New approaches to rational drug design are being explored in the contemporary discovery and development of medicines, with pharmacokinetic parameters being of great importance because unfavorable pharmacokinetic properties are one of the most common causes of clinical failure of new chemical entities. One of the most important physico-chemical properties that affects the pharmacokinetics and pharmacodynamics of the drug is lipophilicity because it is a property that affects drug solubility, drug delivery through cell membranes and blood brain barrier, absorption and oral bioavailability, distribution, drug interaction with transport proteins, activity and drug metabolism. Chromatographic techniques have proven to be a good choice for determining the lipophilic profile of the compounds, especially high performance reverse phase chromatography. Today, there are also chromatographic columns with artificial membranes which, due to their composition and analytical interactions, allow for a more accurate comparison with results performed in in vivo conditions. In this paper high performance reverse phase chromatography was performed for drug lipophilicity testing in the case of an immunosuppressive drug azathioprine and an antineoplastic drug thioguanine. columns and IAM columns with bound phospholipid membranes were used to create conditions similar to biological and predicted better drug permeability. The results show that both drugs are lipophilic, although thiogainine is less lipophilic than azathioprine, and both drugs interact with phospholipids, which is the reason why both are administered perorally in the form of tablets.