Phase 1 Trial

A Phase 1 trial is where an investigational drug or biologic is tested in humans for the first time (excluding any exploratory Early Phase 1/”Phase 0″ microdosing work). Under 21 CFR 312.21, the purpose of Phase 1 is to determine metabolism and pharmacologic action, side effects associated with increasing doses, and, where possible, early evidence of effectiveness — but effectiveness is incidental at this stage, not the primary endpoint. That distinction is what separates Phase 1 from Phase 2’s efficacy focus and Phase 3’s confirmatory focus: a Phase 1 trial is designed to answer ‘is this safe, and at what dose,’ not ‘does it work.’

Dose-escalation designs: 3+3 and rolling six

Most Phase 1 trials use a rule-based dose-escalation design to decide when it’s safe to move to the next, higher dose cohort. The two most common are:

• 3+3 design — the traditional approach. Three participants are treated at a starting dose. If none experience a dose-limiting toxicity (DLT), the trial escalates to the next dose with a new cohort of three. If one of the three experiences a DLT, three more participants are added at that same dose; if no further DLTs occur among those three, escalation continues. If two or more of a cohort experience a DLT, that dose is considered above the maximum tolerated dose and escalation stops.
• Rolling six design — a variant, common in pediatric oncology, that allows three to six participants to be enrolled concurrently at a dose level depending on how many are already enrolled, how many have had a DLT, and how many have toxicity data still pending — letting enrollment continue without pausing for every participant’s full observation window, which shortens trial duration compared to a strict 3+3.

Both are ‘rule-based’ or ‘algorithmic’ designs: the escalation decision follows a fixed set of rules rather than a statistical model. Model-based alternatives, such as the continual reassessment method (CRM), use accumulating dose-toxicity data to estimate the maximum tolerated dose statistically; they are used less often than 3+3 in practice but are increasingly discussed in the methodological literature as more statistically efficient at correctly identifying the MTD.

Determining the MTD and RP2D

A Phase 1 dose-escalation trial’s primary deliverable is the maximum tolerated dose (MTD) — generally the highest dose level at which the observed rate of dose-limiting toxicity stays within the pre-specified threshold (commonly framed as 0 of 6 or 1 of 6 participants experiencing a DLT, with 2 or more DLTs at the next higher dose confirming that dose was too high). In oncology and other settings where toxicity and efficacy don’t cleanly separate, sponsors also define a recommended Phase 2 dose (RP2D) — the dose actually carried forward into Phase 2, which is often but not always the same as the MTD once the full toxicity, pharmacokinetic, and any early activity data are reviewed together.

Who gets enrolled: healthy volunteers vs. patients

Phase 1 participant selection depends on what the investigational product does. Many Phase 1 trials — particularly for drugs with a wide expected safety margin — enroll healthy volunteers, often in single ascending dose (SAD) and multiple ascending dose (MAD) designs, because there’s no therapeutic rationale for the target condition and healthy participants give a cleaner pharmacokinetic and tolerability signal. Oncology and certain other Phase 1 trials instead enroll patients who already have the target condition (e.g., advanced, treatment-refractory cancer), because the doses under investigation are expected to be too toxic to justify exposing healthy volunteers, and because there may be a plausible direct benefit to the patient.

Administrative and regulatory context

Phase 1 carries a disproportionately heavy administrative burden relative to its size. Because the safety profile is still unknown, monitoring is typically intensive (frequently near-100% source data verification), and dose-escalation decisions between cohorts usually require formal safety review committee sign-off. Safety reporting is also at its heaviest: under 21 CFR 312.32, sponsors must report any unexpected fatal or life-threatening suspected adverse reaction within 7 calendar days, and other serious unexpected suspected adverse reactions within 15 calendar days. See CASRAI’s Clinical Trial Phases guide for how Phase 1 fits alongside Early Phase 1 (Phase 0), Phase 2, Phase 3, and Phase 4, and the Investigational New Drug (IND) term for the regulatory filing that must be in place before any Phase 1 dosing can begin.