Optimization of pharmacokinetic properties by modification of a carbazole-based cannabinoid receptor subtype 2 (CB2) ligand.

Recently, the development of the fluorinated PET tracer [18F]1a for imaging of CB2 receptors in the central nervous system was reported. [18F]1a showed high CB2 affinity and selectivity over the CB1 subtype, but rapid biotransformation in mice. In addition to the amide hydrolysis, oxidative N-dealkylation and carbazole oxidation were postulated as main metabolic pathways. Based on these results, novel carbazole derivatives with additional 6-substituents (23a, 24a), modified hydrogenation state (26a) and enlarged fluoroalkyl substituent (13a, 13b) were synthesized and pharmacologically evaluated. The key step in the synthesis of substituted carbazoles 23a, 24a and 26a was a Fischer indole synthesis. Nucleophilic substitution of tosylated lactate 5 by carbazole anion provided the fluoroisopropyl derivatives 13a and 13b. Partial hydrogenation of the aromatic carbazole system (26a) was not tolerated by the CB2 receptor. A methylsulfonyl moiety in 6-position (24a) led to considerably reduced CB2 affinity, whereas a 6-methoxy moiety (23a) was well tolerated. An additional methyl moiety in the fluoroethyl side chain of 1a resulted in fluoroisopropyl derivatives 13 with unchanged high CB2 affinity and CB2: CB1 selectivity. Compared with the fluoroethyl derivative 1a, the carbazole N-atom of the fluoroisopropyl derivative 13a (Ki(CB2) = 2.9 nM) is better shielded against the attack of CYP enzymes as formation of N-oxides was not observed and N-dealkylation took place to a less amount.

Self-assembled uracil complexes containing tautomeric uracils: an IRMPD spectroscopic and computation study of the structures of gaseous uracilnCa2+ (n = 4, 5, or 6) complexes.

The structures of doubly-charged uracil (U) complexes with Ca2+, UnCa2+ (n = 4, 5, 6), were studied by infrared multiple photon dissociation (IRMPD) spectroscopy and computational methods. The ions were produced by electrospray ionization (ESI) and were isolated in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR-MS). The recorded IRMPD spectra in both the fingerprint and the C-H/N-H/O-H stretching regions, combined with computed vibrational spectra, reveal that the structures present in the greatest abundance consist of both canonical uracil as well as the lactam (or colloquially "enol") tautomer of uracil. U4Ca2+ consists of two hydrogen-bonded dimers of uracil, one canonical and one tautomer, with each uracil interacting with Ca2+ through a carbonyl oxygen. The structures most consistent with the vibrational spectrum of U6Ca2+ consist of two hydrogen-bonded uracil trimers, each composed of two canonical and one enolic uracil, with each uracil also interacting with Ca2+ through carbonyl oxygen. U5Ca2+ consists of one of the aforementioned trimers and dimers, each containing one enol tautomerized uracil. The computed structures whose vibrational spectra best agree with the experimental vibrational spectra are also the lowest-energy structures for all three complexes. This study clearly shows that some uracils adopt the normally very high energy enol tautomer in the lowest energy gas phase complexes of uracil with a doubly-charged ion like Ca2+.