Phases of clinical research

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Main article: Clinical trial

The phases of clinical research are the steps in which scientists do experiments with a health intervention in an attempt to find enough evidence for a process which would be useful as a medical treatment. In the case of pharmaceutical study, the phases start with drug design and drug discovery then proceed on to animal testing. If this is successful, they begin the clinical phase of development by testing for safety in a few human subjects and expand to test in many study participants to determine if the treatment is effective.


Clinical trials involving new drugs are commonly classified into four phases. Individual trials may encompass more than one phase. A common example of this is combined phase I/II or phase II/III trials. Therefore, it may be easier to think of early phase studies and late phase studies.[1] The drug-development process will normally proceed through all four phases over many years. If the drug successfully passes through Phases I, II, and III, it will usually be approved by the national regulatory authority for use in the general population. Phase IV are 'post-approval' studies.

Summary of clinical trial phases
Phase Primary goal Dose Patient monitor Typical number of participants Success rate[2] Notes
Preclinical Testing of drug in non-human subjects, to gather efficacy, toxicity and pharmacokinetic information unrestricted scientific researcher not applicable (in vitro and in vivo only)
Phase 0 Pharmacokinetics particularly oral bioavailability and half-life of the drug very small, subtherapeutic clinical researcher 10 people often skipped for phase I
Phase I Testing of drug on healthy volunteers for dose-ranging often subtherapeutic, but with ascending doses clinical researcher 20-100 normal healthy volunteers (or for cancer drugs, cancer patients) approximately 70% determines whether drug is safe to check for efficacy
Phase II Testing of drug on patients to assess efficacy and side effects therapeutic dose clinical researcher 100-300 patients with specific diseases approximately 33% determines whether drug can have any efficacy; at this point, the drug is not presumed to have any therapeutic effect whatsoever
Phase III Testing of drug on patients to assess efficacy, effectiveness and safety therapeutic dose clinical researcher and personal physician 300-3,000 patients with specific diseases 25-30% determines a drug's therapeutic effect; at this point, the drug is presumed to have some effect
Phase IV Postmarketing surveillance – watching drug use in public therapeutic dose personal physician anyone seeking treatment from their physician N/A watch drug's long-term effects

Pre-clinical studies[edit]

Before pharmaceutical companies start clinical trials on a drug, they conduct extensive pre-clinical studies. These involve in vitro (test tube or cell culture) and in vivo (animal) experiments using wide-ranging doses of the study drug to obtain preliminary efficacy, toxicity and pharmacokinetic information. Such tests assist pharmaceutical companies to decide whether a drug candidate has scientific merit for further development as an investigational new drug.

Phase 0[edit]

Phase 0 is a recent designation for optional exploratory trials conducted in accordance with the United States Food and Drug Administration's (FDA) 2006 Guidance on Exploratory Investigational New Drug (IND) Studies.[3] Phase 0 trials are also known as human microdosing studies and are designed to speed up the development of promising drugs or imaging agents by establishing very early on whether the drug or agent behaves in human subjects as was expected from preclinical studies. Distinctive features of Phase 0 trials include the administration of single subtherapeutic doses of the study drug to a small number of subjects (10 to 15) to gather preliminary data on the agent's pharmacokinetics (what the body does to the drugs).[4]

A Phase 0 study gives no data on safety or efficacy, being by definition a dose too low to cause any therapeutic effect. Drug development companies carry out Phase 0 studies to rank drug candidates in order to decide which has the best pharmacokinetic parameters in humans to take forward into further development. They enable go/no-go decisions to be based on relevant human models instead of relying on sometimes inconsistent animal data.

Phase I[edit]

Phase I trials are often referred to as “first-in-man studies” as they are the first stage of testing in human subjects.[5] They are designed to determine the maximum amount of the drug that can be given to a person before adverse effects become intolerable or dangerous.

Normally, a small group of 2–100 healthy volunteers will be recruited.[2][5] These trials are often conducted in a clinical trial clinic, where the subject can be observed by full-time staff. These clinical trial clinics are often run by contract research organization (CROs) who conduct these studies on behalf of pharmaceutical companies or other research investigators. The subject who receives the drug is usually observed until several half-lives of the drug have passed. This phase is designed to assess the safety (pharmacovigilance), tolerability, pharmacokinetics, and pharmacodynamics of a drug. Phase I trials normally include dose-ranging, also called dose escalation studies, so that the best and safest dose can be found and to discover the point at which a compound is too poisonous to administer.[6] The tested range of doses will usually be a fraction[quantify] of the dose that caused harm in animal testing. Phase I trials most often include healthy volunteers. However, there are some circumstances when clinical patients are used, such as patients who have terminal cancer or HIV and the treatment is likely to make healthy individuals ill. These studies are usually conducted in tightly controlled clinics called CPUs (Central Pharmacological Units), where participants receive 24-hour medical attention and oversight. In addition to the previously mentioned unhealthy individuals, “patients who have typically already tried and failed to improve on the existing standard therapies"[1] may also participate in phase I trials. Volunteers are paid a variable inconvenience fee for their time spent in the volunteer center.

Before beginning a phase I trial, the sponsor must submit an Investigational New Drug application to the FDA detailing the preliminary data on the drug gathered from cellular models and animal studies.

Phase I trials can be further divided:

Single ascending dose (Phase Ia)
In single ascending dose studies, small groups of subjects are given a single dose of the drug while they are observed and tested for a period of time to confirm safety.[5][7] Typically, a small number of participants, usually three, are entered sequentially at a particular dose.[1] If they do not exhibit any adverse side effects, and the pharmacokinetic data are roughly in line with predicted safe values, the dose is escalated, and a new group of subjects is then given a higher dose. If unacceptable toxicity is observed in any of the three participants, an additional number of participants, usually three, are treated at the same dose.[1] This is continued until pre-calculated pharmacokinetic safety levels are reached, or intolerable side effects start showing up (at which point the drug is said to have reached the maximum tolerated dose (MTD)). If an additional unacceptable toxicity is observed, then the dose escalation is terminated and that dose, or perhaps the previous dose, is declared to be the maximally tolerated dose. This particular design assumes that the maximally tolerated dose occurs when approximately one-third of the participants experience unacceptable toxicity. Variations of this design exist, but most are similar.[1]
Multiple ascending dose (Phase Ib)
Multiple ascending dose studies investigate the pharmacokinetics and pharmacodynamics of multiple doses of the drug, looking at safety and tolerability. In these studies, a group of patients receives multiple low doses of the drug, while samples (of blood, and other fluids) are collected at various time points and analyzed to acquire information on how the drug is processed within the body. The dose is subsequently escalated for further groups, up to a predetermined level.[5][7]
Food effect
A short trial designed to investigate any differences in absorption of the drug by the body, caused by eating before the drug is given. These studies are usually run as a crossover study, with volunteers being given two identical doses of the drug while fasted, and after being fed.

Phase II[edit]

Once a dose or range of doses is determined, the next goal is to evaluate whether the drug has any biological activity or effect.[1] Phase II trials are performed on larger groups (100-300) and are designed to assess how well the drug works, as well as to continue Phase I safety assessments in a larger group of volunteers and patients. Genetic testing is common, particularly when there is evidence of variation in metabolic rate.[1] When the development process for a new drug fails, this usually occurs during Phase II trials when the drug is discovered not to work as planned, or to have toxic effects.

Phase II studies are sometimes divided into Phase IIA and Phase IIB. There is no formal definition for these 2 sub-categories, but generally:

  • Phase IIA studies are usually pilot studies designed to demonstrate clinical efficacy or biological activity ('proof of concept' studies);
  • Phase IIB studies look to find the optimum dose at which the drug shows biological activity with minimal side-effects (‘definite dose-finding’ studies).

Some trials combine Phase I and Phase II, and test both efficacy and toxicity.

Trial design
Some Phase II trials are designed as case series, demonstrating a drug's safety and activity in a selected group of patients. Other Phase II trials are designed as randomized controlled trials, where some patients receive the drug/device and others receive placebo/standard treatment. Randomized Phase II trials have far fewer patients than randomized Phase III trials.
Example Cancer Design
In the first stage, the investigator attempts to rule out drugs which have no or little biologic activity. For example, the researcher may specify that a drug must have some minimal level of activity, say, in 20% of participants. If the estimated activity level is less than 20%, the researcher chooses not to consider this drug further, at least not at that maximally tolerated dose. If the estimated activity level exceeds 20%, the researcher will add more participants to get a better estimate of the response rate. A typical study for ruling out a 20% or lower response rate enters 14 participants. If no response is observed in the first 14 participants, the drug is considered not likely to have a 20% or higher activity level. The number of additional participants added depends on the degree of precision desired, but ranges from 10 to 20. Thus, a typical cancer phase II study might include fewer than 30 people to estimate the response rate.[1]
Efficacy vs Effectiveness
When a study assesses efficacy, it is looking at whether the drug given in the specific manner described in the study is able to influence an outcome of interest (e.g. tumor size) in the chosen population (e.g. cancer patients with no other ongoing diseases). When a study is assessing effectiveness, it is determining whether a treatment will influence the disease. In an effectiveness study it is essential that patients are treated as they would be when the treatment is prescribed in actual practice. That would mean that there should be no aspects of the study designed to increase patient compliance above those that would occur in routine clinical practice. The outcomes in effectiveness studies are also more generally applicable than in most efficacy studies (for example does the patient feel better, come to the hospital less or live longer in effectiveness studies as opposed to better test scores or lower cell counts in efficacy studies). There is usually less rigid control of the type of patient to be included in effectiveness studies than in efficacy studies, as the researchers are interested in whether the drug will have a broad effect in the population of patients with the disease.

Some researchers argue that phase II studies are generally smaller than they ought to be.[1]

Success rate[edit]

In 2010, the percentage of phase II trials that proceeded to phase III was 18%.[8]

Phase III[edit]

This phase is designed to assess the effectiveness of the new intervention and, thereby, its value in clinical practice.[1] Phase III studies are randomized controlled multicenter trials on large patient groups (300–3,000 or more depending upon the disease/medical condition studied) and are aimed at being the definitive assessment of how effective the drug is, in comparison with current 'gold standard' treatment. Because of their size and comparatively long duration, Phase III trials are the most expensive, time-consuming and difficult trials to design and run, especially in therapies for chronic medical conditions. Phase III trials of chronic conditions or diseases often have a short follow-up period for evaluation, relative to the period of time the intervention might be used in practice.[1] This is sometimes called the "pre-marketing phase" because it actually measures consumer response to the drug.

It is common practice that certain Phase III trials will continue while the regulatory submission is pending at the appropriate regulatory agency. This allows patients to continue to receive possibly lifesaving drugs until the drug can be obtained by purchase. Other reasons for performing trials at this stage include attempts by the sponsor at "label expansion" (to show the drug works for additional types of patients/diseases beyond the original use for which the drug was approved for marketing), to obtain additional safety data, or to support marketing claims for the drug. Studies in this phase are by some companies categorized as "Phase IIIB studies."[9][10]

While not required in all cases, it is typically expected that there be at least two successful Phase III trials, demonstrating a drug's safety and efficacy, in order to obtain approval from the appropriate regulatory agencies such as FDA (USA), or the EMA (European Union).

Once a drug has proved satisfactory after Phase III trials, the trial results are usually combined into a large document containing a comprehensive description of the methods and results of human and animal studies, manufacturing procedures, formulation details, and shelf life. This collection of information makes up the "regulatory submission" that is provided for review to the appropriate regulatory authorities[11] in different countries. They will review the submission, and, it is hoped, give the sponsor approval to market the drug.

Most drugs undergoing Phase III clinical trials can be marketed under FDA norms with proper recommendations and guidelines through a New Drug Application (NDA) containing all manufacturing, pre-clinical, and clinical data. In case of any adverse effects being reported anywhere, the drugs need to be recalled immediately from the market. While most pharmaceutical companies refrain from this practice, it is not abnormal to see many drugs undergoing Phase III clinical trials in the market.[12]

Success rate[edit]

As of 2010, about 50% of drug candidates either fail during the Phase III trial or are rejected by the national regulatory agency.[13]

Phase II/III cost

The amount of money spent on Phase II/III trials in 2015 by Large Sponsors (R&D $500M+) was $465,725,000 on average, while Non-Large Sponsors (R&D <$500M) spent $13,352,000 on average.[14]

Phase IV[edit]

Phase IV trial is also known as postmarketing surveillance Trial. Phase IV trials involve the safety surveillance (pharmacovigilance) and ongoing technical support of a drug after it receives permission to be sold (e.g. after approval under FDA Accelerated Approval Program). Phase IV studies may be required by regulatory authorities or may be undertaken by the sponsoring company for competitive (finding a new market for the drug) or other reasons (for example, the drug may not have been tested for interactions with other drugs, or on certain population groups such as pregnant women, who are unlikely to subject themselves to trials). The safety surveillance is designed to detect any rare or long-term adverse effects over a much larger patient population and longer time period than was possible during the Phase I-III clinical trials. Harmful effects discovered by Phase IV trials may result in a drug being no longer sold, or restricted to certain uses; recent examples involve cerivastatin (brand names Baycol and Lipobay), troglitazone (Rezulin) and rofecoxib (Vioxx).

The minimum time period mandatory for Phase IV clinical trials is 2 years.[citation needed]

Overall cost[edit]

The entire process of developing a drug from preclinical research to marketing can take approximately 12 to 18 years and often cost well over $1 billion.[15][16]


  1. ^ a b c d e f g h i j k DeMets, D., Friedman, L., and Furberg, C. (2010). Fundamentals of Clinical Trials (4th ed.). Springer. ISBN 978-1-4419-1585-6. 
  2. ^ a b "Step 3. Clinical research". US Food and Drug Administration. 14 October 2016. Retrieved 1 February 2017. 
  3. ^ CDER (January 2006). "Exploratory IND Studies" (PDF). Guidance for Industry, Investigators, and Reviewers. Food and Drug Administration. Retrieved 2010-06-15. 
  4. ^ The Lancet (2009). "Phase 0 trials: a platform for drug development?". Lancet. 374 (9685): 176. doi:10.1016/S0140-6736(09)61309-X. PMID 19616703. 
  5. ^ a b c d "Phases of clinical trials". Canadian Cancer Society. 2017. Retrieved 1 February 2017. 
  6. ^ Adil E. Shamoo (2008). "The Myth of Equipoise in Phase 1 Clinical Trials". Medscape J Med. 10 (11): 254. PMC 2605120Freely accessible. PMID 19099004. (registration required)
  7. ^ a b Elizabeth Norfleet, Shayne Cox Gad, "Phase I Clinical Trials", in Shayne Cox Gad, Clinical Trials Handbook, ISBN 978-0-470-46635-3, 2009, p. 247
  8. ^ "New drugs failing Phase II and III clinical trials". MedCity News. 
  9. ^ "Guidance for Institutional Review Boards and Clinical Investigators". Food and Drug Administration. 1999-03-16. Retrieved 2007-03-27. 
  10. ^ "Periapproval Services (Phase IIIb and IV programs)". Covance Inc. 2005. Retrieved 2007-03-27. 
  11. ^ The regulatory authority in the USA is the Food and Drug Administration; in Canada, Health Canada; in the European Union, the European Medicines Agency; and in Japan, the Ministry of Health, Labour and Welfare
  12. ^ Arcangelo, Virginia Poole; Andrew M. Peterson (2005). Pharmacotherapeutics for Advanced Practice: A Practical Approach. Lippincott Williams & Wilkins. ISBN 0-7817-5784-3. 
  13. ^ Arrowsmith, John (1 February 2011). "Trial watch: Phase III and submission failures: 2007–2010". Nat Rev Drug Discov. 10 (2): 87–87. doi:10.1038/nrd3375 – via 
  14. ^ "Phase II/III Study Trends and Market Outlook (2016-2020) - Industry Standard Research". p. 9. Retrieved 2016-07-18. 
  15. ^ Holland, John (2013). "Fixing a broken drug development process". Journal of Commercial Biotechnology. 19. doi:10.5912/jcb588. 
  16. ^ Adams, C. P.; Brantner, V. V. (2006). "Estimating the Cost of New Drug Development: Is It Really $802 Million?". Health Affairs. 25 (2): 420–8. doi:10.1377/hlthaff.25.2.420. PMID 16522582.