In Vivo-to-In Vitro Extrapolation of Transporter-Mediated Renal Clearance: Relative Expression Factor Versus Relative Activity Factor Approach

Abstract

About 30% of approved drugs are cleared predominantly by renal clearance (CL_r). Of these, many are secreted by transporters. For these drugs, in vitro-to-in vivo extrapolation of transporter-mediated renal secretory clearance (CL_sec,plasma) is important to prospectively predict their renal clearance and to assess the impact of drug-drug interactions and pharmacogenetics on their pharmacokinetics. Here we compared the ability of the relative expression factor (REF) and the relative activity factor (RAF) approaches to quantitatively predict the in vivo CL_sec,plasma of 26 organic anion transporter (OAT) substrates assuming that OAT-mediated uptake is the rate-determining step in the CL_sec,plasma of the drugs. The REF approach requires protein quantification of each transporter in the tissue (e.g., kidney) and transporter-expressing cells, whereas the RAF approach requires the use of a transporter-selective probe substrate (both in vitro and in vivo) for each transporter of interest. For the REF approach, 50% and 69% of the CL_sec,plasma predictions were within 2- and 3-fold of the observed values, respectively; the corresponding values for the RAF approach were 65% and 81%. We found no significant difference between the two approaches in their predictive capability (as measured by accuracy and bias) of the CL_sec,plasma or CL_r of OAT drugs. We recommend that the REF and RAF approaches can be used interchangeably to predict OAT-mediated CL_sec,plasma Further research is warranted to evaluate the ability of the REF or RAF approach to predict CL_sec,plasma of drugs when uptake is not the rate-determining step. SIGNIFICANCE STATEMENT: This is the first direct comparison of the relative expression factor (REF) and relative activity factor (RAF) approaches to predict transporter-mediated renal clearance (CL_r). The RAF, but not REF, approach requires transporter-selective probes and that the basolateral uptake is the rate-determining step in the CL_r of drugs. Given that there is no difference in predictive capability of the REF and RAF approach for organic anion transporter-mediated CL_r, the REF approach should be explored further to assess its ability to predict CL_r when basolateral uptake is not the sole rate-determining step.

Fig. 1.

For IVIVE of renal CL_sec,plasma of drugs, the REF approach scales the drug uptake CL into transporter-expressing cells using the REF (transporter abundance in transporter-expressing cells/transporter abundance in the human kidney). In contrast, the RAF approach scales the drug uptake CL into transporter-expressing cells using the RAF (uptake CL of the probe drug in transporter-expressing cells/in vivo probe CL_{int,sec,in vivo}). Eq. X indicates the equation used in the REF approach or the RAF approach (Mathialagan et al., 2017). CL_Filtration, filtration clearance; hOAT, human organic anion transporter.

Fig. 2.

Observed and predicted values of CL_{int,sec,in vivo} (A and B), CL_sec,plasma (C and D), or CL_r,plasma (E and F) when using the REF and RAF approaches. The solid line is the line of identity. Passive diffusion secretory CL of the drugs was assumed to be negligible.

Fig. 3.

Both the REF and RAF approaches were equally precise (RMSE) and unbiased (ME) in predicting the CL_{int,sec,in vivo} (A and B) as demonstrated by the overlapping 95% confidence intervals (lines). This conclusion remained the same for precision and bias of CL_sec,plasma and CL_r,plasma predictions (C and D) by the two approaches. Passive diffusion secretory CL of the drugs was assumed to be negligible.

Fig. 4.

Observed and predicted values of CL_int,sec,in vivo (A and B), CL_sec,plasma (C and D), or CL_r,plasma (E and F) when passive diffusion secretory CL was included in the REF and RAF approaches. The solid line is the line of identity.

Fig. 5.

After including passive diffusion secretory clearance when predicting CL_{int,sec,in vivo}, the REF approach demonstrated a positive bias (ME), whereas the RAF approach did not, but both approaches were equally precise (RMSE) as demonstrated by the overlapping 95% confidence intervals [lines; (A) and (B)]. In addition, both approaches were equally precise (RMSE) and unbiased (ME) in predicting CL_sec,plasma and CL_r,plasma of the drugs (C and D).