A clinical trial is a systematic evaluation of a drug in patients or healthy volunteers, the primary purpose of which is:

• demonstrate the safety and tolerability of the drug,
• to demonstrate and verify the therapeutic effects of the drug,
• find out what its side effects are,
• to determine the pharmacokinetic parameters and the behavior of the drug in the human body.

Clinical trials conducted on humans are subject to very strict rules and follow a predetermined plan, known as a protocol. The clinical trial itself is preceded by preclinical testing, which is conducted on animals or cell cultures and evaluates factors such as acute and chronic toxicity, carcinogenicity, mutagenicity, and others.

Phases of Clinical Trials
Depending on the stage of drug development, there are four phases of clinical trials.

Phase I
The drug is administered to humans for the first time, most often to healthy volunteers. The study determines whether the human body tolerates the new substance and how it is metabolized in the body. At the same time, any adverse effects are monitored. Initially, low doses of the drug are administered, and these are gradually increased to determine the maximum tolerated dose. Research is not conducted on healthy volunteers if administering the substance to a healthy individual is inappropriate and would be dangerous for them (e.g., with cytostatics).

Phase II
In a Phase II clinical trial, the drug is administered for the first time to a small number of carefully selected patients based on very strict criteria. This phase demonstrates the drug’s therapeutic effects, identifies the appropriate dose, and collects further data on the drug. If, at this stage, good efficacy is confirmed that outweighs the risk of adverse effects, it is possible to proceed to Phase III, where hundreds to thousands of patients are already involved, in order to verify the drug’s efficacy. At the same time, it is possible to obtain further information about its safety.

Phase III
In the Czech Republic, Phase III studies are most commonly conducted; these are often internationally organized, involving multiple countries, and include multicenter studies that take place at multiple sites.
Once a drug has passed through the necessary phases of clinical trials, the data obtained can be submitted in an application for its registration, which is handled by the State Institute for Drug Control (SÚKL) in the Czech Republic and by the European Medicines Agency at the European level. Once a drug has valid registration, nothing prevents it from being used in the provision of healthcare. However, this does not mark the end of the investigation into its efficacy and potential risks.

Phase IV
In this phase, the occurrence of adverse effects, the drug’s effects during long-term use, and information on potential interactions with other drugs are carefully monitored. If necessary, additional Phase “III.b” clinical trials are conducted to test new indications, new treatment regimens, or the use of the drug in specific patient groups, such as children or the elderly.