New solvates of the drug naltrexone: protonation, conformation and interplay of synthons

Naltrexone [systematic name: (4R,4aS,7aR,12bS)-3-cyclopropylmethyl-4a,9-dihydroxy-2,4,5,6,7a,13-hexahydro-1H-4,12-methanobenzofuro[3,2-e]isoquinolin-7-one] is an important morphine-related drug used for combating alcoholism and opioid dependence. Of the eight crystal forms of naltrexone known thus far, only one exists in the neutral form and it crystallizes as a monohydrate. We have isolated the naltrexone free base as two new solvate forms, i.e. the ethyl acetate 0.33-solvate, C₂₀H₂₃NO₄·0.33C₄H₈O₂, (I), and the diethyl ether hemisolvate, C₂₀H₂₃NO₄·0.5C₄H₁₀O, (II). While just one solvent molecule is present in the asymmetric unit of each solvate, there are three drug molecules (Z' = 3) in ethyl acetate solvate (I) and two (Z' = 2) in diethyl ether solvate (II). In (I), one of the three crystallographically independent drug molecules is present with its cyclopropyl group disordered over two sets of positions, as is the whole diethyl ether solvent molecule in (II). In all known forms, including the title forms, the naltrexone molecule exhibits the same conformation of the fused rings. The only conformational variability of naltrexone is in the cyclopropylmethyl group. Two conformations can be found around the bond connecting this group to the N-heterocycle, which is directly related to drug protonation. We have calculated, at the B3LYP/6-31G** level of theory, the minimum energy conformations of protonated and neutral naltrexone molecules for a chosen torsion angle about this bond. The lowest energy conformers depend on the protonation state and are in agreement with those found in the solid state. Within the cyclopropylmethyl group, the bond joining the methylene C atom to the cyclopropyl fragment also evidences conformational variability. In the literature, there are two well defined conformations around this bond. A third cyclopropyl conformation around this second bond is observed in the title solvates. Concerning the supramolecular features of the previously reported crystal structures, only one classical hydrogen bond between naltrexone molecules and one C(8) homosynthon is known, pointing to the robustness of this synthon and the difficulty in disrupting it. New R₂²(7) and C₂²(10) homosynthons are found in both (I) and (II), suggesting that their occurrence derives from crystallization of the neutral drug from nonpolar solvents.

Crystal structure of naltrexone chloride solvates with ethanol, propan-2-ol, and 2-methyl-propan-2-ol

Naltrexone [systematic name: 17-(cyclo-propyl-meth-yl)-3,14-di-hydroxy-4,5α-epoxymorphinan-6-one] is an opioid receptor competitive antagonist that has been widely used to prevent relapse in opioid- and alcohol-dependent subjects. Its chloride salt forms non-isomorphic solvates with ethanol (C20H24NO4+·Cl-·C2H5OH) (I), propan-2-ol (C20H24NO4+·Cl-·C3H7OH) (II), and 2-methyl-propan-2-ol (C20H24NO4+·Cl-·C4H9OH) (III). The naltrexone cation can be described as a T-shape made out of two ring systems, a tetra-hydro-2H-naphtho-[1,8-bc]furan system and a deca-hydro-isoquinolinium subunit, that are nearly perpendicular to one another. The flexible cyclo-propyl-methyl group can adopt various different conformations in response to its surroundings: an increase of available space around cyclo-propyl-methyl group may allow it to adopt a more favorable conformation. In all these structures, the alcohol mol-ecules occupy infinite solvent-filled channels. All three compounds described are attractive crystalline forms for unambiguous identification of naltrexone chloride after isolation from a pharmaceutical form. Compound (III) was refined as a two-component twin.

Probes for narcotic receptor mediated phenomena. 41. Unusual inverse μ-agonists and potent μ-opioid antagonists by modification of the N-substituent in enantiomeric 5-(3-hydroxyphenyl)morphans

Conformational restraint in the N-substituent of enantiomeric 5-(3-hydroxyphenyl)morphans was conferred by the addition of a cyclopropane ring or a double bond. All of the possible enantiomers and isomers of the N-substituted compounds were synthesized. Opioid receptor binding assays indicated that some of them had about 20-fold higher μ-affinity than the compound with an N-phenylpropyl substituent (K(i) = 2-450 nM for the examined compounds with various N-substituents). Most of the compounds acted unusually as inverse agonists in the [(35)S]GTP-γ-S functional binding assay using nondependent cells that stably express the cloned human μ-opioid receptor. Two of the N-substituted compounds with a cyclopropane ring were very potent μ-opioid antagonists ((+)-29, K(e) = 0.17 and (-)-30, K(e) =0.3) in the [(35)S]GTP-γ-S functional binding assay. By comparison of the geometry-optimized structures of the newly synthesized compounds, an attempt was made to rationalize their μ-opioid receptor affinity in terms of the spatial position of N-substituents.

Molecular determinants of non-specific recognition of delta, mu, and kappa opioid receptors

Identification of the molecular determinants of recognition common to all three opioid receptors embedded in a single three-dimensional (3D) non-specific recognition pharmacophore has been carried out. The working hypothesis that underlies the computational study reported here is that ligands that bind with significant affinity to all three cloned opioid receptors, delta, mu, and kappa, but with different combinations of activation and inhibition properties at these receptors, could be promising behaviorally selective analgesics with diminished side effects. The study presented here represents the first step towards the rational design of such therapeutic agents. The common 3D pharmacophore developed for recognition of delta, mu, and kappa opioid receptors was based on the receptor affinities determined for 23 different opioid ligands that display no specificity for any of the receptor subtypes. The pharmacophore centers identified are a protonated amine, two hydrophobic groups, and the centroid of an aromatic group in a geometric arrangement common to all 23, non-specific, opioid ligands studied. Using this three-dimensional pharmacophore as a query for searching 3D structural databases, novel compounds potentially involved in non-specific recognition of delta, mu, and kappa opioid receptors were retrieved. These compounds can be valuable candidates for novel behaviorally selective analgesics with diminished or no side effects, and thus with potential therapeutic usefulness.

N-cubylmethyl substituted morphinoids as novel narcotic antagonists

N-Cubylmethylnormorphine (1) and N-cubylmethylnoroxymorphone (2) have been synthesized and found to be more potent ligands at the mu and kappa opioid receptors than morphine and oxymorphone respectively. In the guinea-pig ileum preparation, compounds 1 and 2 were characterized as opioid mu antagonists (Ke = 68 and 16 nM, respectively). Compound 2 also showed effective kappa-antagonism (Ke = 22 nM). The narcotic antagonism activity of 1 has been confirmed by in vivo assays.