zinc pellet), unless the solution is heated to near boiling. The resulting solution contains only trace amounts of aluminum. In an embodiment of a zinc dissolution method using nitric acid, the nitric acid selectively dissolves zinc while the oxidizing properties of nitric acid increase the thickness of the natural oxide layer on metallic aluminum, thus protecting it from attack by the acid. The dissolution of zinc proceeds rapidly, and a wide range of concentrations may be used.
For example, in 8N nitric acid a 10 mm diameter, 200 mg zinc pellet dissolves in approximately 1-2 minutes. Concentrated nitric acid dissolved a similar pellet in less than one minute. In ˜2N HNO3, the dissolution is complete in ≤6 minutes. The resulting nitrate solution may be evaporated to dryness and taken up in hydrochloric acid for standard ion exchange separation.
In this method, in a 35 mm diameter target, aluminum may be dissolved concomitantly from a ˜2.5 gram backing in the range of 0-1.5 mg (0.06%), which may not affect the subsequent Ga-68 purification. The higher the acid concentration, the less aluminum was dissolved.
The aluminum content may be further reduced by not exposing the entire area of the target backing to nitric acid, for example, only the zinc layer on the front surface of the metal disc.
Nitric acid dissolution proceeds much faster than acetic acid dissolution, and is therefore desirable with Ga-68 separation because of the relatively short half-life of Ga-68 (approximately 68 minutes).
The present invention is also directed to a Ga-68 composition of matter made according to the any of the methods discussed above.
Material ratios after separation are determined from the isotope ratios at the end of bombardment, the efficacy of the chosen chemical purification process, and then accounting for decay that occurs for each isotope during the time required to conduct separation.
Methods of the invention can produce Ga-68 compositions that, after purification and following the end of bombardment, have the following quotient of activity ratios:
The impurities present in a Ga-68 composition made from a proton irradiated zinc target depend on the chemical and isotopic composition of the zinc starting material. For example, if the zinc starting material were 100% pure Zn-68, the only expected impurity would be Ga-67 if the proton energy is above 12.7 MeV.
In an experimental example, where a target apparatus 10 with a zinc portion 15 containing the following materials
The proportion of Zn-68 in the target material relative to other materials is directly related to the relative proportion of Ga-68 created in the target material post irradiation. In other words, the greater the percentage of Zn-68 in the target material pre-irradiation, the greater the percentage of Ga-68 in the target material post-irradiation.
Other irradiations yield different results, depending on the composition of the starting material and irradiation time. During irradiation Ga-68 nears saturation before Ga-66 and Ga-67 because the half-life of Ga-68 is shorter than the half-life of Ga-66 and Ga-67.
This Utility patent application claims the benefit of U.S. Provisional Application No. 62/538,954 filed on Jul. 31, 2017, the entirety of which is incorporated herein.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CA2018/000146 | 7/30/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62538954 | Jul 2017 | US |
