How depression staging is being formalised and what that means for trial design

Depression has long been recognised as a heterogeneous condition with patients responding very differently to standard pharmacological and psychological approaches. The treatment-resistant depression label has historically been applied to patients who have failed two or more adequate trials of antidepressant therapy, but the practical thresholds for what counts as adequate, what counts as a trial, and what counts as failure have varied across studies, regulatory frameworks, and clinical practices.

The formalisation of staging - moving from a single 'TRD' bucket to a graded system that distinguishes Stage I (one failed trial), Stage II (two failed trials), Stage III (failed two trials plus failed augmentation), and so on - is happening unevenly but visibly. Several regulatory agencies have signalled preferences for clearer staging language in submissions; several large clinical research networks have aligned around staging frameworks; some payer protocols are increasingly granular about what counts as a covered indication for second-line versus augmentation versus device-based interventions.

The trial-design implications are real. A drug developed for Stage II TRD has a different patient population, a different comparator, a different effect-size expectation, and a different commercial trajectory than a drug developed for Stage III or post-ECT-failure depression. The recent generation of psychedelic-assisted therapy trials has had to negotiate this carefully, since the patient populations recruited and the staging language used in primary publications affect both regulatory readability and clinical-community uptake.

The ketamine and esketamine experience is a useful precedent. Esketamine's approval was specifically for treatment-resistant depression with fairly tight inclusion criteria; the rollout into routine practice has involved a continuing conversation about which patients should access the therapy, in what setting, and with what monitoring. The formal staging language has helped some of those conversations and lagged others.

The staging frame also has implications for how patient-reported outcomes are interpreted. A patient at Stage IV who shows partial improvement on a novel mechanism has experienced a different therapeutic event than a patient at Stage I who shows similar improvement; the patient-experience meaning is not captured by symptom-rating change alone. The newer trials are increasingly using stage-specific endpoint frameworks rather than treating all depression patients as interchangeable for outcome measurement.

For commercial planning, the practical question is not just whether a candidate works in 'depression' but where in the staging frame it is positioned and what comparator it has to beat. The candidates being designed today for moderate Stage II TRD face a different competitive set than the candidates being designed for severe Stage III post-augmentation failure. Both can be commercially viable; they require different development plans.

The convergence is incomplete. Different regulators, different professional societies, and different payers are at different stages of adopting harmonised staging. Sponsors that build their development plans around the most regulator-aligned framework tend to have a smoother path; those that use looser TRD definitions tend to find the regulatory and reimbursement reviews more contested.