203/212Pb Theranostic Radiopharmaceuticals for Image-guided Radionuclide Therapy for Cancer

Receptor-targeted image-guided Radionuclide Therapy (TRT) is increasingly recognized as a promising approach to cancer treatment. In particular, the potential for clinical translation of receptor-targeted alpha-particle therapy is receiving considerable attention as an approach that can improve outcomes for cancer patients. Higher Linear-energy Transfer (LET) of alpha-particles (compared to beta particles) for this purpose results in an increased incidence of double-strand DNA breaks and improved-localized cancer-cell damage. Recent clinical studies provide compelling evidence that alpha-TRT has the potential to deliver a significantly more potent anti-cancer effect compared with beta-TRT. Generator-produced 212Pb (which decays to alpha emitters 212Bi and 212Po) is a particularly promising radionuclide for receptor-targeted alpha-particle therapy. A second attractive feature that distinguishes 212Pb alpha-TRT from other available radionuclides is the possibility to employ elementallymatched isotope 203Pb as an imaging surrogate in place of the therapeutic radionuclide. As direct non-invasive measurement of alpha-particle emissions cannot be conducted using current medical scanner technology, the imaging surrogate allows for a pharmacologically-inactive determination of the pharmacokinetics and biodistribution of TRT candidate ligands in advance of treatment. Thus, elementally-matched 203Pb labeled radiopharmaceuticals can be used to identify patients who may benefit from 212Pb alpha-TRT and apply appropriate dosimetry and treatment planning in advance of the therapy. In this review, we provide a brief history on the use of these isotopes for cancer therapy; describe the decay and chemical characteristics of 203/212Pb for their use in cancer theranostics and methodologies applied for production and purification of these isotopes for radiopharmaceutical production. In addition, a medical physics and dosimetry perspective is provided that highlights the potential of 212Pb for alpha-TRT and the expected safety for 203Pb surrogate imaging. Recent and current preclinical and clinical studies are presented. The sum of the findings herein and observations presented provide evidence that the 203Pb/212Pb theranostic pair has a promising future for use in radiopharmaceutical theranostic therapies for cancer.

Cross-section measurements and production of 72Se with medium to high energy protons using arsenic containing targets

Experiments were performed to evaluate production of 72Se, parent radionuclide of the positron emitter 72As, at high energy at the Brookhaven Linac Isotope Producer (BLIP). Excitation functions for 75As(p, xn)72/75Se in the 52-105 MeV energy range were measured by irradiating thin gallium arsenide (GaAs) wafers. Maximum cross section value for the natAs(p, 4n)72Se reaction in the energy range was 103±9 mb at 52±1 MeV. Production size GaAs and arsenic metal (As°) targets were irradiated with 136 μA and 165 μA beam current possessing an initial Linac energy of 117 MeV. A total of 3.77±0.1 GBq (102±3 mCi) of 72Se was produced from a GaAs target at a calculated target entrance energy of 105.4 MeV, and 13.8±0.3 GBq (373±8 mCi) of 72Se from an As° target at a calculated incident energy of 49.5 MeV irradiated for 116.5 h and 68.9 h, respectively.

Chromatographic separation of germanium and arsenic for the production of high purity (77)As

A simple column chromatographic method was developed to isolate (77)As (94±6% (EtOH/HCl); 74±11 (MeOH)) from germanium for potential use in radioimmunotherapy. The separation of arsenic from germanium was based on their relative affinities for different chromatographic materials in aqueous and organic environments. Using an organic or mixed mobile phase, germanium was selectively retained on a silica gel column as germanate, while arsenic was eluted from the column as arsenate. Subsequently, enriched (76)Ge (98±2) was recovered for reuse by elution with aqueous solution (neutral to basic). Greater than 98% radiolabeling yield of a (77)As-trithiol was observed from methanol separated [(77)As]arsenate [17].

Trithiols and their arsenic compounds for potential use in diagnostic and therapeutic radiopharmaceuticals

INTRODUCTION

Arsenic-72 ((72)As; 2.49MeV β(+), 26h) and (77)As (0.683MeV β(-), 38.8h) have nuclear properties useful for positron emission tomography (PET) and radiotherapy applications, respectively. Their half-lives are sufficiently long for targeting tumors with antibodies, as well as peptides. Potential radiopharmaceuticals based on radioarsenic require development of suitable bifunctional chelates for stable conjugation of arsenic to vectors under in vivo conditions at high dilution.

METHODS

The thiophilic nature of arsenic led to the synthesis and characterization of a simple trithiol ligand and its arsenic complex, and radiolabeling studies at the no carrier added (NCA) (77)As level.

RESULTS

(1)H- and (13)C-NMR spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and single crystal X-ray diffraction were used to characterize the trithiol ligand and its arsenic(III) complex. Radiotracer studies with no carrier added (NCA) (77)As resulted in high radiolabeling yields (>96%) with high in vitro stability.

CONCLUSIONS

The high yield and stability of a single NCA (77)As trithiol complex indicates that this framework is suitable for developing matched pair agents for non-invasive in vivo PET imaging and radiotherapy of tumors with (72,77)As. This is the first reported chelate developed for NCA radioarsenic and studies are underway for developing a trithiol bifunctional chelate conjugated to a targeting vector, such as a peptide or monoclonal antibody.

Chromatographic separation of selenium and arsenic: A potential (72)Se/(72)As generator

An anion exchange method was developed to separate selenium and arsenic for potential utility in a (72)Se/(72)As generator. The separation of the daughter (72)As from the (72)Se parent is based on the relative acid-base behavior of the two oxo-anions in their highest oxidation states. At pH 1.5, selenate is retained on strongly basic anion exchange resin as HSeO4(-) and SeO4(2-), while neutral arsenic acid, H3AsO4, is eluted.