Phase II studies can only begin once all Phase I studies have been completed. Phase II studies are so-called exploratory therapeutic studies that test the drug candidate in patients, i.e. participants suffering from the disease for which the drug is being developed.  

The number of participants in this phase varies from 100 to 500 and they are usually selected according to strict inclusion criteria, which makes the study population fairly homogeneous. This makes it easier to interpret the study results. 

The objective of Phase II studies is to determine whether the drug candidate has beneficial therapeutic effects for patients with the disease in question and whether it is safe.  

Research into therapeutic efficacy is carried out in so-called proof-of-concept studies, in which the drug candidate is compared to a placebo or, if one exists, to an existing treatment (usually the standard of care). These studies are usually randomized and (double)blind, but open label studies are not excluded.  

To test efficacy and maximize effect, proof-of-concept studies generally use the maximum tolerated dose determined in phase I. But Phase II also investigates the optimal dose and frequency of administration of the drug candidate and collects additional information on side effects. This is done in dose-response studies. Dose-response studies are usually randomized, placebo-controlled studies with parallel study groups studying different dose levels. The zero-dose level is then placebo. Crossover studies are also possible. 

In total, Phase II can last several years. The drug candidate can proceed to Phase III if, based on the results of these Phase II studies, it meets predefined criteria. These results form the basis for the design of phase III clinical trials.