Phase I Trial

A Phase I trial is the first stage of testing an experimental treatment in human subjects, primarily designed to evaluate the safety, tolerability, pharmacokinetics (how the body processes the drug), and pharmacodynamics (how the drug affects the body) of the intervention. Phase I trials typically enroll a small number of participants and aim to determine the appropriate dose for further testing.

Phase I trials are the critical gateway between laboratory research and clinical application. They mark the transition from animal studies and preclinical data to human testing, answering the most fundamental question: is this treatment safe enough to continue investigating? Without Phase I trials, promising treatments would never advance to efficacy testing. Despite their small size, Phase I trials provide essential data on dose response relationships, metabolic pathways, drug interactions, and side effect profiles that shape the design of all subsequent clinical development. They also serve an ethical function by identifying unacceptable toxicity before larger numbers of patients are exposed to the treatment.

Phase I trials typically enroll 20 to 80 healthy volunteers (for most drug classes) or patients with the target disease (for oncology and other high risk therapies where exposing healthy volunteers would be unethical). The trial follows a dose escalation design in which sequential cohorts of participants receive increasing doses of the experimental treatment.

The most traditional dose escalation approach is the 3+3 design, in which cohorts of three participants receive a given dose level. If none of the three experiences a dose limiting toxicity (DLT), the next cohort receives a higher dose. If one of three experiences a DLT, three additional participants are enrolled at that dose; if none of the added three has a DLT (one of six in total), escalation continues. If two or more of three (or two or more of six in the expanded cohort) experience DLTs, the maximum tolerated dose (MTD) has been exceeded and escalation stops. The next lower dose is then usually declared the MTD, provided that no more than one of six participants treated at it had a DLT; if only three were treated there, three more are enrolled to check.

Model based designs such as the continual reassessment method (CRM), and model assisted designs such as the Bayesian optimal interval (BOIN) design, use statistical models to guide dose escalation decisions, potentially identifying the MTD more efficiently and exposing fewer participants to subtherapeutic or toxic doses. These designs have gained increasing acceptance; in December 2021 the US FDA determined the local BOIN design, under a non informative prior, fit for purpose for finding the MTD in phase I dose finding trials.

Pharmacokinetic studies conducted during Phase I trials characterize the absorption, distribution, metabolism, and excretion (ADME) profile of the drug. Blood samples are collected at multiple time points to measure drug concentration, and parameters such as maximum concentration (Cmax), time to maximum concentration (Tmax), half life, area under the concentration time curve (AUC), and clearance are calculated. These data inform dosing frequency and help predict drug interactions.

Pharmacodynamic studies assess the biological effects of the drug on the body, such as changes in biomarkers, receptor occupancy, or physiological parameters. Together with pharmacokinetic data, pharmacodynamic findings help establish the dose response relationship and guide dose selection for Phase II trials.

Phase I trials are not designed to assess efficacy, though preliminary signals of activity may be observed and reported, particularly in oncology where participants are patients with the disease. The primary regulatory purpose of Phase I data is to support the design of subsequent trials by establishing a safe dose range and characterizing the drug's behavior in the human body.

Phase I trials are conducted when a new drug, biologic, or combination therapy enters human testing for the first time, and may be needed when an existing treatment is tested in a substantially new formulation, route of administration, or patient population. The trial must be preceded by adequate preclinical data and cleared by a regulatory agency and an institutional review board or ethics committee; in the US, for example, an Investigational New Drug application (IND) goes into effect 30 days after the FDA receives it unless the FDA places the study on clinical hold.

A common mistake is interpreting Phase I results as evidence of efficacy. Phase I trials are not designed or powered to assess whether a treatment works; they assess whether it is safe enough to continue testing. Another error is applying the recommended Phase II dose (RP2D) determined in one patient population to a different population without additional dose finding, as pharmacokinetics and tolerability can vary substantially across populations. In oncology, some Phase I trials enroll heavily pretreated patients whose tolerance may differ from earlier line patients who will comprise the Phase II and III populations. Rushing through dose escalation to complete the trial quickly can miss important safety signals that emerge at specific dose levels.

A pharmaceutical company develops a novel small molecule inhibitor for advanced solid tumors. The Phase I trial enrolls patients with advanced cancers that have progressed on standard therapies. Using a BOIN design, sequential cohorts of three to six patients receive escalating oral doses starting at 50 mg daily. After enrolling 42 patients across seven dose levels, the study identifies a maximum tolerated dose of 300 mg daily, with dose limiting toxicities of grade 3 hepatotoxicity and grade 3 fatigue at 400 mg. Pharmacokinetic data show a half life of 14 hours, so with daily dosing the drug reaches steady state after about three days (roughly five half lives). Three patients show partial tumor responses, providing preliminary evidence of antitumor activity that supports advancement to Phase II.