ENDO-205 Clinical Trial: Phase 1 Status, Who Can Enroll, and How to Get Notified

The ENDO-205 clinical trial is a Phase 1 safety study in healthy volunteers — women without the disease, as far as the public description goes. The company announced FDA clearance for it on March 23, 2026; as of September 11, 2026 it was listed as “Planning” in the AdisInsight drug-pipeline database and had no record on ClinicalTrials.gov, which means no site, start date or enrollment contact has been published. If you have endometriosis and want to take part in ENDO-205 research, the trial you are waiting for is Phase 2, and it would not normally start until this one has reported. This page explains what the Phase 1 is, who it enrolls, how trials like it are announced, and how to be notified when a patient study opens.

In This Guide

• What has been registered for the ENDO-205 clinical trial so far
• Who the Phase 1 enrolls, and why it is not patients
• What a Phase 1 trial measures
• Where and when: what is known about sites and dates
• What the Phase 2 patient trial would look like
• How to get notified when enrollment opens
• Frequently asked questions
• Update log
• Primary sources

What Has Been Registered for the ENDO-205 Clinical Trial So Far

Two public records exist, and one expected record does not.

The IND clearance. On March 23, 2026 EndoCyclic Therapeutics announced that the FDA had cleared its Investigational New Drug application for ENDO-205 and that “the company plans to initiate a Phase 1 clinical study in healthy pre-menopausal women of reproductive age” (PR Newswire). The release did not give a start date, site list, or enrollment target.

The AdisInsight entry. Springer’s AdisInsight database, which tracks drug development from company disclosures, lists the trial with sponsor EndoCyclic Therapeutics, status “Planning,” and focus “Adverse reactions,” last updated April 1, 2026 (AdisInsight 700391838). Design details behind that entry require a subscription and, as far as we can tell, come from the same press release.

No ClinicalTrials.gov record. On September 11, 2026 we queried ClinicalTrials.gov for the term “ENDO-205” and separately for studies sponsored by EndoCyclic. Neither returned a study. US law requires most interventional drug trials to be registered within 21 days of enrolling the first participant, and Phase 1 drug trials are exempt from that requirement (FDAAA Section 801; 42 CFR Part 11) — so the absence of a record is consistent with either “not yet enrolling” or “enrolling but not required to post.” It does mean there is no public listing you can apply through.

Who the Phase 1 Enrolls, and Why It Is Not Patients

The trial title says it: healthy pre-menopausal women of reproductive age. Eligibility criteria have not been published, but “healthy” in a first-in-human study means volunteers without the disease under study — if you have endometriosis, you are almost certainly not the population this study is recruiting, and that is by design.

A first-in-human trial has one job: to find out what a compound does in a person before anyone with the disease is exposed to it. It answers whether the drug is tolerated at the doses planned, how it is absorbed and cleared, where side effects begin, and whether anything unexpected happens. Running that in healthy volunteers keeps the answer clean — an adverse event cannot be blamed on the disease, and a lab abnormality cannot be blamed on prior treatments. The “pre-menopausal women of reproductive age” specification is presumably the one concession to the target population: a drug intended for women with cycling hormones is tested in women with cycling hormones.

The company’s stated premise is that healthy tissue does not take the peptide up at all — “Healthy cells don’t absorb our peptides” (EndoCyclic, Peptide Platform). A Phase 1 in volunteers without lesions is, in effect, the first test of that claim: if the drug is as selective as described, healthy participants should show little beyond the effects of administration itself. That result would be reassuring. It would not tell anyone whether ENDO-205 works, because the participants have no lesions to eliminate.

What a Phase 1 Trial Measures

The AdisInsight entry lists the focus as adverse reactions. In practice a Phase 1 of a new peptide almost always covers the following, and the eventual ClinicalTrials.gov record will spell out which apply here.

1. Safety and tolerability — every adverse event, graded by severity, at each dose level tested. This becomes the first human side-effect data for ENDO-205, and the first numbers we can put on our side-effects section — reported the way our retatrutide side-effects page reports published trial incidence, dose by dose.
2. Dose escalation — small groups receive increasing doses, each group starting only after the previous one is reviewed, to find the range that is tolerated. The doses tested will be the first public information about how much ENDO-205 is given.
3. Pharmacokinetics — how quickly the peptide appears in blood, how long it stays, and how it is cleared. For a peptide designed to act inside tissue, the blood levels are also an indirect check on the selectivity claim.
4. Route of administration — not yet disclosed. The trial record will be the first place it is stated publicly.

What a Phase 1 does not measure is efficacy. There are no lesions to shrink and no pain to score in healthy volunteers. Anyone describing this trial as a test of whether ENDO-205 “works” has misread it.

Where and When: What Is Known About Sites and Dates

Nothing has been published about trial sites, and no start date has been announced. The ESSI commentary noted in March that human trial locations “have not yet been made available” (ESSI, March 31, 2026); five months later that is still the case. The company is based in Irvine, California, which tells you nothing reliable about where a Phase 1 unit would be — early-phase studies are commonly run at specialist clinical pharmacology units under contract.

How these details become public, in the usual order: the sponsor registers the study on ClinicalTrials.gov, which lists sites, a contact, eligibility criteria and status; a press release announces “first participant dosed”; and, for a company that has been active with the press, a trade-press interview fills in the design. Any of those could happen at any time. None had happened by September 11, 2026.

If you want to ask the company directly, its website carries contact details; we do not reproduce them here because they change, and because a Phase 1 in healthy volunteers is not recruiting patients in any case.

What the Phase 2 Patient Trial Would Look Like

This is the trial people with endometriosis are actually asking about when they ask how to sign up. It does not exist yet, and it would not normally begin until the Phase 1 reports and the company designs the next study around those results.

A Phase 2 in endometriosis would enroll women with a confirmed diagnosis — surgically or, increasingly, by imaging — and would need to show the two things the company has claimed: that lesions shrink or disappear, and that symptoms follow. The recent endometriosis drug programs give a sense of the shape. The elagolix (Orilissa) pivotal trials scored dysmenorrhea and non-menstrual pelvic pain daily in an electronic diary over six months, in 872 women in the first of two trials (PMID 28525302); the relugolix combination (Myfembree) trials used similar pain endpoints over 24 weeks in 1,261 women across two studies, 638 and 623 (PMID 35717987); the label’s pooled safety comparison covers 418 on the combination and 416 on placebo. A lesion-eliminating drug would likely add an imaging or surgical endpoint on top of pain, because pain relief alone is what the hormonal drugs already deliver.

The NIH SEED showcase describes the intended treatment as “a short-term treatment course… lasting up to 3 months” (NIH SEED), so a Phase 2 would probably follow participants well past the end of dosing to see whether lesions and symptoms stay away — the recurrence problem that makes surgery unsatisfying, with rates of 21.5% at two years and 40–50% at five (PMID 19279046), is exactly what a disease-modifying drug would have to beat.

Timing: the Phase 1 has to enroll, dose, follow and analyze, then the Phase 2 has to be designed, approved and opened. If the Phase 1 started in late 2026, a Phase 2 opening in 2027 or 2028 would be an ordinary pace. That is an inference from how drug development usually runs, not a company statement.

How to Get Notified When Enrollment Opens

• ClinicalTrials.gov. Search “ENDO-205” and save the search; the site can email you when a matching record is posted or changes. Also search by sponsor name, since a record may be titled by mechanism rather than code name.
• The sponsor. EndoCyclic Therapeutics’ website is where it has announced each milestone so far; the two major releases went out through PR Newswire on September 16, 2025 and March 23, 2026.
• This page. The update log below records every ClinicalTrials.gov and AdisInsight check and every change. The companion ENDO-205 page covers the drug itself, its evidence, side effects and availability.
• Your clinician. Endometriosis specialists learn about recruiting trials through the sponsor and through professional networks before the general public does, and a Phase 2 will almost certainly require a confirmed diagnosis that your own records can supply.

One caution. As interest in ENDO-205 has grown, so has search demand for buying it. There is no legitimate way to obtain the drug outside a trial, its sequence has never been published, and anything sold under its name cannot be verified as the compound — our availability section explains why. A trial is the only route, and this page will say so the day one opens to patients.

Frequently Asked Questions

Can I sign up for the ENDO-205 clinical trial?
Not currently. The only trial cleared so far is a Phase 1 safety study in healthy pre-menopausal women without endometriosis, and as of September 11, 2026 it had no public enrollment listing. Women with endometriosis would be eligible for a later Phase 2 trial, which has not been announced.

Do I need an endometriosis diagnosis to join the ENDO-205 trial?
For the Phase 1, the study is designed for healthy volunteers, so a diagnosis would most likely exclude you; the criteria have not been published. For the eventual Phase 2, a confirmed diagnosis will almost certainly be required. No eligibility criteria for either study have been published.

Where is the ENDO-205 trial taking place?
No sites have been announced. The sponsor, EndoCyclic Therapeutics, is based in Irvine, California, but early-phase trials are usually run at specialist clinical units that may be anywhere. Site information normally appears first on ClinicalTrials.gov.

When will the ENDO-205 trial start?
No start date has been announced. The company announced FDA clearance on March 23, 2026, the AdisInsight database listed the trial as “Planning” on April 1, 2026, and no ClinicalTrials.gov record existed on September 11, 2026.

Is ENDO-205 being tested in Europe or Australia?
No trials outside the United States have been announced. The IND clearance is a US FDA authorization; trials elsewhere would require separate regulatory approval and would typically appear on ClinicalTrials.gov or the EU and Australian registries.

What will the Phase 1 trial tell us about ENDO-205?
Whether it is tolerated at the doses tested, what side effects appear and at what rate, how the body absorbs and clears it, and how it is administered. It will not show whether ENDO-205 shrinks lesions or relieves pain, because the participants do not have endometriosis.

Update Log

• 2026-09-11 — Page published. ClinicalTrials.gov: no record for “ENDO-205” or sponsor “EndoCyclic.” AdisInsight: Phase I, status “Planning,” focus adverse reactions, updated 2026-04-01. No sites, dates, dose levels or route disclosed. No Phase 2 announced.

Primary Sources

• EndoCyclic Therapeutics Announces FDA Clearance of Investigational New Drug (IND) Application for ENDO-205 — PR Newswire, March 23, 2026
• 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
• ClinicalTrials.gov searches for “ENDO-205” and sponsor “EndoCyclic,” September 11, 2026: no records
• EndoCyclic Therapeutics — Peptide Platform, accessed September 2026
• NIH SEED Portfolio Company Showcase: EndoCyclic Therapeutics — seed.nih.gov
• ENDO-205 and the Future of Endometriosis Treatment: Breakthrough or Hype? — ESSI, March 31, 2026
• 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)
• Guo SW. Recurrence of endometriosis and its control. Hum Reprod Update 2009 (PubMed)

Peptide Insider is an independent research publication and is not affiliated with EndoCyclic Therapeutics or any trial site. ENDO-205 is an investigational drug available only within clinical trials. Nothing on this page is medical advice; decisions about joining a trial belong with you and your clinician. See our medical disclaimer and editorial policy.