A Phase 0 study is a very early, exploratory clinical trial with limited human exposure and without therapeutic or diagnostic intent. Its purpose is to rapidly gather decision-relevant data on pharmacokinetics, distribution, target binding, or a pharmacodynamic signal with a small number of participants. Microdosing studies are an important, but not the only, manifestation of this approach.
Objective and typical study design
The focus is on an early development decision, not the treatment of a disease. A Phase 0 study can test whether a mechanism of action observed in nonclinical systems is measurable in humans, how a candidate is distributed in the body, or which of several candidates should be further pursued. Depending on the question, sensitive bioanalytical methods, imaging, or pharmacodynamic markers are used. The study is small, the dosing duration is limited, and the objective is narrowly predefined.
In a microdosing study, the single dose remains so low that no pharmacological effect is intended. ICH M3(R2) describes microdose approaches with a maximum of 100 micrograms and limited proportions of the expected pharmacologically active dose and the nonclinical NOAEL. This can enable early human pharmacokinetics or imaging target investigations. The informational value, however, depends on whether the pharmacokinetics in the microdose range can be extrapolated to subsequent dose ranges; this extrapolability must not be assumed.
Exploratory IND approach and nonclinical support
In the US, the exploratory IND approach refers to a regulatory pathway for very early studies with limited human exposure. The FDA includes under this term screening and microdosing studies, as well as studies with expected pharmacologically active but not toxic doses. The extent of nonclinical support depends on the objective, dose, duration, and planned exposure range of the study. An exploratory IND is therefore not a waiver of safety evaluation, but a data foundation tailored to the limited clinical question.
For European study planning, a scientifically justified protocol, the approval of the clinical trial application, and the protection of the participants remain essential. Phase 0 is not a distinct approval phase in the sense of a uniform European legal concept. Instead, the term describes a development strategy whose nonclinical and clinical documentation must align with the specific risk. The results replace neither the more comprehensive safety evaluation nor the dose finding for subsequent studies.
Differentiation from Phase I and First-in-Human
The core difference to a conventional Phase I study lies in the objective. Phase I typically investigates safety, tolerability, pharmacokinetics, and optionally pharmacodynamics over a dose range relevant to further development. Dose escalations can serve the characterization of exposure and tolerability. A Phase 0 study, on the other hand, remains focused on a narrowly limited exploratory question and very low or otherwise clearly limited exposure; it is not intended to determine a maximum tolerated dose.
First-in-Human does not refer to a dose or study phase, but to the first administration of an investigational product to humans. A Phase 0 study can thus be a First-in-Human study, but it does not have to be, for example, if human exposure from a previous investigation is already available. Conversely, not every First-in-Human study is a Phase 0 study: a conventional first study with ascending doses and a safety or tolerability objective regularly belongs to Phase I. Therefore, these terms must not be used synonymously.
Relevance for clinical trials
The practical relevance lies in robust early go/no-go decisions. A prerequisite is that the planned measurement is analytically sensitive enough and the result actually influences a predefined development decision. In everyday study practice, the nonclinical justification, the selection of participants, the dose rationale, the sampling time points, and the safety monitoring must be consistent. Authorities and ethics committees specifically evaluate whether the limited exposure and the lack of individual benefit are justified by the research question and the risk minimization.
Full-service CROs such as Mediconomics support the feasibility assessment of a Phase 0 study, the preparation of the protocol and investigator’s brochure, and the regulatory submission strategy. Concrete services include the coordination of the nonclinical justification, the selection and management of specialized bioanalytics, the planning of sample logistics and imaging, project management, as well as monitoring and data management for the tightly scheduled execution.
Frequently Asked Questions (FAQ)
Is every Phase 0 study a microdosing study?
No. Microdosing studies are a clearly defined subset. Exploratory studies can also investigate pharmacologically relevant, but not expectedly toxic doses or short repeated administrations, provided the objective and nonclinical support justify this.
Can a Phase 0 study provide proof of efficacy?
It is not designed to provide proof of therapeutic efficacy. A pharmacodynamic signal or target binding can support a hypothesis, but does not replace a controlled efficacy assessment in a clinical trial designed for this purpose.
Who participates in a Phase 0 study?
Depending on the research question, healthy volunteers or patients may be suitable. For a substance with expected significant toxicity or for a question that can only be meaningfully measured in tumor tissue, a patient population may be more appropriate.
Regulatory references
- ICH M3(R2), Nonclinical Safety Studies for the Conduct of Human Clinical Trials and Marketing Authorization for Pharmaceuticals – outlines the principles for exploratory clinical trials and microdose approaches.
- FDA, Exploratory IND Studies – describes the US exploratory IND approach and its limited human exposure.
- Regulation (EU) No 536/2014 on clinical trials on medicinal products for human use – regulates authorization, protection of trial participants, and scientific requirements in the EU.