A novel strategy for analgesic drugs

[Anand]

In what I believe will be a rather recurring theme for my posts, I would like to highlight a recent paper within the pain field. Generating drugs that are specific to nociceptors represents a major goal in pain research at the moment with many different targets proposed. One such target is to block voltage gated sodium channels specific to nociceptors like Nav 1.7, Nav 1.8 and Nav 1.9. I personally feel that this would be an ideal target for analgesics. The labs of Bruce Bean and Clifford Woolf recently published an article in Nature titled “Inihibition of nociceptors by TRPV1-mediated entry of impermeant sodium channel blockers” where they show a novel strategy to achieve this specific block of nociceptor voltage gated sodium channels

How do they achieve specificity?: Make a normally membrane impermeable sodium channel blocker able to specifically cross the membrane of a subset of pain fibre axons.
The strategy involves using a hydrophillic derivative of lidocaine named QX-314. Lidocaine is a commonly used local anaesthetic that has the side effect of blocking non-noxious sensation as well as motor movement. QX-314 being hydrophillic is incapable of crossing the plasma membrane. They utilise this as well as the fact that a certain nociceptor specific ligand gated channel named TRPV1 has a channel pore large enough to allow QX-314 pass through, to specifically target QX-314 to TRPV1 positive cells. This is done by administering QX-314 together with an agonist of TRPV1, capsaicin. Capsaicin opens the channel and QX-314 blocks the fibre. Take a look at this video off the Harvard Medical website to get an idea for how this drug combination works.

What is the effect seen?: The strategy blocks action potential generation in cultured cells. In animals, after 15 minutes there is a specific loss of sensation towards painful heat and touch
In their characterisation they showed that only when used in combination with capsaicin could QX-314 significantly block sodium currents in DRG cells and that this blockage results in the inability to trigger action potential. They then went on to show the behavioural effects of the combination of capsaicin and QX-314 on rats. They looked at the effect of injecting the drugs firstly at the feet and secondly further proximal along the sciatic nerve.  The animals had normal pain behaviour for the initial 5 minutes (keep in view for later) but after 15 minutes there was an analgesic effect seen in both cases which lasted approximately 2 hours.

The authors view on the implications of the study
The authors end the article saying “This strategy for blocking pain should be advantageous for generating pain-restricted local anaesthesia when preserving motor and autonomic responses and nonpainful sensations is desirable, such as in childbirth and some dental procedures, as well as in treating nociceptor-driven chronic pain such as postherpetic neuralgia”.

My personal views

1) What about the first 5 minutes where normal pain behaviour is seen in response to the capsaicin injection?
The authors mention a certain shortcoming of this strategy with regards to application to humans as an analgesic. This is due to the above mentioned fact that the initial 5 minutes after the injection of the combination of QX-314 and capsaicin do not differ from what is seen with capsaicin alone. Capsaicin is a sever pain stimuli and during those initial 5 to 15 minutes the patient would have to bear with this pain, something rather unacceptable. Personally, I do not really see this as much of a hindrance as a pre-treatment with perhaps even lidocaine prior to treatment with the combination should solve this issue. I do wonder about the inflammatory response that would follow, seeing as capsaicin is associated with a rather strong inflammatory response though not a particularly long one.

*Disclaimer: As you go on reading and see me talk about percentages of neurons expressing X or Y within population A or B, I really would like to say that you should treat such staments with scepticism. These are general statements based on small samples and as there is going to be differences between individuals, I personally to do not trust such data completely. Nevertheless take the basic idea that there are different populations and that TRPV1 expression is found in different populations.

2) If it only effects the TRPV1 positive cells, why is there such a strong effect on pain behaviour?

The TRPV1 positive neurons represent 25-40% of the entire DRG population mainly in small and medium diameter neurons. It should be noted that less than 40% of neurons in the DRG are not pain responsive and thus functionally speaking TRPV1 positive cells should represent for 40-60% of the nociceptor population. The majority of TRPV1 positive neurons are within the small diameter neuron population and within this population mainly within the peptidergic neurons rather than the IB4 positive neurons. In short, TRPV1 while not being restricted to a specific population of nociceptive neurons, only represents a certain proportion of nociceptive neurons. I find it quite impressive that silencing just this proportion of neurons can have such a drastic effect on pain sensation both mechanical and thermal.

3) Why blunt mechanical pain when TRPV1 is implicated in heat pain.
The mechanical effects especially intrigue me considering TRPV1 is generally considered to be a transducer of heat pain (though having loads of other potential roles). Admittedly there are a lot of polymodal nociceptors (nociceptors that respond to mechanical and heat stimuli) that would be silenced but there is definitely a population of mechanosensitive nociceptors that should not be affected by this drug combination. What does this result suggest regarding the role of these purely mechanosensitive nociceptors if the ability to sense mechanical pain is lost in response to silencing the TRPV1 positive neurons?

4) What would we see in terms of pain perception in humans?
I am particularly intrigued to see what this drug does towards human pain perception. The evidence above talk about its role in sensing pain by measuring pain reflexes in rats. Admittedly we cannot interview rats and find out what this drug combination does with regards to how they perceive pain. Nevertheless I can imagine that this strategy for treatment may be utilised in humans sometime in the future and I am left wondering what aspect of pain sensation would be lost. Would it just be the quality of the sensation being painful lost? Or would it be a complete loss of sensation of noxious heat and mechanical stimuli. In other words would these people feel something but not think it is painful or would they not feel anything whatsoever?

What are your thoughts on this particular paper or on what I have mentioned about it? Any criticisms or thoughts to add regarding this paper or my thoughts of it. Please do share.

Click here for the official press release article from the Harvard medical school. 

[Julian Neumann]

Hey Anand,
really interesting
I found the mechanism pretty cool and useful for other stuff, as I´m currently thinking about
how to get PHF-tau into and out of neurons, and how to trigger certain pathways of phosphorylation.
Do you know other examples, where such mechanisms are used?

How would the naked mole rat react?

¹Acid transduction by sensory neurons is observed in birds, amphibians, and fish, which suggests that this tranduction mechanism has been selectively disabled in the naked mole-rat in the course of its evolution. In contrast, nociceptors do respond vigorously to capsaicin, and we also show that sensory neurons express a transient receptor potential vanilloid channel-1 ion channel that is capsaicin sensitive. Nevertheless, the activation of capsaicin-sensitive sensory neurons in naked mole-rats does not produce pain-related behavior.

I thought that mechanical pain was triggered by differences in ion concencentrions (mainly K⁺) and mediators as PGE₂ that are a result of cell-death - hmm while I´m writing this, I ask myself if it´s really possible to have enough dying cells for the pain that´s caused by a kick against the shin!?
So what actually are the molecular mechanisms of mechanical pain perception!?

Cheers,
Julian

[Anand]

Hey,

To be honest its the first time I have come across such a method of getting a drug into a cell. Apparently what helps is the fact that the TRPV1 is such a large ion channel though. I think they mention there would be only one other channel in the body where the lidocaine deravtive would be able to enter through. The only thing I can think of really virus vectors maybe???

About the naked mole rat... Well the TRPV1 of the naked mole rat is still capsaicin sensitive even though they are behavioural insensitive. So essentially you would still be able to get lidocaine into these TRPV1 positive fibres. As for its behavioural effect though I am not certain as one of the explanations given by the authors of the paper was that differing circuitry int he naked mole rat is what resulted in this insensitivity to capsaicin. So if these fibres through their circuitry are silenced would silencing them with lidocaine actually have an effect?? I do not know. But it would be an interesting experiment maybe.

About the molecular mechanism of mechanical pain. Honestly I do not beleive that it has been truly identified. There are candidates but nothing concrete has been really truly found. It is a rather hard question to ask really as you can patch directly at the nerves where mechanical stimuli are transduced. Patching from the DRG's only gives you information regardng the spiking on the cell.

To find signal transducers I would say you need to firstly via cell cultures, study the candidate of interest to see if they actually do respond to the stimuli. Then you have to confirm that the KO animal would be behaviourally insensitive. Following these two steps you could be quite confident I guess. The problem is loads of candidates meet the first criteria but not the second.

For example TRPV1 via cell cultures is pretty convincingly linked to thermoception but the KO animal shows no behavioural deficits to acutely sensing pain. It does however not develop thermal hyperalgesia.

Its a hard question to ask.

Anand

[Anand]

A few weeks ago, Prof. Clifford Woolf gave a lecture at seminar where he was presenting data on this experiment, initially going through results already published in the paper and then results of follow up studies.

Firstly he answered the question of whether such a combination treatment would be feasible for use in human being... the answer being no as when tested with large mammals the ínitial pain observed in response to capsaicin would be unbearable for the analgesic effect seen later to be worth while. As a result he suggested that either a new lidocain derivative with faster effect would be needed, a non pungent activator of capsaicin would be required or that an ion channel other than TRPV1 be used for entry of the drug.

He went on to mention two branches of this idea 1) the use of other ion channels for entry of QX-314 and 2) the use of potential non pungent activators of TRPV1. His lab has found that other TRP channels are capable of letting QX-314 into the axon which provides an interesting tool to dissect the role of different primary afferent populations.

The main part of his talk was regarding the use of non pungent activators of TRPV1. In a recent publication it was shown that TRPV1 could be activated by lidocaine itself. This means that lidocaine with its anaesthetic properties could be used to deliver QX-314 into TRPV1 postive cells causing local analgesia. He presented some of the data he has regarding this idea and indeed lidocaine is effective in opening TRPV1 channels so that QX-314 could enter leading to a short lasting local anaesthesia follwed by a long lasting local analgesia.

There is a issue that needs to be resolved with regards to this delivery system. The issue is that of how would QX-314 be eliminated. Lidocaine is eliminated as it is able to diffuse across the membrane but as QX-314 is unable to do so, the question is then whether it would accumulate in the axon on the neuron and whether this poses a potential toxicity issue.