Functional maturation of drug transporters in the developing, neonatal, and postnatal kidney

Abstract

Because renal function in newborns is immature, the pharmacokinetics of drugs administered to neonates vary significantly from adult patients. The establishment of drug transport systems is a key process in the functional maturation of the nephron. However, a thorough examination of the expression of the main drug transporters in the kidney throughout all stages of development (embryonic, postnatal, and mature) has yet to be carried out, and the functional (physiological) impact is not well understood. Using time-series microarray data, we analyzed the temporal behavior of mRNA levels for a wide range of SLC and ABC transporters in the rodent kidney throughout a developmental time series. We find dynamic increases between the postnatal and mature stages of development for a number of transporters, including the proximal tubule-specific drug and organic anion transporters (OATs) OAT1 (SLC22a6) and OAT3 (SLC22a8). The OATs are the major multispecific basolateral drug, toxin, and metabolite transporters in the proximal tubule responsible for handling of many drugs, as well as the prototypical OAT substrate para-aminohippurate (PAH). We therefore performed specific in vivo pharmacokinetic analysis of the transport of PAH in postnatal and maturing rodent kidney. We show that there is a 4-fold increase in PAH clearance during this period. Clearance studies in Oat1 and Oat3 knockouts confirm that, as in the adult, Oat1 is the principle transporter of PAH in the postnatal kidney. The substantial differences observed supports the need for better understanding of pharmacokinetics in the newborn and juvenile kidney compared with the adult kidney at the basic and clinical level.

Fig. 1.

Expression profile of SLC and ABC transporters during kidney development. A, microarray expression profile of probe sets for the SLC family of transporters were plotted for each stage of kidney development. B, expression profiles were also plotted for the ABC family of transporters. The x-axis represents the developmental stages of kidney development (Tsigelny et al., 2008).

Fig. 2.

The SLC22 family of transporters show overall increases in mRNA expression over the course of kidney development. A, the expression of Slc22 (Oat) transporters throughout kidney development highlights a number of transporters that show large increases in expression around the postnatal to mature stage. B, 2-fold filtering identifies the Slc22 genes that have a 2-fold change in expression between the consecutive stages of kidney development. The shaded cells highlight the stage comparison with the largest change in expression. Early emb, early embryonic stage; interm emb, intermediate embryonic stage; late emb, late embryonic stage. C, specific expression of Slc22a6 (broken line) and Slc22a8 (solid line) from e13 through to adulthood highlight maximal expression at adulthood.

Fig. 3.

Clinically relevant SLC and ABC transporters show increased expression throughout the stages of kidney development, with highest expression at the mature stage. A, the expression profiles for the SLC transporters and ABC transporters with emerging clinical importance (Giacomini et al., 2010) were examined during kidney development. B, Fold-change of expression in “clinically relevant” drug transporters. Two-fold filtering identifies the clinically relevant drug transporters, which have a 2-fold change in expression between the consecutive stages of kidney development. The shaded cells indicate the stage comparison with the largest fold change. Early emb, early embryonic stage; interm emb, intermediate embryonic stage; late emb, late embryonic stage.

Fig. 4.

A number of clinically important drug transporters are specifically up-regulated in the developing proximal tubule. A, using the GUDMAP consortium microarray data sets profiling the substructures of the developing mouse kidney, 2-fold filtering was carried out to identify which clinically relevant transporters are specifically up-regulated in the developing proximal tubule. Six transporters show fold changes greater than 2-fold in the developing proximal tubule compared with other structures. GUDMAP substructure abbreviations are listed under Materials and Methods. B, the average fold change of transporters shown in B relative to the proximal tubule sample is tabulated. Average fold change is based on the geometric mean of fold change across all substructures.

Fig. 5.

Age-dependent increase of PAH clearance in wild-type mice. A, elimination constant, volume of distribution, and clearance of PAH were determined by plasma kinetics after intravenous bolus injection of PAH for mice at 1, 2, and 3 weeks of age. Data are means ± S.E.M. in four to six sets of mice per age (four to five mice/set); results were similar for male and female and therefore were pooled. B, plasma kinetics of PAH. C, lower left, ○, ▵, and □ are 1, 2, and 3 weeks of age, respectively. Upper right, circles are from published data for PAH clearance derived from adult mice (8–10 weeks of age) (Eraly et al., 2006; Vallon et al., 2008) and are included for comparison.

Fig. 6.

PAH clearance is attenuated at 2 weeks of age in Oat1(−/−) mice. A, the volume of distribution for PAH and inulin was similar between WT, Oat1(−/−), and Oat3(−/−) mice. B, whereas body weight and the clearance of inulin were similar between genotypes, the clearance of PAH and the difference between PAH and inulin clearance (as an indirect measure of renal secretion) were significantly lower in Oat1(−/−) than in WT mice. C, plasma kinetics of inulin and PAH in WT versus Oat1(−/−) mice. Data are means ± S.E.M. in six to seven sets of mice per genotype (five mice/set); *, P < 0.05 versus WT.