Mark J. Dresser

Transporters involved in the elimination of drugs in the kidney: Organic anion transporters and organic cation transporters

Transporters in the kidney mediate the secretion or reabsorption of many compounds and thereby influence the plasma levels of their substrates. Organic anion transporters and organic cation transporters are two major classes of secretory transporters in the mammalian kidney. During the past decade, significant progress has been made in the cloning, functional expression, and initial characterization of these transporters. To date, five organic cation transporters and nine organic anion transporters have been cloned. In this review, we summarize the available data on organic anion and organic cation transporters, focusing in particular on their molecular characteristics, tissue distribution, and inhibitor and substrate selectivities. Currently we have a good understanding of the inhibitor selectivities for most of these transporters, and with the development of more robust assays, we will soon have a better understanding of their substrate selectivities. Based on the available data, summarized in this review, it appears that many compounds interact with multiple transporters. Furthermore, there appears to be substantial overlap in the selectivities of organic cation transporters, and the same appears true for organic anion transporters. At the present time, it is unclear what the roles of these multiple transporters are in renal drug elimination. With the development of new assays, reagents, and experimental methods, we will soon have a better understanding of the roles of each transporter isoform in the renal elimination of drugs.

Drug absorption interactions between oral targeted anticancer agents and PPIs: is pH-dependent solubility the Achilles heel of targeted therapy?

A majority of the novel orally administered, molecularly targeted anticancer therapies are weak bases that exhibit pH‐dependent solubility, and suppression of gastric acidity with acid‐reducing agents could impair their absorption. In addition, a majority of cancer patients frequently take acid‐reducing agents to alleviate symptoms of gastroesophageal reflux disease, thereby raising the potential for a common but underappreciated drug–drug interaction (DDI) that could decrease the exposure of anticancer medication and result in subsequent failure of therapy. This article is a review of the available clinical literature describing the extent of the interaction between 15 orally administered, small‐molecule targeted anticancer therapies and acid‐reducing agents. The currently available clinical data suggest that the magnitude of this DDI is largest for compounds whose in vitro solubility varies over the pH range 1–4. This range represents the normal physiological gastric acidity (pH ~1) and gastric acidity while on an acid‐reducing agent (pH ~4).

A guide to rational dosing of monoclonal antibodies.

Background and ObjectiveDosing of therapeutic monoclonal antibodies (mAbs) is often based on body size, with the perception that body size-based dosing would reduce inter-subject variability in drug exposure. However, most mAbs are target specific with a relatively large therapeutic window and generally a small contribution of body size to pharmacokinetic variability. Therefore, the dosing paradigm for mAbs should be assessed in the context of these unique characteristics. The objective of this study was to review the current dosing strategy and to provide a scientific rationale for dosing of mAbs using a modelling and simulation approach.MethodsIn this analysis, the body weight-based or body weight-independent (fixed) dosing regimens for mAbs were systematically evaluated. A generic two-compartment first-order elimination model was developed. Individual or population pharmacokinetic profiles were simulated as a function of the body weight effects on clearance (θbw_cl) and on the central volume of distribution (θbw_vl). The variability in exposure (the area under the serum concentration-time curve [AUC], trough serum concentration [Cmin] and peak serum concentration [Cmax]) was compared between body weight-based dosing and fixed dosing in the entire population. The deviation of exposure for light and heavy subjects from median body weight subjects was also measured. The simulation results were then evaluated with clinical pharmacokinetic characteristics of various mAbs that were given either by body weight-based dosing or by fixed dosing in the case study.ResultsResults from this analysis demonstrated that exposure variability was dependent on the magnitude of the body weight effect on pharmacokinetics. In contrast to the conventional assumption, body weight-based dosing does not always offer advantages over fixed dosing in reducing exposure variability. In general, when the exponential functions of θbw_cl and θbw_vl in the population pharmacokinetic model are <0.5, fixed dosing results in less variability and less deviation than body weight-based dosing; when both θbw_cl and θbw_vl are >0.5, body weight-based dosing results in less variability and less deviation than fixed dosing. In the scenarios when either θbw_cl or θbw_vl is >0.5, the impact on exposure variability is different for each exposure measure. The case study demonstrated that most mAbs had little effect or a moderate body weight effect (θbw_cl and θbw_vl <0.5 or ∼0.5). The difference of variability in exposure between body weight-based and fixed dosing is generally less than 20% and the percentages of deviation for light and heavy subpopulations are less than 40%.ConclusionsThe analysis provided insights into the conditions under which either fixed or body weight-based dosing would be superior in reducing pharmacokinetic variability and exposure differences between light and heavy subjects across the population. The pharmacokinetic variability introduced by either dosing regimen is moderate relative to the variability generally observed in pharmacodynamics, efficacy and safety. Therefore, mAb dosing can be flexible. Given many practical advantages, fixed dosing is recommended to be the first option in first-in-human studies with mAbs. The dosing strategy in later stages of clinical development could then be determined based on combined knowledge of the body weight effect on pharmacokinetics, safety and efficacy from the early clinical trials.

Interactions of n-Tetraalkylammonium Compounds and Biguanides with a Human Renal Organic Cation Transporter (hOCT2)

Many clinically used drugs are transported in the liver and kidney by organic cation transporters (OCT) (1). To date, three organic cation transporters in the OCT family have been cloned and characterized (OCT1-3) (2). Of these, OCT1 appears to be an important transporter in the liver and OCT2 appears to be a major transporter in the kidney. With the availability of the cloned transporters, it is now possible to begin investigating their roles in renal and hepatic drug elimination. The human transporters, hOCT1 and hOCT2, share 70% sequence identity, and their predicted secondary structures, based on hydropathy analysis, are essentially the same. This might suggest that paralogous organic cation transporters such as hOCT1 and hOCT2 have similar functional characteristics and are functionally redundant. However, recent chimeric and mutagenesis studies of transporters have shown that changes in even one or two amino acids can dramatically alter specificity (3). Therefore, it is reasonable to propose that these two organic cation transporter homologs serve different functions in vivo. The goal of this study was to compare the substrate and inhibition profiles of hOCT2 and hOCT1 to determine whether these transporters are functionally distinct. Differences in their specificities may provide insights into organ-specific elimination of organic cations. We examined the interactions of n-tetraalkylammonium (nTAA) compounds and biguanides with hOCT2 and compared our results with our previous results for hOCT1 (4,5). Substantial differences between hOCT2 and hOCT1 in their interactions with nTAAs were found, whereas their interactions with the biguanides, metformin and phenformin, were similar. This report demonstrates that there are compound-dependent differences in the specificities of hOCT1 and hOCT2; these differences may contribute to organ-specific elimination of drugs. MATERIALS AND METHODS

Molecular cloning and functional expression of a rabbit renal organic cation transporter

A cDNA encoding an organic cation transporter (rbOCT1) was isolated from rabbit kidney. The cDNA encodes a 554 amino acid protein that is highly homologous to other mammalian organic cation transporters. rbOCT1 mediated 3H-1-methyl-4-phenylpyridinium (3H-MPP+) transport in Xenopus laevis oocytes was saturable, sensitive to membrane potential, and inhibited by various organic cations. rbOCT1 mRNA transcripts are expressed in the kidney, liver, and intestine.

Cloning and Functional Characterization of a Rat Renal Organic Cation Transporter Isoform (rOCT1A)

Polyspecific organic cation transporters in the renal proximal tubule mediate the secretion of many clinically used drugs as well as endogenous metabolites. Recently, two organic cation transporters (rOCT1 and rOCT2) were cloned from rat kidney. In this study, we report the cloning and functional expression of an rOCT1 isoform, rOCT1A, from rat kidney. Genomic DNA cloning and sequencing demonstrated that rOCT1A is an alternatively spliced variant of rOCT1 with a deletion of 104 base pairs near the 5′-end. The uptake of [14C]tetraethylammonium (TEA) in oocytes injected with the cRNA-encoding rOCT1A was increased 16-fold over that in water-injected oocytes (29 ± 2.8 pmol/oocyte/h versus1.8 ± 0.13 pmol/oocyte/h, mean ± S.E., p< 0.05). [14C]TEA uptake in the cRNA-injected oocytes was saturable (K m = 42 ± 11 μm) and was inhibited significantly by organic cations, including cimetidine and N 1-methylnicotinamide. The amino acid sequence was deduced from the cDNA after examination of all three reading frames. Two overlapping open reading frames were found. Studies with synthetic constructs suggest that a functional organic cation transporter is encoded by the larger open reading frame. The larger open reading frame encodes a 430-amino acid protein (termed rOCT1A) that is 92% identical to rOCT1 and 57% identical to rOCT2. From hydropathy analysis, rOCT1A is predicted to have 10 transmembrane domains with both amino and carboxyl termini intracellular. RNase protection assays demonstrate the presence of rOCT1A mRNA transcripts in rat kidney cortex, medulla, and intestine. These studies demonstrate the presence of a functional, alternatively spliced organic cation transporter (rOCT1A) in rat kidney.

BIOCHEMICAL CHARACTERIZATION AND CRYSTALLOGRAPHIC STRUCTURE OF AN ESCHERICHIA COLI PROTEIN FROM THE PHOSPHOTRIESTERASE GENE FAMILY

Phosphotriesterase homology protein (PHP) is a member of a recently discovered family of proteins related to phosphotriesterase, a hydrolytic, bacterial enzyme with an unusual substrate specificity for synthetic organophosphate triesters and phosphorofluoridates, which are common constituents of chemical warfare agents and agricultural pesticides. No natural substrate has been identified for phosphotriesterase, and it has been suggested that the enzyme may have evolved the ability to hydrolyze synthetic compounds in bacteria under selective pressure to meet nutritional needs. PHP, which has 28% sequence identity with phosphotriesterase, may belong to the family of proteins from which phosphotriesterase evolved. Here we report the cloning, expression, initial characterization, and high-resolution X-ray crystallographic structure of PHP. Biochemical analysis shows that PHP is monomeric and binds two zinc ions per monomer. Unlike phosphotriesterase, PHP does not catalyze the hydrolysis of nonspecific phosphotriesters. The structure, similar to that of phosphotriesterase, consists of a long, elliptical alpha/beta barrel and has a binuclear zinc center in a cleft at the carboxy end of the barrel at the location of the presumptive active site.

A physiologically based pharmacokinetic (PBPK) approach to evaluate pharmacokinetics in patients with cancer.

Potential differences in pharmacokinetics (PK) between healthy subjects and patients with cancer were investigated using a physiologically based pharmacokinetic approach integrating demographic and physiological data from patients with cancer. Demographic data such as age, sex and body weight, and clinical laboratory measurements such as albumin, alpha‐1 acid glycoprotein (AAG) and hematocrit were collected in ~2500 patients with cancer. A custom oncology population profile was built using the observed relationships among demographic variables and laboratory measurements in Simcyp® software, a population based ADME simulator. Patients with cancer were older compared with the age distribution in a built‐in healthy volunteer profile in Simcyp. Hematocrit and albumin levels were lower and AAG levels were higher in patients with cancer. The custom population profile was used to investigate the disease effect on the pharmacokinetics of two probe substrates, saquinavir and midazolam. Higher saquinavir exposure was predicted in patients relative to healthy subjects, which was explained by the altered drug binding due to elevated AAG levels in patients with cancer. Consistent with historical clinical data, similar midazolam exposure was predicted in patients and healthy subjects, supporting the hypothesis that the CYP3A activity is not altered in patients with cancer. These results suggest that the custom oncology population profile is a promising tool for the prediction of PK in patients with cancer. Further evaluation and extension of this population profile with more compounds and more data will be needed. Copyright

Single and multiple dose intravenous and oral pharmacokinetics of the hedgehog pathway inhibitor vismodegib in healthy female subjects

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT While recent publications have suggested the pharmacokinetics (PK) of vismodegib appear to be non-linear, there has not been a report describing the mechanisms of non-linearity. WHAT THIS STUDY ADDS This study provides evidence that two separate processes, namely, solubility-limited absorption and concentration-dependent plasma protein binding, can explain the non-linear PK of vismodegib. This study provides quantitative results which can account for the lower than expected accumulation of vismodegib with continuous daily dosing. AIM Vismodegib has demonstrated clinical activity in patients with advanced basal cell carcinoma. The pharmacokinetics (PK) of vismodegib are non-linear. The objective of this study was to determine whether vismodegib PK change following repeated dosing by administering a tracer intravenous (i.v.) dose of (14) C-vismodegib with single and multiple oral doses. METHODS Healthy post menopausal female subjects (n= 6/group) received either a single or daily 150 mg vismodegib oral dose with a (14) C-labelled 10 µg i.v. bolus dose administered 2 h after the single or last oral dose (day 7). Plasma samples were assayed for vismodegib by LC-MS/MS and for (14) C-vismodegib by accelerator mass spectrometry. RESULTS Following a single i.v. dose, mean clearance, volume of distribution and absolute bioavailability were 43.4 ml h(-1) , 16.4 l and 31.8%, respectively. Parallel concentration-time profiles following single oral and i.v. administration of vismodegib indicated elimination rate limited PK. Following i.v. administration at steady-state, mean clearance and volume of distribution were 78.5 ml h(-1) and 26.8 l, respectively. Comparison of i.v. PK parameters after single and multiple oral dosing showed similar half-life, increased clearance and volume of distribution (81% and 63% higher, respectively) and decreased bioavailability (77% lower) after repeated dosing. Relative to single dose, the unbound fraction of vismodegib increased 2.4-fold with continuous daily dosing. CONCLUSION Vismodegib exhibited a long terminal half-life after oral and i.v. administration, moderate absolute bioavailability and non-linear PK after repeated dosing. Results from this study suggest that the non-linear PK of vismodegib result from two separate, non-linear processes, namely solubility limited absorption and high affinity, saturable plasma protein binding.

Absolute bioavailability and effect of formulation change, food, or elevated pH with rabeprazole on cobimetinib absorption in healthy subjects.

Cobimetinib is a potent and highly selective inhibitor of MEK1/2. Since cobimetinib exhibited absorption variability in cancer patients, a series of single-dose studies in healthy subjects were conducted to determine absolute bioavailability and elucidate potential effects of formulation, food, and elevated gastric pH on cobimetinib bioavailability. Three crossover trials were performed with a 20 mg cobimetinib oral dose: absolute bioavailability using a 2 mg intravenous infusion (n = 13), relative bioavailability of tablets versus capsules and food effect (n = 20), and drug interaction with a proton pump inhibitor (20 mg of rabeprazole daily for 5 days prior to cobimetinib administration; n = 20). Absolute bioavailability of cobimetinib was 46.2% (24.2, CV %), likely due to metabolism rather than incomplete absorption. The mean systemic clearance of cobimetinib was low (11.7 L/h [28.2, CV %]). Administration of cobimetinib tablets with a high-fat meal delayed drug absorption (prolonged tmax) but had no statistically significant effect on cobimetinib exposure (Cmax and AUC0-∞). Tablet and capsule formulations of cobimetinib showed comparable exposures. Cobimetinib exhibited delayed absorption (tmax) in the presence of rabeprazole, with no statistically significant effects on drug exposure (Cmax and AUC0-∞) in the fasted state. In conclusion, cobimetinib oral absorption was not affected by change in formulation, food, or elevated gastric pH.