Importance of hereditary factors for the variability of pharmacokinetics of drugs in comparison between oral and intravenous dosing in a twin study

The main objective of this study was to investigate hereditary and non-hereditary differences in the activity of the OATP1B1 transporter and the CYP2C9 and CYP3A4 enzymes between oral and intravenous drug dosing. It was conducted based on an open label, non-randomized pharmacokinetic classical twin study. A total of 58 twins were included in the study. The study consisted of two study periods: In study period I intravenous application was performed, and oral application in study period II. Of the 58 pairs of twins, 37 pairs participated in study period II, of which 29 were monozygotic and 8 were same sex dizygotic twin pairs. During the study phases, torasemide (as a test substance for OATP1B1 and CYP2C9) and midazolam (as a test substance for CYP3A4) were administered to each subject; intravenously in study period I and per os in study period II. Measurements of drug levels in serum and urine at discrete time points over a period of 24h after administration permitted inference of in-vivo-activity of the respective transporter and enzymes. The area under the curve from zero to infinity (AUCinf) was used to measure activity of OATP1B1, CYP2C9 and CYP3A4 after oral and intravenous administration. To calculate the heritability of these pharmacokinetic parameters, three different methods were used: Calculation of correlation coefficients between monozygotic and dizygotic twins, calculation of heritability indices according to Faloner and Vesell, respectively, as well as structural equation modeling. In this study, activity of both OATP1B1 and CYP2C9 after oral administration of torasemide showed high variation depending on hereditary factors. Only 47.6% of this hereditary component, however, could be explained by known gene variants of OATP1B1 and CYP2C9. Genetic variants of CYP2C9 showed a tendency towards higher influence on torasemide metabolism after oral administration than after intravenous. This finding may be confounded, however, by large interindividual variation in expression – and thus activity – of intestinal CYP2C9, providing a basis for further investigation. The absolute oral bioavailability (F) of torasemide was not shown to be influenced by hereditary factors. In particular, there was no statistically significant influence of known genetic variants of OATP1B1 and CYP2C9, which is probably also due to the rarity of some of the gene variants. For CYP3A4 activity after oral administration, no statistically significant influences of hereditary factors on activity were identified in this study as variation in activity was mostly governed by environmental factors. Nevertheless, multiple regression analysis revealed a significant influence of CYP3A4*22 gene activity on AUCinf of 12.5% after oral midazolam application. These findings mirror the conflicting evidence on factors influencing CYP3A4 activity in literature. The small number of dizygotic pairs of twins included in this study may have further limited the power to detect statistically significant differences after oral drug administration. The absolute oral bioavailability (F) of midazolam did not show any significant evidence of genetic influences either, particularly with regard to known CYP3A4 polymorphisms. In conclusion, this study demonstrated a significant impact of hereditary factors on the pharmacokinetics of torasemide after both oral and intravenous intake. More than 50% of the genetic polymorphisms determining OATP1B1 and CYP2C9 activity, however, are still unknown. Variability in CYP3A4 activity seems to be multifactorial and not caused by genetic polymorphisms alone - at least considering the pharmacokinetics of midazolam. Further, environmental factors were shown to cause considerable variability in transporter and enzyme activity after oral administration, with even more of these factors unknown to date. The conflicting findings of absolute oral bioavailability (F) have to be interpreted not only in the light of external confounding factors but also linked to the unequal ratio of monozygotic to dizygotic twins included in the study. For future studies, special attention needs to be paid to ensure a balanced relationship between monozygotic and dizygotic twins, including - if necessary - dizygotic pairs of twins of different sexes. It might also be advisable to include further environmental factors in connection with e.g. CYP3A4 such as nutrition, epigenetic factors or the influence of the human microbiome.2020-10-0