Conformational analysis of some alpha-phenylpropionic acids with anti-inflammatory activity

Mechanism of Hydrogen-Bonded Complex Formation between Ibuprofen and Nanocrystalline Hydroxyapatite

Nanocrystalline hydroxyapatite (nanoHA) is the main hard component of bone and has the potential to be used to promote osseointegration of implants and to treat bone defects. Here, using active pharmaceutical ingredients (APIs) such as ibuprofen, we report on the prospects of combining nanoHA with biologically active compounds to improve the clinical performance of these treatments. In this study, we designed and investigated the possibility of API attachment to the surface of nanoHA crystals via the formation of a hydrogen-bonded complex. The mechanistic studies of an ibuprofen/nanoHA complex formation have been performed using a holistic approach encompassing spectroscopic (Fourier transform infrared (FTIR) and Raman) and X-ray diffraction techniques, as well as quantum chemistry calculations, while comparing the behavior of the ibuprofen/nanoHA complex with that of a physical mixture of the two components. Whereas ibuprofen exists in dimeric form both in solid and liquid state, our study showed that the formation of the ibuprofen/nanoHA complex most likely occurs via the dissociation of the ibuprofen dimer into monomeric species promoted by ethanol, with subsequent attachment of a monomer to the HA surface. An adsorption mode for this process is proposed; this includes hydrogen bonding of the hydroxyl group of ibuprofen to the hydroxyl group of the apatite, together with the interaction of the ibuprofen carbonyl group to an HA Ca center. Overall, this mechanistic study provides new insights into the molecular interactions between APIs and the surfaces of bioactive inorganic solids and sheds light on the relationship between the noncovalent bonding and drug release properties.

Conformational stability of ibuprofen: assessed by DFT calculations and optical vibrational spectroscopy

A thorough conformational analysis of ibuprofen [2-(4-isobutylphenyl) propionic acid] was carried by out, using density functional theory (DFT) calculations coupled to optical vibrational spectroscopy (both Raman and FTIR). Eight different geometries were found to be energy minima. The relative orientations of the substituent groups in the ibuprofen molecule, which can be considered as a para-substituted phenyl ring, were verified to hardly affect its conformational stability. The internal rotations converting the calculated conformers of ibuprofen were studied and the intramolecular interactions governing the conformational preferences of the molecule were analyzed by quantitative potential energy deconvolution using Fourier type profiles. The harmonic vibrational frequencies and corresponding intensities were calculated for all the conformers obtained, leading to the assignment of the spectra, and evidencing the sole presence of one of the lowest energy conformers in the solid state. Vibrational spectroscopic proof of intermolecular hydrogen bonds between the carboxylic groups of adjacent ibuprofen molecules, leading to the formation of dimers, was also obtained.

A combined calorimetric and semiempirical quantum chemical approach to describe the solution thermodynamics of drugs

A combined calorimetric-semiempirical quantum chemical approach is presented to calculate the energy changes for the solution process of drugs. The aim of the presented approach is to understand the elementary steps of the solution process and to propose strategies for an improvement of solubility of drugs. On the basis of the Hess theorem, an alternative route for the solution process via sublimation of the solute, creation of a cavity for the solute in the solvent, transfer into this cavity, and the reorganization of the dipoles of the solute and the solvent can be taken. This approach allows the calculation of all energies describing the direct solution process as well as the alternative route. The approach was tested on 11 substances of different molecular structure by calculating the standard free energy of the specific phase transition. A general way to calculate the energy changes of all phase transitions is given. The complete cycle with DeltaG, DeltaH, and DeltaS was calculated exemplary for the nonsteroidal analgesic ibuprofen. The low solubility of ibuprofen was shown to be due to its high standard free energy of sublimation. Therefore the preparation of solid dispersions could be considered.

QSAR and conformational analysis of the antiinflammatory agent amfenac and analogues

The new nonsteroidal antiinflammatory drug (NSAID) arylacetic amfenac (2-amino-3-benzoylphenylacetic acid) and 19 substituted derivatives were studied in order to correlate the biological activities with the structure-related parameters. The geometry of amfenac in neutral and anionic form was totally optimized, starting from standard geometries and crystallographic data, using semiempirical AM1 and MNDO quantum-mechanical methods. Conformational analysis shows the existence of a rigid structure for rotations of the acetic acid chain (alpha degrees) and the central carbonyl group (gamma degrees) around the bonds with the phenylamine ring, whereas the carboxyl group (beta degrees) and the phenyl ring of the benzoyl group (delta degrees) can rotate almost freely. Electrostatic potential maps were analyzed and showed that the electrostatic orientation effect seems to make an important contribution to the binding of the active compounds to prostaglandin synthase. An electrostatic orientation model of the binding site is proposed. The frontier orbital charge distribution was also described for each compound. On the other hand, steric, electronic and hydrophobic (log P) parameters were calculated and QSAR analysis showed that the most significant parameter for the antiinflammatory activity was the pi-electron density of the HOMO orbital in the second aromatic ring. These results suggest a possible electronic charge transfer between the aromatic fragments and the receptor.

Solid-state nuclear magnetic resonance (NMR) spectra of pharmaceutical dosage forms

Solid-state 13C NMR spectra of tablets or capsules of prednisolone, enalapril maleate, lovastatin, simvastatin, ibuprofen, flurbiprofen, mefenamic acid, indomethacin, diflunisal, sulindac, and piroxicam were obtained in the CP/MAS mode at 50 MHz. These studies show that (1) solid-state NMR spectroscopy can detect the active ingredients in low-dose tablets and capsules; (2) the use of interrupted decoupling often results in suppression of resonances due to excipients, thereby allowing better detection of resonances from the drug; and (3) the technique permits discrimination between two prednisolone polymorphs present in tablets obtained from various manufacturers even though the tablets contain only approximately 5% (w/w) of the drug.

Quantum mechanical study and nuclear magnetic resonance measurements of some alpha-arylcarboxyalkyl acids as anti-inflammatory agents

The CNDO/2 quantum mechanical conformation method of analysis, charge density and protonation energy calculations, as well as 13C and 1H NMR measurements were carried out for ibufenac, ibuprofen, methylibuprofen, and for a series of alpha-arylpropionic acids. It was found that the nature of the terminal lipophilic residue does not significantly influence the conformation of the alpha-arylcarboxyalkyl acid side chain. The preferred conformational angle, for the torsion of the phenyl-C alpha bond, was found to be 90, 120, and 180 degrees in ibufenac, ibuprofen, and methylibuprofen, respectively. This conformational angle is calculated to be the same in all the alpha-arylpropionic acids. The protonation energies of the alpha-arylpropionic acids are correlated with the anti-inflammatory activity. It was found that the smaller the protonation energy, the larger the anti-inflammatory activity.