Phase I: The main aim of this phase is to determine drug safety. At this stage, drugs are tested in a small group of healthy volunteers to determine the drug’s activity. 

Phase II: These trials are aimed at identifying the optimal dose to be used in Phase III trials and, ideally, they identify drugs that will not make it through the next phase of testing. Typically, Phase II trials are double-blinded and have placebo controls.   

phase II clinical trials design: The optimal design of phase II studies continues to be the subject of vigorous debate, especially studies of newer molecularly targeted agents. The observations that many new therapeutics "fail" in definitive phase III studies, coupled with the numbers of new agents to be tested as well as the increasing costs and complexity of clinical trials, further emphasize the critical importance of robust and efficient phase II design. The Design of Phase II Clinical Trials Testing Cancer Therapeutics: Consensus Recommendations from the Clinica l Trial Design Task Force of the National Cancer Institute Investigational Drug Steering Committee. Seymour L, et. al,   Clin Cancer Res. 2010 Mar 9. [Epub ahead of print] http://www.ncbi.nlm.nih.gov/pubmed/20215557

Phase IIa and b: Some pharmaceutical companies further differentiate this phase into phases IIA and IIB. Clinical studies designed to evaluate dosing are referred to as Phase IIA and studies designed to determine the effectiveness of the drug are called phase  IIB. Design and Analysis of Clinical Trials Shein-Chung Chow, Jen-pei Liu Wiley 2004 http://books.google.com/books?id=HXMUEjZ4vyAC&pg=PA14&lpg=PA14&dq=Phase+IIIb+clinical+trials+FDA+DOSING+OR+COMPASSIOANTE&source=bl&ots=Y2P9rfVklb&sig=Idowe-Y

Phase III: These studies, which take several years, can involve thousands of patients at multiple trial centers. They are aimed at definitively determining the drug’s effectiveness and its side- effect profiles. These studies are also typically double- blinded and placebo- controlled.   

Phase III success rates for the pharmaceutical industry are low, but recent advances in simulation & modeling, clinical trial design, proof-of-concept, and pre-clinical decision making are set to reduce the attrition rate.
 
Phase IIIb: 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 categorised as "Phase IIIB studies."^[25][26]  Wikipedia accessed Feb 15 2011 http://en.wikipedia.org/wiki/Clinical_trial#Phase_III

Phase IV Planned post-marketing studies of diagnostic, therapeutic, or prophylactic drugs, devices, or techniques that have been approved for general sale. These studies are often conducted to obtain additional data about the safety and efficacy of a product. This concept includes phase IV studies conducted in both the U.S. and in other countries. MeSH  Clinical trials, Phase IV as topic 2008 (1993) 

Not all Phase IV studies are post-marketing surveillance (PMS) studies but every PMS study is a phase IV study. Phase IV is also an important phase of drug development. In particular, the real world effectiveness of a drug as evaluated in an observational, non-interventional trial in a naturalistic setting which complements the efficacy data that emanates from a pre-marketing randomized controlled trial (RCT). No matter how many patients are studied pre-marketing in a controlled environment, the true safety profile of a drug is characterized only by continuing safety surveillance through a spontaneous adverse event monitoring system and a post-marketing surveillance/non-interventional study.  Phase IV of Drug Development Dr Viraj Suvarna Perspect Clin Res. 2010 Apr-Jun; 1(2): 57–60. PMCID  PMC3148611 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3148611/  See also Post-Marketing Surveillence

phase zero: Phase 0 studies are exploratory studies that often use only a few small doses of a new drug in a few patients. They might test whether the drug reaches the tumor, how the drug acts in the human body, and how cancer cells in the human body respond to the drug. The patients in these studies might need extra tests such as biopsies, scans, and blood samples as part of the study process.The biggest difference between phase 0 and the later phases of clinical trials is that there’s almost no chance the volunteer will benefit by taking part in a phase 0 trial – the benefit will be for other people in the future. Because drug doses are low, there’s also less risk to the patient in phase 0 studies compared to phase I studies. Phase 0 studies help researchers find out whether the drugs do what they’re expected to do. If there are problems with the way the drug is absorbed or acts in the body, this should become clear very quickly in a phase 0 clinical trial. This process may help avoid the delay and expense of finding out years later in phase II or even phase III clinical trials that the drug doesn’t act as expected to based on lab studies. Phase 0 studies aren’t used widely, and there are some drugs for which they wouldn’t be helpful. Phase 0 studies are very small, often with fewer than 15 people, and the drug is given only for a short time. They’re not a required part of testing a new drug. American Cancer Society, Phase Zero Clinical trials https://www.cancer.org/treatment/treatments-and-side-effects/clinical-trials/what-you-need-to-know/phases-of-clinical-trials.html  Related term: Drug discovery & Development microdosing

pivotal clinical trials: The intermediate-sized clinical trials supported through this RFA are a pivotal decision point in the NCI chemoprevention drug development program. The consensus view of a Working Group from the NCI and the FDA acknowledges that "the interim analysis of a validated surrogate endpoint of cancer incidence may facilitate the timely and cost-effective marketing of efficacious drugs (Kelloff et al., Cancer Epidemiol. Biomark. Prev. 4: 1-10, 1995)." Thus, the efficacy and safety data from these studies potentially supports FDA marketing approval (NDA applications) for chemoprevention indications, and certainly facilitates decisions regarding the most appropriate recommendations for subsequent large, community-based efficacy studies. Pivotal clinical trials for chemoprevention clinical development  National Cancer Institute, NIH RFA: CA-98-001 1997 http://grants.nih.gov/grants/guide/rfa-files/rfa-ca-98-001.html  Related term: adaptive clinical trials  

placebo non- responders: Non- responders on placebo] define a group that would never improve their condition unless given the drug. They may be a group that, if we could identify them, could be used to reduce clinical trial size. Using this group in a proof- of -concept, it may be possible to test a drug even without a comparative placebo and determine whether it is likely to be active.   
Related terms: disease resistant individuals, lure of initial value, placebo responders

placebo responders: Most people think of the placebo response as a true response. But much of it is actually regression to the mean.  Clinical trial subjects with more extreme symptoms are often selected because it is desirable to see a dramatic effect upon treatment with the drug.   Related terms: disease resistant individuals, lure of initial value, placebo non- responders 

post-marketing surveillence: Drug safety   https://www.scopesummit.com/Post-Marketing-Studies/

pragmatic clinical trials:  are performed in real-world clinical settings with highly generalizable populations to generate actionable clinical evidence at a fraction of the typical cost and time needed to conduct a traditional clinical trial. They present an opportunity to efficiently address critical knowledge gaps and generate high-quality evidence to inform medical decision-making. However, these trials pose different challenges than are typically encountered with traditional clinical trials. NIH Collaboratory of Pragmatic Clinical Trials http://rethinkingclinicaltrials.org/