Phenoxypropylamines: a new series of squalene synthase inhibitors

Unveiling the Therapeutic Potential of Squalene Synthase: Deciphering Its Biochemical Mechanism, Disease Implications, and Intriguing Ties to Ferroptosis

Squalene synthase (SQS) has emerged as a promising therapeutic target for various diseases, including cancers, owing to its pivotal role in the mevalonate pathway and the antioxidant properties of squalene. Primarily, SQS orchestrates the head-to-head condensation reaction, catalyzing the fusion of two farnesyl pyrophosphate molecules, leading to the formation of squalene, which has been depicted as a highly effective oxygen-scavenging agent in in vitro studies. Recent studies have depicted this isoprenoid as a protective layer against ferroptosis due to its potential regulation of lipid peroxidation, as well as its protection against oxidative damage. Therefore, beyond its fundamental function, recent investigations have unveiled additional roles for SQS as a regulator of lipid peroxidation and programmed cell death pathways, such as ferroptosis-a type of cell death characterized by elevated levels of lipid peroxide, one of the forms of reactive oxygen species (ROS), and intracellular iron concentration. Notably, thorough explorations have shed light on the distinctive features that set SQS apart from other members within the isoprenoid synthase superfamily. Its unique biochemical structure, intricately intertwined with its reaction mechanism, has garnered significant attention. Moreover, considerable evidence substantiates the significance of SQS in various disease contexts, and its intriguing association with ferroptosis and lipid peroxidation. The objective of this report is to analyze the existing literature comprehensively, corroborating these findings, and provide an up-to-date perspective on the current understanding of SQS as a prospective therapeutic target, as well as its intricate relationship with ferroptosis. This review aims to consolidate the knowledge surrounding SQS, thereby contributing to the broader comprehension of its potential implications in disease management and therapeutic interventions.

Antihyperlipidemic morpholine derivatives with antioxidant activity: An investigation of the aromatic substitution

Drugs affecting more than one target could result in a more efficient treatment of multifactorial diseases as well as fewer safety concerns, compared to a one-drug one-target approach. Within our continued efforts towards the design of multifunctional molecules against atherosclerosis, we hereby report the synthesis of 17 new morpholine derivatives which structurally vary in terms of the aromatic substitution on the morpholine ring. These derivatives simultaneously suppress cholesterol biosynthesis through SQS inhibition (IC50 values of the most active compounds are between 0.7 and 5.5 μM) while exhibiting a significant protection of hepatic microsomal membranes against lipid peroxidation (with IC50 values for the most active compounds being between 73 and 200 μM). Further evaluation of these compounds was accomplished in vivo in an animal model of acute experimental hyperlipidemia, where it was observed that compounds reduced the examined lipidemic parameters (TC, TG and LDL) by 15-80%. In order to examine the mode of binding of these molecules in the active catalytic site of SQS, we also performed docking simulation studies. Our results indicate that some of the new compounds can be considered interesting structures in the search for new multifunctional agents of potential application in atherosclerosis.

Cardiac and skeletal muscles show molecularly distinct responses to cancer cachexia

Cancer cachexia is a systemic, paraneoplastic syndrome seen in patients with advanced cancer. There is growing interest in the altered muscle pathophysiology experienced by cachectic patients. This study reports the microarray analysis of gene expression in cardiac and skeletal muscle in the colon 26 (C26) carcinoma mouse model of cancer cachexia. A total of 268 genes were found to be differentially expressed in cardiac muscle tissue, compared with nontumor-bearing controls. This was fewer than the 1,533 genes that changed in cachectic skeletal muscle. In addition to different numbers of genes changing, different cellular functions were seen to change in each tissue. The cachectic heart showed signs of inflammation, similar to cachectic skeletal muscle, but did not show the upregulation of ubiquitin-dependent protein catabolic processes or downregulation of genes involved in cellular energetics and muscle regeneration that characterizes skeletal muscle cachexia. Quantitative PCR was used to investigate a subset of inflammatory genes in the cardiac and skeletal muscle of independent cachectic samples; this revealed that B4galt1, C1s, Serpina3n, and Vsig4 were significantly upregulated in cardiac tissue, whereas C1s and Serpina3n were significantly upregulated in skeletal tissue. Our skeletal muscle microarray results were also compared with those from three published microarray studies and found to be consistent in terms of the genes differentially expressed and the functional processes affected. Our study highlights that skeletal and cardiac muscles are affected differently in the C26 mouse model of cachexia and that therapeutic strategies cannot assume that both muscle types will show a similar response.

Human isoprenoid synthase enzymes as therapeutic targets

In the human body, the complex biochemical network known as the mevalonate pathway is responsible for the biosynthesis of all isoprenoids, which consists of a vast array of metabolites that are vital for proper cellular functions. Two key isoprenoids, farnesyl pyrophosphate (FPP) and geranylgeranyl pyrophosphate (GGPP) are responsible for the post-translational prenylation of small GTP-binding proteins, and serve as the biosynthetic precursors to numerous other biomolecules. The down-stream metabolite of FPP and GGPP is squalene, the precursor to steroids, bile acids, lipoproteins, and vitamin D. In the past, interest in prenyl synthase inhibitors focused mainly on the role of the FPP in lytic bone diseases. More recently pre-clinical and clinical studies have strongly implicated high levels of protein prenylation in a plethora of human diseases, including non-skeletal cancers, the progression of neurodegenerative diseases and cardiovascular diseases. In this review, we focus mainly on the potential therapeutic value of down-regulating the biosynthesis of FPP, GGPP, and squalene. We summarize the most recent drug discovery efforts and the structural data available that support the current on-going studies.

Mechanism of action and inhibition of dehydrosqualene synthase

"Head-to-head" terpene synthases catalyze the first committed steps in sterol and carotenoid biosynthesis: the condensation of two isoprenoid diphosphates to form cyclopropylcarbinyl diphosphates, followed by ring opening. Here, we report the structures of Staphylococcus aureus dehydrosqualene synthase (CrtM) complexed with its reaction intermediate, presqualene diphosphate (PSPP), the dehydrosqualene (DHS) product, as well as a series of inhibitors. The results indicate that, on initial diphosphate loss, the primary carbocation so formed bends down into the interior of the protein to react with C2,3 double bond in the prenyl acceptor to form PSPP, with the lower two-thirds of both PSPP chains occupying essentially the same positions as found in the two farnesyl chains in the substrates. The second-half reaction is then initiated by the PSPP diphosphate returning back to the Mg(2+) cluster for ionization, with the resultant DHS so formed being trapped in a surface pocket. This mechanism is supported by the observation that cationic inhibitors (of interest as antiinfectives) bind with their positive charge located in the same region as the cyclopropyl carbinyl group; that S-thiolo-diphosphates only inhibit when in the allylic site; activity results on 11 mutants show that both DXXXD conserved domains are essential for PSPP ionization; and the observation that head-to-tail isoprenoid synthases as well as terpene cyclases have ionization and alkene-donor sites which spatially overlap those found in CrtM.

Synthesis and biological evaluation of novel propylamine derivatives as orally active squalene synthase inhibitors

Squalene synthase inhibitors are potentially superior hypolipidemic agents. We synthesized novel propylamine derivatives, as well as evaluated their ability to inhibit squalene synthase and their lipid-lowering effects in rats. 1-Allyl-2-[3-(benzylamino)propoxy]-9H-carbazole (YM-75440) demonstrated potent inhibition of the enzyme derived from HepG2 cells with an IC(50) value of 63 nM. It significantly reduced both plasma total cholesterol and plasma triglyceride levels following oral dosing to rats with a reduced tendency to elevate plasma transaminase levels.

Zwitterionic Sulfobetaine Inhibitors of Squalene Synthase

A substantial number of sulfobetaines (e.g., 10) have been synthesized and evaluated as inhibitors of squalene synthase (SS) on the basis of the idea that their zwitterionic structure would have properties conducive both to binding in the active site and to passage through cell membranes. When the simple sulfobetaine moiety is incorporated into compounds containing hydrophobic portions like those in farnesyl diphosphate (1) or presqualene diphosphate (2), inhibition of SS in a rat liver microsomal assay was indeed observed. For example, farnesylated sulfobetaine 10 has IC(50) = 10 &mgr;M and aromatic derivative 35 has IC(50) = 2 &mgr;M for SS inhibition. A wide variety of structural modifications, exemplified by compounds 43, 52, 76, 85, 91, 99, 111, and 115, was investigated. Unfortunately, no inhibitors in the submicromolar range were discovered, and exploration of a different type of zwitterion seems necessary if this appealing approach to inhibition of SS is going to provide a potential antihypercholesterolemic agent.

3-(4-chlorophenyl)-2-(4-diethylaminoethoxyphenyl)-A-pentenonitrile monohydrogen citrate and related analogs. Reversible, competitive, first half-reaction squalene synthetase inhibitors

Squalene synthetase (SQS) catalyzes the head-to-head condensation of two molecules of farnesyl pyrophosphate (FPP) to form squalene. The reaction is unique when compared with those of other FPP-utilizing enzymes, and proceeds in two distinct steps, both of which involve carbocationic reaction intermediates. In this report, we describe the mechanism of action of, and structure-activity relationships within, a series of substituted diethylaminoethoxystilbenes that mimic these reaction intermediates, through characterization of the biochemical properties of 3-(4-chlorophenyl)-2-(4-diethylaminoethoxyphenyl)-A- pentenonitrile monohydrogen citrate (P-3622) and related analogs. As a representative member of this series, P-3622 inhibited SQS reversibly and competitively with respect to FPP (Ki = 0.7 microM), inhibited the enzymatic first half-reaction to the same extent as the overall reaction, exhibited a 300-fold specificity for SQS inhibition relative to protein farnesyltransferase inhibition, inhibited cholesterol synthesis in rat primary hepatocytes (IC50 = 0.8 microM), in cultured human cells (Hep-G2, CaCo-2, and IM-9; IC50 = 0.2, 1.2, and 1.0 microM), and in chow-fed hamsters (62% at 100 mg/kg) without accumulation of post-squalene sterol precursors, and reduced plasma cholesterol in experimental animals. Structure-activity relationships among 72 related analogs suggest that the phenyl residues and central trans-olefin of the stilbene moiety serve as mimics of the three isoprene units of the donor FPP, that substitutions across the central olefin and para-substitutions on the terminal phenyl residue mimic the branching methyl groups of the donor FPP, and that the diethylaminoethoxy moiety of these molecules mimics the various carbocations that develop in the C1-C3 region of the acceptor FPP during reaction. Members of this series of reversible, competitive, first half-reaction SQS inhibitors that show a high degree of specificity for SQS inhibition relative to inhibition of other FPP-utilizing enzymes and other cholesterol synthesis pathway enzymes may serve as useful tools for probing the unique catalytic mechanisms of this important enzyme.

Synthesis and activity of a novel series of 3-biarylquinuclidine squalene synthase inhibitors

Quinuclidines with a 3-biaryl substituent are a new class of potent, orally active squalene synthase (SQS) inhibitors. Variants around these rigid structures indicate key structural requirements for cationic SQS inhibitors. Thus the lower in vitro potency found for quinuclidines bearing 3-substituents, which did not overlay the biphenyl group of 3-(biphenyl-4-yl)-3-hydroxyquinuclidine (2) (IC50 = 16 nM, rat microsomal SQS), implied a directional requirement for the 3-substituent. Similarly, the lower potency of the 3-terphenyl analogue 6 (IC50 = 370 nM) indicated size constraints for this substituent. In compounds with a linking group between the quinuclidine and biphenyl ring, linking groups of lower lipophilicity were less well tolerated (e.g., 17, CH2CH2, IC50 = 5 nM vs 19, NHCO, IC50 = 1.2 microM). Replacement of the distal phenyl ring of 2 with a more polar pyridine heterocycle caused a reduction in in vitro potency. In general, good in vivo activity in the rat was restricted to 3-hydroxy analogues, with the 3-[4-(pyrid-4-yl)phenyl] derivative 39 (IC50 = 161 nM) showing the best inhibition (following oral dosing) of cholesterol biosynthesis from mevalonate (ED50 = 2.7 mg/kg).