on behalf of the International Transporter ConsortiumDrug transporters can govern the absorption, distribution, metabolism, and excretion of substrate drugs and endogenous substances. Investigations to examine their potential impact to pharmacokinetic (PK) drug-drug interactions (DDIs) are an integral part of the risk assessment in drug development. To evaluate a new molecular entity as a potential perpetrator of transporters, use of well characterized and/or clinically relevant probe substrates with good selectivity and sensitivity are critical for robust clinical DDI assessment that could inform DDI management strategy in the product labeling. The availability of endogenous biomarkers to monitor transportermediated DDIs in early phases of clinical investigations would greatly benefit downstream clinical plans. This article reviews the state-of-the-art in transporter clinical probe drugs and emerging biomarkers, including current challenges and limitations, delineates methods and workflows to identify and validate novel endogenous biomarkers to support clinical DDI evaluations, and proposes how these probe drugs or biomarkers could be used in drug development.Drug transporters can modulate the absorption, distribution, metabolism, and excretion (ADME) of substrate drugs and endogenous substances, ultimately determining their exposure in systemic circulation and tissues. 1 Transporter substrate or modulator (inhibitor or inducer) drugs can become clinical victims or perpetrators of drug-drug interactions (DDIs), respectively, when the transporter in question is a substantial contributor to the pharmacokinetics (PK) of the victim drug and can be inhibited or induced in the clinical setting. For example, lapatinib, a P-glycoprotein (P-gp) inhibitor, increased digoxin exposure by 2.8-fold (TYKERB labeling at Drugs@FDA), whereas tipranavir/ ritonavir, a P-gp inducer, decreased saquinavir/ritonavir exposure by 76% (APTIVUS labeling at Drugs@FDA). Understanding DDIs is an integral part of risk assessment in drug development considering the common practice of concomitant use of multiple medications. 1-3

In vivo enzyme levels are governed by the rates of de novo enzyme synthesis and degradation. A current lack of consensus on values of the in vivo turnover half-lives of human cytochrome P450 (CYP) enzymes places a significant limitation on the accurate prediction of changes in drug concentration-time profiles associated with interactions involving enzyme induction and mechanism (time)-based inhibition (MBI). In the case of MBI, the full extent of inhibition is also sensitive to values of enzyme turnover half-life. We review current understanding of CYP regulation, discuss the pros and cons of various in vitro and in vivo approaches used to estimate the turnover of specific CYPs and, by simulation, consider the impact of variability in estimates of CYP turnover on the prediction of enzyme induction and MBI in vivo. In the absence of consensus on values for the in vivo turnover half-lives of key CYPs, a sensitivity analysis of predictions of the pharmacokinetic effects of enzyme induction and MBI to these values should be an integral part of the modelling exercise, and the selective use of values should be avoided.

Although data are available on the change of expression/activity of drug-metabolizing enzymes in liver cirrhosis patients, corresponding data on transporter protein expression are not available.

To predict the impact of liver cirrhosis on hepatic drug clearance using physiologically based pharmacokinetic (PBPK) modeling, we compared the protein abundance of various phase 1 and phase 2 drug-metabolizing enzymes (DMEs) in S9 fractions of alcoholic ( = 27) or hepatitis C (HCV, = 30) cirrhotic versus noncirrhotic (control) livers ( = 25). The S9 total protein content was significantly lower in alcoholic or HCV cirrhotic versus control livers (i.e., 38.3 ± 8.3, 32.3 ± 12.8, vs. 51.1 ± 20.7 mg/g liver, respectively). In general, alcoholic cirrhosis was associated with a larger decrease in the DME abundance than HCV cirrhosis; however, only the abundance of UGT1A4, alcohol dehydrogenase (ADH)1A, and ADH1B was significantly lower in alcoholic versus HCV cirrhotic livers. When normalized to per gram of tissue, the abundance of nine DMEs (UGT1A6, UGT1A4, CYP3A4, UGT2B7, CYP1A2, ADH1A, ADH1B, aldehyde oxidase (AOX)1, and carboxylesterase (CES)1) in alcoholic cirrhosis and five DMEs (UGT1A6, UGT1A4, CYP3A4, UGT2B7, and CYP1A2) in HCV cirrhosis was <25% of that in control livers. The abundance of most DMEs in cirrhotic livers was 25% to 50% of control livers. CES2 abundance was not affected by cirrhosis. Integration of UGT2B7 abundance in cirrhotic livers into the liver cirrhosis (Child Pugh C) model of Simcyp improved the prediction of zidovudine and morphine PK in subjects with Child Pugh C liver cirrhosis. These data demonstrate that protein abundance data, combined with PBPK modeling and simulation, can be a powerful tool to predict drug disposition in special populations.

Cytochromes P450 (P450s) incur phosphorylation. Although the precise role of this post-translational modification is unclear, marking P450s for degradation is plausible. Indeed, we have found that after structural inactivation, CYP3A4, the major human liver P450, and its rat orthologs are phosphorylated during their ubiquitin-dependent proteasomal degradation. Peptide mapping coupled with mass spectrometric analyses of CYP3A4 phosphorylated in vitro by protein kinase C (PKC) previously identified two target sites, Thr 264 and Ser 420 . We now document that liver cytosolic kinases additionally target Ser 478 as a major site. To determine whether such phosphorylation is relevant to in vivo CYP3A4 degradation, wild type and CYP3A4 with single, double, or triple Ala mutations of these residues were heterologously expressed in Saccharomyces cerevisiae pep4⌬ strains. We found that relative to CYP3A4wt, its S478A mutant was significantly stabilized in these yeast, and this was greatly to markedly enhanced for its S478A/T264A, S478A/ S420A, and S478A/T264A/S420A double and triple mutants. Similar relative S478A/T264A/S420A mutant stabilization was also observed in HEK293T cells. To determine whether phosphorylation enhances CYP3A4 degradation by enhancing its ubiquitination, CYP3A4 ubiquitination was examined in an in vitro UBC7/gp78-reconstituted system with and without cAMPdependent protein kinase A and PKC, two liver cytosolic kinases involved in CYP3A4 phosphorylation. cAMPdependent protein kinase A/PKC-mediated phosphorylation of CYP3A4wt but not its S478A/T264A/S420A mutant enhanced its ubiquitination in this system. Together, these findings indicate that phosphorylation of CYP3A4 Ser 478 , Thr 264 , and Ser 420 residues by cytosolic kinases is important both for its ubiquitination and proteasomal degradation and suggest a direct link between P450 phosphorylation, ubiquitination, and degradation.Hepatic cytochromes P450 (P450s) 3 are integral endoplasmic reticulum (ER)-anchored hemoproteins engaged in the oxidative biotransformation of various endo-and xenobiotics. Of these, human CYP3A4 is the most dominant liver enzyme, accounting for Ͼ30% of the hepatic microsomal P450 complement, and responsible for the oxidative metabolism of over 50% of clinically relevant drugs (1). In common with all the other ER-bound P450s, CYP3A4 is a monotopic protein with its N-terminal Ϸ33-residue domain embedded in the ER membrane with the bulk of its structure in the cytosol. Our in vivo studies of the heterologously expressed CYP3A4 in the yeast Saccharomyces cerevisiae as well as of its rat liver CYP3A2/ 3A23 orthologs in primary hepatocytes have revealed that human and rat liver CYPs 3A are turned over via ubiquitin (Ub)-dependent proteasomal degradation (UPD) (2-8). Thus, CYPs 3A represent excellent prototypic substrates of ER-associated degradation (ERAD), specifically of the ERAD-C pathway (6 -11). Consistent with this CYP3A ERAD process, our studies of in vivo and/or in vitro reconstituted systems have led us to conclude tha...

Using positron emission tomography imaging, we determined the hepatic concentrations and hepatobiliary transport of [11C]rosuvastatin (RSV; i.v. injection) in the absence (n = 6) and presence (n = 4 of 6) of cyclosporin A (CsA; i.v. infusion) following a therapeutic dose of unlabeled RSV (5 mg, p.o.) in healthy human volunteers. The sinusoidal uptake, sinusoidal efflux, and biliary efflux clearance (CL; mL/minute) of [11C]RSV, estimated through compartment modeling were 1,205.6 ± 384.8, 16.2 ± 11.2, and 5.1 ± 1.8, respectively (n = 6). CsA (blood concentration: 2.77 ± 0.24 μM), an organic‐anion‐transporting polypeptide, Na+‐taurocholate cotransporting polypeptide, and breast cancer resistance protein inhibitor increased [11C]RSV systemic blood exposure (45%; P < 0.05), reduced its biliary efflux CL (52%; P < 0.05) and hepatic uptake (25%; P > 0.05) but did not affect its distribution into the kidneys. CsA increased plasma concentrations of coproporphyrin I and III and total bilirubin by 297 ± 69%, 384 ± 102%, and 81 ± 39%, respectively (P < 0.05). These data can be used in the future to verify predictions of hepatic concentrations and hepatobiliary transport of RSV.

The monotopic, endoplasmic reticulum (ER)-anchored cytochromes P450 (P450s) undergo variable proteolytic turnover. CYP3A4, the dominant human liver drug-metabolizing enzyme, is degraded via a ubiquitin (Ub)-dependent 26S proteasomal pathway after heterologous expression in Saccharomyces cerevisiae. This turnover involves the Ub-conjugating enzyme Ubc7p and the 19S proteasomal subunit Hrd2p but is independent of Hrd1p/Hrd3p, a major Ub-ligase (E3) involved in ER protein degradation. We now show that CYP3A4 ERAD also involves the Ubc7p-ER anchor Cue1p, because CYP3A4 is significantly stabilized at the stationary growth phase in Cue1p-deficient yeast. To determine whether the other major Ub-ligase Doa10p or Rsp5p involved in ER protein degradation functions in CYP3A4 ERAD, wild type and Doa10p-or Rsp5p-deficient yeast strains were also similarly examined. No appreciable CYP3A4 stabilization was detected in either Doa10p-or Rsp5p-deficient yeast, thereby excluding these E3s and revealing that CYP3A4 ERAD involves a novel or yet to be identified E3. Similar studies also revealed that the Cdc48p-Ufd1p-Hrd4p complex, responsible for the translocation of polyubiquitinated ER proteins was critical for CYP3A4 ERAD. We previously reported that grafting of the C-terminal (CT) CYP3A4 heptapeptide onto the CYP2B1 C terminus switched its proteolytic susceptibility from predominantly vacuolar to proteasomal degradation. To determine the relevance of this CT heptapeptide to CYP3A4 ERAD, CYP3A4 degradation after CT heptapeptidedeletion (CYP3A4⌬CT) was similarly examined in yeast. These findings revealed that CYP3A4⌬CT was also degraded by Ubc7p-26S proteasomal pathway, thereby indicating that this CT heptapeptide is not critical for CYP3A4 proteasomal degradation. Thus, unlike CYP2B1, CYP3A4 harbors additional/ multiple structural degrons for its recruitment into the Ubproteasomal pathway.Mammalian hepatic cytochromes P450 (P450s) are hemoproteins instrumental in the biotransformation of various endo-and xenobiotics. It is now becoming increasingly evident that in addition to induction via transcriptional/ translational activation, exposure to many substrates can alter the hepatic P450 content through substrate-induced hemoprotein stabilization as well as irreversible functional inactivation and/or enhanced degradation. P450s are integral monotopic endoplasmic reticulum (ER) proteins with their relatively hydrophobic N terminus (Ϸ30 -35 residues) embedded in the ER-membrane bilayer and the bulk of their catalytic domain exposed to the cytosol. Despite strikingly similar tertiary structures, individual hepatic P450s not only exhibit differential physiologic turnover with highly variable protein half-lives ranging from 7 to 37 h but also use distinct proteolytic loci and cellular processes (Correia, 2003, and references therein). Thus, the longer-lived CYP2B1 and CYP2C11 (half-lives of 37 and 20 h, respectively) apparently are proteolytic substrates of the autophagic-lysosomal pathway, whereas CYP3A2 and CYP3A23 (t 1/2 Ϸ 14...

Tryptophan 2,3-dioxygenase (TDO), a liver-specific cytosolic hemoprotein, is the rate-limiting enzyme in L-tryptophan catabolism and thus a key serotonergic determinant. Glucocorticoids transcriptionally activate the TDO gene with marked enzyme induction. TDO is also regulated by heme, its prosthetic moiety, as its expression and function are significantly reduced after acute hepatic heme depletion. Here we show in primary rat hepatocytes that this impairment is not due to faulty transcriptional activation of the TDO gene but rather due to its posttranscriptional regulation by heme. Accordingly, in acutely heme-depleted hepatocytes, the de novo synthesis of TDO protein is markedly decreased (Ͼ90%) along with that of other hepatic proteins. This global suppression of de novo hepatic protein syntheses in these heme-depleted cells is associated with a significantly enhanced phosphorylation of the ␣-subunit of the eukaryotic initiation factor eIF2 (eIF2␣), as monitored by the phosphorylated eIF2␣/total eIF2␣ ratio. Heme supplementation reversed these effects, indicating that heme regulates TDO induction by functional control of an eIF2␣ kinase. A cDNA was cloned from heme-depleted rat hepatocytes, and DNA sequencing verified its identity to the previously cloned rat brain heme-regulated inhibitor (HRI). Proteomic, biochemical, and/or immunoblotting analyses of the purified recombinant protein and the immunoaffinity-captured hepatic protein confirmed its identity as a rat heme-sensitive eIF2␣ kinase. These findings not only document that a hepatic HRI exists and is physiologically relevant but also implicate its translational shut-off of key proteins in the pathogenesis and symptomatology of the acute hepatic heme-deficient conditions clinically known as the hepatic porphyrias.Tryptophan 2,3-dioxygenase (TDO) is a liver-specific cytosolic enzyme highly specific for L-tryptophan (L-Trp) as the substrate (Greengard and Feigelson, 1961;Schimke et al., 1965;Knox, 1966). As the rate-limiting enzyme in L-Trp oxidative breakdown to kynurenine, its activity critically determines the relative L-Trp flux into serotonergic (5-HT) and kynurenine [NAD, NADP, poly(ADP-ribose)] pathways (Greengard and Feigelson, 1961;Schimke et al., 1965;Knox, 1966). TDO is a This work was supported by National Institutes of Health (NIH) Grants DK26506 (to M.A.C.) and DK61510 (to J.J.M.). We also acknowledge the UCSF