Two Phase 2 Readouts, Same Endpoint: What Earns a Phase 3?
Why a promising efficacy signal is only the beginning of a development decision.
A Phase 2 trial can produce the result a development team has been waiting for: a measurable improvement in patients, a biological rationale that appears to hold, and enough evidence to justify considering the next stage.
But a positive result does not automatically make a program ready for Phase 3.
The harder question is what that result can support. Does the observed benefit provide a credible basis for a larger, longer, and more demanding trial? Does the evidence establish a treatment effect, or does it leave important uncertainty about whether the effect is real, reproducible, and relevant to the disease patients live with?
Two rheumatoid arthritis (RA) programs help illustrate why these questions matter. Tofacitinib, an oral Janus kinase (JAK) inhibitor, generated a positive Phase 2b efficacy signal and advanced through Phase 3 to regulatory approval. Zunsemetinib, formerly ATI-450, an investigational oral inhibitor of the MK2 pathway, did not meet its primary or secondary efficacy endpoints in a later Phase 2b trial, and its developer discontinued the program.
Both studies used the American College of Rheumatology 20% improvement response, or ACR20, at Week 12 as their primary efficacy endpoint.
That shared endpoint creates a useful starting point for examining how Phase 2 evidence informs development decisions. It does not make the trials directly comparable, and it does not establish why one program advanced while the other stopped. The value of the comparison lies elsewhere: in understanding what a clinical signal establishes, what remains uncertain, and how the next investment should depend on the evidence available.
Start with the patient, not the percentage
Rheumatoid arthritis is a chronic inflammatory disease that can affect joints, physical function, and everyday activities. For patients, improvement is not an abstract percentage. It can mean less pain, less stiffness, better mobility, and a greater ability to manage ordinary responsibilities.
That lived experience also makes RA a demanding disease in which to evaluate treatment. Symptoms and disease activity can vary, and a patient’s response cannot be fully described by a single laboratory value or one isolated measure.
ACR20 provides a standardized way to assess improvement. It generally requires at least a 20% improvement in both tender and swollen joint counts, alongside improvement in at least three of five additional measures, which can include patient and physician assessments, pain, physical function, and an inflammatory marker.
The endpoint is useful because it combines several aspects of disease activity rather than relying on one measurement alone. It also offers a defined outcome that can be assessed across treatment groups.
But ACR20 has boundaries. A patient meeting the response criterion has demonstrated improvement according to the trial definition. That does not necessarily mean the patient has achieved low disease activity, remission, sustained functional recovery, or freedom from future disease progression.
The distinction matters when interpreting an early efficacy result. A response endpoint can establish that a treatment has produced a measurable effect over a specified period. It cannot, by itself, answer every question a development team needs to resolve before committing to a confirmatory trial.
The first task is to understand exactly what the endpoint establishes, without asking it to prove more than it can.
What the tofacitinib Phase 2b trial established
In 2012, Kremer and colleagues reported a 24-week, randomized, double-blind, placebo-controlled Phase 2b dose-ranging study of tofacitinib in 507 patients with active RA who had an inadequate response to methotrexate. Patients continued stable background methotrexate and received placebo or one of several tofacitinib regimens.
The primary endpoint was ACR20 response at Week 12. The results showed a clear separation between placebo and the active treatment groups at doses of at least 3 mg twice daily.
The response rates for the listed active regimens were statistically significantly higher than placebo. The investigators also reported sustained improvements through Week 24 across several clinical measures, including other ACR response levels, physical function, and disease activity measures. [1]
The result gave the development team more than an encouraging number. It provided evidence of a treatment effect against a control group, across multiple active dose regimens, in a defined patient population. The Week 24 findings also offered an early indication that the observed improvement was not limited to the primary Week 12 assessment.
That distinction is important. The trial did not establish long-term disease control or settle every question about safety and appropriate dosing. A 24-week study cannot substitute for the larger and longer evidence base required to characterize a treatment fully. However, it provided a credible foundation for further investigation.
The next question was whether the signal would hold up in larger studies and across the broader evidence requirements for development.
What happened when the evidence moved forward?
Tofacitinib subsequently advanced through Phase 3 testing. In the ORAL Step trial, published in The Lancet in 2013, researchers studied patients with active RA who had an inadequate response to tumor necrosis factor inhibitors, a different and more treatment-refractory population.
At Month 3, ACR20 response rates were 41.7% with tofacitinib 5 mg twice daily, 48.1% with 10 mg twice daily, and 24.4% with placebo. The study also reported improvements in physical function and other disease activity measures. [2]
These results should not be read as a direct replication of the earlier Phase 2b trial. The patient population and study design differed, and the response percentages should not be compared as if they came from one experiment. Nevertheless, the Phase 3 findings provided further evidence that tofacitinib could produce clinically meaningful improvements in a larger, controlled study.
The program continued through clinical development and ultimately received regulatory approval for RA.
The lesson is not that a particular Week 12 percentage guarantees success. Nor does the later outcome prove that every relevant uncertainty was resolved in Phase 2. It is that an early signal can provide a basis for the next investment when it is supported by a coherent body of evidence and can be tested further in a larger development program.
Phase 2 created a rationale for progression. Subsequent trials had to do the work of validating that rationale.
The zunsemetinib readout: when the signal was not enough
Zunsemetinib followed a different path.
The investigational oral MK2 inhibitor was developed to target a different part of the inflammatory signaling pathway. Its earlier Phase 2a study in RA had reported promising pharmacodynamic findings and clinical activity in a small treatment group, supporting further investigation of the mechanism. [3]
Those early observations were encouraging, but they came from a small study and were not equivalent to a definitive demonstration of efficacy. The larger question remained whether the treatment could produce a convincing clinical benefit under the conditions of a Phase 2b trial.
In November 2023, Aclaris Therapeutics announced topline results from a randomized, double-blind, placebo-controlled Phase 2b study involving 251 patients with moderate-to-severe RA who had an inadequate response to methotrexate. Patients received one of two doses of zunsemetinib or placebo, alongside methotrexate. The primary endpoint was ACR20 response at Week 12, with ACR50, ACR70, DAS28-CRP, and other measures among the secondary endpoints. [4]
The company reported that neither zunsemetinib dose met the primary endpoint or any of the secondary efficacy endpoints. It also reported no notable differentiation from placebo across the efficacy measures at Week 12.
Based on the overall program results, Aclaris decided to discontinue further development of zunsemetinib, including halting enrollment in its ongoing Phase 2a psoriatic arthritis trial. [4]
The distinction between the earlier and later readouts is instructive. The Phase 2a findings had supported continued investigation. The larger Phase 2b study did not provide the clinical efficacy evidence needed to justify further development under the program’s existing rationale.
This does not mean the earlier findings were fabricated or that the biological observations had no value. It means that encouraging early observations did not translate into a successful result on the larger trial’s prespecified efficacy endpoints.
A development hypothesis can be biologically plausible and still fail to produce the clinical outcome needed to move forward.
The endpoint was the same. The evidence was not.
It is tempting to compare the two programs by looking at the endpoint and asking which drug produced the bigger effect. That approach is not supported by the available public results.
Although both Phase 2b studies used ACR20 at Week 12 as the primary endpoint, their published reports are not a head-to-head comparison. The programs targeted different mechanisms, and their study designs, treatment regimens, and development contexts were not identical. Aclaris’s topline announcement did not provide a complete set of numerical ACR20 response rates that would support a detailed quantitative comparison with the tofacitinib trial.
We therefore cannot conclude from these studies that one mechanism is superior to the other, that a particular effect-size threshold explains the divergent outcomes, or that durability, patient heterogeneity, payer considerations, or another specific factor caused one program to advance and the other to stop.
Those would require evidence beyond what these readouts establish.
What the comparison does show is a difference in the clinical evidence available to support the next decision. Tofacitinib’s Phase 2b study demonstrated statistically significant ACR20 improvements versus placebo at several doses and reported sustained improvements through Week 24.
Zunsemetinib’s Phase 2b study did not demonstrate differentiation from placebo on its primary or secondary efficacy endpoints at Week 12.
For a development team, those are materially different evidence positions.
The tofacitinib findings supported further testing of a defined clinical signal. The zunsemetinib findings did not support continued development of the program as it stood.
The important question is not whether the two programs had the same endpoint on paper. It is what each trial established, how much uncertainty remained, and whether the results justified the next commitment.
What should a Phase 2 readout change?
A Phase 2 result should do more than produce a positive or negative label. It should change what the team knows and clarify what the next decision requires.
For a program showing an efficacy signal, that means examining the strength and consistency of the treatment effect, the relevance of the endpoint to the disease, the duration of observed benefit, the population in which the effect was measured, and the remaining safety questions. It also means defining what the next trial must demonstrate to confirm or challenge the current hypothesis.
For a program that fails to separate from placebo, the response should be equally evidence-led. The team needs to determine what the trial can and cannot explain. Did the study fail to demonstrate efficacy? What do the available data establish about exposure, biological activity, endpoint performance, and the treatment hypothesis? Which uncertainties can reasonably be investigated, and which findings undermine the rationale for further investment?
These questions do not guarantee that a program can be rescued, and they should not be used to rationalize continuation after a negative readout. Sometimes the evidence supports stopping. In other cases, it may identify a specific uncertainty that can be resolved through a justified next study. The distinction must come from the data, not from a desire to preserve the program.
A useful Phase 2 decision therefore has three parts:
This turns the Phase 2 readout into a development decision rather than a milestone to celebrate or survive.
From an early signal to a defensible investment
The contrast between tofacitinib and zunsemetinib is not a simple story of one drug succeeding and another failing. It is a reminder that clinical development depends on evidence accumulating in stages.
A negative result is also evidence: it can reveal that the current clinical hypothesis has not earned further investment, even when the underlying mechanism once appeared promising.
For clinical development leaders, portfolio strategists, and investors, the practical challenge is to understand what the readout means before committing to the next trial. That requires looking beyond whether the primary endpoint was met and examining the strength of the signal, the boundaries of the evidence, and the specific uncertainties that remain.
The question is not simply whether Phase 2 produced a positive result. It is whether the evidence justifies the next investment, what could still change the decision, and what the next study must prove.
Before you commit to a confirmatory trial, know what your Phase 2 results can support, where the evidence is still thin, and what needs to be tested next.
Connect with Rubix LS to identify the evidence gaps that matter most to your program and build a clearer path from early efficacy signals to confirmatory evidence.
Rethink Research. Evidence that moves.
References
Kremer JM, Bloom BJ, Breedveld FC, et al. A phase IIb dose-ranging study of the oral JAK inhibitor tofacitinib (CP-690,550) versus placebo in combination with background methotrexate in patients with active rheumatoid arthritis and an inadequate response to methotrexate alone. Arthritis & Rheumatism. 2012;64(4):970–981. PubMed
Burmester GR, Blanco R, Charles-Schoeman C, et al. Tofacitinib (CP-690,550) in combination with methotrexate in patients with active rheumatoid arthritis with an inadequate response to tumour necrosis factor inhibitors: a randomised phase 3 trial. The Lancet. 2013;381(9865):451–460. PubMed
Gordon R, et al. Selective Inhibition of the MK2 Pathway: Data From a Phase IIa Randomized Clinical Trial in Rheumatoid Arthritis. ACR Open Rheumatology. Full text
Aclaris Therapeutics, Inc. Aclaris Therapeutics Announces Top-line Results from 12-Week Phase 2b Trial of Oral Zunsemetinib (ATI-450) for Moderate to Severe Rheumatoid Arthritis and Provides Corporate Update. November 13, 2023. Company announcement
