What are the differences between phase 1, 2, and 3 clinical trials?

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MichaelWosnickPhD (Researcher (Verified) ) - 08 / 15 / 2012

Phase I trials are the first stage of testing in humans, usually involving healthy volunteers and usually between 20-80 or so people. The main purpose of a Phase I trial is asses safety and side effects and to determine what looks to be a safe and tolerable dose in the case of a drug or treatment. Under normal circumstances (there are exceptions) the purpose of Phase I is not to actually see outcome improvements – after all the subjects are healthy to begin with - but to determine what regimen and dosage and administration will be safe to use in Phase II and beyond.

Once the initial safety of the study drug has been confirmed in Phase I trials, Phase II trials are performed on larger groups of people, usually patients. Numbers vary but 100-300 subjects would not be uncommon. Now that a safe dose has been established in Phase I it is time to see if the new drug, treatment or intervention actually makes a DIFFERNCE to people with the disease being treated. Sometimes new toxic effects not found in the more limited Phase I trial emerge and the trial is stopped, and sometimes even if side effects are not an issue, it may now seem under further testing in real patients that the new regimen just does not work as hoped for, the trial may also be stopped as it becomes pointless to continue. However, if no new safety/toxicity concerns emerge, AND if there truly is data to support the belief that the new treatment MAY have the desired effect, it is time to begin the really rigorous part of the trial, the Phase III.

Phase III studies are (usually) what are called randomized controlled trials, most often needing several hundred to as many as several thousand patients to be enrolled. That means that they are also usually conducted in multiple centres (according to a strict and consistent protocol from centre to centre) to be able to accrue the number of patients needed to make a statistically reliable decision at the end. Phase III trials are all about answering, is drug “A” better than drug “B”; is diagnostic “X” better than diagnostic “Y”; is prevention intervention “P” better than intervention “Q” and so on. Biostatisticians set very rigorous models in place to make sure that the answer will be meaningful and reliable and that any differences between trial arms can be attributed to the treatments being tested, and not to stray chance factors.

For that reason, Phase III trials in oncology often take several years to complete because the numbers of patients that have to be enrolled is high, especially since most often we are looking for small incremental advances over what is an accepted treatment protocol. There is also a lot of regulatory work that goes on (some say we have swung the pendulum too far and that the bureaucracy is now impeding the conduct of trials) since patient safety is of utmost concern above all else. This makes trials frustratingly slow to start up even after approval, makes them inevitably long, and of course increasingly expensive.

But since clinical trials are a critical part of the research landscape it is essential that we improve the way we do trials or the system will soon implode under its own weight. I recently had the privilege to play a leadership role on an important Task Force to study the threats to the systematic conduct of clinical trials in Canada. You can read more about that here -http://www.ccra-acrc.ca/PDF%20Files/CT%20report%20Oct%202011.pdf.
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