Abstract

Depression and anxiety disorders are very common mental illnesses, affecting about 10-15% of the population throughout their lives¹. Nortriptyline has been used to treat depression for many years, but it has a side effect of increased risk of cardiotoxicity. Binding of nortriptyline to the hERG channel is most likely the cause of these side effects. The focus of the project is to understand the binding of nortriptyline to the hERG channel and look for ways to prevent binding.

Case Study

A 41 year old female presented to a hospital with symptoms of pneumonia. Upon diagnostic confirmation, she was initiated on the antibiotic sulfamethoxazole/trimethoprim 1200mg/240mg oral tablet(s) three times daily. Reviewing the patient's home medications, nortriptyline for the treatment of depression was found. Concurrent use of nortriptyline and sulfamethoxazole/trimethoprim increases the risk of cardiotoxicity and life-threatening events like QT prolongation, Torsades de pointes, or cardiac arrest. The prescribing hospital physician confirmed his awareness of the interaction between the two drugs and mandated their continuation with increased and careful monitoring of the patient.

Introduction

Nortriptyline:
• has been approved for depression by the FDA since 1964²
• belongs to the tricyclic antidepressant class
• has off-label uses including bulimia nervosa, diabetic neuropathy, and irritable bowel syndrome

Nortriptyline, as an antidepressant, specifically enhances neurotransmission by inhibiting the reuptake of the monoamines Serotonin and Dopamine, at the synaptic junction which increases the concentration of these monoamines in the brain, increasing neuron activation. As with any drug, nortriptyline has effects other than the intended effects in the brain. In the case of our patient, nortriptyline is able to bind to the human Ether-à-go-go-Related Gene (hERG) channel in cardiac cells. This is the potassium channel that is necessary for repolarization of the cardiac cells. This binding can cause the phenomenon called QT prolongation. QT prolongation delays the beating of the heart and can lead to other deadly manifestations such as Torsades de Pointes³.

Molecular Story

Nortriptyline⁴
Cycloheptane with 2 benzene rings attached on the sides (Forms the tricyclic rings)
Positively ionizable nitrogen (Secondary Amine)

Drug Targets⁴
SERT - Serotonin Transporter (5-hydroxytryptamine or 5-HT Transporter)
NET - Norepinephrine Transporter

How Nortriptyline Functions⁴
It inhibits the reuptake of SERT transporter and NET transporter at the synaptic junction to increase the concentration of monoamines in the brain and increases neuron activation.
It is non-selective due to its 'privileged structure,' which is a structural motif that fulfills several pharmacophores at once and results in many off-target effects. Nortriptyline's pharmacophore is seen in the ligands for many different channels, such as the SERT, NET, dopamine, and hERG channels.

How Nortriptyline Binds
The dopamine channel in the Drosophila fly was found to have greater than 50% sequence identity with the human SERT and NET. Due to the close resemblance and the fact that it is well studied, interactions between nortriptyline and the Drosophila dopamine channel can be assumed to be very similar to those of the human SERT and NET.
Amino acids that are important for the binding of nortriptyline to the dopamine channel: tyrosine 124A, phenylalanine 325A, and phenylalanine 43A.

First Interaction: Polar region of nortriptyline (secondary amine group) interacts with a phenylalanine (Phe43A). One of the hydrogens on the secondary amine group interacts with the carbonyl backbone of Phe43A. The second hydrogen on the secondary amine binds to the phenyl ring in Phe43A due to the pi-cation interaction that occurs within the phenyl ring.

Second Interaction: In the non-polar region of nortriptyline, two phenylalanine side chains interact with the two outer rings of nortriptyline. This stabilizes nortriptyline in a hydrophobic pocket and allows for a stronger bond between nortriptyline and the dopamine transporter.

hERG Channel - human Ether-à-go-go-Related Gene Channel⁵
One of the off-target binding sites of nortriptyline is the hERG receptor. When nortriptyline binds the hERG receptor, it blocks the channel and slows the electrical conduction required for the contractility of the heart. This induces delayed repolarization of the heart.

It is hypothesized that the pharmacophore for a hERG channel blocker contains three hydrophobic features and one positively ionizable feature with each group being 4.5 to 7.0Å of each other. This is directly seen in nortriptyline's structure.

Amino acids on hERG channel that interact with nortriptylline: Threonine 623, Serine 624, Tyrosine 652, and Phenylalanine 656.

It is proposed that Threonine 623, Serine 624, and Phenylalanine 656 interact with the polar hydrophobic regions while Tyrosine 652 interacts with the nitrogen via a pi-stacking interaction.

The Next Question

To remove the QT prolonging characteristic of nortriptyline, we could potentially make chemical modifications to the structure of nortriptyline. Studies have shown that the hydrophobic features and the positively ionizable feature of the drug have to be within a proximity of 4.5 to 7.0 Å of each other in order for it to bind to the hERG channel.

Therefore, if we could add a few carbon bonds to increase the distance between the amine group and the fused tricyclic rings to more than 7.0 Å, we would push nortriptyline out of the binding pocket and prevent it from binding to hERG while still retain its ability to affect the NET and SERT.

Improvements to the structure of nortriptyline should not alter the tricyclic rings and secondary amine.

Summary

Although nortriptyline is no longer first-line therapy for depression, certain patient populations still take this medication routinely.

In viewing the drug therapy problem and undesirable effects of nortriptyline binding to the hERG receptor, alteration of the chemical structure of antidepressants would eliminate the binding to this receptor and decrease inadvertent cardiac effects.

Pharmacists play an essential role in pharmacotherapy monitoring in terms of QTc prolongation potential. Pharmacists must evaluate the risks and benefits because there currently is no hard cut off for QTc prolongation and must be available to properly educate our patients.


These duties are critical in order to minimize the potential for fatal cardiac arrhythmias, such as Torsades de Pointes, while on nortriptyline.

References

1. Goodman, L. (2011). Drug Therapy of Depression and Anxiety Disorders. In L. Goodman, Goodman & Gilman's the pharmacological basis of therapeutics (p. Chapter 15.). New York: McGraw-Hill, Health Professions Divison.
2. López-Muñoz F, A. C. (2009). Monoaminergic neurotransmission: the history of the discovery of antidepressants from 1950s until today. Current Pharmaceutical Design, 1563-86.
3. Seol-Hee Jeon, J. J.-H.-S.-S.-J. (2011). Effects of nortriptyline on QT prolongation: A safety pharmacology study. Human and Experimental Toxicology, 1649–1656.
4. Aravind Penmatsa, K. H. (2013, Nov 7). X-ray structure of the dopamine transporter in complex with tricyclic antidepressant. Nature, 85-90.
5. Yuko Yamakawa, K. F. (2012). Pharmacophore modeling for hERG channel facilitation. Biochemical and Biophysical Research Communications, 161-166.