phase I trial
Meaning of phase I trial: The first time a new treatment is given to people, with safety and dose as the questions rather than whether it works.
Definition of phase I trial
A phase I trial is the first study of a new treatment in humans, designed to find out whether it is tolerated and at what dose. Small groups receive it, usually starting very low and stepping up only once a lower level has proved safe. Researchers track side effects and measure how the body absorbs, distributes and clears the drug.
What actually happens in one?
Participants are enrolled in small groups, often three or six at a time, called cohorts. The first cohort receives a cautious dose worked out from laboratory and animal data. If nothing serious happens, the next cohort receives more. This continues until side effects of a defined severity appear, which marks the ceiling. The dose carried into later testing sits just below that ceiling. Blood samples taken through the day show how quickly levels rise and fall, and that shapes how often the treatment would be given.
Who takes part?
It depends on the illness. For most medicines the first participants are healthy volunteers, since there is no benefit to be had and the point is to observe safety cleanly. That changes when the treatment is expected to be toxic. In cancer, in serious infections and in advanced heart or nerve disease, exposing healthy people is not acceptable, so the participants are patients, often ones whose disease has progressed despite standard treatment.
How does it differ from the later phases?
| Phase | Main question | Rough size |
|---|---|---|
| I | Is it tolerated, and at what dose | Tens of people |
| II | Does it show a useful effect | Up to a few hundred |
| III | Is it better than current treatment | Hundreds to thousands |
| IV | What emerges in ordinary use | Open ended |
Can a participant benefit?
Benefit is not the design goal, and early doses are deliberately set below the level thought likely to work. Some participants in cancer studies do respond, and that possibility is part of why people enroll, but the consent process treats it as uncertain. Every study of this kind is reviewed by an ethics committee before it opens, and stopping rules are written in advance so that enrollment halts if harm appears. Modern designs have softened the old fixed ladder. Some studies escalate using a statistical model that reweights the estimated safe dose after each cohort reports, which reaches the answer with fewer people exposed to a dose far below any useful level. Others add an expansion group at the chosen dose once the ceiling is known, so a first read on activity arrives earlier than it once did.
Also known as
Used in a sentence
The compound is entering a phase I trial to establish the maximum tolerated dose.
Good to know
Phase I studies are sized for safety, not for proving benefit, and most enroll healthy volunteers; participants with the disease are used chiefly where the treatment is expected to be toxic, as in cancer.