The sphingosine-1-phosphate receptor 1 (S1PR1) radiotracer [11C]CS1P1 has shown promise in proof-of-concept PET imaging of neuroinflammation in multiple sclerosis (MS). Our HPLC radiometabolite analysis of human plasma samples collected during PET scans with [11C]CS1P1 detected a radiometabolite peak that is more lipophilic than [11C]CS1P1. Radiolabeled metabolites that cross the blood-brain barrier complicate quantitative modeling of neuroimaging tracers; thus, characterizing such radiometabolites is important. Here, we report our detailed investigation of the metabolite profile of [11C]CS1P1 in rats, nonhuman primates, and humans. CS1P1 is a fluorine-containing ligand that we labeled with C-11 or F-18 for preclinical studies; the brain uptake was similar for both radiotracers. The same lipophilic radiometabolite found in human studies also was observed in plasma samples of rats and NHPs for CS1P1 labeled with either C-11 or F-18. We characterized the metabolite in detail using rats after injection of the nonradioactive CS1P1. To authenticate the molecular structure of this radiometabolite, we injected rats with 8 mg/kg of CS1P1 to collect plasma for solvent extraction and HPLC injection, followed by LC/MS analysis of the same metabolite. The LC/MS data indicated in vivo mono-oxidation of CS1P1 produces the metabolite. Subsequently, we synthesized three different mono-oxidized derivatives of CS1P1 for further investigation. Comparing the retention times of the mono-oxidized derivatives with the metabolite observed in rats injected with CS1P1 identified the metabolite as N-oxide 1, also named TZ82121. The MS fragmentation pattern of N-oxide 1 also matched that of the major metabolite in rat plasma. To confirm that metabolite TZ82121 does not enter the brain, we radiosynthesized [18F]TZ82121 by the oxidation of [18F]FS1P1. Radio-HPLC analysis confirmed that [18F]TZ82121 matched the radiometabolite observed in rat plasma post injection of [18F]FS1P1. Furthermore, the acute biodistribution study in SD rats and PET brain imaging in a nonhuman primate showed that [18F]TZ82121 does not enter the rat or nonhuman primate brain. Consequently, we concluded that the major lipophilic radiometabolite N-oxide [11C]TZ82121, detected in human plasma post injection of [11C]CS1P1, does not enter the brain to confound quantitative PET data analysis. [11C]CS1P1 is a promising S1PR1 radiotracer for detecting S1PR1 expression in the CNS.

N-X⋅⋅⋅- O-N+ halogen-bonded systems formed by 27 pyridine N-oxides (PyNOs) as halogen-bond (XB) acceptors and two N-halosuccinimides, two N-halophthalimides, and two N-halosaccharins as XB donors are studied in silico, in solution, and in the solid state. This large set of data (132 DFT optimized structures, 75 crystal structures, and 168 1 H NMR titrations) provides a unique view to structural and bonding properties. In the computational part, a simple electrostatic model (SiElMo) for predicting XB energies using only the properties of halogen donors and oxygen acceptors is developed. The SiElMo energies are in perfect accord with energies calculated from XB complexes optimized with two high-level DFT approaches. Data from in silico bond energies and single-crystal X-ray structures correlate; however, data from solution do not. The polydentate bonding characteristic of the PyNOs\' oxygen atom in solution, as revealed by solid-state structures, is attributed to the lack of correlation between DFT/solid-state and solution data. XB strength is only slightly affected by the PyNO oxygen properties [(atomic charge (Q), ionization energy (Is,min ) and local negative minima (Vs,min )], as the σ-hole (Vs,max ) of the donor halogen is the key determinant leading to the sequence N-halosaccharin>N-halosuccinimide>N-halophthalimide on the XB strength.

Nitroethenediamine is an important functional unit, which is present in H2-receptor antagonists. These drugs show low bioavailability due to the bacterial degradation caused by the N-oxide reductase type of enzymes present in the human colon. Quantum chemical studies have been carried out to elucidate the mechanism of metabolic degradation of nitroethenediamine in the active site of N-oxide reductase. Three different pathways have been explored for the N-oxide bond cleavage by the model system, Mo(IV) bis-dithiolene complex [Mo(OMe)(mdt)2](-), (where mdt=1,2-dimethyl-ethene-1,2-dithiolate) using B3LYP/6-311+G(d,p) and M06/6-311+G(d,p) Density Functional Theory methods. The oxygen atom transfer from the nitrogen atom of nitroethenediamine to the Mo(IV) complex, involves simultaneous weakening of the N-oxide bond and the formation of Mo-O bond through a least motion path. During this transfer, Mo center is converted from a square pyramidal geometry to a distorted octahedral geometry, to facilitate the process of oxygen atom transfer. The energy barrier for the oxygen atom transfer from the imine tautomer has been estimated to be 25.9kcal/mol however, the overall reaction has been found to be endothermic. On the other hand, oxygen transfer reaction from the nitronic acid tautomer requires 30.5kcal/mol energy leading to a highly exothermic metabolite (M-1) directly hence, this path can be considered thermodynamically favorable for this metabolite. The alternative path involving the oxygen atom transfer from the enamine tautomer requires comparatively a higher energy barrier (32.6kcal/mol) and leads to a slightly endothermic metabolite. This study established the structural and energetic details associated with the Mo(IV) bis-dithiolene complex that catalyzes the degradation of nitroethenediamine based drug molecules.