BOOK EXCERPT
Cosmetic and Clinical Applications of Botox and Dermal Fillers, Second Edition
William J. Lipham MD, FACS

CHAPTER 2
What Is Botulinum Toxin and How Does It Work?
William J. Lipham, MD, FACS

Physical Properties

There are 7 distinct strains of Clostridium botulinum that have been identified. Each strain is characterized by the type of botulinum neurotoxin that it is capable of producing and has been classified as type A, B, C, D, E, F, or G.¹ While all of these neurotoxins inhibit the release of ACh at the myoneural junction, they all vary in their chemical structure and size as well as their mechanism of action within the nerve terminal itself. Five of these subtypes (A, B, E, F, G) affect the human nervous system, while 2 subtypes (C and D) do not. Types A and B are the 2 most clinically relevant subtypes and, therefore, are commercially produced. Botulinum toxin type-A is felt to exert the most powerful neuromuscular blockade and is also capable of exerting its effect for the longest duration of time.² In contrast, botulinum toxin type-E and type-F are also capable of blocking myoneural transmission, but they have a shorter duration of action when compared to types A and B and, therefore, are not commercially produced.

Both botulinum toxin type-A and type-B are composed of a 150 kD polypeptide consisting of a disulfide bond-linked light chain and heavy chain.³ These disulfide-linked molecules are associated with other non-neurotoxin proteins during their synthesis to form a neurotoxin complex, which is approximately 500 kD in size (Figure 2-1). These non-neurotoxin accessory proteins may serve a beneficial role in stabilizing the fragile botulinum toxin molecule when it is reconstituted.

Figure 2-1. The botulinum toxin molecule consists of a light chain and heavy chain joined by a single disulfide bond. While the heavy chain is responsible for binding to the nerve terminal receptors, the light chain exerts its effect by preventing the release of ACh from the nerve terminal. Figure 2-2. The heavy chain of the botulinum toxin molecule binds selectively to cell membrane receptors on the outer surface of the nerve terminal. (Used by permission © 2003 Allergan, Inc.) Figure 2-3. The entire neurotoxin complex is then internalized into the motor nerve terminal through receptor-mediated endocytosis. (Used by permission © 2003 Allergan, Inc.)

Mechanism of Action

At the neuromuscular junction, the motor nerve terminal lies in close apposition with the adjacent muscle fiber. When botulinum toxin is administered, the heavy chain binds selectively to cell membrane receptors on the outer surface of the presynaptic nerve terminal (Figure 2-2). The entire neurotoxin complex (both light and heavy chains) is then internalized into the nerve terminal via receptor-mediated endocytosis (Figure 2-3). The vesicles containing the botulinum toxin then fuse with digestive vacuoles that cleave the botulinum toxin molecule into separate light and heavy chains.^4,5

The light chain exerts the paralytic effect of botulinum toxin by inactivating a group of proteins that are responsible for the fusion of vesicles containing the neurotransmitter ACh with the nerve cell membrane and thereby blocking the release of ACh into the neuromuscular junction. This group of proteins is referred to as the SNARE complex (soluble N-ethylmalemide-sensitive factor attachment protein receptor), a neural exocytic complex that regulates the membrane docking and fusion of synaptic vesicles and the release of ACh.⁶

Each botulinum toxin serotype acts upon the SNARE complex. Serotypes A, C, and E cleave the synaptic neural-associated protein (SNAP-25) molecule, while serotypes B, D, F, and G cleave synaptobrevin or vesicle-associated membrane protein (VAMP), each at a distinct site (Figure 2-4). In each case, botulinum toxin enzymatically inactivates a specific protein that is required for the docking and fusion of vesicles containing ACh into the neuromuscular junction.⁷ The inhibition of ACh release results in localized muscle weakness (paralysis) that gradually reverses over time. The mechanism by which botulinum toxin-induced muscle weakness is reversed is unknown, but it may involve the intraneural turnover of the affected docking proteins (responsible for the release of ACh into the neuromuscular junction), the sprouting of new nerve terminals, or a combination of both of these mechanisms.⁸

Figure 2-4. The light chain exerts its effect by cleaving the synaptic neural-associated protein (SNAP-25) that is responsible for fusion of vesicles containing ACh with the nerve terminal cell membrane. (Used by permission © 2003 Allergan, Inc.)

The axon begins to expand approximately 2 months after administration of botulinum toxin, and new nerve terminal sprouts emerge and extend toward the muscle surface.⁹ The motor nerve unit is re-established once a new sprout forms a physical synaptic connection with the previous neuromuscular junction. The new nerve sprouts that do not establish a connection to the motor endplate, however, subsequently regress and are spontaneously eliminated while the parent, or former, nerve terminal is re-established (Figure 2-5).¹⁰

An understanding of the mechanism of action of botulinum toxin allows one to understand the time required for the onset of paralysis as well as the duration of clinical effect. Botulinum toxin, once injected, takes approximately 3 to 4 days for its effect to become clinically apparent. This corresponds to the amount of time that is required for the botulinum toxin molecule to bind to the motor nerve terminal, undergo internalization via receptor-mediated endocytosis, and block ACh release through inactivation of the SNAP-25 or VAMP proteins.

In contrast, the clinical duration of effect, which is approximately 3 to 4 months in length, corresponds to the time that is required for new sprouts to grow from the nerve root to re-establish the motor endplate. Therefore, the duration of effect is not dependent on the continued presence of botulinum toxin at the nerve terminal, but rather reflects the length of time that it takes for a particular individual’s nerves to regenerate and develop a functional connection at the myoneural junction.¹¹

Figure 2-5. Approximately 2 months after injection, the nerve terminal begins to expand and new sprouts emerge and extend toward the muscle surface. Additional redundant nerve sprouts are also produced. The motor nerve unit is re-established once a new sprout forms a physical synaptic connection with the previous neuromuscular junction. (Used by permission © 2003 Allergan, Inc.)

Contraindications and Precautions

The only contraindication to the administration of botulinum toxin includes neuromuscular disease such as myasthenia gravis or Eaton-Lambert syndrome, which can similarly potentiate the effects of botulinum toxin.¹² Similarly, botulinum toxin should not be administered to individuals with a cutaneous infection at the proposed site of injection. Since the effects of botulinum toxin on pregnancy in human subjects are unknown, administration during pregnancy or while breast feeding is not recommended. Similarly, botulinum toxin is not recommended for use in children. Finally, both compounds contain human albumin to stabilize the lyopholite; therefore, individuals with allergies to eggs should not receive botulinum toxin because there may be an increased risk for an anaphylactic reaction to the albumin proteins. Epinephrine should always be available if an anaphylactic reaction should occur.

Clinicians should obtain a complete list of current medications from patients undergoing botulinum toxin injections. Drug interactions can occur with botulinum toxin, and special care or postponement of treatment should be considered in patients taking aminoglycosides, which can potentiate the effect of botulinum toxin and produce a botulism-like clinical syndrome. In contrast, aminoquinolines may delay the onset of botulinum toxin.

Adverse Reactions

The most common adverse events in clinical trials of BOTOX included headache, respiratory infection, flu syndrome, blepharoptosis, and nausea.¹³ While weakness of the injected muscle is expected following botulinum toxin administration, inadvertent administration of larger doses to the effected area may induce severe paresis or paralysis of the muscle, which may cause problems for the patient. For example, when botulinum toxin is administered to the periocular region for the treatment of blepharospasm, the rate and strength of the blink response may be compromised or lagophthalmos may develop due to weakness of the orbicularis oculi muscle. This may result in corneal exposure, ad ocular irritation, and redness, which should be treated with aggressive lubrication of the ocular surface. Similarly, weakening of the orbicularis oris muscle may compromise an individual’s ability to purse his or her lips or maintain a symmetric smile.

Botulinum toxin may also inadvertently spread to involve adjacent muscles with a variety of untoward side effects. A thorough understanding of facial muscle anatomy and proper injection technique is the best way to avoid these problems. Transient upper eyelid ptosis is probably the most common example of this type of adverse event and may develop following inactivation of the orbicularis oculi muscle in blepharospasm or hemifacial spasm patients. The ptosis results from diffusion or inadvertent administration of botulinum toxin behind the orbital septum, which weakens the levator palpebrae superioris muscle. Less commonly, injection into the platysmal bands in the neck region may temporarily induce dysphonia or dysphagia.

If eyelid ptosis occurs, a single drop of apraclonidine 0.5% may be administered 3 times daily to temporarily stimulate Müller’s muscle, which elevates the eyelid, until the ptosis resolves. Similarly, over-the-counter preparations containing the vasoconstrictor naphazoline (eg, Naphcon-A® [Alcon Canada, Mississauga, Ontario], naphazoline hydrochloride) may demonstrate a beneficial effect. Patients should be warned, however, about the possibility of rebound vasodilation when vasoconstrictors are discontinued, resulting in slightly inflamed eyes.

References

1. Jankovic J. Botulinum A toxin in the treatment of blepharospasm. Adv Neurol. 1988;49:467-472.
2. Hankins CL, Strimling R, Rogers GS. Botulinum A toxin for glabellar wrinkles: dose and response. Dermatol Surg. 1998;24(11):1181-1183.
3. Lew MF. Review of the FDA-approved uses of botulinum toxins, including data suggesting efficacy in pain reduction. Clin J Pain. 2002;18(6 Suppl):S142-S146.
4. Pearce LB, First ER, MacCallum RD, Gupta A. Pharmacologic characterization of botulinum toxin for basic science and medicine. Toxicon. 1997;35(9):1373-1412.
5. Lowe NJ, Yamauchi PS, Lask GP, Patnaik R, Moore D. Botulinum toxins types A and B for brow furrows: preliminary experiences with type B toxin dosing. J Cosmet Laser Ther. 2002;4(1):15-18.
6. Setler P. The biochemistry of botulinum toxin type B. Neurology. 2000;55(Suppl 5):S22-S28.
7. Popoff MR, Marvaud JC, Raffestin S. [Mechanism of action and therapeutic uses of botulinum and tetanus neurotoxins]. Ann Pharm Fr. 2001;59(3):176-190.
8. Keller JE, Neale EA, Oyler G, et al. Persistence of botulinum neurotoxin action in cultured spinal cord cells. FEBS Lett. 1999;456(1):137-142.
9. Angaut-Petit D, Molgo J, Comello JX, et al. Terminal sprouting in mouse neuromuscular junctions poisoned with botulinum type-A toxin: morphological and electrophysiological features. Neuroscience. 1990;37(3):799-808.
10. Boni R, Kreyden OP, Burg G. Revival of the use of botulinum toxin: application in dermatology. Dermatology. 2000;200(4):287-291.
11. Edelstein C, Shorr N, Jacobs J, Balch K, Goldberg R. Oculoplastic experience with the cosmetic use of botulinum A exotoxin. Dermatol Surg. 1998;24(11):1208-1212.
12. Matarasso SL. Complications of botulinum A exotoxin for hyperfunctional lines. Dermatol Surg. 1998;24(11):1249-1254.
13. Carruthers JA, Lowe NJ, Menter MA, et al. A multicenter, double-blind, randomized, placebo-controlled study of the efficacy and safety of botulinum toxin type-A in the treatment of glabellar lines. J Am Acad Dermatol. 2002;46(6):840-849.