4-Substituted (benzo[b]thiophene-2-carbonyl)guanidines as novel Na+/H+ exchanger isoform-1 (NHE-1) inhibitors

pH regulators and their inhibitors in tumor microenvironment

As an important characteristic of tumor, acidic tumor microenvironment (TME) is closely related to immune escape, invasion, migration and drug resistance of tumor. The acidity of the TME mainly comes from the acidic products produced by the high level of tumor metabolism, such as lactic acid and carbon dioxide. pH regulators such as monocarboxylate transporters (MCTs), carbonic anhydrase IX (CA IX), and Na+/H+ exchange 1 (NHE1) expel protons directly or indirectly from the tumor to maintain the pH balance of tumor cells and create an acidic TME. We review the functions of several pH regulators involved in the construction of acidic TME, the structure and structure-activity relationship of pH regulator inhibitors, and provide strategies for the development of small-molecule antitumor inhibitors based on these targets.

Ion Transport and Inhibitor Binding by Human NHE1: Insights from Molecular Dynamics Simulations and Free Energy Calculations

The human Na+/H+ exchanger (NHE1) plays a crucial role in maintaining intracellular pH by regulating the electroneutral exchange of a single intracellular H+ for one extracellular Na+ across the plasma membrane. Understanding the molecular mechanisms governing ion transport and the binding of inhibitors is of importance in the development of anticancer therapeutics targeting NHE1. In this context, we performed molecular dynamics (MD) simulations based on the recent cryo-electron microscopy (cryo-EM) structures of outward- and inward-facing conformations of NHE1. These simulations allowed us to explore the dynamics of the protein, examine the ion-translocation pore, and confirm that Asp267 is the ion-binding residue. Our free energy calculations did not show a significant difference between Na+ and K+ binding at the ion-binding site. Consequently, Na+ over K+ selectivity cannot be solely explained by differences in ion binding. Our MD simulations involving NHE1 inhibitors (cariporide and amiloride analogues) maintained stable interactions with Asp267 and Glu346. Our study highlights the importance of the salt bridge between the positively charged acylguanidine moiety and Asp267, which appears to play a role in the competitive inhibitory mechanism for this class of inhibitors. Our computational study provides a detailed mechanistic interpretation of experimental data and serves the basis of future structure-based inhibitor design.

KR-33028, a potent inhibitor of the Na+/H+ exchanger NHE1, suppresses metastatic potential of triple-negative breast cancer cells

Hyper-activation of the Na⁺/H⁺ exchanger NHE1 occurs at the onset of oncogenic transformation and plays a critical role in breast cancer carcinogenesis. Dysregulation of NHE1 activity results in intracellular alkalinization and the acidification of the extracellular tumor microenvironment that promotes metastasis. Hence, the use of chemical inhibitors of NHE1 as chemotherapeutic agents is an alluring prospect. We previously demonstrated that two structurally different NHE1 inhibitors, EMD87580 [(2-methyl-4,5-di-(methylsulfonyl)-benzoyl)-guanidine], and HMA [5-(N,N-hexamethylene)-amiloride], were effective as co-adjuvants to potentiate paclitaxel-mediated cytotoxic chemotherapy in triple-negative breast cancer (TNBC) cells. Both these drugs, however, had reduced or minimal anti-cancer effects when used alone. Here, we tested KR-33028 (4-cyano (benzo[b]thiophene-2-carbonyl)guanidine), a potent and selective inhibitor of NHE1, to determine its efficacy in inhibition of metastatic potential of TNBC cells. In highly invasive MDA-MB-231, moderately invasive MDA-MB-468, and lowly invasive Hs578T TNBC cells, KR-33028 considerably reduced rates of cell migration and anchorage-independent colony growth. Invasion of MDA-MB-231 and MDA-MB-468 cells through extracellular matrix was also dramatically decreased in response to KR-33028. We further tested the effect of KR-33028 on MDA-MB-231 cells lacking NHE1 expression (231koNHE1); no differences were observed between untreated control and KR-33028-treated 231koNHE1 cells. Taken together, our results highlight the in vitro efficacy of KR-33028-mediated NHE1 inhibition on limiting cellular functions that are predictive of metastasis in vivo. We suggest that targeting NHE1 in the development of novel chemotherapeutics could be highly effective in combatting triple-negative breast cancer and that KR-33028 is potentially useful in prevention of metastasis.

Inside out: targeting NHE1 as an intracellular and extracellular regulator of cancer progression

The sodium hydrogen exchanger isoform one is a critical regulator of intracellular pH, serves as an anchor for the formation of cytoplasmic signaling complexes, and modulates cytoskeletal organization. There is a growing interest in the potential for sodium hydrogen exchanger isoform one as a therapeutic target against cancer. Sodium hydrogen exchanger isoform one transport drives formation of membrane protrusions essential for cell migration and contributes to the establishment of a tumor microenvironment that leads to the rearrangement of the extracellular matrix further supporting tumor progression. Here, we focus on the potential impact that an inexpensive, $100 genome would have in identifying prospective therapeutic targets to treat tumors based upon changes in gene expression and variation of sodium hydrogen exchanger isoform one regulators. In particular, we will focus on the ezrin, radixin, moesin family proteins, calcineurin B homologous proteins, Ras/Raf/MEK/ERK signaling, and phosphoinositide signaling as they relate to the regulation of sodium hydrogen exchanger isoform one in cancer progression.

Facile synthesis of heterocycles via 2-picolinium bromide and antimicrobial activities of the products

The 2-picolinium N-ylide 4, generated in situ from the N-acylmethyl-2-picolinium bromide 3, underwent cycloaddition to N-phenylmaleimide or carbon disulfide to give the corresponding cycloadducts 6 and 8, respectively similar reactions of compound 3 with some electron-deficient alkenes in the presence of MnO₂ yielded the products 11 and 12. In addition, reaction of 4 with arylidene cyanothioacetamide and malononitrile derivatives afforded the thiophene and aniline derivatives 15 and 17, respectively. Heating of picolinium bromide 3 with triethylamine in benzene furnished 2-(2-thienyl)indolizine (18). The structures of the isolated products were confirmed by elemental analysis as well as by ¹H- and ¹³C-NMR, IR, and MS data. Both the stereochemistry and the regioselectivity of the studied reactions are discussed. The biological activity of the newly synthesized compounds was examined and showed promising results.

Effects of KR-33028, a novel Na+/H+exchanger-1 inhibitor, on ischemia and reperfusion-induced myocardial infarction in rats and dogs

The present study was performed to evaluate the cardioprotective effects of KR-33028, a novel Na+/H+ exchanger subtype 1 (NHE-1) inhibitor, in rat and dog models of coronary artery occlusion and reperfusion. In anesthetized rats subjected to a 45-min coronary occlusion and a 90-min reperfusion, KR-33028 at 5 min before occlusion (i.v. bolus) dose-dependently reduced myocardial infarct size from 58.0% to 46.6%, 40.3%, 39.7%, 33.1%, and 27.8% for 0.03, 0.1, 0.3, 1.0, and 3.0 mg/kg respectively (P < 0.05). In anesthetized beagle dogs that underwent a 1.0-h occlusion followed by a 3.0-h reperfusion, KR-33028 (3 mg/kg, i.v. bolus) markedly decreased infarct size from 45.6% in vehicle-treated group to 16.4% (P < 0.05), and reduced the reperfusion-induced release in creatine kinase myocardial band isoenzyme (MB), lactate dehydrogenase, troponin-I, glutamic oxaloacetic transaminase, and glutamic pyruvic transaminase. In separate experiments to assess the effects of timing of treatment, KR-33028 (1 mg/kg, i.v. bolus) given 10 min before or at reperfusion in rat models also significantly reduced the myocardial infarct size (46.3% and 44.1% respectively) compared with vehicle-treated group. In all studies, KR-33028 caused no significant changes in any hemodynamic profiles. In an isolated rat heart model of hypothermic cardioplegia, KR-33028 (30 mum), which was added to the heart preservation solution (histidin-tryptophan-ketoglutarate) during hypothermic cardioplegic arrest, significantly improved the recovery of left ventricular developed pressure, heart rate and dP/dt(max) after reperfusion. Taken together, these results indicate that KR-33028 significantly reduced the myocardial infarction induced by ischemia and reperfusion in rats and dogs, without affecting hemodynamic profiles.

Dose-dependent pharmacokinetics of a new Na+/H+ exchanger inhibitor KR-33028 in rats

The dose-dependency of the pharmacokinetics of a new Na(+)/H(+) exchanger inhibitor, KR-33028 was evaluated in rats after intravenous and oral administration. After intravenous administration of KR-33028 (1, 5, 10 and 20mg/kg doses), the systemic clearance (Cl) was reduced and AUC was nonlinearly increased as a function of dose. The volume of distribution (V(ss)), however, remained unchanged as the dose was increased, which was consistent with unaltered plasma protein binding in vitro (unbound fraction = 0.09-0.12). Upon oral administration (2, 10 and 20mg/kg doses), KR-33028 was rapidly absorbed, and this was consistent with high Caco-2 P(app) values found in vitro. There were nonlinear increases in AUC and C(max), and the absolute oral bioavailability (F) was significantly increased as the dose was increased (F = 23.3%, 40.7% and 78.2% for 2, 10 and 20mg/kg doses, respectively). The extent of urinary excretion was low for both intravenous (0.5-0.7%) and oral (0.2-0.8%) doses. The reduced systemic clearance and increased oral bioavailability at high doses appears to be due to a saturable first-pass metabolism.

Determination of a selective Na+/H+ exchanger inhibitor, 4-cyano(benzo[b]thiophene-2-carbonyl)guanidine (KR-33028) in rat plasma by liquid chromatography–tandem mass spectrometry

A liquid chromatography-tandem mass spectrometric (LC/MS/MS) method was developed for the determination of a selective Na(+)/H(+) exchanger inhibitor 4-cyano(benzo[b]thiophene-2-carbonyl)guanidine (KR-33028) in rat plasma. KR-33028 and the internal standard, linezolid, were extracted from rat plasma with ethyl acetate at neutral pH. The analytes were separated on an XBridge C(18) column with a mixture of methanol-0.1% formic acid (35:65, v/v) as mobile phase and detected using an electrospray ionization tandem mass spectrometry in the multiple-reaction-monitoring mode. The standard curve was linear (r = 0.9998) over the concentration range of 2.0-1000 ng/mL. The coefficients of variation of intra- and inter-assay were 1.3-6.8% and the relative error was 0.8-5.0%. The recoveries of KR-33028 and linezolid were 70.5 and 84.6%, respectively. The lower limit of quantification for KR-33028 was 2.0 ng/mL using 50 microL plasma sample. This method was successfully applied to the pharmacokinetic study of KR-33028 in rats.

Characterization of the cytochrome P450 enzymes involved in the metabolism of a new cardioprotective agent KR-33028

KR-33028 (N-[4-cyano-benzo[b]thiophene-2-carbonyl]guanidine) is a new cardioprotective agent for preventing ischemia-reperfusion injury. This study was performed to characterize the cytochrome P450 (CYP) enzymes that are involved in the metabolism of KR-33028. Hydroxylation (5-hydroxy- and 7-hydroxy-KR-33028) is major pathways for the metabolism of KR-33028 in human liver microsomes. Among the nine c-DNA expressed CYP isoforms tested, KR-33028 was 5-hydroxylated by CYP3A4 and 7-hydroxylated by CYP1A2, CYP3A4, and CYP2C19. These findings were supported by the combination of chemical inhibition studies in human liver microsomes and correlation analysis. Furafylline and ketoconazole potently inhibited hydroxylation of KR-33028 in human liver microsomes. Correlation analysis between the known CYP enzyme activities and the rates of the formation of 5-hydroxy- and 7-hydroxy-KR-33028 in the 16 human liver microsomes has showed significant correlations with CYP3A4-mediated midazolam 1'-hydroxylation and CYP1A2-mediated phenacetin O-deethylation, respectively. A 7-hydroxy-KR-33028 formation is also weakly correlated with CYP3A4-mediated midazolam 1'-hydroxylation. The kinetics of the major biotransformation of KR-33028 were studied: CYP3A4 mediated the formation of 5-hydroxy-KR-33028 from KR-33028 with Cl(int)=0.22microl/min/pmol CYP. The intrinsic clearance for 7-hydroxy-KR-33028 formation by CYP1A2, CYP2C19, and CYP3A4 were 0.26, 0.19, and 0.03microl/min/pmol CYP, respectively. Taken together, these results provide evidence that CYP3A4 and CYP1A2 are the major isoforms responsible for the hydroxy metabolites formation from KR-33028.

Pharmacological profile of KR-33028, a highly selective inhibitor of Na+/H+ exchanger

We evaluated the cardioprotective effects of 4-cyano (benzo[b]thiophene-2-carbonyl)guanidine (KR-33028), a recently developed inhibitor of the Na+/H+ exchanger (NHE), on hypoxia-induced H9c2 cell death and on perfused rat hearts subjected to ischemia/reperfusion. KR-33028 inhibited in a concentration-dependent manner the recovery from acidosis induced by an NH4Cl prepulse in PS120 fibroblast cells expressing the human NHE-1 isoform (IC50: 2.59 microM). Treatment with KR-33028 (1-10 microM) significantly decreased hypoxia-induced necrotic cell death and apoptotic cell death in H9c2 cells. KR-33028 significantly inhibited hypoxia-induced increases in cytosolic and mitochondrial Ca2+ level and cytochrome c release, and recovered hypoxia-induced Delta psi(m) reduction. In the perfused rat hearts subjected to 30 min of ischemia and 30 min of reperfusion, KR-33028 (1-10 microM) improved cardiac contractility, decreased lactate dehydrogenase release, and increased content of tissue ATP, creatine phosphate and glycogen in a concentration-dependent manner. In addition, KR-33028 did not produce significant acute or subacute toxicity in the rats at doses tested. Our results suggest that a novel NHE-1 inhibitor KR-33028 possesses potent cardioprotective effects with minimal toxicity and that the effects may be mediated by inhibition of intracellular Ca2+ overload and mitochondrial cell death pathway.

In vitro metabolism of a new cardioprotective agent, KR-33028 in the human liver microsomes and cryopreserved human hepatocytes

KR-33028 (N-[4-cyano-benzo[b]thiophene-2-carbonyl]guanidine) is a new cardioprotective agent for preventing ischemia-reperfusion injury. This study was performed to identify the metabolic pathway of KR-33028 in human liver microsomes and to compare its metabolism with that of cryopreserved human hepatocytes. Human liver microsomal incubation of KR-33028 in the presence of NADPH and UDPGA resulted in the formation of four metabolites, M1, M2, M3, and M4. M1 and M2 were identified as 5-hydroxy-KR-33028 and 7-hydroxy-KR-33028, respectively, on the basis of LC/MS/MS analysis with the synthesized authentic standard. M3 and M4 were suggested to be dihydroxy-KR-33028 and hydroxy-KR-33028-glucuronide, respectively. Metabolism of KR-33028 in cryopreserved human hepatocytes resulted in the formation of M1, M2, and M4. These data show a good correlation between major metabolites formed in human liver microsomes and cryopreserved human hepatocytes. In addition, KR-33028 was found to inhibit moderately the metabolism of CYP1A2 substrates. Based on the results obtained metabolic pathway of KR-33028 is proposed.