Modulation of human dorsal root ganglion neuron excitability by Nav1.7 inhibition

This study demonstrates that while selective Nav1.7 inhibition (via AM-2099) significantly alters the threshold and refractory period of human dorsal root ganglion neurons, it fails to effectively suppress repetitive firing compared to Nav1.8 inhibition, potentially explaining the limited clinical efficacy of Nav1.7 inhibitors for pain treatment.

The Gist

The Big Picture: The "Pain Switch" Problem

Imagine your body has a sophisticated alarm system. When you touch something hot or sharp, a specific type of nerve cell (called a nociceptor) acts like a messenger, running a message to your brain screaming, "Ouch! Stop!"

For a long time, scientists thought the main "starter motor" for this alarm was a specific protein called Nav1.7. They believed that if they built a drug to jam this starter motor, the alarm would never go off, and you wouldn't feel pain.

However, when they tested these drugs on humans, they didn't work very well. The pain still happened. Meanwhile, a different drug targeting a different protein (Nav1.8) actually worked quite well in clinical trials.

The Question: Why did the Nav1.7 drug fail while the Nav1.8 drug succeeded?

The Answer: This paper investigates the human nerve cells directly to see exactly what happens when you jam one motor versus the other.

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The Experiment: The "Human Nerve Gym"

The researchers took human nerve cells (from donors) and put them in a lab dish. They heated the dish to body temperature (37°C) to make the cells behave naturally. Then, they used two different tools:

1. AM-2099: A drug that jams the Nav1.7 motor.
2. Suzetrigine (VX-548): A drug that jams the Nav1.8 motor (this was tested in a previous study).

They then gave the nerve cells little electrical "pushes" to see how they reacted. Think of this like tapping a car's gas pedal to see how the engine responds.

The Findings: Two Different Engines

Here is what they discovered, using some analogies:

1. The "Starting Line" (Threshold)

• Nav1.7 (The Starter): When they jammed Nav1.7, the nerve cell became very hard to start. It was like trying to start a car with a dead battery; you had to push the gas pedal much harder just to get the engine to turn over.
  • Result: The "threshold" (the point where the pain signal starts) went way up.
• Nav1.8 (The Runner): When they jammed Nav1.8, the starting line didn't move much. The car still started easily.
  • Result: The threshold stayed mostly the same.

2. The "Sprint" (Upstroke Velocity)

• Nav1.7: When the nerve did fire, it was sluggish. The "sprint" was slow.
• Nav1.8: The sprint was also affected, but not as dramatically as with Nav1.7.

3. The "Marathon" (Repetitive Firing)

This is the most important part. Pain isn't just one "ouch"; it's often a continuous stream of signals (like a fire alarm that won't stop ringing).

• Nav1.7: Even though the nerve was hard to start and slow to sprint, once it got going, it could still run a marathon. If you gave it a strong, continuous push, it kept firing over and over again. Jamming Nav1.7 didn't stop the marathon.
• Nav1.8: When they jammed Nav1.8, the nerve got tired very quickly. It could start, maybe run one lap, but then it collapsed. It couldn't keep firing repeatedly. Jamming Nav1.8 stopped the marathon.

The "Why" Behind the Results

Think of the nerve cell as a relay race team with two runners:

• Runner A (Nav1.7): Is great at the starting blocks. They get the race going quickly and powerfully. But they get tired fast and can't run long distances.

• Runner B (Nav1.8): Is a bit slower to start, but they are the marathon runner. They keep the race going once it's started.

• If you stop Runner A (Nav1.7): The race is hard to start. But if you give them a huge shove to get them moving, Runner B (Nav1.8) takes over and keeps the pain signal running.

• If you stop Runner B (Nav1.8): The race starts okay, but the team runs out of steam almost immediately. The pain signal dies out.

The Conclusion: Why One Drug Works Better Than the Other

The paper explains why the Nav1.8 inhibitor (Suzetrigine) is a better painkiller than the Nav1.7 inhibitors tried so far.

• Nav1.7 inhibitors are like putting a heavy weight on the starter motor. It makes it hard to feel a tiny pinch, but if the pain is strong (like a burn or a cut), the nerve finds a way to keep firing.
• Nav1.8 inhibitors are like cutting the fuel line to the marathon runner. Even if the alarm starts, it can't keep ringing. It stops the sustained pain that makes us suffer.

The Takeaway

The human pain system is complex. While Nav1.7 is crucial for starting the pain signal, Nav1.8 is the engine that keeps the pain signal going. To effectively treat pain, you need to stop the engine from running continuously, not just make it harder to start. This explains why targeting Nav1.8 has been more successful in the clinic so far.

Technical Summary

1. Problem Statement

• Clinical Context: Nav1.7 voltage-gated sodium channels are highly expressed in human primary pain-sensing neurons (nociceptors). Loss-of-function mutations in Nav1.7 cause congenital insensitivity to pain, suggesting it is a prime therapeutic target. However, selective Nav1.7 inhibitors (e.g., PF-05089771) have failed to demonstrate significant efficacy in large-scale clinical trials for pain relief.
• The Contrast: In contrast, a selective Nav1.8 inhibitor (suzetrigine/VX-548) has shown clinical success in treating acute post-operative pain.
• The Knowledge Gap: Human nociceptors co-express both Nav1.7 and Nav1.8 channels. While the roles of these channels in rodent models are partially understood, there is a lack of direct, comparative data on how inhibiting Nav1.7 versus Nav1.8 affects the specific electrophysiological properties (threshold, upstroke, repetitive firing) of human dorsal root ganglion (DRG) neurons at physiological temperatures. Understanding these differences is crucial for explaining the divergent clinical outcomes of inhibitors targeting these two channels.

2. Methodology

• Tissue Source: Dissociated human dorsal root ganglion (DRG) neurons obtained from donors via AnaBios Corporation.
• Experimental Conditions:
  • Temperature: Experiments were conducted at 37°C (physiological temperature), a critical factor for channel kinetics.
  • Recording Technique: Whole-cell patch-clamp electrophysiology.
  • Stimulation Protocols:
    • Action Potential (AP) Shape: Evoked by short (0.5-ms) current injections to isolate channel effects from stimulus artifacts.
    • Threshold/Rheobase: Determined by incremental 0.5-ms current injections.
    • Repetitive Firing: Evoked by 1-second current injections of varying magnitudes.
    • Refractory Period: Measured using paired 0.5-ms current injections with variable inter-stimulus intervals.
• Pharmacology:
  • Nav1.7 Inhibitor: AM-2099 (600 nM), a highly selective inhibitor (>200-fold selectivity for Nav1.7 over Nav1.8).
  • Comparison Data: Results were directly compared to a previous study using Suzetrigine (VX-548) (10–100 nM), a selective Nav1.8 inhibitor, using identical protocols and temperature.

3. Key Contributions

• Direct Human Comparison: This study provides the first direct, side-by-side comparison of Nav1.7 and Nav1.8 inhibition effects on human DRG neurons under identical physiological conditions (37°C).
• Mechanistic Differentiation: It elucidates that while both channels contribute to excitability, they play distinct and non-redundant roles: Nav1.7 is dominant for threshold and upstroke velocity, while Nav1.8 is dominant for repetitive firing.
• Clinical Insight: The findings offer a mechanistic explanation for the clinical disparity between Nav1.7 and Nav1.8 inhibitors, suggesting that blocking Nav1.7 alone may be insufficient to stop high-frequency firing associated with acute pain.

4. Key Results

A. Effects on Action Potential (AP) Shape and Threshold (Nav1.7 Inhibition)

• Threshold Shift: AM-2099 caused a significant depolarizing shift in AP threshold (average +9.4 mV), making it harder to initiate a spike.
• Upstroke Velocity: Maximum upstroke velocity ($dV/dt_{max}$) was drastically reduced (to ~42% of control).
• AP Peak: The AP peak amplitude was reduced by ~16 mV.
• AP Width: Unlike Nav1.8 inhibition (which narrows the AP), Nav1.7 inhibition had no consistent effect on AP width.
• Rheobase: The current required to trigger a spike (rheobase) increased by ~69%.

B. Effects on Repetitive Firing and Refractory Period

• Repetitive Firing: Nav1.7 inhibition had minimal effect on the ability of neurons to fire repetitively in response to strong, sustained stimuli. In 14 of 15 neurons that fired multiple spikes in control, they continued to do so after Nav1.7 inhibition.
• Refractory Period: Nav1.7 inhibition prolonged the refractory period (average increase to 1.8x control). This contrasts with Nav1.8 inhibition, which paradoxically shortened the refractory period.

C. Comparison: Nav1.7 vs. Nav1.8 Inhibition

• Threshold & Upstroke: Nav1.7 inhibition had a larger impact on AP threshold and upstroke velocity than Nav1.8 inhibition.
• Repetitive Firing: Nav1.8 inhibition was much more effective at suppressing repetitive firing than Nav1.7 inhibition.
• AP Peak: Both inhibitors reduced the AP peak amplitude to a similar degree.

5. Significance and Conclusion

• Explaining Clinical Failure: The study suggests that the limited clinical efficacy of Nav1.7 inhibitors may stem from their inability to suppress repetitive firing in nociceptors during strong stimulation. While Nav1.7 inhibition raises the threshold (preventing weak stimuli from triggering pain), it fails to stop the high-frequency firing that correlates with severe pain perception once the threshold is crossed.
• Role of Nav1.8: The superior efficacy of Nav1.8 inhibitors (like suzetrigine) likely arises because Nav1.8 channels are critical for sustaining the high-frequency firing required for pain signaling, particularly during the "shoulder" of the action potential and repetitive bursts.
• Physiological Relevance: The data confirms that in human neurons, Nav1.7 dominates the initial upstroke and threshold determination, whereas Nav1.8 is the primary driver of repetitive firing.
• Future Directions: The authors propose that successful pain therapeutics may require dual inhibition or targeting Nav1.8 specifically to address the repetitive firing component of pain, while Nav1.7 inhibition might be more relevant for conditions involving low-threshold spontaneous firing or specific neuropathic states.

In summary, the paper demonstrates that Nav1.7 inhibition raises the bar for firing but does not lower the ceiling for repetitive firing, whereas Nav1.8 inhibition effectively dampens the repetitive firing that drives pain perception, providing a biophysical basis for the differential clinical success of these drug classes.