Table 1.

General and pain-specific factors influencing the properties and peculiarities of botulinum toxin type A (BoNT/A) action.

General Factors not Specific to Pain
Property of BoNT/A Molecule or Peculiarity of Action	Mechanisms of Action	Contribution to Desirable Pharmacological Properties	Attempted or Potential Improvement	References
Low local diffusion after application	rapid and high affinity binding to neuronal membrane at the injected site	safety, onset of the effect	lower injection volume, intradermal injections	[12,13,14]
Absorption through epithelial barriers	crossing epithelial barriers by transcytosis	application by different routes (e.g., transmucosal)	novel therapeutic systems with incorporated BoNT/A which might improve toxin absorption and extend the contact time with the epithelial tissue/mucosa	[15]
Specificity for hyperactive neurons	Expression of membrane acceptors such as glycosylated SV2C; higher rate of SVs exo/endocytosis favors toxin uptake	safety, selectivity for hyperactive nerve terminals	recombinant chimeras with different neuronal specificities	[16]
Protease specific targeting of SNARE proteins	synaptic localization, disturbance of SNARE supercomplex	potency, safety	- specific point mutations for higher affinity to SNAP-25	[17]
			- recombinant molecules with shorter action or different affinity for SNARE proteins
Protease longevity inside neurons	cellular localization, avoidance of proteasomal degradation	long duration of effects	specific chimeras that change the affinity for intraneuronal degradation system	[18]
Reversibility of the neuroparalysis	recovery of neuronal exocytosis is dependent on nerve terminal type (gain of function)	long duration of effect (from 3 months to more than a year)	interference with the nerve function recovery processes	[19]
Repeatability of neuroparalysis	recovery of neuronal exocytosis can be repeated many times without loss of neuron function	application schedule	repeated application for prolonged period into the same site	[20]
Factors Specifically Related to Pain
Property of BoNT/A Molecule or Peculiarity of Action	Mechanisms of Action	Contribution to Desirable Pharmacological Properties	Attempted or Potential Improvement	References
Selectivity for certain sensory neuron populations	occurrence in TRPV1-expressing neurons	selectivity for chronic or prolonged pain	recombinant chymeras with different receptor specificities	[21,22,23]
	effect on glutamatergic transmission	efficacy in chronic pain (possibly LTP related)
	effect on peptidergic transmitters	efficacy in chronic pain and migraine
Segmental activity in the sensory nucleus/spinal cord dorsal horn	microtubule-dependent neuronal axonal transport	localization of toxin effect	neural block for segmental treatment	[24,25,26]
Interaction with other pain neurotransmitter system	Interaction with endogenous opioid system	synergism with opioid analgesic and avoidance of tolerance development; efficacy in opioid-overused patients	combined use of lower dose opioids and BoNT/A	[27,28,29]
	Restoration of sensitivity to morphine