Terfenadine

Terfenadine is a histamine H1-receptor antagonist, which blocks HERG and KATP (KIR6) channels. The IC values for Terfenadines inhibitory activity against these channels is 204nM for HERG and 1.2µM for KATP. Studies show that Terfenadine inhibits the delayed rectifier K+ in ventricular myocytes. Terfenadine is converted into its more active form, terfenadine carboxylate by the enzyme CYP3A4.

Marmoset cytochrome P450 2J2 mainly expressed in small intestines and livers effectively metabolizes human P450 2J2 probe substrates, astemizole and terfenadine.[Pubmed:26899760]

Xenobiotica. 2016 Nov;46(11):977-85.

1. Common marmoset (Callithrix jacchus), a New World Monkey, has potential to be a useful animal model in preclinical studies. However, drug metabolizing properties have not been fully understood due to insufficient information on cytochrome P450 (P450), major drug metabolizing enzymes. 2. Marmoset P450 2J2 cDNA was isolated from marmoset livers. The deduced amino acid sequence showed a high-sequence identity (91%) with cynomolgus monkey and human P450 2J2 enzymes. A phylogenetic tree revealed that marmoset P450 2J2 was evolutionarily closer to cynomolgus monkey and human P450 2J2 enzymes, than P450 2J forms in pigs, rabbits, rats or mice. 3. Marmoset P450 2J2 mRNA was abundantly expressed in the small intestine and liver, and to a lesser extent in the brain, lung and kidney. Immunoblot analysis also showed expression of marmoset P450 2J2 protein in the small intestine and liver. 4. Enzyme assays using marmoset P450 2J2 protein heterologously expressed in Escherichia coli indicated that marmoset P450 2J2 effectively catalyzed astemizole O-demethylation and Terfenadine t-butyl hydroxylation, similar to human and cynomolgus monkey P450 2J2 enzymes. 5. These results suggest the functional characteristics of P450 2J2 enzymes are similar among marmosets, cynomolgus monkeys and humans.

Virtual Clinical Trial Toward Polytherapy Safety Assessment: Combination of Physiologically Based Pharmacokinetic/Pharmacodynamic-Based Modeling and Simulation Approach With Drug-Drug Interactions Involving Terfenadine as an Example.[Pubmed:27640752]

J Pharm Sci. 2016 Nov;105(11):3415-3424.

A Quantitative Systems Pharmacology approach was utilized to predict the cardiac consequences of drug-drug interaction (DDI) at the population level. The Simcyp in vitro-in vivo correlation and physiologically based pharmacokinetic platform was used to predict the pharmacokinetic profile of Terfenadine following co-administration of the drug. Electrophysiological effects were simulated using the Cardiac Safety Simulator. The modulation of ion channel activity was dependent on the inhibitory potential of drugs on the main cardiac ion channels and a simulated free heart tissue concentration. ten Tusscher's human ventricular cardiomyocyte model was used to simulate the pseudo-ECG traces and further predict the pharmacodynamic consequences of DDI. Consistent with clinical observations, predicted plasma concentration profiles of Terfenadine show considerable intra-subject variability with recorded Cmax values below 5 ng/mL for most virtual subjects. The pharmacokinetic and pharmacodynamic effects of inhibitors were predicted with reasonable accuracy. In all cases, a combination of the physiologically based pharmacokinetic and physiology-based pharmacodynamic models was able to differentiate between the Terfenadine alone and Terfenadine + inhibitor scenario. The range of QT prolongation was comparable in the clinical and virtual studies. The results indicate that mechanistic in vitro-in vivo correlation can be applied to predict the clinical effects of DDI even without comprehensive knowledge on all mechanisms contributing to the interaction.

Liquid microjunction surface sampling of acetaminophen, terfenadine and their metabolites in thin tissue sections.[Pubmed:25411703]

Bioanalysis. 2014;6(19):2599-606.

BACKGROUND: The aim of this work was to evaluate the analytical performance of a fully automated droplet-based surface-sampling system for determining the distribution of the drugs acetaminophen and Terfenadine, and their metabolites, in rat thin tissue sections. RESULTS: The rank order of acetaminophen concentration observed in tissues was stomach > small intestine > liver, while the concentrations of its glucuronide and sulfate metabolites were greatest in the liver and small intestine. Terfenadine was most concentrated in the liver and kidney, while its major metabolite, fexofenadine, was found in the liver and small intestine. CONCLUSION: The spatial distributions of both drugs and their respective metabolites observed in this work were consistent with previous studies using radiolabeled drugs.

Stimulating Effect of Terfenadine on Erythrocyte Cell Membrane Scrambling.[Pubmed:27035465]

Cell Physiol Biochem. 2016;38(4):1425-34.

BACKGROUND/AIMS: The antihistaminic drug Terfenadine may trigger apoptosis of tumor cells, an effect unrelated to its effect on histamine receptors. Similar to apoptosis of nucleated cells, erythrocytes may enter eryptosis, the suicidal death of erythrocytes characterized by cell shrinkage and cell membrane scrambling with phosphatidylserine translocation to the erythrocyte surface. Signaling triggering eryptosis include increase of cytosolic Ca2+ activity ([Ca2+]i), oxidative stress, and ceramide. The present study explored, whether Terfenadine is capable to trigger eryptosis. METHODS: Flow cytometry was employed to estimate phosphatidylserine abundance at the erythrocyte surface from annexin-V-binding, cell volume from forward scatter, [Ca2+]i from Fluo3-fluorescence, abundance of reactive oxygen species (ROS) from 2',7'-dichlorodihydrofluorescein (DCF) diacetate dependent fluorescence, and ceramide abundance at the human erythrocyte surface utilizing specific antibodies. Hemolysis was quantified from haemoglobin concentration in the supernatant. RESULTS: A 48 hours exposure of human erythrocytes to Terfenadine (>/= 5 microM) significantly increased the percentage of annexin-V-binding cells and triggered hemolysis without significantly modifying the average forward scatter. Terfenadine (7.5 microM) significantly increased Fluo3-fluorescence, but did not significantly modify DCF fluorescence or ceramide abundance. The effect of Terfenadine on annexin-V-binding was significantly blunted but not abolished by removal of extracellular Ca2+. Exposure of human erythrocytes to Ca2+ ionophore ionomycin (1 microM, 15 min) triggered annexin-V-binding, an effect augmented by Terfenadine pretreatment (10 microM, 48 hours). CONCLUSIONS: Terfenadine triggers phospholipid scrambling of the human erythrocyte cell membrane, an effect in part due to entry of extracellular Ca2+ and in part due to sensitizing human erythrocyte cell membrane scrambling to Ca2+.

Loratadine blockade of K(+) channels in human heart: comparison with terfenadine under physiological conditions.[Pubmed:10604956]

J Pharmacol Exp Ther. 2000 Jan;292(1):261-4.

Recently, there has been considerable attention focused on drugs that prolong the QT interval of the electrocardiogram, with the H(1)-receptor antagonist class of drugs figuring prominently. Albeit rare, incidences of QT prolongation and ventricular arrhythmias, in particular torsade de pointes, have been reported with the antihistamines astemizole and Terfenadine and more recently with loratadine. The most likely mechanism for these drug-related arrhythmias is blockage of one or more ion channels involved in cardiac repolarization. Several studies have demonstrated block of multiple cardiac K(+) channels by Terfenadine, including I(to), I(sus), I(K1), and I(Kr) or human ether-a-go-go-related gene (HERG). In contrast to Terfenadine, previous studies have shown the antihistamine loratadine to be virtually free of cardiac ion channel-blocking effects. This disparity in the lack of any significant cardiac ion channel-blocking effect and the existence of numerous adverse cardiac event reports for loratadine prompted the comparison of the human cardiac K(+) channel-blocking profile for loratadine and Terfenadine under physiological conditions [37 degrees C, holding potential (V(hold)) = -75 mV] with the whole-cell patch-clamp method. Isolated human atrial myocytes were used to examine drug effects on I(to), I(sus), and I(K1), whereas HERG was studied in stably transfected HEK cells. In contrast to previous studies in nonhuman systems and/or under nonphysiological conditions, Terfenadine (1 microM) had no effect on I(to), I(sus), or I(K1) at pacing rates up to 3 Hz. Similar results were found for 1 microM loratadine. However, both drugs potently blocked HERG current amplitude, with a mean IC(50) of 173 nM for loratadine and 204 nM for Terfenadine (pacing rate, 0.1 Hz). Neither drug exhibited any significant use-dependent blockage of HERG (pacing rates = 0.1-3 Hz). These results point to a similarity in the human cardiac K(+) channel-blocking effects of loratadine and Terfenadine and provide a possible mechanism for the arrhythmias associated with the use of either drug.

Mechanism of terfenadine block of ATP-sensitive K(+) channels.[Pubmed:10928959]

Br J Pharmacol. 2000 Aug;130(7):1571-4.

The ATP-sensitive K(+) (K(ATP)) channel is a complex of a pore-forming inwardly rectifying K(+) channel (Kir6.2) and a sulphonylurea receptor (SUR). The aim of the present study was to gain further insight into the mechanism of block of K(ATP) channels by Terfenadine. Channel activity was recorded both from native K(ATP) channels from the clonal insulinoma cell line RINm5F and from a C-terminal truncated form of Kir6.2 (Kir6.2Delta26), which - in contrast to Kir6.2 - expresses independently of SUR. Kir6.2Delta26 channels were expressed in COS-7 cells, and enhanced green fluorescent protein (EGFP) cDNA was used as a reporter gene. EGFP fluorescence was visualized by a laser scanning confocal microscope. Terfenadine applied to the cytoplasmic side of inside-out membrane patches concentration-dependently blocked both native K(ATP) channel and Kir6.2Delta26 channel activity, and the following values were calculated for IC(50) (the Terfenadine concentration causing half-maximal inhibition) and n (the Hill coefficient): 1.2 microM and 0.7 for native K(ATP) channels, 3.0 microM and 1.0 for Kir6. 2Delta26 channels. Terfenadine had no effect on slope conductance of either native K(ATP) channels or Kir6.2Delta26 channels. Intraburst kinetics of Kir6.2Delta26 channels were not markedly affected by Terfenadine and, therefore, Terfenadine acts as a slow channel blocker on Kir6.2Delta26 channels. Terfenadine-induced block of Kir6. 2Delta26 channels demonstrated no marked voltage dependence, and lowering the intracellular pH to 6.5 potentiated the inhibition of Kir6.2Delta26 channels by Terfenadine. These observations indicate that Terfenadine blocks pancreatic B-cell K(ATP) channels via binding to the cytoplasmic side of the pore-forming subunit. The presence of the pancreatic SUR1 has a small, but significant enhancing effect on the potency of Terfenadine.

Terfenadine ((±)-Terfenadine) is a potent open-channel blocker of hERG with an IC50 of 204 nM. Terfenadine, an H1 histamine receptor antagonist, acts as a potent apoptosis inducer in melanoma cells through modulation of Ca2+ homeostasis. Terfenadine induces ROS-dependent apoptosis, simultaneously activates Caspase-4, -2, -9.

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