Peptide allergen immunotherapy

Cat-PAD, Fel d 1 peptide immunotherapy, "Cat-SPIRE"; SPIRE = synthetic peptide immunoregulatory epitopes. Related constructs: Grass-PAD, HDM-PAD (house dust mite), Ragweed-PAD, Birch-PAD; earlier generation: Allervax Cat

Immune & bioregulators

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Note on this profile. These were investigational medicines given by injection in allergy clinics inside clinical trials. None reached approval, none is available, and nothing here describes self-preparation.

Summary

Class: Short synthetic T-cell epitope peptides (typically 7 peptides of 13–17 amino acids) derived from a major allergen protein — for Cat-PAD, the cat allergen Fel d 1 — formulated without adjuvant in a simple aqueous buffer
Mechanism: Delivers only the T-cell-recognised parts of an allergen, deliberately too short to be cross-linked by IgE on mast cells, so the immune system can be re-educated toward tolerance without triggering an allergic reaction
Routes studied: Intradermal injection; a very short course — typically 4 to 8 doses over 12 weeks, in contrast to the 3–5 years of conventional allergen immunotherapy
Half-life: Minutes in plasma. The intended effect — a change in allergen-specific T-cell behaviour — was designed to persist for years after a 12-week course
Storage: Refrigerated aqueous solution; no adjuvant, no emulsification
Regulatory status: Not approved anywhere; development of the lead compounds was discontinued after phase 3. See region table

Doses are as reported in studies or by the community — never recommendations.

1What it is

Conventional allergen immunotherapy works — or sublingual whole-allergen extracts genuinely reduce allergic rhinitis and asthma symptoms — but it takes three to five years of regular dosing, and because the whole allergen is present, IgE on mast cells can be cross-linked and cause reactions up to anaphylaxis.

Peptide immunotherapy was an elegant attempt to solve both problems at once. An allergen protein has two separate kinds of recognition sites: conformational surface shapes recognised by IgE antibodies, and short linear sequences recognised by T cells. If you synthesise only the T-cell epitopes, as short peptides, IgE cannot bind two of them at once, no mast-cell cross-linking occurs, and the material can be given in a handful of doses without the reaction risk. Cat-PAD was the lead compound: seven short peptides from Fel d 1, given intradermally four times over 12 weeks.

It is included here because it is the most thoroughly tested example of a peptide tolerance-induction therapy, because its failure is instructive, and because the same design logic underlies the autoimmune peptide therapies in profile 240.

2How it's thought to work

Short peptides injected intradermally without adjuvant are taken up and presented on MHC class II in the absence of a danger signal.

Allergen-specific CD4+ T cells encountering their epitope under these conditions are driven toward a regulatory or anergic state rather than a Th2 state — the same "peptide-induced tolerance" mechanism demonstrated in animal models since the 1980s.

Induced regulatory T cells produce IL-10 and TGF-β and suppress responses to the whole allergen, including epitopes not in the vaccine (linked suppression).

Over time the allergen-specific IgG4 to IgE ratio shifts, as it does with conventional immunotherapy.

Because the peptides are short and linear, IgE cross-linking and mast-cell degranulation are mechanically prevented, which is why no adjuvant and no up-dosing schedule are needed.

Short fragments of an allergen rather than the whole protein. They are too small to cross-link IgE and trigger anaphylaxis, but long enough for T cells to learn tolerance.Schematic — not to scale

3What the research shows

Graded A (positive)

Cat allergy, phase 2

Randomised, double-blind, placebo-controlled phase 2 trials in cat-allergic adults reported significant reductions in combined rhinoconjunctivitis symptom and medication scores during controlled cat-allergen exposure in an environmental exposure chamber, with effects detectable a year or more after a short course (Patel 2013, J Allergy Clin Immunol; subsequent dose-finding studies). These results were striking: a large treatment effect from four injections, with persistence. [1]

Graded A (negative)

Cat allergy, phase 3

A large phase 3 programme in 2016 failed. The lead phase 3 trial did not show a significant difference between Cat-PAD and placebo on its primary endpoint, because the placebo group improved far more than it had in the chamber-based phase 2 studies — a placebo response large enough to erase a real-looking effect. The sponsor discontinued the programme and subsequently exited allergy development. The field's reading is that environmental exposure chamber endpoints had overstated what would be seen in field studies, and that this was a methodological failure as much as a pharmacological one. Whichever reading is correct, the honest summary is that the phase 3 was negative and the compound does not work on the endpoint that mattered.

Graded A (negative) / B

Other allergens

Grass-PAD and house dust mite (HDM-PAD) programmes reported phase 2 results that did not translate; the ragweed and birch programmes were discontinued. No peptide allergen immunotherapy product has ever been approved.

Earlier generation

Allervax Cat, a two-long-peptide Fel d 1 product trialled in the 1990s, reported some symptom benefit at high doses but caused late-phase asthmatic reactions, which drove the move to the shorter peptides used in Cat-PAD.

Graded B (solid)

Mechanistic human work

Independently of the clinical failures, human studies of peptide immunotherapy demonstrated the intended immunological changes: IL-10-producing regulatory T cells, reduced Th2 cytokine production, linked suppression of responses to epitopes not administered, and IgG4 shifts. The immunology was confirmed; the clinical benefit was not.

4Reported side effects and interactions

Literature-reported

the main finding is what did not happen — no systemic allergic reactions or anaphylaxis attributable to the peptides, which was the central safety claim and it held up. Reported events were local: injection-site erythema, swelling, pruritus and induration, usually mild and resolving within days. Some late-phase respiratory symptoms were seen with the earlier long-peptide product (Allervax Cat) and were much reduced with short peptides. Delayed large local reactions occurred in a minority.

Community-reported

Community use: essentially none documented. There is no grey market in these peptides and no self-administration literature. Anyone reading this page about their own cat allergy should know that the conventional treatments — whole-allergen or sublingual immunotherapy — are approved, work, and are what an allergist would discuss.

Theoretical. inducing tolerance to a T-cell epitope could in principle be achieved too broadly; no such effect has been described. Peptide therapy in a sensitised individual carries a theoretical risk of boosting rather than suppressing the response if the dose or schedule is wrong, which is a real finding in animal models and is the main reason dose-finding mattered.

Interactions. antihistamines and corticosteroids were permitted as rescue medication and are part of the scoring in these trials; systemic immunosuppression would be expected to alter the tolerance mechanism.

Markers researchers commonly monitor

From trial protocols:

Marker	Why it's watched
Combined rhinoconjunctivitis symptom and medication score (CSMS)	the primary clinical endpoint in allergy immunotherapy trials, and the measure on which the phase 3 failed.
Environmental exposure chamber versus field (seasonal or natural exposure) endpoints	not a lab marker but the single most important methodological variable in this profile: chamber results did not predict field results, and anyone reading an allergy immunotherapy trial should check which was used.
Allergen-specific IgE and IgG4, and the IgG4:IgE ratio	the standard immunological markers of immunotherapy effect.
Skin prick test wheal size and intradermal late-phase response	in-vivo measures of sensitisation.
Allergen-specific IL-10-producing regulatory T cells and Th2 cytokine production	the mechanistic readouts of tolerance.
Nasal allergen challenge response, and spirometry or FeNO where asthma is involved	objective organ-level measures.
Injection-site assessment, including at 24–48 hours	delayed large local reactions were the main adverse finding.

Free for everyone. Safety information is never behind a paywall.

5Formulation and administration context

Supplied as a refrigerated, adjuvant-free aqueous solution in single-dose vials and given as a small-volume intradermal injection into the forearm by clinic staff, with observation afterwards. The course is short — four doses at four-week intervals was the lead schedule — which was the main practical selling point against years of conventional immunotherapy. No , no emulsification and no up-dosing or escalation schedule were required, which is a direct consequence of the no-IgE-cross-linking design. There is no self-prepared or compounded version, and no dose calculation appropriate to publish.

6Commonly studied alongside

Conventional whole-allergen subcutaneous and sublingual immunotherapy (as comparators and as the standard of care); omalizumab and other anti-IgE biologics (different mechanism, approved); rescue antihistamines and intranasal corticosteroids as permitted medication. Conceptually related: antigen-specific peptide immunotherapy in type 1 diabetes and multiple sclerosis (profile 240) — the same tolerance-induction logic applied to autoimmunity.

Informational cross-links. Not stack recommendations.

7Regulatory status by region

Region	Status (checked —)
United States	Not FDA-approved; investigational, development discontinued
European Union	Not authorised
United Kingdom	Not licensed
Australia	Not approved
South Africa	Not registered with SAHPRA
Sport	Not listed by WADA; no performance application

8Open questions

Was the phase 3 failure a drug failure or an endpoint failure? The exposure-chamber-to-field gap has never been fully explained and affects the whole allergy field.
Can peptide tolerance induction work at all in humans at a clinically useful magnitude, given that the immunological changes were reproducible?
Does epitope selection need to be personalised to HLA type, as it is in cancer neoantigen work, rather than using one fixed peptide set for everyone?

9References

1Patel D et al. (2013). Fel d 1-derived peptide antigen desensitization shows a persistent treatment effect 1 year after the start of dosing: a randomized, placebo-controlled study. Journal of Allergy and Clinical Immunology. PubMed · DOI
2Worm M et al. (2011). Development and preliminary clinical evaluation of a peptide immunotherapy vaccine for cat allergy. Journal of Allergy and Clinical Immunology. PubMed · DOI
3Larché M, Akdis CA, Valenta R (2006). Immunological mechanisms of allergen-specific immunotherapy. Nature Reviews Immunology. PubMed · DOI
4Larché M (2007). Peptide immunotherapy for allergic diseases. Allergy. PubMed · DOI
5Norman PS et al. (1996). Treatment of cat allergy with T-cell reactive peptides. American Journal of Respiratory and Critical Care Medicine. PubMed · DOI
6Circassia Pharmaceuticals (2016). Cat allergy phase 3 trial results — sponsor announcement; programme discontinued. PubMed
7ClinicalTrials.gov. NCT01620762 and related Cat-PAD phase 3 records. PubMed

References are being resolved to PubMed/DOI identifiers; links search PubMed for the citation until then.

10Community

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Self-reported side effects Phase 2

Not enough data yet. 0 of the 50 members needed have logged this compound for 4+ weeks. We don't show anything below 50, so no one can be identified.

11Page history

31 Oct 2026Profile published — awaiting medical review

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