Production of No-Carrier-Added<sup>105</sup> Rh from Neutron Irradiated Ruthenium Target
Production of No-Carrier-Added105 Rh from Neutron Irradiated Ruthenium Target
Nuclear medicine radiotherapy involves the administration of a radiolabelled drug whose purpose is tissue damage and/or destruction at the point of localisation. Radionuclides useful for this application are those which emit particles (that is alpha, beta or Auger electrons) because they deposit their decay energy over a relatively short range (for example, at the tumour site). Rhodium-105 is a radionuclide with desirable nuclear properties for therapeutic applications (its half-life is 35.4 hours, the maximum α- energy is 0.56 MeV and it produces a 319 keV γ-ray suitable for imaging). However, this radionuclide is not readily available to most of the interested investigators due to the difficulty in production scale-up. The work reported here was designed to develop a viable method to produce and purify multi-millicurie quantities of 105Rh for radiotherapy research. Rhodium-105 was produced at the University of Missouri Research Reactor by the nuclear reaction, 104Ru (n, γ) → 105Ru (β- decay) → 105Rh and a new procedure was developed to chemically separate the no-carrier-added 105Rh from the neutron irradiated ruthenium target. Rhodium-105 production yields, for 10 runs, averaged about 5 mCi per milligram of ruthenium from a 72-hour irradiation at a thermal neutron flux of 8 × 1013 neutrons cm-2 s-1. Rhodium-105 was successfully isolated from the ruthenium radionuclides and the non-radioactive ruthenium. This new separation technique was fast (a total time of 3 hours) and highly efficient for removing the ruthenium. The decontamination factor of ruthenium averaged 16,600, indicating that only 0.006 per cent of the ruthenium remained after separation.
A therapeutic radiopharmaceutical typically consists of a biomolecule (such as an antibody, hormone or polypeptide) to which is bound a radionuclide of appropriate half-life, high specific activity and high radionuclidic purity. The biomolecule is generally labelled with the radionuclide by the incorporation of a bifunctional chelate which covalently bonds to both.
There has been much recent interest in the use of monoclonal antibody (MAb) and peptide conjugates of 105Rh for radioimmunotherapy (1—6). Bifunctional chelates of 105Rh conjugated to MAbs and peptides have been designed to target cancer cells through interactions with specific receptors on the tumour surface. The specificity of MAbs and peptides for their particular receptor makes them excellent candidates for delivering radionuclides to cancer cells containing the receptor. The kinetic inertness of rhodium(III) complexes (low spin d 6 configuration) is expected to result in a much higher in vivo stability for 105Rh labelled monoclonal antibodies and peptides compared to those labelled with other radionuclides, such as 67Cu, 90Y, 186Re and 188Re. Various multidentate bifunctional ligands have been prepared and shown to form kinetically stable complexes in vitro with 105Rh. Examples of these ligands are 1,7-bis(2-hydroxybenzyl)-4-(p -aminobenzyl)-diethylenetriamine (2), hematoporphyrin (1), 3-[N -(4-aminobenzyl)]amino-3-methyl-2-butanoneoxime (7), N,N′ -bis(2-hydroxybenzyl)-1,3-diarninopropane (3), cysteine (4), p -aminobenzylpropyleneamineoxime (6), and also 16-ane-S4-diol-macrocycle (8).
Rhodium-105 is a reactor-produced radionuclide with moderate beta energy which is suitable for radiotherapy. It decays to stable 105Pd by two β- emissions, of 560 keV (70 per cent) and 250 keV (30 per cent). The 560 keV β- particles have a maximum range in body tissues of 2 mm. Rhodium-105 also emits imageable γ rays of energy 319 keV (19 per cent) and 306 keV (6 per cent) that would allow initial diagnostic experiments prior to administering therapeutic doses of the same preparation.
In addition, the decay half-life (35.4 hours) of 105Rh is well matched to the in vivo pharmacokinetics of many peptides and antibody fragments.
Production of Rhodium-105
Radiotherapeutic research which uses 105Rh requires a route for the production of high specific activity 105Rh. Unfortunately, this radionuclide is not readily available to researchers (primarily due to the relatively stringent conditions and requirements in the isotope production and separation).
Rhodium-105 can be produced in a reactor by several routes, see Figure 1; the simplest of which is via a (2n, γ) reaction from a 103Rh target. Unfortunately, this does not yield 105Rh of high specific activity. 105Rh is also a product (about 1 per cent) of the fission of 235U. However, the most commonly used method for the production of nocarrier-added 105Rh is the indirect (n, γ) reaction using ruthenium metal as the target to produce 105Ru, which then decays to 105Rh. Significant byproducts from the production are 97Ru (T1/2 = 2.89 days), 106Ru (T1/2 = 39.27 days), 105Pd (stable) and 106Pd (stable). The use of enriched ruthenium metal (104Ru > 99 per cent) as the target material minimises the radionuclidic impurities from ruthenium and increases the production yield of 105Rh by five-fold.
A procedure proposed by Troutner and colleague has previously been used for the preparation of 5 to 150 millicuries of no-carrier-added 105Rh at the University of Missouri Research Reactor (MURR) (9). The separation of 105Rh from its 105Ru parent was based on a combination of procedures previously described by Kobayashi and Morris (10, 11). An irradiated metal target of enriched 104Ru is dissolved in alkaline solution in a reaction vial and oxidised to RuO4 using chlorine gas. The RuO4 is then removed from the solution by distillation (b.p. 108°C) into a series of hydrochloric acid (HCl) and sodium hydroxide (NaOH) traps. The 105Rh remains in the reaction vial and is converted to a mixture of Cl- complexes by the addition of HCl solution and heating. However, the major disadvantage of this process is the use of chlorine gas, which is a corrosive material (in the presence of water) and a highly toxic gas which requires a very effective trapping system. These disadvantages have somewhat discouraged the scale-up of this procedure for the production of higher quantities of 105Rh. In addition, the subsequent distillation of the oxidised product RuO4, which is a highly toxic volatile chemical, also requires extreme precautions and a cumbersome trapping system. Although the system can be designed to trap both distilled RuO4 and excess chlorine gas by the use of a series of HCl and NaOH solutions, the amount of liquid used in the traps would increase the volume of the radioactive waste, which mainly contains 103Ru (T1/2 = 39.27 days). This would be a major disadvantage when scaling-up production.
A new separation procedure is reported here using magnesium oxide, MgO, as the adsorbent to purify no-carrier-added 105Rh in quantities of 10 to 100 mCi. The method eliminates the use of chlorine gas and the distillation of ruthenium tetraoxide, and has thus significantly increased the feasibility for scale-up (12).
Production of 105Ru and 105Rh
Rhodium-105 was produced in quantities of 5 to 100 mCi at MURR using isotopically enriched 104Ru (99.08 per cent, natural abundance: 28.2 per cent). The thermal and epithermal capture cross-sections of 105Ru are 0.47 barns and 6 barns, respectively, while the burn-up thermal cross-section of 105Ru toward the formation of 106Ru is 0.3 barns. Rhodium-105 is produced from the radioactive decay of the 105Ru intermediate (T1/2 = 4.44 hours) both during and after the irradiation. The irradiation samples were prepared by accurately weighing the target material into clean quartz vials, which were then sealed under vacuum. The sealed quartz vials were encapsulated in high purity aluminum capsules and loaded into reflector positions within the MURR reactor for 72 hour irradiations at neutron fluxes of 8 × 1013 n cm-2s-1. The irradiated samples were allowed to decay for more than 12 hours and processed in a dedicated glove box. For shorter irradiations, natural ruthenium metal and potassium ruthenate were used as target materials for process development. Ruthenium metal samples were prepared in quantities of 1 to 10 mg and sealed in high purity polyethylene vials (of diameter 11 mm and length 23 mm). These vials were embedded in Styrofoam cylinders to cushion shock and placed in high density polyethylene containers known as “rabbits” for neutron irradiation for up to 1 hour in the pneumatic tube facility at MURR. The activities of the 105Rh produced from these irradiations were in μCi quantities (less than 0.5 mCi).
Separation and Purification of 105Rh
The irradiated ruthenium metal was placed into the 105Rh glovebox and transferred into a reaction vial which was connected to a trapping system containing HCl and NaOH solution. Sodium hypochlorite (NaOCl) (4 ml, 5 per cent) was added into the reaction vial and the mixture was stirred at room temperature for 30 minutes. About 2 ml of 2 M NaOH solution was added into the vial after the ruthenium target was dissolved. While stirring, 100 μl of 0.1 M magnesium chloride solution was carefully added into the ruthenium solution. The magnesium hydroxide suspension was then filtered through a 0.45 μm Teflon filter. After washing twice with water, the magnesium hydroxide precipitate and adsorbed 105Rh were dissolved with 0.5 M HCl in a new reaction vial. About 1 ml of NaOCl solution was added to the new vial and the 105Rh solution was stirred for 5 minutes. The 105Rh was re-precipitated on hydrous MgO on the addition of 1 ml of 5 M NaOH. The re-precipitated 105Rh and MgO were filtered, washed with water, and dissolved with 2 ml of 0.5 M HCl solution. This 105Rh solution was mixed with 1 ml of 4 M NaCl solution in a clean reaction vial and boiled for approximately 30 minutes. After cooling, the 105Rh solution was passed through a cation exchange column to remove Mg2+. The final pH of 105Rh solution, as measured with pH paper, was ca. 1.
Aliquots of the sample were taken throughout the process and analysed to assess the recovery and radionuclidic purity of the 105Rh. Gamma spectra were obtained using an intrinsic hyperpure germanium (HPGe) spectrometer attached to a 4000 channel analyser. All fractions for spectral analysis were prepared as 10 ml solutions in glass scintillation vials.
Trace quantities of 103Ru in the 105Rh product processed from the enriched, irradiated ruthenium target were determined on the HPGe detector after the decay of 105Rh. Ruthenium decontamination factors were calculated by comparing the initial ratio of 103Ru:105Rh prior to chemical separation to the ratio of 103Ru:105Rh in the final 105Rh sample after the separation.
The radionuclide production resulting from the irradiation of an enriched ruthenium target in a reflector position in the MURR is shown in Figure 2.
The average perturbed neutron flux at this position is about 6.5 × 1013n cm-2s-1. Ruthenium-105 reaches maximum activity after the first 20 hours of irradiation due to its short decay half-life (T1/2 = 4.4 hours). Its decay product, 105Rh, gradually builds up and approaches saturation activity after 140 hours.
One important factor, which needs to be considered in the production, is that the product, 105Rh, has very high neutron cross-sections (thermal: σth = 16,000 barns and epithermal: σepi = 17,000 barns) and thus will tend to capture a neutron to form 106Rh, a short-lived radionuclide with T1/2 = 30 seconds. Thus, 106Rh is produced in fairly high quantity and the actual production yield of 105Rh becomes significantly lower than the theoretical calculation which does not take into account such an “isotope burn-up” process.
The short-lived 105Ru and 106Rh, produced during the production process of 105Rh, are allowed to decay before the irradiated samples are radiochemically processed. The maximum activity of 105Rh is obtained about 14 hours after the end of irradiation (EOI), see Figure 3. This is, indeed, ideal “timing”, since the short-lived and high dose rate radionuclides, such as 105Ru, 106Rh and 31Si (2.62 hours, from irradiation of the quartz vial) are decaying and approaching their minimum quantities. The radiochemical separation of the samples can then usually begin 12 to 20 hours after the EOI.
Hydrous MgO adsorbed or co-precipitated rhodium(III) with very high efficiency. Rhodium-105 was collected and concentrated with a small quantity (typically ∼500 μg) of solid MgO/ Mg(OH)2 which was then readily separated from the alkaline solution containing radioactive ruthenate. Both natural and enriched 104Ru metal targets were used to develop the MgO separation procedure, and the distillation procedure which uses chlorine gas was used as a control for comparison of method efficiency. A decontamination factor of 11,300 was obtained from this control procedure (distillation) using chlorine gas as the oxidant and a natural ruthenium target. The decontamination factor from the procedure employing hydrous MgO averaged 16,600 for both natural and enriched ruthenium targets, for over 3 trials each. Total 105Rh recovery averaged above 85 per cent from either procedure.
The solution chemistry of ruthenium is complicated since, in most cases, ruthenium is present as more than one chemical species (13). Therefore it is necessary to use a multi-step separation procedure to remove these various ruthenium species. The use of MgO to separate the 105Rh from the Ru (ruthenate) solution followed by a re-precipitation and column separation step for purification of 105Rh seems to be a good choice. The decontamination factor obtained from this procedure was as good or better than that of the distillation procedure (control).
This new procedure is superior to the distillation procedure in terms of simplicity and the radionuclidic purity of the isolated 105Rh. In addition, this procedure can be readily scaled up to processing multi-curie quantities of 105Rh, because the use of hazardous materials, such as chlorine gas and volatile ruthenium tetraoxide, is avoided.
One important concept that this paper is intended to address is that there are always isotopie carriers and radiochemical contaminants present in a “pure” or “no-carrier-added” sample, which are produced during the currently available separation methodologies. Because of the trace quantity of the “no-carrier-added” radionuclide (the concentration for 10 mCi of 105Rh in 5 ml solution is about 2 × 10-11 M), the major components or elements present in the product may not be the specific radionuclides of interest. The ratio of ruthenium:rhodium at the end of a 72-hour irradiation is calculated to be around 200,000, from a comparison of the number of atoms of 105Rh produced during the irradiation and the total number of ruthenium atoms available. The theoretical ratio of ruthenium:rhodium in the 105Rh sample separated from a ruthenium target using the MgO adsorption method is in the region of ∼ 12, assuming a decontamination factor of 16,600 is achieved.
In other words, in the separated 105Rh sample, the major metal element is ruthenium, rather than rhodium, even though nearly 100 per cent of the ruthenium has been chemically removed.
Additionally, it is probably worth mentioning that the 105Rh is recovered as a mixture of chloroaquorhodium (III) complexes. In dilute HCl solution where chloride [Cl-] is less than 1 M, there is a mixture of RhCl4(H2O)2-, RhCl5(H2O)2- and RhCl63-. RhCl3(H2O)3 may also exist in the solution (14).
It has been reported that at high temperature (> 50°C and at a high concentration of chloride ([Cl-1] = 1.5—3 M), the above reaction is effectively unidirectional toward the formation of RhCl5(H2O)2- and RhCl63- which may undergo aquation by the release of chloride to form neutral or cationic species when reaction conditions are changed (14). It is difficult, in most cases, to directly use the processed 105Rh for radiolabelling due to its unknown speciation. The problem of converting all rhodium chloroaquo complexes to one chemical form for the subsequent radiosynthesis with bifunctional ligands or proteins – in order to obtain a uniform product with high efficiency – remains.
Rhodium-105 can be produced in quantities and purities suitable for radiotherapy research using a novel separation method based on MgO adsorption. The contamination from residual ruthenium does not apparently cause a problem for the practical use of 105Rh. Ruthenium removal is less an issue than making sure all the rhodium is in the same chemical form.
This adsorption method eliminates the use of chlorine gas and the production of ruthenium tetraoxide, and thus is desirable for use in large-scale isotope production. This will lead to the potential availability of 105Rh in multicurie quantities. However, at present no clinical trials are underway or are planned.
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Wei Jia is currently working to develop collaboration between the U.S. and China in the production of radioisotopes, primarily for medical purposes.
Dangshe Ma is currently a postdoctoral fellow at the Memorial Sloan-Kettering Cancer Institute in New York City. His work involves use of Bi-213 (an alpha emitter) for radiotherapy of blood-borne malignancies.
Eric Volkert earned a business degree at the University of Missouri and now works in industry in Kansas.
Alan Ketring is Associate Director of Development, MURR. His interests include development of all aspects of production of isotopes for radiotherapeutic use as well as other income-generating products.
Gary Ehrhardt is a Senior Research Scientist at MURR. His interests include radioisotope generators, radiotherapeutic (infra-arterial) microspheres, and preparation of large amounts of radiochemical lanthanides.
Silvia Jurisson is Associate Professor of Chemistry at the University of Missouri. Her interests include radiopharmaceutical chemistry and radioenvironmental chemistry.