NMDA receptor;

Inwardly rectifying K+ channels (GIRK) that are activated by G proteins[1].The effects of the atypical N-methyl-D-aspartate (NMDA) receptor antagonist Ifenprodil were investigated by voltage-clamp recording of Xenopus oocytes expressing heteromeric NMDA receptors from cloned NR1 and NR2 subunit RNAs. In oocytes voltage-clamped at -70 mV, ifenprodil inhibited NMDA-induced currents at NR1A/NR2B receptors with high affinity (IC50 = 0.34 microM). The affinity of NR1A/NR2A receptors for Ifenprodil (IC50 = 146 microM) was 400-fold lower than that of NR1A/NR2B receptors. The rate of onset of inhibition by low concentrations of ifenprodil acting at NR1A/NR2B receptors was considerably slower than the onset of inhibition seen with high concentrations of ifenprodil acting at NR1A/NR2A receptors. The onset and recovery of blockade by ifenprodil at NR1A/NR2B receptors were not activity dependent. The inhibitory effects of low concentrations of ifenprodil at NR1A/NR2B receptors were not voltage dependent. In contrast, the inhibitory effects of high concentrations of ifenprodil at NR1A/NR2A receptors were partially voltage dependent, and a greater inhibition of NMDA-induced currents was seen at hyperpolarized membrane potentials than at depolarized membrane potentials. The reversal potential of NMDA currents was not altered in the presence of ifenprodil. Ifenprodil may act as a weak open-channel blocker of NR1A/NR2A receptors. The degree of inhibition seen with 100 microM ifenprodil at NR1A/NR2A receptors was not altered by changes in the concentration of extracellular glycine. However, the inhibitory effect of 1 microM ifenprodil at NR1A/NR2B receptors was reduced by increasing the concentration of glycine. Thus, part of the mechanism of action of ifenprodil at NR1A/NR2B receptors may involve noncompetitive antagonism of the effects of glycine. These results indicate that the mechanism of action of ifenprodil, as well as the potency of this antagonist, is different at NR1A/NR2B and NR1A/NR2A receptors expressed in Xenopus oocytes [1].

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G protein-activated inwardly rectifying K+ channels (GIRK, also known as Kir3) are regulated by various G-protein-coupled receptors. Activation of GIRK channels plays an important role in reducing neuronal excitability in most brain regions and the heart rate. Ifenprodil, which is a clinically used cerebral vasodilator, interacts with several receptors, such as alpha1 adrenergic, N-methyl-D-aspartate, serotonin and sigma receptors. However, the molecular mechanisms underlying the various clinically related effects of ifenprodil remain to be clarified. Here, we examined the effects of Ifenprodil on GIRK channels by using Xenopus oocyte expression assays. In oocytes injected with mRNAs for GIRK1/GIRK2, GIRK2 or GIRK1/GIRK4 subunits, ifenprodil reversibly reduced inward currents through the basal GIRK activity. The inhibition was concentration-dependent, but voltage- and time-independent, suggesting that ifenprodil may not act as an open channel blocker of the channels. In contrast, Kir1.1 and Kir2.1 channels in other Kir channel subfamilies were insensitive to ifenprodil. Furthermore, GIRK current responses activated by the cloned kappa-opioid receptor were similarly inhibited by ifenprodil. The inhibitory effects of ifenprodil were not observed when Ifenprodil was applied intracellularly, and were not affected by extracellular pH, which changed the proportion of the uncharged to protonated ifenprodil, suggesting its action from the extracellular side. The GIRK currents induced by ethanol were also attenuated in the presence of ifenprodil. Our results suggest that direct inhibition of GIRK channels by ifenprodil, at submicromolar concentrations or more, may contribute to some of its therapeutic effects and adverse side effects [2].

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In the CPE-based antiviral assay, Ifenprodil was comparable to nylidrin, with EC50 values of 6.6 and 16.9 μM for PR8 and HK, respectively (Table 1). Clenbuterol was potently active only against PR8 with an EC50 value of 9.4 μM, whereas labetalol and eliprodil were only marginally effective against HK alone with EC50 values above 44.0 μM. In contrast, ritodrine, fenoterol, and bambuterol had no antiviral activity against any viral strains. In further experiments, we tested the inhibitory compounds, even partially including nylidrin, Ifenprodil, labetalol, eliprodil, and clenbuterol, against additional influenza A and B viruses (Table 2). Nylidrin, ifenprodil, and clenbuterol were consistently effective against A/H1N1 strains, but their efficacy varied among the H3N2 viral strains, and none had antiviral activity in influenza B-infected cells. As expected, the remaining two compounds, labetalol and eliprodil, had little effect against any of these viral strains, except for A/Brisbane/59/2007 (H1N1), which was partially sensitive to eliprodil (EC50, 53.1 μM). These results indicate that nylidrin and its analogues, Ifenprodil and clenbuterol, can reliably inhibit the infection of H1N1 strains of the influenza A virus at subtoxic concentrations [4].

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Interaction of Ifenprodil with the NMDA receptor reduces the opening state of the ion channel and inhibits the influx of Ca2+ ions. With respect to this mechanism ifenprodil leads to neuroprotective, anticonvulsant and analgesic effects. The binding site of ifenprodil was first supposed at the amino terminal domain (ATD) of the GluN2B subunit. Later it could be found at the surface between the GluN1 and the GluN2B subunits. According to the first developed ligand it has been termed “ifenprodil binding site” and can cross-talk with different other binding sites of the receptor. The NMDA affinity of ifenprodil is very high (IC50 = 13.3 nM, Ki = 10 nM), but its selectivity is rather poor [5].

We showed that either Ifenprodil or bupivacaine produced spinal blockades of motor function and nociception dose-dependently. On the ED50 basis, the potency of Ifenprodil (0.42(0.38-0.46) μmol; 0.40(0.36-0.44) μmol) was equal (p>0.05) to that of bupivacaine (0.38(0.36-0.40) μmol; 0.35(0.32-0.38) μmol) in motor function and nociception, respectively. At the equianesthetic doses (ED25, ED50, and ED75), duration produced by Ifenprodil was greater than that produced by bupivacaine in motor function and nociception (p<0.05 for the differences). Furthermore, both Ifenprodil and bupivacaine showed longer duration of sensory blockade than that of motor blockade (p<0.05 for the differences). Conclusions: The resulting data demonstrated that Ifenprodil produces a dose-dependent local anesthetic effect in spinal anesthesia. Ifenprodil shows a more sensory-selective duration of action over motor block, whereas the duration of anesthesia is significantly longer with Ifenprodil than with bupivacaine. [3]

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The results of the present study indicated for the first time that intrathecal Ifenprodil elicited spinal block of motor function and nociception. The spinal anesthetic effect of Ifenprodil was comparable to that of the long-acting local anesthetic bupivacaine. Ifenprodil as well as bupivacaine showed greater duration of nociceptive/sensory block than motor block. At the equianesthetic doses (ED25, ED50, and ED75), the duration of spinal anesthesia with ifenprodil was greater than that of...
Our preclinical data showed that intrathecal Ifenprodil produced a dose-dependent local anesthetic effect in spinal anesthesia. The potency of ifenprodil in spinal anesthesia was similar to that of bupivacaine, whereas duration of spinal anesthesia with ifenprodil was greater than that of bupivacaine. Furthermore, Ifenprodil as well as bupivacaine exhibited significantly sensory-specific over motor block. The neural block of ifenprodil is worth testing in further [3].

Microsomal incubations (phase I) [5]
Ifenprodil (235 μg) was dissolved in phosphate buffer (250 μL, pH 7.4, 0.1 M) containing magnesium chloride (100 μL, 45.5 mM). After adding 150 μL of the microsome preparation (7 mg protein/mL) and 5 mg NADPH/H+ the mixture was incubated in a shaker at room temperature. After 120 min, the incubation was terminated by adding an equal volume of cold acetonitrile (−20 °C). The mixture was stored in an ice bath for 10 min. Subsequently, the sample was centrifuged at 13,000 rpm for 8 min and the resulting supernatant was analyzed by HPLC–MS without further sample pre-treatment.

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Rate of metabolic degradation of Ifenprodil [5]
For calibration studies, Ifenprodil (117.5 μg) was dissolved in sodium phosphate buffer (125 μL, pH 7.4, 0.1 M). Magnesium chloride (50 μL, 45.5 mM) and rat liver microsome suspension (100 μL) were added. The calibration was performed with 100, 80, 60, 40, 20 and 0% of the parent ifenprodil concentration (117.5 μg) which was diluted with sodium phosphate buffer to obtain the desired concentration. After 120 min the reaction was stopped by addition of cold CH3CN (500 μL, −20 °C) and the internal standard eliprodil (62.75 μg, dissolved in 32.5 μL CH3CN) was added. The mixture was stored in an ice bath for 10 min and subsequently centrifuged (13,000 rpm, 8 min). The supernatant was diluted 1:40 with CH3CN/H2O (1/1) and injected into HPLC system 1. Each calibration step was carried out three times. The resulting TICs (m/z = 326.2 and m/z = 348.15) were integrated manually. Calculation of the eliprodil/Ifenprodil ratio and the resulting calibration curve were carried out using Microsoft Excel 2010. Metabolic degradation was carried out with ifenprodil (117.5 μg) dissolved in sodium phosphate buffer (125 μL, pH 7.4, 0.1 M). Magnesium chloride (50 μL, 45.5 mM), rat liver microsomes (100 μL), NADPH (2.5 mg) and UDPGA (2 mg) were added. This reaction mixture was shaken for 15, 30, 60, 90 and 120 min. Then, the reaction was stopped by addition of cold acetonitrile (500 μL, −20 °C) and eliprodil (62.75 μg, dissolved in 32.5 μL CH3CN). The mixture was stored in an ice bath for 10 min and subsequently centrifuged (13,000 rpm, 8 min). The supernatant was diluted 1:40 with acetonitrile/H2O (1/1) and injected into HPLC system 1. Each step was carried out three times. The resulting TICs (m/z 326.2 and m/z 348.15) were integrated manually. Calculation of the eliprodil/ifenprodil ratio and the resulting amount for Ifenprodil were carried out using Microsoft Excel 2010.

For examining the effect of intracellular Ifenprodil, 23 nl of 10 mM ifenprodil or 30 mM lidocaine N-ethyl bromide (QX-314) dissolved in distilled water was administered to an oocyte through an additional pipette by pressure injection using a Nanoliter injector as described previously (Kobayashi et al, 2003), and the oocyte currents were then continuously recorded for approximately 30–40 min. As the volume of a Xenopus oocyte used is ∼1 μl, the intracellular concentration of Ifenprodil or QX-314 was presumed as ∼225 or ∼674 μM, respectively. Data were fitted to a standard logistic equation by using KaleidaGraph for analysis of concentration–response relationships. The EC50 value, which is the concentration of a drug that produces 50% of the maximal current response for that drug; the IC25 and IC50 values, which are the concentrations of a drug that reduces control current responses by 25 and 50%, respectively; and the Hill coefficient (nH) were obtained from the concentration–response relationships.[2]

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Compounds and Cytopathic Effect (CPE) Reduction Assay [4]
The test compounds nylidrin hydrochloride (~95%), Ifenprodil (+)–tartrate salt (≥98%), labetalol hydrochloride (≥98%), ritodrine hydrochloride (99.6%), fenoterol hydrobromide (≥98%), eliprodil (≥98%), clenbuterol hydrochloride (≥95%), and bambuterol hydrochloride (≥98%), the M2 inhibitor amantadine hydrochloride (AMT; ≥98%) and the polymerase inhibitor ribavirin (RBV; ≥98%), oseltamivir carboxylate (OSV-C; ≥98%) were used.

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Cell-based antiviral assays were performed as previously described [19,20]. Briefly, MDCK cells were seeded on 96-well plates (3 × 104 cells per well) and grown overnight. The cells were mock-infected or infected with individual influenza strains at a multiplicity of infection (MOI) of 0.001 to 0.005 for 1 h at 33 °C or 35 °C. After the removal of unadsorbed virus, test and control compounds were serially 3-fold diluted from 100 to 0.01 µM (total ten concentration points) in serum-free MEM with 2 µg/mL tosyl phenylalanyl chloromethyl ketone (TPCK)-treated trypsin and used to treat the cells. To synchronize the reading time point, at three days post-infection (p.i.), the plates were incubated at the following temperatures: 33 °C for A/California/7/2009, A/Brisbane/59/2007, A/Perth/17/2009, A/Brisbane/10/2007, A/Victoria/361/2011-like, and B/Brisbane/60/2008); and 35 °C for PR8, HK, Lee, and A/Seoul/11/1988. The half-maximal cytotoxic concentration (CC50) and half-maximal effective concentration (EC50) were determined by measuring cell viability using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT). Immunoassays [4]
To detect viral proteins, western blot analysis was performed as described previously. MDCK cells grown to 100% confluence in 6-well plates were infected with PR8 virus at an MOI of 0.001 for 24 h at 35 °C in the presence of nylidrin, OSV-C, or RBV. Viral proteins NP, HA, and M1 were detected by immunoblotting the appropriate antibodies: mouse anti-NP, rabbit anti-HA2, and mouse anti-M1, respectively. Secondary antibodies were horseradish peroxidase (HRP)-conjugated goat anti-mouse or anti-rabbit IgG. Cellular β-actin was used as the loading control.

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Plaque Assay [4]
PR8-infected MDCK cells were mock-treated or treated with the individual compounds. After 24 h at 35 °C, culture supernatants were harvested to prepare 10-fold serial dilutions, which were used to infect naïve MDCK cells seeded in 48-well plates for 1 h. After PBS washing, they were incubated in overlay medium (MEM with 0.6% carboxymethylcellulose and 2 µg/mL TPCK-trypsin) at 33 °C for three days. Plaques were counted by crystal violet staining. For time-of-addition experiments, MDCK cells in 48-well plates were infected with PR8 (MOI, 0.001) at 4 °C for 1 h in the presence or absence of nylidrin. After washing with PBS to remove chemicals and unadsorbed virus, nylidrin was administered at 1, 2, and 4 h p.i. (−)-Epigallocatechin gallate was used as a control for blocking viral entry. Subsequently, all samples were washed with PBS at 5 h p.i., followed by incubation under the overlay medium for three days prior to crystal violet staining.

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Confocal Microscopy [4]
For fluorescence microscopy, PR8-infected MDCK cells were incubated for 5 h at 37 °C in the presence or absence of nylidrin. After fixation and permeabilization, viral NP was visualized using anti-NP antibody and Alexa Fluor 488-conjugated goat anti-mouse IgG; nuclear DNA was counterstained using 4´,6-diamidino-2-phenylindole. Images were captured on a Zeiss LSM 700 confocal microscope with analysis using the ZEN software. For investigating colocalization of viral NP with cellular Rab5 or Rab7, A549 cells were seeded in 4-well slides (4 × 104 cells per well). On the next day, cells were transfected with 0.4 μg of pEGFP-Rab5 or pEGFP-Rab7 using lipofectamine 2000, according to the manufacturer’s instructions. After 24 h, they were mock-infected or infected with PR8 at an MOI of 10 in the absence or presence of 100 μM nylidrin at 4 °C for 30 min and then at 37 °C for an additional 4 h. Viral NP protein was probed using the same primary antibody above-mentioned, but with Alexa Fluor 633-conjugated secondary antibody.

For isolating liver microsomes and for in vivo metabolism studies of Ifenprodil, Wistar rats weighing 200–320 g from a local strain were used. [5]
In vivo metabolism studies of Ifenprodil [5]
Ifenprodil was dissolved in 0.9% sterile saline solution. A single dose of 6.2 mg Ifenprodil tartrate was administered i.p. to one female rat (310 g, Wistar rat), which results in a dose of 20 mg/kg. A dose of 20 mg/kg ifenprodil was chosen, since traxoprodil, which has similar structure and pharmacological properties, was used in previous experiments in a comparable amount [12]. The rat was housed individually in a metabolism cage with water and powdered feed ad libitum. Urine was collected for 48 h in three intervals of 0–8 h, 8–24 h and 24–48 h after i.p. application of ifenprodil. The volumes of urine samples were recorded and the samples were stored at −20 °C until analysis or directly used.
Extraction was carried out with RP-18 SPE cartridges, which were pre-conditioned with 1 mL H2O and 1 mL EtOH. After application of the urine samples, the cartridge was rinsed with 10 mL of H2O. Afterwards, Ifenprodil and the corresponding metabolites were eluted with 10 mL of EtOH. The solvent was removed under reduced pressure. The resulting solid was dissolved in CH3CN:H2O 1:1 and analyzed with the different HPLC systems.

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The aim of the study was to compare the proposed spinal anesthetic effect of Ifenprodil, an a1 adrenergic receptor antagonist, with that of the long-acting local anesthetic bupivacaine. Methods: After intrathecally injecting the rats with five different doses of each drug, the dose-response curves of Ifenprodil and bupivacaine were constructed to obtain the 50% effective dose (ED50). The spinal blockades of motor function and nociception of Ifenprodil were compared with that of bupivacaine.[3]

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Male Sprague–Dawley rats (300–350 g) were used. Intrathecal Ifenprodil caused significant motor or nociceptive blockade. Intrathecal Ifenprodil, as well as the long-lasting local anesthetic bupivacaine produced a dose-dependent local anesthetic effect in spinal anesthesia in rats (Fig. 1). The ED25s, ED50s, and ED75s of Ifenprodil and bupivacaine are presented in Table 1. On the ED50 basis, the potency of Ifenprodil in motor function and nociception was comparable to that of bupivacaine (Table 1, p > 0.05). [3]

The NMDA receptor antagonist Ifenprodil is an important lead structure for developing new GluN2B selective NMDA receptor antagonists. Ifenprodil itself has a high affinity to the GluN2B subunit but a poor selectivity for the NMDA receptor. This aspect and the fast biotransformation are the major drawbacks of ifenprodil. In order to optimize the development of new and more selective GluN2B (NMDA) receptor antagonists, the identification of the main metabolic pathways of ifenprodil is necessary. Herein the in vitro and in vivo phase I and phase II metabolites of Ifenprodil were generated and analyzed via LC-MS(n) experiments. In vitro experiments were carried out with rat liver microsomes and various co-factors to generate phase I and phase II metabolites. The application of ifenprodil to a rat and the analysis of its urine led to the identification of diverse formed in vivo metabolites. The phenol represents the metabolically most labile structural element since glucuronide 7 and 8 appeared as main metabolites. [5]

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It has been reported that the bioavailability of Ifenprodil is rather low. The maximum plasma level in humans was found after ∼30 min [9]. However the biotransformation of ifenprodil and the structure of its metabolites have not been described so far. Therefore, identification of the metabolically labile positions of ifenprodil might contribute to the development of metabolically more stable drugs interacting with the ifenprodil binding site. Herein we report on our studies of phase I and phase II metabolism of ifenprodil in vitro and in vivo. [5]

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Fragmentation of Ifenprodil [5]
MSn experiments of ifenprodil revealed a fragmentation pattern which can be considered quite typical for the structure of ifenprodil (Fig. 1). The first fragmentation step is the dehydration of the secondary alcohol (m/z 308.2003). Further fragmentation leads to a fragment with m/z 293.1752, which can be explained by the loss of a methyl group. Another important fragmentation of Ifenprodil is the cleavage of the Csingle bondN (piperidine) bond leading to the formation of the benzylpiperidine moiety with m/z 176.1424. Release of the tropylium ion (m/z 91.0511) further proves this fragment. This fragmentation pattern of the parent compound ifenprodil represents the basis for the interpretation of the fragmentation studies of the produced metabolites, providing information about the structures of the formed metabolites.

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Identification and fragmentation of the phase I metabolites in vitro [5]
Phase I transformation led to several metabolites including two N-dealkylation products and several M+O metabolites (Fig. 2, supporting information). The N-dealkylated metabolites were identified as 4-benzyl-piperidine (1) and oxidized 4-benzyl-piperidin-2-on (2). The structures of the piperidine metabolites 1 and 2 were identified by fragmentation experiments. The main fragment of both metabolites is the tropylium ion with an exact mass of m/z 91.0510 and m/z 91.0518, respectively (see sup. inf.). Furthermore, six phase I metabolites were identified with m/z 342 [M+O+H]+, caused by introduction of an additional O-atom (Fig. 3a). The metabolites were analyzed with MSn experiments. According to this analysis oxidation took place at the piperidine ring (3), in ortho-, meta- and para-position of the phenyl moiety (4a–4c), in ortho position of the phenol (5), and at the N-atom forming the N-oxide (6, Fig. 3b). The three metabolites 4a–4c showed identical fragmentation pattern, indicating very similar structures.
The hydroxy group of the monohydroxylated metabolite 3 was assigned to the piperidine heterocycle. The first hint was the fragment m/z 192. This mass is 16 amu higher than the mass of the corresponding fragment of the parent compound. The subsequent fragmentation led to a fragment with m/z 174, which represents the dehydrated benzylpiperidine. These fragments prove the position of the hydroxy group because the fragment m/z 174 is only possible when the hydroxy group was located in the piperidine moiety (Fig. 4). The N-oxide 6 was identified by the fragments m/z 192.1358 (oxidized benzylpiperidine) and m/z 174 [benzylpiperidine+O+H–H2O]+. Thus the additional O-atom can only be localized at the benzylpiperidine part of Ifenprodil. The slightly increased retention time (10.8 min) compared to Ifenprodil (9.4 min) indicates an N-oxide. N-Oxides are known to have longer retention times then their parent compounds due to their slightly increased lipophilicity.

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Phase II metabolites found in vitro [5]
In phase II reactions Ifenprodil can theoretically be converted into glucuronides, sulfates and methylated catechol derivates as described for traxoprodil. β-Glucuronides and sulfates could be formed directly by reaction of the hydroxy groups present in ifenprodil. Additionally, metabolites from phase I reactions can be conjugated in phase II reactions. Methylated catechol derivates are only possible after formation of catechol derivatives by hydroxylation in o-position of the phenol (compare metabolite 5).

The glucuronidation was investigated in vitro by addition of UDPGA to the microsomal incubation mixture without addition of NADPH/H+. The observed signal for the glucuronide 7 [M+Glu+H]+ was recorded in positive and negative ion mode. Fragmentation of the β-glucuronide metabolite 7 in positive mode showed two main fragmentation pathways. Similar to Ifenprodil and its derivatives, loss of water was observed (m/z 484.2346 in positive ion mode). On the other hand glucuronic acid was cleaved off resulting in the fragment m/z 326.2134 ([ifenprodil+H]+). Both fragmentations took place in parallel and led to the same further fragment m/z 308.2040 (see SI). In the negative ion mode analogous fragments were observed. Additionally, the fragments of glucuronic acid were identified in negative ion mode. The fragment m/z 193 represents the glucuronate anion, whilst m/z 175 corresponds to a dehydrated glucuronic acid. Subsequent loss of water and CO2 led to fragment m/z 113 which is characteristic for glucuronides (Fig. 5).

The incubation of Ifenprodil with NADPH/H+ and UDPGA resulted in the additional glucuronide 8 (Fig. 2), which occurred after glucuronidation of the catechol 5, which was produced during phase I biotransformation. The same catechol 5 could also be methylated by catechol O-methyl transferases. The addition of S-adenosyl methionine (SAM) and NADPH/H+ to the incubation mixture resulted in the formation of a compound with m/z 356.1965, corresponding to the mass of the methylated catechol 9. The fragmentation of 9 (Fig. 6) proceeded analogously to the fragmentation of ifenprodil (see Fig. 1). The formation of the fragment m/z 176.1444 corresponding to the unsubstituted benzylpiperidine dearly proves that the methoxylation had occurred at the phenol moiety and is not the result of a methoxylation of the benzyl moiety. Moreover, the mass of the fragments m/z 338.2129, 163 and 137 are 30 amu higher than the mass of the corresponding ifenprodil fragments.

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Identification of metabolites formed in vivo [5]
For in vivo metabolites the urine of one rat was collected for 48 h in three periods (0–8 h, 8–24 h and 24–48 h) after i.p. injection of 20 mg Ifenprodil/kg rat. The identified metabolites are shown in Fig. 7. Three types of metabolites (11, 12 and 13) were found only in vivo. Metabolite 11 is the result of o-hydroxylation, methylation and glucuronidation. The regioisomeric glucuronides 12a and 12b have the same mass (m/z 518) as 8, but show a different fragmentation pattern (see supporting information). They were generated starting from hydroxylated metabolites 4 which were additionally glucuronidated. The fragmentation patterns comparable to metabolites 4 allowed the identification of the corresponding metabolites. Furthermore, two metabolites 13 with an additional O-atom were observed, but the fragmentation pattern did not allow the unequivocal assignment of the position of the O-atom. The sulfate 10, which has been identified in the in vitro system after addition of PAPS, was not detected in the urine of the rat. However, whether this metabolite was not formed or was decomposed in the sample urine or during storage remains to be elucidated.

Ifenprodil was excreted rapidly during the first period (0–8 h). In this period, ifenprodil is one of the main compounds, together with the glucuronides 7 and 11, assuming comparable ionization factors of all compounds. Glucuronidation was observed as the main metabolic pathway within the first two periods (0–8 h, 8–24 h). The detection of ifenprodil even in the last period of urine collection (24–48 h) indicates the unmodified excretion of ifenprodil even after 24 h (Fig. 8).

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Metabolic stability of Ifenprodil [5]
In in vitro experiments the metabolic stability of ifenprodil in the presence of rat liver microsomes, NADPH/H+ and UDPGA was determined. These co-factors were chosen to generate the main metabolites observed in vivo. For the exact quantification of ifenprodil, a calibration curve was recorded first. Ifenprodil was incubated in different amounts (20, 40, 60, 80 and 100% of the amount used for the incubation with microsomes) with the reaction mixture without NADPH/H+. Additionally, eliprodil was added as internal standard (IS). The ratio ifenprodil/IS was plotted against the employed Ifenprodil amount, which resulted in a good regression coefficient (see supporting information). The ifenprodil concentration in the metabolic active mixture was determined after six periods of incubation up to 120 min.
Unexpectedly, incubation with NADPH/H+ and UDPGA resulted in increasing Ifenprodil concentrations after 60 min. This result was explained with fast decomposition of the glucuronide 7, which leads to regeneration of ifenprodil. This theory was confirmed by incubation of ifenprodil only with NADPH/H+ without UDPGA which resulted in continuously decreasing ifenprodil amounts (Fig. 9). After an incubation period of 60 min with both co-factors, 86 ± 2% of ifenprodil remained to be detected. Generation of only phase I metabolites reduced the amount of ifenprodil to 92.8 ± 2% after 60 min. Thus, the main biotransformation of Ifenprodil was caused by glucuronidation reactions. This observation corresponds well with the in vivo experiments, leading to glucuronides 7 and 8 as main metabolites, too.

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The biotransformation of Ifenprodil, an important lead compound for the development of potent and selective GluN2B selective NMDA receptor antagonists, was analyzed. In in vitro experiments, N-dealkylation, hydroxylation of both aromatic rings and the piperidine moiety were identified as possible reactions. In phase II experiments, glucuronidation of the phenol was observed. Additionally, after incubation with the corresponding co-factors SAM und PAPS, methoxylated and sulfated metabolites were detected. The analysis of a rat urine sample led to the identification of glucuronides, hydroxylated and methoxylated metabolites. The glucuronide 7 was identified as main metabolite.
Altogether, the phenol of Ifenprodil is the main structural element susceptible for biotransformation. Especially glucuronidation of the OH group was identified as the main metabolic pathway in vitro and in vivo. These results clearly indicate that the phenol should be replaced bioisosterically in order to obtain metabolically more stable GluN2B selective NMDA receptor antagonists. [5]

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The interaction of Ifenprodil with other receptors in the CNS (α1, 5-HT, σ1, σ2 receptor), leads to undesired side effects, e.g. impaired motor function and reduced blood pressure. Nevertheless, ifenprodil serves as an important lead structure for the rational design of novel GluN2B selective antagonists bearing the potential of becoming drugs for life-threatening CNS diseases [5].

[1]. Ifenprodil discriminates subtypes of the N-methyl-D-aspartate receptor: selectivity and mechanisms at recombinant heteromeric receptors. Mol Pharmacol. 1993 Oct;44(4):851-9.

[2]. Inhibition of G protein-activated inwardly rectifying K+ channels by ifenprodil. Neuropsychopharmacology. 2006 Mar;31(3):516-24.

[3]. Ifenprodil for prolonged spinal blockades of motor function and nociception in rats. Pharmacol Rep. 2016 Apr;68(2):357-62.

[4]. In Vitro and In Vivo Antiviral Activity of Nylidrin by Targeting the Hemagglutinin 2-Mediated Membrane Fusion of Influenza A Virus. Viruses. 2020 May 25;12(5):581.

[5]. Metabolism studies of ifenprodil, a potent GluN2B receptor antagonist. J Pharm Biomed Anal . 2014 Jan:88:96-105.

4-[1-hydroxy-2-[4-(phenylmethyl)-1-piperidinyl]propyl]phenol is a member of piperidines.
N-methyl-D-aspartate (NMDA) receptors (NMDARs) are members of the ionotropic glutamate receptor family, with key roles in brain development and neurological function. NMDARs are heterotetramers that typically involve a dimer of dimers of both GluN1 and GluN2A-D subunits, with each subunit itself composed of an N-terminal domain (NTD), a ligand-binding domain (LBD), a transmembrane domain, and a C-terminal cytoplasmic domain. Binding at the LBD of the agonists glycine (or D-serine) to the GluN1 subunits and of glutamate to the GluN2 subunits is a regulatory mechanism for channel activation. In addition, allosteric modulators are known to bind at the NTDs and form another layer of regulation. One such allosteric regulator is Ifenprodil, which was first shown to bind the NMDARs in the 1990s, and specifically to those NMDARs containing the GluN2B subunit. Further studies elucidated that ifenprodil binds strongly at the inter-subunit interface of adjacent GluN1 and GluN2B NTDs, where it acts as a non-competitive antagonist. Although ifenprodil has received considerable interest in its potential neuromodulatory activities in psychiatric conditions, including dependency and depression, it has also been shown to have an immunomodulatory effect. In an unbiased screen for compounds capable of reducing cell death induced by infection with the influenza strain H5N1, ifenprodil was found to have a protective effect against H5N1-induced lung damage, in part through its ability to alleviate the H5N1-induced cytokine storm and reduce pulmonary infiltration of neutrophils, natural killer cells, and T cells. Ifenprodil is being investigated for its potential utility in treating COVID-19 in an ongoing phase 2b/3 clinical trial (NCT04382924).
Ifenprodil is an orally bioavailable, N-methyl-D-aspartate (NMDA) receptor antagonist, with potential central nervous system (CNS) stimulating, neuroprotective, anti-inflammatory and anti-fibrotic activities. Upon administration, ifenprodil targets, binds to and inhibits glutamanergic NMDA receptors (NMDARs), specifically the glycine-binding NMDA receptor subunit 1 (GluN1) and 2 (glutamate-binding NMDA receptor subunit 2; NMDA-type subunit 2B; GluN2B), thereby preventing NMDAR signaling. This inhibits neuronal excitotoxicity, and thereby potentially enhancing cognitive function. Additionally, ifenprodil inhibits G protein-coupled inwardly-rectifying potassium (GIRK) channels, and interacts with alpha1 adrenergic, serotonin, and activates sigma receptors. Ifenprodil exerts its anti-inflammatory effect through its effect on NMDA and possibly sigma-1 receptors. Although the exact mechanism has not fully been elucidated, this agent reduces the infiltration of neutrophils and T-cells into the lungs and prevents the release of pro-inflammatory cytokines. This may result in the reduction of the lung inflammatory response, inhibit fibrosis in the lungs and may reduce the severity of cough. NMDA receptors are multimeric ionotropic glutamate receptors composed of four subunits and are expressed on various cells and organs, such as in the brain, lungs, and on T-cells and neutrophils.

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Characteristics of GIRK Channel Inhibition by Ifenprodil [2]
The present study demonstrated that Ifenprodil inhibited brain-type GIRK1/2 and GIRK2 channels and cardiac-type GIRK1/4 channels at nanomolar concentrations or more in a distinctive manner. The inhibition of GIRK channels by ifenprodil was concentration-dependent, but voltage-independent and time-independent with a primarily significant effect on the instantaneous current and a steady percentage inhibition during each voltage pulse. Our results also suggest that ifenprodil acted at the channels from the extracellular side of the cell membrane. On the other hand, blockade by extracellular Ba2+ and Cs+, typical of Kir channel blockers that occlude the pore of the open channel, shows a concentration-dependence, a strong voltage-dependence, and a time-dependence with a comparatively small effect on the instantaneous current but a marked inhibition on the steady-state current at the end of voltage pulses (Lesage et al, 1995). These observations suggest that ifenprodil probably causes a conformational change in the GIRK channels, but does not act as an open channel blocker of the channels, as Ba2+ and Cs+ do. The action mechanism may also be involved in the incomplete blockade of GIRK currents by ifenprodil. In the present study, ifenprodil similarly inhibited GIRK currents induced by basally free G-protein βγ subunits present in oocytes, by G-proteins mediated by κOR activation, or by ethanol. Further studies using single channel experiments may be useful for understanding the mechanism of the action of ifenprodil on GIRK channels.

In addition, the potency of inhibition by Ifenprodil of GIRK1/4 channels was higher than that of GIRK1/2 and GIRK2 channels. Although the rank order of the effectiveness by ifenprodil at the highest concentrations tested was GIRK2>GIRK1/2⩾GIRK1/4 channels, the differences were not statistically significant. Moreover, Kir1.1 and Kir2.1 channels in other Kir channel subfamilies were insensitive to ifenprodil. Further studies using GIRK/Kir1.1 and GIRK/Kir2.1 chimeric channels and mutant GIRK channels may clarify the critical sites mediating the effects of ifenprodil on GIRK channels. Furthermore, high-resolution structure analysis of GIRK channels may allow characterization of the binding sites. Additionally, although haloperidol is structurally related to ifenprodil (Williams, 2001), haloperidol weakly inhibits GIRK1/2 and GIRK1/4 channels in a similar manner (Kobayashi et al, 2000). The different effectiveness of these drugs on GIRK channels may be due to the different chemical structures between them or to their different binding sites on GIRK channels. Studies on the relationship between the structures of GIRK channels and the structure of ifenprodil may provide the basis for designing candidates for potent GIRK inhibitors.

Clinical and Pharmacological Implications [2]
The human plasma concentrations of Ifenprodil are reported to be approximately 0.1 μM. after a single administration of its clinical dosage (Aventis Pharma's data). In animals, the radioactive ifenprodil in the brain and heart after its intramuscular administration was approximately 5–8 times and 5–10 times higher, respectively, than that in blood (Nakagawa et al, 1975). Therefore, the present findings suggest that GIRK channels in the brain and heart may be inhibited by ifenprodil at clinically relevant concentrations in these tissues. Activation of GIRK channels in physiological conditions induces K+ efflux, leading to membrane hyperpolarization (North, 1989), whereas inhibition of GIRK channels leads to a depolarization of the membrane potential, resulting in an increase in cell excitability (Kuzhikandathil and Oxford, 2002). Therefore, in clinical use ifenprodil might affect various brain and heart functions via the inhibition of GIRK channels, which are expressed widely in the nervous system and the atrium (Kobayashi et al, 1995; Karschin et al, 1996).

GIRK2 knockout mice show spontaneous seizures and are more susceptible to seizures induced by pentylenetetrazol, a GABAA receptor antagonist, than wild-type mice (Signorini et al, 1997). In addition, the resting membrane potentials of neurons in GIRK knockout mice were depolarized compared to those in wild-type mice (Lüscher et al, 1997; Torrecilla et al, 2002). High doses of Ifenprodil potentiated seizures induced by some convulsants including pentylenetetrazol (Mizusawa et al, 1976), although ifenprodil has been shown to have anticonvulsant effects (Thurgur and Church, 1998; Yourick et al, 1999), probably due to inhibition of NMDA receptor channels (Williams, 2001) and Ca2+ channels (Church et al, 1994; Bath et al, 1996). In spite of its anticonvulsant property, potent blockade of neuronal GIRK channels by ifenprodil may contribute to the increased susceptibility to seizure by causing an increase in neuronal excitability.

Interestingly, GIRK2 knockout mice show reduced anxiety with an increase in motor activity in three tests for anxiety: the elevated plus-maze, light/dark box, and canopy test (Blednov et al, 2001). Ifenprodil had an anxiolytic property with an increase in locomotion in MF1 mice in the elevated plus-maze test (Fraser et al, 1996), although it had no anxiolytic effect in the light/dark exploratory test and caused no change in locomotor activity in Wistar rats (Mikolajczak et al, 2003). This discrepancy might have been caused by differences in the behavioral tests including difference in the ratio of the two light/dark compartments in the apparatus and/or in animal species. A clinical study showed that Ifenprodil improved anxiety, a decrease in spontaneity, and melancholy in patients with sequelae of cerebrovascular diseases (Otomo et al, 1976). Therefore, inhibition of neuronal GIRK channels by ifenprodil might partly contribute to the clinical effects on anxiety and decreased activity, which are observed in some neuropsychiatric disorders as well.

In the heart, acetylcholine opens atrial GIRK channels via activation of the M2 muscarinic acetylcholine receptor, and ultimately causes slowing of the heart rate (Brown and Birnbaumer, 1990). Sinus tachycardia during treatment with ifenprodil is observed along with its hypotensive effect (Carron et al, 1971; Young et al, 1983; Yajima et al, 1987). Ifenprodil exhibits no significant affinity for the muscarinic acetylcholine receptor (Chenard et al, 1991). The present study demonstrated that Ifenprodil, at submicromolar concentrations or more, inhibited cardiac-type GIRK1/4 channels, which are abundantly present in the atrium of the heart (Krapivinsky et al, 1995). Therefore, atrial GIRK channels may also be inhibited by ifenprodil in clinical practice. GIRK1 or GIRK4 knockout mice show mild tachycardia (Bettahi et al, 2002). Additionally, the hypotensive effect of ifenprodil may induce compensational activation of the sympathetic nervous system, which plays an important role in the stimulatory regulation of the heart rate. Taken together, our data suggest that sinus tachycardia during treatment with ifenprodil may be partly related to inhibition of atrial GIRK channels.

Ifenprodil influenced ethanol-related behavioral changes in animals, such as suppression of amnestic effects and withdrawal signs including convulsions (Malinowska et al, 1999; Napiórkowska-Pawlak et al, 2000; Narita et al, 2000). Ethanol activates GIRK channels (Kobayashi et al, 1999; Lewohl et al, 1999). The present study demonstrated that ifenprodil inhibited GIRK1/2 currents induced by ethanol. Interestingly, GIRK2 knockout mice show reduced ethanol-induced conditioned taste aversion and conditioned place preference (Hill et al, 2003), and are less sensitive to some of acute ethanol effects, including anxiolysis, habituated locomotor stimulation and handling-induced convulsions after an acute administration of ethanol, than wild-type mice (Blednov et al, 2001). Taken together, ifenprodil might suppress GIRK-related ethanol effects.

Morphine, a commonly used potent analgesic, preferentially binds to the μ-opioid receptor, and exerts various pharmacological effects, including analgesia, euphoria, and dependence (Gutstein and Akil, 2001). The μ-opioid receptor is coupled to G-protein-mediated signal transductions involving GIRK channels, adenylyl cyclase, Ca2+ channels, and phospholipase C (Ikeda et al, 2002). Although morphine produces a conditioned place preference in animals, indicating its rewarding effect, pretreatment with ifenprodil suppresses the rewarding effect produced by morphine (Suzuki et al, 1999). However, Ifenprodil exhibits no significant affinity for the opioid receptors (Chenard et al, 1991). The present study demonstrated that ifenprodil inhibited G-protein-mediated GIRK currents. It may be important to determine whether GIRK channel function contributes to the rewarding effect of morphine. Interestingly, GIRK knockout mice show reduced self-administration of cocaine (Morgan et al, 2003). In a clinical report, desipramine, which acts as an inhibitor of GIRK channels as well as of norepinephrine transporters (Kobayashi et al, 2004b), facilitated initial abstinence from cocaine (Gawin et al, 1989). Thus, selective GIRK inhibitors might be potential agents for the treatment of abusers of cocaine. Further studies on the effects of ifenprodil on GIRK knockout mice might clarify the roles of the GIRK-mediated effects of ifenprodil in addiction to morphine and cocaine. [2]

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Decisively, the polykaryon formation assay verified that nylidrin targets the HA2-mediated membrane fusion by blocking pH-dependent conformational change of HA. As above-mentioned, additional evidence supporting this hypothesis was provided by subtype dependency on the antiviral efficacy: nylidrin, Ifenprodil, and clenbuterol were active against all A/H1N1 strains tested; in contrast, nylidrin and Ifenprodil were limitedly effective only against A/Hong Kong/8/1968 and A/Seoul/11/1988 among the H3N2 strains tested (Table 1 and Table 2). This observation is intriguing because most HA2 fusion inhibitory agents, either therapeutic antibodies or small molecules, tend to exhibit antiviral potency in a viral subtype- or HA group-dependent manner; group I consists of H1, H2, and H5, whereas group 2 consists of H3 and H7. The strain-specific activity of the compounds against the H3N2 subtype viruses indicated that optimization of nylidrin through chemical modifications could be a plausible approach for identifying a broad-spectrum fusion inhibitor.[4]

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Influenza A virus, one of the major human respiratory pathogens, is responsible for annual seasonal endemics and unpredictable periodic pandemics. Despite the clinical availability of vaccines and antivirals, the antigenic diversity and drug resistance of this virus makes it a persistent threat to public health, underlying the need for the development of novel antivirals. In a cell culture-based high-throughput screen, a β2-adrenergic receptor agonist, nylidrin, was identified as an antiviral compound against influenza A virus. The molecule was effective against multiple isolates of subtype H1N1, but had limited activity against subtype H3N2, depending on the strain. By examining the antiviral activity of its chemical analogues, we found that Ifenprodil and clenbuterol also had reliable inhibitory effects against A/H1N1 strains. Field-based pharmacophore modeling with comparisons of active and inactive compounds revealed the importance of positive and negative electrostatic patterns of phenyl aminoethanol derivatives. Time-of-addition experiments and visualization of the intracellular localization of nucleoprotein NP demonstrated that an early step of the virus life cycle was suppressed by nylidrin. Ultimately, we discovered that nylidrin targets hemagglutinin 2 (HA2)-mediated membrane fusion by blocking conformational change of HA at acidic pH. In a mouse model, preincubation of a mouse-adapted influenza A virus (H1N1) with nylidrin completely blocked intranasal viral infection. The present study suggests that nylidrin could provide a core chemical skeleton for the development of a direct-acting inhibitor of influenza A virus entry.[4]

C21H27NO2.1/2C4H6O6

800.98

325.204

C, 68.98; H, 7.55; N, 3.50; O, 19.97

23210-58-4

Ifenprodil glucuronide;66516-92-5

3689

White to off-white solid powder

493.5ºC at 760mmHg

178-180ºC

248.7ºC

0mmHg at 25°C

1.584

2

3

5

24

353

0

CC(C(C1=CC=C(C=C1)O)O)N2CCC(CC2)CC3=CC=CC=C3.CC(C(C1=CC=C(C=C1)O)O)N2CCC(CC2)CC3=CC=CC=C3.[C@@H]([C@H](C(=O)O)O)(C(=O)O)O

DMPRDSPPYMZQBT-CEAXSRTFSA-N

InChI=1S/2C21H27NO2.C4H6O6/c2*1-16(21(24)19-7-9-20(23)10-8-19)22-13-11-18(12-14-22)15-17-5-3-2-4-6-17;5-1(3(7)8)2(6)4(9)10/h2*2-10,16,18,21,23-24H,11-15H2,1H3;1-2,5-6H,(H,7,8)(H,9,10)/t;;1-,2-/m..1/s1

4-(2-(4-benzylpiperidin-1-yl)-1-hydroxypropyl)phenol; (2R,3R)-2,3-dihydroxysuccinate (2:1)

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: Please store this product in a sealed and protected environment, avoid exposure to moisture.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

Solubility in Formulation 1: ≥ 1 mg/mL (2.50 mM) (saturation unknown) in 1% DMSO + 99% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: 12.22 mg/mL (30.51 mM) in 0.5% CMC-Na/saline water (add these co-solvents sequentially from left to right, and one by one), suspension solution; with ultrasonication.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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 (Please use freshly prepared in vivo formulations for optimal results.)