Thymosin beta-4 eye drops for dry eye: the 72-person trial missed its main goals

In a phase 2 trial of 72 people with moderate to severe dry eye, thymosin beta-4 eye drops used twice daily for 28 days didn't beat placebo on either of the two outcomes the trial was designed to test [1]. The authors then ran further analyses and found improvements in some other measures of corneal damage and discomfort. The drops caused no worrying side effects, but the evidence that they work is weaker than the paper's conclusion suggests.

Why this study matters

Thymosin beta-4 (Tβ4) is a 43-amino-acid peptide found in platelets and white blood cells, where it helps control the actin scaffolding inside cells [1]. In rats and mice with injured corneas, Tβ4 drops sped up healing and lowered inflammatory signals [1]. Whether that translates into people is exactly the kind of question a placebo-controlled trial exists to answer.

It's also relevant to anyone who has come across Tβ4 in the injectable peptide world, where the name “TB-500” gets attached to it. This trial tested one thing only: a sterile, preservative-free 0.1% eye-drop formulation of synthetic Tβ4, called RGN-259 [1]. It tells you nothing about injected products.

We picked this trial over the other published phase 2 study, a nine-patient trial in severe dry eye [2], because it's eight times larger and pre-specified its main outcomes. That second trial comes up again below.

What they did

The trial used a “controlled adverse environment” chamber, which standardises the conditions that aggravate dry eye and, according to the authors, gives a treatment more room to show an effect [1]. Participants went into the chamber before starting treatment and again on day 28, with check-ups the day after and two days after.

The two primary outcomes, fixed in advance, were inferior corneal staining (a dye test that shows damaged surface cells on the lower cornea) and ocular discomfort, both measured on day 29, the day after the second chamber visit [1]. Each person's worse eye was analysed.

What they found

On the pre-specified outcomes, Tβ4 didn't win. Mean inferior corneal staining was 2.08 on Tβ4 and 1.90 on placebo (p = 0.26); ocular discomfort was 1.6 versus 1.3 (p = 0.22) [1]. Both numbers were slightly higher, meaning slightly worse, in the Tβ4 group, though neither gap was meaningful.

The authors then adjusted for baseline differences in “several ad hoc statistical analyses”, in their words, and found some significant results [1]:

The superior staining p value is reported as 0.0210 in the abstract and 0.0109 or 0.0181 in the paper's table, depending on the test; the discomfort result appears as 0.0244 in the abstract and 0.0224 in the text [1]. Small inconsistencies like these don't change the picture, but they're the kind of detail that makes us read a paper's conclusions more carefully.

The core problem is statistical. When the main outcomes fail and several alternative analyses are run afterwards, some will cross p = 0.05 by chance. These results are worth testing in a new trial. They aren't proof.

Safety

Here the paper's own summary and its results section don't quite agree. The abstract says no adverse events were observed. The full text reports that two people on Tβ4 (5.6%) and five on placebo (13.9%) had at least one eye-related adverse event, all mild on Tβ4: one case each of eye pain, blurred vision and a stye [1]. One placebo-group participant withdrew because of vomiting judged unrelated to treatment. Vision, eye pressure, corneal sensitivity and eye examinations showed no clinically significant changes [1].

The fair reading is that 28 days of these drops caused no safety concerns in 36 treated people. That's reassuring, and also a small, short exposure.

The nine-patient trial

The other phase 2 study randomised nine patients with severe dry eye, some with graft-versus-host disease, at two US sites [2]. They used 0.1% Tβ4 or vehicle drops six times a day for 28 days, then were followed for another 28 days. At day 56, the Tβ4 group (12 eyes) had 35.1% less ocular discomfort than the vehicle group (6 eyes, p = 0.014) and 59.1% less corneal staining (p = 0.011) [2]. The authors reported the drops as safe and well tolerated [2].

Those are bigger effects than the 72-person trial found, in sicker patients, but with nine people the estimates are fragile. A single responder can move the averages a long way.

Caveats

• Primary outcomes failed. Everything positive in the larger trial comes from secondary or post hoc analyses [1].
• Short and small. 28 days, 72 people, a single site [1]. Dry eye is a chronic condition, a point the authors make themselves [1].
• Artificial setting. The chamber model is designed to provoke symptoms and may not reflect everyday dry eye.
• Who's behind it. The drug was RegeneRx's RGN-259 formulation [1]. The authors declared no conflicts of interest, and the text we read doesn't include a separate funding statement.
• Mostly White women. 75% women and 93% White [1].

What it means in practice

Tβ4 eye drops looked safe over four weeks, and a handful of secondary measures hinted at less corneal damage and less discomfort under stress. But the one trial big enough to be taken seriously didn't meet its own targets. On this evidence, Tβ4 drops for dry eye are a plausible idea that hasn't been proven, and neither trial says anything about Tβ4 given any other way.

What we still don’t know

• Whether the secondary improvements hold up in a trial that makes them the primary outcomes. A PubMed search for RGN-259 in September 2026 turned up no peer-reviewed phase 3 dry-eye results.
• Whether effects last beyond four weeks, or what longer use does to the eye.
• Which patients, if any, benefit most. The nine-patient trial enrolled sicker people and saw larger effects, but it's far too small to say whether severity is the reason.

What readers report

First-hand accounts, shared with permission. They are self-reported, not evidence, and sit alongside the research above rather than in place of it.

Educational content only — not medical advice. Many peptides discussed on HPR are not approved for human use. Talk to a qualified clinician before making any decision about your health.

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