**8. Iohexol**

#### **8.1 Physiological and analytical data**

Iohexol is a non-ionic contrast product, mainly used for myelography. Its molecular weight is 821 Da (Olsson et al., 1983; Schwartz et al., 2006). Iohexol is chronologically the last marker proposed for measuring GFR. Actually, the first human was receiving iohexol in 1980 (Aakhus et al., 1980). In this study, it was shown that the substance was safe and fully excreted by the kidneys (this assertion will be criticized thereafter, see below). However, these authors also describe (but data are not available) a higher urinary clearance of iohexol than 51Cr-EDTA (Aakhus et al., 1980). The details of these comparison studies were published three years after (see clinical data)(Olsson et al., 1983). In the same study, the authors confirm that iohexol is distributed through the extracellular volume, which will be confirmed by other authors (including in CKD patients and in obese subjects) (Friedman et al., 2010; Nossen et al., 1995; Edelson et al., 1984; Back et al., 1988b; Olsson et al., 1983). Iohexol has not effect *per se* on GFR (Olofsson et al., 1996). Binding to protein seems very limited for iohexol. The first study described a binding to protein of only 1.5% (Mutzel et al., 1980). This will be thereafter confirmed (Back et al., 1988b; Krutzen et al., 1984). Physical properties of iohexol make it a good candidate to be used in simplified protocols like plasma clearance (Thomsen & Hvid-Jacobsen, 1991; Gaspari et al., 1995; Edelson et al., 1984). Contrary to the prior studies (Aakhus et al., 1980), several authors have shown that extrarenal clearance of iohexol is limited but not null (Arvidsson & Hedman, 1990; Krutzen et al., 1984). Back calculated at 6.2 mL/min the difference between total and urinary clearance of iohexol in healthy subjects (Back et al., 1988b). Frennby observed an extra-renal clearance lower than 2 mL/min in 6 anuric dialysis patients (Frennby et al., 1994; Frennby et al., 1995). These last very low results were also found by Nossen in 16 patients with severe CKD. Their mean measured GFR was 14 mL/min and the extra-renal clearance was estimated at 10% (Nossen et al., 1995). In 16 healthy subjects, Edelson estimated the extrarenal clearance of iohexol at 5% (Edelson et al., 1984). Contrary to iothalamate, there are very few physiological studies on the renal tubular handling of iohexol.

As for iothalamate, iohexol can be measured by several different techniques. Among these, HPLC and XRF are the most used ones. HPLC was historically the first method used (Aakhus et al., 1980) and described (Krutzen et al., 1984). As we have shown, iohexol measurements by HPLC are sensitive, specific and reproducible (Back et al., 1988c; Farthing et al., 2005; Cavalier et al., 2008). The high performance of such dosage notably enables the use of iohexol low doses and the measurement on finger-prick samples (Krutzen et al., 1990; Niculescu-Duvaz et al., 2006; Mafham et al., 2007; Cavalier et al., 2008; Aurell, 1994). Iohexol measurement is also pretty stable at room temperature and at -20°C(Krutzen et al., 1984; O'Reilly et al., 1988). Measurement of iohexol by XRF method is less validated and probably less performing, especially in low plasma concentrations (O'Reilly et al., 1986; Back & Nilsson-Ehle, 1993; Effersoe et al., 1990; Brandstrom et al., 1998; Aurell, 1994). We will not discuss into details the other methods for measuring iohexol: capillary electrophoresis (Shihabi & Constantinescu, 1992) and mass spectrometry (Lee et al., 2006; Annesley & Clayton, 2009; Denis et al., 2008; Stolz et al., 2010). The safety of iohexol is now confirmed (Heron et al., 1984; Aurell, 1994), notably by the largest series of iohexol measurements in Sweden (1500 GFR measurements/y)(Nilsson-Ehle & Grubb, 1994; Nilsson-Ehle, 2002). This safety profile is, at least in part, explained by the low dose of iohexol injected, and by the exclusion of patients with contrast products allergy.
