Alto's ALTO-300 targets a biomarker-defined MDD subgroup in Phase 2b test
The adjunctive antidepressant's primary endpoint reads MADRS change in a pre-defined subgroup, not the full randomized population, setting a narrower bar than a standard depression trial.
Executive Summary
- Alto Neuroscience's Phase 2b trial of ALTO-300 in major depressive disorder is built around a subgroup-specific primary endpoint rather than a straightforward placebo comparison across all enrolled patients, which changes what a positive topline result will actually prove.
- The trial's expected completion date has slipped substantially since it first registered, moving through two rounds of revision, while the enrollment target has stayed unchanged through the most recent registry update.
- No trial in the current major depressive disorder landscape shares ALTO-300's target or mechanism, leaving the readout without a direct mechanistic comparator, even as the broader depression pipeline stays active with multiple late-stage programs testing other mechanisms.
- A topline result that holds up in both the pre-defined subgroup and the full randomized population would be the outcome that meaningfully de-risks the program; a result confined to the subgroup alone would leave open how much of the signal reflects patient selection rather than drug effect.
The trial
NCT05922878 is a randomized, placebo-controlled Phase 2b study testing ALTO-300 as an add-on to standard antidepressants in adults with moderate to severe MDD who have been on a stable dose of an SSRI, SNRI, or bupropion for at least six weeks. The trial is designed with one experimental arm and one placebo comparator arm, and its registered primary endpoint measures the change in MADRS (Montgomery-Asberg Depression Rating Scale, a clinician-rated depression severity score) through week 6, evaluated in a pre-defined subgroup of participants. A secondary endpoint repeats the same MADRS analysis across all randomized participants, and another secondary endpoint tracks MADRS response rate, defined as a 50% or greater improvement from baseline. That structure means the trial is designed to test whether ALTO-300 works in a specific, likely biomarker- or feature-defined slice of MDD patients first, with the broader population analysis carried as a secondary measure. NCT05922878Study of ALTO-300 in MDDNCT05922878
Probability of SuccessBased on the AppliedXL Probability of Success model. For more information about the methodology, read the research here.
Timing and enrollment
The trial's primary completion date has moved twice since it started enrolling in June 2023: first from July 2024 to March 2025, then from March 2025 to December 2026. That second revision, filed in July 2025, also reset the enrollment target from 200 to 321 participants. Alto Neuroscience has since guided investors to expect topline data in the second half of 2026, a window that runs from July 1 to December 31 and comfortably contains the registered December 1, 2026 primary completion date. The enrollment figure has not changed since that July 2025 update, a flat trajectory the operational risk model treats as within the routine band for a trial of this design. NCT05922878+1Study of ALTO-300 in MDDNCT05922878Alto Neuroscience Reports Third Quarter 2025 Financial Results and Recent Business HighlightsNov 12, 2025
The competitive field
The MDD trial landscape is active but mechanistically scattered: named late-stage programs include vortioxetine (a serotonin reuptake inhibitor from Takeda), seltorexant and esketamine (from Janssen), solriamfetol (from Axsome Therapeutics), and NBI-1065845 (from Neurocrine Biosciences), none of which share ALTO-300's target or mechanism class. ALTO-300's own target and mechanism are not established in the available data, so no direct mechanistic comparator exists for this readout; the nearest available context is indication-level, not mechanism-level. Alto's own pipeline includes ALTO-101 for cognitive impairment in schizophrenia, which was granted Fast Track Designation by the FDA in October 2025, and ALTO-207 for treatment-resistant depression, which the company is accelerating toward a Phase 3 trial after a successful FDA meeting. Those programs sit alongside ALTO-300 as part of the same near-term data cadence Alto has guided to investors. AltoAlto Neuroscience Reports Third Quarter 2025 Financial Results and Recent Business HighlightsNov 12, 2025
What the readout can establish
A randomized, placebo-controlled Phase 2b design with a MADRS-based primary endpoint in a pre-defined subgroup can establish whether ALTO-300 produces a depression-severity signal distinct from placebo in that selected population within six weeks. It cannot, on its own, establish that the same effect generalizes to the full randomized population without the secondary all-comers MADRS analysis also showing directional agreement. The trial's own secondary endpoint set, which includes response rate and multiple safety measures across blood pressure, heart rate, weight, and suicidality tracking via the CHRT-SR12 scale, gives the readout enough structure to assess tolerability alongside efficacy. NCT05922878Study of ALTO-300 in MDDNCT05922878
This analysis was produced using AI-assisted reporting systems, AppliedXL data, and official public records. These systems undergo editorial review, quality checks, and regular audits by human experts. Errors may still occur, as with any automated system. Always consult the linked primary sources. Read our AI Editorial Policy.