ENDO-205 Peptide: What the Endometriosis Drug Is, Where the Trial Stands, and What Has Not Been Shown

ENDO-205 peptide is an investigational, non-hormonal drug for endometriosis from EndoCyclic Therapeutics of Irvine, California. The company announced FDA clearance for a first human trial on March 23, 2026. It is not approved, not available, and — as of the date on this page — has no published human or animal data behind it. This page covers what the company says the peptide does, what the Phase 1 trial will and will not tell you, what is known about side effects (very little), why nothing sold under this name can be verified, and what a realistic timeline to a prescription looks like.

In This Guide

• What ENDO-205 is, and what it is not
• How ENDO-205 is supposed to work
• Why “non-hormonal” is the whole pitch
• What the evidence actually is (September 2026)
• The Phase 1 trial: what is being tested
• ENDO-205 side effects: what is and is not known
• Is ENDO-205 available or for sale?
• When will ENDO-205 be available?
• Who is EndoCyclic Therapeutics?
• What people are asking
• What we are watching
• Frequently asked questions
• Glossary
• Primary sources

What ENDO-205 Is, and What It Is Not

ENDO-205 is the lead drug candidate of EndoCyclic Therapeutics, a small women’s-health biotech in Irvine, California. In the company’s words it is “a first-in-class, non-hormonal targeted peptide therapeutic for endometriosis” (PR Newswire, March 23, 2026). That release announced that the FDA had cleared the company’s Investigational New Drug application, which is the regulatory permission to begin testing a compound in people.

Three things follow from that sentence, and each one matters for how you should read everything else you find about this drug.

It is a drug in development, not a research chemical. The peptides covered elsewhere on this site — in our compound index, retatrutide, eloralintide, emideltide — have published sequences, published trials, and vials moving through the grey market whose contents can at least be checked against a known molecule. ENDO-205 has none of those. Its amino-acid sequence has not been disclosed. Its route of administration has not been stated. Its dose is unknown outside the company and the FDA.

IND clearance is not approval. It is the first of several gates. The FDA reviewed the company’s animal safety data and manufacturing information and agreed that a first-in-human study could begin. It has not reviewed any evidence that ENDO-205 works in a person, because no such evidence exists yet. The Endometriosis Specialist Surgical Institute made the same point plainly in its commentary: “This is not FDA approval. It is the first step into human testing” (ESSI, March 31, 2026).

It is a peptide, but not the kind you reconstitute. The company describes its platform as cyclic peptides that occupy “a middle ground” between small molecules and antibodies (EndoCyclic, Peptide Platform). Whether ENDO-205 will be an injection, an infusion, or something else has not been said publicly. Nothing on this page, and nothing on this site, will tell you how to take it, because nobody outside the trial can.

How ENDO-205 Is Supposed to Work

Every mechanism statement in this section is the company’s description or a journalist’s paraphrase of it. No peer-reviewed paper describes ENDO-205, so there is nothing independent to check it against yet. That is a statement about the record, not an accusation — most Phase 1 drugs have no publications at this stage. It simply means the word “reportedly” belongs in front of every claim below.

Selective uptake: the lesion takes the drug in, healthy tissue does not

The company says its peptides are “absorbed selectively by diseased tissue” through “a specific endocytic pathway that is exclusively active in diseased tissue,” and that once inside they activate “in response to local pH” (EndoCyclic, Peptide Platform). The claim that matters most is the negative one: “Healthy cells don’t absorb our peptides.” If that holds in humans, it is the property that would separate ENDO-205 from every existing endometriosis drug, all of which act on the whole body.

What it does once inside: apoptosis

Drug Discovery News, drawing on an interview with founder Tanya Petrossian, describes ENDO-205 as “a peptide engineered for selective uptake into endometriotic cells, where it triggers programmed cell death, or apoptosis” (Drug Discovery News). Petrossian is quoted describing the preclinical result: “When we began our studies and went in to analyze the on-target lesions, we found that they were no longer present.” The company’s own website puts it differently — ENDO-205 “eliminates lesions at their origin, allowing pain and inflammation to naturally subside as the disease resolves,” and the platform page frames the effect as cells that “quietly recede, not through destruction, but through correction.” Whether that is a difference of emphasis or of mechanism is one of the things a publication would settle.

The target

The company does not name a molecular target in its public materials. The Endometriosis Specialist Surgical Institute’s commentary says that public patent filings “strongly suggest that the platform includes peptides that bind to β-catenin,” and it describes the Wnt/β-catenin pathway’s role in lesion cell migration, fibrosis and proliferation (ESSI). ESSI does not state that ENDO-205 itself acts on that pathway, and neither does the company, so treat the target as an informed inference from patents rather than a disclosed fact. A self-described researcher in the July Reddit thread voiced skepticism that a pathway of that kind had produced results when targeted before — see what people are asking.

The intended course

The NIH’s portfolio showcase for the company describes the intended regimen as “a short-term treatment course… lasting up to 3 months” (NIH SEED). If that is how the drug is ultimately used, it is a different proposition from the current standard: months of treatment aimed at removing lesions, rather than years of hormone suppression aimed at quieting them.

Why “Non-Hormonal” Is the Whole Pitch

Endometriosis affects an estimated 10% of reproductive-age women — about 190 million people worldwide — and “there is currently no cure,” in the World Health Organization’s words; treatment “aims to control symptoms and limit long-term impacts” (WHO fact sheet, October 2025). The disease is estrogen-dependent, which is why nearly every drug for it works by lowering or blocking estrogen (PMID 32212520; PMID 28525302). Surgery removes lesions but does not stop them coming back: a review of recurrence data put the rate at 21.5% at two years and 40–50% at five (PMID 19279046).

The two most recent FDA-approved oral options are both GnRH-receptor antagonists, and their labels are the clearest picture of what “hormonal” costs a patient. The numbers below are the adverse reactions reported in the pivotal endometriosis trials, taken from the current prescribing information for each drug.

Sources: Orilissa (elagolix) prescribing information, AbbVie, revised 12/2025, Studies EM-1 and EM-2 (DailyMed; trial report PMID 28525302). Myfembree (relugolix, estradiol, norethindrone acetate) prescribing information, Sumitomo Pharma America, revised 8/2025, Studies S1 and S2 at 24 weeks, N=418 vs 416 (DailyMed; trial report PMID 35717987). Myfembree also carries a boxed warning for thrombotic and thromboembolic events. Bone-density figures are absolute mean changes from baseline; the Orilissa ranges span the two trials. Label terms differ slightly: Orilissa lists “mood altered, mood swings” and Myfembree “decreased sexual desire and arousal.”

Read that table and the appeal of ENDO-205 explains itself. Orilissa is limited to 24 months at the lower dose and six months at the higher because the bone loss “may not be completely reversible after stopping treatment”; Myfembree is limited to 24 months because bone loss “may not be reversible” (both from the current labels). Nearly half of women on the higher Orilissa dose get hot flushes. Neither drug touches the lesions; both quiet them by turning down the estrogen that feeds them, and both are limited to “moderate to severe pain associated with endometriosis” rather than the disease itself. A drug that removed lesions in a three-month course, with no effect on healthy tissue, would be a different category of treatment. That is the promise. Nothing in the public record yet tests it in a human being.

One clarification that comes up constantly in searches: neither Orilissa nor Myfembree is a peptide. Elagolix and relugolix are small molecules — Orilissa’s label describes elagolix as a “nonpeptide small molecule, GnRH receptor antagonist,” and Myfembree’s describes relugolix as a “non-peptide small molecule, GnRH receptor antagonist.” The approved peptides in this space are GnRH agonists such as leuprolide, goserelin and nafarelin, which suppress estrogen by a different route and carry their own hormonal side-effect profiles.

What the Evidence Actually Is (September 2026)

This is the section most pages about ENDO-205 skip, so here it is in full.

Published studies: none

On September 11, 2026 we searched PubMed for “ENDO-205” and got zero results. A search for the company name returns only organic-chemistry papers about “endocyclic” double bonds. A search for the founder’s surname against endometriosis returns nothing. There is no peer-reviewed publication, poster or preprint on ENDO-205 that we could locate. We will re-run the search and update this page when that changes.

Preclinical results: reported, not shown

The IND press release says that “in preclinical studies, ENDO-205 demonstrated elimination of endometriosis lesions and associated inflammation, with no safety signals observed in GLP toxicology studies” (PR Newswire). Neither the animal model, the number of animals, the dose, the duration, nor the measure of “elimination” is stated anywhere public. The FDA has seen this data; you and we have not. That is normal for a company at this stage, and it is also the reason nobody outside the company can tell you whether the effect is large, durable, or real in tissue that behaves like human disease.

Human data: none, by definition

The Phase 1 trial is the first time ENDO-205 will enter a human body. Until it reports, every statement about how the drug behaves in people — efficacy, safety, dosing, how long it lasts — is a projection from animals. The company’s own toxicology summary is one sentence long: “we’ve seen no systemic effects in toxicology studies, the peptide simply doesn’t act where it doesn’t belong” (EndoCyclic, Peptide Platform).

What would change this section

Three things, in rough order of likelihood: a poster or paper on the preclinical work at a reproductive-medicine meeting; the Phase 1 record appearing on ClinicalTrials.gov with the design, dose levels and sites; and, eventually, Phase 1 safety results. Any one of them would give this page real numbers to report. Until then, the honest summary of the evidence for ENDO-205 is: a credible mechanism, an experienced regulator’s permission to test it, and no data you can read.

The Phase 1 Trial: What Is Being Tested

The trial is titled “A Phase I clinical trial of ENDO-205 for treatment of endometriosis in healthy pre-menopausal women of reproductive age.” As of its last update on April 1, 2026, Springer’s AdisInsight drug-pipeline database listed its status as “Planning” and its focus as adverse reactions (AdisInsight 700391838). On September 11, 2026 a search of ClinicalTrials.gov for ENDO-205, and separately for EndoCyclic as sponsor, returned no study record.

The participants are healthy volunteers — which in practice means women without the disease, though the eligibility criteria are unpublished. That surprises people, and it is standard. A first-in-human trial exists to find out whether a compound is safe at the doses planned, what the body does with it, and where the side effects start. It does that in healthy volunteers so that disease and drug effects cannot be confused. Whether ENDO-205 works comes later, in a Phase 2 trial that enrolls patients — and a Phase 2 would not normally start until Phase 1 has reported.

What this means if you have endometriosis and want to be part of the research, how trial sites are announced, what a Phase 2 in patients would look like, and how to get notified when enrollment opens is on its own page: the ENDO-205 clinical trial — status, eligibility and how to enroll.

ENDO-205 Side Effects: What Is and Is Not Known

ENDO-205 has no known side-effect profile, because no human has received it in a reported study. This section exists to say that clearly, to record what the company has said, and to set out the questions the trials exist to answer — not to predict.

What the company has said

Two statements, both unpublished: “no safety signals observed in GLP toxicology studies” (IND release) and “we’ve seen no systemic effects in toxicology studies” (company website). GLP toxicology means the animal safety studies were run to the FDA’s Good Laboratory Practice standard, which an IND must document (21 CFR 312.23), not a finding about people.

What the trials will have to show

A drug whose entire premise is selective uptake by diseased cells has to demonstrate the selectivity. The questions any reader of the eventual data should look for are whether healthy endometrium and ovarian tissue take up the peptide; what happens to fertility and menstrual cycles during and after a course; whether the immune system reacts to a foreign peptide on repeat dosing; and, if it turns out to be injected, what the local reactions look like. None of these has an answer today. When Phase 1 reports, this section will carry the incidence numbers with the same precision as the Orilissa and Myfembree table above.

Why this page is not a side-effects page yet

Our retatrutide side-effects page can tell you the exact percentage of trial participants who had nausea at each dose because the trials were published. Our emideltide page can draw on forty years of human studies. ENDO-205 has neither, and a standalone side-effects page with no data would be a page with a title and nothing under it. When the first human safety results exist, we will give them their own page and link it here.

Is ENDO-205 Available or For Sale?

No. ENDO-205 is an investigational drug. The only way a person can lawfully receive it is inside a clinical trial run by or for the company, and that trial has not yet enrolled anyone publicly. There is no approved product, no compassionate-use program announced, and no manufacturer other than the sponsor.

Why a vial labelled ENDO-205 cannot be checked

Research-chemical vendors sell peptides by name. For a compound like retatrutide that works, imperfectly, because the molecule is public: a certificate of analysis can confirm that what is in the vial has the right mass and purity, and you can read the COA against a known standard. ENDO-205’s sequence has not been published. There is no reference standard, no public mass to match, and no way for any third-party lab to say “this is ENDO-205.” A COA attached to such a listing can only tell you that a peptide of some kind is present at some purity. It cannot tell you which peptide.

What listings will look like when they appear

They are likely to appear. Ahrefs first recorded US searches for “endo 205 for sale,” “how to get endo 205” and “endo 205 price” between May and August 2026, months after the IND announcement, and vendor listings tend to follow that kind of demand. Expect the usual framing: “for research use only,” a purity figure, a stock photo of a lyophilized vial, and a price. Everything on our guide to spotting a fake peptide website applies, with one addition specific to this compound: for ENDO-205 there is no version of the listing that can be verified, however professional it looks, because the thing it claims to contain is not publicly defined.

What we will do

If and when a vendor lists a product as ENDO-205, this section will record what is being sold under the name, what its documentation claims, and what — if anything — can be checked. We do not link to, price, or compare such listings on this page, and we will not until there is a molecule to compare them against — our price tracking covers only compounds with a published structure. Our lab-testing coverage and vendor reviews explain how we handle compounds where verification is possible.

When Will ENDO-205 Be Available? A Realistic Timeline

Nobody has given a date, and anyone who does is guessing. What can be said is the sequence every new drug follows and where ENDO-205 sits in it.

1. Phase 1 (now, in planning). Safety and pharmacology in healthy volunteers. Typically dozens of participants and well under a year once enrollment starts. The ClinicalTrials.gov entry, first dosing, and the safety readout are the three events to watch.
2. Phase 2. The first trial in women with endometriosis: does it reduce lesions, pain, or both, at which dose, over what course. This is where the “eliminates lesions” claim gets its first human test. Usually one to two years including enrollment.
3. Phase 3. Larger trials designed to support approval. Multiple years.
4. NDA review. The FDA’s formal review; its PDUFA goal is ten months from filing for a standard review and six for a priority review, and new molecular entities get two months more.

Added together, that is the several-year path that applies to essentially every new drug; a Reddit commenter’s estimate of ten to twelve years is at the pessimistic end, and expedited pathways exist for serious conditions with unmet need, but the company has announced none. The first public milestone that would shorten anyone’s uncertainty is a ClinicalTrials.gov record with a start date. It did not exist on September 11, 2026.

Who Is EndoCyclic Therapeutics?

EndoCyclic Therapeutics is a woman-owned biotechnology company in Irvine, California, founded in 2017 by biochemist Tanya Petrossian, who serves as CEO (Drug Discovery News; NIH SEED). Its development has been funded substantially by the National Institutes of Health: the company reports completing Phase I, II and IIB NIH Small Business Innovation Research grants, and in September 2025 it announced a Commercialization Readiness Pilot award from the National Institute of Child Health and Human Development with a perfect overall impact score of 10 (PR Newswire, September 16, 2025). The company says it has been recognized as an NIH SBIR Success Story.

ENDO-205 is one of four programs on the company’s pipeline page. The others are FemLUNA, an imaging agent intended as a non-invasive diagnostic for endometriosis (the NIH showcase refers to an MRI diagnostic agent as ENDO-210); ENDO-995, a therapeutic for solid tumors; and ENDO-311, an imaging diagnostic for solid tumors (EndoCyclic, Pipeline). All are described as investigational, and the company does not publish development stages for them. If you have seen the query “EndoCyclic stock”: we found no record of publicly traded shares.

What People Are Asking

There are no user reports of ENDO-205, because no one outside a trial has taken it. What exists is discussion, and three threads in r/endometriosis carry most of it. We summarize the themes below with links to the threads; no usernames are reproduced, and nothing here is evidence about the drug.

• The announcement thread (March 24, 2026; about 460 upvotes and 60 comments) is mostly relief that a non-hormonal candidate exists at all. A commenter identifying as an endometriosis surgeon said they were meeting the company. The recurring worry, even on day one, was cost and insurance coverage if it ever reaches market.
• The Tia-article thread (April 28, 2026; about 380 upvotes) turned quickly to volunteering — “where do we sign up” was the question, and one that this page’s trial companion answers as far as anyone can. Several commenters drew the parallel to cancer therapy, since a drug that selectively kills a tissue is closer in concept to targeted oncology than to hormone treatment. One commenter cautioned the thread that IND clearance means testing can begin, not that the drug is coming soon.
• The July thread (July 13, 2026) is where the timeline reality set in: an estimate of ten to twelve years to availability, questions about whether it would ever reach Europe or Australia, and one self-described researcher’s skepticism that the pathway they believe is being targeted had produced results when tried before. That last point is exactly the kind of question a Phase 2 trial exists to settle.

What is missing from all three threads, and from the search queries around this drug, is anyone reporting having used it. If you encounter such a report, treat it with the skepticism the for-sale section above explains: there is currently no way for a person to have obtained the actual compound.

What We Are Watching

This page is updated when the record changes. The log below is dated so you can see what was known when.

• 2026-09-11 — Page published. PubMed: 0 results for “ENDO-205.” ClinicalTrials.gov: no record for the compound or the sponsor. AdisInsight: Phase I status “Planning,” last updated 2026-04-01. No vendor listings observed. No route of administration or dose disclosed.

Triggers that will produce a new dated line: a ClinicalTrials.gov registration; a company announcement of first dosing; any peer-reviewed publication or conference abstract; Phase 1 safety results; a vendor listing under the ENDO-205 name; a Phase 2 announcement.

Frequently Asked Questions

What is ENDO-205?
ENDO-205 is an investigational non-hormonal peptide drug for endometriosis developed by EndoCyclic Therapeutics of Irvine, California. It is built on what the company describes as a cyclic-peptide platform and is designed, in the company’s and its founder’s account, to be taken up selectively by endometriosis lesion cells and eliminate them. The company announced FDA clearance for a first human trial on March 23, 2026. It is not approved and has no published human or animal data.

What does ENDO-205 do?
According to the company, it enters endometriotic cells through a pathway active only in diseased tissue and activates in response to local pH; in the founder’s account to Drug Discovery News it then triggers programmed cell death, eliminating lesions rather than suppressing the hormones that drive them. These descriptions rest on preclinical results that have not been published or independently reviewed.

When will ENDO-205 be available?
No date exists. The drug is entering Phase 1, a safety trial in healthy volunteers, which had no ClinicalTrials.gov record as of September 11, 2026. It would then need a Phase 2 trial in patients, Phase 3 trials, and FDA review — a multi-year path for any new drug. The company has not announced an expedited pathway.

Is ENDO-205 available for sale?
No. It is an investigational drug available only within a clinical trial. Its amino-acid sequence has not been published, so any product sold under the name ENDO-205 cannot be verified as the actual compound by any laboratory test. We do not link to or price such listings.

What are the side effects of ENDO-205?
Unknown. No human has received ENDO-205 in a reported study. The company states that no safety signals were observed in GLP toxicology studies, but that data is unpublished. The Phase 1 trial exists to establish the first human safety profile, and this page will report its incidence figures when they are released.

Is ENDO-205 hormonal?
The company describes it as non-hormonal. Unlike Orilissa, Myfembree, leuprolide and progestin-based treatments, which lower or block estrogen throughout the body, ENDO-205 is designed to act only inside endometriosis lesion cells and leave hormone levels alone. Whether it achieves that in people is what the clinical trials will test.

What is the best peptide for endometriosis?
There is no approved peptide that treats endometriosis itself. GnRH-agonist peptides such as leuprolide, goserelin and nafarelin are approved for endometriosis and work by suppressing estrogen. ENDO-205 is the only peptide we are aware of with FDA clearance to begin trials as a lesion-targeting treatment, and it has no human data yet. Research peptides marketed for endometriosis pain, such as BPC-157, have no clinical evidence in this condition.

Glossary

• IND (Investigational New Drug application) — the FDA filing that permits a drug to be tested in humans. Clearance means testing may begin; it is not approval.
• Phase 1 — the first human trial of a drug, usually in healthy volunteers, designed to establish safety, tolerability and how the body handles the compound.
• Cyclic peptide — a peptide whose chain is joined into a ring, which generally makes it more stable in the body than a linear peptide; the modality the company describes for its platform.
• Apoptosis — programmed cell death; the mechanism by which, in the account given to Drug Discovery News, ENDO-205 eliminates lesion cells.
• Endometriotic lesion — tissue resembling the uterine lining growing outside the uterus; the defining feature of endometriosis.
• GnRH antagonist — a drug that blocks gonadotropin-releasing hormone receptors in the pituitary, lowering estrogen; the class that includes Orilissa and Myfembree.
• Disease-modifying — a treatment that changes the course of a disease rather than relieving its symptoms; the company’s claim for ENDO-205, untested in humans.
• GLP toxicology — animal safety studies run under the FDA’s Good Laboratory Practice regulations, required before an IND.

Primary Sources

• EndoCyclic Therapeutics Announces FDA Clearance of Investigational New Drug (IND) Application for ENDO-205 — PR Newswire, March 23, 2026
• EndoCyclic Therapeutics Awarded Rare NIH “Perfect 10” Grant for Endometriosis Therapeutic — PR Newswire, September 16, 2025
• EndoCyclic Therapeutics — Peptide Platform and Pipeline pages, accessed September 2026
• NIH SEED Portfolio Company Showcase: EndoCyclic Therapeutics — seed.nih.gov
• A Phase I clinical trial of ENDO-205 for treatment of endometriosis in healthy pre-menopausal women of reproductive age — AdisInsight trial 700391838, updated April 1, 2026
• Is this the first disease-modifying treatment for endometriosis? — Drug Discovery News
• FDA clears ENDO-205 Investigational New Drug application for endometriosis — Contemporary OB/GYN
• ENDO-205 and the Future of Endometriosis Treatment: Breakthrough or Hype? — Endometriosis Specialist Surgical Institute, March 31, 2026
• Endometriosis fact sheet — World Health Organization, October 15, 2025
• Zondervan KT, Becker CM, Missmer SA. Endometriosis. N Engl J Med 2020 (PubMed)
• Guo SW. Recurrence of endometriosis and its control. Hum Reprod Update 2009 (PubMed)
• Taylor HS et al. Treatment of Endometriosis-Associated Pain with Elagolix, an Oral GnRH Antagonist. N Engl J Med 2017 (PubMed)
• Giudice LC et al. Once daily oral relugolix combination therapy versus placebo in patients with endometriosis-associated pain (SPIRIT 1 and 2). Lancet 2022 (PubMed)
• ORILISSA (elagolix) prescribing information, AbbVie, revised 12/2025 — DailyMed
• MYFEMBREE (relugolix, estradiol, and norethindrone acetate) prescribing information, Sumitomo Pharma America, revised 8/2025 — DailyMed
• PubMed search “ENDO-205,” September 11, 2026: 0 results. ClinicalTrials.gov searches for “ENDO-205” and sponsor “EndoCyclic,” September 11, 2026: no records.

Peptide Insider is an independent research publication. ENDO-205 is an investigational drug that is not approved for any use and is not available outside clinical trials. Nothing on this page is medical advice; decisions about endometriosis treatment belong with you and your clinician. Reddit discussion is paraphrased and linked to its threads; no usernames are reproduced. Company statements are quoted and attributed as such and have not been independently verified. See our medical disclaimer and editorial policy.