Figure 4.6 – Longitudinal biomarker analysis of children with CF treated with courses of tobramycin. Representative figures demonstrating the longitudinal quantification of the biomarkers KIM-1 (green; ng/mg Cr), NGAL (red; ng/mg Cr) and serum creatinine (blue; µmol/L) for four children treated with tobramycin. Tobramycin treatment episodes and length of treatment (days) are indicated by the yellow boxes on each figure.
Representative longitudinal plots of biomarker concentration are provided for four individuals demonstrating change in biomarker concentration over the period of follow-up
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(Figure 4.6). The individual plots do suggest that there is some acute change in biomarker concentration during exposure to tobramycin.
One approach to describing the change in biomarker concentration during exposure to tobramycin is to calculate an absolute change from pre-tobramycin (day 0) value to the peak concentration during treatment. However, due to the wide variation in pre- tobramycin value, a better approach seemed to be to describe a fold-change in the biomarker concentration by dividing the peak value on tobramycin by the pre-tobramycin (day 0) value. For both KIM-1 and NGAL the distribution of peak fold-change is skewed (Figure 4.7). Median peak fold-change for KIM-1 is 2.28 (IQR 2.69) and for NGAL is 4.02 (IQR 7.29).
Figure 4.7 – Distribution of peak fold-change in biomarker concentration during exposure to tobramycin. Peak fold-change was calculated for each individual by dividing the peak biomarker concentration measured during tobramycin exposure by their pre-tobramycin (day 0) concentration. Plots are included for KIM-1 (A) and NGAL (B). The bottom and top of the box represent first and third quartile respectively, while the darker line in the box is the median, and outliers are represented by the small circle points.
Figure 4.8 – Longitudinal analysis of changes in biomarker values during and after exposure to tobramycin. Plots are from day 0 to day 30 with a course of tobramycin starting on day 0 and usually lasting for 14 days. The day 0 urine sample for biomarker analysis was collected before the first dose of tobramycin was given. Each line represents measured biomarker concentrations for a different individual, and are for their first exposure to tobramycin in this study. Biomarker values are plotted as log of fold change from the day 0 value and as log of absolute value for KIM-1 (A&C) and NGAL (B&D). Daily mean value is plotted (black triangles), and a mean line, produced using locally weighted regression (lowess), with 95% CI is plotted to demonstrate the overall trend.
Longitudinal profile plots (Figure 4.8) demonstrate a high degree of intra-and inter- individual variability. However, the mean trend suggests that both biomarkers increase during exposure to tobramycin. KIM-1 appears to increase earlier, with a peak at 3-5 days, with NGAL peaking later, at 9-11 days. After completing tobramycin (usually at 14 days), NGAL has returned to its pre-tobramycin (day 0) value, whereas KIM-1 remains elevated.
4.4
Discussion
This chapter has demonstrated that, in children with CF, acute changes are seen in both urinary KIM-1 and NGAL during exposure to IV tobramycin. Furthermore, baseline KIM-1 is associated with previous exposure to IV aminoglycosides, with an increasing concentration with increasing number of exposures. In contrast to previously published literature in adults (Al-Aloul et al., 2005a), eGFR was not found to be correlated with previous aminoglycoside exposure.
In accordance with our expectations from the previous study in preterm neonates (McWilliam et al., 2012), elevations in both KIM-1 and NGAL were observed during exposure to tobramycin. Exploratory plots suggest that KIM-1 rises earlier and reaches a peak at 3-5 days, whereas NGAL rises later and reaches a peak at 9-11 days. Furthermore, KIM-1 remains elevated throughout tobramycin treatment and for some time afterwards. NGAL, however, appears to return to its pre-tobramycin level at the end of treatment. Given the large degree of inter- and intra-individual variability it is clear that not all
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participants follow the same trend, and the variability is such that it would not be appropriate to make concrete conclusions from these observations. Furthermore, novel statistical techniques employing generalized estimating equations and regression splines could provide more insight, and provide a consideration of the effects of tobramycin exposure on biomarker value, along with the effects of covariates. However, another issue that also needs to be considered is the power of the study, and these effects may become more evident with larger numbers.
A novel finding from this chapter is that baseline KIM-1 in children with CF is associated with cumulative lifetime exposure to IV aminoglycosides. A previous study measuring KIM- 1 in children with CF also found a significant correlation with cumulative exposure to aminoglycosides (Lahiri et al., 2014). Our data suggest that KIM-1 becomes elevated during acute episodes of proximal tubule epithelial cell cytotoxicity caused by aminoglycoside exposure, but then remains elevated. Its chronic elevation suggests that the proximal tubule does not fully recover from the acute event, and that KIM-1 is playing a role in this longer term response to toxicity. This interpretation is supported by a growing body of literature which suggests that KIM-1 may be a useful marker for the development of chronic kidney disease (CKD) (Gardiner et al., 2012). In contrast, it may be argued that the relationship between previous aminoglycoside exposure and KIM-1 elevation may not be a causative one. Could KIM-1 simply be a marker for more severe or poorly controlled CF? However, the specificity of KIM-1 for the proximal tubule makes this unlikely.
Baseline urinary NGAL concentration was not associated with previous aminoglycoside exposure. It was associated with sex, which is consistent with our findings in a cohort of healthy children (Chapter 3) (McWilliam et al., 2014), and with published literature in adults (Cullen et al., 2012, Zhang et al., 2013), children (Pennemans et al., 2013), and in
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very low birthweight infants (Huynh et al., 2009, Askenazi et al., 2011a). Baseline serum creatinine was strongly correlated with age as has been well described previously in children (Schwartz et al., 1976a).
Estimated GFR was calculated using the Schwartz formula. No correlation was found between eGFR and cumulative lifetime exposure to aminoglycosides. This is in contrast to observations made in a cohort of adult patients with CF (Al-Aloul et al., 2005a). In this previous study, both measured creatinine clearance, and eGFR calculated using the Cockroft-Gault formula (Cockcroft and Gault, 1976), were shown to decrease with increasing lifetime exposure to aminoglycosides. However, the present findings are in agreement with a previous study in a French paediatric CF cohort (Andrieux et al., 2010). In the current paediatric cohort it may be that too few have been exposed to sufficient courses of aminoglycosides for an effect on eGFR to be seen. Perhaps the observed elevation in urinary KIM-1 in children reflects chronic impairment of the proximal tubule epithelial cells which, with time, will result in a global impairment of renal function and a reduction in eGFR. Advances in management, especially the advent of once daily dosing of aminoglycosides, may also account for differences between the current paediatric cohort, and an adult cohort recruited around a decade before (Al-Aloul et al., 2005a). It is also important to note that whilst the Schwartz formula (Schwartz and Work, 2009) is widely used for the calculation of eGFR in children, it is not validated for use in children with CF (Al-Aloul et al., 2007), although it has been used in this population previously (Soulsby et al., 2010, Andrieux et al., 2010). Indeed, estimates of GFR which depend on serum creatinine may overestimate renal function in CF due to the reduced muscle mass in these patients (Al-Aloul et al., 2007). Therefore any further investigation should involve measurement of GFR by a ‘gold standard’ test (such as Iohexol clearance) alongside calculation of eGFR.
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In children with CF significant acute changes occur in the urinary biomarkers KIM-1 and NGAL during exposure to tobramycin. In addition, baseline KIM-1 concentration increases with greater cumulative exposure to aminoglycosides. KIM-1 in particular may be a useful, non-invasive, biomarker of acute and chronic proximal tubular injury associated with exposure to aminoglycosides in children with CF. The clinical utility of KIM-1 should be further evaluated in prospective studies. However, it has immediate potential for use as a surrogate outcome marker in clinical trials looking at interventions to treat or prevent aminoglycoside-induced nephrotoxicity.
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