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Scholarly Works (12 results)

  • Thesis
  • Peer Reviewed
UC San Francisco Electronic Theses and Dissertations (2019)

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.

    Cover page: Improvements for the in Vitro to in Vivo Extrapolation of Hepatic Clearance
    • Article
    • Peer Reviewed
    UC San Francisco Previously Published Works (2016)
    Predicting in vivo pharmacokinetic parameters such as clearance from in vitro data is a crucial part of the drug-development process. There is a commonly cited trend that drugs that are highly protein-bound and are substrates for hepatic uptake transporters often yield the worst predictions. Given this information, 11 different data sets using human microsomes and hepatocytes were evaluated to search for trends in accuracy, extent of protein binding, and drug classification based on the Biopharmaceutics Drug Disposition Classification System (BDDCS), which makes predictions about transporter effects. As previously reported, both in vitro systems (microsomes and hepatocytes) gave a large number of inaccurate results, defined as predictions falling more than 2-fold outside of in vivo values. The weighted average of the percentage of inaccuracy was 66.5%. BDDCS class 2 drugs, which are subject to transporter effects in vivo unlike class 1 compounds, had a higher percentage of inaccurate predictions and often had slightly larger bias. However, since the weighted average of the percentage of inaccuracy was still high in both classes (81.9% for class 2 and 62.3% for class 1), it may be currently hard to use BDDCS class to predict potential accuracy. The results of this study emphasize the need for improved in vitro to in vivo extrapolation experimental methods, as using physiologically based scaling is still not accurate, and BDDCS cannot currently help predict accurate results.
      Cover page: Hepatic clearance predictions from in vitro-in vivo extrapolation and BDDCS
      • Article
      • Peer Reviewed
      UC San Francisco Previously Published Works (2019)
      Accurately predicting the hepatic clearance of compounds using in vitro to in vivo extrapolation (IVIVE) is crucial within the pharmaceutical industry. However, several groups have recently highlighted the serious error in the process. Although empirical or regression-based scaling factors may be used to mitigate the common underprediction, they provide unsatisfying solutions because the reasoning behind the underlying error has yet to be determined. One previously noted trend was intrinsic clearance-dependent underprediction, highlighting the limitations of current in vitro systems. When applying these generated in vitro intrinsic clearance values during drug development and making first-in-human dose predictions for new chemical entities though, hepatic clearance is the parameter that must be estimated using a model of hepatic disposition, such as the well-stirred model. Here, we examine error across hepatic clearance ranges and find a similar hepatic clearance-dependent trend, with high clearance compounds not predicted to be so, demonstrating another gap in the field.
        Cover page: In Vitro-In Vivo Extrapolation and Hepatic Clearance-Dependent Underprediction
        • Article
        • Peer Reviewed
        UC San Francisco Previously Published Works (2019)

        Purpose

        To examine the interlaboratory variability in CLint values generated with human hepatocytes and determine trends in variability and clearance prediction accuracy using physicochemical and pharmacokinetic parameters.

        Methods

        Data for 50 compounds from 14 papers were compiled with physicochemical and pharmacokinetic parameter values taken from various sources.

        Results

        Coefficients of variation were as high as 99.8% for individual compounds and variation was not dependent on the number of prediction values included in the analysis. When examining median values, it appeared that compounds with a lower number of rotatable bonds had more variability. When examining prediction uniformity, those compounds with uniform in vivo underpredictions had higher CLint, in vivo values, while those with non-uniform predictions typically had lower CLint, in vivo values. Of the compounds with uniform predictions, only a small number were uniformly predicted accurately. Based on this limited dataset, less lipophilic, lower intrinsic clearance, and lower protein binding compounds yield more accurate clearance predictions.

        Conclusions

        Caution should be taken when compiling in vitro CLint values from different laboratories as variations in experimental procedures (such as extent of shaking during incubation) may yield different predictions for the same compound. The majority of compounds with uniform in vitro values had predictions that were inaccurate, emphasizing the need for a better mechanistic understanding of IVIVE. The non-uniform predictions, often with low turnover compounds, reaffirmed the experimental challenges for drugs in this clearance range. Separating new chemical entities by lipophilicity, intrinsic clearance, and protein binding may help instill more confidence in IVIVE predictions.
          Cover page: Interlaboratory Variability in Human Hepatocyte Intrinsic Clearance Values and Trends with Physicochemical Properties
          • Article
          • Peer Reviewed
          Journal of Education and Teaching in Emergency Medicine, Volume 8, Issue 1 (2023)
            Cover page: Electrocardiogram Abnormalities Following Diphenhydramine Ingestion A Case ReportCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC San Francisco Previously Published Works (2019)
            When predicting hepatic clearance using in vitro to in vivo extrapolation (IVIVE), microsomes or hepatocytes are commonly used. Here, we examine intrinsic clearance values and IVIVE results in human hepatocytes and microsomes for compounds metabolized by a variety of enzymes. The great majority of CYP3A4 substrates examined had higher intrinsic clearance values in microsomes compared with hepatocytes, whereas the values were more similar between the two incubations for substrates of other enzymes. We hypothesize that this may be due to interplay between CYP3A4 and the efflux transporter P-glycoprotein, as they have been shown to exhibit coordinated regulation. When examining the prediction accuracy for substrates of other enzymes between microsomes and hepatocytes, average fold errors as well as overall error were similar, demonstrating once again that IVIVE methods are not adequately defined and understood. SIGNIFICANCE STATEMENT: For CYP3A4 substrates, microsomes give markedly higher predictive in vitro to in vivo extrapolation than for other metabolic enzymes, which is not found for hepatocytes. We hypothesize that this could be a result of CYP3A4-P-glycoprotein interplay or coordinated regulation in hepatocytes that would not be observed in microsomes.
              Cover page: In Vitro–In Vivo Inaccuracy: The CYP3A4 Anomaly
              • Article
              • Peer Reviewed
              UC San Francisco Previously Published Works (2019)
              When predicting hepatic clearance using in vitro to in vivo extrapolation (IVIVE), microsomes or hepatocytes are commonly used. Here, we examine intrinsic clearance values and IVIVE results in human hepatocytes and microsomes for compounds metabolized by a variety of enzymes. The great majority of CYP3A4 substrates examined had higher intrinsic clearance values in microsomes compared with hepatocytes, whereas the values were more similar between the two incubations for substrates of other enzymes. We hypothesize that this may be due to interplay between CYP3A4 and the efflux transporter P-glycoprotein, as they have been shown to exhibit coordinated regulation. When examining the prediction accuracy for substrates of other enzymes between microsomes and hepatocytes, average fold errors as well as overall error were similar, demonstrating once again that IVIVE methods are not adequately defined and understood. SIGNIFICANCE STATEMENT: For CYP3A4 substrates, microsomes give markedly higher predictive in vitro to in vivo extrapolation than for other metabolic enzymes, which is not found for hepatocytes. We hypothesize that this could be a result of CYP3A4-P-glycoprotein interplay or coordinated regulation in hepatocytes that would not be observed in microsomes.
                Cover page: In Vitro–In Vivo Inaccuracy: The CYP3A4 Anomaly
                • Article
                • Peer Reviewed
                UC San Francisco Previously Published Works (2019)
                Accurately predicting hepatic clearance is an integral part of the drug-development process, and yet current in vitro to in vivo (IVIVE) extrapolation methods yield poor predictions, particularly for highly protein-bound transporter substrates. Explanations for error include inaccuracies in protein-binding measurements and the lack of recognition of protein-facilitated uptake, where both unbound and bound drug may be cleared, violating the principles of the widely accepted free drug theory. A new explanation for protein-facilitated uptake is proposed here, called a transporter-induced protein binding shift High-affinity binding to cell-membrane proteins may change the equilibrium of the nonspecific binding between drugs and plasma proteins, leading to greater cellular uptake and clearance than currently predicted. The uptake of two lower protein-binding organic anion transporting polypeptide substrates (pravastatin and rosuvastatin) and two higher binding substrates (atorvastatin and pitavastatin) were measured in rat hepatocytes in incubations with protein-free buffer versus 100% plasma. Decreased unbound K m values and increased intrinsic clearance values were seen in the plasma incubations for the highly bound compounds, supporting the new hypothesis and mitigating the IVIVE underprediction previously seen for highly bound transporter substrates.
                  Cover page: The Presence of a Transporter-Induced Protein Binding Shift: A new Explanation for Protein-Facilitated Uptake and Improvement for In Vitro-In Vivo Extrapolation
                  • Article
                  • Peer Reviewed
                  UC San Francisco Previously Published Works (2019)
                  The emergence and continued evolution of the transporter field has caused re-evaluation and refinement of the original principles surrounding drug disposition. In this paper, we emphasize the impact that transporters can have on volume of distribution and how this can affect the other major pharmacokinetic parameters. When metabolic drug-drug interactions or pharmacogenomic variance changes the metabolism of a drug, the volume of distribution appears to be unchanged while clearance, bioavailability, and half-life are changed. When transporters are involved in the drug-drug interactions or pharmacogenomic variance, the volume of distribution can be markedly affected causing counterintuitive changes in half-life. Cases are examined where a volume of distribution change is significant enough that although clearance decreases, half-life decreases. Thus, drug dosing decisions must be made based on CL/F changes, not half-life changes, as such volume of distribution alterations will also influence the half-life results.
                    Cover page: How Transporters Have Changed Basic Pharmacokinetic Understanding
                    • Article
                    • Peer Reviewed
                    UC San Francisco Previously Published Works (2018)

                    Purpose

                    To derive the theoretical basis for the extended clearance model of organ elimination following both oral and IV dosing, and critically analyze the approaches previously taken.

                    Methods

                    We derived from first principles the theoretical basis for the extended clearance concept of organ elimination following both oral and IV dosing and critically analyzed previous approaches.

                    Results

                    We point out a number of critical characteristics that have either been misinterpreted or not clearly presented in previously published treatments. First, the extended clearance concept is derived based on the well-stirred model. It is not appropriate to use alternative models of hepatic clearance. In analyzing equations, clearance terms are all intrinsic clearances, not total drug clearances. Flow and protein binding parameters should reflect blood measurements, not plasma values. In calculating the AUCR-factor following oral dosing, the AUC terms do not include flow parameters. We propose that calculations of AUCR may be a more useful approach to evaluate drug-drug and pharmacogenomic interactions than evaluating rate-determining steps. Through analyses of cerivastatin and fluvastatin interactions with cyclosporine we emphasize the need to characterize volume of distribution changes resulting from transporter inhibition/induction that can affect rate constants in PBPK models. Finally, we note that for oral doses, prediction of systemic and intrahepatic drug-drug interactions do not require knowledge of fu,H or Kp,uu for substrates/victims.

                    Conclusions

                    The extended clearance concept is a powerful tool to evaluate drug-drug interactions, pharmacogenomic and disease state variance but evaluating the AUCR-factor may provide a more valuable approach than characterizing rate-determining steps.
                      Cover page: The Extended Clearance Concept Following Oral and Intravenous Dosing: Theory and Critical Analyses
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