Hot for HAT and HAT for HOT (labels): Photochemical methods for isotopic labeling

Nine out of ten chemists agree—photochemistry can do some pretty Hot things (Ref 1). If you are reading this, then you’re probably one of the nine. Single Electron Transfer (SET) and Hydrogen Atom Transfer (HAT) reactions driven by light have significantly impacted small molecule synthesis, energetic materials, in vivo and in vitro protein, DNA and cell labeling methods, and water treatment applications among others. This month, we want to discuss an application that connects two of our favorite interests, drug metabolism and photochemistry.  Specifically, photochemical methods for isotopic labeling  by Volker Derdau and coworkers at Sanofi using HAT reactions to undergo Hydrogen Isotope Exchange (HIE) reactions to make deuterium and tritium labeled compounds for drug development (Ref 2).

Isotope labeled drugs play an incredibly important role in ADME (absorption, distribution, metabolism, and excretion) studies. Inject your favorite compound in a rat, let it work its way through all the blood and guts and liver and kidneys, and then want to quantify where it goes? It is helpful to have both the drug and a deuterated version along the way to help with analysis of the LC-MS data as an isotopic internal standard. Alternatively, want to quantify where the compound went without having to deal with the messy work of identifying and quantifying up all the metabolites of your drug? Count the radioactivity of your tritium labeled drug in all the blood, tissues, urine, and feces. Furthermore, labeled compounds find important uses in both imaging and radiolabeled ligand binding studies. Does all this sound too good to be true? Well, access to the compound that you might need to do that study can be incredibly difficult. Ok, so where can I order up my tritium labeled compound?

Labeling with hydrogen isotopes

Many isotopically labeled compounds require multi-step synthesis or a preactivated precursor of your compound of interest than can facilitate labeling. Carrying a radioactive label like tritium through multiple low yielding reactions step can be impractical, due to cost (since this has never been something we’ve ever needed a quote for, we’ll take Wikipedia at their word and say that tritium gas can cost more than $30,000 a gram), and safety concerns (radioactivity). Late-stage C-H functionalization methods are specifically well suited to resolve these concerns affording labeled material in a single step, aiding in their use and utility. Transition metal catalyzed hydrogen isotope exchange methods for labelling drug compounds at aromatic positions are well established (Ref 3). Think Ir(I) catalysis or the pioneering work from Paul Chirik with iron (Ref 4), for aromatic C-H functionalization. As clomipramine is a common model pharmaceutical, aromatic C-H positions accessible with transition metal catalysis are shown in pink in Figure 1. However, exchange with aliphatic C-H bonds remained a challenge until photochemistry entered the scene.

Figure 1: Summary of isotopic labeling options for Clomipramine

Photochemical methods for isotopic labeling

The first photochemical version of this type of reaction was introduced by David Macmillan and Merck in 2017. And it’s no less remarkable a reaction than it was at the time 6 years ago. Working with either an iridium photocatalyst or 4CzIPN and a thiol HAT co-catalyst, they install D or T at α-amino C-H bonds (Figure 1, purple dots) via photocatalysis with blue LEDs. For the isotope label source, they use either D2O, or T2O that they generated from T2 in situ over a platinum catalyst. With this reaction, they were able to achieve high degrees of deuterium incorporation into numerous drug compounds directly with yields greater than 80% at gram scale. For safety and cost concerns, tritium reactions were run on a much smaller scale and blew past minimum radiation incorporation necessary for follow on experiments. This represents a significant achievement and affords hot labels in complex compounds containing no aromatic positions like Azithromycin.

This brings us to a recent work that we want to discuss by Volker Derdau and coworkers at Sanofi Germany entitled “In situ Generated Iridium Nanoparticles as Hydride Donors in Photoredox-Catalyzed Hydrogen Isotope Exchange Reactions with Deuterium and Tritium Gas” (Ref 2). Derdau and his group at Sanofi have a long history of highly useful synthetic methods for isotopic labeling including a recent iridium nanoparticle method for aromatic C-H labeling (Ref 6). Here they look to extend their nanoparticles to a photochemical hydrogen isotope exchange reaction for aliphatic C-H bonds. This paper brings two big advantages to the field of photochemical aliphatic C-H labeling; heterogenous catalysts and direct labeling with D2 and T2 gas as opposed to heavy water.

Figure 2: Model reaction for photocatalytic HIE reaction using heterogeneous nanoparticles (Ref 2)

Photochemical methods for isotopic labeling

For the clomipramine model reaction (Figure 2), the authors screened a series of nanoparticle hydride pre-catalysts (NHP) containing iridium, rhodium, ruthenium, palladium or platinum complexes with 4CzIPN as the photocatalyst, 450 nm LEDs and D2 gas. For their photoreactor, the authors used the EvoluChem™ Photoredox Box. [Ir(Cl)(COD)]2 nanoparticles gave the highest degree of deuterium incorporation at 5.3 D/molecule and were selected as the nanoparticle for further optimization. An extensive optimization was undertaken screening photocatalysts, solvent, NHP loading, and D2 equivalents ultimately pushing deuterium incorporation to 7.6 D/molecule with an extreme excess of D2 (100 equivalents). A temperature survey at elevated temperature was undertaken with the EvoluChem™ PhotoRedox Box TC. Ultimately, the optimal conditions included 10 mol% 4CzIPN, 5 mol% [Ir(Cl)(COD)]2 nanoparticle at RT for 17 hours with 450 nm LEDs. Controls demonstrated that without nanoparticles pre-catalyst, no isotope exchange was observed while the addition of H2O blocked any deuterium incorporation.

Next the authors tested the scope of the deuteration reaction on 13 drug compounds with a wide range of success, with yields ranging from 10-90% and D incorporation (1.3 to 5.8 D/molecule). A broad range of C-H bonds were deuterated across the complex series, suggesting that the reaction with NHP can proceed through both the photoredox catalyzed HAT reaction and classical surface chemistry. Finally, the reaction was adapted for use with tritium by lowering the reaction scale and decreasing the gas pressure (8 equiv. of T2). As an example, see Repaglinide (Figure 3) which demonstrated high T incorporation 2.9 T/molecule (83.4 Ci/mmol) more than sufficient for future use as a radiolabeled compound.

Figure 3: Repaglinide labeling with deuterium and tritium using iridium nanoparticles.

Photochemical methods for isotopic labeling

To better understand the extreme differences observed between different nanoparticles, the authors used Transmission Electron Microscopy (TEM) to investigate the shape of the pre-catalysts. Based on the shape and distribution of size of the particles formed with [Ir(Cl)(COD)]2, it is proposed that the smaller nanoparticles are reactive for the HAT photocatalytic cycle. Additionally, an extensive description of the proposed mechanism between the photocatalytic cycle, the deuterium atom transfer and C-H activation can be found if you the read the full article.

Ultimately, this is nice technology platform likely to find unlimited use within Sanofi for their development programs. With the added benefit of demonstrating the EvoluChem Photoredox Box and PhotoRedox Box TC in full use. Have an idea for where a labeled compound might play a role in your study and need access to the labeled drug? Next time, maybe you can give photochemistry a try.

(1)  Reference not found.

(2)  Kramp, H.; Weck, R.; Sandvoss, M.; Sib, A.; Mencia, G.; Fazzini, P.-F.; Chaudret, B.; Derdau, V. In-Situ Generated Iridium Nanoparticles as Hydride Donors in Photoredox-Catalyzed Hydrogen-Isotope Exchange Reactions with Deuterium and Tritium Gas. Angew. Chemie – Int. Ed. 2023, ASAP.

(3)  Atzrodt, J.; Derdau, V.; Kerr, W. J.; Reid, M. C−H Functionalisation for Hydrogen Isotope Exchange. Angew. Chem. Int. Ed. Engl. 2018, 57 (12), 3022–3047.

(4)  Renyuan Pony Yu, Hesk, D.; Rivera, N.; Pelczer, I.; Chirik, P. J. Iron-Catalysed Tritiation of Pharmaceuticals. Nature 2016, 529, 195–199.

(5)  Loh, Y. Y.; Nagao, K.; Hoover, A. J.; Hesk, D.; Rivera, N. R.; Colletti, S. L.; Davies, I. W.; Macmillan, D. W. C. Photoredox-Catalyzed Deuteration and Tritiation of Pharmaceutical Compounds. Science (80), 2017, 358, 1182–1187.

(6)  Valero, M.; Bouzouita, D.; Palazzolo, A.; Atzrodt, J.; Dugave, C.; Tricard, S.; Feuillastre, S.; Pieters, G.; Chaudret, B.; Derdau, V.; NHC-Stabilized Iridium Nanoparticles as Catalysts in Hydrogen Isotope Exchange Reactions of Anilines. Angew. Chemie – Int. Ed. 2020, 59 (9), 3517–3522.