Efficient Synthesis of Nicotinamide-1-¹⁵N for Ultrafast NMR Hyperpolarization Using Parahydrogen.

Nicotinamide (a vitamin B3 amide) is one of the key vitamins as well as a drug for treatment of M. tuberculosis, HIV, cancer, and other diseases. Here, an improved Zincke reaction methodology is presented allowing for straightforward and scalable synthesis of nicotinamide-1-(15)N with an excellent isotopic purity (98%) and good yield (55%). (15)N nuclear spin label in nicotinamide-1-(15)N can be NMR hyperpolarized in seconds using parahydrogen gas. NMR hyperpolarization using the process of temporary conjugation between parahydrogen and to-be-hyperpolarized biomolecule on hexacoordinate iridium complex via the Signal Amplification By Reversible Exchange (SABRE) method significantly increases detection sensitivity (e.g., >20,000-fold for nicotinamide-1-(15)N at 9.4 T) as has been shown by Theis T. et al. (J. Am. Chem. Soc. 2015, 137, 1404), and hyperpolarized in this fashion, nicotinamide-1-(15)N can be potentially used to probe metabolic processes in vivo in future studies. Moreover, the presented synthetic methodology utilizes mild reaction conditions, and therefore can also be potentially applied to synthesis of a wide range of (15)N-enriched N-heterocycles that can be used as hyperpolarized contrast agents for future in vivo molecular imaging studies.

NMR hyperpolarization increases nuclear spin polarization (P) by several orders of magnitude above equilibrium thermal polarization of nuclear spins achieved by high-field magnets (3–21 T).[1−3] This significant P increase enables concomitant gains in detection sensitivity. Small biomolecules with sufficiently slowly relaxing nuclear spins of 13C and 15N atoms (i.e., with T1 on the order of tens of seconds or more) can be hyperpolarized and used in vivo to probe metabolism and function.[4−6] Isotopic enrichment of these slowly relaxing spins is mandatory to maximize the payload of hyperpolarization for MRI detection.[5] During one decade, hyperpolarized NMR and MRI progressed from a proof-of-principle concept[7−11] to clinical trials in human volunteers.[12,13]

The use of 15N sites in hyperpolarized MRI[14] has a translational advantage over 13C based hyperpolarized contrast agents: spin–lattice relaxation time can be significantly longer, up to tens of minutes vs approximately 1 min.[5,15] Moreover, we and others have recently demonstrated an additional advantage of hyperpolarization process speed: 15N sites of N-heterocyclic compounds can be hyperpolarized in seconds via very simple and instrumentationally nondemanding approach of signal amplification by reversible exchange (SABRE[16]) in shield enables alignment transfer to heteronuclei (SABRE-SHEATH)[17−20] and other RF-based approaches.[21,22] While 15N enrichment of prototype molecule 15N-pyridine can be achieved by several techniques,[23] this compound itself has no significant biomedical relevance.

On the other hand, substituted pyridine-based 15N-heterocycles represent key biomolecules and therefore can be potentially employed as molecular contrast agents. For example, they can be utilized for pH[24] and ion[25] sensing. Nicotinamide-1-15N (vitamin B3 amide), in particular, is potentially an attractive molecular imaging target, because it has low in vivo toxicity: LD50 of nicotinamide is 1.6 g/kg with intravenous injection in dogs.[26] Nicotinamide is a safe active ingredient for treatment of hyperlipidemia in doses of up to 2 g/day,[27] and it is was generally well tolerated at up to 8 g single dose in human volunteers in a dose-escalating study for treatment of Friedreich’s ataxia.[28] The latter high dose corresponds to ∼0.8 mmol/kg dose, which is 8–25 times greater than the 0.03–0.1 mmol/kg dose used in a recent successful clinical MRI trial with hyperpolarized 13C-pyruvate injection.[12] Furthermore, encouraging reports on indirect proton detection of 13C[29,30] and 15N[31,32] hyperpolarized compounds, which increase the detection sensitivity by several-fold compared to direct 13C or 15N detection, can significantly reduce the required dose of hyperpolarized contrast agent (to below 0.1 mmol/kg) required for image acquisition or can be utilized to significantly improve spatial or temporal resolution of future in vivo studies with hyperpolarized 15N-nicotinamide.

More importantly, nicotinamide is used as a drug or a molecular framework for other drugs offering a wide range of potential applications in biomedicine.[33] For example, it was used directly for treatment of M. tuberculosis, HIV[33] and cancer,[34] traumatic brain injury,[35] and its two modifications (isoniazid and pyrazinamide) are the only two small-molecule drugs currently used for M. tuberculosis treatment.[33] Other potential imaging applications related to the differential uptake of hyperpolarized 15N heterocycles (including nicotinamide) can include detecting tuberculosis drug resistance to pyrazinamide[36] and isoniazid[37] and detecting tumors.[38] The agent uptake or pH sensing typically happens on a scale of seconds,[39] because these are relatively fast metabolic processes, which are certainly compatible with slow relaxing hyperpolarized 15N spins.[14,15,25]

Because of the low natural abundance (∼0.3%) of NMR active 15N nuclear spin, efficient isotopic enrichment is essential for the use of nicotinamide as a 15N hyperpolarized contrast agent. A previously published procedure for nicotinamide-1-15N synthesis[40] was based on the Zincke[41,42] two-step methodology. During the first reaction step the Zincke salt is formed, followed by ring opening and displacement by ammonia (or a primary amine) in the second step. The generalized scheme for synthesis of this class of compounds is shown in Figure A.

Figure 1

(A) Mechanism of Zincke reaction. (B) Reaction scheme for the improved preparation of nicotinamide-1-15N.

Unfortunately, this methodology[40] based on our experience produced only fair 15N isotopic enrichment (66%[18] to 85%). Similar results were obtained by Burgos et al.[43] forcing a tedious second round of isotopic enrichment (i.e., the 87% 15N enriched product obtained after first round of isotopic enrichment was used again and the entire synthetic procedure was repeated) in order to achieve final 98% isotopic purity. It could be speculated that the accuracy of instrumentation technique used in ref (40) in 1977 was likely insufficient to properly determine the percentage of 15N isotopic enrichment. Decomposition of nicotinamide-Zincke salt (back to the unlabeled nicotinamide) during its purification as well as during slow replacement reaction most likely caused the substantial loss of the isotopic purity. Here, we present a significant improvement for both reaction steps of this methodology.

Despite some recent advances of the Zincke reaction,[44] its first step is often performed by melting neat starting materials at elevated temperatures (>100 °C). Several attempts to conduct this reaction with nicotinamide either by heating the reaction mixture with a mantel or microwave irradiation as heat sources were made. In all cases, production of a large amount of resin was observed, which required a substantial amount of purification. In order to circumvent this problem, we utilized anhydrous solvents and mild, preferably room temperature, conditions.

First, nicotinamide solubility (semiquantitative) in several commercially available anhydrous solvents was investigated (Table S1). Due to the high solubility of nicotinamide and 2,4-dinitrochlorobenzene in anhydrous dimethyl sulfoxide (DMSO), it was chosen as a reaction medium for the Zincke salt formation. During the preliminary experiments, nicotinamide and 2,4-dinitro-chlorobenzene in the molar ratio 1:3 were dissolved in anhydrous DMSO. After 3 days, 66% conversion of nicotinamide to its corresponding Zincke salt was observed with no side or decomposition products accompanying the transformation (Figures S1 and S2). While overall conversion can be improved by increasing reagent concentration and reaction time, the purity of the conversion was very high. This in turn allowed us to simplify purification procedure and to decrease the handling time of the relatively unstable Zincke salt. Therefore, Zincke salt of nicotinamide was prepared by mixing nicotinamide and 2,4-dinitrochlorobenzene in the molar ratio 1:3 at their maximum concentration and allowing them to react for 5 days (Figure B), followed by a rapid reaction mixture decanting into anhydrous acetone in order to remove excess of the starting material. In order to further minimize possible decomposition of the Zincke salt, substoichiometric amounts of 15NH3, prepared from ammonium-15N chloride (15NH4Cl, 98% 15N, Sigma-Aldrich-Isotec, P/N 299251) and sodium methoxide, were used instead of previously described 15NH4Cl/triethylamine (Et3N) system.[40] Purification involved several cycles of filtration with activated carbon, solvent evaporation, and pH adjustment followed by recrystallization. Absence of laborious flash-chromatography additionally makes this procedure highly scalable—a welcomed advantage for biomedical applications. The final product nicotinamide-1-15N was produced with a 55% yield and the 15N isotopic purity of 98% (Figure B), which was estimated by means of high-resolution mass spectrometry (Figure S7). Note that 98% product isotopic purity corresponds to a theoretical maximum, because reagent (15NH4Cl) isotopic purity was 98%. While the undesirable loss of 15N isotope label (in the form of approximately one 15NH4Cl equivalent per every equivalent of nicotinamide-Zincke salt) is unavoidable in the presented methodology, the ease of preparation and high isotopic purity of the product compensates for the cost of this relatively inexpensive (15NH4Cl costs less than $20 per gram) 15N isotope enrichment source.

In 15N SABRE-SHEATH hyperpolarization, activated by H2[45] Ir-IMes SABRE catalyst[46] forms a hexacoordinate complex with nicotinamide-1-15N and parahydrogen (para-H2), Figure A. Chemical exchange of equatorial nicotinamide-1-15N and para-H2 in μT magnetic fields[19] enables spontaneous polarization transfer from nascent para-H2 singlet. Importantly, SABRE-SHEATH hyperpolarization of activated complex requires only several seconds.[18,19] For example, 15N signal (ε15N) and P15N enhancement by ∼7300-fold is shown in Figure . We note that 50% para-H2 gas was employed here resulting in 1/3 of the maximum effect. If ∼100% para-H2 gas was utilized, ε15N would be effectively tripled to ∼22 000-fold corresponding to %P15N of ∼ 7.2%. The latter number is in quantitative agreement with %P15N ∼ 7% reported earlier for nicotinamide with 66% 15N enrichment.[18] While the efficient hyperpolarization of 15N-nicotinamide for future in vivo experiments requires further hardware advances (e.g., improving the efficiency of para-H2 mixing) and advances in chemistry to make hyperpolarized material biologically compatible to take full advantage of increased enrichment levels demonstrated here,[17] the fundamental parameters of polarization transfer efficiency from para-H2 to 15N spins and the 15N polarization enhancements appear to be approximately the same for ∼66% enriched 15N,[18] and the ∼98% 15N enriched nicotinamide (Figure B) under nearly identical experimental conditions and preparation protocols. Therefore, the payload of produced hyperpolarized 15N magnetization is effectively increased by ∼49% when using 98% 15N enriched nicotinamide vs the previously used 66% 15N enriched one. Furthermore, we additionally report that 15N SABRE-SHEATH hyperpolarization efficiency for nicotinamide can be further improved through the use of SABRE catalyst activation with natural abundance pyridine and the use of natural abundance methanol vs deuterated methanol-d4 employed in the original SABRE-SHEATH demonstration (ref (18) and Figure B). Figure C shows the 15N spectrum of hyperpolarized 15N enriched (98% enrichment) nicotinamide with ε15N ∼ 11 000-fold, which was achieved using 50% para-H2. If nearly 100% para-H2 was utilized, ε15N would be effectively tripled to ∼33 000-fold corresponding to %P15N ∼ 11%. As a result of this improvement, the effective payload of produced hyperpolarized 15N magnetization is further increased by ∼50% compared to the previously employed procedure at the same nicotinamide and catalyst concentrations. The reported ε15N and %P15N do not take into account T1 relaxation losses, which likely occurred,[900,901] because 15N T1 of nicotinamide is only 20.2 ± 0.3 s at 9.4 T (Figure D)—in accord with a previous report of 22 ± 0.3 s in aqueous solution at the same magnetic field strength.[24] Overall, the results presented in this study show that the hyperpolarization payload of 15N nicotinamide is ∼2.2 times greater than that previously shown.[18]

Figure 2

(A) Schematics of 15N SABRE-SHEATH hyperpolarization process[18,19] for nicotinamide-1-15N. Note that Nic stands for axial nicotinamide-1-15N not participating in SABRE process, and IMes stands for 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene. (B) 15N NMR spectroscopy of 15N SABRE-SHEATH of a 50 mM sample of 15N-nicotinamide (15N enrichment of ∼98%) in methanol-d4 using 3 mM of activated IMes catalyst. (C) 15N NMR spectroscopy of 15N SABRE-SHEATH of a 50 mM sample of 15N-nicotinamide (15N enrichment of ∼98%) in methanol using 3 mM of IMes catalyst activated using pyridine (see SI for details). Enhancements ε15N of ∼7300-fold and ε15N of ∼11 000-fold were achieved, respectively, in B and C in comparison to E. (D) 15N T1 signal decay of hyperpolarized nicotinamide-15N in methanol-d4. (E) Spectrum of thermally polarized sample of 12.5 M 15N-Py (note the vertical axis is scaled by 32-fold). The spectra of HP nicotinamide-1-15N show the expected resonances of free and catalyst-bound species.[18]

15N SABRE-SHEATH has been successfully applied to several pyridine-based 15N-heterocycles to date including nicotinamide-1-15N,[18] which can potentially be used for pH sensing[24] or as a reporting molecular probe to study HIV, M. tuberculosis, and others. Furthermore, because conventional proton SABRE of isoniazid and pyrazinamide,[47] SABRE in aqueous media,[45,48,49] and heterogeneous SABRE[50,51] have been successfully demonstrated, the 15N SABRE-SHEATH method can be likely extended to these[47] and other important biomolecules[52] for biomedical applications.

In summary, we have developed a straightforward, scalable method of preparation of isotopically pure (98% 15N) nicotinamide-1-15N. Because of mild conditions developed for the Zincke salt formation, this methodology is likely applicable to a wide range of potential 15N-hyperpolarized contrast agents containing N-heterocycles such as the drug isoniazid,[47] potent in vivo pH sensor 2,6-lutidine,[24] ion sensors,[25] and others.