Inhibition of release of neurotransmitters from rat dorsal root ganglia by a novel conjugate of a Clostridium botulinum toxin A endopeptidase fragment and Erythrina cristagalli lectin

Engineering botulinum neurotoxin to extend therapeutic intervention

Clostridium botulinum neurotoxins (BoNTs) are effective therapeutics for a variety of neurological disorders, such as strabismus, blepharospam, hemificial spasm, and cervical dystonia, because of the toxin's tropism for neurons and specific cleavage of neuronal soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptors (SNARE) proteins. Modifying BoNT to bind nonneuronal cells has been attempted to extend therapeutic applications. However, prerequisite to develop nonneuronal therapies requires the retargeting the catalytic activity of BoNTs to nonneuronal SNARE isoforms. Here, we reported the engineering of a BoNT derivative that cleaves SNAP23, a nonneuronal SNARE protein. SNAP23 mediates vesicle-plasma membrane fusion processes, including secretion of airway mucus, antibody, insulin, gastric acids, and ions. This mutated BoNT/E light chain LC/E(K(224)D) showed extended substrate specificity to cleave SNAP23, and the natural substrate, SNAP25, but not SNAP29 or SNAP47. Upon direct protein delivery into cultured human epithelial cells, LC/E(K(224)D) cleaved endogenous SNAP23, which inhibited secretion of mucin and IL-8. These studies show the feasibility of genetically modifying LCs to target a nonneuronal SNARE protein that extends therapeutic potential for treatment of human hypersecretion diseases.

Engineered toxins: new therapeutics

Clostridial neurotoxins possess discrete structural domains with distinct pharmacological properties. Aspects of neurotoxin function with therapeutic potential include specific neuronal binding, intracellular (cytosolic) delivery of biologically active protein and inhibition of SNARE-mediated secretion. Understanding the structure function relationship of the neurotoxin protein enables the creation of recombinant proteins incorporating select domains of the neurotoxins to produce novel proteins with therapeutic potential in a range of clinical applications.

Botulinum neurotoxin devoid of receptor binding domain translocates active protease

Clostridium botulinum neurotoxin (BoNT) causes flaccid paralysis by disabling synaptic exocytosis. Intoxication requires the tri-modular protein to undergo conformational changes in response to pH and redox gradients across endosomes, leading to the formation of a protein-conducting channel. The approximately 50 kDa light chain (LC) protease is translocated into the cytosol by the approximately 100 kDa heavy chain (HC), which consists of two modules: the N-terminal translocation domain (TD) and the C-terminal Receptor Binding Domain (RBD). Here we exploited the BoNT modular design to identify the minimal requirements for channel activity and LC translocation in neurons. Using the combined detection of substrate proteolysis and single-channel currents, we showed that a di-modular protein consisting only of LC and TD was sufficient to translocate active protease into the cytosol of target cells. The RBD is dispensable for cell entry, channel activity, or LC translocation; however, it determined a pH threshold for channel formation. These findings indicate that, in addition to its individual functions, each module acts as a chaperone for the others, working in concert to achieve productive intoxication.

The use of botulinum neurotoxin type A (BoNTA) in urology

The use of Botulinum neurotoxin type A (BoNT/A) in the lower urinary tract was pioneered as early as 20 years ago with injections into the urethral sphincter reducing bladder voiding pressures, urethral pressures, and post-void residual urine. Over the past 9 years, the use of BoNT/A has revolutionised the treatment of intractable symptoms associated with the neurogenic or idiopathic overactive bladder, both in adults and children. The duration of clinical improvement is 6-11 months, is accompanied by significant amelioration of patients' quality of life and repeat bladder treatments appear to have sustained effects. Despite evidence for an effect on the afferent pathways, its mode of action in the human bladder remains largely unknown. The use of BoNT/A has also expanded into the painful bladder syndrome and in benign prostatic diseases, with promising preliminary results. This review aims to provide an insight of the use of BoNT/A in the lower urinary tract, addressing issues such as treatment outcomes and safety, mechanisms of action and potential for future research.

Resiniferatoxin and botulinum toxin type A for treatment of lower urinary tract symptoms

Resiniferatoxin (RTX) and botulinum toxin subtype A (BTX-A) are increasingly viewed as potential treatments for lower urinary tract symptoms (LUTS) refractory to conventional therapy. RTX, a capsaicin analogue devoid of severe pungent properties, acts by desensitizing the transient receptor potential vanilloid type 1 (TRPV1) receptor and inactivating C-fibers. BTX-A cleaves soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in afferent and efferent nerve endings, therefore impeding the fusion of synaptic vesicles with the neuronal membrane necessary for the release of neurotransmitters. In patients with neurogenic and idiopathic detrusor overactivity, RTX and BTX-A have been shown to increase the volume to first detrusor contraction, increase bladder capacity, and improve urinary incontinence and quality of life. Recent data also suggest a role for these neurotoxins in treating urgency, the primary symptom in overactive bladder (OAB) syndrome. Furthermore, experimental data strongly support the use of both neurotoxins in the treatment of pain and frequency in patients with interstitial cystitis/painful bladder syndrome (IC/PBS), although the results from available clinical trials for this use are still inconclusive. In spite of promising results overall, it should be made clear that the administration of these neurotoxins is still considered an experimental procedure and that more clinical studies are necessary before a license for their use will be issued by health authorities.

Botulinum toxin for pain

Botulinum toxin (BTX) injection is being increasingly used 'off label' in the management of chronic pain. Data support the hypothesis of a direct analgesic effect of BTX, different to that exerted on muscle. Although the pain-reducing effect of BTX is mainly due to its ability to block acetylcholine release at the synapse, other effects on the nervous system are also thought to be involved. BTX affects cholinergic transmission in both the somatic and the autonomic nervous systems. Proposed mechanisms of action of BTX for pain relief of trigger points, muscular spasms, fibromyalgia and myofascial pain include direct action on muscle and indirect effects via action at the neuromuscular junction. Invitro and invivo data have shown that BTX has specific antinociceptive activity relating to its effects on inflammation, axonal transport, ganglion inhibition, and spinal and suprasegmental level inhibition. Our review of the mechanisms of action, efficacy, administration techniques and therapeutic dosage of BTX for the management of chronic pain in a variety of conditions shows that although muscular tone and movement disorders remain the most important therapeutic applications for BTX, research suggests that BTX can also provide benefits related to effects on cholinergic control of the vascular system, autonomic function, and cholinergic control of nociceptive and antinociceptive systems. Furthermore, it appears that BTX may influence the peripheral and central nervous systems. The therapeutic potential of BTX depends mainly on the ability to deliver the toxin to the target structures, cholinergic or otherwise. Evidence suggests that BTX can be administered at standard dosages in pain disorders, where the objective is alteration of muscle tone. For conditions requiring an analgesic effect, the optimal therapeutic dosage of BTX remains to be defined.

Intraprostatic Botulinum Toxin Type A Injection in Patients Unfit for Surgery Presenting with Refractory Urinary Retention and Benign Prostatic Enlargement. Effect on Prostate Volume and Micturition Resumption

OBJECTIVES

To evaluate the effect of intraprostatic injection of botulinum toxin A (BoNTA) on prostate volume and refractory urinary retention in patients with benign prostatic enlargement.

METHODS

Twenty-one men with benign prostatic enlargement on chronic indwelling catheter for at least 3 mo who were not candidates for surgery because of poor general condition received 200 U BoNTA in the transition zone by transrectal approach under ultrasound guidance. Patients were reevaluated at 1 and 3 mo posttreatment.

RESULTS

Patients had a mean age of 80+/-2 yr. Injections were done without anaesthetic support as an outpatient procedure. No significant local effects occurred. Baseline prostate volume of 70+/-10 ml decreased to 57+/-10 ml (p<0.0006) at 1 mo and to 47+/-7 ml (p=0.03 against 1 mo) at 3 mo. At 1 mo, 16 patients (76%) could resume voiding with a mean Qmax of 9.0+/-1.2 ml/s. At 3 mo, 17 patients (81%) voided with a mean Qmax of 10.3+/-1.4 ml/s. Residual urine was 80+/-19 ml and 92+/-24 ml at the two time points, respectively. Mean serum total PSA decreased from 6.0+/-1.1 ng/ml at baseline to 5.0+/-0.9 ng/ml at 3 mo (p=0.04).

CONCLUSIONS

BoNTA injection into the prostate swiftly reduces prostate volume and may be a promising treatment for refractory urinary retention in patients with benign prostatic enlargement who are unfit for surgery. Future studies will determine the duration of BoNTA effect.

Future aspects of botulinum neurotoxins

The future of botulinum neurotoxin (BoNT) development is expected to proceed along two lines: the development of novel indications and the development of novel products. New indications will likely be based on the neuromuscular mode of action of BoNTs, as well as action on primary sensory fibers and other neuronal types. Novel BoNT products may be designed for increased specificity or enhanced duration. As new products enter the market, it will be important for each to demonstrate efficacy and safety. Unfortunately, the future of BoNTs will also likely include attempts to obtain and distribute unlicensed and illegal BoNT products that may pose substantial risks to patients.

Status on the use of botulinum toxin for headache disorders

PURPOSE OF REVIEW

Reports and studies on botulinum toxin A in headache treatment are increasing. The studies available from reference systems and published congress contributions on the prophylactic treatment of idiopathic and symptomatic headache with botulinum toxin were analyzed with respect to the study design, the headache diagnosis, and the significance of results.

RECENT FINDINGS

For the prophylactic treatment of tension-type headache and migraine, no sufficient positive evidence for a treatment with botulinum toxin A is obtained from randomized, double-blind, and placebo-controlled trials to date. For the treatment of chronic daily headache (including medication overuse headache), there is inconsistent positive evidence for subgroups (e.g. patients without other prophylactic treatment).

SUMMARY

The majority of double-blind and placebo-controlled studies do not confirm the assumption that botulinum toxin A is efficacious in the treatment of idiopathic headache disorders. It is possible that subgroups of patients with chronic daily headache benefit from a long-term treatment with this substance. Future clinical trials should focus on these defined patient groups.

Re-engineering the target specificity of Clostridial neurotoxins - a route to novel therapeutics

The ability to chemically couple proteins to LH(N)-fragments of clostridial neurotoxins and create novel molecules with selectivity for cells other than the natural target cell of the native neurotoxin is well established. Such molecules are able to inhibit exocytosis in the target cell and have the potential to be therapeutically beneficial where secretion from a particular cell plays a causative role in a disease or medical condition. To date, these molecules have been produced by chemical coupling of the LH(N)-fragment and the targeting ligand. This is, however, not a suitable basis for producing pharmaceutical agents as the products are ill defined, difficult to control and heterogeneous. Also, the molecules described to date have targeted neuroendocrine cells that are susceptible to native neurotoxins, and therefore the benefit of creating a molecule with a novel targeting domain has been limited. In this paper, the production of a fully recombinant fusion protein from a recombinant gene encoding both the LH(N)-domain of a clostridial neurotoxin and a specific targeting domain is described, together with the ability of such recombinant fusion proteins to inhibit secretion from non-neuronal target cells. Specifically, a novel protein consisting of the LH(N)-domains of botulinum neurotoxin type C and epidermal growth factor (EGF) that is able to inhibit secretion of mucus from epithelial cells is reported. Such a molecule has the potential to prevent mucus hypersecretion in asthma and chronic obstructive pulmonary disease.

Botulinum neurotoxin structure, engineering, and novel cellular trafficking and targeting

Botulinum neurotoxins are multifaceted molecules, which are truly unique not only in their mode of action, but also their utility as a drug carrier either across the gut wall or to the nerve terminals. The molecule is divided in clear functional domains that can operate independently. This feature can be used to employ them as cargo carrier by linking other drugs or vaccines with the binding and translocation domains of BoNT. While the domain structures are largely independent of each other, the dynamic structure of these domains, especially that of the enzymatic domain (L chain), is quite different from the reported crystal structures for several BoNT serotypes and their enzymatic domain. This review discusses the comparative structures of BoNT in crystal and solution for their relevance to the molecular mechanism of BoNT action, especially in view of our recent discovery that the enzymatically active structure of the BoNT exists as a molten-globule and that of the endopeptidase domain as a novel PRIME conformation. Finally, a non-exhaustive discussion has been included to explain the long-lasting biological effects of certain serotypes of BoNT, based on the current knowledge of the structure-function of different serotypes of botulinum neurotoxins.

Botulinum-A toxin injections into the detrusor muscle decrease nerve growth factor bladder tissue levels in patients with neurogenic detrusor overactivity

PURPOSE

We investigated the effects of BTX-A on visceral afferent nerve transmission by measuring bladder tissue NGF levels in patients with neurogenic detrusor overactivity before and after intravesical treatment with BTX-A. We also compared the bladder tissue NGF content with clinical and urodynamic data.

MATERIALS AND METHODS

A total of 23 patients underwent clinical evaluation and urodynamics with detection of the UDC threshold, maximum pressure and maximum cystometric capacity before, and at the 1 and 3-month followups. Endoscopic bladder wall biopsies were also obtained at the same time points. NGF levels were measured in tissue homogenate by enzyme-linked immunosorbent assay (Promega, Madison, Wisconsin).

RESULTS

At 1 and 3 months mean catheterization and incontinent episodes were significantly decreased (p <0.05 and <0.001, respectively). On urodynamics we detected a significant increase in the UDC threshold and maximum cystometric capacity, and a significant decrease in UDC maximum pressure at the 1 and 3-month follow-ups compared to baseline (each p <0.001). At the same time points we detected a significant decrease in NGF bladder tissue content (each p <0.02).

CONCLUSIONS

BTX-A intravesical treatment induces a state of NGF deprivation in bladder tissue that persists at least up to 3 months. As caused by BTX-A, the decrease in acetylcholine release at the presynaptic level may induce a decrease in detrusor contractility and in NGF production by the detrusor muscle. Alternatively BTX-A can decrease the bladder level of neurotransmitters that normally modulate NGF production and release.

[Potential antinociceptive mechanisms of botulinum toxin]

Botulinum toxin has been used in pain therapy for several years. Its application in migraine and headaches is particularly interesting. Clinical results have not yet been definitely conclusive, and a uniform model of the mode of action has not been established either. Apart from a purely muscular effect, a direct antinociceptive effect of botulinum toxin has been found in patients, in the preclinical model, and in a clinical pain model. This is contradicted by negative observations in the clinical model of pain, which might be related to methodological deficits. Further basics need to be worked out before arriving at any final result. Clinical studies with patients and pain models should then follow. Studying botulinum toxin within the context of pain will also provide many new insights into pain therapy in general. In which pain model botulinum toxin may play a role in the future, has to be awaited.

The effects of Botulinum Toxin type A on capsaicin-evoked pain, flare, and secondary hyperalgesia in an experimental human model of trigeminal sensitization

The trigeminovascular system is involved in migraine. Efficacy of Botulinum Toxin type A (BoNT-A) in migraine has been investigated in clinical studies but the mechanism of action remains unexplored. It is hypothesized that BoNT-A inhibits peripheral sensitization of nociceptive fibers and indirectly reduces central sensitization. We examined the effect of intramuscular injection of BoNT-A on an experimental human model of trigeminal sensitization induced by intradermal capsaicin injection to the forehead. BoNT-A (BOTOX) or saline was injected intramuscularly in precranial, neck and shoulder muscles to 32 healthy male volunteers in a double blind-randomized manner. Intradermally capsaicin-induced pain, flare and secondary hyperalgesia were obtained before and 1, 4 and 8 weeks after the above treatments. A significant suppressive effect of BoNT-A on pain, flare and hyperalgesia area was observed. The pain intensity area was significantly smaller in BoNT-A group (9.16+/-0.83 cm x s) compared to saline group (15.41+/-0.83cm x s) (P=0.011). The flare area was also reduced significantly in BoNT-A group (29.81+/-0.69 cm2) compared to saline group (39.71+/-0.69 cm2) (P<0.001). Similarly, the mean area of secondary hyperalgesia was significantly smaller in BoNT-A group (4.25+/-0.91 cm2) compared to saline group (7.03+/-0.91 cm2) (P=0.040). Post hoc analysis showed significant differences across the trials with a remarkable suppression effect of BoNT-A on capsaicin-induced sensory and vasomotor reactions as early as week1 (P<0.001). BoNT-A presented suppressive effects on the trigeminal/cervical nociceptive system activated by intradermal injection of capsaicin to the forehead. The effects are suggested to be caused by a local peripheral effect of BoNT-A on cutaneous nociceptors.

Botulinum A Toxin Intravesical Injections in the Treatment of Painful Bladder Syndrome: A Pilot Study

OBJECTIVE

We evaluated the efficacy and tolerability of botulinum A toxin (BTX-A) intravesical injections in patients affected by painful bladder syndrome with increased urinary frequency, refractory to conventional treatment modalities.

METHODS

Twelve women and two men were prospectively included in the study. Under short general anaesthesia patients were given injections of 200 U of commercially available BTX-A diluted in 20 ml 0.9% NaCl. Injections were performed submucosally in the trigone and bladder floor under cystoscopic control. Voiding chart, the Visual Analog Scale (VAS) for pain, and urodynamics were performed before treatment and 1 and 3 mo afterward.

RESULTS

Overall, 12 patients (85.7%) reported subjective improvement at 1 and 3 mo follow-up. The mean VAS score was significantly reduced at 1 and 3 mo after treatment (p<0.05 for both); at the same time points daytime and nighttime urinary frequency significantly decreased (p<0.01 and p<0.05, respectively), and bladder cystometric capacity significantly increased (p<0.01). Two patients reported incomplete bladder emptying. We did not detect any systemic side effects during or after treatment.

CONCLUSIONS

The results of this pilot study indicate that BTX-A intravesical injections are effective in the short-term management of painful bladder syndrome. By modulating afferent C-fiber activity within the bladder walls, BTX-A significantly improves urodynamic parameters and reduces bladder pain and urinary frequency.

Proposed mechanism for the efficacy of injected botulinum toxin in the treatment of human detrusor overactivity

BACKGROUND

Treatment of human bladder overactivity with intradetrusor Botulinum-A neurotoxin (BoNT/A) injections temporarily blocks the presynaptic release of acetylcholine from the parasympathetic innervation and produces a paralysis of the detrusor smooth muscle. The efficacy of the treatment exceeds that expected from simple detrusor muscle paralysis, however, and its effect of reducing urgency is greatly welcomed by patients.

OBJECTIVES

To examine whether BoNT/A has a complex effect on sensory mechanisms by inhibiting vesicular release of multiple excitatory neurotransmitters by urothelial and suburothelial nerves and reducing axonal expression of SNARE-complex dependent proteins.

METHODS

A literature review.

CONCLUSIONS

We propose that a primary peripheral effect of BoNT/A is the inhibition of release of acetylcholine, ATP, substance P, and reduction in the axonal expression of the capsaicin and purinergic receptors. This may be followed by central desensitization through a decrease in central uptake of substance P and neurotrophic factors. The summation of these effects is a profound and long-lasting inhibition of those afferent and efferent mechanisms that are thought to be the pathophysiological basis for DO.

A new wrinkle on pain relief: re-engineering clostridial neurotoxins for analgesics

Botulinum neurotoxins are used to treat of a range of chronic neuromuscular conditions and, increasingly, conditions involving non-neuromuscular transmission, both cholinergic and non-cholinergic, including chronic pain. However, their clinical use is limited by the potential for adverse effects related to the neuromuscular activity, which results from the selectivity of the toxin for the neuromuscular junction. The elucidation of the structure of the botulinum toxin molecule and its relationship to neurotoxin function has enabled the design of novel molecules incorporating selected aspects of toxin function. This review considers the suitability of engineered neurotoxins as the basis for novel therapeutic proteins and the opportunity of developing analgesics based on these neurotoxins.

Preparation of specifically activatable endopeptidase derivatives of Clostridium botulinum toxins type A, B, and C and their applications

Clostridium botulinum neurotoxins are potently toxic proteins of 150 kDa with specific endopeptidase activity for SNARE proteins involved in vesicle docking and release. Following treatment with trypsin, a fragment of botulinum neurotoxin serotype A that lacks the C-terminal domain responsible for neuronal cell binding, but retains full catalytic activity, can be obtained. Known as the LH(N) fragment, we report the development of a recombinant expression and purification scheme for the isolation of comparable fragments of neurotoxin serotypes B and C. Expressed as maltose-binding protein fusions, both have specific proteolytic sites present between the fusion tag and the light chain to facilitate removal of the fusion, and between the light chain endopeptidase and the H(N) translocation domains to facilitate activation of the single polypeptide. We have also used this approach to prepare a new variant of LH(N)/A with a specific activation site that avoids the need to use trypsin. All three LH(N)s are enzymatically active and are of low toxicity. The production of specifically activatable LH(N)/A, LH(N)/B, and LH(N)/C extends the opportunities for exploitation of neurotoxin fragments. The potential utility of these fragments is discussed.

Botulinum neurotoxin type D enables cytosolic delivery of enzymatically active cargo proteins to neurones via unfolded translocation intermediates

Multi-domain bacterial protein toxins are being explored as potential carriers for targeted delivery of biomolecules. Previous approaches employing isolated receptor binding subunits disallow entry into the cytosol. Strategies in which catalytic domains are replaced with cargo molecules are presumably inefficient due to co-operation of domains during the endosomal translocation step. Here, we characterize a novel transport vehicle in which cargo proteins are attached to the amino terminus of the full-length botulinum neurotoxin type D (BoNT/D). The intrinsic enzymatic activity of the neurotoxin allowed quantification of the efficacy of cargo delivery to the cytosol. Dihydrofolate reductase and BoNT type A (BoNT/A) light chain (LC) were efficiently conveyed into the cytosol, whereas attachment of firefly luciferase or green fluorescent protein drastically reduced the toxicity. Luciferase and BoNT/A LC retained their catalytic activity as evidenced by luciferin conversion or SNAP-25 hydrolysis in the cytosol of synaptosomes, respectively. Conformationally stabilized dihydrofolate reductase as cargo considerably decreased the toxicity indicative for the requirement of partial unfolding of cargo protein and catalytic domain as prerequisite for efficient translocation across the endosomal membrane. Thus, enzymatically inactive clostridial neurotoxins may serve as effective, safe carriers for delivering proteins in functionally active form to the cytosol of neurones.

Novel application of an in vitro technique to the detection and quantification of botulinum neurotoxin antibodies

Detection of Clostridium botulinum neurotoxin (BoNT) neutralising antibodies is currently achieved using the mouse lethality assay (MLA). This technique has provided the majority of the data for vaccine development and, with the increasing use of BoNT as a therapeutic agent, the MLA is the assay of choice to evaluate 'non-responder' antisera. However, the MLA is semi-quantitative and has an animal consumption rate that raises ethical concerns. The development of an alternative is therefore desirable. Here, we describe an in vitro neuronal release assay that may represent such an alternative in terms of both its sensitivity and ability to produce quantitative data. Initially recognised in the course of assessing a novel vaccine candidate, the suitability of this assay has been further explored using an International standard. The results support the conclusion that the detection of neutralising antibodies in human sera should be attempted using this method.

Retargeted clostridial endopeptidases: inhibition of nociceptive neurotransmitter release in vitro, and antinociceptive activity in in vivo models of pain

Clostridial neurotoxins potently and specifically inhibit neurotransmitter release in defined cell types. Previously reported data have demonstrated that the catalytically active LH(N) endopeptidase fragment of botulinum neurotoxin type A (termed LH(N)/A) can be retargeted to a range of cell types in vitro to lead to inhibition of secretion of a range of transmitters. Here, we report the synthesis of endopeptidase conjugates with in vitro selectivity for nociceptive afferents compared to spinal neurons. Chemical conjugates prepared between Erythrina cristagalli lectin and LH(N)/A are assessed in vitro and in in vivo models of pain. Chemical conjugates prepared between E. cristagalli lectin and either natively sourced LH(N)/A, or recombinant LH(N)/A purified from Escherichia coli are assessed, and equivalence of the recombinant material is demonstrated. The duration of action of inhibition of neurotransmitter release by the conjugate in vitro is also assessed and is comparable to that observed with Clostridium botulinum neurotoxin. Selectivity of targeting and therapeutic potential have been confirmed by in vivo electrophysiology studies. Furthermore, the analgesic properties of the conjugate have been assessed in in vivo models of pain and extended duration effects observed. These data provide proof of principle for the concept of retargeted clostridial endopeptidases as novel analgesics.

Botulinum toxin and the management of chronic headaches

PURPOSE OF REVIEW

There is an increasing number of reports on botulinum toxin in pain therapy, in particular in headache treatment. Therefore, the studies available from reference systems and published congress contributions on the prophylactic treatment of idiopathic and cervicogenic headache with botulinum toxin were analyzed with respect to study design, headache diagnosis, and the significance of results.

RECENT FINDINGS

For the prophylactic treatment of tension-type headache, migraine, and cervicogenic headache, no sufficient positive evidence for treatment with botulinum toxin is obtained from randomized, double-blind, placebo-controlled trials to date. For the treatment of miscellaneous headache, there is some but no consistent positive evidence.

SUMMARY

Most open studies and case reports suggest an efficacy of botulinum toxin in headache prophylaxis but double-blind, placebo-controlled studies do not confirm this assumption. Larger controlled studies are needed for a definite evaluation of subgroups that might possibly benefit from such a treatment. Migraine, tension-type headache, and cervicogenic headache cannot be regarded as a general indication for a treatment with botulinum toxin.

Regulation of calcitonin gene-related peptide secretion from trigeminal nerve cells by botulinum toxin type A: implications for migraine therapy

OBJECTIVES

To determine the effect of botulinum toxin type A on calcitonin gene-related peptide secretion from cultured trigeminal ganglia neurons.

BACKGROUND

The ability of botulinum toxins to cause muscle paralysis by blocking acetylcholine release at the neuromuscular junction is well known. Previous studies and clinical observations have failed to demonstrate sensory changes related to botulinum toxins or the disease of botulism. Recent studies, however, have suggested that botulinum toxin type A injected into pericranial muscles may have a prophylactic benefit in migraine. This observation has renewed the debate of a mechanism of sensory inhibition mediated by botulinum toxin type A.

METHODS

Primary cultures of rat trigeminal ganglia were utilized to determine whether botulinum toxin type A could directly decrease the release of calcitonin gene-related peptide, a neuropeptide involved in the underlying pathophysiology of migraine. Untreated cultures or cultures stimulated with a depolarizing stimulus (potassium chloride) or capsaicin, an agent known to activate sensory C fibers, were treated for 3, 6, or 24 hours with clinically effective doses of botulinum toxin type A or a control vehicle. The amount of calcitonin gene-related peptide secreted into the culture media following the various treatments was determined using a specific radioimmunoassay.

RESULTS

A high percentage (greater than 90%) of the trigeminal ganglia neurons present in 1- to 3-day-old cultures was shown to express calcitonin gene-related peptide. Treatment with depolarizing stimuli (potassium chloride), a mixture of inflammatory agents, or capsaicin caused a marked increase (4- to 5-fold) in calcitonin gene-related peptide released from the trigeminal neurons. Interestingly, overnight treatment of trigeminal ganglia cultures with therapeutic concentrations of botulinum toxin type A (1.6 or 3.1 units) did not affect the amount of calcitonin gene-related peptide released from these neurons. The stimulated release of calcitonin gene-related peptide following chemical depolarization with potassium chloride or activation with capsaicin, however, was greatly repressed by the botulinum toxin, but not by the control vehicle. A similar inhibitory effect of overnight treatment with botulinum toxin type A was observed with 1.6 and 3.1 units. These concentrations of botulinum toxin type A are well within or below the range of tissue concentration easily achieved with a local injection. Incubation of the cultures with toxin for 24, 6, or even 3 hours was very effective at repressing stimulated calcitonin gene-related peptide secretion when compared to control values.

CONCLUSIONS

These data provide the first evidence that botulinum toxin type A can directly decrease the amount of calcitonin gene-related peptide released from trigeminal neurons. The results suggest that the effectiveness of botulinum toxin type A in the treatment of migraine may be due, in part, to its ability to repress calcitonin gene-related peptide release from activated sensory neurons.

Detection of antibodies against botulinum toxins

After immunisation with botulinum vaccine, antibodies to multiple epitopes are produced. Only some of these will have the capacity to neutralise the toxin activity. In fact, the ability of toxoid vaccine to induce toxin neutralising antibodies has provided the basis for the use of therapeutic antitoxins and immunoglobulins for the prophylaxis and treatment of diseases caused by bacterial toxins. Increasing indications for the chronic use of botulinum toxin for therapy have inevitably resulted in concern for patients becoming unresponsive because of the presence of circulating toxin-specific antibodies. Highly sensitive and relevant assays to detect only clinically relevant toxin neutralising antibodies are essential. Although immunoassays often provide the sensitivity, their relevance and specificity is often questioned. The mouse protection LD(50) bioassay is considered most relevant but can often only detect 10 mIU/ml of antitoxin. This sensitivity, although sufficient for confirming protective immunity, is inadequate for patients undergoing toxin therapy. An intramuscular paralysis assay improves the sensitivity to ca. 1 mIU/ml, and a mouse ex vivo diaphragm assay, with sensitivity of < 0.5 mIU/ml, is the most sensitive functional assay to date for this purpose. Alternative approaches for the detection of antibodies to botulinum toxin have included in vitro endopeptidase activity neutralisation. Unlike any other functional assay, this approach is not reliant on serotype-specific antibodies for specificity. Most recent promising developments are focused on cellular assays utilising primary rat embryonic cord cells or more conveniently in vitro differentiated established cell lines such as human neuroblastoma cells.

Isolation of the gene and large-scale expression and purification of recombinant Erythrina cristagalli lectin

Using polymerase chain reaction, the coding sequence for Erythrina cristagalli lectin (ECL) has been cloned and expressed in Escherichia coli. The amplified DNA sequence of ECL is highly homologous to that previously reported for Erythrina corallodendron lectin (ECorL), confirming the absence of introns in the ECL gene. The polypeptide sequences of ECL and ECorL have been compared and five amino acids have been identified that differentiate the two proteins. Recombinant E. cristagalli lectin (recECL) was expressed in E. coli from a genomic clone encoding the mature E. cristagalli lectin gene. Constitutive expression localised recombinant protein in inclusion bodies, which were solubilised, and recECL, subsequently refolded and purified by lactose affinity chromatography. Significant advantages were observed for purification from inclusion bodies rather than from a clone optimised to express soluble protein. A large-scale purification scheme has been developed that can prepare functional recECL from inclusion bodies with a yield of 870 mg/l culture. By the range of characterisation methods employed in this study, it has been demonstrated that recECL is functionally equivalent to native ECL obtained from the E. cristagalli plant. In addition, characterisation of the binding of radiolabelled recECL to cultured dorsal root ganglia demonstrated that recECL binds to a single pool of receptors.

Botulinum toxin for the treatment of musculoskeletal pain and spasm

The impressive pain relief experienced by sufferers of dystonia and spasticity from intramuscular injections of botulinum toxin suggested that patients with other chronic, musculoskeletal pain conditions also may benefit. However, there have been relatively few placebo-controlled studies of botulinum toxin in such non-neurologic conditions as myofascial pain syndrome, chronic neck and low back pain, and fibromyalgia; the results of these studies have not been impressive. One explanation for the lack of positive findings may be the lack of clinically evident muscle spasms (overactivity), despite the presence of muscle tenderness, tightness, or trigger points. Clinical observations of pain relief from injections of botulinum toxin for dystonia and spasticity and its apparent efficacy in treating migraine suggest an anti-nociceptive action independent of its neuromuscular junction-blocking action. Evidence from animal experiments supports this notion, and other data provide plausible physiologic mechanisms in the periphery and central nervous systems. These involve modulation of the activity of the neurotransmitters glutamate, substance P, calcitonin gene-related peptide, enkephalins, and others. However, even if botulinum toxin is firmly established as an analgesic, there is insufficient clinical evidence of its efficacy in treating non-neurologic, chronic, musculoskeletal pain conditions.