Sequential first-pass elimination of a metabolite derived from a precursor

Hepatic clearance concepts and misconceptions: Why the well-stirred model is still used even though it is not physiologic reality?

The liver is the most important drug metabolizing organ, endowed with a plethora of metabolizing enzymes and transporters to facilitate drug entry and removal via metabolism and/or biliary excretion. For this reason, much focus surrounds the development of clearance concepts, which are based on normalizing the rate of removal to the input or arterial concentration. By so doing, some authors have recently claimed that it implies one specific model of hepatic elimination, namely, the widely used well-stirred or venous equilibration model (WSM). This commentary challenges this claim and aims to provide a comprehensive discussion of not only the WSM but other currently applied hepatic clearance models - the parallel tube model (PTM), the dispersion model (DM), the zonal liver model (ZLM), and the heterogeneous capillary transit time model of Goresky and co-workers (GM). The WSM, PTM, and DM differ in the patterns of internal blood flow, assuming bulk, plug, and dispersive flows, respectively, which render different degrees of mixing within the liver that are characterized by the magnitudes of the dispersion number (D_N), resulting in different implications concerning the (unbound) substrate concentration in liver (Cu_H). Early models assumed perfusion rate-limited distribution, which have since been modified to include membrane-limited transport. The recent developments associated with the misconceptions and the sensitivity of the models are hereby addressed. Since the WSM has been and will likely remain widely used, the pros and cons of this model relative to physiological reality are further discussed.

Unequivocal evidence supporting the segregated flow intestinal model that discriminates intestine versus liver first-pass removal with PBPK modeling

Merits of the segregated flow model (SFM), highlighting the intestine as inert serosa and active enterocyte regions, with a smaller fractional (f_Q < 0.3) intestinal flow (Q_I ) perfusing the enterocyte region, are described. Less drug in the circulation reaches the enterocytes due to the lower flow (f_Q Q_I ) in comparison with drug administered into the gut lumen, fostering the idea of route-dependent intestinal removal. The SFM has been found superior to the traditional model (TM), which views the serosa and enterocytes totally as a well-mixed tissue perfused by 100% of the intestinal flow, Q_I . The SFM model is able to explain the lower extents of intestinal metabolism of enalapril, morphine and midazolam with i.v. vs. p.o. dosing. For morphine, the urine/bile ratio of the metabolite, morphine glucuronide MGurineMGbile for p.o. was 2.6× that of i.v. This was due to the higher proportion of intestinally formed morphine glucuronide, appearing more in urine than in bile due to its low permeability and greater extent of intestinal formation with p.o. administration. By contrast, the TM predicted the same MGurineMGbile for p.o. vs. i.v. The TM predicted that the contributions of the intestine:liver to first-pass removal were 46%:54% for both p.o. and i.v. The SFM predicted same 46%:54% (intestine:liver) for p.o., but 9%:91% for i.v. By contrast, the kinetics of codeine, the precursor of morphine, was described equally well by the SFM- and TM-PBPK models, a trend suggesting that intestinal metabolism of codeine is negligible. Fits to these PBPK models further provide insightful information towards metabolite formation: available fractions and the fractions of hepatic and total clearances that form the metabolite in question. The SFM-PBPK model is useful to identify not only the presence of intestinal metabolism but the contributions of the intestine and liver for metabolite formation. Copyright © 2016 John Wiley & Sons, Ltd.

Metabolite Kinetics: The Segregated Flow Model for Intestinal and Whole Body Physiologically Based Pharmacokinetic Modeling to Describe Intestinal and Hepatic Glucuronidation of Morphine in Rats In Vivo

We used the intestinal segregated flow model (SFM) versus the traditional model (TM), nested within physiologically based pharmacokinetic (PBPK) models, to describe the biliary and urinary excretion of morphine 3β-glucuronide (MG) after intravenous and intraduodenal dosing of morphine in rats in vivo. The SFM model describes a partial (5%-30%) intestinal blood flow perfusing the transporter- and enzyme-rich enterocyte region, whereas the TM describes 100% flow perfusing the intestine as a whole. For the SFM, drugs entering from the circulation are expected to be metabolized to lesser extents by the intestine due to the segregated flow, reflecting the phenomenon of shunting and route-dependent intestinal metabolism. The poor permeability of MG crossing the liver or intestinal basolateral membranes mandates that most of MG that is excreted into bile is hepatically formed, whereas MG that is excreted into urine originates from both intestine and liver metabolism, since MG is effluxed back to blood. The ratio of MG amounts in urine/bile [Formula: see text] for intraduodenal/intravenous dosing is expected to exceed unity for the SFM but approximates unity for the TM. Compartmental analysis of morphine and MG data, without consideration of the permeability of MG and where MG is formed, suggests the ratio to be 1 and failed to describe the kinetics of MG. The observed intraduodenal/intravenous ratio of [Formula: see text] (2.55 at 4 hours) was better predicted by the SFM-PBPK (2.59 at 4 hours) and not the TM-PBPK (1.0), supporting the view that the SFM is superior for the description of intestinal-liver metabolism of morphine to MG. The SFM-PBPK model predicts an appreciable contribution of the intestine to first pass M metabolism.

Mechanistic Modeling to Predict Midazolam Metabolite Exposure from In Vitro Data

Methods to predict the pharmacokinetics of drugs in humans from in vitro data have been established, but corresponding methods to predict exposure to circulating metabolites are unproven. The objective of this study was to use in vitro methods combined with static and dynamic physiologically based pharmacokinetic (PBPK) models to predict metabolite exposures, using midazolam and its major metabolites as a test system. Intrinsic clearances (CLint) of formation of individual metabolites were determined using human liver microsomes. Metabolic CLintof hydroxymidazolam metabolites via oxidation and glucuronidation were also determined. Passive diffusion intrinsic clearances of hydroxymidazolam metabolites were determined using sandwich cultured human hepatocytes and the combination of this term along with the metabolic CLint, and liver blood flow was used to estimate the fraction of the metabolite that can enter the systemic circulation after formation in the liver. The metabolite/parent drug area under the plasma concentration-time curve ratio (AUCm/AUCp) was predicted using a static model relating the fraction of midazolam clearance to each metabolite, the clearance rates of midazolam and hydroxymidazolam metabolites, and the availability of the metabolites. Additionally, the human disposition of midazolam metabolites was simulated using a SimCYP PBPK model. Both approaches yielded AUCm/AUCpratios that were in agreement with the in vivo ratios. This study shows that in vivo midazolam metabolite exposure can be predicted from in vitro data and PBPK modeling. This study emphasized the importance of metabolite systemic availability from its tissue of formation, which remains a challenge to quantitative prediction.

Development of a Rat Plasma and Brain Extracellular Fluid Pharmacokinetic Model for Bupropion and Hydroxybupropion Based on Microdialysis Sampling, and Application to Predict Human Brain Concentrations

Administration of bupropion [(±)-2-(tert-butylamino)-1-(3-chlorophenyl)propan-1-one] and its preformed active metabolite, hydroxybupropion [(±)-1-(3-chlorophenyl)-2-[(1-hydroxy-2-methyl-2-propanyl)amino]-1-propanone], to rats with measurement of unbound concentrations by quantitative microdialysis sampling of plasma and brain extracellular fluid was used to develop a compartmental pharmacokinetics model to describe the blood-brain barrier transport of both substances. The population model revealed rapid equilibration of both entities across the blood-brain barrier, with resultant steady-state brain extracellular fluid/plasma unbound concentration ratio estimates of 1.9 and 1.7 for bupropion and hydroxybupropion, respectively, which is thus indicative of a net uptake asymmetry. An overshoot of the brain extracellular fluid/plasma unbound concentration ratio at early time points was observed with bupropion; this was modeled as a time-dependent uptake clearance of the drug across the blood-brain barrier. Translation of the model was used to predict bupropion and hydroxybupropion exposure in human brain extracellular fluid after twice-daily administration of 150 mg bupropion. Predicted concentrations indicate that preferential inhibition of the dopamine and norepinephrine transporters by the metabolite, with little to no contribution by bupropion, would be expected at this therapeutic dose. Therefore, these results extend nuclear imaging studies on dopamine transporter occupancy and suggest that inhibition of both transporters contributes significantly to bupropion's therapeutic efficacy.

Prediction of relative in vivo metabolite exposure from in vitro data using two model drugs: dextromethorphan and omeprazole

Metabolites can have pharmacological or toxicological effects, inhibit metabolic enzymes, and be used as probes of drug-drug interactions or specific cytochrome P450 (P450) phenotypes. Thus, better understanding and prediction methods are needed to characterize metabolite exposures in vivo. This study aimed to test whether in vitro data could be used to predict and rationalize in vivo metabolite exposures using two model drugs and P450 probes: dextromethorphan and omeprazole with their primary metabolites dextrorphan, 5-hydroxyomeprazole (5OH-omeprazole), and omeprazole sulfone. Relative metabolite exposures were predicted using metabolite formation and elimination clearances. For dextrorphan, the formation clearances of dextrorphan glucuronide and 3-hydroxymorphinan from dextrorphan in human liver microsomes were used to predict metabolite (dextrorphan) clearance. For 5OH-omeprazole and omeprazole sulfone, the depletion rates of the metabolites in human hepatocytes were used to predict metabolite clearance. Dextrorphan/dextromethorphan in vivo metabolite/parent area under the plasma concentration versus time curve ratio (AUC(m)/AUC(p)) was overpredicted by 2.1-fold, whereas 5OH-omeprazole/omeprazole and omeprazole sulfone/omeprazole were predicted within 0.75- and 1.1-fold, respectively. The effect of inhibition or induction of the metabolite's formation and elimination on the AUC(m)/AUC(p) ratio was simulated. The simulations showed that unless metabolite clearance pathways are characterized, interpretation of the metabolic ratios is exceedingly difficult. This study shows that relative in vivo metabolite exposure can be predicted from in vitro data and characterization of secondary metabolism of probe metabolites is critical for interpretation of phenotypic data.

Physiologically-based pharmacokinetic modeling for absorption, transport, metabolism and excretion

The seminal paper on the liver physiologically-based pharmacokinetic (PBPK) model by Rowland et al. (J Pharmacokinet Biopharm 1:123-136, 1973) that described the influence of blood flow, intrinsic clearance, and binding on hepatic clearance had inspired further development of PBPK modeling of the liver, kidney and intestine as well as whole body. Shortly thereafter, a series of papers from Pang and Rowland compared the well-stirred and parallel-tube liver models and sparked further development on clearance concepts in the liver, including those described by the dispersion model. From 2005 onwards, several seminal papers by Rodgers and Rowland, in their recognition of the binding of molecules to tissue acidic and neutral phospholipids, improved the methodology in providing estimates of the tissue-to-plasma coefficient and rendering easy calculation of these hard-to-get constants. The improvement has strongly consolidated the basic premise on PBPK modeling and simulations and these basics have allowed scientists to focus on other important variables: membrane barriers, and transporter and enzyme and their heterogeneities that further impact drug disposition. In particular, the PBPK models have delved into sequential metabolism and futile cycling to illustrate how transporters and enzymes could affect the metabolism of drugs and metabolites. PBPK models that are especially pertinent to metabolite kinetics are being utilized in drug studies and risk assessment. These types of PBPK modeling reveal differences in kinetics between the formed vs. preformed metabolite, showing special considerations for membrane barriers, and the influence of competing pathways and competing organs.

Physiological modeling to understand the impact of enzymes and transporters on drug and metabolite data and bioavailability estimates

PURPOSE

To obtain mathematical solutions that correlate drug and metabolite exposure and systemic bioavailability (F (sys)) with physiological determinants, transporters and enzymes.

METHODS

A series of physiologically-based pharmacokinetic (PBPK) models that included renal excretion and sequential metabolism within the intestine and/or liver as metabolite formation organs were developed. The area under the curve for drug (AUC) and formed metabolite (AUC{mi,P}) were solved by matrix inversion.

RESULTS

The PBPK models revealed that AUC{mi,P} was dependent on dispositional parameters (transport and elimination) for the drug and metabolite. The solution was unique for each metabolite formation organ and was dependent on the type of drug and metabolite elimination organs. The AUC ratio of the formed metabolite after oral and intravenous drug dosing was useful for determination of the fraction absorbed (F (abs)) and not the systemic bioavailability (F (sys)) when either intestine or liver was the only drug elimination organ.

CONCLUSIONS

The AUC ratio of the formed metabolite after oral and intravenous drug dosing differed from that for drug and would not provide F (sys). However, the AUC ratio of the formed metabolite for oral and intravenous drug dosing furnished the estimate of F (abs) when intestine or liver was the only drug metabolic organ.

Formed and preformed metabolites: facts and comparisons

The administration of metabolites arising from new drug entities is often employed in drug discovery to investigate their associated toxicity. It is expected that administration of metabolites can predict the exposure of metabolites originating from the administration of precursor drug. Whether exact and meaningful information can be obtained from this has been a topic of debate. This communication summarizes observations and theoretical relationships based on physiological modelling for the liver, kidney and intestine, three major eliminating organs/tissues. Theoretical solutions based on physiological modelling of organs were solved, and the results suggest that deviations are expected. Here, examples of metabolite kinetics observed mostly in perfused organs that did not match predictions are provided. For the liver, discrepancies in fate between formed and preformed metabolites may be explained by the heterogeneity of enzymes, the presence of membrane barriers and whether transporters are involved. For the kidney, differences have been attributed to glomerular filtration of the preformed but not the formed metabolite. For the intestine, the complexity of segregated flows to the enterocyte and serosal layers and differences in metabolism due to the route of administration are addressed. Administration of the metabolite may or may not directly reflect the toxicity associated with drug use. However, kinetic data on the preformed metabolite will be extremely useful to develop a sound model for modelling and simulations; in-vitro evidence on metabolite handling at the target organ is also paramount. Subsequent modelling and simulation of metabolite data arising from a combined model based on both drug and preformed metabolite data are needed to improve predictions on the behaviours of formed metabolites.

Safety testing of metabolites: Expectations and outcomes

Metabolites arising from chemical entities, old or new, are often mediators of toxicity. Frequently, metabolites are investigated in test animals, with the expectation that the resultant toxicity or activity will mimic the exposure of their formed counterparts. This communication described observations that showed discrepant kinetics between formed and preformed metabolites in the liver, intestine, and kidney, major drug removal organs. Differences in the observed areas under the curve (AUCs) or the extraction ratios (Es) of formed and preformed metabolites in the liver had been attributed to zonal, enzyme heterogeneity, membrane barriers, or transporters. Preformed and formed metabolite also differed in their handling by the kidney; only the preformed and not the formed metabolite would be filtered. In the intestine, differences in the absorption of the precursor and the metabolite and the flow pattern in the intestine would bring about discrepancy in the time-courses of the formed vs. preformed metabolites. Analytical solutions of the AUCs of the metabolites and extraction ratios, based on physiological modeling of the liver, kidney, and intestine, showed that the AUC of the preformed, administered metabolite was dependent only on metabolite parameters, whereas the AUC of the formed metabolite was modulated additionally by the metabolic, secretory and intestinal absorptive intrinsic clearances of the precursor drug. Hence, administration of the synthetic metabolite would not reflect the toxicity associated with the metabolite formed via bioactivation. However, data on preformed metabolite may be used for simultaneous fitting by a combined model of drug and metabolite. Such a strategy is shown to be successful in risk assessment of environmental chemicals. Upon refinement of the resultant model with data on metabolite transport and handling by modeling and simulations, the resultant model would be more robust to provide improved predictions on metabolite toxicity pursuant to drug administration.

Pharmacokinetics of desethylamiodarone in the rat after its administration as the preformed metabolite, and after administration of amiodarone

The pharmacokinetics of desethylamiodarone (DEA), the active metabolite of amiodarone (AM), were studied in the rat after administration of AM or preformed metabolite. Rats received 10 mg/kg of either intravenous or oral AM HCl or DEA base. Blood samples were obtained via a surgically implanted jugular vein cannula. Plasma concentrations were measured by a validated LC/MS method. In all AM treated rats, AM plasma concentrations greatly exceeded those of the formed DEA. The fraction of AM converted to DEA after i.v. administration was 14%. Amiodarone had a significantly lower (approximately 50%) clearance than DEA, although the volume of distribution and terminal phase half-life did not differ significantly. The hepatic extraction ratio of DEA was 0.48, similar to that of AM (0.51). Oral AM demonstrated higher plasma AUC (5.6 fold) and higher C(max) (6.1 fold) than oral DEA and oral bioavailability of AM (46%) was greater than DEA (17%). The estimated fraction of the oral dose of AM converted to DEA was 4.5 fold higher than after i.v. administration, suggesting first-pass formation of DEA from AM. Amiodarone and DEA differed in their pharmacokinetic characteristics mostly due to a higher CL of DEA. With oral dosing, AM appeared to undergo significant presystemic first-pass metabolism within the intestinal tract.

Extrapolation of diclofenac clearance from in vitro microsomal metabolism data: role of acyl glucuronidation and sequential oxidative metabolism of the acyl glucuronide

Diclofenac is eliminated predominantly (approximately 50%) as its 4'-hydroxylated metabolite in humans, whereas the acyl glucuronide (AG) pathway appears more important in rats (approximately 50%) and dogs (>80-90%). However, previous studies of diclofenac oxidative metabolism in human liver microsomes (HLMs) have yielded pronounced underprediction of human in vivo clearance. We determined the relative quantitative importance of 4'-hydroxy and AG pathways of diclofenac metabolism in rat, dog, and human liver microsomes. Microsomal intrinsic clearance values (CL(int) = V(max)/K(m)) were determined and used to extrapolate the in vivo blood clearance of diclofenac in these species. Clearance of diclofenac was accurately predicted from microsomal data only when both the AG and the 4'-hydroxy pathways were considered. However, the fact that the AG pathway in HLMs accounted for ~75% of the estimated hepatic CL(int) of diclofenac is apparently inconsistent with the 4'-hydroxy diclofenac excretion data in humans. Interestingly, upon incubation with HLMs, significant oxidative metabolism of diclofenac AG, directly to 4'-hydroxy diclofenac AG, was observed. The estimated hepatic CL(int) of this pathway suggested that a significant fraction of the intrahepatically formed diclofenac AG may be converted to its 4'-hydroxy derivative in vivo. Further experiments indicated that this novel oxidative reaction was catalyzed by CYP2C8, as opposed to CYP2C9-catalyzed 4'-hydroxylation of diclofenac. These findings may have general implications in the use of total (free + conjugated) oxidative metabolite excretion for determining primary routes of drug clearance and may question the utility of diclofenac as a probe for phenotyping human CYP2C9 activity in vivo via measurement of its pharmacokinetics and total 4'-hydroxy diclofenac urinary excretion.

Carrier-mediated entry of 4-methylumbelliferyl sulfate: characterization by the multiple-indicator dilution technique in perfused rat liver

The hepatocellular entry of 4-methylumbelliferyl sulfate (4MUS) a highly ionized and highly bound anion capable of futile cycling, was examined in the single-pass albumin-free perfused rat liver preparation. Desulfation of 4MUS to 4-methylumbelliferone (4MU) was verified in vitro to be a low-affinity, high-capacity process (Km = 731 micromol/L; Vmax = 414 nmol min(-1) g(-1) liver). With 4MUS given to the perfused rat liver, sulfation of 4MU, the formed metabolite, was attenuated in the presence of 2,6-dichloro-4-nitrophenol (DCNP), a sulfation inhibitor, and when sulfate ion was substituted by chloride ion. 4MU sulfation, being a high-affinity system, was reduced most effectively at the lowest 4MUS concentration (15 micromol/L) used, evidenced by the increased (24%) net hepatic extraction ratio of 4MUS and reduced utilization (72%) of infused tracer 35SO4(2-) by 4MU for 4MU35S formation. Single-pass multiple indicator dilution (MID) studies were thus conducted under identical conditions (DCNP and absence of inorganic sulfate), with injection of [3H]4MUS and a set of noneliminated vascular and cellular reference indicators into the portal vein (prograde) or hepatic vein (retrograde), against varying background bulk concentrations of 4MUS (5 to 900 micromol/L). The steady-state removal rate of 4MUS and formation rates of 4MU and its glucuronide conjugate (4MUG) were not altered with perfusion flow direction, suggesting the presence of even or parallel distributions of 4MUS desulfation and 4MU glucuronidation activities. When the outflow dilution profile of [3H]4MUS was evaluated with the barrier-limited model of Goresky, a slight red cell carriage effect was found for 4MUS. The permeability surface area product for cellular entry for prograde showed a dramatic concentration-dependent decrease (from 0.13 to 0.01 mL sec(-1) g(-1), or 7.4 to 0.56 times the blood perfusate flow rate) and was resolved as saturable and nonsaturable components, while data for retrograde were more scattered, varying from 2.8 to 1 times the blood perfusate flow rate. Efflux (coefficient = 0.0096 +/- 0.0024 and 0.0088 +/- 0.0062 mL sec(-1) g(-1), respectively) was relatively insensitive to concentration and flow direction. The same was observed for the removal capacity for metabolism and excretion (sequestration coefficient: for prograde, 0.0056 +/- 0.0017 mL sec(-1) g(-1); for retrograde, 0.0056 +/- 0.003 mL sec(-1) g(-1)). The decrease in the apparent partition coefficient (ratio of 4MUS concentration estimated in tissue to unbound plasma concentration) and the increase in relative throughput component with concentration further substantiate the claim on the presence of concentrative processes at the sinusoidal membrane.

New anticonvulsant drugs. Focus on flunarizine, fosphenytoin, midazolam and stiripentol

In the past decade, several new antiepileptic drugs have been tested. Most recently, 5 new antiepileptic drugs have been launched onto European and US markets. These include vigabatrin, oxcarbazepine and lamotrigine in Europe, and felbamate and gabapentin in the US. In addition to these, 3 additional drugs are in the clinical investigational stage: flunarizine, fosphenytoin and stiripentol. A fourth agent is midazolam, which was originally introduced in 1986, but recently has shown effectiveness in the treatment of status epilepticus. Flunarizine is a selective calcium channel blocker that has shown anticonvulsant properties in both animal and human studies. It is a long-acting anticonvulsant that clinical studies have shown to have effects similar to those of phenytoin and carbamazepine in the treatment of partial, complex partial and generalised seizures. Fosphenytoin was developed to eliminate the poor aqueous solubility and irritant properties of intravenous phenytoin. It is rapidly converted to phenytoin after intravenous or intramuscular administration. In clinical studies, this prodrug showed minimal evidence of adverse events and no serious cardiovascular or respiratory adverse reactions. It may have a clear advantage over the present parenteral formulation of phenytoin. Midazolam is a benzodiazepine that is more potent than diazepam as a sedative, muscle relaxant and in its influence on electroencephalographic measures. It has been shown to be an effective treatment for refractory seizures in status epilepticus. Stiripentol has anticonvulsant properties as well as the ability to inhibit the cytochrome P450 system. There are significant metabolic drug interactions between stiripentol and phenytoin, carbamazepine and phenobarbital (phenobarbitone). Stiripentol has been studied in patients with partial seizures, refractory epilepsy and refractory absence seizures with some efficacious results.

A comparative investigation of hepatic clearance models: predictions of metabolite formation and elimination

Liver clearance models serve to improve our understanding of the relationships between the physiological determinants and hepatic clearance and predict changes in the disposition of substrates when homeostasis of the organ is perturbed. Their ability to describe metabolism was presently extended to the sequential formation and elimination of primary (M1), secondary (M2), and tertiary (M3) metabolites during a single passage of drug (P) across the liver, under steady state and first-order conditions. The well-stirred model is distinct from other models in that metabolite formation and elimination is independent of enzymic distributions, the number of steps involved in metabolite formation, and the intrinsic clearances of the precursors. This model predicts that the extraction ratio of a formed primary metabolite derived from drug (E[M1, P]) is identical to that for the preformed primary metabolite (E[M1]), and that the extraction ratios of a secondary metabolite derived from drug (E[M2, P]) and primary metabolite (E[M2, M1]) or preformed secondary metabolite (E[M2]) are identical. For the more physiologically acceptable, parallel-tube and dispersion models, metabolite sequential elimination is highly influenced by the intrinsic clearances of the precursors and the enzymic distributions that mediate removal of precursor species and the metabolites. Furthermore, the extent of sequential metabolism recedes as the number of steps involved for metabolite formation increases. These models predict that E[M1, P] less than E[M1], and E[M2, P] less than E[M2, M1] less than E[M2], with the magnitude of the changes being less for the dispersion model than for the parallel-tube model. Competing pathways that divert substrate from entering the sequential pathway were found to exert only minimal influence on the sequential pathway.

Stereoselective pharmacokinetics and inversion of suprofen enantiomers in humans

The stereoselective pharmacokinetics of suprofen enantiomers has been studied in humans by means of stable isotope-labeled pseudoracemate-diastereomer methodology. After a single oral dose of a near equimolar mixture of unlabeled-(R)-(-)- and [2H3]-(S)-(+)-suprofen [or unlabeled-(S)- and [2H3]-(R)-suprofen] to three healthy male subjects, the plasma concentrations of drug were determined by a stereospecific gas chromatography-mass spectrometry method. Racemic [2H7]suprofen was used as an internal standard. The method involved chiral derivatization with (S)-(-)-1-(naphthyl)ethylamine to form the diastereomeric amide. The plasma concentrations were consistently higher for the (R)-isomer than the (S)-isomer. No significant difference in the elimination half-life of the enantiomers was observed. An average of 6.8% of an administered dose of the (R)-isomer was stereospecifically inverted to the (S)-isomer. There was no measurable inversion of the (S)- to (R)-isomer. The present stable isotope-labeled pseudoracemate-diastereomer methodology has made it possible to evaluate the pharmacokinetics of each enantiomer, including the estimation of chiral inversion after administration of the racemic mixture.

Disposition of methylprednisolone and its sodium succinate prodrug in vivo and in perfused liver of rats: nonlinear and sequential first-pass elimination

The disposition of methylprednisolone (MP) and its prodrug succinate ester, methylprednisolone sodium succinate (MS), were examined both in vivo and in situ (perfused livers) in rats. In vivo studies included iv and oral dosing of 10 or 50 mg/kg of MP in both forms, while liver perfusion involved initial perfusate concentrations of 5 and 25 micrograms/mL of either compound. Steroid concentrations were measured by HPLC. In the intact rat, clearance (CL) values of both compounds were high, twice the hepatic plasma flow, and decreased by one-half after the high dose, indicating nonlinear kinetics. The volumes of distribution of MS and MP were essentially constant with dose. Incomplete availability of MP from iv MS (52-55%) and from the oral dose (10%) was found. Sequential first-pass metabolism was investigated in situ. Extensive hepatic extraction of MP (84%) occurred at the low dose, but decreased to 48% at the high dose, supporting in vivo observations of high CL and nonlinearity. Extraction of MS was also high (83%), but MP availability was slight (8%). The MS and MP data were fitted to a sequential first-pass model yielding an average fraction of MS metabolized-to-MP value of 0.22. The prodrug MS and the active metabolite MP thus demonstrate both systemic and hepatic nonlinearity in rats, and the low availability of MP from iv MS was due, in part, to sequential first-pass elimination. This factor is more extensive in rats than in other species.

Physiological modeling of drug and metabolite: disposition of oxazepam and oxazepam glucuronides in the recirculating perfused mouse liver preparation

The disposition of tracer doses of 3H-oxazepam was studied in the recirculating perfused mouse liver preparation. 3H-Oxazepam was biotransformed primarily to the diastereomeric 3H-oxazepam glucuronides, which either effluxed into the circulation or underwent biliary excretion. Three additional, unknown metabolites constituted a small fraction (5-10%) of the total radioactivity recovered in bile (7% of dose); no other metabolite was detected in perfusate. A physiologically based model, comprising the reservoir, liver blood and tissue, and bile, was fitted to reservoir concentrations of 3H-oxazepam and 3H-oxazepam glucuronides, and the cumulative amount excreted into bile. The model allowed for consideration of elimination pathways other than glucuronidation and the presence of a transport barrier for the oxazepam glucuronides across the hepatocyte membrane. The fitted results suggest a slight barrier existing for the transport of metabolites across the sinusoidal membrane, inasmuch as the transmembrane clearance was comparable to liver blood flow rate. Upon further comparison of estimates of formation, biliary, and transmembrane clearances for the oxazepam glucuronides, the rate-limiting step in the overall (biliary) clearance appears to be a poor capacity for biliary excretion. The influence of the cumulative volume loss that a recirculating perfused organ system incurs upon repeated sampling was discussed, and a compartmental method of correcting the observed concentrations of drug and generated metabolite was presented.

Stable isotope coadministration methodology for the estimation of the fraction of imipramine metabolized to desipramine

The application of a stable isotope coadministration technique for estimating the fraction (fm) of imipramine (IP) that is converted to desipramine (DMI) is described. Four healthy male subjects received 25 mg of IP-d4 hydrochloride orally with 25 mg of DMI hydrochloride. The plasma concentrations of IP-d4, DMI-d4, and DMI were determined by capillary gas chromatography-mass spectrometry-selected ion monitoring using d8 analogues as internal standards. The fm values, calculated from the ratio of the area under the plasma concentration-time curve of DMI-d4 to that of DMI, varied from 0.54 to 0.85.

Primary, secondary, and tertiary metabolite kinetics

Because of the propensity of nascently formed metabolites towards sequential metabolism within formation organs, theoretical and experimental treatments that achieve mass conservation must recognize the various sources contributing to primary, secondary, and tertiary metabolite formation. A simple one-compartment open model, with first-order conditions and the liver as the only organ of drug disappearance and metabolite formation, was used to illustrate the metabolism of a drug to its primary, secondary, and tertiary metabolites, encompassing the cascading effects of sequential metabolism. The concentration-time profiles of the drug and metabolites were examined for two routes of drug administration, oral and intravenous. Formation of the primary metabolite from drug in the gut lumen, with or without further absorption, and metabolite formation arising from first-pass metabolism of the drug and the primary metabolite during oral absorption were considered. Mass balance equations, incorporating modifications of the various absorption and conversion rate constants, were integrated to provide the explicit solutions. Simulations, with and without consideration of the sources of metabolite formation other than from its immediate precursor, were used to illustrate the expected differences in circulating metabolite concentrations. However, a simple relationship between the area under the curve of any metabolite, M, or [AUC (m)], its clearance [CL(m)], and route of drug administration was found. The drug dose, route, fraction absorbed into the portal circulation, Fabs, fraction available of drug from the liver, F, availabilities of the metabolites F(m) from formation organs, and CL(m) are determinants of the AUC(m)'s. After iv drug dosing, the area of any intermediary metabolites is determined by the iv drug dose divided by the (CL(m)/F(m] of that metabolite. When a terminal metabolite is not metabolized, its area under the curve becomes the iv dose of drug divided by the clearance of the terminal metabolite since the available fraction for this metabolite is unity. Similarly, after oral drug administration, when loss of drug in the gut lumen does not contribute to the appearance of metabolites systematically, the general solution for AUC(m) is the product of Fabs and oral drug dose divided by [CL(m)/F(m)]. A comparison of the area ratios of any metabolite after po and iv drug dosing, therefore, furnishes Fabs. When this fraction is divided into the overall systemic availability or Fsys, the drug availability from the first-pass organs, F, may be found.(ABSTRACT TRUNCATED AT 400 WORDS)

A general model of metabolite kinetics following intravenous and oral administration of the parent drug

A model of metabolite pharmacokinetics is developed in terms of residence time distributions and derived non-compartmental measures. It provides quantitative insight into factors determining the concentration-time curve of metabolite following intravenous and oral administration of the precursor drug. The AUCs and higher curve moments (mean residence times and relative dispersions) are calculated/predicted and their dependence on mean absorption time, fraction of first-pass metabolism and intrinsic disposition residence times of the parent drug and metabolite, respectively, is discussed. An AUC-based method for the determination of the first-pass effect is proposed which is not influenced by drug absorption. The approach is valid for linear pharmacokinetic systems exhibiting hepatic and renal elimination of the precursor drug; it is not restricted to specific compartmental models. Limitations of previous concepts of metabolite kinetics are defined. Criteria are presented for the appearance of concave metabolite curves in a semi-logarithmic scale.

Functional group metabolism of dopamine-2 agonists: conversion of 4-(2-di-n-propylaminoethyl)-2-(3H)-indolone to 4-(2-di-n-propylaminoethyl)-7-hydroxy-2-(3H)-indolone

In the previous report, we reported the results of absorption, protein binding, and pharmacokinetics of the dopamine-2 agonists (D2-agonists) 4-(2-di-n-propylaminoethyl)-7-hydroxy-2-(3H)-indolone, N-(2'-hydroxy-5'-[N,N-di-n-propylaminoethylphenyl])methanesulfonamide, and 4-(di-n-propylaminoethyl)-2-(3H)-indolone. Both phenolic compounds, 1 and 2, were subject to more rapid metabolism than the nonphenol 3. In the present study, we investigated the metabolic basis of the differences in the pharmacokinetics of these compounds. In both rats and dogs, the principal urinary metabolite of 1 and 2 was the corresponding glucuronide. In contrast, 3 was first converted to 1 which then was converted to a glucuronide. On the basis of the urinary excretion of 1 and its glucuronide after intravenous administration of 1 and 3, approximately 78% of the dose of 3 in rats and 58% in dogs was converted to 1. The depropyl analogue of 3 was identified as a minor urinary metabolite. 4-(2-Di-n-propylaminoethyl)-7-hydroxy-2-(3H)-indolone was found in the plasma of rats, dogs, and cynomolgus monkeys treated with 3. The concentration of 1 declined in parallel with that of 3 in dogs and monkeys, indicating that the true half-life of 1 is shorter than or equal to that of 3. On the basis of plasma concentrations of 1 in dogs, the apparent conversion of 3 to 1 was 9%.(ABSTRACT TRUNCATED AT 250 WORDS)

A review of metabolite kinetics

The importance of metabolites as active and toxic entities in drug therapy evokes the need for an examination of metabolite kinetics after drug administration. In the present review, emphasis is placed on single-compartmental characteristics for a drug and its primary metabolites under linear kinetic conditions. The determination of the first-order elimination rate constants for drug and metabolite are also detailed. For any ith primary metabolite mi formed solely in liver, kinetic parameters with respect to primary metabolite formation under first-order conditions require a comparison of the areas under the metabolite concentration-time curve after drug and preformed metabolite administrations. These area ratios hold regardless of the number of noneliminating compartments for the drug and metabolite. These parameters include fmi and gmi, the fractions of total body clearance that respectively furnishes mi to the general circulation and forms mi, and hmi, the fraction of hepatic clearance responsible for the formation of mi. Moreover, the fraction of dose dmi converted to form mi is defined with respect to the route of drug administration. The inherent assumption of these estimates, however, requires that the extent of sequential elimination of the generated mi be identical to the extent of metabolism of preformed mi. Discrepancies have been found, and may be attributed mostly to the uneven distribution of drug-metabolizing activities as well as to the presence of diffusional barriers. Other linear systems that involve mi formation from multiple organs are briefly described.

Inhibitory effects of intravenous chloramphenicol sodium succinate on the disposition of phenylbutazone in horses

The effects of i.v. chloramphenicol sodium succinate on the pharmacokinetics of i.v. phenylbutazone in six healthy adult horses were investigated. Administration of chloramphenicol sodium succinate to mares reduced mean (+/- SD) phenylbutazone clearance from 0.600 +/- 0.222 to 0.339 +/- 0.123 ml/min per kg and increased mean (+/- SD) half life from 244 +/- 59.8 to 371 +/- 80.8 min and mean residence time from 333 +/- 86.2 to 533 +/- 124 min. The mean steady-state volume of distribution of phenylbutazone was unchanged, with mean (+/- SD) values of 187 +/- 28.9 ml/kg in control animals and 170 +/- 32.4 ml/kg after chloramphenicol sodium succinate.

Inhibition of epoxidation of carbamazepine by valproic acid in the isolated perfused rat liver

The effect of valproic acid on carbamazepine epoxidation in the perfused liver was investigated in two separate studies. In study I, significant decreases were observed both in the intrinsic clearance of carbamazepine and the intrinsic formation clearance of carbamazepine-epoxide in the presence of therapeutic concentrations of valproate. The same inhibitory effect of valproate was also observed in liver preparations from a group of animals pretreated with carbamazepine. Study II focused on the effect of valproate and carbamazepine on the apparent Michaelis-Menten parameters (Vmax,m, Km,m) associated with the intrinsic formation clearance of carbamazepine-epoxide in the perfused liver. Valproate had no statistically significant effect on either the Vmax,m or the Km,m of epoxidation, although the Km,m value was 43% higher in the presence of valproate. However, the ratio of Vmax,m and Km,m (intrinsic formation clearance) was significantly reduced by valproate. The Vmax,m and Km,m values obtained in study II predicted a significant decrease in the intrinsic formation clearance of carbamazepine-epoxide, consistent with the results of study I. Carbamazepine pretreatment was associated with significant increases in apparent Vmax,m and Km,m of epoxide formation.

Fractions metabolized in a triangular metabolic system: cinromide and two metabolites in the rhesus monkey

A previous study of the metabolic fate of cinromide (3-bromo-N-ethylcinnamamide) in rhesus monkey established that half of a dose is metabolized by N-deethylation to an active metabolite, 3-bromocinnamamide. Both cinromide and its proximal metabolite can be metabolized by amide hydrolysis to a second metabolite, 3-bromocinnamic acid, resulting in a triangular metabolic problem. This investigation was undertaken to distinguish between these two nonexclusive possibilites. A preliminary study was carried out to characterize the pharmacokinetics of 3-bromocinnamic acid. In the main study, six monkeys received an intravenous dose of cinromide, 3-bromocinnamamide, and 3-bromocinnamic acid in a randomized order. The time courses of compound administered and corresponding metabolites were followed. The following fractions of dose metabolized (mean +/- SD) were obtained: cinromide to 3-bromocinnamide: 0.53 +/- 0.24; 3-bromocinnamamide to 3-bromocinnamic acid: 0.53 +/- 0.21; cinromide to 3-bromocinnamic acid directly: 0.48 +/- 0.32. Thus, it was found that 3-bromocinnamic acid is formed directly from cinromide and from 3-bromocinnamamide. Also, as primary metabolites, 3-bromocinnamic acid and 3-bromocinamamide account for all of a cinromide dose with a mean value of 1.00 +/- 0.34. The observed variability in these fractions metabolized was explained by the fact that in the solution of the triangular metabolic problem, three clearances are assumed to remain constant over three studies.

Phenytoin Prodrugs V: In Vivo Evaluation of Some Water-Soluble Phenytoin Prodrugs in Dogs

Phenytoin bioavailability was evaluated in beagle dogs after oral and intravenous administrations of sodium phenytoin and two amino acyl esters and a disodium phosphate ester of 3-(hydroxymethyl)phenytoin (three prodrugs of phenytoin). Phenytoin displayed nonlinear pharmacokinetics in the dogs, complicating the determination of the absolute bioavailability of phenytoin from sodium phenytoin and the prodrugs. All three prodrugs essentially released phenytoin after intravenous administration in a quantitative manner, and all gave plasma levels of phenytoin after oral administration greater than those found after administration of sodium phenytoin. Based on the behavior in dogs and the earlier determination of the physicochemical properties of the prodrugs, it was concluded that one of the amino acyl esters, 3-(hydroxymethyl)-5,5-diphenylhydantoin N,N-dimethylglycine ester methanesulfonate, would be the most useful prodrug for oral administration, while 3-(hydroxymethyl)-5,5-diphenylhydantoin disodium phosphate ester would be the most useful for parenteral administration.

Drug metabolite concentration-time profiles: influence of route of drug administration

In order to assess the contribution of an active metabolite to the overall pharmacological response following drug administration it is necessary to characterise the metabolite concentration-time profile. The influence of route of drug administration on metabolite kinetics has been investigated by computer simulation. Comparisons between simulated profiles and published concentration-time data have been carried out. A route dependence in metabolite concentration-time curves is readily apparent provided the metabolite kinetics are formation rate limited and the hepatic clearance of drug is greater than 25 l/h (medium to highly cleared). Oral drug administration produces a triphasic metabolite concentration-time profile whereas only two phases are discernable after intravenous drug administration. The magnitude of the difference in maximum metabolite concentration is directly proportional to the hepatic clearance of drug due to first-pass metabolite production. The route dependence in the shape of the metabolite concentration-time curves is most dramatic when the absorption and distribution of drug and the elimination of metabolite is rapid. A reduction in the rate of either of these processes alters the shape of the metabolite concentration-time profile such that the consequence of first-pass metabolite formation may be reduced.

An understanding of the role of enzyme localization of the liver on metabolite kinetics: a computer simulation

The metabolic sequence of drug, D, to its primary (MI) and terminal (MII) metabolites as mediated by enzymes A and B, respectively, was chosen to illustrate metabolizing activities among hepatocytes in different regions of the liver lobule. Six models of distributions of the hepatocellular activities (intrinsic clearances for A and B) were defined with respect to the flow path in liver, and the concentrations D, MI, and MII in the liver were simulated. The extent of sequential metabolism of the primary metabolite was compared for these six models of enzymic distributions. It was found that when the average hepatic intrinsic clearances of A and B were high (almost complete extraction of both drug and primary metabolite during their single passage through the liver), the distributions of A and B were not important determinants of metabolite kinetics. By contrast, when the average hepatic intrinsic clearances of A and B were both low, the distributions of A and B exerted profound effects on metabolite kinetics. The sensitivity to enzymic distribution in this region, however, was difficult to assess due to difficulties in detecting low levels of MI and MII. The effects of enzymic distributions on metabolite disposition would be better detected in compounds (drug and metabolite) with intermediate extraction ratios.

Metabolism of isosorbide dinitrate in the isolated perfused rabbit lung

The uptake and metabolism of isosorbide dinitrate was investigated in the recirculating isolated perfused rabbit lung and in lung homogenate 9000 X g supernatant. Concentration versus time profiles from the isolated lung experiments indicate rapid metabolism of isosorbide dinitrate and corresponding increases in the metabolites 5-isosorbide mononitrate, 2-isosorbide mononitrate, and isosorbide. The data suggest that the mononitrates formed in the lung tissue were converted to isosorbide at an extraordinarily high rate. Surprisingly, the rate of appearance of completely denitrated isosorbide was greater when isosorbide dinitrate was administered to the lung than when the mononitrate metabolites of isosorbide dinitrate were administered. The results suggest rapid metabolism of a substantial portion of the mononitrates formed endogenously from isosorbide dinitrate before partitioning of mononitrates into the perfusion medium could occur. The metabolism of isosorbide dinitrate in lung homogenate 9000 X g supernatant exhibited a metabolic scheme kinetically different from the intact lung studies, as isosorbide was formed slowly from a mononitrate intermediate and not by a near-simultaneous cleavage of both nitrate ester groups. Intravascular multiple-dose studies did not demonstrate any inhibition between isosorbide dinitrate and the mononitrates.

Pharmacokinetics of triamterene and its metabolite in man

The pharmacokinetic profiles of triamterene and hydroxytriamterene sulfuric acid ester, the major metabolite of triamterene, were studied in six normal male volunteers using a newly developed specific HPLC analytical method. Following a 100 mg oral dose of triamterene, the plasma concentration time course of the sulfate conjugate parallels that of triamterene in all subjects, but concentrations of the metabolite were more than 10 times higher than unchanged triamterene concentrations at identical sampling times. Interestingly, the renal clearance of the sulfate conjugate was less than that of triamterene. These characteristic features of triamterene disposition were fitted to a compartment model incorporating a first-pass metabolic process. Unbound fractions of triamterene and metabolite in plasma were 0.39 and 0.10 (mean of 6 subjects), respectively. The low unbound fraction of the metabolite in plasma most probably accounts for the low renal clearance of the sulfate conjugate as compared with triamterene.

Metabolite pharmacokinetics: the area under the curve of metabolite and the fractional rate of metabolism of a drug after different routes of administration for renally and hepatically cleared drugs and metabolites

A model comprised of four compartments, a central and liver compartment for a drug, and a central and liver compartment for a metabolite, is presented to describe the interrelationships between the area under the curve of the metabolite and physiological parameters after intravenous and intraportal administration of the drug. The model includes renal and hepatic eliminatory mechanisms for both drug and metabolite as long as the metabolite is formed only by the liver. It is found that when competing renal eliminatory pathways exist for a drug, the area under the curve for the metabolite will change according to the route of drug administration. Also, the fractional rate of metabolism of a drug to form the metabolite will be underestimated by the normal use of the ratio areas under the curve of the metabolite. Other properties of the model are also discussed.

Metabolite to parent drug concentration ratio as a function of parent drug extraction ratio: cases of nonportal route of administration

The ratio of metabolite to parent drug concentration (Cm/Cp) of a medium or high extraction ratio (E greater than 0.1) drug administered intravenously has been shown to depend on intrinsic clearance of drug by other metabolic routes (CLr,int) as well as on organ blood flow (Q). In contrast, for a low extraction ratio drug given intravenously or for any drug given by a portal route, this ratio is equal to the ratio of formation clearance (CLf,int) and metabolite clearance (CLm,int). The sensitivity of Cm/Cp to changes in CLr,int and CLf,int has been analyzed quantitatively. It was shown to be dependent on the fraction metabolized to that particular metabolite (fm).

Saturable metabolism and its relationship to toxicity

Metabolism plays a central role in regulating the toxicity of a variety of chemicals. Relatively innocuous substances can be converted to highly toxic metabolites. Conversely, toxic substances can be biotransformed to less harmful metabolites or be excreted, thus limiting their duration of biological action. Virtually all metabolism and many excretory processes utilize specific binding proteins, i.e., enzymes and carrier proteins. These metabolic and carrier-mediated excretory clearance pathways are capacity-limited, becoming saturated at sufficiently high substrate concentrations. Saturable metabolic clearance processes lead to dose-dependent pharmacokinetics for many chemicals. When dose-dependent pharmacokinetics prevail, internally significant parameters, such as area under the curve for concentration of toxicant at active sites and the amount of metabolite formed during inhalation exposure, are not linearly related to externally significant parameters such as administered dose or inspired concentration. Dose-response curves should relate observed effects to some internally significant parameter. Toxic response should often be indexed to area under the curve relationships or total amount metabolized, instead of dose or inspired concentration. The former parameters are complexly related to the latter. The nature of the relationship depends on the kinetic constants for metabolic and excretory clearance. Pharmacokinetic analyses of dose-dependent clearance mechanisms provide an understanding of how one transforms externally significant parameters to internally significant parameters under various exposure conditions. Consideration of metabolic clearance at the organ level illuminates the importance of physiological factors, showing unequivocally that blood flow may be rate-limiting for metabolism under many exposure conditions. Recognition of the potential for this behavior is essential to the proper design and evaluation of certain toxicological experimentation. Development of comprehensive pharmacokinetic descriptions of the influence of saturable clearance on delivery of active chemical to target sites augurs well for improving both intraspecies and interspecies extrapolation of toxicity data. This is a critical area of contemporary toxicology. Dose selection for chronic studies could also be improved by knowledge of the dose-dependence of pharmacokinetic parameters in proposed test species. The field of toxicology reviewed here represents an interface between pharmacokinetic research and studies on basic mechanisms of toxic action. It entails utilization of quantitative concepts to better understand the physiological and biochemical controls which regulate the expression of the toxicity of various chemicals. Much work remains to be accomplished in this exciting area of toxicological research. Some of the predictions of the pharmacokinetic analyses are still tentative and require more definitive experimentation...