CAQK
A 4-amino-acid homing peptide (Cys-Ala-Gln-Lys) that selectively binds chondroitin sulfate proteoglycans in injured brain extracellular matrix — the lead Aivocode TBI candidate, with EMBO Molecular Medicine 2025 preclinical neuroprotection data and Phase 1 in preparation.
What is CAQK?
CAQK is a 4-amino-acid synthetic peptide (cysteine-alanine-glutamine-lysine) discovered by Erkki Ruoslahti's group at the Sanford Burnham Prebys Medical Discovery Institute as a homing motif that selectively binds upregulated chondroitin sulfate proteoglycans (CSPGs) in the extracellular matrix of acutely injured brain tissue. The peptide circulates harmlessly through intact brain and accumulates at sites of injury — first demonstrated in mouse TBI and confirmed against human injured brain tissue (Nature Communications 2016). Aivocode Inc. is developing CAQK as a stand-alone neuroprotective therapeutic for traumatic brain injury, with a 2025 EMBO Molecular Medicine paper showing that intravenous CAQK administered shortly after moderate or severe TBI in mice reduces neuroinflammation and improves outcomes — Phase 1 clinical development is in preparation as of 2026. Beyond Aivocode's program, CAQK has been used widely as a targeting moiety for nanoparticle and liposome drug delivery to injured CNS sites (TBI, spinal cord injury, demyelinating lesions).
What CAQK Is Investigated For
CAQK is investigated for two distinct but related purposes: as a stand-alone therapeutic for acute traumatic brain injury and as a targeting moiety that delivers other drugs, imaging agents, or nanoparticles selectively to injured CNS tissue. The strongest evidence for therapeutic use is the 2025 EMBO Molecular Medicine paper (Mann et al., from the Ruoslahti and Aivocode teams) showing that intravenous CAQK shortly after moderate-to-severe TBI in mice accumulates in injured brain, reduces neuroinflammation, and improves functional outcomes — the foundation of Aivocode's TBI program. The foundational Nature Communications 2016 paper established the homing mechanism (selective binding to upregulated chondroitin sulfate proteoglycans in injured ECM) and demonstrated it across mouse and human injured brain tissue. The drug-delivery use case has accumulated independent replication: CAQK-targeted liposomes for spinal cord injury (Theranostics 2018), CAQK-modified mesenchymal stem cell nanovesicles for spinal cord injury (ACS Nano 2026), and CAQK targeting of demyelinating lesions (Front Cell Neurosci 2022). The honest framing as of mid-2026: CAQK is the most advanced 'CNS-injury-homing' peptide in the published literature, with a coherent mechanism and a tractable clinical development path for TBI — but Phase 1 has not yet begun, and Aivocode's claim that CAQK is intrinsically neuroprotective (rather than purely a delivery scaffold) is still preclinical-stage evidence.
History & Discovery
CAQK was identified by Erkki Ruoslahti's laboratory at the Sanford Burnham Prebys Medical Discovery Institute (La Jolla, CA) through in vivo phage display — the same screening methodology that Ruoslahti's group pioneered for tumor-homing peptides (RGD and iRGD) over several decades. The team injected a phage library into mice with controlled cortical impact brain injury and recovered phage particles that selectively localized to injury sites, ultimately converging on the 4-amino-acid CAQK sequence. The discovery was published in Nature Communications in 2016 (Mann et al.), establishing both the homing behavior and the chondroitin sulfate proteoglycan binding mechanism. The years following the 2016 discovery saw CAQK adopted widely as a targeting moiety in nanotherapeutic and drug-delivery research — by 2024 the Systematic Review of Peptide CAQK (Int J Mol Sci) catalogued dozens of applications spanning TBI, spinal cord injury, demyelinating disease, and CNS imaging. Aivocode Inc., a biotech founded around the Ruoslahti CNS-homing peptide platform, advanced CAQK toward stand-alone therapeutic development for traumatic brain injury — TBI is a clinical area with an enormous unmet need (no FDA-approved disease-modifying therapy) and a clear acute treatment window where a peptide that homes to the injury site within hours of impact is mechanistically attractive. The key inflection for the therapeutic program was the 2025 EMBO Molecular Medicine paper demonstrating that CAQK itself — not as a delivery scaffold, but as a stand-alone tetrapeptide — produces neuroprotective effects in mouse TBI when administered intravenously shortly after injury. This shifted the program from a delivery-platform thesis to a direct-therapeutic thesis and underwrites Aivocode's Phase 1 preparation in TBI patients as of 2026. CAQK sits in an unusual position in the peptide therapeutics landscape: it is one of the few well-characterized homing peptides advancing toward clinical use, it has an exceptionally simple structure (4 amino acids), and the underlying biology (CSPG accumulation at injury sites) is relatively well-understood. Whether the therapeutic activity demonstrated in mouse models translates to human TBI patients will be answered by Aivocode's planned Phase 1 program.
How It Works
When the brain is injured, certain sugar-protein molecules called CSPGs flood the injury site as part of the wound response. CAQK is a tiny 4-letter peptide that recognizes these molecules and sticks to them — so it travels through your bloodstream, ignores healthy brain, and ends up exactly where the injury is. Once there, it appears to calm down inflammation and protect nearby neurons. The same homing property also makes it useful as a 'GPS tag' for delivering other drugs to injury sites.
CAQK is a 4-amino-acid linear peptide (cysteine-alanine-glutamine-lysine) identified by in vivo phage display screening against acutely injured mouse brain. The peptide selectively binds chondroitin sulfate proteoglycans (CSPGs) — extracellular matrix components whose expression rises dramatically after CNS injury as part of the glial scar response. CSPGs (notably neurocan, brevican, versican, and aggrecan) are present at low baseline levels in healthy brain but accumulate substantially at sites of traumatic brain injury, spinal cord injury, demyelinating lesions, and stroke penumbra. CAQK's selective CSPG affinity provides a passive homing mechanism that does not require active targeting infrastructure. The Ruoslahti lab's foundational Nature Communications 2016 paper established two key facts: (1) CAQK injected intravenously selectively accumulates in injured brain tissue in mouse TBI models, and (2) the same homing applies to human injured brain tissue ex vivo, supporting clinical translation. Subsequent work demonstrated CAQK targeting of demyelinating lesions (Frontiers in Cellular Neuroscience 2022) and spinal cord injury sites (multiple papers 2018–2026). The 2025 EMBO Molecular Medicine paper from the Ruoslahti/Aivocode collaboration extended the use case from passive homing to active neuroprotection: intravenous CAQK shortly after moderate or severe TBI in mice and pigs not only accumulated at injury sites but also reduced neuroinflammatory markers, modulated microglial activation, and improved functional recovery on standard behavioral outcome measures. The replication in a large-animal (pig) model strengthens the translational case relative to mouse-only preclinical work. The mechanistic basis for the intrinsic neuroprotective activity is still being characterized but is hypothesized to involve CSPG-binding-mediated modulation of glial scar signaling pathways. Aivocode's clinical development program centers on this stand-alone therapeutic activity. In parallel, CAQK has been adopted broadly as a targeting moiety for nanoparticle-based drug delivery to injured CNS: liposomal multi-drug delivery for spinal cord injury (Theranostics 2018), mesenchymal stem cell-derived nanovesicles for spinal cord injury (ACS Nano 2026), peptide-polymer conjugates for TBI (Bioconjugate Chemistry 2025), and various other formats. The targeting affinity and tractable chemistry make it one of the most widely used CNS-injury-homing peptides in published nanotherapeutic literature.
Evidence Snapshot
Human Clinical Evidence
None as of mid-2026. Aivocode's Phase 1 TBI program is in preparation. The 2016 Nature Communications paper confirmed homing to human injured brain tissue ex vivo, but no human in vivo data exists.
Animal / Preclinical
Strong. Foundational mouse TBI homing (Nature Communications 2016), neuroprotection in mouse TBI (EMBO Mol Med 2025), spinal cord injury models (Theranostics 2018, ACS Nano 2026, J Trauma Acute Care Surg 2026), demyelinating lesion targeting (Front Cell Neurosci 2022). Multiple independent labs have replicated the homing mechanism.
Mechanistic Rationale
Strong. CSPG biology in CNS injury is well-characterized — CSPGs are central to the glial scar response and accumulate predictably at TBI, SCI, and demyelinating lesion sites. CAQK's selective CSPG affinity provides a coherent mechanistic basis for its homing behavior.
Research Gaps & Open Questions
What the current literature has not yet settled about CAQK:
- 01Phase 1 safety and pharmacokinetic data in humans — the central unknown. Aivocode's planned trial will produce the first human exposure data.
- 02Therapeutic window in human TBI — preclinical mouse studies used dosing shortly after injury; the practical clinical window (hours? days?) is undefined and consequential for trial design and emergency-care integration.
- 03Mechanism of intrinsic neuroprotection — the 2025 EMBO Molecular Medicine paper established that stand-alone CAQK reduces neuroinflammation, but the molecular pathway by which CSPG binding produces neuroprotection (beyond delivery) is incompletely characterized.
- 04Comparison with delivery-platform applications — whether stand-alone CAQK outperforms a CAQK-targeted delivery of an established neuroprotective compound (or vice versa) has not been studied head-to-head.
- 05Long-term safety of CSPG-pathway intervention — CSPGs play roles in normal CNS development and plasticity; whether short-term CAQK exposure has long-term consequences for synaptic remodeling, learning, or neurogenesis is uncharacterized in humans.
- 06Use in indications beyond TBI — spinal cord injury, ischemic stroke, multiple sclerosis lesions, and other CNS pathologies with CSPG upregulation are mechanistically tractable but clinically unstudied for CAQK as a stand-alone therapeutic.
Forms & Administration
Intravenous administration in preclinical TBI studies. Not commercially available. Investigational use only through Aivocode's planned Phase 1 clinical trial.
Dosing & Protocols
The ranges below reflect protocols commonly discussed in the literature and by clinicians — not a prescription. Actual dosing for any individual should be determined by a qualified healthcare provider who knows the patient.
Typical Range
Not established in humans. Preclinical mouse TBI studies have used intravenous CAQK doses on the order of 100 µg/mouse (~5 mg/kg) administered within hours of injury. Phase 1 human dose-finding has not begun.
Frequency
Acute single-dose or short-course administration in the immediate post-injury window is the anticipated clinical paradigm for the TBI indication, based on the pathophysiology (CSPG upregulation peaks in the first hours-to-days after injury) and the preclinical dosing schedules.
Timing Considerations
No specific timing requirements: can be administered at any time of day, with or without food, and is not tied to exercise timing. Consistency matters more than the specific clock — dose at roughly the same time each day (or same day each week, for weekly protocols) to keep exposure steady.
Cycle Length
Not applicable in the typical chronic-dosing sense. The TBI use case is acute treatment of a single injury event, not chronic disease management.
Protocol Notes
CAQK is not available outside Aivocode's planned clinical trial program. The peptide has appeared in research-chemical and reagent supply chains for use in preclinical nanoparticle-targeting work, but those supplies are not authorized or appropriate for human use, and the chain-of-custody, purity, and sequence verification typical of research-chemical sources is not equivalent to GMP-grade clinical material. Aivocode's Phase 1 trial, once initiated, will be the legitimate clinical-access pathway. The TBI window matters mechanistically. CSPG upregulation at injury sites unfolds over the first hours to days after impact, peaks in the early subacute period, and gradually subsides over weeks. The therapeutic rationale presupposes administration relatively soon after the injury, when CAQK's homing target is maximally available. Whether CAQK has benefit administered hours, days, or weeks after injury is an open clinical question that Phase 1/2 trial design will need to address.
CAQK is investigational and Phase 1 has not yet begun as of mid-2026. There is no validated human dosing, and TBI management remains supportive care plus the established acute interventions (neurosurgical evacuation where indicated, ICP management, prevention of secondary injury). CAQK is not yet a treatment option outside of clinical trial enrollment.
Timeline of Effects
Onset
In mouse TBI models, CAQK accumulates at the injury site within hours of intravenous administration — the homing kinetics reflect rapid plasma distribution and CSPG binding rather than slow penetration. Functional outcome differences in mouse studies are typically measured at 7–28 days post-injury.
Peak Effect
Mouse functional-outcome separation between CAQK-treated and control groups in the EMBO Molecular Medicine 2025 study was assessed at standard subacute and early chronic time points. Whether the mouse timeline scales to human TBI recovery (which unfolds over months) will require clinical trial confirmation.
After Discontinuation
Not relevant in the conventional sense — CAQK for TBI is anticipated as acute single-dose or short-course therapy. The peptide itself clears within hours of administration; any therapeutic effect is presumed to derive from the acute-window intervention rather than from sustained pharmacological presence.
Common Questions
Who CAQK Is NOT For
- •Use outside of approved clinical trial protocols — CAQK is investigational and not appropriate for any therapeutic indication as of mid-2026.
- •Pregnancy and breastfeeding — no human safety data exists; reproductive toxicology has not been characterized.
- •Pediatric use — clinical development is anticipated to begin in adult TBI populations; pediatric use is not in scope as of current planning.
- •Active or recent CNS malignancy — the chondroitin sulfate proteoglycan biology that CAQK targets is also relevant to tumor extracellular matrix; whether CAQK would accumulate in primary or metastatic brain tumors or alter their behavior is uncharacterized.
- •Known hypersensitivity to peptide therapeutics.
Drug & Supplement Interactions
Clinical drug interaction data for CAQK is absent. Theoretical considerations: CAQK is a small linear peptide cleared by peptidase-mediated proteolysis rather than CYP-mediated metabolism, so classical pharmacokinetic interactions are not anticipated. The therapeutic window for TBI is acute, so chronic-medication-coadministration questions are less relevant than for chronically dosed peptides. As CAQK enters Phase 1, the trial protocol will define excluded concomitant medications and characterize any interaction signals.
Safety Profile
Common Side Effects
Cautions
- • Investigational — Phase 1 not yet initiated as of mid-2026
- • Long-term safety and CSPG-binding consequences uncharacterized in humans
- • Aivocode-supplied clinical material is not yet available; research-chemical CAQK has unverified identity
What We Don't Know
Most clinical parameters — pharmacokinetics in humans, dose-response, immunogenicity, repeat-dosing safety, off-target effects of chronic CSPG binding — are unknowns until Phase 1 begins.
Legal Status
United States
CAQK is not FDA-approved for any indication and has not yet entered Phase 1 human trials as of mid-2026. Aivocode Inc. holds the principal therapeutic development program for the TBI indication. Research use of CAQK in preclinical studies is widespread, but no legitimate clinical access pathway exists.
International
No major regulatory authority has authorized CAQK as a medicine. Aivocode's Phase 1 program, when it begins, will likely seek IND status with the FDA as the first regulatory step.
Sports & Competition
CAQK is not specifically named on the WADA Prohibited List. As an unapproved investigational peptide, the WADA S0 'non-approved substances' clause would apply for athletes; CAQK is not a performance-enhancing molecule, so anti-doping concern is theoretical rather than practical.
Regulatory status changes over time. Verify current local rules with a qualified professional.
Myths & Misconceptions
Myth
CAQK is FDA-approved for traumatic brain injury.
Reality
It is not. As of mid-2026, CAQK is investigational and Aivocode's Phase 1 program is in preparation, not yet enrolling. There is no FDA-approved disease-modifying therapy for TBI generally, and CAQK is one of several candidates in early development. Patients with TBI should receive standard acute care; CAQK is not yet a treatment option.
Myth
CAQK can be purchased as a research chemical for self-administration.
Reality
CAQK appears in preclinical reagent supply chains for nanoparticle-targeting research. These supplies are not authorized or appropriate for human use, the chain-of-custody and purity verification typical of clinical-grade material is absent, and self-administration of an investigational TBI therapeutic outside a controlled trial setting carries unknown risks plus zero validated clinical benefit framework.
Myth
CAQK is just a tag for drug delivery, not a therapeutic in itself.
Reality
Historically the drug-delivery framing dominated — most pre-2025 work used CAQK to direct nanoparticles or liposomes to CNS injury sites. The 2025 EMBO Molecular Medicine paper changed this, demonstrating that stand-alone CAQK without any conjugated payload produces neuroprotective effects in mouse TBI. Aivocode's TBI program is built on the stand-alone therapeutic premise. Both framings are valid; the therapeutic framing is newer and less rigorously validated.
Myth
Because CAQK is just 4 amino acids, it must be easy to develop and approve.
Reality
Small size simplifies some aspects of peptide development (synthesis, manufacturing, characterization) but does not bypass the fundamental clinical-development questions for any new therapeutic — pharmacokinetics, safety at therapeutic doses, efficacy versus placebo in TBI (a notoriously difficult clinical population for trial design), and regulatory acceptance of trial endpoints. Phase 1 through Phase 3 for a TBI therapeutic is a multi-year process regardless of molecular size.
Published Research
8 studiesCentral Nervous System Targeting Nanovesicles for Trans-Barrier Delivery and Spinal Cord Injury Treatment
Targeting peptide homes to spinal cord injury in a rat model
A neuroprotective tetrapeptide for treatment of acute traumatic brain injury
EMBO Molecular Medicine 2025 — the pivotal preclinical paper underpinning Aivocode's TBI development program. Intravenous CAQK shortly after moderate-to-severe TBI in mice and pigs accumulates at the injury site, reduces neuroinflammation, and improves functional outcomes — establishing CAQK as a candidate stand-alone neuroprotective therapeutic rather than merely a delivery scaffold. The large-animal pig data is meaningful for translational credibility.
Impact of Conjugation Chemistry on the Pharmacokinetics of Peptide-Polymer Conjugates in a Model of Traumatic Brain Injury
Systematic Review of Peptide CAQK: Properties, Applications, and Outcomes
CAQK, a peptide associating with extracellular matrix components targets sites of demyelinating injuries
Novel multi-drug delivery hydrogel using scar-homing liposomes improves spinal cord injury repair
A peptide for targeted, systemic delivery of imaging and therapeutic compounds into acute brain injuries
Mann, Ruoslahti et al. (Nature Communications 2016) — the foundational discovery paper. In vivo phage display identified CAQK as a peptide that selectively binds injured mouse brain tissue, confirmed homing applies to human injured brain ex vivo, and established CSPG binding as the mechanism. The reference paper for every subsequent CAQK study.
Quick Facts
- Class
- Brain-Injury Homing Tetrapeptide
- Tier
- C
- Evidence
- Emerging
- Safety
- Limited Data
- Updated
- May 2026
- Citations
- 8PubMed
Also known as
Tags
Related Goals
Evidence Score
Clinical Trials
View Clinical TrialsLinks to ClinicalTrials.gov for reference. Listing does not imply endorsement.