The advent of molecular radiopharmaceuticals (Sullivan et al., 2024) has been transformative in treatment for neuroendocrine cancer (Strosberg et al., 2017) and metastatic castration-resistant prostate cancer (Sartor et al., 2021). These radiopharmaceuticals (or tracers) consist of a radionuclide appended to a pharmacophore or biomolecule that targets receptors over-expressed on the surface of cancer cells. There are many metallic radionuclides that have utility for molecular diagnostic imaging or systemic radiotherapy. These radioactive metal centres are attached to biomolecules via chelators (Jackson et al., 2020).

In a “theranostic” approach with pairs of “look and treat” radiopharmaceuticals a radioactive imaging tracer first provides a whole body diagnostic imaging scan of receptor expression in a patient (Langbein et al., 2019; Weber et al., 2023; Bodei et al., 2022). If the scan shows that the patient’s disease is positive for the target receptor, a second radiotherapeutic tracer can be administered to the patient. For both the imaging and therapeutic tracers, the same pharmacophore is employed to enable delivery of the radioactive cargo to diseased tissue (Sullivan et al., 2024; Strosberg et al., 2017).

In neuroendocrine cancer, the somatostatin 2 receptor (SSTR2) is targeted by ⁶⁸Ga-DOTA-octreotate (Figure 1), (DOTA, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) for diagnostic PET (Positron Emission Tomography) imaging, and ¹⁷⁷Lu-DOTA-octreotate for delivery of a cytotoxic does of β^--irradiation for systemic radiotherapy (Strosberg et al., 2017). In the past - and indeed still in centres where PET imaging is unavailable - ¹¹¹In-DTPA-octreotide (DTPA, diethylenetriaminepentaacetic acid) has been used to diagnose SSTR2-positive neuroendocrine cancers using γ-scintigraphy or SPECT (Single Photon Emission Computed Tomography) imaging (Shi et al., 1998).

In prostate cancer, the prostate specific membrane antigen (PSMA) is targeted most typically with a urea-containing dipeptide, appended to a PET isotope (⁶⁸Ga, or ¹⁸F) or a radiotherapeutic isotope. The most common pairing is “⁶⁸Ga-PSMA-11”, using a derivative of the HBED (bis(2-hydroxybenzyl)ethylenediaminediacetic acid) chelator, for diagnostic PET imaging, alongside companion “¹⁷⁷Lu-PSMA-617”, using the DOTA chelator, for systemic radiotherapy (Figure 1) (Sartor et al., 2021).

There are a range of new therapeutic radionuclides that are being investigated for the utility of their β^-, α and Auger electron particle emissions in cancer treatment. This is likely to have further impact in oncology treatment, and pharmaceutical investment in receptor-targeted cancer theranostics is in the multibillion dollar range (Sullivan et al., 2024). The availability of companion diagnostic agents will be critical to the clinical success of these future radiotherapy treatments. Currently, receptor-targeted PET agents are the state-of-the-art in molecular imaging, however, there are limitations here:

There are shortfalls in availability of ⁶⁸Ga, as a result of demand for ⁶⁸Ga outstripping supply (Kumar, 2020).

There is an alternative to complex radiopharmaceutical production: one-step, rapid, kit-based radiosyntheses of radiopharmaceuticals, which decrease the need for infrastructure, personnel and expense. This approach is already applied with “traditional” technetium-99m (^99mTc). ^99mTc emits γ-photons (t _½ = 6 h; 90% γ, 140 keV) and is widely used in γ-scintigraphy and SPECT imaging radiopharmaceuticals for perfusion/function imaging. ^99mTc is universally available from ⁹⁹Mo/^99mTc generators, with well-established supply chains from nuclear reactors, and ^99mTc radiopharmaceuticals are routinely prepared by technicians in radiopharmacies, using aqueous ^99mTc, commercial “kits” (containing reagents and chelator) and a syringe. The chelators rapidly bind ^99mTc, enabling simple radiosyntheses (Jackson et al., 2020; Rivas et al., 2021).

Due to all these existing resources, ^99mTc imaging is significantly more accessible than PET imaging. For example, it is estimated that ^99mTc is used in over 30 million scans worldwide per year. In the UK, there are 252 National Health Service (NHS) nuclear medicine departments (as of 2021), which routinely undertake ^99mTc imaging procedures. From 2013 to 2023 within NHS England, there were 4,263,180 γ-scintigraphy and SPECT scans undertaken, and 1,621,835 PET/CT scans (NHS England, 2023).

The development of molecular ^99mTc radiopharmaceuticals would leverage existing healthcare and infrastructure resources, enabling increased population-wide access to receptor-targeted molecular imaging. These resources include SPECT and γ-scintigraphy cameras, which are more prevalent than PET/CT cameras (Weber et al., 2023; Bodei et al., 2022; Shi et al., 1998). Essential resources, including trained staff, radiopharmacies and nuclear medicine infrastructure, already exist.

The β^--emitting radioisotope, ¹⁸⁸Re, is also available from a benchtop ¹⁸⁸W/¹⁸⁸Re generator, and the β^- decay properties (t _½ = 17 h; 100% β⁻, E _max = 2.12 MeV; 15% γ, 155 keV) of ¹⁸⁸Re are suitable for systemic radiotherapy (Lepareur et al., 2019). Furthermore, systemic radiotherapeutics are potentially economically viable and accessibly in Lower and Middle Income Countries (LMICs), where there are significant barriers to accessing radiopharmaceuticals (Shinto, 2017; Bernal et al., 2008). Recognising the potential of ¹⁸⁸Re as an affordable basis for systemic radiotherapies, in the early part of this century, the International Atomic Energy Agency (IAEA) sponsored multinational clinical trials of ¹⁸⁸Re-labelled Lipiodol for treatment of inoperable liver cancer in LMICs (including India, Mongolia, Philippines, Thailand and Vietnam). ¹⁸⁸Re-labelled Lipiodol proved effective and inexpensive (Bernal et al., 2008; Bernal et al., 2007). ¹⁸⁸Re-labelled bisphosphonates have also been extremely beneficial in palliative treatment of bone metastases in LMICs (Shinto et al., 2018). ¹⁸⁸W/¹⁸⁸Re generators currently supply ¹⁸⁸Re for “Rhenium-SCT” brachytherapy (Baxi et al., 2024) for highly prevalent basal and squamous cell carcinomas, the most common skin cancers. “Rhenium-SCT” has been recently approved in Europe, South Africa and Australia.

Tc and Re often form isostructural and isoelectronic complexes, leading to the possibility of pairs of complexes with near identical biodistributions, including accumulation patterns in diseased tissue, in vivo. Pairs of ^99mTc and ¹⁸⁸Re radiopharmaceuticals are often considered “theranostic pairs”, with the ^99mTc radiotracer providing a diagnostic image of disease, to stratify patients for systemic radiotherapy using an analogous ¹⁸⁸Re radiotracer (Blower et al., 2000; Bordoloi et al., 2015; Spyrou et al., 2021).

^99mTc chemistry and its application has been a dynamic and international field of research for the last 8 decades, and alongside the routine clinical use of ^99mTc perfusion and anatomical radiopharmaceuticals, there are many elegant examples of ^99mTc tracers that have entered first-in-human studies (Alberto, 2023a; Alberto, 2023b; Riondato et al., 2023; Duatti, 2021; Shi and Liu, 2024). This review will summarise how well-established ^99mTc chelator and coordination chemistry, such as that used for ^99mTc perfusion imaging agents, has been applied to develop the latest state-of-the-art radiopharmaceuticals that have recently entered clinical evaluation for receptor targeted imaging. It will also describe selected innovations in ligands designed specifically for ^99mTc that demonstrate the breadth and scope of Tc coordination chemistry.

The “MAG₃” (mercaptoacetyl-triglycine) chelator consists of a mercapto function attached to three glycine amino acids. The deprotonated amide groups in combination with the thiol donor group provide a N₃S tetradentate chelator that coordinates a [^99mTc^VO]³⁺ motif, to furnish a five-coordinate, distorted square pyramidal complex, [^99mTcO(MAG₃)]^- (Grummon et al., 1995), used routinely for diagnostic renal imaging (Figure 2) (Marta et al., 2013).

The chemistry of [^99mTcO(MAG₃)]^- has been successfully adapted for molecular imaging. Early studies established that glycine residues can be substituted for other amino acids without ameliorating [^99mTc^VO]³⁺ coordination (Cantorias et al., 2007; Cyr et al., 2007). NMR experiments have indicated that in this case, in which glycine is substituted for optically active amino acids, two distinct isomers are formed, as the amino acid sidechains can be positioned either syn or anti relative to the Tc=O bond (Cantorias et al., 2007; Cyr et al., 2007).

The most significant application of this chemistry to ^99mTc molecular imaging involves replacement of the three glycine amino acids for serine amino acids, and concomitant functionalisation with PSMA-targeted peptide sequences (Figure 2) (Robu et al., 2017). It is notable that incorporation of “unnatural” D-serine in place of biogenic L-serine results in increased metabolic stability of the ^99mTc radiotracer.^{33 99m}Tc-PSMA-I&S is the most clinically studied of these mas₃-derived PSMA-targeted tracers to date. ^99mTc-PSMA-I&S has demonstrated utility in detecting tumour lesions at various stages of prostate cancer, including primary and advanced disease, and in biochemically recurrent prostate cancer (Werner et al., 2020). However, when lesions are small or the biochemical markers of recurrence are low, ^99mTc-PSMA-I&S imaging detects fewer lesions than PSMA PET imaging (Albalooshi et al., 2020). ^99mTc-PSMA-I&S has also been clinically assessed for detection of prostate cancer metastases using a hand-held γ-detector, during surgery. The aim of this endeavour is to improve surgical resection of prostate cancer tissue (Maurer et al., 2019).

More recently, a new PSMA-targeted mas₃-peptide derivative, PSMA-GCK01,³⁷ has been developed for coordination of both the [^99mTcO]³⁺ and [¹⁸⁸ReO]³⁺ motifs. In a first in human study, both tracers showed equivalent biodistributions in a prostate cancer patient, as evidenced by γ-scintigraphy (Figure 3). Such innovation shows the feasibility and possibility of chemically analogous ^99mTc/¹⁸⁸Re dual molecular imaging/systemic radiotherapy tracers.

New directions, including application of mercapto-peptide chelating derivatives to alternative pharmacophores and their receptor targets – for example, small molecules that target cancer-associated fibroblasts of aggressive, hard-to-treat cancers — will further expand the clinical utility of this chelator chemistry (Xu et al., 2025).

Like MAG3, HMPAO (hexamethylpropylene amine oxine) coordinates a [^99mTc^VO]³⁺ core (Figure 4). The resulting neutral, lipophilic complex can pass the blood-brain barrier, and [^99mTcO(HMPAO)] is routinely used to detect changes in cerebral perfusion in stroke patients (Neirinckx et al., 1987).

The radiopharmaceutical “^99mTc-maraciclatide” is a derivative of HMPAO. In this ^99mTc-labelled compound, the HMPAO chelator contains two additional methyl substituents, and is appended to a cyclic “RGD” peptide that targets the αvβ3 integrin receptor (Figure 4), which is associated with inflammation and neovasculature. ^99mTc-maraciclatide has been assessed in multiple first-in-human studies, and has shown utility for clinical imaging in imaging rheumatoid arthritis (Attipoe et al., 2020), cancer imaging (Cook et al., 2018) and more recently, in endometriosis, for which it has recently received FDA fast track designation.

The HYNIC motif, 2-hydrazinonicotinamide, was pioneered in the 1990s (Abrams et al., 1990), and acts as both the linker and a chelator. The coordination environment of ^99mTc complexes has not been elucidated despite HYNIC’s prevalent use. Scientific literature often depicts HYNIC coordinating to Tc^V in a monodentate fashion (Figure 5), with five co-ligands for stabilization — for example, ethylenediaminediacetic acid (EDDA) — although existing data suggests that it is more likely to interact with Tc as a bidentate chelator, via the terminal hydrazine and pyridine nitrogen atom, with the latter requiring fewer co-ligands to fill the Tc coordination sphere (King et al., 2007; Meszaros et al., 2010). Attempts to prepare Re analogues of Tc complexes have largely been unsuccessful. Despite this, the radiopharmaceutical ^99mTc-EDDA/HYNIC-octreotide is in occasional use for receptor targeted imaging of the somatostatin receptor over-expressed in neuroendocrine cancers (Gabriel et al., 2003), and ^99mTc-HYNIC derivatives targeting αvβ3 integrin (Zhu et al., 2012) and PSMA (Lawal et al., 2017) have recently undergone clinical studies in cancer imaging.

Organometallic complexes of ^99mTc^I have exhibited sufficient kinetic stability to enable application in radiopharmaceuticals, and indeed, are useful exemplars in demonstrating the application of organometallic chemistry to healthcare. The homoleptic ^99mTc complex, ^99mTc-sestamibi, consists of six isonitrile ligands coordinated to a Tc^I metal centre in an octahedral environment (Figure 6) (Kronauge and Mindiola, 2016). The resulting lipophilic complex bearing a single positive charge, diffuses across the lipid bilayer of cell and then mitochondrial membranes, driven by membrane potentials. As ^99mTc-sestamibi accumulates in tissue rich in mitochondria, such as the heart, ^99mTc-sestamibi is routinely used for imaging cardiac perfusion.

Although they have not been assessed in the clinical, isonitrile ligands bearing receptor-targeted peptides have been developed for coordination to ^99mTc^I (Mizuno et al., 2016). This approach, whereby multiple copies of a targeting motif are incorporated onto a single radioactive metal centre, is potentially advantageous in increasing the affinity of a molecular radiopharmaceutical for its cognate receptor.

At the turn of the century, the d⁶ fac-[^99mTc^I(CO)₃(H₂O)₃]⁺ complex was developed (Alberto et al., 2001; Alberto et al., 1998). The three water ligands undergo relatively rapid substitution, enabling coordination of chelator-pharmacophore bioconjugates. The fac-[^99mTc^I(CO)₃]⁺ motif is used in the radiopharmaceutical ^99mTc-MIP-1404 (also known as ^99mTc-Trofolastat), in which the PSMA-targeted dipeptide is appended to a tridentate N₃ chelator consisting of a tertiary amine with two pendant imidazole groups, which in turn is coordinated to the fac-[^99mTc^I(CO)₃]⁺ motif, to form an octahedral complex (Figure 6) (Hillier et al., 2013) The radiosynthesis and purification of ^99mTc-MIP-1404 is currently time-consuming compared to kit formulations of ^99mTc perfusion radiopharmaceuticals: the former requires (i) formation of an intermediate fac-[^99mTc(CO)₃(H₂O)₃]⁺ precursor prior to (ii) chelation with the tridentate MIP-1404 chelator-peptide, and finally (iii) purification and formulation before administration.

^99mTc-MIP-1404 has demonstrated clinical utility for detection of prostate cancer lesions in patients with intermediate and high grade prostate cancer before prostatectomy (Goffin et al., 2017), and in patients with biochemical recurrence of prostate cancer (after primary treatment with either prostatectomy or radiotherapy) (Schmidkonz et al., 2018). Recent clinical studies have confirmed its suitability as a companion diagnostic imaging agent to inform stratification of patients for systemic radiotherapy with ¹⁷⁷Lu-DOTA-PSMA (Derlin et al., 2025; Cook et al., 2023). As a result, ^99mTc-MIP-1404 has recently been approved by the United Kingdom’s MHRA for clinical use in imaging and detecting high risk prostate cancer, biochemical recurrence of prostate cancer, and in assessing eligibility for ¹⁷⁷Lu-DOTA-PSMA systemic radiotherapy.

Other elegant organometallic ^99mTc complexes have been developed, and although, like isonitrile-peptide derivatives for coordination of ^99mTc^I (Mizuno et al., 2016), they have not yet been assessed in clinical studies, their ingenuity compels a mention. This includes piano-stool fac-[^99mTc^I(CO)₃]⁺ complexes based on cyclopentadiene motifs bearing receptor-targeted peptides (Frei et al., 2019), dinuclear Tc complexes such as [^99mTc₂(μ₂-SR)₃(CO)₆]^- (R = S(CH₂)₂NEt₂), bridged by alkyl thiols (Bolliger et al., 2019), and [Tc(η⁶-arene)₂]⁺ sandwich complexes (Nadeem et al., 2020), including those of functionalised arenes that have exhibited anti-tumour activity (Battistin et al., 2023).

The ^99mTc^V complex, ^99mTc-tetrofosmin, consists of a [TcO₂]⁺ motif coordinated to two bidentate diphosphine ligands, to furnish a lipophilic, singly charged cation, which, like ^99mTc-sestamibi, is routinely used for imaging cardiac perfusion (Jones and Hendel, 1993). Like ^99mTc-HMPAO, ^99mTc-sestamibi and ^99mTc-MAG3, ^99mTc-tetrafosmin can be prepared using a single step kit, in which all the non-radioactive reagents are contained in a lyophilised, sterile vial, to which generator-produced ^99mTcO₄ ^- is added, to furnish ^99mTc-tetrofosmin, typically in quantitative radiochemical yield.

Inspired by the efficiency of the kit-based radiosynthesis of ^99mTc-tetrofosmin, we have recently developed diphosphine-derivatised maleic anhydride platforms to prepare bioconjugates of diphosphines with receptor-targeted molecules (targeting αvβ3 integrin (Hungnes et al., 2021), PSMA receptors (Hungnes et al., 2023; Pham et al., 2024; Nuttall et al., 2025)) and other receptors (Nuttall et al., 2025) (Figure 7). These diphosphine-based bioconjugates can be incorporated into a kit formulation (Hungnes et al., 2021; Hungnes et al., 2023; Pham et al., 2024; Nuttall et al., 2025; Nuttall et al., 2023). Addition of ^99mTcO₄ ^- in saline to a lyophilised mixture of the diphosphine-peptide, reducing agent (stannous chloride), intermediate chelator (tartrate) and buffer (carbonate), yields the desired radiotracer, [^99mTcO₂(DP-peptide)₂]⁺ in high radiochemical yields.

By tuning phosphine ligand substituents of a diphosphine maleic anhydride derivative to increase electron donation of the phosphine ligands, we have shown that we can increase the radiochemical yield of the desired [^99mTcO₂(DP-peptide)₂]⁺ radiotracer (Hungnes et al., 2023; Nuttall et al., 2025). Furthermore, chemically analogous [¹⁸⁸ReO₂(DP-peptide)₂]⁺ compounds can also be accessed, using generator-produced ¹⁸⁸Re, and pairs of ^99mTc and ¹⁸⁸Re radiotracers show near-equivalent biodistributions in mouse models of cancer (Pham et al., 2024). To date, the radiotracers we have prepared all show high tumour accumulation, fast clearance via a renal pathway and low off-target/healthy organ retention (Hungnes et al., 2021; Pham et al., 2024; Nuttall et al., 2025; Nuttall et al., 2023), suggesting that diphosphine platforms have utility for simple, kit-based preparation of ^99mTc radiopharmaceuticals.

Chemical technology enabling development and clinical adoption of ^99mTc receptor-targeted molecular imaging has demonstrated feasibility for clinical translation, and new, fit-for-purpose innovations in ^99mTc and ¹⁸⁸Re chelator chemistry could enable the development of ^99mTc and ¹⁸⁸Re receptor-targeted theranostic pairs. These advances match the availability of existing nuclear medicine infrastructure, including ^99mTc generator supply chains, γ-scintigraphy and SPECT cameras, and radiopharmacies and nuclear medicine departments. The latter include staff skilled in formulation of kit-based ^99mTc radiopharmaceuticals. Leveraging ^99mTc chemistry alongside these resources and infrastructure could increase economical and accessible patient access to the benefits of molecular radiopharmaceuticals.