The definite article can most certainly be omitted in this situation, but it does not really matter one way or the other.
The paragraph immediately preceding the passage we are discussing is intended to serve as an introduction to the general topic of pharmacokinetic studies. (The whole of chapter 5 of the document deals with clinical studies. Section 5.1 introduces pharmacokinetic clinical studies, stating that conducting pharmacokinetic studies is "normally the first step" in the clinical studies part of development that the whole document addresses, while section 5.2 deals with pharmacodynamic clinial studies and section 5.3 deals with safety evaluations in clinical studies. The passage we are discussing, subsection 5.1.1, addresses the design of pharmacokinetic studies.)
The writer is using the definite article here as a way to say to the reader, "Hey, we are now going to continue discussing those pharmacokinetic studies that I said are usually the first step in development." This is a perfectly acceptable rhetorical choice.
However, using the definite article in this way is entirely optional. It does not make the meaning of the passage any clearer, within the context of the document as a whole. The "the" can be left in, or it can be removed; it simply does not really matter.