Neosaxitoxin

NSTX, and other PSTs, are produced by several species of marine dinoflagellates (eukaryotes) and freshwater cyanobacteria, blue-green algae (prokaryotes), which can form extensive blooms around the world.{{cite journal |vauthors=Wiese M, D'Agostino PM, Mihali TK, Moffitt MC, Neilan BA |title=Neurotoxic Alkaloids: Saxitoxin and Its Analogs |journal=Marine Drugs |date=July 2010 |volume=8 |issue=7 |pages=2185–2211 |pmid=20714432 |doi=10.3390/md8072185 |pmc=2920551|doi-access=free }} Under special conditions, during harmful algal blooms (HAB) or red tide, all these toxins may build up in filter-feeding shellfish, such as mussels, clams and oysters, and can produce an outbreak of Paralytic Shellfish Poisoning (PSP).Centers for Disease Control and Prevention (CDC). CDC's Laboratory Response to Toxins (accessed: May 8, 2012) [https://www.cdc.gov/biomonitoring/pdf/Toxins_Fact_Sheet.pdf]

Saxitoxin analogues associated to PSP can be divided into three categories:{{cite journal |author=Wang DZ |title=Neurotoxins from Marine Dinoflagellates: A Brief Review |journal=Marine Drugs |date=March 2008 |volume=6 |issue=3 |pages=349–71 |doi=10.3390/md6020349|pmid=18728731 |pmc=2525493 |doi-access=free }}

• Carbamate compounds, including saxitoxin, neosaxitoxin and gonyautoxins 1–4.
• N-sulfocarbamoyl compounds, including C and B toxins.
• Decarbamoyl compounds with respect to the presence or absence of 1-N-hydroxyl, 11-hydroxysulfate, and 21-N-sulfocarbamoyl substitutions as well as epimerization at the C-11 position.

NSTX is quite similar to saxitoxin, like all the neurotoxins associated to PSP, the only difference is that NSTX shows one hydroxyl group bonded to nitrogen "1", where saxitoxyn contains one hydrogen.{{cite journal |vauthors=Hu SL, Kao CY |title=Interactions of neosaxitoxin with the sodium channel of the frog skeletal muscle fiber |journal=The Journal of General Physiology |date=March 1991 |volume=97 |issue=3 |pages=561–78 |pmid=1645395 |pmc=2216488 |doi=10.1085/jgp.97.3.561}}

This purine is highly hydrophilic{{cite journal |author=Etheridge SM |title=Paralytic shellfish poisoning: seafood safety and human health perspectives |journal=Toxicon |date=August 2010 |volume=56 |issue=2 |pages=108–22 |pmid=20035780 |doi=10.1016/j.toxicon.2009.12.013|bibcode=2010Txcn...56..108E |url=https://zenodo.org/record/1259399 }} and thermostable, it is not destroyed by cooking.{{cite journal |vauthors=Lawrence JF, Maher M, Watson-Wright W |title=Effect of cooking on the concentration of toxins associated with paralytic shellfish poison in lobster hepatopancreas |journal=Toxicon |date=January 1994 |volume=32 |issue=1 |pages=57–64 |pmid=9237337 |doi=10.1016/0041-0101(94)90021-3|bibcode=1994Txcn...32...57L }} Moreover, is very stable in usual storage, specially in acidic condition.{{cite journal |vauthors=Alfonso A, Louzao MC, Vieytes MR, Botana LM |title=Comparative study of the stability of saxitoxin and neosaxitoxin in acidic solutions and lyophilized samples |journal=Toxicon |date=December 1994 |volume=32 |issue=12 |pages=1593–8 |pmid=7725328 |doi=10.1016/0041-0101(94)90318-2 |bibcode=1994Txcn...32.1593A }}

NSTX blocks the extracellular portion,{{cite journal |vauthors=Tarnawa I, Bölcskei H, Kocsis P |title=Blockers of Voltage-Gated Sodium Channels for the Treatment of Central Nervous System Diseases |journal=Recent Patents on CNS Drug Discovery |date=January 2007 |volume=2 |issue=1 |pages=57–78 |pmid=18221218 |doi=10.2174/157488907779561754}} the outer vestibule,{{cite journal |vauthors=Penzotti JL, Lipkind G, Fozzard HA, ((Dudley SC Jr)) |title=Specific neosaxitoxin interactions with the Na⁺ channel outer vestibule determined by mutant cycle analysis |journal=Biophysical Journal |date=February 2001 |volume=80 |issue=2 |pages=698–706 |pmid=11159437 |doi=10.1016/S0006-3495(01)76049-3 |pmc=1301268|bibcode=2001BpJ....80..698P }} of some voltage gated sodium channels in a very powerful and reversible manner, without affection of other ion channels.

"Voltage-gated", "voltage-sensitive" and "voltage-dependent" sodium channels - also known as "VGSCs" or "NaV" ("Na_v") channels" - are crucial elements of normal physiology in a variety of animals, including flies, leeches, squid and jellyfish, as well as mammalian and non-mammalian vertebrates. This large integral membrane protein plays an essential role in the initiation and propagation of action potentials in neurons, myocytes and other excitable cells.{{cite journal |author=Catterall WA. |title=Voltage-Gated Sodium Channels at 60: Structure, Function, and Pathophysiology |journal=The Journal of Physiology |date=April 2012 |pmid=22473783 |doi=10.1113/jphysiol.2011.224204 |volume=590 |issue=Pt 11 |pages=2577–89|pmc=3424717 }}

NaV channels form the basis of electrical excitability in animal cells. Like many other neuronal channels and receptors, NaV channels pre-date neurons; having evolved from Ca²⁺ channels - likely permeable to Na⁺ and Ca²⁺ - present in the common ancestor of choanoflagellates and animals. Invertebrates possess two NaV channels - Na_v1 and Na_v2 - while vertebrates only possess Na_v1 family channels.{{cite journal |author=Zakon H. |title=Adaptive evolution of voltage-gated sodium channels: the first 800 million years |journal=Proceedings of the National Academy of Sciences of the United States of America |date=June 2012 |pmid=22723361 |doi=10.1073/pnas.1201884109 |volume=109 |issue=Suppl 1 |pages=10619–25 |pmc=3386883|bibcode=2012PNAS..10910619Z |doi-access=free }}

Sodium-channel proteins in the mammalian brain comprise one alpha subunit and one or more auxiliary beta subunits. Nine types of alpha subunits, Na_v1.1 to Na_v1.9, have been described, and a tenth isoform, Na_x, is suspected to perform some NaV-channel-like activity.{{cite journal |vauthors=Yu FH, Catterall WA |title=Overview of the voltage-gated sodium channel family |journal=Genome Biology |date=July 2003 |volume=4 |issue=3 |pages=207 |pmid=12620097 |doi=10.1186/gb-2003-4-3-207 |pmc=153452 |doi-access=free }} Between five{{cite journal |vauthors=Fainzilber M, Kofman O, Zlotkin E, Gordon D |title=A new neurotoxin receptor site on sodium channels is identified by a conotoxin that affects sodium channel inactivation in molluscs and acts as an antagonist in rat brain |journal=Recent Patents on CNS Drug Discovery |date=January 1994 |volume=269 |issue=4 |pages=2574–80 |pmid=8300586 |access-date=May 9, 2012 |url=http://www.jbc.org/content/269/4/2574.full.pdf }} and six{{cite journal |vauthors=Wang J, Yarov-Yarovoy V, Kahn R, Gordon D, Gurevitz M, Scheuer T, Catterall WA |title=Mapping the receptor site for alpha-scorpion toxins on a Na⁺ channel voltage sensor |journal=PNAS |date=September 2011 |issue=37 |pages=15426–31 |pmid=21876146 |doi=10.1073/pnas.1112320108 |volume=108 |pmc=3174582|bibcode=2011PNAS..10815426W |doi-access=free }} neurotoxin receptor sites have been recognised between the seven receptor sites {{cite journal |vauthors=Catterall WA, Goldin AL, Waxman SG |title=International Union of Pharmacology. XLVII. Nomenclature and structure-function relationships of voltage-gated sodium channels |journal=Pharmacological Reviews |date=December 2005 |issue=4 |pages=397–409 |pmid=16382098 |doi=10.1124/pr.57.4.4 |volume=57|s2cid=7332624 }} in vertebrate sodium channel receptor alpha subunits:

• Site 1 binds the sodium channel blockers tetrodotoxin and saxitoxin.
• Site 2 binds lipid-soluble sodium channel activators such as veratridine.
• Site 3 binds alpha-scorpion and sea anemone toxins, which slow sodium channel inactivation.
• Site 4 binds beta-scorpion toxins, which affect sodium channel activation.
• Site 5 binds the polyether ladder brevetoxins and ciguatoxin.
• Site 6 binds delta-conotoxin.
• Local anesthetic receptor site binds local anesthetics, antiarrhythmic drugs and antiepileptic drugs

NSTX and other site 1 blockers have high affinity (very low dissociation constant) and high specificity for NaV channels. The action of NSTX produces minimal effect on cardiac NaV channels, exhibiting around 20–60 fold less affinity than NaV channels in skeletal muscle and the brain of rats.{{cite journal |vauthors=Guo XT, Uehara A, Ravindran A, Bryant SH, Hall S, Moczydlowski E |title=Kinetic basis for insensitivity to tetrodotoxin and saxitoxin in sodium channels of canine heart and denervated rat skeletal muscle |journal=Biochemistry |date=December 1987 |volume=26 |issue=24 |pages=7546–56 |pmid=2447944 |doi=10.1021/bi00398a003}} Most data emphasize the role of "STX resistant" NaV channel 1.5 in human heart muscles.{{cite journal |vauthors=Lowe JS, Palygin O, Bhasin N, Hund TJ, Boyden PA, Shibata E, Anderson ME, Mohler PJ |title=Voltage-gated Nav channel targeting in the heart requires an ankyrin-G dependent cellular pathway |journal=The Journal of Cell Biology |date=January 2008 |issue=1 |pages=173–86 |pmid=18180363 |doi=10.1083/jcb.200710107 |volume=180 |pmc=2213608}}{{cite journal |author=Abriel H |title=Cardiac Sodium Channel Na_v1.5 Mechanosensitivity is Inhibited by Ranolazine |journal=Circulation |date=May 7, 2012 |volume=125 |issue=22 |pages=2681–3 |pmid=22565937 |doi=10.1161/CIRCULATIONAHA.112.110908|doi-access=free }}

Toxins such as neosaxitoxin and tetrodotoxin have a lower affinity for most cardiac Na_v channels than nerve tissue Na_v channels. Moreover, the affinity of NSTX for nerve Na_v channels exhibits a potency roughly a million-fold that of lidocaine.{{cite journal |author=Butterworth JF 4th |title=Will conventional local anesthetics soon be replaced by neurotoxins? |journal=Regional Anesthesia and Pain Medicine |date=March–April 2011 |volume=36 |issue=2 |pages=101–2 |pmid=21326065 |doi=10.1097/AAP.0b013e31820db23e|s2cid=33189422 }}

This mechanism of action can produce two well known kinds of effects in humans:

It can be approximately described using one of the classical model of neurotoxic disease, known from ancient times as red tide, the most harmful algal bloom (HAB). This well known clinical model is the "paralytic shellfish poisoning".FAO FOOD AND NUTRITION PAPER 80. Food and Agriculture Organization of the United Nations. Chapter 2. Paralytic Shellfish Poisoning (PSP). Rome, 2004(accessed: May 6, 2012)[ftp://ftp.fao.org/docrep/fao/007/y5486e/y5486e02.pdf]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}}

Of course, there are great differences between different algal blooms,{{cite journal |vauthors=Salfate O, Vazquez J, Galván J, Sánchez A, Nazar A |journal=Salud Pública de México |title=Intoxicaciones por toxina paralizante de molusco en Oaxaca |date=May–June 1991 |volume=33 |issue=3 |pages=240–7 |language=es |access-date=May 10, 2012 |url=https://www.redalyc.org/pdf/106/10633306.pdf }}{{cite journal |author=Anderson DM |journal=Scientific American |title=Red tides |date=August 1994 |volume=271 |issue=2 |pages=62–8 |pmid=8066432 |doi=10.1038/scientificamerican0894-62|bibcode=1994SciAm.271b..62A }} (accessed: May 9, 2012) [http://www.whoi.edu/cms/files/Anderson_1994_SciAm-redtides_31132.pdf]{{cite book |title=Toxic Cyanobacteria in Water: A guide to their public health consequences, monitoring and management |year=1999 |publisher=World Health Organization |isbn=0-419-23930-8 |chapter-url=https://www.who.int/water_sanitation_health/resourcesquality/toxicyanbact/en/index.html |vauthors=Sivonen K, Jones G |veditors=Chorus I, Bartram J |access-date=10 May 2012 |archive-url=https://web.archive.org/web/20130330145224/http://www.who.int/water_sanitation_health/resourcesquality/toxicyanbact/en/index.html |archive-date=30 March 2013 |chapter=3. Cyanobacterial Toxins |url-status=bot: unknown }} [https://www.who.int/entity/water_sanitation_health/resourcesquality/toxcyanobacteria.pdf]{{cite journal |vauthors=Glibert PM, Anderson DM, Gentien P, Granéli E, Sellner KG |title=The Global, Complex Phenomena of Harmful Algal Blooms |journal=Oceanography |date=June 2005 |volume=18 |issue=2 |pages=136–47 |doi=10.5670/oceanog.2005.49|doi-access=free |hdl=1912/2790 |hdl-access=free }}(accessed: May 11, 2012) [http://www.tos.org/oceanography/issues/issue_archive/issue_pdfs/18_2/18.2_glibert1_et_al.pdf] {{Webarchive|url=https://web.archive.org/web/20060510115011/http://tos.org/oceanography/issues/issue_archive/issue_pdfs/18_2/18.2_glibert1_et_al.pdf |date=2006-05-10 }} because of the mix of species included in each HAB, usually related to environmental conditions;{{cite journal |vauthors=Hayashi M, Yanagi T |journal=Marine Pollution Bulletin |title=Analysis of change of red tide species in Yodo River estuary by the numerical ecosystem model |year=2008 |volume=57 |issue=1–5 |pages=103–7 |pmid=18513758 |doi=10.1016/j.marpolbul.2008.04.015 |bibcode=2008MarPB..57..103H |s2cid=21200491 |url=http://www.lib.kobe-u.ac.jp/repository/90000799.pdf |access-date=2019-12-11 |archive-date=2018-11-03 |archive-url=https://web.archive.org/web/20181103170112/http://www.lib.kobe-u.ac.jp/repository/90000799.pdf |url-status=dead }} because of the levels and quality of PSTs produced in each HAB, that may be modulated by concurrent microorganism;{{cite journal |vauthors=Gallacher S, Flynn KJ, Franco JM, Brueggemann EE, Hines HB |title=Evidence for production of paralytic shellfish toxins by bacteria associated with Alexandrium spp. (Dinophyta) in culture |journal=Applied and Environmental Microbiology |date=January 1997 |volume=63 |issue=1 |pages=239–45 |doi=10.1128/AEM.63.1.239-245.1997 |pmid=9065273 |pmc=168316|bibcode=1997ApEnM..63..239G }}{{cite journal |vauthors=Grzebyk D, Denardou A, Berland B, Pouchus YF |journal=Journal of Plankton Research |title=Evidence of a new toxin in the red-tide dinoflagellate Prorocentrum minimum |date=August 1997 |volume=19 |issue=8 |pages=1111–24 |doi=10.1093/plankt/19.8.1111|doi-access=free }}{{cite journal |vauthors=Zheng TL, Su JQ, Maskaoui K, Yu ZM, Hu Z, Xu JS, Hong HS |journal=Marine Pollution Bulletin |title=Microbial modulation in the biomass and toxin production of a red-tide causing alga |year=2005 |volume=51 |issue=8–12 |pages=1018–25 |pmid=16291201 |doi=10.1016/j.marpolbul.2005.02.039|bibcode=2005MarPB..51.1018Z }}{{cite journal |vauthors=Alonso Rodríguez R, Ochoa JL, Uribe Alcocer M |journal=Revista Latinoamericana de Microbiología |title=Grazing of heterotrophic dinoflagellate Noctiluca scintillans (Mcartney) Kofoid on Gymnodinium catenatum Graham |date=January–June 2005 |volume=47 |issue=1–2 |pages=6–10 |pmid=17061541}} and, last but not least, because of the specific properties of each kind of PST, for example:

• Brevetoxins are lipid-soluble (hydrophobic) polyether marine toxins; their predominant effect is excitatory (blocked by tetrodotoxin), mediated by the enhancement of cellular Na⁺ influx; and bind to site 5 on Nav (like ciguatoxin).{{cite journal |author=Baden DG |title=Brevetoxins: unique polyether dinoflagellate toxins |journal=The FASEB Journal |date=May 1989 |volume=3 |issue=7 |pages=1807–17 |pmid=2565840 |doi=10.1096/fasebj.3.7.2565840|doi-access=free |s2cid=44847897 }}
• Tetrodotoxin (TTX) toxicity is associated with marked and surprising cardiovascular effects (i.e.: hypotension and bradycardia).{{cite journal |vauthors=How CK, Chern CH, Huang YC, Wang LM, Lee CH |title=Tetrodotoxin poisoning |journal=The American Journal of Emergency Medicine |date=January 2003 |volume=21 |issue=1 |pages=51–4 |pmid=12563582 |doi=10.1053/ajem.2003.50008}} Those effects are unexpected because of notorious TTX-resistance observed in vertebrate cardiac Nav channel. Moreover, this characteristic of the mammalian cardiac Nav channel is attributed to the cardiac predominance of the TTX-resistant Nav channel isoform (Na_v1.5).{{cite journal |vauthors=Vornanen M, Hassinen M, Haverinen J |title=Tetrodotoxin sensitivity of the vertebrate cardiac Na⁺ current |journal=Marine Drugs |year=2011 |volume=9 |issue=11 |pages=2409–22 |pmid=22163193 |doi=10.3390/md9112409 |pmc=3229242|doi-access=free }} On the contrary, as presumed on physiologic basis, NSTX produces just mild and transient cardiovascular abnormalities during experimental intoxication (there are no data on pure NSTX clinical toxicity).
• STX has two positive charges, in contrast to TTX's single charge and GTX2/3, a naturally occurring STX congener with a net +1 charge. In view of their rather different structures, it is not surprising that STX and TTX bind in a different fashion to VGSCs. In fact, when Phe 385 near the selectivity filter of Na_v1.2 is mutated to Cys, the channel's affinity for TTX is reduced 3,000-fold, whereas that for STX is reduced (only) 340-fold.{{cite journal |vauthors=Zhang MM, Gruszczynski P, Walewska A, Bulaj G, Olivera BM, Yoshikami D |title=Cooccupancy of the outer vestibule of voltage-gated sodium channels by micro-conotoxin KIIIA and saxitoxin or tetrodotoxin |journal= Journal of Neurophysiology|date=July 2010 |volume=104 |issue=1 |pages=88–97 |pmid=20410356 |doi=10.1152/jn.00145.2010 |pmc=2904204}}
• There are very limited data on the relative potency of different PSTs, and developing alternative methods to animal bioassays for marine-toxin detection is an urgent need.{{cite journal |vauthors=Perez S, Vale C, Botana AM, Alonso E, Vieytes MR, Botana LM |title=Determination of toxicity equivalent factors for paralytic shellfish toxins by electrophysiological measurements in cultured neurons |journal=Chemical Research in Toxicology |date=July 2011 |volume=24 |issue=7 |pages=1153–7 |pmid=21619049 |doi=10.1021/tx200173d}}

In spite of its heterogeneous and poorly understood epidemiology, the clinical picture of PSP could be useful to anticipate clinical effects of systemic NSTX.

• In the most frequent and benign situation, the patient suffers just mild, short-lived paresthesias of the mouth or extremities.
• In moderate cases perioral tingling progressing to numbness spreading to face and neck can be observed.Woods Hole Oceanographic Institution. Paralytic Shellfish Poisoning. Fleming LE. Last updated: May 7, 2008 (accessed: May 8, 2012)[http://www.whoi.edu/redtide/page.do?pid=9679&tid=523&cid=27690]
• In severe cases, patient can suffer apnea secondary to motor block, requiring mechanical ventilation.{{cite journal |vauthors=Yuen CW, Ng MH |title=Respiratory distress after consumption of sea snails |journal=Hong Kong Journal of Emergency Medicine |date=April 2002 |volume=9 |issue=3 |pages=159–61 |access-date=May 6, 2012 |url=http://www.hkcem.com/html/publications/Journal/2002-3/159-161.pdf|doi=10.1177/102490790200900308 |s2cid=115428542 |doi-access=free }}

Usually, the victims of mild and severe acute intoxications eliminate the toxin in urine during the first 24 hours after ingestion, and improve to full recovery in the first day of intrahospital care (when vital support is provided in a timely manner).{{cite journal |author=Centers for Disease Control and Prevention (CDC) |title=Paralytic Shellfish Poisoning Southeast Alaska, May–June 2011 |journal=Morbidity and Mortality Weekly Report |date=December 2011 |volume=60 |issue=45 |pages=1554–56|pmid=22089968 }}(accessed: May 8, 2012)[http://www.medscape.com/viewarticle/754163]

When outbreaks of PSP occur in remote locations, where medical assistance is limited, reported lethality is under 10% in adults, but can reach 50% in children younger than six years old. This difference could be secondary to dissimilar doses and composition of involved mixes of PSTs; delay in medical support; or some kind of susceptibility of children.{{cite journal |vauthors=Rodrigue DC, Etzel RA, Hall S, de Porras E, Velasquez OH, Tauxe RV, Kilbourne EM, Blake PA |title=Lethal paralytic shellfish poisoning in Guatemala |journal=American Journal of Tropical Medicine and Hygiene |date=March 1990 |volume=42 |issue=3 |pages=267–71 |pmid=2316796|doi=10.4269/ajtmh.1990.42.267 }} More recent information suggest that lethality could be around 1% of symptomatic patients,{{cite journal |vauthors=Cheng HS, Chua SO, Hung JS, Yip KK |title=Creatine kinase MB elevation in paralytic shellfish poisoning |journal=Chest |date=April 1991 |volume=99 |issue=4 |pages=1032–3 |pmid=2009759 |doi=10.1378/chest.99.4.1032 }} including cases where air transportation was required from remote locations of Alaska.{{cite journal |vauthors=Gessner BD, Middaugh JP |title=Paralytic shellfish poisoning in Alaska: a 20-year retrospective analysis |journal=American Journal of Epidemiology |date=April 1995 |volume=141 |issue=8 |pages=766–70 |pmid=7709919|doi=10.1093/oxfordjournals.aje.a117499 }}

Electrophysiologic observations demonstrated sub clinical abnormalities lasting for some days{{cite journal |vauthors=Long RR, Sargent JC, Hammer K |title=Paralytic shellfish poisoning. A case report and serial electrophysiologic observations |journal=Neurology |date=August 1990 |volume=40 |issue=8 |pages=1310–2 |pmid=2381544 |doi=10.1212/wnl.40.8.1310|s2cid=33896985 }} or weeks{{cite journal |vauthors=de Carvalho M, Jacinto J, Ramos N, de Oliveira V, Pinho e Melo T, de Sá J |title=Paralytic Paralytic shellfish poisoning: clinical and electrophysiological observations |journal=Journal of Neurology |date=August 1998 |volume=245 |issue=8 |pages=551–4 |pmid=9747920 |doi=10.1007/s004150050241|s2cid=29180723 }} after clinical recovery .

Some evidence suggest the presence of metabolic pathways for the sequential oxidation and glucuronidation of PST in vitro, both being the initial detoxication reactions for the excretion of these toxins in humans.{{cite journal |vauthors=García C, Barriga A, Díaz JC, Lagos M, Lagos N |title=Paralytic Route of metabolization and detoxication of paralyticshellfishtoxins in humans |journal=Toxicon |date=January 2010 |volume=55 |issue=1 |pages=135–44 |pmid=19632259 |doi=10.1016/j.toxicon.2009.07.018|bibcode=2010Txcn...55..135G |hdl=10533/141436 |hdl-access=free }}

Forensic analysis of fatalities after severe cases, conclude that PSP toxins are metabolically transformed by humans and that they are removed from the body by excretion in the urine and feces like any other xenobiotic compound.{{cite journal |vauthors=García C, del Carmen Bravo M, Lagos M, Lagos N |title=Paralytic shellfish poisoning: post-mortem analysis of tissue and body fluid samples from human victims in the Patagonia fjords |journal=Toxicon |date=February 2004 |volume=43 |issue=2 |pages=149–58 |pmid=15019474 |doi=10.1016/j.toxicon.2003.11.018|bibcode=2004Txcn...43..149G |hdl=10533/175125 |hdl-access=free }}

Considering the heterogeneous nature of toxins mixes contained in contaminated bivalve molluscs, the safe limit of toxin content in shellfish adequate for human ingestion is expressed in "saxitoxin equivalents". According to the Food and Agriculture Organization of the United Nations (FAO) and European Parliament, this limit is 80 microgram of saxitoxin equivalent per 100 gram of mussel meat (each mussel weights around 23 g).FAO Food and Nutrition Paper 80. Food and Agriculture Organization of the United Nations. Chapter 8. Risk Assessment. Rome, 2004. (accessed: May 6, 2012) [ftp://ftp.fao.org/docrep/fao/007/y5486e/y5486e08.pdf]{{dead link|date=May 2025|bot=medic}}{{cbignore|bot=medic}}{{cite journal |author=The European Parliament and the Council of the European Union |title=Regulation (EC) N° 853/2004 of the European Parliament and of the Council of 29 April 2004 laying down specific hygiene rules for food of animal origin |journal=Off J Eur Comm |date=April 2004 |volume=139 |page=61}} (accessed: May 6, 2012)[http://www.fsai.ie/uploadedFiles/Reg853_2004(1).pdf] The U.S. Food and Drug Administration extends the same definition to "fish" quality, but the term "fish" refers to fresh or saltwater fin fish, crustaceans, other forms of aquatic animal life other than birds or mammals, and all mollusks; and incorporate the use of "ppm" as another measure for saxitoxin equivalent concentration in mentioned foods.U.S. Food and Drug Administration. Fish and Fishery Products Hazards and Controls Guidance, Fourth Edition, November 2011. Chapter 6: Natural Toxins (p. 99–112). (accessed: May 6, 2012) [https://web.archive.org/web/20110429192526/http://www.fda.gov/downloads/Food/GuidanceComplianceRegulatoryInformation/GuidanceDocuments/Seafood/UCM251970.pdf] [https://web.archive.org/web/20110429192701/http://www.fda.gov/downloads/Food/GuidanceComplianceRegulatoryInformation/GuidanceDocuments/Seafood/FishandFisheriesProductsHazardsandControlsGuide/UCM252395.pdf]

Paradoxically, the chronic and/or repeated exposure to marine seafood toxins, which is a much more realistic phenomenon, has not been fully examined.Woods Hole Oceanographic Institution. Marine Biotoxins and Harmful Algae: A National Plan. Chapter II. THE TOXINS. Risk Assessment. (accessed: May 8, 2012)[http://www.whoi.edu/science/B/redtide/nationplan/s-kplan/toxins.html]{{cite journal |vauthors=Batoréu MC, Dias E, Pereira P, Franca S |title=Risk of human exposure to paralytic toxins of algal origin |journal=Environmental Toxicology and Pharmacology |date=May 2005 |volume=19 |issue=3 |pages=401–6 |pmid=21783504 |doi=10.1016/j.etap.2004.12.002|bibcode=2005EnvTP..19..401B }} One study in rats exposed to chronic (12 weeks) NSTX administration demonstrated some reduction in water and food intake, and a mild degree of transient cholestasis, probably associated to fasting, without other abnormalities.{{cite journal |vauthors=Zepeda RJ, Candiracci M, Lobos N, Lux S, Miranda HF |title=Chronic Toxicity Study of Neosaxitoxin in Rats |journal=Marine Drugs |date=September 2014 |volume=12 |issue=9 |pages=5055–71 |pmid=25257789 |doi=10.3390/md12095055 |pmc=4178483|doi-access=free }}

This action has been demonstrated in animals{{cite journal |vauthors=Kohane DS, Lu NT, Gökgöl-Kline AC, Shubina M, Kuang Y, Hall S, Strichartz GR, Berde CB |title=The local anesthetic properties and toxicity of saxitonin homologues for rat sciatic nerve block in vivo |journal=Regional Anesthesia and Pain Medicine |date=Jan–Feb 2000 |volume=25 |issue=1 |pages=52–9 |pmid=10660241 |doi=10.1097/00115550-200001000-00010}} and humans.{{cite journal |vauthors=Rodriguez-Navarro AJ, Lagos N, Lagos M, Braghetto I, Csendes A, Hamilton J, Berger Z, Wiedmaier G, Henriquez A |title=Intrasphincteric neosaxitoxin injection: evidence of lower esophageal sphincter relaxation in achalasia |journal=American Journal of Gastroenterology |date=November 2006 |volume=101 |issue=11 |pages=2667–8 |doi=10.1111/j.1572-0241.2006.00809_6.x |pmid=17090291 |s2cid=1548289 }}{{cite journal |vauthors=Rodriguez-Navarro AJ, Lagos N, Lagos M, Braghetto I, Csendes A, Hamilton J, Figueroa C, Truan D, Garcia C, Rojas A, Iglesias V, Brunet L, Alvarez F |title=Neosaxitoxin as a local anesthetic: preliminary observations from a first human trial |journal=Anesthesiology |date=February 2007 |volume=106 |issue=2 |pages=339–45 |pmid=17264729 |doi=10.1097/00000542-200702000-00023|s2cid=19507764 |doi-access=free }}{{cite journal |vauthors=Rodríguez-Navarro AJ, Berde CB, Wiedmaier G, Mercado A, Garcia C, Iglesias V, Zurakowski D |title=Comparison of neosaxitoxin versus bupivacaine via port infiltration for postoperative analgesia following laparoscopic cholecystectomy: a randomized, double-blind trial |journal=Regional Anesthesia and Pain Medicine |date=March–April 2011 |volume=36 |issue=2 |pages=103–9 |pmid=21425506 |doi=10.1097/aap.0b013e3182030662|s2cid=37539195 |hdl=10533/134301 |hdl-access=free }}{{cite journal |vauthors=Manríquez V, Castro Caperan D, Guzmán R, Naser M, Iglesia V, Lagos N |title=First evidence of neosaxitoxin as a long-acting pain blocker in bladder pain syndrome |journal=Int Urogynecol J |date=2015 |volume=26 |issue=6|pages=853–8 |pmid=25571865 |doi=10.1007/s00192-014-2608-2|s2cid=22432232 }}Coppens SJR, Zawodny Z, Dewinter G, Neyrinck A, Balocco AL, Rex S. In search of the Holy Grail: Poisons and extended release local anesthetics. Best Pract Res Clin Anaesthesiol. 2019 Mar;33(1):3-21. {{doi|10.1016/j.bpa.2019.03.002}} {{PMID|31272651}}

The medical use of the NSTX anesthetic effect is supported by three reasons:

1. NSTX anesthetic duration:
2. * Any current available local anesthetic hardly produces clinical effects 12 hours after a single injection.{{cite journal |vauthors=Leone S, Di Cianni S, Casati A, Fanelli G |title=Pharmacology, toxicology, and clinical use of new long acting local anesthetics, ropivacaine and levobupivacaine |journal=Acta Biomedica |date=August 2008 |volume=79 |issue=2 |pages=92–105 |pmid=18788503}} (accessed: May 10, 2012) [http://www.actabiomedica.it/data/2008/2_2008/leone.pdf] {{Webarchive|url=https://web.archive.org/web/20111216062303/http://www.actabiomedica.it/data/2008/2_2008/leone.pdf |date=2011-12-16 }} Then, in cases of severe or prolonged pain, some patients need repeated injections, catheters, pumps and opioids{{cite journal |vauthors=Aubrun F, Mazoit JX, Riou B |title=Postoperative intravenous morphine titration |journal=British Journal of Anaesthesia |date=February 2012 |volume=108 |issue=2 |pages=193–201 |pmid=22250276 |doi=10.1093/bja/aer458|doi-access=free }}{{cite journal |vauthors=Beilin Y, Halpern S |title=Focused review: ropivacaine versus bupivacaine for epidural labor analgesia |journal=Anesthesia & Analgesia |date=August 2012 |volume=111 |issue=2 |pages=482–7 |pmid=20529986 |doi=10.1213/ANE.0b013e3181e3a08e|doi-access=free }} to feel comfortable, with different kinds of side effects, costs and risks.{{cite journal |vauthors=Wu CL, Raja SN |title=Treatment of acute postoperative pain |journal=Lancet |date=June 2011 |volume=377 |issue=9784 |pages=2215–25 |pmid=21704871 |doi=10.1016/S0140-6736(11)60245-6|s2cid=13140529 }}
3. * On the other hand, NSTX local infiltration produces long lasting anesthesia, well over all the current available local anesthetics. Some investigations demonstrated anesthetic effect lasting over one week after single injection in rodents, using extended release formulation, without histologic or functional sequelae.{{cite journal |vauthors=Epstein-Barash H, Shichor I, Kwon AH, Hall S, Lawlor MW, Langer R, Kohane DS |title=Prolonged duration local anesthesia with minimal toxicity |journal=Proceedings of the National Academy of Sciences of the United States of America |date=April 2009 |volume=106 |issue=17 |pages=7125–30 |pmid=19365067 |doi=10.1073/pnas.0900598106 |pmc=2678453|bibcode=2009PNAS..106.7125E |doi-access=free }}
4. * Additionally, two human reports demonstrated strong potentiation between NSTX anesthetic effect, bupivacaine and epinephrine.{{cite journal |vauthors=Rodriguez-Navarro AJ, Lagos M, Figueroa C, Garcia C, Recabal P, Silva P, Iglesias V, Lagos N |title=Potentiation of local anesthetic activity of neosaxitoxin with bupivacaine or epinephrine: development of a long-acting pain blocker |journal=Neurotoxicity Research |date=November 2009 |volume=16 |issue=4 |pages=408–15 |pmid=19636660 |doi=10.1007/s12640-009-9092-3|s2cid=23287251 |hdl=10533/141060 |hdl-access=free }}{{cite journal |vauthors=Lobo K, Donado C, Cornelissen L, Kim J, Ortiz R, Peake RW, Kellogg M, Alexander ME, Zurakowski D, Kurgansky KE, Peyton J, Bilge A, Boretsky K, McCann ME, Berde CB, Cravero J |title=A Phase 1, Dose-escalation, Double-blind, Block-randomized, Controlled Trial of Safety and Efficacy of Neosaxitoxin Alone and in Combination with 0.2% Bupivacaine, with and without Epinephrine, for Cutaneous Anesthesia |journal=Anesthesiology |date=October 2015 |volume=123 |issue=4 |pages=873–85 |pmid=26275090 |doi=10.1097/ALN.0000000000000831|s2cid=22740054 }}
5. NSTX local safety:
6. * All available local anesthetic are associated with local damage in different models.{{cite journal |vauthors=Zink W, Graf B |title=Review Articles: Local Anesthetic Myotoxicity |journal=Regional Anesthesia and Pain Medicine |date=July–August 2004 |volume=29 |issue=4 |pages=333–40 |pmid=15305253 |doi=10.1016/j.rapm.2004.02.008|s2cid=26185858 }}{{cite journal |vauthors=Bogatch MT, Ferachi DG, Kyle B, Popinchalk S, Howell MH, Ge D, You Z, Savoie FH |title=Cytotoxicity of lidocaine or bupivacaine on corneal endothelial cells in a rabbit model |journal=Cornea |date=June 2006 |volume=25 |issue=5 |pages=590–6 |pmid=16783149 |doi=10.1097/01.ico.0000220775.93852.02|s2cid=21454799 }}{{cite journal |vauthors=Perez-Castro R, Patel S, Garavito-Aguilar ZV, Rosenberg A, Recio-Pinto E, Zhang J, Blanck TJ, Xu F |title=Cytotoxicity of local anesthetics in human neuronal cells |journal=Anesthesia & Analgesia |date=March 2009 |volume=108 |issue=3 |pages=997–1007 |pmid=19224816 |doi=10.1213/ane.0b013e31819385e1|s2cid=41982515 |doi-access=free }}{{cite journal |vauthors=Nouette-Gaulain K, Dadure C, Morau D, Pertuiset C, Galbes O, Hayot M, Mercier J, Sztark F, Rossignol R, Capdevila X |title=Age-dependent bupivacaine-induced muscle toxicity during continuous peripheral nerve block in rats |journal=Anesthesiology |date=November 2009 |volume=111 |issue=5 |pages=1120–7 |pmid=19809284 |doi=10.1097/ALN.0b013e3181bbc949|doi-access=free }}{{cite journal |vauthors=Bogatch MT, Ferachi DG, Kyle B, Popinchalk S, Howell MH, Ge D, You Z, Savoie FH |title=Is chemical incompatibility responsible for chondrocyte death induced by local anesthetics? |journal=The American Journal of Sports Medicine |date=March 2010 |volume=38 |issue=3 |pages=520–6 |pmid=20194957 |doi=10.1177/0363546509349799|s2cid=8766478 }} This undesired effect could be enhanced by sustained release formulations.{{cite journal |vauthors=Padera R, Bellas E, Tse JY, Hao D, Kohane DS |title=Local Myotoxicity from Sustained Release of Bupivacaine from Microparticles |journal=Anesthesiology |date=May 2008 |volume=108 |issue=5 |pages=921–8 |pmid=18431129 |doi=10.1097/ALN.0b013e31816c8a48|pmc=3939710 }}
7. * On the contrary, several investigations show local safety of saxitoxin-related neurotoxins, including very sensitive models, and there is no reason to presume otherwise for NSTX.{{cite journal |vauthors=Gabliks J, Barter S |title=Comparative cytotoxicity of aflatoxin B1 and saxitoxin in cell cultures |journal=Molecular Toxicology |date=April–September 1987 |volume=1 |issue=2–3 |pages=209–16 |pmid=3130568}}{{cite journal |vauthors=Schwartz DM, Duncan KG, Fields HL, Jones MR |title=Tetrodotoxin: anesthetic activity in the de-epithelialized cornea |journal=Graefe's Archive for Clinical and Experimental Ophthalmology |date=October 1998 |volume=236 |issue=10 |pages=790–4 |pmid=9801896 |doi=10.1007/s004170050160|s2cid=25050842 }}{{cite journal |vauthors=Duncan KG, Duncan JL, Schwartz DM |title=Saxitoxin: an anesthetic of the deepithelialized rabbit cornea |journal=Cornea |date=August 2001 |volume=20 |issue=6 |pages=639–42 |pmid=11473167 |doi=10.1097/00003226-200108000-00016|s2cid=24304236 }}{{cite journal |vauthors=Padera RF, Tse JY, Bellas E, Kohane DS |title=Tetrodotoxin for prolonged local anesthesia with minimal myotoxicity |journal=Muscle Nerve |date=December 2006 |volume=34 |issue=6 |pages=747–53 |pmid=16897761 |doi=10.1002/mus.20618|s2cid=22726109 }}
8. NSTX systemic safety:
9. * In spite of advances of ultrasound guided injections, acute systemic local anesthetic toxicity is still an unsolved clinical problem, and can produce devastating consequences, related to the neurologic and cardiovascular effects of all available local anesthetics.{{cite journal |vauthors=Dillane D, Finucane BT |title=Local anesthetic systemic toxicity |journal=Canadian Journal of Anesthesia |date=April 2010 |volume=57 |issue=4 |pages=368–80 |pmid=20151342 |doi=10.1007/s12630-010-9275-7|doi-access=free }}{{cite journal |vauthors=Neal JM, Bernards CM, Butterworth JF, Di Gregorio G, Drasner K, Hejtmanck MR, Mulroy MF, Rosenquist RW, Weinberg GL |title=ASRA practice advisory on local anesthetic systemic toxicity. |journal=Regional Anesthesia and Pain Medicine |date=March–April 2010 |volume=35 |issue=2 |pages=152–61 |pmid=20216033 |doi=10.1097/AAP.0b013e3181d22fcd|s2cid=8548084 }}
10. * Otherwise, clinical experience and animal models shows the relative safety of accidental and experimental NSTX intoxication (when appropriate support therapy is provided in a timely manner).{{cite journal |vauthors=Popkiss ME, Horstman DA, Harpur D |title=Paralytic shellfish poisoning. A report of 17 cases in Cape Town |journal=South African Medical Journal |date=June 1979 |volume=55 |issue=25 |pages=1017–23 |pmid=573505}} (accessed: May 8, 2012)[http://archive.samj.org.za/1979%20VOL%20LV%20Jan-Jun/Articles/06%20June/3.5%20PARALYTIC%20SHELLFISH%20POISONING%20-%20A%20REPORT%20OF%2017%20CASES%20IN%20CAPE%20TOWN.%20M.E.E.%20Popkiss,%20D.A.%20Horst.pdf]
11. * Recent investigation in sheep shows a safe limit, due to motor block, over 1 μg/kg for intravenous injection of NSTX, with full recovery after a brief course of mechanical ventilation.{{cite journal |vauthors=Wylie MC, Johnson VM, Carpino E, Mullen K, Hauser K, Nedder A, Kheir JN, Rodriguez-Navarro AJ, Zurakowski D, Berde CB |title=Respiratory, neuromuscular, and cardiovascular effects of neosaxitoxin in isoflurane-anesthetized sheep |journal=Regional Anesthesia and Pain Medicine |date=March 2012 |volume=37 |issue=2 |pages=152–8 |pmid=22330260 |doi=10.1097/AAP.0b013e3182424566|s2cid=205432781 }}
12. * Regarding systemic safety, saxitoxins diffuse through the blood–brain barrier,{{cite journal |vauthors=Funari E, Testai E |title=Human Health Risk Assessment Related to Cyanotoxins Exposure |journal=Critical Reviews in Toxicology |date=February 2008 |volume=38 |issue=2 |pages=97–125 |pmid=18259982 |doi=10.1080/10408440701749454|s2cid=19506251 }} but, because of Nav channel specificity, acute toxicity is associated to a very low risk of seizures. This establishes an important difference with current local anesthetic toxicity.{{cite journal |author=Guay J |title=Adverse events associated with intravenous regional anesthesia (Bier block): a systematic review of complications |journal=Journal of Clinical Anesthesia |date=December 2009 |volume=21 |issue=8 |pages=585–94 |pmid=20122591 |doi=10.1016/j.jclinane.2009.01.015}}
13. * As could be predicted from its ion channel selectivity,{{cite journal |vauthors=Guo XT, Uehara A, Ravindran A, Bryant SH, Hall S, Moczydlowski E |title=Kinetic basis for insensitivity to tetrodotoxin and saxitoxin in sodium channels of canine heart and denervated rat skeletal muscle |journal=Biochemistry |date=December 1987 |volume=26 |issue=24 |pages=7546–56 |pmid=2447944 |doi=10.1021/bi00398a003}} NSTX intoxication clinical picture is almost devoid of arrhythmias, establishing another difference with available local anesthetic's numerous cardiac effects.{{cite journal |vauthors=Sheets MF, Fozzard HA, Lipkind GM, Hanck DA |title=Sodium channel molecular conformations and antiarrhythmic drug affinity |journal=Trends in Cardiovascular Medicine |date=January 2010 |volume=20 |issue=1 |pages=16–21 |pmid=20685573 |doi=10.1016/j.tcm.2010.03.002 |pmc=2917343}}
14. * And last but not least, some degree of improving in therapeutic index of NSTX can be observed when is mixed with bupivacaine and/or epinephrine.{{cite journal |vauthors=Templin JS, Wylie MC, Kim JD, Kurgansky KE, Gorski G, Kheir J, Zurakowski D, Corfas G, Berde C |title=Neosaxitoxin in Rat Sciatic Block: Improved Therapeutic Index Using Combinations with Bupivacaine, with and without Epinephrine. |journal=Anesthesiology |date=October 2015 |volume=123 |issue=4 |pages=886–98 |pmid=26280473 |doi=10.1097/ALN.0000000000000832 |s2cid=39363279 }}

In conclusion, NSTX is a well defined molecule with a long-lasting and sometimes dangerous relationship with human subjects. Recent investigations suggest a clinical application as a new local anesthetic that sounds "too good to be true", but more investigation is required.{{cite journal |vauthors=Ip VH, Tsui BC |title=Novelty without toxicity: a quest for a safer local anesthetic |journal=Canadian Journal of Anesthesia |date=January 2011 |volume=58 |issue=1 |pages=8–13 |pmid=21042902 |doi=10.1007/s12630-010-9409-y |doi-access=free }}

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Category:Local anesthetics

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