Exploring the coordination chemistry of thallium-201 and its therapeutic potential for use in molecular radionuclide therapy

Abstract

²⁰¹Tl (t_1/2 = 73 hours) can be tracked in vivo by SPECT via its gamma emissions, whilst simultaneously delivering a potentially therapeutic dose to tumours via its low-energy Auger electron emissions (~37 Auger electrons/decay). Their high linear energy transfer (LET) and short range (typically <1 micrometre) make Auger electron-emitters attractive for molecular radionuclide therapy (MRT). Targeted delivery of ²⁰¹Tl has been hindered due to lack of suitable bifunctional chelator chemistry. We aimed to find a chelator that forms a kinetically stable complex with Tl³⁺ and move on to developing a bioconjugate to selectively deliver radiotherapy to cancer cells.

A wide range of oxidation methods and chelators for Tl³⁺ have been evaluated, including DOTA, DTPA and H₄pypa. The DNA damaging potential of [²⁰¹Tl]Tl⁺ and [²⁰¹Tl]Tl³⁺ was demonstrated using a cell free isolated plasmid DNA damage assay. Serum stability of radiolabelled [201Tl]Tl3+ complexes, incubated 37 °C, were monitored by RP-TLC at various time points (up to 144 h). These complexes were also investigated using MS, NMR and X-ray crystallography. Furthermore, a PSMA-targeting bioconjugate of H4pypa coupled to a PSMA peptide via a lipophilic linker, based on the structure of PSMA-617, was developed. This was subsequently radiolabelled with ^[201Tl]Tl³⁺. SPECT imaging was used to probe the in vivo biodistribution and stability of the [²⁰¹Tl]Tl-pypa-PSMA bioconjugate in healthy SCID/beige mice, and compared to [²⁰¹Tl]TlCl and [²⁰¹Tl]TlCl₃ . Additionally, DU145 PSMA-positive and PSMA-negative prostate cancer tumour xenograft models were used to investigate the bioconjugate’s biodistribution. 

Using iodobeads and HCl (0.5 M), [²⁰¹T]Tl³⁺ was generated from commercially available [²⁰¹Tl]Tl⁺ in a quick, simple and biologically friendly way. Gel electrophoresis results showed that [²⁰¹Tl]Tl⁺ and [²⁰¹Tl]Tl³⁺ both cause single and double strand breaks in DNA after incubation up to 144 hours. [^nat/201Tl]Tl³⁺ was then shown to be chelated by various commercially available chelators such as DTPA and DOTA, as well as novel chelators such as H₄pypa and H₄noneunpa. H₄pypa was radiolabelled in high radiochemical yields (98 ± 2 %) at RT within 15 minutes. The complex remained intact in serum for an hour, but integrity dropped to 61 % after 24 h and 44 % after 48 h. In vivo experiments using [²⁰¹Tl]Tl-pypa-PSMA showed excretion through the kidneys and bladder with little heart uptake observed at early time points, suggesting adequate in vivo stability for delivery to tumours. In contrast, [²⁰¹Tl]TlCl and [²⁰¹Tl]TlCl₃ both showed high heart uptake at 15 minutes, and renal excretion over the following hour. The DU145 PSMA positive and negative tumour models showed higher uptake in the positive tumour compared to the negative. However, due to the reduction potential of Tl³⁺, the radiometal is likely to be reduced, dissociate from the chelator and then be effluxed from the tumour cells.

[²⁰¹Tl]TlCl₃ can be readily produced from [^nat/201Tl]TlCl and both oxidation states can cause SSBs and DSBs to DNA. [²⁰¹Tl]Tl³⁺ can be chelated at RT within 15 minutes using a number of chelators. The [²⁰¹Tl]Tl-pypa complex showed good serum stability in human serum. A PSMA bioconjugate was synthesised and radiolabelled, followed by in vivo biodistribution studies that revealed fast renal clearance with no evidence of [²⁰¹Tl]Tl³⁺ release. Promising radiobiological evaluation of the ²⁰¹Tl in a plasmid assay highlights the potential use of ²⁰¹Tl for use as a theragnostic agent to deliver a therapeutic dose, while being tracked within the body using SPECT imaging.

Date of Award	1 May 2022
Original language	English
Awarding Institution	King's College London
Supervisor	Samantha Terry (Supervisor) & Philip Blower (Supervisor)