Systemic concentrations can limit the oral absorption of poorly soluble drugs: an investigation of non-sink permeation using physiologically based pharmacokinetic modeling.

In the early drug discovery environment, poorly soluble compounds with suboptimal potency are often used in efficacy studies to demonstrate in vivo preclinical proof-of-concept for new drug discovery targets and in preclinical toxicity studies to assess chemical scaffold safety. These compounds present a challenge to formulation scientists who are tasked with improving their oral bioavailability because high systemic concentrations are required. Despite the use of enabling formulations, increases in systemic exposure following oral delivery are often not achieved. We hypothesize that in some cases non-sink intestinal permeation can occur for poorly soluble compounds where their high systemic concentrations can act to inhibit their own oral absorption. Rats were given a 30 mg/kg oral dose of 1,3-dicyclohexyl urea (DCU) alone or concurrently with deuterated DCU (D8-DCU) intravenous infusions at rates of 13, 17, and 22 mg/kg/h. D8-DCU infusions dose dependently inhibited DCU oral absorption up to a maximum of 92%. Physiologically based pharmacokinetic modeling was utilized to understand the complex interaction between high DCU systemic concentrations and its effect on its own oral absorption. We show that high systemic concentrations of DCU act to suppress its own absorption by creating a condition where intestinal permeation occurs under non-sink conditions. More importantly, we identify relevant DCU concentrations that create the concentration gradient driving the intestinal permeation process. A new parameter, the maximum permeation extraction ratio, is proposed and provides a simple means to assess the extent of non-sink permeation.