Neurokinin A Preclinical

What It Is

Neurokinin A (NKA), historically also called Substance K and Neuromedin L, is a 10-amino-acid mammalian tachykinin neuropeptide with the sequence His-Lys-Thr-Asp-Ser-Phe-Val-Gly-Leu-Met-NH₂. The C-terminal pentapeptide Phe-Val-Gly-Leu-Met-NH₂ is the canonical tachykinin signature (Phe-X-Gly-Leu-Met-NH₂), shared with Substance P (SP, Neurokinin 1 family), Neurokinin B (NKB), hemokinin-1, and the endokinins. The N-terminal four residues (His-Lys-Thr-Asp) give NKA its preferred engagement of NK2R over NK1R — position 3 (Thr) and 4 (Asp) are particularly important for NK2R affinity.

NKA is produced from the preprotachykinin-A gene (TAC1) by alternative splicing. TAC1 generates α-, β-, γ-, and δ-PPT-A transcripts; α-PPT-A produces only Substance P, while β- and γ-PPT-A produce both SP and NKA from a single precursor via subsequent prohormone-convertase processing. NKA is therefore cleaved from the same primary transcript as SP in most tachykinin-producing neurons. An N-terminally extended form, neuropeptide K (NPK, 36 AA), and neuropeptide γ (NPγ, 21 AA), are also produced from these transcripts and share NK2R affinity with NKA.

Endogenous NKA is released from unmyelinated sensory C-fibers (and some Aδ fibers) upon noxious stimulation, capsaicin challenge, chemical irritation, or inflammatory stimuli. It is also expressed in enteric neurons throughout the gastrointestinal tract, in airway sensory nerves, in the lower urinary tract, in spinal cord dorsal-horn interneurons, and in multiple brain regions including the amygdala and hypothalamus. Peripheral NKA release drives the clinically recognizable pattern of neurogenic inflammation — localized vasodilation, plasma extravasation, smooth muscle contraction, and immune cell recruitment — that characterizes asthma exacerbation, migraine, IBS, interstitial cystitis, and certain inflammatory dermatoses.

NKA is never used therapeutically. Its principal role in human-subjects research is as a bronchoprovocation agent — inhalation of escalating doses of NKA produces dose-dependent bronchoconstriction in asthmatics, making it an ideal challenge molecule for evaluating NK2R antagonist pharmacology in controlled respiratory-lab studies. The translational reward of the tachykinin field has been the antagonist class: aprepitant (NK1R antagonist, FDA-approved for chemotherapy-induced nausea and vomiting), saredutant and ibodutant (NK2R antagonists developed for IBS and depression), and osanetant / fezolinetant (NK3R antagonist — the latter recently FDA-approved for menopausal vasomotor symptoms).

Mechanism of Action

• NK2R (TACR2) preferred agonist — NKA binds NK2R with higher affinity than the other tachykinin receptors (NK1R, NK3R), though at high concentrations it engages all three (receptor cross-talk is dose-dependent).
• Gq-PLC-IP₃-Ca²⁺ smooth muscle contraction — NK2R is a Gq-coupled GPCR. Agonism activates phospholipase C-β, hydrolyzing PIP₂ into IP₃ and DAG; IP₃ releases calcium from sarcoplasmic reticulum; DAG activates PKC; the net effect is sustained smooth muscle contraction in airways, gut, bladder, and vasculature.
• Bronchoconstriction — NK2R expressed on airway smooth muscle. NKA is one of the most potent endogenous bronchoconstrictors known — tenfold more potent than histamine on a molar basis in several human airway preparations. Released from sensory nerves during asthma exacerbation, contributing to airway hyperresponsiveness.
• Neurogenic inflammation — Peripheral NKA release from C-fibers drives vasodilation (via endothelial NK2R and downstream NO/prostaglandin release), plasma extravasation (post-capillary venule leakage), and mast-cell recruitment — the classical triad of neurogenic inflammation.
• Gut smooth muscle contraction — NK2R on enteric smooth muscle mediates phasic and tonic contractions along the gut. Physiological contribution to peristalsis; pathologic contribution to IBS-D hypercontractility.
• Visceral hypersensitivity — NK2R on primary afferent terminals and spinal dorsal-horn neurons amplifies visceral pain signaling; this is the mechanism targeted by the NK2R-antagonist ibodutant in IBS-D.
• Urinary bladder contraction — NK2R on detrusor smooth muscle; contributes to urinary urgency in conditions including interstitial cystitis.
• Vascular effects — Dose-dependent; direct NK2R-mediated vascular smooth muscle constriction opposed by endothelium-dependent vasodilation (NO release). The net clinical signature at typical experimental doses is modest cutaneous vasodilation with plasma extravasation.
• Cross-receptor activity — At pharmacological doses, NKA also engages NK1R (SP's preferred receptor) and NK3R; however, tachykinin receptor selectivity is preserved at low endogenous concentrations.
• Metabolism — NKA is rapidly metabolized by neutral endopeptidase (NEP/neprilysin) and angiotensin-converting enzyme in plasma and tissue. Circulating half-life is very short (minutes).
• NEP pharmacology crosstalk — Because NEP is the principal enzymatic inactivator of NKA (as well as SP, bradykinin, and natriuretic peptides), NEP inhibitors (sacubitril in the sacubitril/valsartan combination Entresto, approved for heart failure) raise circulating NKA and SP. The clinical relevance of elevated tachykinin tone from NEP inhibition is incompletely characterized.
• Endokinin and hemokinin context — The tachykinin family extends beyond SP/NKA/NKB to include the TAC4-gene products endokinins A/B/C/D and the related hemokinin-1. These peripheral / immune tachykinins share NK1R binding affinity and are implicated in hematopoiesis and immune modulation; NKA itself does not extend into these indications.
• Receptor internalization kinetics — NK2R undergoes rapid agonist-induced internalization and desensitization — clinically relevant for sustained-infusion or repeated-exposure paradigms and contributing to the characteristic tachyphylaxis observed with repeated tachykinin challenges.
• Alpha and beta NK2R splice variants — The NK2R gene produces alpha and beta splice variants with distinct intracellular C-tail sequences and slightly different signaling / desensitization kinetics. Pharmacodynamic relevance in humans is incompletely characterized but may underlie tissue-specific NK2R antagonist effects.
• Structural basis of binding — Catalioto et al. (2009) characterized ibodutant binding at the human NK2R through radioligand-competition and mutagenesis studies, mapping key interactions to residues in transmembrane helices 4 (Cys167), 5 (Ile202, Tyr206), 6 (Phe270, Tyr266, Trp263), and 7 (Tyr289). This molecular-interaction map defines the extended transmembrane binding pocket critical for NK2R activation.
• NKA vs neuropeptide gamma vs neuropeptide K — Longer N-terminally extended tachykinins (NPγ, 21 AA; NPK, 36 AA) derived from the same TAC1 transcripts are also NK2R-preferring; their physiological contribution relative to the 10-AA NKA is tissue- and context-dependent.
• Central NK2R distribution — Beyond peripheral smooth muscle, NK2R is expressed in amygdala, hippocampus, and hypothalamus. Central NK2R engagement has been implicated in fear, anxiety, and depression pharmacology — the mechanistic rationale for the earlier clinical evaluation of NK2R antagonists in depression (saredutant, discontinued).
• Visceral nociception — spinal dorsal horn — NK2R on primary afferent terminals entering the spinal dorsal horn and on second-order neurons in laminae I–II mediates ascending visceral pain signaling. This anatomically localized NK2R activity is the substrate targeted by ibodutant in IBS-D; ibodutant does not cross the blood-brain barrier, meaning its clinical effect is mediated through peripheral / primary-afferent NK2R rather than central supraspinal sites.

What the Research Shows

• Inhaled NKA bronchoprovocation — Inhalation of nebulized NKA causes dose-dependent bronchoconstriction in asthmatics. NKA challenge is a standardized respiratory-lab paradigm for evaluating NK2R antagonist pharmacology (Joos 1996; Van Schoor 1998).
• NK2R antagonists reverse NKA bronchoconstriction — saredutant — Van Schoor 1998 (PMID 9701408) demonstrated that oral SR 48968 (saredutant, 100 mg) significantly inhibited NKA-induced bronchoconstriction in 12 asthmatic patients — the first human evidence for NK2R antagonist airway effect.
• FK224 inhibition of NKA — Joos 1996 (PMID 8665034) inhaled FK224, a dual NK1/NK2 cyclopeptide antagonist, inhibited NKA-induced bronchoconstriction in asthmatics. Early proof-of-concept for tachykinin antagonism in airway disease.
• Dual NK1/NK2 antagonism — DNK333 — Joos 2004 (Eur Respir J; PMID 14738235) single-dose DNK333 inhibited NKA-induced bronchoconstriction in asthma with a 4.08-doubling-dose shift — large pharmacodynamic effect.
• Triple neurokinin antagonist — CS-003 — Schelfhout 2006 (PMID 16364669) demonstrated efficacy of CS-003 in inhibiting NKA bronchoprovocation.
• MEN11420 nepadutant — Schelfhout 2009 (PMID 19880429) — selective NK2R antagonist effect on NKA-induced bronchoconstriction.
• Ibodutant in IBS-D — Phase 2 (Tack 2017, PMID 27196574) — Multinational double-blind placebo-controlled Phase 2 of ibodutant (selective NK2R antagonist, 1 / 3 / 10 mg) in 559 IBS-D patients. Dose-dependent symptom response; statistical significance at 10 mg in female patients (p=0.003). Established peripheral NK2R as a relevant target in IBS-D.
• Ibodutant mechanism (Catalioto 2009, J Pharmacol Exp Ther) — Characterization of ibodutant as a potent competitive NK2R antagonist with high oral bioavailability and fast receptor association / slow dissociation kinetics.
• Tachykinins and gut pharmacology — Lecci & Maggi — Long-running body of work from the Menarini / Florence group establishing NKA–NK2R as the dominant tachykinin axis in human colonic smooth muscle physiology and pathophysiology.
• Airway hyperresponsiveness — Joos review 2000 — Tachykinins in asthma comprehensive review; NKA positioned as a major effector of airway hyperresponsiveness alongside SP.
• Visceral pain models — Extensive preclinical literature using NKA to characterize visceral nociceptive pathways in bladder and gut; NK2R antagonists dose-dependently reverse nocifensive behavior.
• Capsaicin co-release — Capsaicin-sensitive C-fibers release both SP and NKA; the neurogenic inflammation signature represents the combined action of both peptides at NK1R and NK2R.

Human Data

• Joos 1996 (PMID 8665034) — Inhaled FK224 inhibits NKA-induced bronchoconstriction in asthmatics.
• Van Schoor 1998 (PMID 9701408) — Oral saredutant (SR 48968) 100 mg inhibits NKA bronchoconstriction in 12 asthmatics; first NK2R-selective antagonist human proof-of-concept.
• Joos 2004 (PMID 14738235) — Single-dose DNK333 dual NK1/NK2 antagonist, 4.08-doubling-dose shift in NKA challenge.
• Schelfhout 2006 (PMID 16364669) — CS-003 triple-tachykinin antagonist in NKA bronchoprovocation.
• Schelfhout 2009 (PMID 19880429) — Nepadutant (MEN11420) NK2R-selective antagonist in NKA bronchoprovocation.
• Tack 2017 (PMID 27196574) — Ibodutant Phase 2 in IBS-D; 559 patients; dose-response efficacy at 10 mg in females (p=0.003). Advanced to Phase 3 in female IBS-D.
• Catalioto 2009 (J Pharmacol Exp Ther; PMID 19218528) — Ibodutant NK2R pharmacology characterization.
• NKA in asthma bronchoalveolar lavage — Elevated NKA documented in BAL fluid of asthmatic patients; biomarker evidence for tachykinin involvement.
• NKA in migraine — Plasma NKA elevations during migraine attacks reported in multiple cohort studies; contributes to the neurogenic-inflammation model of migraine pathophysiology.
• NKA in interstitial cystitis — Elevated urine and bladder tissue NKA in some IC patient cohorts; NK2R implicated in bladder pain syndrome.

There are no published human therapeutic trials of exogenous NKA administration. All human exposure occurs in bronchoprovocation challenge studies under controlled respiratory-lab supervision.

Dosing from the Literature

Neurokinin A has no therapeutic dose. The following summarizes standard bronchoprovocation and preclinical research protocols for context only.

Reconstitution & Storage

NKA is supplied as a lyophilized powder for research use, typically in 100 µg, 500 µg, or 1 mg vial sizes. The C-terminal methionine amide is sensitive to oxidation; proper handling preserves bioactivity.

• Reconstitution — Sterile water, PBS, or 0.1% acetic acid depending on downstream application. Swirl gently.
• Oxidation protection — The C-terminal Met-NH₂ is oxidation-sensitive; store under argon or nitrogen where possible; protect from light.
• Lyophilized storage — −20°C or below, desiccated, stable 12+ months.
• Reconstituted storage — 4°C for 24–48 hours; aliquot and freeze at −80°C for longer storage. Avoid repeated freeze-thaw.
• Inspection — Clear solution. Discard if cloudy, discolored, or visible particulate.
• Not for human administration — Research-reagent reconstitution is not equivalent to pharmaceutical-grade sterile compounding for clinical use.

→ Use the Kalios Dosing Calculator for research reconstitution math

Side Effects & Risks

• Bronchoconstriction — Primary pharmacological effect. In asthmatic patients or individuals with airway hyperresponsiveness, even small inhaled doses can produce clinically significant bronchoconstriction requiring bronchodilator rescue.
• Cough and throat irritation — Expected at bronchoprovocation doses.
• Visceral pain / cramping — Systemic administration produces gut cramping and urgency.
• Hypotension — Vasodilation from tachykinin activity can produce transient blood-pressure drops.
• Facial flushing and wheal-and-flare — Intradermal administration produces classical neurogenic-inflammation local response.
• Urinary urgency — NK2R on detrusor muscle.
• Anxiety / psychiatric effects — Central NK2R activation has been implicated in fear and anxiety circuits; not typically observed at peripheral research doses.
• Contraindications (for any administration) — Asthma (unless supervised bronchoprovocation), COPD, severe cardiovascular disease, active IBS-D or functional bowel disease, pregnancy, lactation, uncontrolled anxiety disorders.
• Emergency-response capability required — Any human administration requires immediate access to short-acting bronchodilator, IV access, and emergency airway management.
• Purity considerations — Research-grade peptide purity is not regulated to pharmaceutical standards.

Bloodwork & Monitoring

Monitoring considerations apply to research-lab bronchoprovocation protocols or hypothetical therapeutic study — NKA is not a self-administered compound.

• Spirometry (FEV1, FVC, PEF) — Baseline and serial post-challenge; PC20 (provocative concentration causing 20% FEV1 fall) is the standard bronchoprovocation endpoint.
• Pulse oximetry — Continuous during any inhaled challenge.
• Blood pressure and heart rate — Baseline and serial; tachykinin activity produces modest autonomic shifts.
• Comprehensive metabolic panel — Baseline renal and hepatic function prior to any research administration.
• Complete blood count — Baseline; eosinophil count relevant for asthma-research context.
• Methacholine or histamine bronchoprovocation (reference) — Some protocols compare NKA challenge to methacholine challenge for airway-hyperresponsiveness phenotyping.
• IBS symptom measures (gut research) — IBS-SSS, Bristol stool scale, abdominal pain VAS for any gut-research protocol.
• Validated respiratory lab requirement — Bronchoprovocation studies require institutional spirometry capability, emergency response equipment, and appropriate regulatory approvals (IRB, local ethics).

Commonly Stacked With

NKA is not a therapeutic compound and is not "stacked" in any clinical context. The research-relevant compound adjacencies are:

→ Check compound compatibility in the Stack Builder

Research Context — The Tachykinin Drug-Discovery Arc

Neurokinin A's therapeutic importance is as a mechanistic anchor for the tachykinin antagonist drug class. Understanding NKA pharmacology clarifies the larger arc of tachykinin-system drug development:

• NK1R antagonists — the first approved tachykinin drug class — Aprepitant (Emend, 2003) and fosaprepitant received FDA approval for chemotherapy-induced nausea and vomiting (CINV). The clinical success validated the tachykinin antagonist concept and paved the way for NK2R and NK3R programs.
• NK3R antagonists — fezolinetant (Veozah) for menopausal hot flashes — FDA-approved in May 2023 for vasomotor symptoms of menopause. The NK3R / neurokinin B / KNDy-neuron axis in the hypothalamic arcuate nucleus controls thermoregulatory set point; NK3R antagonism by fezolinetant directly targets hot-flash physiology. A landmark non-hormonal therapy in menopause.
• NK2R antagonists — incomplete translational story — Saredutant for depression and IBS (discontinued), ibodutant for IBS-D (advanced to Phase 3; female-specific efficacy signal). The class has technical validation (consistent NKA bronchoprovocation reversal) but has not produced a widely approved product in respiratory or GI indications to date.
• Orvepitant and other second-generation NK1R antagonists — Investigated for chronic cough, pruritus, and anxiety; the NK1R / SP axis extends beyond CINV to multiple non-nausea indications.
• Nausea and vomiting physiology — The SP–NK1R interaction in the area postrema / chemoreceptor trigger zone is the classical mechanism underlying aprepitant's CINV indication; NKA itself is less involved in emesis than SP.
• Neurogenic inflammation model — Cutaneous, airway, and meningeal neurogenic inflammation models have historically used SP + NKA together to produce the composite plasma-extravasation / vasodilation / mast-cell activation signature.
• Migraine — Tachykinin system is implicated in migraine pathophysiology; the CGRP / CGRP receptor antagonist class (rimegepant, ubrogepant, atogepant, the monoclonal antibody gepants) became the translational success in migraine rather than NK1/NK2 antagonists.
• Bladder / interstitial cystitis — Preclinical NK2R involvement in bladder pain syndrome; therapeutic translation has not matured.
• Depression / anxiety (central NK1/NK2/NK3) — Multiple tachykinin antagonists have been evaluated for mood and anxiety; despite early promise, the class has not produced broadly effective psychiatric therapeutics at the scale predicted by preclinical data.
• Asthma / COPD — Despite clean mechanism (NK2R-mediated airway smooth muscle contraction), tachykinin antagonists have not displaced β-agonists, muscarinic antagonists, or biologics in asthma / COPD care.

Supportive Nutrition & Clinical Adjacency

NKA has no therapeutic indication. The following contextualizes validated clinical care for conditions where tachykinin pharmacology is relevant:

• Asthma — Approved controller (ICS, LABA, LAMA, biologics) and reliever therapy is the validated pathway. Tachykinin-related investigational agents are not a substitute for evidence-based asthma control.
• IBS-D — Approved therapies including rifaximin, eluxadoline, alosetron (restricted), and dietary/lifestyle interventions (low-FODMAP, CBT). NK2R antagonists (if approved) would add to this pharmacologic toolkit rather than replacing it.
• Migraine — Validated acute (triptans, gepants, lasmiditan) and preventive (CGRP antibodies, topiramate, propranolol) therapies.
• CINV — NK1R antagonists (aprepitant, fosaprepitant, rolapitant) are established standard of care in combination with 5-HT3 antagonists and dexamethasone.
• Menopausal vasomotor symptoms — NK3R antagonist fezolinetant is FDA-approved; HRT remains first-line where not contraindicated.
• Interstitial cystitis / bladder pain syndrome — Multimodal validated care including behavioral, intravesical, oral, and (in refractory cases) procedural therapy.
• Chronic cough — Investigational P2X3 antagonists (gefapixant) and targeted antitussive approaches; NK1 antagonism has been evaluated but not established as standard care.
• Pruritus — Validated systemic antipruritics (antihistamines, neurokinin-pathway investigational agents like serlopitant, aprepitant in specific contexts).

Regulatory Status

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Key References

1. Van Schoor J, Joos GF, Chasson BL, Brouard RJ, Pauwels RA. The effect of the NK2 tachykinin receptor antagonist SR 48968 (saredutant) on neurokinin A-induced bronchoconstriction in asthmatics. Eur Respir J. 1998;12(1):17-23. PMID: 9701408.
2. Joos GF, Van Schoor J, Kips JC, Pauwels RA. The effect of inhaled FK224, a tachykinin NK-1 and NK-2 receptor antagonist, on neurokinin A-induced bronchoconstriction in asthmatics. Am J Respir Crit Care Med. 1996;153(6 Pt 1):1781-1784. PMID: 8665034.
3. Joos GF, Vincken W, Louis R, Schelfhout VJ, Wang JH, Shaw MJ, Cioppa GD, Pauwels RA. Dual tachykinin NK1/NK2 antagonist DNK333 inhibits neurokinin A-induced bronchoconstriction in asthma patients. Eur Respir J. 2004;23(1):76-81. PMID: 14738235.
4. Schelfhout V, Louis R, Lenz W, Heyrman R, Pauwels R, Joos G. The triple neurokinin-receptor antagonist CS-003 inhibits neurokinin A-induced bronchoconstriction in patients with asthma. Pulm Pharmacol Ther. 2006;19(6):413-418. PMID: 16364669.
5. Schelfhout V, Van De Velde V, Maggi C, Pauwels R, Joos G. The effect of the tachykinin NK(2) receptor antagonist MEN11420 (nepadutant) on neurokinin A-induced bronchoconstriction in asthmatics. Ther Adv Respir Dis. 2009;3(5):219-226. PMID: 19880429.
6. Tack J, Schumacher K, Tonini G, Scartoni S, Capriati A, Maggi CA. The neurokinin-2 receptor antagonist ibodutant improves overall symptoms, abdominal pain and stool pattern in female patients in a phase II study of diarrhoea-predominant IBS. Gut. 2017;66(8):1403-1413. PMID: 27196574.
7. Catalioto RM, Cucchi P, Renzetti AR, Criscuoli M, Maggi CA, Giuliani S. Multifaceted approach to determine the antagonist molecular mechanism and interaction of ibodutant at the human tachykinin NK2 receptor. J Pharmacol Exp Ther. 2009;329(2):486-495. PMID: 19218528.
8. Joos GF, De Swert KO, Pauwels RA. Airway inflammation and tachykinins: prospects for the development of tachykinin receptor antagonists. Eur J Pharmacol. 2001;429(1-3):239-250. PMID: 11698043.
9. Maggi CA. The mammalian tachykinin receptors. Gen Pharmacol. 1995;26(5):911-944. PMID: 7557266.
10. Lecci A, Capriati A, Altamura M, Maggi CA. Tachykinins and tachykinin receptors in the gut, with special reference to NK2 receptors in human. Auton Neurosci. 2006;126-127:232-249. PMID: 16616700.
11. Pennefather JN, Lecci A, Candenas ML, Patak E, Pinto FM, Maggi CA. Tachykinins and tachykinin receptors: a growing family. Life Sci. 2004;74(12):1445-1463. PMID: 14729395.
12. Nawa H, Hirose T, Takashima H, Inayama S, Nakanishi S. Nucleotide sequences of cloned cDNAs for two types of bovine brain substance P precursor. Nature. 1983;306(5938):32-36. PMID: 6195531.
13. Almeida TA, Rojo J, Nieto PM, Pinto FM, Hernandez M, Martín JD, Candenas ML. Tachykinins and tachykinin receptors: structure and activity relationships. Curr Med Chem. 2004;11(15):2045-2081. PMID: 15279567.
14. Joos GF, Pauwels RA. Pro-inflammatory effects of substance P: new perspectives for the treatment of airway diseases? Trends Pharmacol Sci. 2000;21(4):131-133. PMID: 10740285.
15. Steinhoff MS, von Mentzer B, Geppetti P, Pothoulakis C, Bunnett NW. Tachykinins and their receptors: contributions to physiological control and the mechanisms of disease. Physiol Rev. 2014;94(1):265-301. PMID: 24382888.