Chittaranjan, as per your knowledge, you don't seem to know what you are talking about. This is a clinical research forum. Your wikipedia definition isn't helpful.
A proof of principle study is done before a proof of concept study. Typically, it is a Phase I study that is primarily looking at safety in healthy volunteers but also examines the intended pharmacological effect. Eg, an intended diabetes drug might look at the effect on blood sugar in healthy volunteers. It might also look at an expected change in a relevant biomarker. Proof of principle therefore provides the first measurable evidence that an investigational product might work in humans, but in this case healthy volunteers.
Proof of concept studies are typically Phase II studies and are performed in a larger number of subjects, and in this case the subjects are patients with the disease. Proof of concept often include several doses and are used to establish the dose and patient population to be used in the subsequent confirmatory Phase III studies.
These are are the classic definitions, and different disease may are may not be amenable to this approach. Are they regulatory requirements? In most cases health authorities and ethics committees would require such trials, such a step wise development, to insure patients safety in subsequent trials. Also, it is in the pharmaceutical company's best interest to get this information before going on to bigger and more expensive trials.