Radiochemical Quality Control Methods for Radium-223 and Thorium-227 Radiotherapies

Abstract

Background: The majority of radiopharmaceuticals for use in disease detection and targeted treatment undergo a single radioactive transition (decay) to reach a stable ground state. Complex emitters, which produce a series of daughter radionuclides, are emerging as novel radiopharmaceuticals. The need for validation of chemical and radiopurity with such agents using common quality control instrumentation is an area of active investigation. Here, we demonstrate novel methods to characterize ²²⁷Th and ²²³Ra. Materials and Methods: A radio-TLC scanner and a γ-counter, two common and widely accessible technologies, as well as a solid-state α-particle spectral imaging camera were evaluated for their ability to characterize and distinguish ²²⁷Th and ²²³Ra. We verified these results through purity evaluation of a novel ²²⁷Th-labeled protein construct. Results: The γ-counter and α-camera distinguished ²²⁷Th from ²²³Ra, enabling rapid and quantitative determination of radionuclidic purity. The radio-TLC showed limited ability to describe purity, although use under α-particle-specific settings enhanced resolution. All three methods were able to distinguish a pure from impure ²²⁷Th-labeled protein. Conclusions: The presented quality control evaluation for ²²⁷Th and ²²³Ra on three different instruments can be applied to both research and clinical settings as new alpha particle therapies are developed.

Keywords: quality control; radio-thin layer chromatography; α particle therapy; γ counter.

FIG. 1.

(A) Decay chain of ²²⁷Ac, which decays into ²²⁷Th and ²²³Ra. (B) Depiction of the anion exchange column used for the purification of ²²³Ra and ²²⁷Th. (C) Antibody radiolabeling with ²²⁷Th. ²²⁷Th is chelated to an antibody and purified using size exclusion chromatography.

FIG. 2.

(A) National Institute of Standards and Technology calibrated dilution series for ²²³Ra standards. (B) Quantitation of activity from traceable ²²³Ra source summed across channels. (C) Measurements of a ²²³Ra source at equilibrium over time, days indicated. (D) Measurements of ²²⁷Th over time. Ingrowth of ²²³Ra and daughters is noted. CPM, counts per minute.

FIG. 3.

Readings at 1500 V (A–C) and at 1000 V (D–F) of each radioisotope and a mixed source spotted on cellulose support, measured over time. (A, D) ²²⁷Th CPM were plotted and fit according to a one-phase exponential decay. (B, E) ²²³Ra CPM were plotted and fit to a one-phase exponential decay. (C, F) Mixed source of ²²³Ra and ²²⁷Th. For all curves, the initial point is excluded from the exponential fitting to allow the system to reach equilibrium. Error bars indicate standard deviation between triplicates. CPM, counts per minute.

FIG. 4.

A strip of DGA-coated chromatographic iTLC paper was spotted with a mixture of ²²³Ra/²²⁷Th at 10 mm and migrated with a mobile phase of 1 M HNO₃ up to 100 mm. TLC readings over high voltage 1500 V at 20 min (A), 1 h (B), and 3 h postmigration (C). TLC readings of the same mixture at 1000 V at 5 min (D), 1 h (E), and 3 h postmigration (F). The middle peak seen at both high-voltage settings immediately after purification gradually decreases until it completely disappears 3 h postmigration. CPM, counts per minute; DGA, N,N,N′,N′-tetra-n-octyldiglycolamide; iTLC, instant thin layer chromatography; TLC, thin layer chromatography.

FIG. 5.

Total CPM acquired in an open window channel measured for ²²⁷Th (A), ²²³Ra (B), and a mixture (C); Deconvoluted activity (Bq) calculated for a pure ²²⁷Th sample (D) showing ²²⁷Th decay fitting a single-phase decay (R = 0.9980), as well as ²²³Ra ingrowth. (E) Activity (Bq) for ²²³Ra fit with a single-phase decay (R = 0.9986). (F) ²²⁷Th and ²²³Ra mixed sample activity (Bq) plot. CPM, counts per minute.

FIG. 6.

(A) A ²⁴¹Am/²³⁷Np/²⁴⁴Cm standard source read using Minipix spectral imaging camera. From left to right, the three peaks correspond to the α-particles emitted by ²³⁷Np, ²⁴¹Am, and ²⁴⁴Cm. (B, C) ²²³Ra and ²²⁷Th imaging spectra over one complete half-life. (D, E) Plotted counts acquired at 5.43 and 7.45 MeV for pure ²²⁷Th (D) and ²²³Ra (E). 5.43 MeV depicts the most significant α particle contribution emitted by ²²³Ra, and 7.45 MeV corresponds to the α particle released by ²¹¹Po and relates to the second peak seen in the graphs. Both increase over time for ²²⁷Th and decrease for ²²³Ra. (F) For the mixed spectra, the day 0 measurements of the radionuclides from (B) and (C) were normalized and plotted together. (G) The α spectra of ²²⁷Th and ²²³Ra plus daughters when spotted on an aluminum support.

FIG. 7.

²²⁷Th-labeled antibody migrated on silica-coated chromatographic iTLC with DTPA (10 mM). Purified (A–D) and nonpurified (E–H) material were compared using radio-TLC scanner (A, B, E, F), γ counting (C, G), and α spectral imaging (D, H). The TLC scanner of purified and nonpurified ²²⁷Th-labeled protein was acquired immediately after migration at a high-voltage setting of 1000 V (A, E) and 1500 V (B, F), showing a main peak at the spotted area characteristic of ²²⁷Th-labeled material and migration of unchelated ²²³Ra at the top of the strip. (C, G) Purified (C) and unpurified (G) ²²⁷Th-labeled protein measured using γ counting. ²²⁷Th was found primarily nonmigrated at the bottom of the strip, confirming protein labeling. ²²³Ra was detected at the top of the strip for the unpurified material (G); (D, H) α spectral imaging of the purified (D) and nonpurified (H) ²²⁷Th-labeled antibody showed ²²⁷Th α emission at the bottom of the strip for both materials and ²²³Ra contribution at the top of the strip for the nonpurified material. CPM, counts per minute; DTPA, diethylenetriamine pentaacetate; iTLC, instant thin layer chromatography; TLC, thin layer chromatography.