Successful drug delivery will achieve an appropriate drug concentration at the target to elicit a desired level of response. The concentration of circulating free drug is dependent upon the pharmacokinetic processes of absorption, distribution, metabolism and elimination. The response usually results from interaction of a drug with a target protein. The genetic basis that underlies pharmacokinetic and pharmacodynamic interindividual variability is an important consideration in the design of drug delivery systems. Genetic polymorphism has been identified in drug-metabolizing enzymes, transporters and targets. Potentially, these pharmacogenomic factors can affect the concentration of free drug available for delivery and the subsequent elicited response.