Introduction

The world of botulinum toxin has changed dramatically in the 30 years of our involvement. In 1982 the only available product was called ‘Oculinum’ and was available only from Alan Scott, MD, at the Smith-Kettlewell Institute for Visual Sciences in San Francisco. The dramatic concept of using ‘the most deadly poison’ to treat muscle disorders had, of course, already been published in Readers’ Digest and was soon to be published in the National Enquirer and other popular publications. At that time the indication for use was the treatment of strabismus although dystonia, especially blepharospasm, and hemifacial spasm were soon added to the list. Many other indications have since followed especially after the FDA’s first approval in the USA in 1989.

When discussing the future of this dramatic molecule it is hard to stay ahead of reality and common usage. We shall endeavor to tread the fine line between documenting what has been shown scientifically and wild speculation. However, the field is expanding so rapidly that this will be difficult.

One of the problems with discussing the clinical use of botulinum toxin is that the regulatory pathway and the rigorous scientific investigation of the efficacy and safety of new indications have consistently lagged behind off-label clinical use. We were using BoNT-A to treat glabellar lines more than 10 years before the regulatory studies began and 15 years before US FDA approval for glabellar lines. So it is not easy to keep track of the many areas where BoNT-A is being used and sort out those areas where its use is a major therapeutic advance. Some years ago we were in communication with a physician who was using aerosolized BoNT-A to treat asthma, but have heard nothing of this indication since. The same is true for the treatment of allergic rhinitis. By comparison, the use of BoNT-A to treat detrusor overactivity in individuals with paraplegia or other neurological problems appeared as an indication very early in the clinical development of BoNT-A. Urologists have subsequently gone on to use the toxin for overactive bladder in females and benign prostatic hypertrophy in males, and both of these indications should be approved by the FDA in the near future.

This short chapter will be speculative and opinionated. In addition, it may well be proven wrong! But we are close to the expanding body of knowledge about BoNT and hope that we shall be able to suggest at least some of the important areas of future knowledge and clinical use.

Newer indications

As the understanding of the mechanism of action of BoNT expands and as the number of clinicians exposed to the toxin increases, there have been great extensions in the clinical use of the toxin. For example, the use by physicians of the toxin for cosmetic purposes led to recognition that some of the treated individuals experienced improvement in headache and migraine. This led to a wider understanding of the mechanism of action of BoNT and subsequently use of the toxin to treat a range of conditions associated with pain. The recognition that BoNT treatment of hyperhidrosis would shrink sweat glands has led other physicians to treat other glands, thus improving benign prostatic hypertrophy and sialorrhea.

The range of conditions involving abnormal muscle function, pain, and excessive glandular function that is now being treated is extremely wide and is expected to expand further. Some of these are relatively specialized. The conditions that dermatologists recognize as being worsened by excessive sweating, such as benign familial pemphigus, can be improved by BoNT-A injection into the area. This is important, providing relief for those individuals but not being as important in sheer numerical terms as, for example headache and migraine.

Topical versus injectable

We had long presumed that the large size of the BoNT-A molecule would prevent it from passing through intact skin. That assumption has been proven incorrect and the effectiveness of topically applied BoNT-A has now been established (see Ch. 10). This will have important implications beyond the current clinical development both in axillary hyperhidrosis and in lateral orbital wrinkles and lines (crow’s feet). We would expect that the advantage of a needleless approach will encourage many more individuals to have excessive sweating treated, especially that of the palms and soles, face, and scalp. Also, those individuals who have normal levels of sweating but who choose not to sweat in these areas can be safely and effectively treated.

There has been discussion about the injection of tiny doses of BoNT-A in the face with multiple injections (‘MesoBotox’) and arguments as to whether or not this is effective. The ability to be able to apply BoNT-A topically to the face will answer these questions and we shall know what effect to expect on sebaceous glands, flushing, telangiectasias, rosacea, etc. So far the clinical development program has experimented with different concentrations of toxin in order to produce the desired effect. What will become available to clinicians will be a single concentration and we shall have to work out how long to apply the gel in different areas and for different indications. It might be anticipated that the application time in the cheeks for flushing / rosacea would be less than in the lateral orbital area, but we do not know and it will be an exciting new era in botulinum toxin research when the topical product becomes available to clinicians.

Changes in the toxin molecule

Changing both the target and the duration of action of the toxin have long been dreams of clinicians and basic scientists involved in toxin research. Some of this has been attainable for many years. For example, when the effectiveness of BoNT-A was established, resistance to the toxin also occurred, because clinicians did not understand the immunogenicity of the molecule and the original preparations available contained amounts of ineffective botulinum toxin, increasing the treated individual’s exposure to the antigenic epitopes. For Botox^® this changed dramatically in 1997 with the change from original Botox^® to current Botox^® with approximately 20% of the immunogenic potential. However, the existence of patients who were clinically resistant to BoNT-A stimulated examination of other serotypes. As a result, BoNT-B (rimabotulinumtoxinB, Myobloc^®) was approved for the treatment of cervical dystonia in the US almost coincidentally with Botox^®. BoNT-B has a shorter duration than BoNT-A. Some preliminary work was done with BoNT-F, but the duration of effect proved to be much too short. BoNT-E has a similarly short duration of effect.

Short duration of effect is not necessarily a bad thing. For example, it is possible to imagine many conditions such as post-surgery states or post-traumatic conditions, where short duration of effect would actually be advantageous. It might be necessary to rest an area for 2 weeks but not beyond that time. An individual will tolerate having weakness on one side of the face for 2 weeks but not for 3 months. Some of the post-traumatic indications appear to respond so rapidly to BoNT-A that a 2-week duration of effect might be all that is needed. We hope that this potential area of research will get the attention it deserves at some time in the near future.

The other direction for duration of effect is to increase it. This is obviously desirable and many of our patients would be delighted if the BoNT-A effect could be increased by 50% or more. There is some preliminary evidence by Stone and colleagues that this may be possible. These authors were exploring the action of the peptide-added BoNT-A molecule that is the active ingredient of the Revance Therapeutics topical preparation. They injected the gastrocnemius muscle in mice with various doses of the Revance product (RT002) and Botox^®. When they looked at the tibialis anterior muscle of the injected mice, there was a significant degree of spread of effect in the Botox^®-injected group but less in the RT002-injected group. When they attempted to work out the dose that produced approximately equal degrees of spread to tibialis anterior the respective doses were 2 U/kg of Botox^® and 5 U/kg of RT002. When the authors went back to look at the effect on gastrocnemius they observed an approximate doubling in the duration of effect in the RT002-injected animals. This is very preliminary evidence, but it does suggest that it might be possible to lengthen the duration of effect of BoNT-A.

The next part of the story is even more speculative, probably owing to the commercial pressures involved. The neurotoxin molecule consists of three parts: the binding domain, the translocation domain, and the endoprotease. It has long been speculated that changing one of these domains to change the target or effect would produce interesting results. For example, adding the BoNT-A endopeptidase to different heavy chains would change the target of the enzyme but not its intracellular action; it appears that this concept may be coming into more practical use. Allergan has announced that it is collaborating with a company (Syntaxin) that has created engineered forms of BoNT using the endopeptidase portion of the toxin in fusion with various targeting moieties which allow them to selectively target a specific cell type and then block exocytotic release of desired mediators (e.g. neurotransmitters involved in nociception, excessive growth hormone secretion for acromegaly, etc.). This would have a dramatic effect on the clinical use of molecules derived from the neurotoxin!

Central effects of the neurotoxin

In 2008, Antonucci and co-workers reported what appeared to be central effects of BoNT-A resulting from peripheral injection. Great concern was expressed about the validity of their results, but they have subsequently expanded on this original work and presented data to back it up at the Toxins 2011 meeting in Santa Fe, NM.

An early report by Blood and colleagues, which suggested an intracranial effect of BoNT-A, came from the Department of Neurology, Massachusetts General Hospital. These researchers used diffusion tensor imaging to screen for white matter abnormalities in regions between the basal ganglia and the thalamus in cervical and hand dystonia patients. All patients exhibited an abnormal hemispheric asymmetry in a focal region between the pallidum and the thalamus. This asymmetry was absent 4 weeks after the same patients were treated with intramuscular botulinum toxin injections. In other words, a peripheral injection of botulinum toxin produced an observable change inside the brain.

The report from Pisa by Antonucci and co-workers demonstrated that catalytically active BoNT-A is retrogradely transported by central neurons and motoneurons and is then transcytosed to afferent synapses, in which it cleaves SNAP-25. SNAP-25 cleavage by BoNT-A was observed in the contralateral hemisphere after unilateral BoNT-A delivery to the hippocampus. Appearance of cleaved SNAP-25 resulted in blockade of hippocampal activity in the untreated hemisphere. These findings reveal a novel pathway of BoNT-A trafficking in neurons and have important implications for the clinical uses of this neurotoxin.

Subsequent to the Pisa reports other authors have reported similar findings. Dr Ed Chapman from the University of Wisconsin reported at Toxins 2011 a new hypothesis in which BoNT-A (and other serotypes) are trafficked along neuronal processes away from the initial site of injection in the periphery to exert many of their clinical effects by entering neurons in the central nervous system! Lawrence and colleagues from Dublin and Allergan, using compartmented cultures of rat sympathetic neurons, demonstrated cleaved SNAP-25 through intraneuronal migration of BoNT-A. However, they pointed out that ‘neither spontaneous nor evoked transmission to cell bodies were inhibited by retrogradely-migrated BoNT-A, concurring with the lack of evidence for a central action when used clinically’.

This and other evidence points to the central effect of BoNT-A. What is the significance of this? When we look at new evidence suggesting that BoNT-A gets into the brain it is easy to be concerned about the effects of such central transcytosis. However, we prefer to bear in mind the more than 30 years of clinical, safe use in humans when thinking on this evidence. We presume that this accumulating evidence helps us to better understand the mechanism of action of BoNT and not that it is inherently worrying. This understanding should help us to use the neurotoxins more safely and to the greater benefit of our patients.

Depression

Some indications for the clinical use of BoNT-A rapidly become obvious because of both the safety and efficacy of the indication. Treatment of the glabella is a good example. In other indications, such as migraine, it is more difficult to establish efficacy. The treatment of depression comes into this latter category. The problem is the presence of a significant degree of placebo effect. Despite the considerable amount of research done on the use of BoNT-A for migraine, there are still clinicians (e.g. Solomon) who believe this to be a placebo effect.

The effect of glabellar BoNT-A injections on depression was first reported by dermatologists Finzi & Wasserman who described improvement in mood in 10 women after glabella BoNT-A. A further paper by Lewis & Bowler demonstrated improvement in depression after glabella B0NT-A but not after other cosmetic procedures. The problem with this research is the difficulty in truly controlling the placebo effect. It is a fact that individuals who have had their glabella treated with BoNT-A know whether they have received active drug or placebo within a few days. A properly controlled study of the effect of BoNT-A on depression will be a challenge. It may be that the effect is a manifestation of the facial feedback hypothesis where it is postulated by Alam and colleagues that individuals who cannot frown do not look grumpy so others interacting with them will respond in a more positive manner, improving the mood of the treated individual.

Conclusion

BoNT-A is without doubt an amazing molecule. Each time we think we understand its actions and its diverse clinical effects we are illuminated by yet another action and interaction. The molecular modifications will permit even more specific targets to be treated.

The recent explosion of new neurotoxin research – both clinical and basic science – heralds another generation of exciting advances. There has never been a more carefully researched molecule and the future is bright indeed. We are still just in neuromodulator kindergarten.