Kelatorphan
Also known as: H-Arg-Arg-Arg-Phe-Lys-Arg-Arg-Arg-NH2, KEL, Kelatorphan, N-[(R,S)-2-benzyl-3-(hydroxyaminocarbonyl)propanoyl]-L-alanine, RB 38A
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Summary
Kelatorphan (also known as RB 38A) is a synthetic peptide‑like inhibitor of several zinc‑metallopeptidases that degrade endogenous enkephalins, chiefly neutral endopeptidase (NEP) and aminopeptidase N (APN). By blocking these enzymes it preserves enkephalins in the nervous system, producing analgesic effects in animal models. It has also been reported to inhibit leukotriene A4 hydrolase, suggesting possible anti‑inflammatory activity. Kelatorphan is used only in research settings and has not been approved for clinical use.
Mechanism of Action
Kelatorphan binds to the active sites of NEP and APN, two metallopeptidases that rapidly hydrolyze Met‑enkephalin and other opioid peptides. The compound chelates the catalytic zinc ion, preventing substrate turnover and thereby increasing the local concentration of endogenous enkephalins. The heightened enkephalin tone activates μ‑opioid receptors, producing analgesia. In addition, kelatorphan shows inhibitory activity against leukotriene A4 hydrolase, a zinc‑dependent enzyme that generates the pro‑inflammatory mediator LTB4, which may contribute to anti‑inflammatory effects.
What the Research Shows
Early work showed kelatorphan completely protects Met‑enkephalin from enzymatic degradation in vitro and in vivo, yielding analgesic potency greater than a mixture of thiorphan and bestatin (J Pharmacol 1985). Subsequent studies described it as a mixed NEP/APN inhibitor that fully blocks enkephalin metabolism, producing strong analgesia after intracerebroventricular or systemic dosing without the tolerance, dependence, or respiratory depression typical of morphine (Ann Pharm Fr 1991; Neurophysiol Clin 1990). Animal experiments reported weak dependence and no significant respiratory effects, while chronic central administration of related mixed inhibitors also avoided antinociceptive tolerance. A 2001 review identified kelatorphan as a potent inhibitor of leukotriene A4 hydrolase, linking it to potential anti‑inflammatory applications. Throughout, evidence remains confined to rodent models and central administration routes.
Reported Benefits
Animal studies indicate kelatorphan provides potent analgesia that exceeds that of conventional enkephalinase inhibitor combinations, while producing minimal opioid‑related side effects such as tolerance, dependence, or respiratory depression. Its broad inhibition of NEP and APN preserves endogenous opioid peptides, allowing physiologic pain modulation without exogenous opioids. The additional inhibition of leukotriene A4 hydrolase suggests a dual anti‑inflammatory potential, broadening its therapeutic relevance in preclinical models.
Limitations of the Evidence
All published data are limited to pre‑clinical rodent experiments; no human trials or safety assessments have been reported. Effective analgesia has required central delivery (intracerebroventricular or intrathecal), raising questions about systemic bioavailability. The compound’s non‑selective inhibition of multiple metallopeptidases could interfere with physiological processes such as blood pressure regulation and natriuresis. Consequently, translational potential remains uncertain and further pharmacokinetic and toxicology studies are needed.
Safety Considerations
In animal models kelatorphan produced only weak physical dependence and did not cause measurable respiratory depression, but comprehensive toxicity data are lacking. Inhibition of NEP, APN, and leukotriene A4 hydrolase could affect peptide hormone metabolism, renal function, and inflammatory pathways, potentially leading to off‑target effects. Until systematic safety evaluations are performed, caution is warranted when interpreting its risk profile.
How It Is Administered
Research investigations have administered kelatorphan intracerebroventricularly, intrathecally, and intravenously in rodents. Formulations were not described in detail, and systemic (e.g., intraperitoneal) delivery has been less effective for analgesia. The compound is used experimentally rather than therapeutically, and no approved clinical formulation exists.
Routes of Administration
Goals & Uses
- Reducing tolerance to exogenous opioidsResearchLow
- Study peptidase functionEnzyme BiologyHigh
- Pharmacological tool for opioid system researchResearchModerate
- Elevate endogenous enkephalin levelsAnalgesia ResearchModerate
- Analgesia via endogenous opioid potentiationPain ManagementLow
Contraindications
- Human clinical use (unapproved compound)RegulatoryHigh
Adverse Effects
- Local Injection Site ReactionsDermatologicalUnknown
- HypotensionCardiovascularUnknownLow blood pressure
- Opioid-like side effects (potential)Central Nervous SystemUnknown
Drug Interactions
- Neprilysin inhibitors (e.g., sacubitril)Moderate
- Opioid analgesics (e.g., morphine)Moderate
Population Constraints
- Human subjects (general population)Regulatory/SafetyAbsolute
Regulatory Status
- European UnionUnapprovedNo clinical authorization.
- United StatesUnapprovedResearch chemical only.
- United KingdomUnapprovedNo MHRA approval; research use only.
Not approved for human use in any jurisdiction; classified as a research chemical.
Evidence & Sources
- Journal ArticleModerateYamamoto Y, et al.2002-01-01T00:00:00.000000Z
- Journal ArticleModerateRoques BP1991-01-01T00:00:00.000000Z
- Journal ArticleModerateRoques BP, et al.1990-01-01T00:00:00.000000Z
- Journal ArticleModerateRoques BP1985-01-01T00:00:00.000000Z
- Journal ArticleModerateNoble F, Roques BP1992-01-01T00:00:00.000000Z
- Journal ArticleModeratePenning TD2001-01-01T00:00:00.000000Z
Frequently Asked Questions
How does kelatorphan differ from other enkephalinase inhibitors?
Kelatorphan is described as a complete inhibitor of enkephalin metabolism, blocking both NEP and APN simultaneously. This broader inhibition yields stronger analgesia than single‑enzyme inhibitors such as thiorphan, and it does so without the tolerance and dependence seen with morphine.
Has kelatorphan been tested in humans?
No. All published studies involve animal models, primarily rodents, and the compound remains classified as a research tool with no clinical approval or human safety data.
Can kelatorphan be given as an oral medication?
Research has only used central (intracerebroventricular or intrathecal) or intravenous routes. There is no evidence that oral administration achieves sufficient systemic exposure or analgesic effect.
What are the main safety concerns with kelatorphan?
While animal data show limited dependence and no respiratory depression, kelatorphan inhibits several metallopeptidases that regulate hormone and inflammatory mediator levels. Potential off‑target effects on blood pressure, renal function, and inflammation have not been fully evaluated.
Could kelatorphan have anti‑inflammatory uses?
A 2001 review identified kelatorphan as a potent inhibitor of leukotriene A4 hydrolase, an enzyme that generates the pro‑inflammatory mediator LTB4. This suggests possible anti‑inflammatory activity, but it remains untested in disease models beyond the enzymatic assay.
What is Kelatorphan?
Kelatorphan (also known as RB 38A) is a synthetic peptide‑like inhibitor of several zinc‑metallopeptidases that degrade endogenous enkephalins, chiefly neutral endopeptidase (NEP) and aminopeptidase N (APN). By blocking these enzymes it preserves enkephalins in the nervous system, producing analgesic effects in animal models. It has also been reported to inhibit leukotriene A4 hydrolase, suggesting possible anti‑inflammatory activity. Kelatorphan is used only in research settings and has not been approved for clinical use.
What is Kelatorphan used for?
Kelatorphan is educationally associated with: Reducing tolerance to exogenous opioids, Study peptidase function, Pharmacological tool for opioid system research, Elevate endogenous enkephalin levels, Analgesia via endogenous opioid potentiation. Educational only — not medical advice.
How is Kelatorphan administered?
Recorded routes of administration: Intracerebroventricular, Intraperitoneal, Intrathecal, Intravenous.
What are the potential side effects of Kelatorphan?
Reported adverse effects include: Local Injection Site Reactions, Hypotension, Opioid-like side effects (potential). This list is not exhaustive — consult a qualified clinician.
Who should avoid Kelatorphan?
Recorded contraindications: Human clinical use (unapproved compound). Consult a qualified clinician before use.