Recent advances in the ontogeny of drug disposition.

Developmental changes that occur throughout childhood have long been known to impact drug disposition. However, pharmacokinetic studies in the paediatric population have historically been limited due to ethical concerns arising from incorporating children into clinical trials. As such, much of the early work in the field of developmental pharmacology was reliant on difficult-to-interpret in vitro and in vivo animal studies. Over the last 2 decades, our understanding of the mechanistic processes underlying age-related changes in drug disposition has advanced considerably. Progress has largely been driven by technological advances in mass spectrometry-based methods for quantifying proteins implicated in drug disposition, and in silico tools that leverage these data to predict age-related changes in pharmacokinetics. This review summarizes our current understanding of the impact of childhood development on drug disposition, particularly focusing on research of the past 20 years, but also highlighting select examples of earlier foundational research. Equally important to the studies reviewed herein are the areas that we cannot currently describe due to the lack of research evidence; these gaps provide a map of drug disposition pathways for which developmental trends still need to be characterized.

The Respective Roles of CYP3A4 and CYP2D6 in the Metabolism of Pimozide to Established and Novel Metabolites.

Pimozide is a dopamine receptor antagonist indicated for the treatment of Tourette syndrome. Prior in vitro studies characterized N-dealkylation of pimozide to 1,3-dihydro-1-(4-piperidinyl)-2H-benzimidazol-2-one (DHPBI) via CYP3A4 and, to a lesser extent, CYP1A2 as the only notable routes of pimozide biotransformation. However, drug-drug interactions between pimozide and CYP2D6 inhibitors and CYP2D6 genotype-dependent effects have since been observed. To reconcile these incongruities between the prior in vitro and in vivo studies, we characterized two novel pimozide metabolites: 5-hydroxypimozide and 6-hydroxypimozide. Notably, 5-hydroxypimozide was the major metabolite produced by recombinant CYP2D6 (Km ∼82 nM, V max ∼0.78 pmol/min per picomoles), and DHPBI was the major metabolite produced by recombinant CYP3A4 (apparent Km ∼1300 nM, V max ∼2.6 pmol/min per picomoles). Kinetics in pooled human liver microsomes (HLMs) for the 5-hydroxylation (Km ∼2200 nM, V max ∼59 pmol/min per milligram) and N-dealkylation (Km ∼3900 nM, V max ∼600 pmol/min per milligram) reactions were also determined. Collectively, formation of DHPBI, 5-hydroxypimozide, and 6-hydroxypimozide accounted for 90% of pimozide depleted in incubations of NADPH-supplemented pooled HLMs. Studies conducted in HLMs isolated from individual donors with specific cytochrome P450 isoform protein abundances determined via mass spectrometry revealed that 5-hydroxypimozide (r 2 = 0.94) and 6-hydroxypimozide (r 2 = 0.86) formation rates were correlated with CYP2D6 abundance, whereas the DHPBI formation rate (r 2 = 0.98) was correlated with CYP3A4 abundance. Furthermore, the HLMs differed with respect to their capacity to form 5-hydroxypimozide relative to DHPBI. Collectively, these data confirm a role for CYP2D6 in pimozide clearance via 5-hydroxylation and provide an explanation for a lack of involvement when only DHPBI formation was monitored in prior in vitro studies. SIGNIFICANCE STATEMENT: Current CYP2D6 genotype-guided dosing information in the pimozide label is discordant with available knowledge regarding the primary biotransformation pathways. Herein, we characterize the CYP2D6-dependent biotransformation of pimozide to previously unidentified metabolites. In human liver microsomes, formation rates for the novel metabolites and a previously identified metabolite were determined to be a function of CYP2D6 and CYP3A4 content, respectively. These findings provide a mechanistic basis for observations of CYP2D6 genotype-dependent pimozide clearance in vivo.

Polymorphic Human Sulfotransferase 2A1 Mediates the Formation of 25-Hydroxyvitamin D3-3-O-Sulfate, a Major Circulating Vitamin D Metabolite in Humans.

Metabolism of 25-hydroxyvitamin D3 (25OHD3) plays a central role in regulating the biologic effects of vitamin D in the body. Although cytochrome P450-dependent hydroxylation of 25OHD3 has been extensively investigated, limited information is available on the conjugation of 25OHD3 In this study, we report that 25OHD3 is selectively conjugated to 25OHD3-3-O-sulfate by human sulfotransferase 2A1 (SULT2A1) and that the liver is a primary site of metabolite formation. At a low (50 nM) concentration of 25OHD3, 25OHD3-3-O-sulfate was the most abundant metabolite, with an intrinsic clearance approximately 8-fold higher than the next most efficient metabolic route. In addition, 25OHD3 sulfonation was not inducible by the potent human pregnane X receptor agonist, rifampicin. The 25OHD3 sulfonation rates in a bank of 258 different human liver cytosols were highly variable but correlated with the rates of dehydroepiandrosterone sulfonation. Further analysis revealed a significant association between a common single nucleotide variant within intron 1 of SULT2A1 (rs296361; minor allele frequency = 15% in whites) and liver cytosolic SULT2A1 content as well as 25OHD3-3-O-sulfate formation rate, suggesting that variation in the SULT2A1 gene contributes importantly to interindividual differences in vitamin D homeostasis. Finally, 25OHD3-3-O-sulfate exhibited high affinity for the vitamin D binding protein and was detectable in human plasma and bile but not in urine samples. Thus, circulating concentrations of 25OHD3-3-O-sulfate appear to be protected from rapid renal elimination, raising the possibility that the sulfate metabolite may serve as a reservoir of 25OHD3 in vivo, and contribute indirectly to the biologic effects of vitamin D.

Development of a microphysiological model of human kidney proximal tubule function.

The kidney proximal tubule is the primary site in the nephron for excretion of waste products through a combination of active uptake and secretory processes and is also a primary target of drug-induced nephrotoxicity. Here, we describe the development and functional characterization of a 3-dimensional flow-directed human kidney proximal tubule microphysiological system. The system replicates the polarity of the proximal tubule, expresses appropriate marker proteins, exhibits biochemical and synthetic activities, as well as secretory and reabsorptive processes associated with proximal tubule function in vivo. This microphysiological system can serve as an ideal platform for ex vivo modeling of renal drug clearance and drug-induced nephrotoxicity. Additionally, this novel system can be used for preclinical screening of new chemical compounds prior to initiating human clinical trials.

Functional Comparison of Human Colonic Carcinoma Cell Lines and Primary Small Intestinal Epithelial Cells for Investigations of Intestinal Drug Permeability and First-Pass Metabolism.

To further the development of a model for simultaneously assessing intestinal absorption and first-pass metabolism in vitro, Caco-2, LS180, T84, and fetal human small intestinal epithelial cells (fSIECs) were cultured on permeable inserts, and the integrity of cell monolayers, CYP3A4 activity, and the inducibility of enzymes and transporters involved in intestinal drug disposition were measured. Caco-2, T84, and fSIECs all formed tight junctions, as assessed by immunofluorescence microscopy for zonula occludens-1, which was well organized into circumscribing strands in T84, Caco-2, and fSIECs but was diffuse in LS180 cells. The transepithelial electrical resistance value for LS180 monolayers was lower than that for Caco-2, T84, and fSIECs. In addition, the apical-to-basolateral permeability of the paracellular marker Lucifer yellow across LS180 monolayers was greater than in fSIECs, T84, and Caco-2 monolayers. The transcellular marker propranolol exhibited similar permeability across all cells. With regard to metabolic capacity, T84 and LS180 cells showed comparable basal midazolam hydroxylation activity and was inducible by rifampin and 1α,25(OH)2D3 in LS180 cells, but only marginally so in T84 cells. The basal CYP3A4 activity of fSIECs and Caco-2 cells was much lower and not inducible. Interestingly, some of the drug transporters expressed in LS180 and Caco-2 cells were induced by either 1α,25(OH)2D3 or rifampin or both, but effects were limited in the other two cell lines. These results suggest that none of the cell lines tested fully replicated the drug disposition properties of the small intestine and that the search for an ideal screening tool must continue.

Effect of Crohn's Disease on Villous Length and CYP3A4 Expression in the Pediatric Small Intestine.

Changes in absorptive capacity and first-pass metabolism in the small intestine affect oral drug bioavailability. Characterization of such changes as a consequence of inflammation is important for developing physiologically-based pharmacokinetic (PBPK) models for inflammatory bowel disease. We sought to elucidate the impact of small intestinal Crohn's disease (CD) on villous length and CYP3A4 expression in children. Freshly frozen duodenal and terminal ileum (TI) biopsies from 107 children (1-19 years) with and without CD were evaluated for active inflammation. Villous length and CYP3A4 mRNA/protein expression were compared among regions of active and inactive inflammation in CD and controls. A twofold reduction in villous length was observed in inflamed duodena and ilia of children with CD, but in the absence of regional inflammation, villi in CD were comparable in length to controls. Expression of CYP3A4 mRNA correlated significantly with villous length in the TI (P = 0.0003), with a trend observed in the duodenum that did not reach statistical significance. In the presence of active inflammation, a significant decrease in CYP3A protein expression was confirmed in the duodenum, where protein expression also correlated significantly with villous length across diagnoses (P < 0.0001). Our findings suggest that previous observations of decreased CYP3A4 expression and function in inflamed intestine may not be due solely to downregulation by inflammatory cytokines, but also to villous blunting and subsequent loss of surface area for protein expression. This information is relevant for PBPK model development and could aid with dose adjustment decisions for oral CYP3A4 substrates administered during CD flare (e.g., budesonide).

An Improved Vascularized, Dual-Channel Microphysiological System Facilitates Modeling of Proximal Tubular Solute Secretion.

A vascularized human proximal tubule model in a dual-channel microphysiological system (VPT-MPS) was developed, representing an advance over previous, single-cell-type kidney microphysiological systems. Human proximal tubule epithelial cells (PTECs) and human umbilical vein endothelial cells (HUVECs) were cocultured in side-by-side channels. Over 24 h of coculturing, PTECs maintained polarized expression of Na+/K+ ATPase, tight junctions (ZO-1), and OAT1. HUVECs showed the absence of ZO-1 but expressed endothelial cell marker (CD-31). In time-lapse imaging studies, fluorescein isothiocyanate (FITC)-dextran passed freely from the HUVEC vessel into the supporting extracellular matrix, confirming the leakiness of the endothelium (at 80 min, matrix/intravessel fluorescence ratio = 0.2). Dextran-associated fluorescence accumulated in the matrix adjacent to the basolateral aspect of the PTEC tubule with minimal passage of the compound into the tubule lumen observed (at 80 min, tubule lumen/matrix fluorescence ratio = 0.01). This demonstrates that the proximal tubule compartment is the rate-limiting step in the secretion of compounds in VPT-MPS. In kinetic studies with radiolabeled markers, p-aminohippuric acid (PAH) exhibited greater output into the tubule lumen than did paracellular markers mannitol and FITC-dextran (tubule outflow/vessel outflow concentration ratio of 7.7% vs 0.5 and 0.4%, respectively). A trend toward reduced PAH secretion by 45% was observed upon coadministration of probenecid. This signifies functional expression of renal transporters in PTECs that normally mediate the renal secretion of PAH. The VPT-MPS holds the promise of providing an in vitro platform for evaluating the renal secretion of new drug candidates and investigating the dysregulation of tubular drug secretion in chronic kidney disease.

Reevaluating the role of megalin in renal vitamin D homeostasis using a human cell-derived microphysiological system

The role of megalin in the regulation of renal vitamin D homeostasis has previously been evaluated in megalin-knockout mice and rat proximal tubule epithelial cells. We revisited these hypotheses that were previously tested solely in rodent models, this time using a 3-dimensional proximal tubule microphysiological system incorporating primary human proximal tubule epithelial cells. Using this human cell-derived model, we confirmed that 25OHD3 is transported into the human proximal tubule epithelium via megalin-mediated endocytosis while bound to vitamin D binding protein. Building upon these findings, we then evaluated the role of megalin in modulating the cellular uptake and biological activity of 1α,25(OH)2D3. Inhibition of megalin function decreased the 1α,25(OH)2D3-mediated induction of both cytochrome P450 24A1 protein levels and 24-hydroxylation activity following perfusion with vitamin D binding protein and 1α,25(OH)2D3. The potential for reciprocal effects from 1α,25(OH)2D3 on megalin expression were also tested. Contrary to previously published observations from rat proximal tubule epithelial cells, 1α,25(OH)2D3 did not induce megalin gene expression, thus highlighting the potential for meaningful interspecies differences in the homeostatic regulation of megalin in rodents and humans. These findings challenge a recently promoted hypothesis, predicated on the rodent cell data, that attempts to connect 1α,25(OH)2D3-mediated regulation of renal megalin expression and the pathology of chronic kidney disease in humans. In addition to providing specific insights related to the importance of renal megalin in vitamin D homeostasis, these results constitute a proof-of-concept that human-derived microphysio­logical systems are a suitable replacement for animal models for quantitative pharmacology and physiology research.