Clinical Trial Phases Explained: From Preclinical to Phase 4

Clinical research is the foundation of modern medicine. There are multiple phases that can be confusing unless they are properly explained and understood. By advancing successfully through all phase trials, new therapies, medical devices, and treatments can be brought to market to treat patients safely and effectively.

For healthcare providers and others new to clinical research or considering participation in clinical trials, understanding the different phases of research is essential. Site management organizations such as Tristar Clinical Research Consulting (TriStar CRC), can help explain the clinical trial phases and also help providers and clinics navigate this process with a low-burden, fully supported approach to initiate or expand your capabilities.

Clinical Trial Phases Explained?

Clinical trials are typically initiated by sponsors wanting to bring a new investigational product (IP) such as a drug, device, or biologic to market. These are conducted in a series of phases designed to answer specific questions about an investigational product (IP) and to ensure its safe and ethical testing and use. 

Each phase builds on the previous one and include:

• Preclinical Research: Initial laboratory and animal testing 

• Phase 1: Focus on safety and dosing 

• Phase 2: Focus on early effectiveness and side effects 

• Phase 3: Focus on large-scale confirmation and comparison 

• Phase 4: Post-marketing and real-world data 

Preclinical Research

Before a treatment is ever tested in humans it must undergo preclinical research. This is typically done at the bench level in the research laboratories and groups and in animal models to approximate the effect in humans. 

The goals of this stage are to understand the safety and biological activity of the IP, identify any potential toxicity it may exhibit, and determine if the IP is appropriate for use in human trials. Providers typically are not involved in this stage of research, though it is a foundational step for all future phases of clinical trials. 

Phase 1: First-in-Human & Safety

The first phase of clinical trials, Phase 1, is the first time the IP is tested in humans. This trial is done in healthy individuals, with the exception of certain studies such as in oncology where it may be investigated with individuals with the disease. There are about 20-100 participants recruited for this phase which can be done across 1-3 investigative sites. This phase of the trials can take a few days to multiple weeks of monitoring per patient.

The goal of this phase is to determine the acceptable dosing ranges for the IP, including the maximum tolerated dose (MTD). This helps determine ranges that humans can tolerate and any side effects and adverse events. By studying the effects on healthy individuals this helps investigators understand how the body processes the IP. 

Phase 1 Key characteristics:

• Participants: 20–100 often healthy volunteers

• Sites: Typically 1–3 specialized research centers 

• Duration: A few days to several weeks 

Phase 1 Primary objectives:

• Determine IP dosing including the maximum tolerated dose (MTD) 

• Identify side effects and adverse events 

• Understand how the body processes the treatment 

Phase 2: Early Efficacy & Dose Optimization

Phase 2 trials are where a key question about the IP is answered. Does the investigated treatment actually work in the populations with the target condition. These trials are conducted in about 100-300 individuals with the target condition which can engage 10-30 clinics. Participants are monitored longer in this phase and the trials can take from 4 weeks to 6 months. 

The goal of this phase is to evaluate the efficacy of the IP within the previously established acceptable dosage ranges. There is also additional monitoring to determine any side effects or adverse effects in the target population. This provides additional information on the efficacy of the IP and if it is worth moving to the next phase. 

In this phase there are also specific inclusion and exclusion criteria established. This is to outline the eligibility determination for participants in this phase of the trial to help reduce risk and isolate treatment effects. This may not provide a full reflection of the real-world populations. 

In this phase, placebo controls can be introduced as well. These help to evaluate if the IP is having any effect in patients compared to no treatment at all. These trials are often randomized and double blinded to prevent emergence of any bias in the study.  

Phase 2 Key characteristics:

• Participants: 100–300 individuals with the target condition that follow strict inclusion and exclusion criteria to determine participant eligibility

• Sites: Approximately 10–30 clinics 

• Duration: ~4 weeks to 6 months 

Phase 2 Primary objectives:

• Evaluate IP efficacy within established dose ranges on target populations

• Continue monitoring safety and adverse effects 

• Refine optimal dosing strategies 

Phase 2 Study design considerations:

• Randomization (participants assigned to different groups) 

• Double-blinding (neither participants nor study staff know assignments) 

• Comparisons between: 

  • Investigational product (IP) vs placebo

  • Different dose levels 

  • Existing standard-of-care treatments 

This can often be a great entry point for new research sites with specialization in a specific target population of interest. At TriStar CRC, we support sites through study startup, regulatory processes, patient recruitment, and ongoing trial management—helping reduce administrative burden while maintaining compliance.

Phase 3: Pivotal Trials for Approval

Phase 3 trials are typically the last trial before the IP is approved for market. This is a large-scale study that can involve anywhere from 300 to over 1000 patients from multiple sites, depending on the indication. These trials can last from 6 months to a year and half or even up to five years. 

These trials are designed to evaluate and confirm the outcomes of phase 2 trials in much larger diverse target populations. The inclusion criteria can be broader in this phase to get a better representation of use of the IP in demographics closer to real world representations.   

Open-Label extension periods can occur at the end of phase 3 trials as well. This is where participants that are part of the study can continue the treatment or be moved from a placebo control to the actual treatment. This means that all participants can receive the IP and remain a part of the trial for long term safety and efficacy monitoring. 

Phase 3 Key characteristics:

• Participants: 300–1,000+ 

• Duration: 6 months to 18 months (sometimes longer depending on indication) 

• Design: Randomized, double-blind, often multi-site

Phase 3 Primary objectives:

• Confirm clinical efficacy in a broader population 

• Monitor long-term safety 

• Compare against placebo or standard treatments 

Notable differences from Phase 2:

• Broader inclusion criteria for better real-world representation 

• Larger, more diverse populations 

• Increased regulatory scrutiny 

Open-Label Extension Periods

• Participants who complete the study may continue treatment 

• All participants receive the investigational product 

• Long-term safety and efficacy data are collected 

If all phases are successfully completed up to this point, the IP receives regulatory approval and can be brought to market. 

Phase 4: Post-Marketing & Real-World Evidence

Once a product is approved and taken to market, the investigation does not stop. Phase 4 trials, referred to as post-marketing studies, continue p

ast phase 3. This allows the product to be continuously monitored and to determine its performance in the real-world.

This phase can monitor over 2000 patients using the IP long-term to determine long-term safety issues. In this phase the sponsors are also seeking to evaluate improvements to the patients quality of life and overall satisfaction with the product. They are also evaluating the health economic outcomes that can positively impact the patients. They are also seeking out other potential benefits or uses for the IP that they can include in their marketing to try and sell more of their new product. 

Phase 4 Key characteristics:

• Participants: 2,000+ often in open-label and observational study in real-world clinical environments 

Phase 4 Primary objectives:

• Monitor long-term safety 

• Assess quality of life and patient satisfaction 

• Evaluate health economics and outcomes 

• Identify additional benefits or use cases 

Phase 4 studies are typically less restrictive, making them more accessible for newer research sites. This makes them an excellent starting point for providers looking to enter clinical research with lower complexity and broader patient eligibility. 

Clinical Trial Phases at a Glance

Phase	Focus	Participants	Key Goal
Preclinical	Lab research	N/A	Safety & feasibility
Phase 1	Safety & dosing	20–100	Identify side effects & MTD
Phase 2	Early efficacy	100–300	Does it work?
Phase 3	Confirmation	300–1,000+	Prove safety & effectiveness
Phase 4	Real-world use	2,000+	Long-term outcomes

Ready to Explore Clinical Research for Your Practice?

Whether you're new to clinical trials or looking to expand your current capabilities, the right support can make all the difference. TriStar CRC partners with providers to simplify the process by reducing burden while maximizing opportunity. Contact us today or schedule an appointment to learn how your site can get started in clinical research.