Abstract View

Introduction

Nicotine is a chiral alkaloid that exists as (S)- and (R)-enantiomers, which exhibit marked stereoselective differences in receptor affinity, pharmacological potency, and systemic disposition. While (S)-nicotine is the predominant and biologically active form in tobacco, (R)-nicotine is increasingly present in synthetic nicotine products, yet its in vivo pharmacokinetics and brain distribution remain poorly defined. Here, we report, the enantioselective radiolabeling, characterization, and in vivo PET evaluation of (R)- and (S)-[¹¹C]nicotine with high radiochemical fidelity establishing a robust platform for stereospecific molecular imaging for the first time.

Methods

Desmethyl-nornicotine (R or S) precursors (2 mg) was reacted with [¹¹C]MeI at 100℃ for 5 min in ACN:DMF (0.25 mL : 0.15 mL). Enantiomericly pure radiotracer was purified via semi-preparative HPLC, formulated over C18 SepPak, and eluted with 10% ethanol in saline directly into a sterile final product vial. Radiochemical purity and identity were assessed on an analytical chiral HPLC.

All eight healthy male rats (6 mo, LongEvans) underwent dual dynamic (30 min) brain scans with (R)- and (S)-[¹¹C]nicotine  (~7.4 MBq, iv)  using a TriFoil PET/CT scanner. Standard uptake values (SUV) were calculated on co-registered PET/CT images using PMOD software, by defining regions of interests (ROIs) for whole brain, thalamus, nucleus accumbens, striatum, and cortex regions.

Results

[¹¹C]nicotine (R & S) were obtained in high chemical and radiochemical purity (>95%), molar activity (~180 GBq/µmol), and minimal residual solvent levels. Notably, both radiotracers exhibited excellent stability (>98% radiochemical purity) in human serum ex vivo, demonstrating their suitability for in vivo applications and further translational imaging.

PET scans demonstrated that both tracers exhibit rapid blood–brain barrier penetration, peak brain uptake at ~5 minutes post-injection, and efficient washout by ~30 minutes, indicating favorable pharmacokinetics for quantitative analysis. (R)-[¹¹C]nicotine exhibited significantly lower whole-brain uptake compared to (S)-[¹¹C]nicotine (~43% reduction; SUV (g/mL): 0.82 Vs. 1.52 *p=0.031. ROI analysis further revealed consistent reductions across nAChR-rich brain regions, including the thalamus, nucleus accumbens, cortex, and striatum (28-32%, *p<0.05) highlighting diminished central exposure of the (R)-enantiomer.

These findings provide the first in vivo evidence that nicotine chirality governs brain uptake and regional distribution, directly linking radiochemical stereochemistry to pharmacokinetic and neurobiological outcomes. Importantly, the establishment of a high-purity, enantiomer-specific [¹¹C]nicotine radiochemistry platform opens new opportunities for investigating chiral pharmacology using PET.

Conclusion

In conclusion, this study delivers a first-in-class enantioselective radiochemistry and imaging framework for nicotine, demonstrating that (R)-nicotine exhibits significantly reduced brain exposure relative to (S)-nicotine. These advances provide a strong chemical and translational foundation for future applications, including nicotine self-administration models, receptor occupancy studies, and regulatory evaluation of chiral drug mixtures.

Figure 1. A. Radiosynthetic scheme of (S)- and (R)-[¹¹C]nicotine (left to right). B. Representative dynamic brain PET/CT images of a male rat upon administration of  [¹¹C]-(S)-nicotine (left) and [¹¹C]-(R)-nicotine (right).