Predictive Performance of Vancomycin Trough Concentrations in Patients With Diabetes With Microalbuminuria.

BACKGROUND: The performance of a population pharmacokinetic model in predicting trough concentrations after the initial vancomycin dose was evaluated in patients with albuminuria compared with patients who did not have albuminuria. METHODS: Data were collected from 52 patients infected with methicillin-resistant Staphylococcus aureus (excluding patients undergoing dialysis and acute kidney injury) and treated with vancomycin. The data included urinary albumin concentration. RESULTS: The calculated mean prediction error and mean absolute error for the serum trough concentrations of vancomycin (with 95% confidence intervals) were 4.65 (4.13-5.17) and 6.1 (5.65-6.51), respectively, in microalbuminuria and 0.33 (-0.2 to 0.86) and 4.02 (3.59-4.45), respectively, in those without. There was no significant difference observed in serum creatinine concentration, age, weight, estimation of vancomycin trough concentration in serum, and actual trough concentration of vancomycin in serum between individuals with microalbuminuria and those without albuminuria. CONCLUSIONS: Microalbuminuria in patients with diabetes is a marker of the difference between predicted vancomycin trough concentrations and actual vancomycin trough concentrations.

A toxicity risk index, an index for warning idiosyncratic drug toxicity.

Drug toxicity impedes drug development and its clinical use. In the present study, a toxicity risk index (TRI), which is an index for warning idiosyncratic drug toxicity (IDT), was proposed. The TRI of drugs was defined as a function of dose, pharmacokinetic parameters, and toxicokinetic data from covalent binding experiment. Twenty drugs, which were classified into three categories by a report (Nakayama S, Atsumi R, Takakusa H, Kobayashi Y, Kurihara A, Nagai Y, Nakai D, Okazaki O. 2009. Drug Metab Dispos 37:1970-1977), were studied with TRI. The three categories were BBW (drugs with a block box warning for IDT), WNG (drugs without a black box warning but with a warning for IDT), and SAFE (drugs without any warning). The TRIs of drugs classified as SAFE were distinctly different from those classified as BBW. The TRI of the SAFE drugs were lower than 0.456 (nmol/mg protein). In contrast, the TRI of the BBW drugs were higher than 1.10 (nmol/mg protein). These results warned us that a drug candidate, where the TRI is higher than 1.0 nmol/mg protein, should be categorized as a BBW drug. Further study with more data of TRI will give a cutoff value with a statistical meaning. Thus, TRI may be useful for decision making in drug development and its clinical use.

Kinetic assessment of luminal degradation of orally effective prodrugs for rational drug development.

Although prodrugging (prodrug derivatization) is a powerful technique for improving the pharmacokinetic characteristics of drugs, the intestinal pharmacokinetics of prodrugs has yet to be elucidated fully. A previous article reported the kinetic requirement of prodrugs to overcome membrane barriers. In the present article, the luminal degradation of prodrugs was kinetically assessed to understand crucial factors in the intestinal absorption of prodrugs and to show a rational development procedure. A kinetic model equation involving luminal degradation clearance (CL(deg)) was derived, and CL(deg) was estimated according to the equation with in vitro and in vivo reported data of two kinds of ampicillin prodrugs (lenampicillin and pivampicillin) and one acyclovir prodrug (valacyclovir). For lenampicillin ((2,2-dimethyl-1-oxopropoxy)methyl ester derivative), CL(deg) was approximately 1.7 times as large as absorption clearance (CL(abs)), whereas for pivampicillin ((5-methyl-2-oxo-1,3-dioxol-4-yl)methyl ester derivative), CL(deg) was approximately one tenth of CL(abs). For valacyclovir (acyclovir prodrug), CL(deg) was negligible. These results indicate that not only membrane permeability but also luminal stability should be assessed for the rational development of orally effective prodrugs, and that luminal stabilization can improve the intestinal absorption of prodrugs. A procedure was proposed to develop orally effective prodrugs considered for luminal degradation as well as membrane permeability.

Intestinal glucuronidation metabolism may have a greater impact on oral bioavailability than hepatic glucuronidation metabolism in humans: a study with raloxifene, substrate for UGT1A1, 1A8, 1A9, and 1A10.

The kinetic impact of intestinal glucuronidation metabolism on oral bioavailability (F) was assessed using reported human data of raloxifene, of which oral bioavailability was only 2%. Kinetic analysis showed that presystemic intestinal availability (Fpg) was 5.4%, whereas fraction absorbed (Ff) and hepatic availability (Fh) were 63% and 59.3%, respectively. Thus, Fpg was the lowest among factors, which affect oral bioavailability. In addition, Fpg was much lower than Fh, suggesting that intestinal glucuronidation metabolism has a greater impact on oral bioavailability than hepatic glucuronidation metabolism. It has been reported that UDP-glucuronosyltransferase (UGT) 1A1, UGT1A8, UGT1A9, and UGT1A10 are enzymes for raloxifene glucuronidation, and UGT1A8 and UGT1A10 are absent in the human liver, whereas UGT1A1, UGT1A8, UGT1A9, and UGT1A10 are present in the human intestine. Therefore, it is also suggested that intestinal glucuronidation catalyzed by UGTs, particularly UGT1A8 and UGT1A10, may play important roles in the first-pass metabolism, causing low oral bioavailability.

Assessment of presystemic and systemic intestinal availability of orally administered drugs using in vitro and in vivo data in humans: intestinal sulfation metabolism impacts presystemic availability much more than systemic availability of salbutamol, SULT1A3 substrate.

Although hepatic availability has been extensively studied to assess the oral bioavailability of drugs, intestinal availability has not, especially that related to conjugative metabolism (phase II metabolism). The present study assessed intestinal presystemic availability by integrating the reported metabolism data in vitro and in vivo of salbutamol, SULT1A3 substrate, in humans. Intrinsic clearance from each organ was calculated with the reported kinetic parameters for salbutamol sulfation metabolism in vitro. Then, presystemic (except intestine) and systemic organ availability and organ clearance were estimated by scaling up the in vitro data using biochemical and physiological data. Scaling factors used were one third, one and three times. Intestinal presystemic availability was calculated without assumptions of a well-stirred model. Presystemic availability of the intestine is much lower than systemic availability of the intestine. In addition, presystemic availability of the intestine is lower than presystemic availability of other organs, the liver or lung. SULT1A3 is expressed primarily in the intestine; therefore, it should be noted that intestinal metabolism affects the oral bioavailability of drugs, which are metabolized by SULT1A3.

Kinetic characterization of glycosidase activity from disaccharide conjugate to monosaccharide conjugate in Caco-2 cells.

Glycosidase activity influences the intestinal absorption of glycosides. Our previous study in rats suggested that disaccharide conjugates might be prototypes for pre-prodrugs aiming at the Na(+)/glucose co-transporter-mediated transport of prodrugs (drug glucoside) as a novel absorption pathway. One of the crucial factors is the formation of a glucoside drug from the disaccharide conjugate. Since there is a large species difference in metabolism, it is necessary to examine the cells and/or enzymes derived from human tissue to confirm this concept. In this paper, we kinetically characterized the glycosidase activity of disaccharide conjugates in Caco-2 cells. Disaccharide conjugates of p-nitrophenol (p-NP) (p-NP beta-cellobioside, p-NP beta-lactoside and p-NP beta-maltoside) were hydrolysed to p-NP beta-glucoside. beta-glucosidase or beta-galactosidase (lactase/phloridzin hydrolase, LPH) and alpha-glucosidase (sucrase-isomaltase) had different pH-dependent activities for disaccharide conjugates. At neutral pH, LPH has low affinity and low capacity, and sucrase-isomaltase has high affinity and high capacity, whereas at acid pH, LPH has high affinity and low capacity, and sucrase-isomaltase has low affinity and high capacity. The hydrolysis clearance calculated with Vmax/Km indicated that sucrase-isomaltase activity is much higher than LPH activity at either neutral or acid pH in Caco-2 cells. Since the hydrolysis rate of the disaccharide conjugate was highly dependent on the pH value and type of glycoside linkage, the appropriate selection of a glycoside form after consideration of these differences is the key to designing a sugar-conjugate prodrug.

Intestinal SGLT1-mediated absorption and metabolism of benzyl beta-glucoside contained in Prunus mume: carrier-mediated transport increases intestinal availability.

The intestinal absorption of benzyl beta-glucoside (BNZ beta glc) contained in the fruit of Prunus mume SIEB. et ZUCC. (Rosaceae), which is traditionally used as a medicinal food in Japan, was studied in rat intestines. BNZ beta glc was absorbed from the mucosal to serosal sides. Its metabolite, benzyl alcohol (BAL), was also detected on both the mucosal and serosal sides. In the presence of phloridzin (Na(+)/glucose cotransporter (SGLT1) inhibitor) or in the absence of Na+ (driving force), BNZ beta glc absorption was significantly decreased. Transport clearance of BNZ beta glc across the brush border membrane decreased as its concentration increased. These results indicate that BNZ beta glc is transported by SGLT1. Metabolic clearance of BNZ beta glc also decreased as its concentration increased. The amount ratio of BNZ beta glc to BAL on the serosal side increased with the increase of BNZ beta glc concentration. The intestinal availability of BNZ beta glc was lower in the absence of Na+ than in the presence of Na+, indicating that the SGLT1-mediated transport of BNZ beta glc increases intestinal availability by decreasing the intestinal extraction ratio. This neutraceutical study concluded that intestinal carrier-mediated transport across the brush border membrane improves the intestinal availability of nutritionally, pharmacologically or physiologically active compounds that undergo intestinal metabolism (first-pass effect).

Differentiation of organ availability by sequential and simultaneous analyses: intestinal conjugative metabolism impacts on intestinal availability in humans.

The impact of intestinal conjugative metabolism on oral bioavailability was assessed by sequential and simultaneous analyses of the reported data in humans. The data were retrieved from reports on drugs that are metabolized by sulfate conjugation, and the organ availabilities affecting oral bioavailability were differentiated. Sequential analysis gave the following results. The intestinal availability (Fg) of salbutamol was 0.700, whereas hepatic availability (Fh) and bioavailability (F) were 0.893 and 0.493, respectively. Fg of (+)-terbutaline, (-)-terbutaline, and (+/-)-terbutaline was 0.128, 0.254, and 0.250, respectively. In contrast, Fh of (+)-terbutaline, (-)-terbutaline, and (+/-)-terbutaline was 0.979, 0.971, and 0.946, respectively. Fg and Fh of ethynylestradiol were 0.536 and 0.780, respectively. Simultaneous analysis also gave similar results, although the sequential analysis overestimated the intestinal availability. These results indicate that intestinal sulfation metabolism has more impact on intestinal availability than on hepatic availability, resulting in low bioavailability in humans.

Does the well-stirred model assess the intestinal first-pass effect well?

The pre-systemic intestinal extraction ratio (E(g)) has been estimated by an equation based on the well-stirred model, which does not have a term of membrane transport. In this report, we have identified the application limitations of the well-stirred model equation to assess the pre-systemic intestinal extraction ratio. The E(g) of metoprolol (CYP2D6 substrate) was assessed by three methods. Intrinsic clearances for metoprolol metabolism in hepatic and gastrointestinal microsomes were from a published report. Method 1 (model-independent method): the E(g) of 0.228 was obtained according to the equation, F = F(f) x (1 - E(g)) x F(h), where F, F(f) and F(h) were the bioavailability, the fraction entering the intestinal tissue and the hepatic availability, respectively. Method 2: the E(g) of 0.0071 was calculated according to the well-stirred model equation, and was much lower than the value of 0.228. Method 3: the E(g) of 0.213 was obtained by the transport-metabolism-flow (TMF) model equation, and was much closer to the value of 0.228 obtained by the model-independent method than the E(g) of 0.0071 calculated by the well-stirred model equation. Therefore, we propose that the factor of membrane transport process be incorporated into the pharmacokinetic model for the assessment of the pre-systemic intestinal extraction ratio.

Factors affecting glucuronidation activity in Caco-2 cells.

Presystemic intestinal metabolism reduces the intestinal absorption and bioavailability of orally administered drugs. The factors affecting glucuronidation activity in Caco-2 cells seeded in Transwell (4.7 cm(2)) require clarification to establish an in-vitro system to assess intestinal glucuronidation metabolism for novel drug development. alpha-Naphthol (alpha-NA), a substrate for UGT1A6 in Caco-2 cells, has often been used as a model substrate for gluruonidation. alpha-Naphthol glucuronidation activity increased from 7 to 21 culture days after seeding in Transwell and stabilized after 21 days. The higher the passage number of Caco-2 cells, the larger the variance of glucuronidation activity, but apical pH did not significantly influence glucuronidation in the pH range of 5.5 to 7.4. When the passage number ranged from 83 to 159, Km,app was highest at passage number 130. In contrast, Vmax,app increased with the passage number. This indicates that the kinetic parameters for glucuronidation in Caco-2 cells are dependent on the passage number of the cells. These results should be useful for establishing the experimental conditions for Caco-2 cells that predict intestinal glucuronidation activity in vivo.

Dietary polyphenols (-)-epicatechin and chrysin inhibit intestinal glucuronidation metabolism to increase drug absorption.

The effect of dietary polyphenols on the intestinal glucuronidation and absorption of a model phenolic drug, alpha-naphthol (alpha-NA), was studied in isolated rat small intestine. (-)-Epicatechin significantly inhibited the glucuronidation of alpha-NA. Chrysin, (-)-epigallocatechin galleate (EGCG), and quercetin decreased the rate of glucuronidation, although not significantly. Baicalin did not affect the glucuronidation. The rate of absorption of alpha-NA in the presence of these polyphenols also varied. The absorption clearance (CLabs) and the metabolic clearance (CLmet) were inversely correlated, and this relationship was well explained in the metabolic inhibition model with kinetic parameters (knowledge-based prediction) which characterizes the relationship between the CLabs and CLmet of alpha-NA (Biochim Biophys Acta 1998, 1425, 398.). These results indicate that the concomitant intake of some polyphenols can increase the absorption of a phenolic drug, and the effect is predictable. (-)-Epicatechin and chrysin are effective for the inhibition of glucuronidation and promotion of intestinal drug absorption.

Concentration-dependent atypical intestinal absorption of cyclic phenylalanylserine: small intestine acts as an interface between the body and ingested compounds.

Intestinal absorption of peptides in linear form has been studied extensively, but there is little knowledge of peptides in a cyclic form. In this report, intestinal absorption of cyclic phenylalanylserine (cyclo(Phe-Ser)), a precursor of gliotoxin, was studied in isolated rat small intestine as a model cyclic dipeptide. Absorption clearance (CLabs) decreased in the presence of glycylsarcosine, cephalexin or cephradine, substrates for H+/oligopeptide cotransporter (PEPT1). CLabs of cyclo(Phe-Ser) also decreased at 4 degrees C, thus indicating that cyclo(Phe-Ser) is in part transported by PEPT1. However, the Eadie-Hofstee plot of absorption revealed an atypical profile at lower concentrations of cyclo(Phe-Ser) (around 0.1 mM). Moreover, comparative experiments of absorptive and excretive transport showed that excretive transport from the serosal to mucosal side of isolated intestinal tissue at a 0.1 mM cyclo(Phe-Ser) was superior to absorptive transport from the mucosal side to the serosal side, and vice versa at a 1 mM cyclo(Phe-Ser). A kinetic model was constructed, in which cyclo(Phe-Ser) concentration for excretive transport was assumed to be at the binding site of excretive transporter, but not the unbound cytoplasmic concentration. These results as well as the results of kinetic analysis indicate that intestinal absorption consists of passive transport, carrier-mediated absorptive transport by PEPT1 and carrier-mediated excretive transport, resulting in atypical absorption. Although cyclic dipeptides have potentials as drugs, their intestinal absorption may be complex. The results of this study lead us to conclude that absorptive and excretive transport by the small intestine acts as an interface between the body and ingested compounds.

Uptake of cyclic dipeptide by PEPT1 in Caco-2 cells: phenolic hydroxyl group of substrate enhances affinity for PEPT1.

Uptake of cyclic dipeptides by H+/oligopeptide cotransporter (PEPT1) was studied in monolayers of the human intestinal cell line, Caco-2. The cyclic dipeptides studied were cyclic glycylphenylalanine (cyclo(Gly-Phe)), cyclic phenylalanylserine (cyclo(Phe-Ser)), cyclic seryltyrosine (cyclo(Ser-Tyr)) and cyclic glycyltyrosine (cyclo(Gly-Tyr)). These molecules have both peptide bonds and aromatic rings, and are similar in structure to cephalexin and cephadroxil, which are transported by PEPT1. Cellular uptake of these cyclic dipeptides was pH dependent, and was inhibited by the addition of PEPT1 substrates such as glycylsarcosine, indicating PEPT1-mediated transport. Michaelis constants (Km) for these cyclic dipeptides were cyclo(Ser-Tyr) < cyclo(Phe-Ser), and cyclo(Gly-Tyr) < cyclo(Gly-Phe), indicating that tyrosine possessing phenol moiety has higher affinity for PEPT1 than phenylalanine possessing benzen moiety. The Km for cephadroxil possessing phenol moiety was reportedly lower than that for cephalexin possessing benzen moiety. Therefore, it was concluded that the phenolic hydroxyl group of the substrate may enhance affinity for PEPT1.

Concentration-dependent preferences of absorptive and excretive transport cause atypical intestinal absorption of cyclic phenylalanylserine: small intestine acts as an interface between the body and ingested compounds.

Intestinal absorption of cyclic phenylalanylserine (cyclo(Phe-Ser)), a precursor of gliotoxin, was studied in isolated rat small intestine as a model cyclic dipeptide. Absorption clearance (CLabs) decreased in the presence of glycylsarcosine, cephalexin or cephradine, substrates for H+/oligopeptide cotransporter (PEPT1). CLabs of cyclo(Phe-Ser) also decreased at 4 degrees C. These indicate that cyclo(Phe-Ser) is in part transported by PEPT1. However, Eadie-Hofstee plot of absorption revealed an atypical profile at lower concentrations of cyclo(Phe-Ser) (around 0.1 mM). Moreover, comparative experiments of absorptive and excretive transport showed that excretive transport from the serosal to mucosal side of isolated intestinal tissue at a 0.1 mM cyclo(Phe-Ser) was superior to absorptive transport from the mucosal side to the serosal side, and vise versa at a 1 mM cyclo(Phe-Ser). These results as well as the results of kinetic analysis indicate that intestinal absorption consists of passive transport, carrier-mediated absorptive transport by PEPT1 and carrier-mediated excretive transport, resulting in atypical absorption. Although cyclic dipeptides have potentials for drug, their intestinal absorption may be complex. The results of this study lead us conclude that absorptive and excretive transport by the small intestine acts as an interface between the body and ingested compounds.

Kinetic impact of presystemic intestinal metabolism on drug absorption: experiment and data analysis for the prediction of in vivo absorption from in vitro data.

Orally administered drugs suffer from attack by metabolic enzymes not only in the liver, but also in the gastrointestine during the absorption process across the intestinal tissue. Although kinetic study on hepatic metabolism has been done well, the intestinal metabolism has not been well focused on compared with hepatic metabolism. In order to emphasize the role of intestinal metabolism in drug absorption and bioavailability, I have reviewed the experimental methods for intestinal absorption and metabolism, and the data analysis. Since Klippert et al. reported the prediction of intestinal first-pass effect of phenacetin in the rat from enzyme kinetic data in 1982, several reports have showed a good prediction, but others have not. Although intestinal absorption is an integrated process of transport (transporters) and metabolism (metabolic enzymes), most of the researchers missed the pathway of intestinal drug absorption and applied the kinetic model effective on only systemic metabolism to presystemic intestinal metabolism for their analysis of intestinal metabolism of orally administered drugs. A kinetic model, which incorporated factors of membrane transport, metabolic activity and protein binding, was structured to compare the equations in the reported models. In conclusion, we need more studies including kinetic modeling and experiments to understand the impact of intestinal metabolism on drug absorption. That knowledge must lead to the construction of ADME in silico (e-ADME).

The Metabolic Inhibition Model Which Predicts the Intestinal Absorbability and Metabolizability of Drug: Theory and Experiment.

The intestinal absorption of analgesic peptides (leucine enkephalin and kyotorphin) and modified peptides in rat were studied. Although these peptides were not absorbed, the absorbability (absorption clearance) of these peptides were increased in the presence of peptidase inhibitors. In order to kinetically analyze these phenomena, we proposed the metabolic inhibition model, which incorporated the metabolic clearance (metabolizability) with the absorption clearance. Metabolic activity was determined with intestinal homogenates. The higher the metabolic clearance was, the lower was the absorption clearance. The relationships between the absorption clearance and the metabolic clearance of the experimental data as well as of the theoretical values were hyperbolic. This model predicted the maximum absorption clearances of cellobiose-coupled leucine enkephalin (0.654 &mgr;l/min/cm) and kyotorphin (0.247 &mgr;l/min/cm). Details of the experimental methods are described.