Validating Low-Dose Iohexol as a Marker for Glomerular Filtration Rate by In Vitro and In Vivo Studies

Abstract

Clearance of an intravenous iohexol dose of 3235 mg is used to assess glomerular filtration rate (GFR), although systematic assessment of its pharmacokinetic (PK) properties is incomplete. The objectives of the present investigations were (i) to assess potential interactions of iohexol with important drug transporters, and (ii) whether a 259 mg dose could replace the current standard dose. In vitro, we evaluated whether iohexol inhibits or is transported by renal transporters (hOAT1/3, hOCT2, and hMATE1/2K) or other transporters (hOATP1B1/3, hOCT1, and hMDR1) using cell-based and vesicle-based systems. In vivo, we conducted a clinical trial with 12 volunteers with the administration of single intravenous doses of 3235 mg ("reference") and 259 mg ("test") using a changeover design. Plasma and urine samples were collected up to 24 h postdose. We assessed the dose linearity of iohexol pharmacokinetics using the standard bioequivalence approach and conducted a population PK analysis to characterize its profile. Our in vitro findings indicate that iohexol is neither a substrate nor a significant inhibitor of the transporters, suggesting it is unlikely to participate in transporter-mediated drug-drug interactions in vivo. In the clinical trial, the test/reference ratio for plasma clearance, calculated as dose divided by the area under the plasma concentration-time curve, was 1.01 (90% confidence interval 0.968-1.05), confirming dose linearity. Population PK analysis further supported these results, showing no significant effect of dose on renal clearance and negligible nonrenal clearance of iohexol. Low-dose iohexol is a suitable marker for precise GFR measurement, even when coadministered with other drugs.

Keywords: Iohexol; clinical trial; glomerular filtration rate; transporter‐mediated drug–drug interactions.

FIGURE 1

Ratio of iohexol uptake in transporter‐expressing cells vs. nonexpressing cells. Stably transfected 293 cells, either expressing (expression on; n = 3 or 4) or not expressing (expression off; n = 3 or 4) hOAT1, hOAT3, hOCT1, hOCT2, hMATE1, or hMATE2K, were incubated with 10 μM iohexol for 10 and 30 min, respectively. Each experiment was performed in triplicate, with each assay conducted on a separate day. In each assay, the ratio of iohexol uptake rates in transporter‐expressing cells (expression on) compared with nonexpressing cells (expression off) was calculated through element‐wise division within each experimental group. The dots and error bars represent the median values with 95% confidence intervals (CIs) of the log‐scaled ratios across three independent experiments.

FIGURE 2

Inhibitory effects of prototypical inhibitors and iohexol on transporter‐mediated transport of standard substrates. Relative transporter activity was calculated as the ratio of activity in the presence of inhibitors or iohexol to the activity in their absence, using element‐wise division within each experiment. Transporter activity (net uptake rate) was derived by element‐wise subtraction of the probe substrate concentrations under “expression off” conditions from those under “expression on” conditions for each transporter and study group. Columns and error bars represent the median values and 95% CIs of relative transporter activity across all respective experiments.

FIGURE 3

Semi‐logarithmic plots of (a) plasma concentration–time profiles of iohexol and (b) cumulative urinary excretion following single reference or test doses. Symbols and error bars represent geometric means and geometric standard deviations, respectively. Data from 12 subjects are included: 10 completed all sample collections, providing both plasma and urine data for the reference and test periods. One provided complete plasma data for both periods and urine data only in the reference period, while another provided complete plasma data for both periods and urine data only in the test period.

FIGURE 4

Bland–Altman plots comparing individual empirical Bayesian estimates (EBEs) of iohexol renal clearance following reference and test doses. The plot illustrates the difference between EBEs for renal clearance obtained after the test dose and those obtained after the reference dose. The mean bias of 0.00311 L/h is shown by the thin solid line, while the dashed lines denote the upper and lower limits of agreement.