Recent Advances in [¹¹C]HCN Radiochemistry and Applications to (Pre)clinical PET Imaging

Abstract

The identification of efficient radiochemical methods has been a long-standing challenge in the development and production of positron emission tomography (PET) tracers. Carbon-11, being a short-lived (t_1/2 = 20.4 min) isotope, offers a unique set of challenges for regulatory-compliant radiochemistry with exceptional time constraints. Simultaneously, the diversity of radiochemical scaffolds available from carbon-11 maintains its importance in clinical PET radiosynthesis. [¹¹C]Hydrogen cyanide ([¹¹C]HCN) represents a unique example of a versatile ¹¹C-synthon that can deliver broad chemical diversity for the synthesis of ¹¹C-labeled PET radiopharmaceuticals. This review article describes recent investigations with [¹¹C]HCN including its production, radiochemistry, and use in developing new PET radiopharmaceuticals.

Graphical Abstract

Introduction

Positron emission tomography (PET) is a non-invasive imaging modality commonly used in the staging and diagnosis of disease. PET employs biologically active compounds labeled with positron-emitting radioisotopes (e.g. ¹⁸F, ¹¹C, ⁶⁸Ga) to track dynamic biological and physiological processes in vivo.^1-5 As opposed to anatomical imaging approaches (Computed Tomography, CT; Magnetic Resonance Imaging, MRI), PET enables direct imaging of biological target engagement allowing physicians to gather clinically actionable insight about otherwise complex biochemical processes like metabolism, protein expression, and enzyme activity.⁶ Apart from clinical applications, PET also finds utility in drug discovery where it can provide important information concerning target engagement and pharmacokinetic attributes of drug candidates.^7,8

From the chemist’s perspective, PET introduces divergent concerns in leveraging extant chemistry for radiopharmaceutical production.^9,10 Many PET tracers are small molecules that are radiolabeled by means of incorporating a radionuclide into their chemical structure. Frequently, fluorine-18 (t_1/2 = 109.7 min) and carbon-11 (t_1/2 = 20.4 min) are employed for these purposes. Given the ubiquity of carbon in both endogenous molecules and pharmaceutically relevant scaffolds, carbon-11 is particularly attractive with respect to PET radiochemistry.^11-14 Additionally, carbon-11 is available as several radiochemical synthons allowing diverse transformations for radiochemical incorporation (Scheme 1).^15,16

Scheme 1 -.

Abbreviated examples of gas phase transformations from [¹¹C]CO₂ and [¹¹C]CH₄.¹⁵

[¹¹C]Hydrogen cyanide ([¹¹C]HCN) is a particularly attractive synthon for carbon-11 radiochemistry. As opposed to other radiosynthons which often do not afford chemical diversity, [¹¹C]HCN is unique in that it can be rapid transformed into numerous other useful synthons (Scheme 2) and can thus be further converted into various functionalities when installed into organic molecules. Radiochemistry with [¹¹C]HCN has been profiled in prior review articles.^17,18 However, in recent years, new methods in [¹¹C]HCN production and radiochemistry as well as novel ¹¹C-PET tracers have necessitated an updated review and prospective of [¹¹C]HCN. This article discusses the past 5 years of advancements in [¹¹C]HCN radiochemistry for PET, including examples that were not covered in prior reviews, and addresses future areas of research with [¹¹C]HCN radiochemistry to fully exploit its utility.

Scheme 2 –

Common production methods for [¹¹C]HCN and derivatization from [¹¹C]HCN.^35-39

[¹¹C]HCN

Hydrogen cyanide (HCN) is a highly volatile liquid which is known to be highly toxic to most living organisms.^19,20 While HCN itself is infrequently used in organic syntheses, its anion CN⁻ is more common as cyanide salts are generally wieldier from a synthetic perspective. CN⁻ is used in diverse chemical transformations, namely in the production of medicinally valuable nitriles as well as amino acids and diverse acyl species through nitrile intermediates.^21-26 Retrosynthetically, the nitrile (R–CN) moiety represents a C1 synthon with an oxidation state of C³⁺, making it isoelectronic with common functionalities such as acids (R–CO₂H), amides (R–CONR₂), and esters (R–CO₂R) which can all be formed from nitrile intermediates through hydrolytic means.²⁷

[¹¹C]HCN similarly offers several advantages from a radiosynthetic perspective. In addition to the diversity of chemical functionalities available from [¹¹C]HCN, in comparison to alternative synthons it can be manipulated more readily. [¹¹C]HCN, while volatile, can be trapped as non-volatile [¹¹C]CN⁻ with a sufficiently strong base (pK_a(HCN) = 9.2), allowing for high temperature reactions without the activity loss apparent with other common gaseous or volatile ¹¹C synthons (e.g. [¹¹C]CH₃I, [¹¹C]CO₂, [¹¹C]CO). Further, [¹¹C]HCN may offer an advantage in molar activity (A_M). While both CO₂ and CO are present in the atmosphere at parts per million (ppm) levels, HCN is generally found in parts per billion (ppb) or lower, suggesting less concern regarding isotopic dilution could be suspected when using [¹¹C]HCN over [¹¹C]CO₂ and [¹¹C]CO for certain transformations.^28-30

Production of [¹¹C]HCN

Several methods have been reported for the accelerator-based production of [¹¹C]HCN. The first reported production of [¹¹C]HCN in 1964 from Dubrin et al. describes irradiating nitrogen oxides on a heavy ion accelerator with ¹¹C-ions produced from the neutron stripping of ¹²C.³¹ Since then, cyclotron-based methods have been extensively developed, primarily via the ¹⁴N(p,α)¹¹C nuclear reaction. The most common methods for [¹¹C]HCN production involve the oxidation of gaseous [¹¹C]CH₄ and NH₃ with a Pt catalyst at 900-1000°C (Scheme 2). For this purpose, [¹¹C]CH₄, can be generated either directly in ¹⁴N₂ targets containing ~ 5% H₂ or from reduction of [¹¹C]CO₂ performed with Ni and H₂ at 400°C.³² Most commonly, the stepwise reduction of [¹¹C]CO₂ to [¹¹C]CH₄ and subsequent oxidation to [¹¹C]HCN is used as it allows ready generation of a larger variety of ¹¹C-synthons ([¹¹C]CO₂, [¹¹C]CO, [¹¹C]CH₃I, [¹¹C]CH₃OTf, [¹¹C]HCN) from a single ¹¹C target.^15,33,34 Despite this, direct oxidation of cyclotron-produced [¹¹C]CH₄ can offer higher molar activity [¹¹C]HCN given the lower atmospheric concentration of [¹²C]CH₄ (< 2 ppm) compared to [¹²C]CO₂ (> 400 ppm) leading to less isotopic dilution.

[¹¹C]HCN may also serve as a synthetic lynchpin to produce other valuable ¹¹C-synthons. [¹¹C]Cyanogen bromide ([¹¹C]CNBr) can be generated from bromination of [¹¹C]HCN in the gas phase. [¹¹C]CNBr can also be further transformed to [¹¹C]cyanamide ([¹¹C]H₂NCN) and [¹¹C]guanidine ([¹¹C]HNC(NH₂)₂) under residence in supercritical ammonia.^35,36 In addition, [¹¹C]cyanate ([¹¹C]OCN⁻) and [¹¹C]thiocyanate ([¹¹C]SCN⁻) ions can be readily produced through the oxidation of [¹¹C]HCN with KMnO₄ and S₈ respectively.^37-40 Meanwhile, the reduction of [¹¹C]HCN under H₂ allows the production of high A_M [¹¹C]methylamine ([¹¹C]H₂NCH₃).⁴¹

While [¹¹C]HCN production methods have largely remained consistent, a recent advance has enabled more widespread use of the synthon. Given the requirement of dedicated infrastructure and carbon-11 processing units for the formation of [¹¹C]HCN, Zhang and Gee et. al. in 2022 developed a method to produce [¹¹C]HCN from the more commonly available [¹¹C]CH₃I (Scheme 3).⁴² This procedure follows a high-yielding two-step transformation with an ammonium N-oxide to form intermediate [¹¹C]CH₂O which can be further oxidized to [¹¹C]HCN with the use of hydroxylamine-O-sulfonic acid (HOSA). This was shown to be applicable to automated radiosynthesis using an in-line column approach, employing a diphenyl sulfoxide (DPSO) as a solvent and oxymatrine (OMT) as an N-oxide. Importantly, the solvent and N-oxide reagents were selected to maximize molar activity by reducing the potential thermal liberation of [¹²C]CH₂O from unlabeled methyl groups present with standard reagents including DMSO and trimethylamine N-oxide. Pleasingly, the production of [¹¹C]HCN through [¹¹C]CH₃I results in higher A_M of resulting radiolabeled products than traditional methods, owing to the chemoselectivity of the transformation and thoughtful reagent choices.

Scheme 3 –

[¹¹C]HCN production from [¹¹C]CH₃I and synthetic example from Zhang and Gee et al.⁴²

Reactions from [¹¹C]HCN

¹¹C-Aryl Radiocyanation

Arguably, the area which [¹¹C]HCN has seen the most development in recent years is toward the synthesis of ¹¹C-labeled aryl nitriles. The aryl nitrile moiety features regularly in bioactive compounds and pharmaceuticals, where it can act as a hydrogen bond acceptor and an electron-withdrawing group to tune π-stacking interactions. Aryl nitriles may also serve as important bioisosteres for halogenated arenes and phenols, enabling similar interactions while navigating certain problematic physiochemical properties.^43-47 Further, aryl nitriles can act synthetic handles in route to other pharmaceutically relevant moieties including amides, acids, and triazoles.²⁵ This section will cover the conception of ¹¹C-aryl radiocyanation and recent advances toward the synthesis of ¹¹C-aryl nitriles through novel methodologies.

¹¹C-Aryl Radiocyanation by Nucleophilic Aromatic Substitution

The first example of aryl radiocyanation in 1989 by Balatoni et al. describes the nucleophilic aromatic substitution of fluoro-arenes with the aid of Cr(CO)₃.⁴⁸ This chromium complex enables the formation of an η⁶ complex with the substrate to activate arenes for nucleophilic substitution via electron withdrawal. The authors showed that trapping of [¹¹C]HCN in aqueous NaOH followed by treatment with a preformed Cr(CO)₃– fluorobenzene complex in DMSO and heating at 150°C provided efficient labeling of [¹¹C]benzonitrile. Later experiments with carrier-added NaCN expanded the substrate scope of this transformation to a few simple arenes with modest yields. Critically, the addition of Cr(CO)₃ is required for this reaction and, without it, no product is observed.

To date, this example remains the only use of direct S_NAr to achieve radiocyanation. However, the operational simplicity and accessibility of precursors make this an attractive area for reconsideration, especially considering the recent advances in ¹⁸F- S_NAr and techniques discovered therein.^49-55

Metal-Mediated ¹¹C-Aryl Radiocyanation

Rosenmund-von Braun-Type Radiocyanation

The Rosenmund-von Braun (RVB) reaction is a common transformation in organic synthesis for converting aryl halides into aryl nitriles.^56-58 This reaction was first developed in 1914 where it was found that CuCN, when reacted with an aryl halide at high temperatures (> 200°C), afforded aryl nitriles in moderate to high yields. In analogy to many metal-mediated electrophile-nucleophile cross-coupling reactions, RVB is proposed to proceed through a Cu^III-aryl species formed from the oxidative addition of CuCN into an aryl halide (Scheme 4). Subsequent reductive elimination of the benzonitrile is thought to proceed rapidly, though, competitive reductive elimination of the aryl halide can lead to a degenerate catalytic cycle. This general paradigm can proceed using various transition metals including Pd, Cu, and Ni, and does not distinctly require a pre-generated metal-cyanide species, allowing the addition of alternative cyanide sources to generate a higher-valent metal-cyanide species in-situ.

Scheme 4 -.

General mechanistic scheme for Rosenmund-von Braun-Type radiocyanation from aryl halide precursors.

The RVB manifold was first translated to ¹¹C-radiocyanation in 1997 by Crouzel et al.⁵⁹ The authors found [¹¹C]CuCN could be trapped from [¹¹C]HCN in an aqueous solution of CuSO₄ with Na₂S₂O₅ as a sacrificial reductant. From this, the trapped [¹¹C]CuCN treated with a solution of an aryl iodide precursor in DMF could afford the corresponding ¹¹C-aryl nitrile after heating at 180°C for 5 minutes. Notably, the authors showed that these aryl nitriles could be further converted to amides, acids, and tetrazoles in similar time scales, adapting this to the synthesis of [¹¹C]irbesartan through cycloaddition of NaN₃ onto the ¹¹C-aryl nitrile precursor, albeit with relatively low yields.

In 2022, Sanford and Scott et al. published an improvement on this reaction with the introduction of a ligated Cu^I species.⁶⁰ Prior reports had suggested that diamine ligands in traditional RVB procedures permit milder cyanation conditions, potentially expanding the scope of this transformation (Scheme 5). With this, the authors screened a variety of primary, secondary, and tertiary diamine ligands. In comparison to a ‘ligandless’ CuI, they found that the bidentate nitrogen ligand DMEDA dramatically reduced the requisite temperature for high-yielding labeling (120°C from 180°C). Additionally, the introduction of DMEDA gave access to less-activated aryl halide substrates (i.e. bromides and chlorides). Untypical of a transition metal-mediated reaction, this method was also unexpectedly accelerated by the addition of water which likely improves the solubility of the CuI and K₃PO₄ in DMF.

Scheme 5 –

Work from Scott and Sanford et. al. 2022 showcasing an improved RVB manifold with DMEDA, including an nocipetin analog and PET images in a primate subject.⁶⁰

This procedure displayed a broad functional group tolerance to electron-rich and poor substrates in addition to heteroaryl scaffolds that are otherwise difficult to access from organometallic precursors (covered in next section). The authors applied this reaction to the preparation of various bioactive aryl nitrile containing molecules including [¹¹C]perampanel which has been explored for the imaging of AMPA receptors in neurodegenerative disorders. Further, the reaction was tolerant to unprotected aromatic amino acids, which was translated to the radiosynthesis of a ¹¹C-labeled nocicpetin analog, an oligopeptide for which the introduction of a nitrile at the phenylalanine position had minimal perturbation on binding to nociceptin/orphanin FQ peptide receptors. PET imaging in non-human primates show the ¹¹C-labeled nocicpetin analog distribution correlated with known NOP receptor expression.

This method was further explored with C–H activation by Sanford and Scott in 2023 via a sequential C–H iodination/radiocyanation approach (Scheme 6).⁶¹ During their study, the authors screened multiple I⁺ reagents, finding that N-iodo-succinimide enabled the best balance of iodination yield and subsequent radiochemical conversion (RCC), while the slightly more reactive IPy₂BF₄ diminished RCC. The authors explored the scope of this reaction toward the labeling of small molecules and peptides with good to excellent RCYs for readily iodinated electron-rich (hetero)arenes. In the case of peptides, efficient labeling was observed for protected tyrosine and tryptophan derivatives, while phenylalanine could not be labeled. Similarly, in a substrate competition study tryptophan appeared to outcompete tyrosine in the two-step telescoped halogenation-radiolabeling sequence. The approach was leveraged in the radiosynthesis of an unprotected 10-amino acid peptide bearing phenylalanine, tryptophan, and tyrosine residues, and which resulted in site-specific radiocyanation of the tryptophan residue.

Scheme 6 –

Scott and Sanford’s sequential iodination-radiocyanation including site-selective C–H functionalization of a peptide example.⁶¹

Another extension of the RVB manifold from Hosoya in 2023 described the radiocyanation of aryl fluorides through Ni catalysis.⁶² Nickel, being capable of oxidative addition into otherwise strong covalent bonds, was seen as an attractive candidate for converting stable aryl fluorides to ¹¹C-aryl nitriles, avoiding the isolation of more labile aryl halide precursors (Scheme 7). Given the inertness of the C–F bond, oxidative addition via Ni⁰ is relatively slow, making direct radiocyanation less feasible on standard ¹¹C radiosynthetic timescales. Instead, the researchers used a two-step procedure through LiCl-promoted generation of the Ni^II-aryl oxidative addition complex under inert conditions for 1 hour, followed by addition of gaseous [¹¹C]HCN into the pretreated solution. This method was applicable to a large array of functionalities including heteroarenes with minimal dependence on substrate electronics, though competing nucleophilic functionalities, such as alcohols, required protecting groups. This method was applied to functionally dense aryl fluoride pharmaceutical compounds including flumazenil and atorvastatin which proceeded in moderate to high RCCs. Despite the requirement of inert conditions for pre-generation of the operant Ni^II-aryl species, it was observed that these complexes could be stored long-term for potential use in clinical radiosyntheses.

Scheme 7 –

Examples from Hosoya et al. 2023 in a nickel-mediated radiocyanation of aryl fluorides via oxidative addition adducts.⁶⁰

Cross-Nucleophile Chan-Lam-Type Radiocyanation

The Chan-Evans-Lam reaction is a C–N forming transformation which offers a mild and efficient means for cross-nucleophile coupling. First described in 1998, this method employs nucleophilic aryl-boronic acids with a corresponding heteroatom (N, O) nucleophile to furnish aryl amines/ethers in the presence of stoichiometric Cu^II under air.^63-66 The reaction is thought to operate under a Cu^II/III manifold with O₂ as a recycling oxidant from Cu^I generated from the reductive elimination of transient Cu^III (Scheme 8). In analogy, the Gouverneur, Scott, and Sanford groups translated this manifold to nucleophilic (radio)fluorination.^67-69 Given the operational simplicity and resilience to ambient moisture/oxygen, similar Cu-mediated radiohalogenations have become commonplace for radiopharmaceutical production.^70,71

Scheme 8 –

General Scheme for Chan-Lam-type Cu-mediated radiocyanation and mechanism.^61-67

The first example of ¹¹C-radiocyanation using Chan-Lam-type reactivity was disclosed in 2017 from Vasdev, Hooker, and Liang et al.⁷² Therein, the authors describe screening a host of Cu^I/Cu^II sources and nitrogen ligands toward radiocyanation of aryl boronic acids (Scheme 9A). The researchers found, similar to RBV-type reactions, that the combination of CuI and DMEDA in a mixture of H₂O in DMF facilitates efficient coupling of aryl boronates and [¹¹C]CsCN. A proof-of-concept automation of the method with a model substrate showed feasibility for potential PET radiosynthesis. Shortly after, the Sanford and Scott groups published a similar Cu-mediated radiocyanation of organometallic precursors (Scheme 9B).⁷³ In this case, using a pyridine-Cu(OTf)₂ mediator, the authors were able to access ¹¹C-aryl nitriles in improved yields and lower temperatures from prior conditions. Alternative organometallic functionalities including aryl-tributylstannanes (−SnBu₃), boronic pinacol esters (−Bpin), and aryl trifluoroborate salts (−BF₃K) were also tolerated. The method was shown to have broad chemical compatibility and was ultimately demonstrated in the automated radiosynthesis of [¹¹C]perampanel.

Scheme 9 –

RVB-type ¹¹C-cyanation methods covered in prior reviews. (A) – Cu^I-mediated ¹¹C-cyanation of aryl boronic acids from Vasdev, Hooker, and Liang et al.⁶⁸ (B) Cu^II-mediated ¹¹C-cyanation of aryl boron and aryl stannane precursors from Scott and Sanford et al.⁶⁹ (C) – Pd-mediated ¹¹C-cyanation from organoboron species from Hosoya et al.⁷⁰

Following this, Hosoya et al. published a similar method employing Pd catalysis in 2018.⁷⁴ In this case, air-stable Pd^II-precatalysts offered higher RCY with improved functional group tolerance compared the analogous Cu-mediated method. Additionally, the presence of an excess of NH₃ from [¹¹C]HCN production had little effect on reaction efficiency, with the addition of NH₃ to the trapping solution improving RCY. This procedure was applied to the automated radiosynthesis of two aromatase inhibitors including [¹¹C]cetrozole, which proceeded with high RCY. A proof-of-concept example also shows a Rh-catalyzed decyanative borylation which enables a two-step formal neutron loss from the ¹²C-aryl nitrile to its ¹¹C-isotopologue for the herbicide cyhalofop-butyl.

Recently, Scott, Sanford, and Wright et al. have expanded the repertoire of aryl organometallic precursors available for Cu-mediated radiocyanation to aryl silanes.⁷⁵ With concerns about the toxicity of organostannanes and the instability of certain heteroaryl organoboron precursors, aryl silanes were envisioned more stable and less toxic alternatives for Cu^II-mediated radiocyanation. Various aryl silane moieties were screened for reactivity in this regime which uncovered heptamethyltrisiloxanes (−Si(OTMS)₂Me) as precursors with the greatest balance of reactivity and synthetic accessibility (Scheme 10). This class of aryl-silanes, available from Ir-catalyzed C–H functionalization and Miyaura-type coupling, showed a broad range of a functional group tolerance in Cu-mediated radiolabeling. Crucially, substrates susceptible to proto-demetallation as the corresponding organoboron species, such as 2-pyridyl substrates, proved to be efficient. This method was applied to the radiosynthesis of [¹¹C]LY2795050, a kappa opioid antagonist and clinical PET tracer which proceeds in higher conversion than the corresponding unstable boronate ester. Through mechanistic studies, it was suggested that this reaction likely proceeds through fluoride-mediated activation of the central silicon motif, enabling the transmetalation of the otherwise unreactive aryl-silane onto Cu.

Scheme 10 –

Cu-mediated radiocyanation following a Chan-Evans-Lam manifold from Wright et al. employing heptamethyltrisiloxanes as aryl nucleophiles.⁷¹

Radical Aryl ¹¹C-Radiocyanations

Single electron reaction manifolds provide divergent reactivity to ‘polar’ or two-electron transformations and can frequently offer milder conditions in reduced timeframes. Many standard metal-mediated radiocyanations require high temperatures to proceed under the short time constraints of ¹¹C-radiosynthesis which often leads to side-product formation, complicating translation. Further, many extant radiocyanation methods employ superstoichiometric quantities of transition metal mediators (Cu, Pd, Ni) to increase reaction rates which require trace-metal testing to verify removal for clinical production. Conversely, the improved kinetics of single electron transfer over polar mechanisms allow reduced metal loadings and, in some cases, complete removal. Thus, the discovery and optimization of radical-mediated ¹¹C-radiolabeling methods is potentially intriguing for improving radiopharmaceutical production workflows and an active area of radiochemistry research.

Reductive Strategies

In 2023, Scott and Sanford et al. showed the potential of radical strategies toward ¹¹C-aryl nitriles with a Sandmeyer-type radiocyanation of aryl diazonium salts.⁷⁶ In analogy to RVB-type radiocyanation reactions wherein the oxidative addition of Cu^I into aryl halides is rate-limiting and requires high temperatures, aryl–N₂⁺ species are readily reduced to aryl radicals (Ar·) at room temperature which recombine with Cu^II–CN to produce aryl nitriles following reductive elimination (Scheme 11). The authors showed this to be feasible radiochemically when employing [¹¹C]Pyr·HCN and a Cu(MeCN)₄PF₆ mediator in the presence of a bidentate nitrogen ligand. Owing to the high reactivity of aryl–N₂⁺ species, this reaction was observed to be exceptionally efficient, producing high RCY in under 1 minute at room temperature. Notably, owing to the instability of heteroaryl diazonium salts, the transformation’s scope was mainly limited to carbocyclic substrates, albeit with high efficiency. The authors also showed the viability of a telescoped diazotization-radiocyanation for unstable or otherwise difficult-to-isolate diazonium salts using the corresponding aniline treated with NOBF₄ prior to labeling. This procedure was employed in the automated radiosynthesis of a small molecule derived from Fast Blue RR which proceeded in high RCY and A_M. Further, a similar approach was preliminarily explored for aryl iodides as potentially more stable precursors. Owing to their higher reduction potential, aryl iodides required the introduction of a phenothiazine photocatalyst and irradiation at 395 nm with LED lights for efficient labeling. While this resulted in moderate RCCs, the reliability and scope suggest further optimization could be revisited for this transformation.

Scheme 11 –

Aryl radical-mediated radiocyanation method from Scott and Sanford et al. utilizing aryl diazonium precursors and aryl iodide precursors under photocatalytic conditions and a general mechanistic scheme for Sandmeyer-type chemistry from aryl diazoniums.⁷²

Building from their work on aryl diazonium and aryl iodide radical radiocyanation approaches, Sanford and Scott et al. continued exploring radical approaches with the radiocyanation of aryl thianthrenium salts in 2024.⁷⁷ This work sought to strike a balance between the unstable and potentially energetic aryl–N₂⁺ species and poorly-reactive aryl iodides from their initial work. Aryl thianthreniums proved to be easily accessible, shelf stable precursors with reductive potentials amenable to simple Ar· generation. Therein, it was shown that through a similar manifold with the addition of 395 nm light, aryl thianthreniums showed remarkable labeling efficiency (Scheme 12). Likely owing to a photoexcited Cu^I species, this reaction did not require the introduction of a photocatalyst and could be completed at very low substrate loadings (5 μmol). The authors demonstrate the compatibility of this method with various polar functionalities in electron-rich and deficient arenes. Crucially, this manifold was functional for heterocyclic moieties for which the corresponding aryl–N₂⁺ is expected to be unstable. This reaction was applied to the automated radiosynthesis of [¹¹C]Febuxostat Ethyl Ester using a purpose-built photoredox setup compatible with GE TracerLab FX_M radiosynthesis module with minimal deviation from manual results.

Scheme 12 –

Cu-mediated radiocyanation from aryl thianthrenium salts under photocatalytic conditions, including a custom photoirradiation manifold used in the automation of the procedure.⁷³

Oxidative Strategies

In 2022, Nicewicz and Li et al. developed an oxidative approach using photoredox catalysis to facilitate direct C–H radiocyanation of arenes.^78,79 This approach exploits highly-oxidizing photocatalysts to allow single-electron oxidation of arenes to the corresponding delocalized aryl radical cation (Ar^·+) to form ¹¹C-aryl nitriles through cation radical-accelerated nucleophilic aromatic substitution (CRA-S_NAr). Utilizing an acridinium photocatalyst system, the authors demonstrate labeling electron-rich arenes and heteroarenes with moderate RCC and varying regiochemical selectivity. Pleasingly, this reaction was observed to function under ambient air and with commercial 456 nm LEDs (Scheme 13). Interestingly, the authors found many cases in which alkoxy arenes (Ar–OR) demonstrated site-selective deoxycyanation which could be exploited for radiolabeling in follow-up work. This approach could be adapted to the automated radiosynthesis of a biphenyl model substrate, displaying good RCY and A_M.

Scheme 13 –

Nicewicz and Li et al. 2022 – direct C–H radiocyanation of electron-rich arenes under oxidative photocatalytic conditions, including generally understood mechanism of cation-radical accelerated nucleophilic aromatic substitution.^74,75

Related to this prior approach, in 2023 Nicewicz and Li et al. reported a follow up demethoxy-radiocyanation to facilitate radiolabeling highly electron-dense arenes.⁸⁰ This method exploits the apparent deoxycyanation seen in prior work where -OMe acts as a nucleofuge in CRA-S_NAr (Scheme 14). The authors describe screening less oxidizing photocatalysts to form Ar^·+ species, finding riboflavin tetraacetate an inexpensive and accessible option. In combination with demonstrating the method with [¹²C]CN⁻ and [¹³C]CN⁻, this reaction was also suitable for ¹¹C-radiocyanation. This procedure offers a higher regioselectivity and generally higher yields compared to the C–H approach with only a small number of substrates favoring C–H radiocyanation over demethoxycyanation. Additionally, when using more oxidizing acridinium photocatalysts, a larger substrate scope could be accessed toward less electron-dense scaffolds. These conditions were applied to several highly electron-rich bioactive molecules with an automated procedure being developed and showcased in the radiolabeling of colchicine. Preliminary small animal PET studies with the resulting [¹¹C]demethoxynitrilo-colchicine matched expected distribution profiles in vivo.

Scheme 14 –

Nicewicz and Li et al. 2023 – demethoxy-radiocyanation of arenes under oxidative photocatalysis, including the small animal PET imaging studies from the ¹¹C-demethoxy-cyano analog of colchicine.⁷⁶

Other Aryl Radiocyanations

In recent work, Sanford and Scott et al. show the radiocyanation of arenes through Friedel-Crafts-type reactivity.⁸¹ [¹¹C]Cyanide can readily be oxidized to cyanogen species ([¹¹C]CN–X) with the addition of electrophilic halogenating reagents. While [¹¹C]CN–X species like [¹¹C]CNBr have been exploited in the synthesis of ¹¹C–guanidines, few other examples demonstrate their use in radiochemistry.^82-87 In this work, [¹¹C]KCN was readily transformed into [¹¹C]cyanogen chloride ([¹¹C]CNCl) by addition of NCS. [¹¹C]CNCl facilitated direct C–H Lewis acid-mediated ¹¹C-cyanation of unfunctionalized arenes, reminiscent of Friedel-Crafts acylation (Scheme 15). In some cases, the reaction was shown to be improved with the addition of organometallic precursors, including aryl-trimethylgermanes (Ar–GeMe₃) and aryl-trimethylstannanes (Ar–SnMe₃), which are thought to increase the nucleophilicity of the arene toward [¹¹C]CNCl. This transformation was shown to produce moderate to high RCCs from electron-dense (hetero)arenes. The method was applied to select electron-rich bioactive molecules where site-selective late-stage C–H radiocyanation proceeded in high RCCs.

Scheme 15 –

Friedel-Crafts-type radiocyanation of electron-rich arenes from Scott and Sanford et al. – selected substrate examples and hypothesized mechanism.⁷⁷

Other Transformations from [¹¹C]HCN

¹¹C–Guanidines

Guanidines are highly basic nitrogen-based acyl moieties found commonly in pharmaceuticals and natural products. In 1994, Långstrom et al. published the first radiosynthesis of ¹¹C-guanidines using [¹¹C]CNBr formed from the oxidation of [¹¹C]HCN with Br₂. Subsequently, the same group developed on-line gas-phase production methods for [¹¹C]CNBr synthesis using a quartz column containing pyridinium tribromide and antimony metal to prevent Br₂ breakthrough into reaction/trapping solutions. This method was demonstrated with primary amines which, upon treatment with [¹¹C]CNBr, furnished the corresponding symmetric ¹¹C-guanidine in moderate conversions (Scheme 16A).^82-84 Access to terminal ¹¹C-guanidines could also be achieved when this reaction was performed in supercritical ammonia, a method that was later used in the production of [¹¹C]MIBG and [¹¹C]GG167.⁸⁵ Additional work by Raffel et. al. employed similar methods toward the synthesis of ¹¹C-phenethylguanidines for imaging sympathetic and adrenergic neurons.^86,87

Scheme 16 –

[¹¹C]BrCN for the formation of ¹¹C-guanidines. A – Fully automated protocol for the synthesis of terminal ¹¹C-guanidines from Xiao and Scott et al.⁸⁴ B – Follow up work from Xiao and Scott et al. describing the synthesis of asymmetric ¹¹C-guanidines.⁸⁵

Methods for ¹¹C-guanidine synthesis have proven difficult to translate for routine production either due to their reliance on supercritical ammonia or manual pressurization. Recently, Shao and Scott et al. published an improvement on these syntheses using an on-line-produced [¹¹C]CNBr.⁸⁸ The authors found that a 3-minute generation of the ¹¹C-cyanamide intermediate at room temperature, followed by addition of an ammonia source and N₂ pressurization on a GE TraerLab FX_M reactor module enabled formation of terminal ¹¹C-guanidines in high RCY. Notably, this automated method could be reliably reproduced and does not demand specialized equipment making it more applicable to routine production of radiopharmaceuticals for clinical use. This was employed in labeling a diverse set of aliphatic guanidines including the potential cardiac tracer [¹¹C]3F-PHPOG which shows promising myocardial uptake in rabbits.

Until recently, extant methods with [¹¹C]CNBr have only established the synthesis of symmetric and terminal ¹¹C-guanidines. Follow up work from Scott and Raffel et al. has augmented standard protocols to access asymmetric ¹¹C-guanidines.⁸⁹ In this case, modification of the reaction solvent from water to hindered alcohols (iPrOH, 3-pentanol) was shown to improve cyanate intermediate conversion, likely by reducing hydrolytic side reactions (Scheme 16B). Further, by addition of 40x (v/v) the amount of the second amine relative to starting material, the generation of the asymmetric product was shown to outcompete the formation of its symmetric counterpart. The authors applied this method to the synthesis of a tracer combining structural aspects of the cardiac sympathetic neuronal imaging agent [^123/131I]mIBG and the dopamine transporter (DAT) PET agent [¹¹C]PE2I. Using this method, >100 MBq of this analog was formulated and preliminary PET imaging in rodents were acquired showing the expected myocardial uptake and slow clearance from the heart.

In additional preliminary studies from Scott and Sanford et al., [¹¹C]CNBr was used to enable intracyclic ¹¹C-heterocyclic labeling.⁹⁰ From bis-nucleophilic precursors, [¹¹C]CNBr was employed in heterocycle-forming condensation reactions to internally incorporate ¹¹C. Toward the synthesis of [¹¹C]2-aminobenzoxazole, it was found that generation of [¹¹C]CNBr from [¹¹C]KCN using substoichiometric N-bromo-saccharin produced high RCCs and the addition of Lewis acids including Y(OTf)₃. This method was applicable to alternative heterocyclic frameworks including 5-membered, 5,6-fused and 6,6-fused heterocycles with minimal modification to conditions. The authors applied this method to the generation of several pharmaceutical relevant molecules including the kinase inhibitors [¹¹C]sapanisertib and [¹¹C]serabelisib which proceed with good conversions (Scheme 17).

Scheme 17 –

Preliminary studies from Scott and Sanford et al. describing internal heterocyclic ¹¹C labeling used in-situ [¹¹C]BrCN formation from [¹¹C]KCN.⁸⁶

Aliphatic Decarboxylative (Radio)cyanation

In 2024, Li and Nicewicz et al. expanded on their aromatic radiocyanation approaches discussed above and disclosed a new method for installing [¹¹C]CN⁻ into aliphatic systems by a photo-mediated decarboxylation.⁹¹ The authors employed a acridinium-photocatalyzed radical decarboxylation toward generating aliphatic radicals which could either be abstracted by a Cu center or directly attacked by a nucleophile in the presence of a proximal stabilizing group (Scheme 18). This approach was shown to be functional in primary, secondary, tertiary, and benzylic systems with moderate to high RCC under a Cu-based manifold. The method was applied to the radiosynthesis of racemic [¹¹C]flurbiprofen via a two-step decarboxylative radiocyanation-hydrolysis approach, enabling a formal neutron loss with high RCC; the tracer showed expected uptake in in vivo and ex vivo imaging of inflamed ear samples. This approach was also appropriate for the isotopic labeling of amino acid derivatives. Owing to the α-amine moiety in amino acids, these compounds likely form a stabilized iminium cation under oxidative photochemical conditions, enabling the removal of Cu from these reactions. Generally, in both mechanistic regimes, these reactions performed well in the absence of oxidizable functionalities found in unprotected amines and similar moieties.

Scheme 18 –

Decarboxylative ¹¹C-cyanation from Li and Nicewicz et al.; hypothesized mechanism and example of 2-step formal neutron loss in the automated radiosynthesis of [¹¹C]Rac-Flurbiprofen.⁸⁷

Recent PET Radiotracer Development with [¹¹C]HCN

Coincident with radiolabeling methodology developments, [¹¹C]HCN has also been used in several (pre)clinical radiotracer syntheses since this subject was last reviewed. This section will cover recent examples of investigational ¹¹C-labeled PET radiotracers derived from [¹¹C]HCN, including instances not covered in prior reviews. The radiochemistry involved in the synthesis of these examples will also be discussed, including nuances that can arise in large-scale research- and clinical-batch radiosyntheses.

A 2024 report from Scott and Sanford et al. highlights the importance of novel radiochemical methodology in translating PET radiotracers for clinical use.⁹² The group compared the preparation of the clinical PET tracer [¹¹C]LY2795050 by four distinct techniques. [¹¹C]LY2795050 is a kappa opioid receptor antagonist used to image relevant neuropsychiatric disorders including addiction, epilepsy, and Alzheimer’s disease (AD).^93-95 Despite its prior clinical use, the production of [¹¹C]LY2795050 had not yet been profiled to optimize routine production. The authors tested four methods including Cu-mediated cross-nucleophile coupling from an aryl boronate ester (A), both Cu- and Pd-mediated nucleophile-electrophile coupling from an aryl iodide (B, C) and Pd-mediated ¹¹C-carbonylation of an aryl iodide (D). In all cases, initial radiolabeling was followed by a hydrolysis step to yield the final ¹¹C-amide of [¹¹C]LY2795050. The authors found that Cu-mediated radiocyanation from the aryl boronate precursor (Scheme 19A) gave good RCY and A_M but was undesirable as a routine strategy based on the instability of the precursor on prolonged storage. Radiocyanation strategies from the more-stable aryl-iodide precursor also provided high RCYs, with Pd (Scheme 19B) outperforming Cu (Scheme 19C) regarding ease of synthesis, A_M, and yield. Meanwhile carbonylation strategies (Scheme 19D), while delivering the most reliable and high A_M, proved to require more skilled handling than radiocyanation methods. In all, the authors concluded that methods A and C provided the most robust methodologies for the clinical production of [¹¹C]LY2795050, with Pd-mediated radiocyanation of the stable aryl iodide precursor (Method C) being the most reasonable starting point for reliable dose delivery.

Scheme 19 –

Comparison of four preparations of [¹¹C]LY2795050 from Scott and Sanford et al. including RCY and A_M of each method.⁸⁸

In 2024, Zhang and Liang et al. reported the development of ¹¹C- and ¹⁸F-labeled tracers targeting Leucine-rich Repeat Kinase 2 (LRRK2).⁹⁶ LRRK2 is a multifaceted kinase serving maintenance functions in neurons. In genome-wide association studies of inherited Parkinson’s Disease (PD), activating mutations of LRRK2 have been delineated as possible contributive factors in disease progression. To support clinical work in the development of LRRK2 inhibitors and illuminate the role of LRRK2 in inherited PD, the authors were interested in the labeling and translation of a Pfizer lead LRRK2 inhibitor PF-06447475 (Scheme 20, 1) which has known potency and attractive pharmacokinetic properties for PET imaging. PF-06447475 also contains an aryl-nitrile moiety which engages in a critical hydrogen bonding interaction which led the authors to choose this as the site of radiolabeling. From the corresponding unprotected aryl-bromide precursor, they were able to prepare [¹¹C]PF-06447475 using a ligandless Cu^I-mediated radiocyanation procedure in 4.6% decay-corrected RCY with A_M of 92.5 GBq/μmol. Initial in vitro autoradiography studies of [¹¹C]PF-06447475 in rat brains showed specific heterogeneous binding in line with expected LRRK2 expression which diminished with self-blocking controls. Further, in vivo PET images acquired in LRRK2 wild-type rats showed [¹¹C]PF-06447475 to be brain penetrant. Despite this, the corresponding ¹⁸F analog, [¹⁸F]PF06455943 (Scheme 20, 2), developed in concert, was chosen for further investigation, given its improved brain distribution and longer half-life. [¹⁸F]PF06455943 displayed favorable imaging qualities in validation studies in LRRK2 G2019S-mutant mice wherein significant increases of [¹⁸F]PF06455943 uptake were observed.

Scheme 20 -.

Examples of investigative PET tracers from [¹¹C]HCN discussed in this section.^92-101

A 2016 report from Antoni et al. discusses the development of PET ligands for amyloid aggregates with ¹¹C- and ¹⁸F-labeled examples.⁹⁷ While typically discussed in the context of AD, amyloid aggregates are indicated in several systemic pathologies. Meanwhile, existing PET radioligands for imaging amyloid plaques, such as [¹¹C]Pittsburgh Compound B ([¹¹C]PiB), have proven inefficient in detecting systemic amyloid plaques given high brain uptake and clearance through disease-relevant tissues including the liver and kidneys. The researchers, embarking to develop a systemic amyloid imaging agent, chose previously reported oligothiophene scaffolds as tentative ¹¹C- or ¹⁸F-radioligands. ¹¹C-Ligands containing four- and five-repeating thiophene units were synthesized from the corresponding 2-bromo-thiophene precursors using Pd-mediated radiocyanation in 27 ± 4% and 26 ± 3% decay-corrected RCY, respectively. Both compounds showed a high degree of specific binding to amyloid in human samples; blocking studies also suggest that oligothiophenes likely bind in a disparate manner to [¹¹C]PiB as self-blocking displayed greater radioligand displacement than for [¹¹C]PiB. Further PET-CT imaging studies in nonhuman primates showed high systemic distribution of these tracers, with very low brain uptake. High uptake in myocardial tissue was observed with the pentameric ¹¹C-tracer (Scheme 20, 3), suggesting the potential of this radioligand for systemic amyloid imaging, however the high blood fraction of the tracer requires further interrogation for continued investigation of these scaffolds.

Another utilization of [¹¹C]HCN was reported by the discovery chemistry team at Merck in 2020.⁹⁸ To support discovery efforts in the development of positive allosteric modulators (PAMs) of the M4 muscarinic acetylcholine receptor (M4 MAChR) as a supportive therapy in AD, the researchers aimed to develop a corresponding MAChR subtype-specific radioligand. Initial tracer candidate screening produced pyridyl-nitrile containing compounds 4 and 5 (Scheme 20). While 5 was accessed through standard radiomethylation, 4 was produced via Pd-catalyzed ¹¹C-cyanation from the corresponding 2-pyridyl bromide using Pd(PPh₃)₄ in DMF, though further information about RCY and A_M were excluded. Compound 5 was shown to have higher non-specific binding in in vivo primate studies compared to 4 and showed high brain uptake and heterogenous distribution sufficient to support discovery efforts for M4 MAChR PAMs.

Wang et. al. described the radiosynthesis and analysis of an [¹¹C]HCN-derived radioligand in a 2020 publication.⁹⁹ The authors detail the development of a PET radiotracer targeting the Enhancer of Zeste Homolog 2 (EZH2), a histone methylating enzyme responsible for the downstream repression of tumor suppressing genes which is overexpressed in many cancers. The tracer, [¹¹C]EI1 (Scheme 20, 6) was based on a previously reported EZH2 inhibitor and was synthesized via Pd-catalyzed ¹¹C-cyanation from the corresponding aryl bromide using Pd(PPh₃)₄ in a non-decay-corrected RCY of 18% and A_M = 5.3 MBq/μmol. Notably, the biological half-life of EI1 (3.4 min) was amenable to radiolabeling with carbon-11, considering its short half-life. [¹¹C]EI1 showed rapid uptake in line with known EZH2 distribution in healthy mice, and self-blocking studies showed expected displacement. The short biological half-life of [¹¹C]EI1 was apparent in preclinical PET-CT studies, where fast elimination was observed. Further studies in disease models aim to build upon these results.

A report by James et al. in 2022 detailed the evaluation of [¹¹C]AZD1283 (Scheme 20, 7) for imaging microgial activation in neurological diseases including AD and multiple sclerosis.¹⁰⁰ AZD1283 was chosen as a potential ligand which antagonizes the ADP receptor P2Y12R which is associated with certain phenotypes of chronic microglial activation. Having a short biological half-life (37-159 min), the researchers chose to label the candidate at the nitrile in the 3 position of the pyridyl core of the molecule. [¹¹C]AZD1283 was accessed through Pd-catalyzed ¹¹C-cyanation with Pd(PPh₃)₄ from the 3-pyridyl bromide in 469.5 ± 393.7 MBq non-decay-corrected RCY with 42.3 ± 18.7 GBq/μmol A_M. Unfortunately, despite having desirable metabolic stability, [¹¹C]AZD1283 was found to have low brain uptake in wild-type and P2Y12R-KO mice and further studies in non-human primates showed that this was not an anomaly of the animal model. The authors attribute this low uptake to the acidic sulfonamide moiety which is likely deprotonated in vivo leading to a charged species with low BBB penetrance. The authors conclude that further structure-activity relationship studies are warranted for these scaffolds to improve CNS penetrance.

In 2024 Nag et al. described the radiochemical optimization of a novel ¹¹C-aryl nitrile tracer [¹¹C]BIIB104 (Scheme 20, 8) for imaging AMPA receptors.¹⁰¹ AMPA receptors are ionophores which play a role in several neurological processes including memory and learning and are thought to play key roles in schizophrenia, epilepsy, and AD. BIIB104 was chosen as a positive allosteric modulator of the AMPA receptor which the authors hoped would provide higher specificity than prior attempts to radiolabel orthosteric ligands. [¹¹C]BIIB104 was prepared from the 2-bromo-thiophene precursor under standard Pd(PPh₃)₄ conditions, albeit initially with low RCY. During their study, the authors describe further optimization of the radiocyanation protocol. Specifically, they discovered that RCYs could be improved by reducing NH₃ gas concentrations and increasing the temperature of the Pt oven used for [¹¹C]HCN production to 990°C. This effect can likely be attributed to the poisoning of the Pd⁰ catalyst with NH₃ which is known in transition metal-mediated cross coupling reactions. Further, the authors found that solvent and temperature effects were substantial in this system, with DMSO outperforming DMF and temperatures above and below 135°C leading to reduced conversions. [¹¹C]BIIB104 was shown to have high brain uptake but showed minimal effects from blocking, suggesting insufficient specific binding to AMPA. Alternative scaffolds and confirmatory blocking studies will be required to fully profile [¹¹C]BIIB104 in the future.

2024 work from Xu et al. elucidated the discovery of a ¹¹C-aryl nitrile radioligand of sirtuin-1.¹⁰² Sirtuins are post-translational modifying deacetylases thought to be operative in the processes of aging as well as certain metabolic disorders, with Sirtuin-1 (Sirt-1) activators showing promising effects for certain disorders. To support research on Sirt-1, the authors chose a precedented benzoxazine scaffold with high affinity for Sirt-1 relative to other Sirts as a potential tracer candidate with expected brain uptake. [¹¹C]1 (Scheme 20, 9) was radiolabeled from an aryl iodide precursor with Pd(PPh₃)₄ in 31 ± 4% decay-corrected RCY and 312 ± 85 GBq/μmol A_M. PET-CT imaging in mice confirmed [¹¹C]1 was brain penetrant with homogeneous brain accumulation and the tracer was subject to rapid urinary and hepatobiliary excretion. Self-blocking studies surprisingly showed increased brain uptake of [¹¹C]1 which the authors attribute to systemic displacement of the tracer into blood resulting in increased brain uptake. Higher order animal imaging and translation to disease models are required for further evaluation of [¹¹C]1 as a Sirt-1 radioligand.

Another 2024 study from Sun and Tang et al. used [¹¹C]HCN in the production of the non-proteogenic D-enantiomer of the amino acid glutamine.¹⁰³ [¹¹C]D-Glutamine (Scheme 20, 10) was conceptualized as a way of imaging infection in the case of orthopedic implant failures. Standard holistic non-invasive imaging approaches such [¹⁸F]FDG and ¹¹¹In-labeled leucocytes have shown mixed efficacy in discriminating infection and sterile inflammation. D-Glutamine was chosen as an essential amino acid for both gram-positive and gram-negative bacteria which is generally absent in mammalian hosts. Both [¹¹C]L- and D- Glutamine were synthesized from nucleophilic substitution with [¹¹C]CsCN/18-crown-6 from the doubly protected enantiopure alkyl iodide precursors to ostensibly form a nitrile intermediate which was deprotected with simultaneous hydrolysis with TFA/H₂SO₄ to afford 40% and 37% decay-corrected RCY respectively. [¹¹C]D-Glutamine showed high uptake in infected tissues in rat models and was shown to be effective even when biofilm formation was apparent. Further, in line with the author’s hypothesis, [¹¹C]D-Glutamine showed much higher bacterial uptake relative to [¹¹C]L-Glutamine. In addition, [¹¹C]D-Glutamine uptake was shown to correlate more closely with bacterial count than [¹⁸F]FDG and was similarly able to better differentiate infection from sterile inflammation with immunosuppressed infected rats.

A publication from Zhang et al. in 2022 described the discovery of a [¹¹C]thiocyanate ([¹¹C]SCN⁻) surrogate.¹⁰⁴ Thiocyanate is an environmental toxin which, when accumulated in the brain, can affect the efficacy of neurological drugs and is thought to be an operative element in several neurological disorders. Thiocyanate trafficking has previously been the focus of PET imaging studies through direct intracerebral injection of [¹¹C]KSCN.³⁸ The authors of this study intended to produce a ¹¹C-PET tracer through a pro-drug strategy inspired by the observation of in vivo dehalogenation of 6-halo-purines. A N⁹-pentyl-purine scaffold was radiolabeled through two approaches from [¹¹C]KSCN (derived from [¹¹C]CS₂) and [¹¹C]NH₄CN. The ¹¹C-thiocyanatopurine (Scheme 20, 11) was accessed through a Cu^I-mediated approach from the aryl iodide precursor in 3.9 ± 1.7% decay-corrected RCY. Alternatively, utilizing a disulfide precursor, the ¹¹C-product could be accessed in 13 ± 6.1% decay-corrected RCY while reducing the total synthesis time from 50 to 30 minutes. The researchers went on to confirm the hydrolytic capability of the [¹¹C]thiocyanatopurine in both mouse brain homogenate and phosphate buffer, suggesting both enzymatic and non-enzymatic [¹¹C]SCN liberation mechanisms. Imaging with [¹¹C]thiocyanatopurine in mice showed higher brain uptake relative to [¹¹C]KSCN in support of the authors’ prodrug hypothesis. Additionally, mice treated with monovalent anions (i.e. ClO₄⁻) showed slower [¹¹C]SCN⁻ efflux from the brain relative to non-treated mice; the same was shown for sodium iodide transporter knockout mice. On balance, these results suggest [¹¹C]SCN⁻ may be in part trafficked via the sodium iodide transporter in a competitive manner with other biologically relevant monovalent anions.

Lastly, in a pending patent from Brugarolas et al.,¹¹C- and ¹⁸F-labeled radiotracers for imaging hypoxia inducible factor 2 alpha (HIF2a) are described.¹⁰⁵ HIF2a is a protein involved in regulating gene expression in hypoxic tissues and, when overexpressed, can accelerate angiogenesis and tumor survival in many forms of cancer. The inventors of the patent describe the generation of isotopologs of PT-2385, a first generation HIF2a inhibitor. The synthesis of [¹¹C]PT-2385 (Scheme 20, 12) by Pd⁰-mediated radiocyanation and [¹⁸F]PT-2385 by Cu^II-mediated radiofluorination are described. [¹¹C]PT-2385 is accessed from an aryl bromide precursor with Pd(PPh₃)₄ in 12-13% non-decay-corrected RCY in 35 minutes. While information is limited concerning the ¹¹C-isotopologue, both [¹¹C]PT-2385 and [¹⁸F]PT-2385 were shown to possess high uptake in HIF2a positive tumor grafts in mice, with correspondingly limited uptake in HIF2a inhibitor-resistant tumors. Thus, an ¹⁸F- or ¹¹C-HIF2a PET radiotracer may be promising for monitoring treatment response and resistance to emerging HIF2a inhibitor treatments.

Summary and Future Perspectives

Recent advances in radiochemistry with [¹¹C]HCN and its use in radiopharmaceutical development have shown its utility and versatility as a radiosynthon. In the past 5 years, several new chemical methods employing [¹¹C]HCN have been reported. In particular, the preparation of ¹¹C-aryl nitriles has seen many new methods emerge including those which leverage manifolds like photochemistry which have been potentially undervalued in radiochemistry. Additionally, increasing consideration to GMP constraints and generalizability are notable with a greater apparent emphasis on [¹¹C]HCN production and suitability for routine clinical production.

The prospective role of [¹¹C]HCN in radiopharmaceutical development is difficult to predict, but potentially substantial. Innovations in radiotherapeutics made possible by approval of ¹⁷⁷Lu-labeled beta therapies like Pluvicto^® and Lutathera^® have sparked increasing investment and interest in this area.^106-108 In turn, companion PET diagnostics for new radiotherapeutic interventions are an important corollary under the current theranostic paradigm. The fate of carbon-11, with its short half-life, remains to be seen, particularly where commercialization for theranostic innovations is concerned. Despite this, the versatility and availability of carbon-11 arguably maintain its importance in cyclotron-based facilities for novel tracer and drug discovery programs and, reflecting this, [¹¹C]choline has been approved by the U.S. FDA for PET imaging of prostate cancer.¹⁰⁹ Additionally, innovations in PET imaging systems and reconstruction algorithms which afford high-resolution outputs with lower activities could be impactful implements for shorter-lived isotopes that so far have resisted longer distance distribution.^110-115

In this context, there remain several underexplored areas of [¹¹C]HCN radiochemistry. Chiefly, although the dedicated laboratory infrastructure for [¹¹C]HCN production has become more commonplace in recent years, it remains a large hurdle to its routine adoption at many smaller PET facilities. With this in mind, methods using widely accessible ¹¹C-synthons ([¹¹C]CH₃I, [¹¹C]CO₂) to form [¹¹C]HCN in the absence of specialized hardware are highly valuable but remain less exampled.⁴² Similarly, many methods employ [¹¹C]HCN as [¹¹C]CN⁻ but little investigation has been reported as to the relevance of trapping bases and counterions in different nucleophilic transformations. Analogous [¹⁸F]F⁻ preparations have been discussed at length in the literature and show profound impacts on radiofluorination reactions, but [¹¹C]CN⁻ preparations often lack theoretical motivation.^116-119 Finally, a major advantage to [¹¹C]HCN is the diversity of labeled functionalities available from it and its derivatives. Recent methods have focused primarily on ¹¹C-cyanation with [¹¹C]HCN; more emphasis on delivering otherwise inaccessible ¹¹C moieties from [¹¹C]HCN and expanding methodology with its derivatives like [¹¹C]SCN⁻, [¹¹C]OCN⁻, [¹¹C]CNBr would be a valuable means of expanding the breadth of radiochemical space and delivering novel radiopharmaceuticals.