Improvements for the in Vitro to in Vivo Extrapolation of Hepatic Clearance
Clearance, or a measure of the body’s ability to remove drug, is a crucial pharmacokinetic parameter. Since clearance is linked to a drug’s dosing regimen as well as its efficacy and toxicity, accurately predicting the parameter early in the drug discovery process is important to reduce the time and cost associated with drug development. To predict hepatic clearance, in vitro to in vivo extrapolation (IVIVE) is commonly used where an in vitro clearance measure generated in hepatocytes or microsomes is scaled to an in vivo prediction using biological scaling factors and a model of hepatic disposition.
The goal of this work was to evaluate the current state of IVIVE and examine ways to improve hepatic clearance predictions. After a literature search, we found that in human microsomes and hepatocytes, on average, 66.5% of drugs were predicted inaccurately. We also found that ≤ 25% of high extraction ratio compounds were predicted to have high extraction ratios. Examining the Biopharmaceutics Drug Disposition Classification System found that class 2 drugs have poorer predictions than class 1 drugs (81.9% vs. 62.3% inaccuracy), however these percentages of inaccuracy were still high in both cases. We conclude that the IVIVE of hepatic clearance needs to be improved through a better understanding of mechanisms.
We go on to propose a new hypothesis called a transporter induced protein binding shift (TIPBS) that is a new explanation for protein-facilitated uptake and can help mitigate current IVIVE error. When reviewing previous explanations for protein facilitated uptake, we noted that the hypotheses did not include the potential role of hepatic uptake transporters, and the poorest IVIVE predictions often occur for compounds that have high protein binding and are substrates of transporters. We hypothesized that for highly protein bound drugs with high affinity to transporters, protein-binding may not actually be restricting the drug’s access to the hepatocyte, which would lead to greater uptake and clearance values than currently predicted. We found support for our new TIPBS hypothesis by measuring the uptake of compounds in protein-free buffer vs. 100% plasma using rat hepatocytes as well as human embryonic kidney (HEK)293 cells overexpressing uptake transporters.
Finally we examined trends in clearance prediction accuracy with physiochemical and pharmacokinetic parameters. After noting high interlaboratory variability, we found that less lipophilic, lower intrinsic clearance, and lower protein binding compounds may yield more accurate predictions. These findings highlight the current errors associated with IVIVE and provide suggestions for predictions moving forward.