Photoredox-catalyzed deuteration and tritiation of pharmaceutical compounds

Article abstract of DOI:10.1126/science.aap9674

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

Full text of DOI:10.1126/science.aap9674

REPORTS
Cite as: Y. Y. Loh et al., Science
10.1126/science.aap9674 (2017).
Photoredox-catalyzed deuteration and tritiation of
pharmaceutical compounds
Yong Yao Loh,1* Kazunori Nagao,1* Andrew J. Hoover,2 David Hesk,2 Nelo R. Rivera,2 Steven L. Colletti,3 Ian W.
Davies,1,2 David W. C. MacMillan1†
1Merck Center for Catalysis at Princeton University, Princeton, NJ 08544, USA. 2Labeled Compound Synthesis Group, Department of Process R&D, MRL, Merck & Co., Inc.,
Rahway, NJ 07065, USA. 3Department of Discovery Chemistry, MRL, Merck & Co., Inc., Kenilworth, NJ 07033, USA.
*These authors contributed equally to this work.
†Corresponding author. Email: dmacmill@princeton.edu
Deuterium- and tritium-labeled pharmaceutical compounds are pivotal diagnostic tools in pharmaceutical
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 (HAT) protocol
can efficiently and selectively install deuterium and tritium at α-amino sp3 C–H 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 (ADME) studies.
An essential component of the drug discovery and develop- deuterium or tritium, enabling the synthesis of labeled com-
ment process is the thorough elucidation of a drug molecule’s pounds in a single step without the need for resynthesis (12–
action and toxicity (1–9). Despite rapid advancements in an- 14). This straightforward approach has recently found wide-
alytical techniques over the past 20 years, the introduction of spread application in the pharmaceutical industry, where the
isotopic labels, which integrates a unique signal into a mole- increasing impact of early-stage ligand binding assays and
cule without drastically altering its function, remains the ADME studies on drug discovery has led to a rising demand
most effective method to detect and quantify drugs and drug for the efficient synthesis of isotopically labeled pharmaceu-
metabolites in both in vivo and in vitro studies (1–6). Re- tical compounds (2).
cently, the ability to introduce hydrogen isotopes at non-la-
Currently, transition metal-catalyzed HIE reactions at ar-
bile C–H moieties has seen the emergence of deuterium (2H) omatic C(sp2)–H moieties are well established (Fig. 1A). For
and tritium (3H) as cheaper and more readily accessible alter- example, cationic iridium(I) complexes have long been used
natives to 13C and 14C for the synthesis of isotopically labeled to selectively label sites ortho to directing groups on aromatic
drug analogs, especially when high isotopic incorporation is rings (12, 13). More recently, Chirik and co-workers reported
desired (1–6). In particular, highly deuterated analogs of drug the use of an iron catalyst for the deuteration and tritiation
molecules are used in absorption, distribution, metabolism of pharmaceutical drugs at aromatic C–H moieties without
and excretion (ADME) studies, where they are ideal internal the need for a directing group, enabling orthogonal site se-
standards for mass spectrometry-based quantification in an- lectivity relative to the iridium catalyst (14). In contrast, the
imal and human samples, as they negate matrix effects which direct HIE at aliphatic C(sp3)–H moieties remains a challenge
can interfere with accurate quantification (10, 11). On the in the field (15–20). With over 50% of the top selling commer-
other hand, high specific activity tritium analogs are critical cial drugs containing at least one alkyl amine moiety (21), the
for the accurate quantification of nanomolar ligand binding development of an HIE reaction targeting α-amino C(sp3)–H
affinity studies, as well as the imaging of in vivo compound bonds could potentially provide a general method for the iso-
distribution via autoradiography (7–9).
topic labeling of the aliphatic positions of a drug molecule.
Currently, a large majority of deuterium- and tritium-la- Indeed, the site of the isotopic label can be of vital im-
beled pharmaceutical compounds are synthesized via multi- portance depending on the nature of the study and the met-
step procedures involving the reduction of halogenated or abolic pathways of the substrate of interest (1–6). In addition,
unsaturated drug precursors (1–6). However, advances in the inherently larger number of exchangeable hydrogens at
transition metal-catalyzed C–H activation have allowed for
the direct hydrogen isotope exchange (HIE) of C–H bonds for
α-amino C–H moieties relative to aromatic C–H moieties en-
ables high incorporation of the desired isotopic label, which
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release) 1

is often necessary for the labeled compound to be of practical vs S–H BDE = 87.0 kcal mol−1 (37)). Both catalytic cycles can
use in pharmaceutical studies. Therefore, the development of then converge to undergo a second single-electron transfer
a mild and efficient HIE reaction targeting α-amino C(sp3)–H between 5 and 9 to regenerate photocatalyst 1 and tritiated
bonds is particularly attractive for the pharmaceutical indus- thiol 7 via protonation of thiol anion 10 (E1/2red [IrIII/IrII] = –
try.
1.50 V vs. SCE in acetonitrile (31), E1/2red [thiol] = –0.85 V vs.
Visible light-mediated photoredox catalysis has emerged SCE for cysteine) (38).
in recent years as an enabling platform to access new organic
We began our investigations into the proposed isotopic
transformations through single-electron transfer events (22– labeling protocol by first examining the deuteration of clom-
24). In particular, our laboratory and others have demon- ipramine (11) hydrochloride (Anafranil), a commercially
strated that tertiary amines can be activated via single-elec- available antidepressant. For the application of deuterated
tron oxidation to give an amine radical cation, which can compounds as internal standards in pharmaceutical studies,
each compound should ideally have an isotopic incorporation
of more than 4.0 deuteriums per molecule, with less than
0.1% of the unlabeled compound remaining, so as to avoid
peak overlaps on the mass spectrum to allow for accurate
quantification (11). In addition, to facilitate long term studies,
the program-scale synthesis of a uniformly deuterated batch
of compound is also highly desirable (Fig. 2B, left). A variety
of photoredox catalysts and thiol catalysts, as well as their
respective loadings were evaluated, using N-methyl-2-pyrrol-
idone (NMP) as solvent (see figs. S1 and S2). In our prelimi-
nary studies conducted at 0.1 mmol scale, we were delighted
to observe that the use of 2 mol % of organic photocatalyst,
4Cz-IPN (12) (4Cz-IPN = 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-di-
cyanobenzene, excited state τ = 5.1 μs, E1/2red[*4Cz-IPN/ 4Cz
IPN-] = +1.35 V, E1/2red[4Cz-IPN-/ 4Cz IPN] = +1.21 V) (39) and
30 mol % triisopropylsilanethiol 13 along with 1.2 equivalents
of lithium carbonate afforded the corresponding deuterated
product in 79% yield with 9.2 deuteriums incorporated per
molecule and less than 0.1% of unlabeled compound remain-
ing. Scaling the reaction up resulted in a comparable effi-
ciency of deuterium incorporation, enabling the program-
scale synthesis of [2H]11 with 7.2 deuteriums per molecule,
no detectable unlabeled compound remaining and an iso-
lated yield of 76% as its HCl salt (1.06 g) (Fig. 2B, left). The
use of the sterically hindered triisopropylsilanethiol was crit-
ical to prevent a deleterious thiol-substrate coupling pathway
(see fig. S2). Control experiments also revealed that, light,
photoredox catalyst, and thiol are all essential for deuteration
to proceed (see table S1).
Next, we sought to extend this HIE protocol to the high
specific activity tritiation of amine-containing drugs with
T2O. In contrast to the deuterium HIE reactions, the analo-
gous tritium HIE reactions are predominantly run on mi-
cromolar scale as i) the tritium isotope is easily detectable
and only a small amount of radioactive product is required,
and ii) safety and cost concerns favor limiting the amount of
tritium used to 1.0 Ci (17.2 μmol) per reaction. An additional
complication is that commercially available T2O is typically
highly diluted with natural abundance H2O to prevent the de-
composition of T2O via autoradiolysis (Fig. 2C) (3). To over-
come this issue, we set out to establish a convenient process
undergo facile α-deprotonation to yield carbon-centered α-
amino radicals. The synthetic utility of α-amino radicals has
been showcased by its ability to couple with various electro-
philic partners (25–27). On this basis, we questioned if we
could exploit α-amino radicals to access α-deuterated or α-
tritiated products (Fig. 1B). Specifically, we hypothesized
that, instead of trapping the α-amino radical with a carbon
electrophile, the use of an appropriate hydrogen atom trans-
fer (HAT) catalyst in equilibrium with D2O or T2O could facil-
itate the abstraction of deuterium or tritium to yield labeled
products. We recognized that the choice of the HAT catalyst
would be heavily influenced by thermodynamic factors, in
particular the bond dissociation energy (BDE) relative to that
of the α-amino C–H bond, as well as its pKa relative to water
(Fig. 1B) (28–30). With these considerations in mind, we pos-
tulated that thiols, which have been demonstrated to be good
hydrogen atom donors (31), would be suitable HAT catalysts
for this transformation. Herein, we describe the direct HIE of
α-amino C(sp3)–H bonds mediated by the synergistic merger
of photoredox and HAT catalysis. This methodology was
readily applied to the program-scale deuteration (32) and
high specific activity tritiation of 18 representative drugs and
drug candidates.
A proposed mechanism for the photoredox and HAT cat-
alyzed HIE of α-amino C(sp3)–H bonds is shown in Fig. 2A
(33). Initial photoexcitation of the iridium (III) photocatalyst
Ir(F-Meppy)2(dtbbpy)PF6 [F-Meppy = 2-(4-fluorophenyl)-5-
(methyl)pyridine, dtbbpy = 4,4’-di-tert-butyl-2,2’-bipyridine]
(1) would generate the long lived (τ = 1.1 μs) triplet excited
state IrIII complex 2 (34). This species is a strong single-elec-
red
tron oxidant (E1/2 [*IrIII/IrII] = +0.94 V vs. SCE in acetoni-
trile) (34) and can oxidize amine 3, which undergoes facile
deprotonation at the α-position to give α-amino radical 4. At
the same time, a thiol HAT catalyst (6) would undergo ex-
change with T2O to give the tritiated thiol 7, which would
serve as the source of tritium. In addition to the reported BDE
for typical α-amino C–H and thiol S–H bonds, we reasoned
that HAT can proceed between the polarity matched (35) nu-
cleophilic α-amino radical 4 and tritiated thiol 7, which
would furnish α-tritiated amine product 8 and the electro-
philic thiol radical 9 (α-amino C–H BDE = 93.0 kcal mol−1 (36)
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release)
2

to synthesize high specific activity T2O using only 1 Ci of T2 These high molecular weight macrocycles with no C(sp2)–H
gas, which could be performed in a common laboratory set- bonds are particularly challenging substrates for HIE via con-
ting and used immediately in our tritium HIE reaction. In- ventional transition metal catalysis. The preparation of deu-
dustrially, neat T2O is produced in bulk from the reaction terated azithromycin would have previously involved a multi-
between T2 and PtO2 before being distilled and diluted (40). step sequence and the use of costly deuterated reagents (42).
To facilitate the transfer of micromolar scale T2O into our In contrast, our photoredox protocol enables access to
photoredox reaction mixture, we proposed to adapt this pro- [2H]25 in a single step using D2O, a readily available source
cedure for the generation of T2O in a potential reaction sol- of deuterium. Remarkably, stereochemistry was retained
vent. Among the solvents evaluated, we were pleased to even when H/D exchange occurred at chiral centers on these
observe that the use of NMP as the reaction solvent enabled macrocycles, likely due to substrate control in the HAT pro-
the formation of high specific activity tritiated water in 61% cess. In addition to α-amino positions, benzylic and β-amino
yield (0.51 Ci, 8.8 μmol) from 1.0 Ci T2 gas and PtO2 (see fig. positions were also deuterated in certain instances ([2H]15,
S3).
[2H]16, [2H]18, [2H]22), see figs. S4 and S5). In summary,
With this convenient method to access T2O in hand, we all substrates gave excellent deuterium incorporations on
proceeded to optimize the proposed photoredox-HAT cata- program-scale, with incorporations of more than 5.0 deuteri-
lyzed tritium labeling protocol. In general, high specific ac- ums per molecule, and importantly, less than 0.1% of the un-
tivity tritium labeled compounds are required to have a labeled compound, meeting the minimum requirement for
specific activity of at least 15 Ci mmol−1 (equivalent to an in- their use as internal standards.
corporation of 0.5 tritiums per molecule), and are isolated in
With the same library of 13 pharmaceutical drugs, we
an appreciable radiochemical yield of at least 10 mCi (Fig. 2B, were able to demonstrate similar functional group tolerance
right). Under the dilute micromolar conditions (using ap- for the photoredox-mediated tritium HIE protocol (Fig. 4). As
proximately 4.4 equivalents of T2O), we observed that the use with the deuteration reaction, the best conditions were sub-
of 11 in its free base form, along with increased loadings of strate dependent, and combinations of the two photocata-
the photoredox (4 mol %) and thiol (60 mol %) catalysts, and lysts, 1 and 12, and the two thiols, triisopropylsilanethiol (13)
the use of the integrated photoreactor developed at Merck & and methyl thioglycolate (14) were used. Despite the smaller
Co., Inc. (41) as the visible light source greatly enhanced the excess of T2O in the reaction, the presence of acidic hydrogens
incorporation of tritium, yielding [3H]11 with a high specific from carbamate ([3H]19), amide ([3H]22, [3H]24), carbox-
activity of 40.2 Ci mmol−1 and isolated yield of 39.8 mCi (Fig. ylic acid ([3H]21) and alcohol ([3H]23, [3H]25, [3H]26)
2B, right).
functionalities did not have a major impact on tritium incor-
With the above optimized conditions in hand, we explored poration. Again, we were pleased to see high tritium incorpo-
the generality of the photoredox-mediated deuteration and rations with the high molecular weight macrocycles
tritiation protocols with a library of commercially available azithromycin ([3H]25) and clarithromycin ([3H]26), as
drugs containing a variety of alkyl amine scaffolds. For the these substrates have no aromatic rings and cannot be triti-
deuteration of these substrates (Fig. 3), the best conditions ated using existing HIE protocols. For [3H]22, when the
were found to be substrate dependent, given a choice of two amount of T2 gas used was increased to 2 Ci, specific activity
photocatalysts, Ir(F-Meppy)2(dtbbpy)PF6 (1) and 4-CzIPN was boosted from 11.5 to 14.6 Ci mmol−1, with a large increase
(12), two thiols (13 and 14) and two light sources, a 34 W in yield (8.6 mCi to 19.6 mCi), demonstrating the tunability
blue LED or the integrated photoreactor (see the supplemen- of our protocol with respect to the amount of T2 used, ena-
tary materials). Acyclic trialkyl amines ([2H]15–[2H]17) bling single step tritiation even for particularly recalcitrant
worked well, as did piperidine ([2H]18, [2H]19) and piper- substrates. We were delighted to achieve high specific activi-
azine rings ([2H]20–[2H]24). The reaction also showed ties for all substrates, meeting the typical standards for the
good tolerance for a variety of functional groups. Aryl fluo- preparation of high specific activity pharmaceutical drugs in
ride ([2H]16) and aryl chloride ([2H]18, [2H]22) substitu- appreciable yield for their use in ligand binding studies.
ents were well tolerated under the reaction conditions.
In addition to the library of commercially available drug
Carboxylic acid ([2H]21), ester ([2H]17), amide ([2H]20, molecules, we further evaluated the utility of this photore-
[2H]22, [2H]24) and nitrile ([2H]15, [2H]16) functionali- dox-catalyzed tritiation with a series of potential GPR40 ago-
ties were also amenable to the photoredox conditions, as were PAMs (Fig. 4) developed in the research laboratories of Merck
free hydroxyl groups ([2H]23, [2H]25, [2H]26). In addi- & Co., Inc. (43). The characterization of GPCR allostery is crit-
tion, chiral centers away from the reactive sites were unper- ical to advancing this agoPAM chemical class. Augmentation
turbed during the reaction ([2H]16, [2H]17, [2H]21). Our of binding in the presence of orthosteric ligands has been pre-
deuteration protocol was also applicable to macrolide drugs viously demonstrated using radiolabeled analogs in radiolig-
like azithromycin ([2H]25) and clarithromycin ([2H]26). and binding assays, and subsequently visualized in the GPR-
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release)
3

rhodium, platinum and palladium catalysts. Chemistry 15, 10397–10404 (2009).
doi:10.1002/chem.200901107 Medline
40 ternary crystal structure (44). As such, we envision that
the radiolabeling methodology described herein might be
used to enable metabolite identification both in vitro and in
vivo, as well as the elucidation of the extent of covalent pro-
tein binding in human hepatocytes to derisk any bioactiva-
tion toward reactive electrophiles (45).
12. D. Hesk, P. R. Das, B. Evans, Deuteration of acetanilides and other substituted
aromatics using [Ir(COD)(Cy3P)(Py)]PF6 as catalyst. J. Labelled Comp.
Radiopharm. 36, 497–502 (1995). doi:10.1002/jlcr.2580360514
13. J. R. Heys, Organoiridium complexes for hydrogen isotope exchange labeling. J.
Labelled Comp. Radiopharm. 50, 770–778 (2007). doi:10.1002/jlcr.1428
14. R. P. Yu, D. Hesk, N. Rivera, I. Pelczer, P. J. Chirik, Iron-catalysed tritiation of
pharmaceuticals. Nature 529, 195–199 (2016). doi:10.1038/nature16464
Medline
Initially, photoredox-mediated decarboxylation of the ali-
phatic carboxylic acid moiety in the drug candidates was ob-
served. However, this was overcome by using a less oxidizing
15. For transition metal catalyzed deuteration of C(sp3)–H bonds, see (16–19). For
transition metal-catalyzed tritiation of C(sp3)–H bonds, see (20).
16. M. Takahashi, K. Oshima, S. Matsubara, Ruthenium catalyzed deuterium labelling
of α-carbon in primary alcohol and primary/secondary amine in D2O. Chem. Lett.
34, 192–193 (2005). doi:10.1246/cl.2005.192
17. L. Neubert, D. Michalik, S. Bähn, S. Imm, H. Neumann, J. Atzrodt, V. Derdau, W.
Holla, M. Beller, Ruthenium-catalyzed selective α,β-deuteration of bioactive
amines. J. Am. Chem. Soc. 134, 12239–12244 (2012). doi:10.1021/ja3041338
Medline
18. G. Pieters, C. Taglang, E. Bonnefille, T. Gutmann, C. Puente, J. C. Berthet, C.
Dugave, B. Chaudret, B. Rousseau, Regioselective and stereospecific deuteration
of bioactive aza compounds by the use of ruthenium nanoparticles. Angew. Chem.
Int. Ed. 53, 230–234 (2014). doi:10.1002/anie.201307930 Medline
19. L. V. A. Hale, N. K. Szymczak, Stereoretentive deuteration of α-chiral amines with
D2O. J. Am. Chem. Soc. 138, 13489–13492 (2016). doi:10.1021/jacs.6b07879
Medline
20. W. J. S. Lockley, D. Hesk, Rhodium- and ruthenium-catalysed hydrogen isotope
exchange. J. Labelled Comp. Radiopharm. 53, 704–715 (2010).
doi:10.1002/jlcr.1815
21. N. A. McGrath, M. Brichacek, J. T. Njardarson, A graphical journey of innovative
organic architectures that have improved our lives. J. Chem. Educ. 87, 1348–1349
(2010) doi:10.1021/ed1003806
22. C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Visible light photoredox catalysis with
transition metal complexes: Applications in organic synthesis. Chem. Rev. 113,
5322–5363 (2013). doi:10.1021/cr300503r Medline
23. M. H. Shaw, J. Twilton, D. W. C. MacMillan, Photoredox catalysis in organic
chemistry. J. Org. Chem. 81, 6898–6926 (2016). doi:10.1021/acs.joc.6b01449
Medline
24. M. D. Kärkäs, J. A. Porco Jr., C. R. J. Stephenson, Photochemical approaches to
complex chemotypes: Applications in natural product synthesis. Chem. Rev. 116,
9683–9747 (2016). doi:10.1021/acs.chemrev.5b00760 Medline
25. A. McNally, C. K. Prier, D. W. C. MacMillan, Discovery of an α-amino C–H arylation
reaction using the strategy of accelerated serendipity. Science 334, 1114–1117
(2011). doi:10.1126/science.1213920 Medline
red
photocatalyst, Ir(ppy)2(dtbbpy)PF6 (E1/2 [*IrIII/IrII] = +0.66
V vs. SCE in acetonitrile) (34) or a Lewis acid additive (see
the supplementary materials). We were also pleased to ob-
serve that sterically hindered amines ([3H]27 and [3H]28)
and azetidine rings ([3H]29) were also amenable substrates
for this protocol. Tritiation was also observed at a C(sp2)–H
moiety in [3H]27, possibly due to conjugation of the benzylic
α-amino radical. For [3H]31, epimerization likely occurred
at the chiral center adjacent to nitrogen as a result of the in-
troduction of tritium (46). All 5 compounds were satisfacto-
rily labeled and met the requirements for use in preclinical
candidate selection studies.
We anticipate that the facile access to highly deuterated
and tritiated compounds based on the method described
herein will enable accelerated and broader interrogation of
the biological activity of molecules in the pursuit of the de-
velopment of new molecular therapies.
REFERENCES AND NOTES
1. E. M. Isin, C. S. Elmore, G. N. Nilsson, R. A. Thompson, L. Weidolf, Use of radiolabeled
compounds in drug metabolism and pharmacokinetic studies. Chem. Res. Toxicol.
25, 532–542 (2012). doi:10.1021/tx2005212 Medline
2. W. J. S. Lockley, A. McEwen, R. Cooke, Tritium: A coming of age for drug discovery
and development ADME studies. J. Labelled Comp. Radiopharm. 55, 235–257
(2012). doi:10.1002/jlcr.2928
3. R. Voges, J. R. Heys, T. Moenius, Preparation of Compounds Labeled with Tritium
and Carbon-14 (John Wiley & Sons, 2009).
4. P. H. Allen, M. J. Hickey, L. P. Kingston, D. J. Wilkinson, Metal-catalysed isotopic
exchange labelling: 30 years of experience in pharmaceutical R&D. J. Labelled
Comp. Radiopharm. 53, 731–738 (2010). doi:10.1002/jlcr.1825
5. C. S. Elmore, R. A. Bragg, Isotope chemistry; a useful tool in the drug discovery
26. A. Noble, D. W. C. MacMillan, Photoredox α-vinylation of α-amino acids and N-aryl
amines. J. Am. Chem. Soc. 136, 11602–11605 (2014). doi:10.1021/ja506094d
Medline
arsenal.
Bioorg.
Med.
Chem.
Lett.
25,
167–171
(2015).
27. C. K. Prier, D. W. C. MacMillan, Amine α-heteroarylation via photoredox catalysis:
A homolytic aromatic substitution pathway. Chem. Sci. 5, 4173–4178 (2014).
doi:10.1039/C4SC02155J Medline
28. S. J. Blanksby, G. B. Ellison, Bond dissociation energies of organic molecules. Acc.
Chem. Res. 36, 255–263 (2003). doi:10.1021/ar020230d Medline
29. W. N. Olmstead, Z. Margolin, F. G. Bordwell, Acidities of water and simple alcohols
in dimethyl sulfoxide solution. J. Org. Chem. 45, 3295–3299 (1980).
doi:10.1021/jo01304a032
30. H.-Z. Yu, Y.-M. Yang, L. Zhang, Z.-M. Dang, G.-H. Hu, Quantum-chemical
predictions of pKa’s of thiols in DMSO. J. Phys. Chem. A 118, 606–622 (2014).
doi:10.1021/jp410274n Medline
31. S. Escoubet, S. Gastaldi, N. Vanthuyne, G. Gil, D. Siri, M. P. Bertrand, Thiyl radical
mediated racemization of nonactivated aliphatic amines. J. Org. Chem. 71, 7288–
7292 (2006). doi:10.1021/jo061033l Medline
doi:10.1016/j.bmcl.2014.11.051 Medline
6. C. S. Elmore, The use of isotopically labeled compounds in drug discovery. Annu.
Rep. Med. Chem. 44, 515–534 (2009). doi:10.1016/S0065-7743(09)04425-X
7. E. C. Hulme, M. A. Trevethick, Ligand binding assays at equilibrium: Validation and
interpretation. Br. J. Pharmacol. 161, 1219–1237 (2010). doi:10.1111/j.1476-
5381.2009.00604.x Medline
8. C. Meleza, B. Thomasson, C. Ramachandran, J. W. O’Neill, K. Michelsen, M. C. Lo,
Development of a scintillation proximity binding assay for high-throughput
screening of hematopoietic prostaglandin D2 synthase. Anal. Biochem. 511, 17–
23 (2016). doi:10.1016/j.ab.2016.07.028 Medline
9. W. E. Stumpf, Drug localization and targeting with receptor microscopic
autoradiography. J. Pharmacol. Toxicol. Methods 51, 25–40 (2005).
doi:10.1016/j.vascn.2004.09.001 Medline
10. J. Atzrodt, V. Derdau, T. Fey, J. Zimmermann, The renaissance of H/D exchange.
Angew. Chem. Int. Ed. 46, 7744–7765 (2007). doi:10.1002/anie.200700039
Medline
32. Scale that will allow enough material to support a drug discovery program (in this
case over 1 g of material).
33. Although we favor the mechanism outlined in Fig. 2A, we cannot rule out the
possibility that a mechanism similar to that outlined in fig. S4, is a competing
11. V. Derdau, J. Atzrodt, J. Zimmermann, C. Kroll, F. Brückner, Hydrogen-deuterium
exchange reactions of aromatic compounds and heterocycles by NaBD4-activated
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release)
4

pathway. In this instance, thiol radical generated in situ can abstract from α-amino
C–H bonds in the substrate to form the key α-amino radical intermediate.
34. M. S. Lowry, J. I. Goldsmith, J. D. Slinker, R. Rohl, R. A. Pascal, G. G. Malliaras, S.
Bernhard, Single-layer electroluminescent devices and photoinduced hydrogen
production from an ionic iridium(III) complex. Chem. Mater. 17, 5712–5719
(2005).
51. B. M. Choudary, N. S. Chowdari, S. Madhi, M. L. Kantam, A trifunctional catalyst for
one-pot synthesis of chiral diols via Heck coupling-N-oxidation-asymmetric
dihydroxylation: Application for the synthesis of diltiazem and taxol side chain. J.
Org. Chem. 68, 1736–1746 (2003). doi:10.1021/jo026687i Medline
52. L. A. Mitscher, P. N. Sharma, D. T. W. Chu, L. L. Shen, A. G. Pernet, Chiral DNA
gyrase inhibitors. 2. Asymmetric synthesis and biological activity of the
35. C. Le, Y. Liang, R. W. Evans, X. Li, D. W. C. MacMillan, Selective sp3 C–H alkylation
via polarity-match-based cross-coupling. Nature 547, 79–83 (2017).
doi:10.1038/nature22813 Medline
enantiomers
of
9-fluoro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-2,3-
dihydro-7H- pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid (ofloxacin). J.
Med. Chem. 30, 2283–2286 (1987). doi:10.1021/jm00395a017 Medline
36. D. D. M. Wayner, K. B. Clark, A. Rauk, D. Yu, D. A. Armstrong, C−H bond 53. F. G. Glansdorp, R. J. Spandl, J. E. Swatton, O. Loiseleur, M. Welch, D. R. Spring,
dissociation energies of alkyl amines: Radical structures and stabilization
Using chemical probes to investigate the sub-inhibitory effects of azithromycin.
energies. J. Am. Chem. Soc. 119, 8925–8932 (1997). doi:10.1021/ja971365v
Org. Biomol. Chem. 6, 4120–4124 (2008). doi:10.1039/b813157k Medline
37. C. M. Hadad, P. R. Rablen, K. B. Wiberg, C−O and C−S bonds: Stability, bond 54. P. Allevi, A. Longo, M. Anastasia,
A new convenient transformation of
dissociation energies, and resonance stabilization. J. Org. Chem. 63, 8668–8681
(1998). doi:10.1021/jo972180+
erythromycin A into clarithromycin. Bioorg. Med. Chem. 7, 2749–2752 (1999).
doi:10.1016/S0968-0896(99)00230-8 Medline
38. L. G. Shaidarova, S. A. Ziganshina, G. K. Budnikov, Electrocatalytic oxidation of
cysteine and cystine at a carbon-paste electrode modified with ruthenium(IV)
oxide. J. Anal. Chem. 58, 577–582 (2003). doi:10.1023/A:1024124421268
39. J. Luo, J. Zhang, Donor–acceptor fluorophores for visible-light-promoted organic
synthesis: Photoredox/Ni dual catalytic C(sp3)–C(sp2) cross-coupling. ACS Catal.
6, 873–877 (2016). doi:10.1021/acscatal.5b02204
40. H. Morimoto, P. G. Williams, Design and operations at the National Tritium
Labelling Facility. Fus. Sci. Technol. 21, 256–261 (1992). doi:10.13182/FST92-
A29754
ACKNOWLEDGMENTS
Research reported in this publication was supported by the National Institutes of
Health (NIH) under award number R01 GM103558-04 (D.W.C.M., Y.Y.L., and
K.N.). Y.Y.L. thanks the Agency for Science, Technology and Research (A*STAR)
for a graduate fellowship. K.N. thanks the Japan Society for the Promotion of
Science for an overseas postdoctoral fellowship. Additional funding was provided
by kind gifts from Merck, Abbvie, BMS, and Janssen. The content is solely the
responsibility of the authors and does not necessarily represent the official views
of the NIH. Additional data supporting the conclusions are available in the
supplementary materials.
41. C. C. Le, M. K. Wismer, Z.-C. Shi, R. Zhang, D. V. Conway, G. Li, P. Vachal, I. W.
Davies, D. W. C. MacMillan,
A general small-scale reactor to enable
standardization and acceleration of photocatalytic reactions. ACS Cent. Sci. 3,
647–653 (2017). doi:10.1021/acscentsci.7b00159 Medline
SUPPLEMENTARY MATERIALS
www.sciencemag.org/cgi/content/full/science.aap9674/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S5
Tables S1 and S2
NMR Spectra
References (47–54)
42. A. W. Czarnik, U.S. Patent 20,090,062,220 (2009).
43. C. W. Plummer, M. J. Clements, H. Chen, M. Rajagopalan, H. Josien, W. K.
Hagmann, M. Miller, M. E. Trujillo, M. Kirkland, D. Kosinski, J. Mane, M. Pachanski,
B. Cheewatrakoolpong, A. F. Nolting, R. Orr, M. Christensen, L. C. Campeau, M. J.
Wright, R. Bugianesi, S. Souza, X. Zhang, J. Di Salvo, A. B. Weinglass, R. Tschirret-
Guth, R. Nargund, A. D. Howard, S. L. Colletti, Design and synthesis of novel,
selective GPR40 agoPAMs. ACS Med. Chem. Lett. 8, 221–226 (2017).
doi:10.1021/acsmedchemlett.6b00443 Medline
14 September 2017; accepted 30 October 2017
Published online 9 November 2017
10.1126/science.aap9674
44. J. Lu, N. Byrne, J. Wang, G. Bricogne, F. K. Brown, H. R. Chobanian, S. L. Colletti, J.
Di Salvo, B. Thomas-Fowlkes, Y. Guo, D. L. Hall, J. Hadix, N. B. Hastings, J. D.
Hermes, T. Ho, A. D. Howard, H. Josien, M. Kornienko, K. J. Lumb, M. W. Miller, S.
B. Patel, B. Pio, C. W. Plummer, B. S. Sherborne, P. Sheth, S. Souza, S. Tummala,
C. Vonrhein, M. Webb, S. J. Allen, J. M. Johnston, A. B. Weinglass, S. Sharma, S. M.
Soisson, Structural basis for the cooperative allosteric activation of the free fatty
acid receptor GPR40. Nat. Struct. Mol. Biol. 24, 570–577 (2017).
doi:10.1038/nsmb.3417 Medline
45. T. A. Baillie, Future of toxicology-metabolic activation and drug design: Challenges
and opportunities in chemical toxicology. Chem. Res. Toxicol. 19, 889–893
(2006). doi:10.1021/tx060062o Medline
46. For the use of tracers or reagents in early in vitro screening, epimerization is less
of an issue as the experimental error can be high (>2x in discovery studies for
binding or ex vivo occupancy studies). It should be noted that racemic tracers
have been used in preclinical and clinical settings e.g., PET.
47. D. D. Perrin, W. L. F. Armarego, Purification of Laboratory Chemicals (Pergamon
Press, ed. 3, 1988).
48. W. C. Still, M. Kahn, A. Mitra, Rapid chromatographic technique for preparative
separations with moderate resolution. J. Org. Chem. 43, 2923–2925 (1978).
doi:10.1021/jo00408a041
49. C. C. Gruber, G. Oberdorfer, C. V. Voss, J. M. Kremsner, C. O. Kappe, W. Kroutil, An
algorithm for the deconvolution of mass spectroscopic patterns in isotope
labeling studies. Evaluation for the hydrogen-deuterium exchange reaction in
ketones. J. Org. Chem. 72, 5778–5783 (2007). doi:10.1021/jo070831o Medline
50. P. Zhang, G. Cyriac, T. Kopajtic, Y. Zhao, J. A. Javitch, J. L. Katz, A. H. Newman,
Structure-activity relationships for
a novel series of citalopram (1-(3-
(dimethylamino)propyl)-1-(4-fluorophenyl)-1,3-dihydroisobenzofuran-5-
carbonitrile) analogues at monoamine transporters. J. Med. Chem. 53, 6112–6121
(2010). doi:10.1021/jm1005034 Medline
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release)
5

Fig. 1. Photoredox-catalyzed deuteration and tritiation of
pharmaceutical compounds. (A) The merger of photoredox and hydrogen
atom transfer (HAT) catalysis enables α-amino C(sp3)–H selective HIE of
alkylamine-based drugs. (B) Hypothesis for the proposed photoredox-
catalyzed deuteration and tritiation.
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release)
6

Fig. 2. Reaction development. (A) Proposed catalytic cycle for the photoredox-catalyzed HAT
protocol is shown. SET, single-electron transfer; HAT, hydrogen atom transfer; LED, light-emitting
diode. (B) Procedures and requirements for program-scale deuteration and high specific activity
tritiation. The colored dots (maroon or green) and numbers denote the positions of the C–H bonds that
are labeled and the % incorporation of the hydrogen isotope, respectively. NMP, N-methyl-2-
pyrrolidone; rt, room temperature (C) High specific activity T2O is accessible from T2 and PtO2 at
micromolar scale and can be used for photoredox-catalyzed tritiation in a one-pot procedure.
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release)
7

Fig. 3. Scope of program-scale deuteration. Reaction conditions: photoredox catalyst 1 or 12 (2 mol %),
triisopropylsilanethiol 13 (30 mol %), D2O (50 equiv.), Li2CO3 (1.2 equiv., added when substrate is used as its
acid salt) NMP, room temperature, 34W blue LED or integrated photoreactor. The products were isolated as
the appropriate acid salts shown in the parentheses. Ac, acetyl group. *The two protons at the α-amino
methylene position are diastereotopic and are labeled to different extents.
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release)
8

Fig. 4. Scope of high specific activity tritiation. Reaction conditions: substrate (2 μmol), photoredox
catalyst 1 or 12 (4 mol %), thiol catalyst 13 or 14 (60 mol %), T2O (preformed from 1Ci T2 and PtO2), NMP,
room temperature, 34W blue LED or integrated photoreactor. tBu, tert-butyl group. ‡Incorporation depicted
is for the 2 Ci reaction with [3H]22.
First release: 9 November 2017
www.sciencemag.org
(Page numbers not final at time of first release)
9

Photoredox-catalyzed deuteration and tritiation of pharmaceutical compounds
Yong Yao Loh, Kazunori Nagao, Andrew J. Hoover, David Hesk, Nelo R. Rivera, Steven L. Colletti, Ian W. Davies and David W. C.
MacMillan
published online November 9, 2017
ARTICLE TOOLS
http://science.sciencemag.org/content/early/2017/11/08/science.aap9674
SUPPLEMENTARY
MATERIALS
http://science.sciencemag.org/content/suppl/2017/11/08/science.aap9674.DC1
REFERENCES
This article cites 47 articles, 1 of which you can access for free
http://science.sciencemag.org/content/early/2017/11/08/science.aap9674#BIBL
PERMISSIONS
http://www.sciencemag.org/help/reprints-and-permissions
Use of this article is subject to the Terms of Service
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published by the American Association for the Advancement of
Science, 1200 New York Avenue NW, Washington, DC 20005. 2017 © The Authors, some rights reserved; exclusive licensee
American Association for the Advancement of Science. No claim to original U.S. Government Works. The title Science is a
registered trademark of AAAS.