Smart Medical Imaging

EPSRC Centre for Doctoral Training

Research

2019_004 - One-step, site-specific labelling of His-tagged proteins with technetium-99m and rhenium-188

1st supervisor: Phil Blower
2nd supervisor: Michelle Ma

From the 1990s to the present time, nuclear medicine has moved into a new era of molecular imaging (as opposed to the functional imaging radiopharmaceuticals based on Tc-99m complexes of unknown structure: Tc-99m-bisphosphonates, DMSA, DTPA etc.) and barely understood mechanism of uptake. In particular, the use of biomolecules (mainly peptides and proteins) as molecular targeting vectors has become the mainstay of molecular imaging during this period. Chemistry for radiolabelling such biomolecules with positron emitting radionuclides such as Ga-68 and F-18 for PET imaging has been a major focus of research and development in the last 20 years, including particularly efficient chelator chemistry developed at King’s, using our proprietary tris(hydroxypyridinone) (THP) chelators,1-6 Fig. 1). However, the parallel development of Tc-99m-labelling methods for them has been almost entirely neglected since 2000. Despite the growth of PET, Tc-99m radiopharmaceutical development remains a high priority for several reasons. PET scanners, and the provision of radiotracers for PET, are more costly than has historically been the case for SPECT with Tc-99m. Both developed countries and low-to-middle income countries are likely to continue to depend on Tc-99m for the foreseeable future. The recent crises in Mo-99 production, leading to shortages of Tc-99m, have galvanised nations and industry into development of new production methods and implementation of new production facilities for Mo-99 and Tc-99m. At the same time, improvements in commercial SPECT scanner design have continued, leading to improvements in quantification, resolution and sensitivity of scanning as well as truly dynamic SPECT. There is therefore now a need to develop new chemistry for this new age of molecular imaging with SPECT, to compensate for the neglect of the last 20 years.

The major gap to fill is in methodology to make radiolabelling of sensitive biomolecules simple, efficient, and undemanding in terms of infrastructure such as automated synthesis equipment. To make the new generation of radiotracers readily accessible and economic to the medical community and its patients, including those in lower and middle income countries, the ideal radiolabelling would be similar in procedure to the well-established kit-based Tc-99m chemistry developed in the 1970s, involving operations as simple as adding Tc-99m generator eluate to a kit vial containing the required reducing agents and other ingredients. Purification and other additional steps should be avoided. These requirements are not met by current chemistry. The same principles apply to the development of methods for radiolabelling biomolecules with radioisotopes for targeted radionuclide therapy.

1. Badar A, Williams JD, Martin de Rosales R, Tavaré R, Kampmeier F, Blower PJ, Mullen, GED. Optimising the radiolabelling properties of technetium tricarbonyl and His-tagged proteins. Eur J Nucl Med Mol Imaging Res, 2014;4:14. Manuscript ID: bc-2014-00029n. PMC4015829
2. Blower PJ. A nuclear chocolate box: the periodic table of nuclear medicine. Dalton Trans 2015;44:4819-4844.
3. Choudhry U, Greenland WEP, Goddard WA, Maclennan TAJ, Teat SJ, Blower PJ. New routes to bioconjugates of rhenium using the oxobis(dithiolato)rhenate(V) core. Dalton Trans 2003:311-317.
4. Lewis JS, Zweit J, Dearling JLJ, Rooney BC, Blower PJ. Copper(I) bis(diphosphine) complexes as a basis for radiopharmaceuticals for positron emission tomography and targeted radiotherapy. Chem Commun 1996;1093-1094.
5. Prakash S, Went MJ, Blower PJ. Cyclic and acyclic polyamines as chelators of rhenium-186 and rhenium-188 for therapeutic use. Nucl Med Biol 1996;23:543-549.
6. Abiraj K, Mansi R, Tamma ML, Forrer F, Cescato R, Reubi JC, et al. Tetraamine-derived bifunctional chelators for technetium-99m labelling: synthesis, bioconjugation and evaluation as targeted SPECT imaging probes for GRP- receptor-positive tumours. Chem Eur J 2010; 16: 2115-2124. https://doi.org/10.1002/chem.200902011

 

From left to right: structure of radiolabelling species (M = ,99mTc or ,188Re); labelled peptide array showing wide variation in labelling efficiency among different peptides; comparison of labelling efficiency of optimised His-tag sequence (green) with standard literature His-tag sequences; SPECT image of mouse bearing PSMA-positive (left) PSMA-negative DU145 prostate tumour using scFv antibody incorporating the optimal His-tag and labelled with ,99mTc