Phases of Clinical Trials

There are five main phases of clinical trials. Phase 0 and Phase IV are optional. In fact, each phase represents a separate study, but at the same time, one phase may imply more than one study.

Phase 0

  • Number of participants: usually less than fifteen. 
  • Duration: several days. 
  • Purpose: to assess the drug’s pharmacokinetic and pharmacodynamic profile. 

Phase 0 was introduced by the FDA to boost the drug development process and filter out unsuitable candidates in the early stages of a trial. This phase is also characterized by several important features. It requires a smaller confidence interval (CI) compared to Phase I. The study involves a small number of patients who receive subtherapeutic doses of the drug. Meanwhile, both the risk of side effects and therapeutic value tend to zero. If the drug turns out to be effective, patients in Phase 0 will not benefit from it.

Phase 0 helps to quickly figure out whether it's worth investing in further work. 

Phase I

  • Number of participants: usually several dozen. 
  • Duration: several months. 
  • Purpose: to identify the maximum safe dosage.

The first phase of a clinical study involves healthy volunteers or people displaying any signs of pathology. At first, participants receive the minimum dose of the drug. In the absence of side effects, the doses are gradually increased to the maximum allowable quantity. Then the research group conducts a study with multiple increasing doses, meaning that each patient receives several doses of the drug. In this case, placebo control is not used. 

Phase I explores pharmacodynamics and pharmacokinetics, an impact on the effectiveness and side effects of the drug, food intake, other drugs, related diseases. If the drug is considered to be safe, it proceeds to the second phase.

According to the FDA, 70% of drugs successfully pass the first phase of clinical trials.

Phase II

  • Number of participants: from several dozen to several hundred. 
  • Duration: from several months to two years. 
  • Purpose: to test the effectiveness and side effects. 

The second phase of clinical trials is conducted among patients suffering from certain diseases. In contrast to the first phase, it usually features a placebo control, randomization and a double-blind method. It is necessary to examine if the drug is more effective than the placebo. Increasing the number of participants provides an opportunity to identify uncommon side effects that had not been found during Phase I.

The FDA states that only 33% of the drugs that participated in Phase II successfully undergo clinical trials and move to the next phase.

Phase III

  • Number of participants: from several hundred to several thousand. 
  • Duration: from one year to several years. 
  • Objective: to confirm the effectiveness and identify side effects that were not detected in the previous stages. 

The third phase of clinical trials is the most difficult and expensive. Thousands of patients from different countries can take part in this phase. Everything has to be planned down to the last detail. The effectiveness and safety of a new drug should be finally confirmed during Phase III. Based on the outcome, developers decide on whether to initiate a market entry process.

Most drugs fail at this stage. The FDA claims that the third phase of clinical trials shows positive results in only 25-30% of cases. 

Phase IV

  • Number of participants: usually several thousand. 
  • Duration: different. 
  • Purpose: to provide additional checks of safety and effectiveness.

The fourth phase of clinical trials is post-marketing studies, which are carried out in parallel with the active use of the drug. The fourth phase is optional for the entry into the market, but in some cases it is important.

Watch this Video to Learn about the Three Phases of Clinical Trials

What do Participants Need to Know about a Clinical Trial?

Generally, people participate in clinical trials for one of two main reasons (or both of them): the lack of effective treatment methods or the desire to contribute to science. Both cases imply an entirely voluntary decision.

The specialists who conduct the research are obliged to provide the patient with written information about the purpose of the CT, the testing process, type of drug, safety, and course of action in case of side effects. All these details are stated in a special document called Informed Consent.

Another important point is confidentiality. Researchers are not allowed to disclose personal information of patients. Any patient can quit the study at any time without explanation.  

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