Introduction: In hypo-albuminemia, the extent of albumin binding of a drug decreases. The resulting change in plasma protein binding only rarely leads to clinically relevant changes in unbound drug exposure. Nevertheless, in the critically ill, a tendency to increase dosing of anti-infective therapy is seen in patients experiencing hypo-albuminemia. To reconcile basic pharmacological principles with current clinical practice, this work presents a pharmacologically-based pharmacokinetic simulation study to emphasize the (lack of) effect of altered plasma protein binding on a drug's concentration-time profile and associated pharmacokinetic parameters.
Methods: Four virtual compounds, representing a broad chemical space (low/high clearance/volume of distribution), were created and administered to a virtual population of normal patients and three types of hypo-albuminemic patients in Simcyp®. The influence of decreased plasma protein binding in hypoalbuminemia on the pharmacokinetic parameters and profiles of these four compounds was investigated.
Results: Simulation results showed that while high-clearance compounds suffer from increased unbound exposure with decreased plasma protein binding, the unbound exposure of low-clearance compounds was unaffected. However, for the subset of low-clearance compounds with a small volume of distribution, it appeared that there were still alterations in their plasma concentration-time profiles. Since this may lead to different times above a minimum inhibitory concentration value, this might affect the bacterial killing for some anti-infective drugs. Overall, for any compound involved in the simulations, the unbound exposure did not decrease in plasma protein binding subjects relative to normal plasma protein binding subjects.
Discussion: This finding is in line with the few case-controlled studies in the literature. Hence, increasing the dose/dosing frequency seems futile and might reduce the benefit-risk ratio for narrow therapeutic index drugs. Moreover, these simulations indicate that when only total plasma concentrations and derived pharmacokinetic parameters are considered, incorrect conclusions will be drawn.