Synthesis of bridged piperazines with sigma receptor affinity

Recent Advances in the Development of Sigma Receptor Ligands as Cytotoxic Agents: A Medicinal Chemistry Perspective

Since their discovery as distinct receptor proteins, the specific physiopathological role of sigma receptors (σRs) has been deeply investigated. It has been reported that these proteins, classified into two subtypes indicated as σ₁ and σ₂, might play a pivotal role in cancer growth, cell proliferation, and tumor aggressiveness. As a result, the development of selective σR ligands with potential antitumor properties attracted significant attention as an emerging theme in cancer research. This perspective deals with the recent advances of σR ligands as novel cytotoxic agents, covering articles published between 2010 and 2020. An up-to-date description of the medicinal chemistry of selective σ₁R and σ₂R ligands with antiproliferative and cytotoxic activities has been provided, including major pharmacophore models and comprehensive structure–activity relationships for each main class of σR ligands.

(+)-and (-)-Phenazocine enantiomers: Evaluation of their dual opioid agonist/σ1 antagonist properties and antinociceptive effects

cis-N-Substituted N-normetazocine enantiomers possess peculiar pharmacological profiles. Indeed, dextro enantiomers bind with high affinity σ₁ receptor while opposite enantiomers bind opioid receptors. In spite of their stereochemistry, cis-N-2-phenylethyl N-normetazocine (phenazocine) enantiomers showed mixed opioid/σ₁ receptor profiles and a significant in vivo analgesia. To the best of our knowledge, there is no information available regarding the evaluation of σ₁ pharmacological profile in the antinociceptive effects of (+)- and (-)-phenazocine. Therefore, the present study was designed to ascertain this component by in vitro and in vivo studies. In particular, we tested the σ₁ affinity of both enantiomers by a predictive binding assay in absence or presence of phenytoin (DPH). Our results showed that DPH (1 mM) did not increase the σ₁ receptor affinity of (+)-and (-)-phenazocine (K_i = 3.8 ± 0.4 nM, K_i = 85 ± 2.0 nM, respectively) suggesting a σ₁ antagonist profile of both enantiomers. This σ₁ antagonistic component of two phenazocine enantiomers was corroborated by in vivo studies in which the selective σ₁ receptor agonist PRE-084, was able to unmask their σ₁ antagonistic component associated with the opioid activity. The σ₁ antagonistic component of (+)- and (-)-phenazocine may justify their analgesic activity and it suggests that they may constitute useful lead compounds to develop new ligands with this dual activity.

4-Nitro-2,1,3-benzoxadiazole derivatives as potential fluorescent sigma receptor probes

New fluorescent derivatives forσreceptors were designed and synthesized.

Synthesis and pharmacological evaluation of a potent and selective σ1 receptor antagonist with high antiallodynic activity

Based on the pharmacophore model of Glennon the conformationally restricted σ(1) receptor ligand 2 with a 1,3-dioxane moiety has been designed and synthesized. The three step synthesis (transacetalization with pentane-1,3,5-triol, tosylation, and nucleophilic substitution with benzylamine) provided diastereoselectively the cis-configured 1,3-dioxane 2 in good yields. The 1,3-dioxane 2 represents a potent σ(1) receptor ligand (K(i) = 19 nM) with moderate selectivity over the σ(2) subtype (K(i) = 92 nM) and excellent selectivity against more than 60 other targets. Additionally the hERG K(+) channel is not affected by 2. In the capsaicin assay 2 showed extraordinarily high analgesic activity with more than 70% analgesia at the very low dose of 0.25 mg/kg body weight, which indicates σ(1) antagonistic activity. Since 2 does only interact with σ(1) receptors, the in-vivo antiallodynic activity of 2 must be attributed to the σ(1) antagonistic activity.

Synthesis and biological evaluation of conformationally restricted σ(1) receptor ligands with 7,9-diazabicyclo[4.2.2]decane scaffold

The key step in the synthesis of the 7,9-diazabicyclo[4.2.2]decane system was a modified Dieckmann condensation of piperazinebutyrate 11, which makes use of trapping the first cyclized intermediate with TMS-Cl. Reduction of the bicyclic ketone 14 with LiBH(4) at -90 °C provided diastereoselectively (>99 : 1) the syn-configured alcohol 15a, which was converted into the final alcohol and ethers 16a-g. The configuration at the 2-position was established by X-ray structure analysis of methyl and ethyl ethers 15b and 15c. In contrast to bicyclic systems with a three-carbon bridge, inversion of the configuration at the 2-position of the alcohol 15a failed to give the inverted alcohol 19a. However, an unselective reduction of the ketone 24 with L-Selectride led to the diastereomeric alcohols 16a and 25a in the ratio 36 : 64. LiAlH(4) reduction of the tosylate 20 and the alkene 18 yielded the diazabicyclo-decane 26 and -decene 27 without further substituents at the four-carbon bridge. The σ(1) and σ(2) receptor affinities were investigated in receptor binding studies with radioligands. All test compounds showed a lower σ(1) affinity than the corresponding bicyclic derivatives with a three-membered bridge. The reduced σ(1) receptor affinity is attributed to the larger four-membered bridge. This hypothesis is supported by the alkene 27, which represents the most potent σ(1) ligand of this series (K(i) = 7.5 nM). In the alkene 27 the size and flexibility of the bridge is considerably reduced by the double bond. The methyl ether 25b and the unsubstituted derivatives 26 and 27 revealed moderate inhibition of the growth of the human tumor cell lines A-427, 5637 and MCF-7. Again, these compounds are less potent than the analogues with a three-membered bridge. The IC(50)-value of the most potent σ(1) ligand 27 against the small cell lung cancer cell line A-427 (IC(50) = 10 μM) should be emphasized, since this cell line is particularly sensitive to homologues with a three-carbon bridge.

5D-QSAR for spirocyclic sigma1 receptor ligands by Quasar receptor surface modeling

Based on a contiguous and structurally as well as biologically diverse set of 87 sigma(1) ligands, a 5D-QSAR study was conducted in which a quasi-atomistic receptor surface modeling approach (program package Quasar) was applied. The superposition of the ligands was performed with the tool Pharmacophore Elucidation (MOE-package), which takes all conformations of the ligands into account. This procedure led to four pharmacophoric structural elements with aromatic, hydrophobic, cationic and H-bond acceptor properties. Using the aligned structures a 3D-model of the ligand binding site of the sigma(1) receptor was obtained, whose general features are in good agreement with previous assumptions on the receptor structure, but revealed some novel insights since it represents the receptor surface in more detail. Thus, e.g., our model indicates the presence of an H-bond acceptor moiety in the binding site as counterpart to the ligands' cationic ammonium center, rather than a negatively charged carboxylate group. The presented QSAR model is statistically valid and represents the biological data of all tested compounds, including a test set of 21 ligands not used in the modeling process, with very good to excellent accuracy [q(2) (training set, n=66; leave 1/3 out) = 0.84, p(2) (test set, n=21)=0.64]. Moreover, the binding affinities of 13 further spirocyclic sigma(1) ligands were predicted with reasonable accuracy (mean deviation in pK(i) approximately 0.8). Thus, in addition to novel insights into the requirements for binding of spirocyclic piperidines to the sigma(1) receptor, the presented model can be used successfully in the rational design of new sigma(1) ligands.

Bicyclic sigma receptor ligands by stereoselective Dieckmann analogous cyclization of piperazinebutyrate

Starting from racemic 2-aminoadipic acid (6) the piperazinedione 10 with a butyrate side chain was synthesized in four reaction steps. The four-carbon bridge was established upon deprotonation of 10 with LHMDS and subsequent trapping of the lithium alcoholate with Me(3)SiCl to give diastereoselectively the mixed methyl silyl ketal 12 in 94% yield. The relative configuration of the new center of chirality was determined by X-ray crystal structure analysis of 12. The high diastereoselectivity during the conversion of the butyrate 10 into the mixed methyl silyl ketal 12 supports the formation of the lithium alcoholate 11 as central intermediate. Stereoselective reduction of the ketone 13 with LiBH(4) led to the alcohol 14, which was benzylated and reduced to provide the final bicyclic products 16 and 17. Whereas the alcohol 16 shows only moderate affinity to both sigma receptor subtypes, the benzyl ether 17 represents a potent and selective sigma(1) receptor ligand (K(i) = 47 nM). Comparison of the sigma receptor affinities of 16 and 17 with the smaller homologues 5a and 5c clearly indicates that the size of the bridge and the O-substituent determines subtype selectivity.

An unambiguous assay for the cloned human sigma1 receptor reveals high affinity interactions with dopamine D4 receptor selective compounds and a distinct structure-affinity relationship for butyrophenones

The ability of the sigma(1) receptor to interact with a huge range of drug structural classes coupled with its wide distribution in the body has contributed to it being implicated as a possible therapeutic target for a broad array of disorders ranging from substance abuse to depression to Alzheimer's disease. Surprisingly, the reported affinity values for some sigma(1) receptor ligands vary more than 50-fold. The potential of the sigma(1) receptor as a pharmacotherapeutic target prompted us to develop an unambiguous assay system for measuring the affinity of ligands to the cloned human sigma(1) receptor. In the course of characterizing this system and determining the true affinity values for almost three dozen compounds, it was discovered that some dopamine D(4) receptor selective compounds bind sigma(1) receptors with high affinity. A systematic analysis of haloperidol-like compounds revealed a clear structure-affinity relationship amongst clinically relevant butyrophenones. The antidepressant fluvoxamine, the drug of abuse methamphetamine, and the neurosteroid progesterone were amongst the many ligands whose interactions with the sigma(1) receptor were confirmed with our screening assay.

Synthesis of 2,5-Diazabicyclo[2.2.2]octanes by Dieckmann Analogous Cyclization

Starting with (S)-aspartate, methyl (S)-2-[1-allyl-4-(4-methoxybenzyl)-3,6-dioxopiperazin-2-yl]acetate 10 was synthesized in a four-step synthesis. Deprotonation of 10 and subsequent trapping of the first cyclization product led to the bicyclic mixed acetal 13 in 15% yield. The low yield of 13, compared with the yield of the corresponding glutamate derivatives, is explained by the higher energy (strain) of the bicyclo[2.2.2]octane system and the lower conformational flexibility of the shorter acetate side chain. The formation of a six-membered Na+-chelate 12 as intermediate is responsible for the high diastereoselectivity of the cyclization step.