Targeted Auger electron-emitter therapy: Radiochemical approaches for thallium-201 radiopharmaceuticals

Abstract

Introduction Thallium-201 is a radionuclide that has previously been used clinically for myocardial perfusion scintigraphy. Although in this role it has now been largely replaced by technetium-99 m radiopharmaceuticals, thallium-201 remains attractive in the context of molecular radionuclide therapy for cancer micrometastases or single circulating tumour cells. This is due to its Auger electron (AE) emissions, which are amongst the highest in total energy and number per decay for AE-emitters. Currently, chemical platforms to achieve this potential through developing thallium-201-labelled targeted radiopharmaceuticals are not available. Here, we describe convenient methods to oxidise [ 201 Tl]Tl(I) to chelatable [ 201 Tl]Tl(III) and identify challenges in stable chelation of thallium to support future synthesis of effective [ 201 Tl]-labelled radiopharmaceuticals. Methods A plasmid pBR322 assay was carried out to determine the DNA damaging properties of [ 201 Tl]Tl(III). A range of oxidising agents (ozone, oxygen, hydrogen peroxide, chloramine-T, iodogen, iodobeads, trichloroisocyanuric acid) and conditions (acidity, temperature)were assessed using thin layer chromatography. Chelators EDTA, DTPA and DOTA were investigated for their [ 201 Tl]Tl(III) radiolabelling efficacy and complex stability. Results Isolated plasmid studies demonstrated that [ 201 Tl]Tl(III) can induce single and double-stranded DNA breaks. Iodo-beads, iodogen and trichloroisocyanuric acid enabled more than 95% conversion from [ 201 Tl]Tl (I) to [ 201 Tl]Tl(III) under conditions compatible with future biomolecule radiolabelling (mild pH, room temperature and post-oxidation removal of oxidising agent). Although chelation of [ 201 Tl]Tl(III) was possible with EDTA, DTPA and DOTA, only radiolabeled DOTA showed good stability in serum. Conclusions Decay of [ 201 Tl]Tl(III) in proximity to DNA causes DNA damage. Iodobeads provide a simple, mild method to convert thallium-201 from a 1+ to 3+ oxidation state and [ 201 Tl]Tl(III) can be chelated by DOTA with moderate stability. Of the well-established chelators evaluated, DOTA is most promising for future molecular radionuclide therapy using thallium-201; nevertheless, a new generation of chelating agents offering resistance to reduction and dissociation of [ 201 Tl]Tl(III) complexes is required.

Searching for a Paradigm Shift in Auger-Electron Cancer Therapy with Tumor-Specific Radiopeptides Targeting the Mitochondria and/or the Cell Nucleus

“…Thus, we considered the same propylenic linker to attach the AO to the dual-targeted chelator AO-Pz-BBN used to obtain the M-AO-BBN (M = Re and 99m Tc) complexes evaluated in this work (Figure 1). As for the BBN peptide, it contains the G3-BBN [7][8][9][10][11][12][13][14] sequence with a triglycine (G3) linker cleavable by cathepsin B, as we recently demonstrated for the Re-BBN complex [27]. The presence of the G3 linker is crucial to obtain an augmented accumulation of 99m Tc or AO chromophore in the nucleus of PC3 cells for dual-targeted M(I) (M = Re and 99m Tc) tricarbonyl complexes similar to M-AO-BBN.…”

“…The dual-targeted bifunctional chelator AO-Pz-BBN was synthesized, as we have previously reported for similar compounds [30]. As shown in Scheme 1, the synthesis of AO-Pz-BBN was done in solution by the in situ activation of the pendant carboxylic acid function of a pyrazolyl diamine prochelator (compound 4), followed by an amidation reaction with the terminal amino group from the G3-BBN [7][8][9][10][11][12][13][14] peptide. The synthesis of the respective pyrazolyldiamine Re(I) complex, Re-AO-BBN, was also performed in a solution by reacting the Re-AO-COOH complex with the G3-BBN [7][8][9][10][11][12][13][14] solution by reacting the Re-AO-COOH complex with the G3-BBN [7][8][9][10][11][12][13][14] sequence (Scheme 1).…”

“…As shown in Scheme 1, the synthesis of AO-Pz-BBN was done in solution by the in situ activation of the pendant carboxylic acid function of a pyrazolyl diamine prochelator (compound 4), followed by an amidation reaction with the terminal amino group from the G3-BBN [7][8][9][10][11][12][13][14] peptide. The synthesis of the respective pyrazolyldiamine Re(I) complex, Re-AO-BBN, was also performed in a solution by reacting the Re-AO-COOH complex with the G3-BBN [7][8][9][10][11][12][13][14] solution by reacting the Re-AO-COOH complex with the G3-BBN [7][8][9][10][11][12][13][14] sequence (Scheme 1). The compounds AO-Pz-BBN and Re-AO-BBN were recovered by precipitation from the respective reaction mixtures with diethyl ether, followed by semipreparative HPLC purification of the crude solids.…”

“…The specific delivery of AE emitters to these radiosensitive organelles (nucleus or mitochondria) in cancer cells might enhance the radiotherapeutic effects at lower doses, thus minimizing any undesired side effects (e.g., hematological toxicity and kidney damage). Towards this goal, a variety of AE-emitting radionuclides have been studied in the past few years by several research groups, using a variety of chemical forms spanning from simple radiochemical precursors (e.g., 99m TcO 4 − or 201 TlCl) to target-specific compounds obtained based on small molecules, monoclonal antibodies or peptides recognizing antigens, such as, receptors or enzymes overexpressed in cancer cells [4,8,9].…”

To Chelate Thallium(I) - Synthesis and Evaluation of Kryptofix-based Chelators for 201Tl

“…We recently described a more robust protocol for the oxidation of commercially available stability in human serum, ruling out the other two chelates for in vivo applications. [10] While the ability to chelate [ 201 Tl](III) with sufficient stability required for in vivo applications is promising (though yet to be fully realized), this approach will always be susceptible to in vivo reduction of [ 201 Tl]Tl(III) to [ 201 Tl]Tl(I), and likely subsequent metal dissociation, as has been indicated by the work of Hijnen. We therefore started to explore the possible chelation of "native" [ 201 Tl]Tl(I).…”