Chapter 25 The Use of Isotopically Labeled Compounds in Drug Discovery

Combining Umpolung and Carbon Isotope Exchange Strategies for Accessing Isotopically Labeled α-Keto Acids

The integration of umpolung and carbon isotope exchange for accessing isotopically labeled α-keto acids through photoredox catalysis is elucidated. This process involves the carbonyl umpolung of C(sp2)-α-keto acids to yield C(sp3)-α-thioketal acids, followed by the carbon isotope exchange of C(sp3)-α-thioketal acids, and ultimately, deprotection to generate carbon-labeled α-keto acids.

Transition-metal-free silylboronate-mediated cross-couplings of organic fluorides with amines

C-N bond cross-couplings are fundamental in the field of organic chemistry. Herein, silylboronate-mediated selective defluorinative cross-coupling of organic fluorides with secondary amines via a transition-metal-free strategy is disclosed. The cooperation of silylboronate and potassium tert-butoxide enables the room-temperature cross-coupling of C-F and N-H bonds, effectively avoiding the high barriers associated with thermally induced S_N2 or S_N1 amination. The significant advantage of this transformation is the selective activation of the C-F bond of the organic fluoride by silylboronate without affecting potentially cleavable C-O, C-Cl, heteroaryl C-H, or C-N bonds and CF₃ groups. Tertiary amines with aromatic, heteroaromatic, and/or aliphatic groups were efficiently synthesized in a single step using electronically and sterically varying organic fluorides and N-alkylanilines or secondary amines. The protocol is extended to the late-stage syntheses of drug candidates, including their deuterium-labeled analogs.

Pipeline Impact of Radiolabeled Compounds in Drug Discovery and Development

The long-lived radionuclides tritium and carbon-14 have been used for many years in pharmaceutical research and development for making key efficacy and toxicological decisions. Early discovery utilizes radiolabels for compound selection through radioligand binding assays and autoradiography. In preclinical safety evaluation, the use of labeled compounds for adsorption, distribution, metabolism, and excretion studies is often preferred for the added detection sensitivity. As the drug substance proceeds to the clinic, human metabolism studies are reliant on the use of labeled materials to fulfill required regulatory applications.

C(sp2)–H Activation with Bis(silylene)pyridine Cobalt(III) Complexes: Catalytic Hydrogen Isotope Exchange of Sterically Hindered C–H Bonds

The bis(silylene)pyridine cobalt(III) dihydride boryl, trans-[ ptol SiNSi]Co(H)2BPin (ptolSiNSi = 2,6-[EtNSi(NtBu)2CAr]2 C5H3N, where Ar = C6H5CH3, and Pin =pinacolato) has been used as a precatalyst for the hydrogen isotope exchange (HIE) of arenes and heteroarenes using benzene-d 6 as the deuterium source. Use of D2 as the source of the isotope produced modest levels of deuterium incorporation and stoichiometric studies established modification of the pincer ligand through irreversible addition of H2 across the silylene leading to catalyst deactivation. High levels of deuterium incorporation were observed with benzene-d 6 as the isotope source and enabled low (0.5 - 5 mol%) loadings of the cobalt precursor. The resulting high activity for C-H activation enabled deuterium incorporation at sterically encumbered sites previously inaccessible with first-row metal HIE catalysts. The cobalt-catalyzed method was also compatible with aryl halides, demonstrating a kinetic preference for chemoselective C(sp2)-H activation over C(sp2)-X (X = Cl, Br) bonds. Monitoring the catalytic reaction by NMR spectroscopy established cobalt(III) resting states at both low and high conversions of substrate and the overall performance was inhibited by the addition of HBPin. Studies on precatalyst activation with cis-[ ptol SiNSi]Co(Bf)2H and cis-[ ptol SiNSi]Co(H)2Bf (where Bf = 2-benzofuranyl), support the intermediacy of bis(hydride)aryl cobalt intermediates as opposed to bis(aryl)hydride cobalt complexes in the catalytic HIE method. Mechanistic insights resulted in an improved protocol using [ ptol SiNSi]Co(H)3 NaBHEt3 as the precatalyst, ultimately translating onto higher levels of isotopic incorporation.

Photoredox-Catalyzed Synthesis of Remote Fluoroalkylated Azaarene Derivatives and the α-Deuterated Analogues via 1,n-Hydrogen-Atom-Transfer-Involving Radical Reactions

A modular strategy to access the remote fluoroalkylated azaarene derivatives and the α-deuterated analogues, which are the isosteres of many pharmaceutically important compounds, is reported. Transformations under the sustainable photoredox catalysis platform could efficiently experience cascade radical addition, 1,n-hydrogen atom transfer (HAT), and single-electron reduction to offer the crucial anions α to azaarenes. Through reacting with H₂O or the inexpensive D₂O, two series of valuable products were obtained in high yields with substantial deuterium incorporation. The work demonstrates that the HAT of the α-sp³ C-H of the electron-withdrawing azaarenes with alkyl radicals is viable.

Direct Carbon Isotope Exchange of Pharmaceuticals via Reversible Decyanation

The incorporation of carbon-14 allows tracking of organic molecules and provides vital knowledge on their fate. This information is critical in pharmaceutical development, crop science, and human food safety evaluation. Herein, a transition-metal-catalyzed procedure enabling carbon isotope exchange on aromatic nitriles is described. By utilizing the radiolabeled precursor Zn([¹⁴C]CN)₂, this protocol allows the insertion of the desired carbon tag without the need for structural modifications, in a single step. By reducing synthetic costs and limiting the generation of radioactive waste, this procedure will facilitate the labeling of nitrile containing drugs and accelerate ¹⁴C-based ADME studies supporting drug development.

Fast Carbon Isotope Exchange of Carboxylic Acids Enabled by Organic Photoredox Catalysis

Carbazole/cyanobenzene photocatalysts promote the direct isotopic carboxylate exchange of C(sp³) acids with labeled CO₂. Substrates that are not compatible with transition-metal-catalyzed degradation-reconstruction approaches or prone to thermally induced reversible decarboxylation undergo isotopic incorporation at room temperature in short reaction times. The radiolabeling of drug molecules and precursors with [¹¹C]CO₂ is demonstrated.

The impact of radiochemistry in drug projects: The use of C-14 label in the AZD8529, AZD7325, and AZD6280 projects

Understanding the metabolic transformations of a potential drug molecule is important to understanding the safety profile of a drug candidate. Liquid chromatography-mass spectrometry is a standard method for detecting metabolites in the drug discovery stage, but this can lead to an incomplete understanding of the molecule's metabolism. In this manuscript, we highlight the role radiolabeling played in determining the metabolism and in quantifying the metabolites of AZD8529, AZD7325, and AZD6280. A quantitative whole-body autoradiography study can detect covalent adducts in vivo as was the case with AZD5248 in which the compound was bound to the aorta. Ultimately another compound free of aortic binding was developed, AZD7986.

4,4,16-Trifluoropalmitate: Design, Synthesis, Tritiation, Radiofluorination and Preclinical PET Imaging Studies on Myocardial Fatty Acid Oxidation

Fatty acid oxidation (FAO) produces most of the ATP used to sustain the cardiac contractile work, although glycolysis is a secondary source of ATP under normal physiological conditions. FAO impairment has been reported in the advanced stages of heart failure (HF) and is strongly linked to disease progression and severity. Thus, from a clinical perspective, FAO dysregulation provides prognostic value for HF progression, the assessment of which could be used to improve patient monitoring and the effectiveness of therapy. Positron emission tomography (PET) imaging represents a powerful tool for the assessment and quantification of metabolic pathways in vivo. Several FAO PET tracers have been reported in the literature, but none of them is in routine clinical use yet. Metabolically trapped tracers are particularly interesting because they undergo FAO to generate a radioactive metabolite that is subsequently trapped in the mitochondria, thus providing a quantitative means of measuring FAO in vivo. Herein, we describe the design, synthesis, tritium labelling and radiofluorination of 4,4,16-trifluoro-palmitate (1) as a novel potential metabolically trapped FAO tracer. Preliminary PET-CT studies on [¹⁸ F]1 in rats showed rapid blood clearance, good metabolic stability - confirmed by using [³ H]1 in vitro - and resistance towards defluorination. However, cardiac uptake in rats was modest (0.24±0.04 % ID/g), and kinetic analysis showed reversible uptake, thus indicating that [¹⁸ F]1 is not irreversibly trapped.

Facile H/D Exchange at (Hetero)Aromatic Hydrocarbons Catalyzed by a Stable Trans-Dihydride N-Heterocyclic Carbene (NHC) Iron Complex

Earth-abundant metal pincer complexes have played an important role in homogeneous catalysis during the last ten years. Yet, despite intense research efforts, the synthesis of iron PC_carbeneP pincer complexes has so far remained elusive. Here we report the synthesis of the first PC_NHCP functionalized iron complex [(PC_NHCP)FeCl₂] (1) and the reactivity of the corresponding trans-dihydride iron(II) dinitrogen complex [(PC_NHCP)Fe(H)₂N₂)] (2). Complex 2 is stable under an atmosphere of N₂ and is highly active for hydrogen isotope exchange at (hetero)aromatic hydrocarbons under mild conditions (50 °C, N₂). With benzene-d₆ as the deuterium source, easily reducible functional groups such as esters and amides are well tolerated, contributing to the overall wide substrate scope (e.g., halides, ethers, and amines). DFT studies suggest a complex assisted σ-bond metathesis pathway for C(sp²)–H bond activation, which is further discussed in this study.

Semiconductor photocatalysis to engineering deuterated N-alkyl pharmaceuticals enabled by synergistic activation of water and alkanols

Precisely controlled deuterium labeling at specific sites of N-alkyl drugs is crucial in drug-development as over 50% of the top-selling drugs contain N-alkyl groups, in which it is very challenging to selectively replace protons with deuterium atoms. With the goal of achieving controllable isotope-labeling in N-alkylated amines, we herein rationally design photocatalytic water-splitting to furnish [H] or [D] and isotope alkanol-oxidation by photoexcited electron-hole pairs on a polymeric semiconductor. The controlled installation of N-CH3, -CDH2, -CD2H, -CD3, and -13CH3 groups into pharmaceutical amines thus has been demonstrated by tuning isotopic water and methanol. More than 50 examples with a wide range of functionalities are presented, demonstrating the universal applicability and mildness of this strategy. Gram-scale production has been realized, paving the way for the practical photosynthesis of pharmaceuticals.

Transition-Metal-Free Carbon Isotope Exchange of Phenyl Acetic Acids

A transition-metal-free carbon isotope exchange procedure on phenyl acetic acids is described. Utilizing the universal precursor CO2 , this protocol allows the carbon isotope to be inserted into the carboxylic acid position, with no need of precursor synthesis. This procedure enabled the labeling of 15 pharmaceuticals and was compatible with carbon isotopes [14 C] and [13 C]. A proof of concept with [11 C] was also obtained with low molar activity valuable for distribution studies.

Visible-Light-Enabled Aminocarbonylation of Unactivated Alkyl Iodides with Stoichiometric Carbon Monoxide for Application on Late-Stage Carbon Isotope Labeling

A visible-light-mediated late-stage aminocarbonylation of unactivated alkyl iodides with stoichiometric amounts of carbon monoxide is presented. The method provides a mild, one-step route to [carbonyl-^13/14C] alkyl amides, thereby reducing radioactive waste, and handling of radioactive materials. Easily accessible and low-cost equipment and a palladium catalyst were successfully used for the synthesis of a wide range of alkyl amides.

Scalable direct N-methylation of drug-like amines using 12CO2/13CO2 by simple inorganic base catalysis

With the growing urgency of potential catastrophic climate changes due to anthropogenic CO₂ emissions, numerous efforts have been devoted to development of synthetic protocols using CO₂ as a building block in organic reactions, but the general applicability to complex drug-like substrates remains a challenge. We develop a general protocol for scalable direct N-methylation of a wide-scope drug-like amines using CO₂ and polymethylhydrosiloxane-a nontoxic, aerobically-stable hydrosilane considered as an industrial waste-via simple inorganic base catalysis. A rare application of the Sabatier principle in organic chemistry led to the discovery of cheap, nontoxic K₃PO₄ as an efficient catalyst. Preparations of a wide-scope drug-like amines with carbon-isotope label were also successfully achieved, enabling direct use of CO₂ in studies of drug absorption, distribution, metabolism and excretion.

Catalytic Generation and Chemoselective Transfer of Nucleophilic Hydrides from Dihydrogen

Copper(I)-N-heterocyclic-carbene (NHC) complexes enabled the catalytic generation of nucleophilic hydrides from dihydrogen (H₂ ) and their subsequent transfer to allylic chlorides. The highly chemoselective catalyst displayed no concomitant hydrogenation reactivity; in fact, the terminal double bond formed in the hydride transfer remained intact. Switching to deuterium gas (D₂ ) allowed for regioselective monodeuteration with excellent isotope incorporation.

Direct Carbon Isotope Exchange through Decarboxylative Carboxylation

A two-step degradation-reconstruction approach to the carbon-14 radiolabeling of alkyl carboxylic acids is presented. Simple activation via redox-active ester formation was followed by nickel-mediated decarboxylative carboxylation to afford a range of complex compounds with ample isotopic incorporations for drug metabolism and pharmacokinetic studies. The practicality and operational simplicity of the protocol were demonstrated by its use in an industrial carbon-14 radiolabeling setting.

One-Pot Synthesis of Deuterated Aldehydes from Arylmethyl Halides

A facile, one-pot approach for synthesizing deuterated aldehydes from arylmethyl halides was developed using D₂O as the deuterium source. The efficient process is realized by a sequence of formation, H/D exchange, and oxidation of pyridinium salt intermediates. The mild and air-compatible reaction conditions enable efficient synthesis of diverse deuterated aldehydes with high deuterium incorporation.

A General Strategy for Site-Selective Incorporation of Deuterium and Tritium into Pyridines, Diazines, and Pharmaceuticals

Methods to incorporate deuterium and tritium atoms into organic molecules are valuable for medicinal chemistry. The prevalence of pyridines and diazines in pharmaceuticals means that new ways to label these heterocycles will present opportunities in drug design and facilitate absorption, distribution, metabolism, and excretion (ADME) studies. A broadly applicable protocol is presented wherein pyridines, diazines, and pharmaceuticals are converted into heterocyclic phosphonium salts and then isotopically labeled. The isotopes are incorporated in high yields and, in general, with exclusive regioselectivity.

Photoredox-catalyzed deuteration and tritiation of pharmaceutical compounds

Deuterium- and tritium-labeled pharmaceutical compounds are pivotal diagnostic tools in drug discovery research, providing vital information about the biological fate of drugs and drug metabolites. Herein we demonstrate that a photoredox-mediated hydrogen atom transfer protocol can efficiently and selectively install deuterium (D) and tritium (T) at α-amino sp3 carbon-hydrogen bonds in a single step, using isotopically labeled water (D2O or T2O) as the source of hydrogen isotope. In this context, we also report a convenient synthesis of T2O from T2, providing access to high-specific-activity T2O. This protocol has been successfully applied to the high incorporation of deuterium and tritium in 18 drug molecules, which meet the requirements for use in ligand-binding assays and absorption, distribution, metabolism, and excretion studies.

New approach toward the synthesis of deuterated pyrazolo[1,5-a]pyridines and 1,2,4-triazolo[1,5-a]pyridines

An efficient and operationally simple synthesis of 7-deuteropyrazolo[1,5-a]pyridine and 7-deutero-1,2,4-triazolo[1,5-a]pyridine derivatives using α-H/D exchange of 1-aminopyridinium cations in basic D2O followed by a 1,3-cycloaddition of acetylenes and nitriles is presented. A high regioselectivity and a high degree of deuterium incorporation were achieved. The procedure was applied for several 4-R-1-aminopyridinium cations (R = H, Me, OMe).

Fe-Catalyzed C-C Bond Construction from Olefins via Radicals

This Article details the development of the iron-catalyzed conversion of olefins to radicals and their subsequent use in the construction of C–C bonds. Optimization of a reductive diene cyclization led to the development of an intermolecular cross-coupling of electronically-differentiated donor and acceptor olefins. Although the substitution on the donor olefins was initially limited to alkyl and aryl groups, additional efforts culminated in the expansion of the scope of the substitution to various heteroatom-based functionalities, providing a unified olefin reactivity. A vinyl sulfone acceptor olefin was developed, which allowed for the efficient synthesis of sulfone adducts that could be used as branch points for further diversification. Moreover, this reactivity was extended into an olefin-based Minisci reaction to functionalize heterocyclic scaffolds. Finally, mechanistic studies resulted in a more thorough understanding of the reaction, giving rise to the development of a more efficient second-generation set of olefin cross-coupling conditions.