An Experimental Generator for Production of High-Purity ²¹²Pb for Use in Radiopharmaceuticals

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Abstract

The feasibility, performance, and radiation safety of an experimental generator were evaluated to efficiently produce ²¹²Pb intended for radiopharmaceuticals. Methods: The generator consisted of a flask with a removable cap containing a source of ²²⁴Ra or ²²⁸Th absorbed on quartz wool. Gaseous ²²⁰Rn emanated from the decaying source, which subsequently decayed to ²¹²Pb, which was adsorbed on the flask’s interior surface. The ²¹²Pb was collected by washing the flask with 0.5–1 mL of 0.1 M HCl. Results: The generator collector flask trapped 62%–68% of the ²¹²Pb, of which more than 87% (tested up to 26 MBq) could be harvested. The obtained ²¹²Pb solution had a high purity (>99.98%) and could be used for the preparation of radioconjugates with more than 97% radiochemical purity. Future designs of the generator should aim to further reduce the risk of radon and γ-energy exposure to operators. Conclusion: The presented technology is a promising method for easy and convenient ²¹²Pb production.

Lead-212 (²¹²Pb; half-life, 10.6 h), a β-emitter itself, is an in vivo generator of α-particles through the α-emitting progenies ²¹²Bi and ²¹²Po. Convenient chelation chemistry makes ²¹²Pb suitable for targeted α-therapy (1). However, the radiolabeling of targeting agents should preferably be performed on-site because of the short half-life of ²¹²Pb. Rapid and efficient processes are required to ensure sufficient ²¹²Pb availability for end users.

As a member of the thorium series (Fig. 1), ²¹²Pb can be obtained from generators that contain the longer-lived mother nuclides ²²⁸Th (half-life, 1.9 y) or ²²⁴Ra (half-life, 3.6 d). Current generators are based on isolating ²¹²Pb from ²²⁴Ra or ²²⁸Th through several purification steps. ²²⁴Ra has become the preferred radionuclide source over ²²⁸Th to minimize radiation hazards (1). A generator used to supply ²¹²Pb for clinical trials by Orano Med is based on ²²⁴Ra immobilized on a cation-exchange column from which ²¹²Pb can be eluted (2–4). The eluate is then evaporated and treated several times with concentrated acid before the final solution is ready for radiolabeling (3). A similar generator purchased from Oak Ridge National Laboratory was integrated into an automated synthesis module, where ²¹²Pb was eluted in dilute HCl for labeling of peptides (5). An alternative method that avoids purification of ²¹²Pb from the generator source material uses a solution of ²²⁴Ra/²¹²Pb in equilibrium directly for the radiolabeling process (6,7). However, this procedure still requires a final purification step to remove ²²⁴Ra and unconjugated daughters.

FIGURE 1.

²²⁸Th series, with ²²⁰Rn and ²¹²Pb highlighted.

A second approach is based on radon emanation, which involves obtaining ²¹²Pb from gaseous ²²⁰Rn (half-life, 55.6 s) emanated from the decaying (²²⁸Th/)²²⁴Ra parent. Thus, ²¹²Pb can be isolated from parent nuclides without the need for dedicated equipment for the separation process. Hassfjell and Hoff reported a generator comprising a ²²⁸Th source distributed within barium stearate and stored in a housing chamber connected to a vacuum pump (8). The source could be slid into the collection chamber via a gate valve. The generator experienced a relatively poor yield (11%–50%) because of radiation damage of the source when 40–50 MBq of ²²⁸Th were used. Other examples are based on 2-compartment systems in which ²²⁰Rn is transferred from a source chamber with parent nuclides into a collector chamber by airflow (9–11). These generators require significant effort and advanced equipment and have been tested in only small-scale production (≤2 MBq). Another drawback of such 2-compartment systems is that ²²⁰Rn may decay before reaching the collector chamber, potentially resulting in low ²¹²Pb yields (9).

Here, we report a novel single-chamber generator based on ²²⁰Rn emanation from decaying ²²⁴Ra or ²²⁸Th to produce high yields of ²¹²Pb for radiolabeling of ligands and monoclonal antibodies (mAbs). The generator is compact and user-friendly—key considerations for a shippable device that can be operated by the staff at a nuclear medicine facility.

MATERIALS AND METHODS

The ²²⁸Th/²²⁴Ra/²¹²Pb Generator

An earlier generation of the generator was previously reported (12), but the recent version was optimized to increase output capacity and reduce the risk of cross-contamination. The generator, consisting of a 100-mL glass flask standing upside down, with the radionuclide source contained in the screw cap (Fig. 2) (13), was kept at room temperature the entire time. ²²⁸Th (Eckert and Ziegler or Oak Ridge National Laboratory) or ²²⁴Ra (prepared as previously described (7)) in 100–200 μL of 0.1–1 M HCl was applied to approximately 0.2 g of porous quartz wool (ProQuarz GmbH). The quartz wool was placed on a small plastic cap covered in aluminum foil to minimize ²²⁰Rn retention and secured inside the screw cap (Fig. 2). During ²²⁸Th/²²⁴Ra decay, the short-lived ²²⁰Rn emanated from the quartz wool, followed by adsorption of the longer-lived ²¹²Pb daughter onto the interior surfaces of the flask. After approximately 2 d, the flask was carefully replaced with a clean flask to harvest ²¹²Pb and reuse the generator, ensuring no cross-contamination from the source. To extract the ²¹²Pb, 0.5–1 mL of 0.1 M HCl solution was added, and the flask was carefully swirled to cover the inner surface for about 5 min before the solution was collected.

FIGURE 2.

Single-chamber ²¹²Pb generator consisting of glass flask and removable cap that contains ²²⁸Th or ²²⁴Ra source fixed onto porous quartz wool.

Radioactivity Measurements

A pure source of ²²⁴Ra reaches transient equilibrium with ²¹²Pb after 2 d. We evaluated the yield of the ²²⁴Ra-based generator when ²¹²Pb was harvested after 2–3 d, or as the average yield for the ²²⁸Th-based generator when 1 generator was used multiple times with at least a 2-d interval. The yield was defined as the percentage of ²¹²Pb activity adsorbed to the flask relative to parent ²²⁴Ra or ²²⁸Th. The yield was also evaluated for generators that were milked for the second time. Radioactivity was quantified by a radioisotope dose calibrator (CRC-25R; Capintec Inc.) (12).

The breakthrough of ²²⁴Ra or ²²⁸Th in the washout solution at harvesting was quantified indirectly through the ²¹²Pb activity of decayed samples—activity that was measured in the 60- to 110-keV window on a γ-counter (automatic γ-counter; Hidex Oy) (12). The details of the measurements and calculations are described in Supplemental Section 1 (supplemental materials are available at http://jnm.snmjournals.org).

²²⁰Rn emanation from the generator and the dose rate resulting from x-rays and γ-rays were evaluated for radiation safety purposes as described in detail in Supplemental Section 2.

Radiolabeling

To evaluate the quality of the extracted ²¹²Pb, the tumor-targeting ligand NG001 (PSMA617-TCMC TFA; MedKoo Biosciences Inc.) and the S-2-(4-Isothiocyanatobenzyl)-1,4,7,10-tetraaza-1,4,7,10-tetra(2-carbamoylmethyl)cyclododecane (TCMC)–conjugated cetuximab (Erbitux; Merck Group) and rituximab (MabThera; Roche) were radiolabeled and the radiochemical purity was measured as described in Supplemental Section 3.

RESULTS

Generator Yield, Performance, and Feasibility

The single-chamber ²¹²Pb generator was easy to use and handle. The ²¹²Pb solution could be extracted at regular intervals, and the generator cap could be transferred to a clean flask each time for reuse. Its small size allowed measurement in a standard ionization chamber dose calibrator. The yield was approximately 62% for the tested ²²⁴Ra/²¹²Pb generators of 2–22 MBq, of which 87%–91% of the deposited ²¹²Pb could be extracted with 0.5–1 mL of 0.1 M HCl (Table 1; Supplemental Table 1). The ²²⁸Th-based generator of approximately 3.5 MBq had a stable yield of 67%–70% (Table 1; Supplemental Table 1). Hence, approximately 262 MBq of ²²⁴Ra and 163 MBq of ²²⁸Th are necessary initially per 100 MBq of ²¹²Pb to be obtained after 2 d.

TABLE 1.

Data on Performance of Generators Based on ²²⁰Rn Emanation from Source of ²²⁴Ra or ²²⁸Th in Equilibrium with Its Daughters

Type of generator	Yield	Available 212Pb in washout	Radioactivity breakthrough of parent source nuclide
224Ra source	62% (56%*)	91%	0.02%† (0.0004%–0.14%)
228Th source	68%‡	87%	0.0001%–0.005%¶

^*Second-time use of generator (n = 3).

^† Average of 9.

^‡ Average of multiple uses of single generator (n = 8).

^¶ In 6/8 samples, measured radioactivity was below quantification limit after >2 mo (<0.0015% breakthrough; Supplemental Section 1).

The breakthroughs of ²²⁴Ra and ²²⁸Th were attributed to cross-contamination from the source. The radioactivity of ²²⁴Ra and ²²⁸Th was 0.0004%–0.14% and 0.0001%–0.005% relative to ²¹²Pb, respectively, at the time of harvesting ²¹²Pb (Table 1; Supplemental Table 1). In 6 of 8 samples (65–269 kBq of ²¹²Pb initially) from the ²²⁸Th-based generator, the measured radioactivity was below the quantification limit of the instrument.

Radiation Safety Aspects

Our evaluation of generator integrity did not indicate any escape of ²²⁰Rn when the generator was closed. However, radon exposure from the generator is a potential radiation safety concern when the generator is opened, because the half-life of ²²⁰Rn is long enough for the gas to reach its surroundings. In the experimental setup in which a 1-MBq ²²⁴Ra-based generator was opened inside a sealed bag for 10 s, approximately 11% of the available ²²⁰Rn escaped (Supplemental Section 2).

Exposure to x-rays and γ-rays is another potential safety concern. The measurements on the surface of a 2-cm lead shield showed an average dose rate of 20 μSv/h per MBq of ²²⁸Th. The dose rate was considerably reduced to 2.3 μSv/h per MBq for a 5-cm lead shield and to 0.7 μSv/h per MBq for a 7-cm lead shield.

Radiochemical Purity of Radioconjugates

The ²¹²Pb extracted from ²²⁴Ra-based generators was used to radiolabel TCMC-conjugated ligands and mAbs with a high and reproducible radiochemical purity for all tested compounds (Table 2).

TABLE 2.

Radiochemical Purity of Various Radioconjugates After Radiolabeling with ²¹²Pb

Radiolabeled substance	Radiochemical purity (average ± SD)
212Pb-NG001 (n = 11)	97% ± 2%
212Pb-TCMC-cetuximab (n = 7)	99% ± 1%
212Pb-TCMC-rituximab (n = 6)	99% ± 1%

DISCUSSION

Here, we present an experimental ²¹²Pb generator that is compact, easy to use, and operable without advanced equipment or hazardous chemicals. These considerations are important for the convenient and efficient routine production of ²¹²Pb in clinical applications. To our knowledge, there are no existing ²¹²Pb generators that meet these criteria entirely (1,14,15). The ²²⁸Th-based generator bypasses the ²²⁴Ra separation step from ²²⁸Th while being a longer-lived device that facilitates upscaled production of ²¹²Pb at an industrial scale. Results show that a single ²²⁸Th-based generator could be milked every 2–5 d to routinely supply high-purity ²¹²Pb for research and development. Radiopharmacies and hospitals must consider the exemption limit of ²²⁸Th—which is a tenth of that of ²²⁴Ra and ²¹²Pb in the European Union and United States—when applying for permits for certified use. No well-defined criteria for an acceptable level of ²²⁸Th impurity in a radiopharmaceutical exist, but for 7 of 8 samples, the values were below the acceptance limit (<0.002%) for the impurity level of another therapeutic radiopharmaceutical that is described in the European Pharmacopoeia (16). Assuming a 100-MBq patient dose, the value is comparable to the effective dose-derived annual limit of intake of ²²⁸Th (17). The breakthrough of ²²⁴Ra from the ²²⁴Ra-based generator was comparable to the current state of the art (14). Hence, a clinically relevant purity is achievable with the presented technology.

Decay of ²²⁸Th/²²⁴Ra results in an increasing accumulation of the stable daughter nuclide ²⁰⁸Pb in the generator, which potentially competes with ²¹²Pb in radiolabeling procedures. The cumulative amount of ²⁰⁸Pb in the extracted ²¹²Pb can be estimated on the basis of the generator yield (Fig. 3; Supplemental Section 4). In terms of mAb binding, these fractions may not influence radiochemical purity, as only 1 in about 2,000 mAbs needs to be bound by a ²¹²Pb atom for a clinically relevant specific activity (18).

FIGURE 3.

Relationship between radioactive ²¹²Pb and stable daughter nuclide ²⁰⁸Pb as function of time, shown as abundance of ²¹²Pb relative to total amount of lead (black curve) and mass concentration of ²⁰⁸Pb per 1 MBq of extracted ²¹²Pb (red curve), assuming generator yield of 62%.

The current generator is a prototype from which a limited number of ²¹²Pb extractions have been performed. Along with upscaling, the radiation safety and yield may be areas for improvement in future studies. Both issues can be addressed by design considerations. The source-holding material, its size and volume, and the inner surface area of the generator can be optimized to increase the levels of ²¹²Pb depositing onto the surface. It should be verified that the holding material is not affected when one is working with higher radioactivity levels. A closed system with an integrated shielding unit for the source in which the source or the shielding unit is movable (e.g., by a plunger) would facilitate operation without exposing the source during ²¹²Pb extraction. Handling the generator inside hot cells or inside glove boxes or bags, or the use of tongs or similar equipment to protect the operator, is an important measure when working with clinically relevant activity levels (e.g., 100 MBq). Automation of the extraction process is considered feasible given that it entails only a surface-washing step and subsequent recovery of the solution.

CONCLUSION

²²⁰Rn emanation can be exploited to create a simple and effective generator that produces high-purity ²¹²Pb without the need for advanced equipment, labor-intensive steps, or hazardous chemicals. Future versions of the presented technology should include simple modifications to shield the source during extraction of the ²¹²Pb. The generator represents a promising method for efficient ²¹²Pb production.

DISCLOSURE

Sciencons AS, owned by Roy Larsen, holds intellectual property rights for the presented technology under a patent application. Ruth Li and Vilde Stenberg were industrial PhD students financially supported by the Norwegian Research Council (grants 291228 and 290639) at the time of contributing to the article, at which Vilde Stenberg was also a shareholder at ArtBio AS. Ruth Li is employed at Oncoinvent AS, Vilde Stenberg is employed at ArtBio AS, and Roy Larsen is the chairman of the board of both companies, which use the presented technology for research-and-development projects. Roy Larsen owns stock directly or indirectly in Sciencons AS, Oncoinvent AS, and ArtBio AS. No other potential conflict of interest relevant to this article was reported.

KEY POINTS

QUESTION: Can ²¹²Pb, intended for radiopharmaceuticals, be produced by a simple generator based on ²²⁰Rn emanation from a ²²⁸Th or ²²⁴Ra source?

PERTINENT FINDINGS: The proposed generator was easy to handle and could routinely be used to produce ²¹²Pb of high purity, suitable for radiolabeling of antibodies and ligands.

IMPLICATIONS FOR PATIENT CARE: Rapid and efficient production methods such as the one proposed are important for ²¹²Pb to be available for patients with metastatic cancer.