Individual variation in first-pass metabolism

Pharmacokinetic characterisation of a valproate Autism Spectrum Disorder rat model in a context of co-exposure to α-Hexabromocyclododecane

Assessing the role of α-hexabromocyclododecane α-HBCDD as a factor of susceptibility for Autism Spectrum disorders by using valproic acid-exposed rat model (VPA) required characterizing VPA pharmacokinetic in the context of α-HBCDD-co-exposure in non-pregnant and pregnant rats. The animals were exposed to α-HBCDD by gavage (100 ng/kg/day) for 12 days. This was followed by a single intraperitoneal dose of VPA (500 mg/kg) or a daily oral dose of VPA (500 mg/kg) for 3 days. Exposure to α-HBCDD did not affect the pharmacokinetics of VPA in pregnant or non-pregnant rats. Surprisingly, VPA administration altered the pharmacokinetics of α-HBCDD. VPA also triggered higher foetal toxicity and lethality with the PO than IP route. α-HBCDD did not aggravate the embryotoxicity observed with VPA, regardless of the route of exposure. Based on this evidence, a single administration of 500 mg/kg IP is the most suitable VPA model to investigate α-HBCDD co-exposure.

Communication Protocols Integrating Wearables, Ingestibles, and Implantables for Closed-Loop Therapies

Body-conformal sensors and tissue interfacing robotic therapeutics enable the real-time monitoring and treatment of diabetes, wound healing, and other critical conditions. By integrating sensors and drug delivery devices, scientists and engineers have developed closed-loop drug delivery systems with on-demand therapeutic capabilities to provide just-in-time treatments that correspond to chemical, electrical, and physical signals of a target morbidity. To enable closed-loop functionality in vivo, engineers utilize various low-power means of communication that reduce the size of implants by orders of magnitude, increase device lifetime from hours to months, and ensure the secure high-speed transfer of data. In this review, we highlight how communication protocols used to integrate sensors and drug delivery devices, such as radio frequency communication (e.g., Bluetooth, near-field communication), in-body communication, and ultrasound, enable improved treatment outcomes.

Androgen aggravates aortic aneurysms via suppressing PD-1 in mice

Androgen has long been recognized for its pivotal role in the sexual dimorphism of cardiovascular diseases, including aortic aneurysms, a devastating vascular disease with a higher prevalence and mortality rate in men than women. However, the molecular mechanism by which androgen mediates aortic aneurysms is largely unknown. Here, we report that male but not female mice develop aortic aneurysms in response to aldosterone and high salt (Aldo-salt). We demonstrate that both androgen and androgen receptors (AR) are crucial for the sexually dimorphic response to Aldo-salt. We identify T cells expressing programmed cell death protein 1 (PD-1), an immune checkpoint molecule important in immunity and cancer immunotherapy, as a key link between androgen and aortic aneurysms. We show that intraperitoneal injection of anti-PD-1 antibody reinstates Aldo-salt-induced aortic aneurysms in orchiectomized mice. Mechanistically, we demonstrate that AR binds to the PD-1 promoter to suppress its expression in the spleen. Hence, our study reveals an important but unexplored mechanism by which androgen contributes to aortic aneurysms by suppressing PD-1 expression in T cells. Our study also suggests that cancer patients predisposed to the risk factors of aortic aneurysms may be advised to screen for aortic aneurysms during immune checkpoint therapy.

A Nanoparticle's Journey to the Tumor: Strategies to Overcome First-Pass Metabolism and Their Limitations

Simple Summary

Traditional cancer therapeutics suffer from off-target toxicity, limiting their effective dose and preventing patients’ tumors from being sufficiently treated by chemotherapeutics alone. Nanomedicine is an emerging class of therapeutics in which a drug is packaged into a nanoparticle that promotes uptake of the drug at a tumor site, shielding it from uptake by peripheral organs and enabling the safe delivery of chemotherapeutics that have poor aqueous solubility, short plasma half-life, narrow therapeutic window, and toxic side effects. Despite the advantages of nanomedicines for cancer, there remains significant challenges to improve uptake at the tumor and prevent premature clearance from the body. In this review, we summarize the effects of first-pass metabolism on a nanoparticle’s journey to a tumor and outline future steps that we believe will improve the efficacy of cancer nanomedicines.

Abstract

Nanomedicines represent the cutting edge of today’s cancer therapeutics. Seminal research decades ago has begun to pay dividends in the clinic, allowing for the delivery of cancer drugs with enhanced systemic circulation while also minimizing off-target toxicity. Despite the advantages of delivering cancer drugs using nanoparticles, micelles, or other nanostructures, only a small fraction of the injected dose reaches the tumor, creating a narrow therapeutic window for an otherwise potent drug. First-pass metabolism of nanoparticles by the reticuloendothelial system (RES) has been identified as a major culprit for the depletion of nanoparticles in circulation before they reach the tumor site. To overcome this, new strategies, materials, and functionalization with stealth polymers have been developed to improve nanoparticle circulation and uptake at the tumor site. This review summarizes the strategies undertaken to evade RES uptake of nanomedicines and improve the passive and active targeting of nanoparticle drugs to solid tumors. We also outline the limitations of current strategies and the future directions we believe will be explored to yield significant benefits to patients and make nanomedicine a promising treatment modality for cancer.

A Pharmacokinetic and Metabolism Study of the TRPC6 Inhibitor SH045 in Mice by LC-MS/MS

TRPC6, the sixth member of the family of canonical transient receptor potential (TRP) channels, contributes to a variety of physiological processes and human pathologies. This study extends the knowledge on the newly developed TRPC6 blocker SH045 with respect to its main target organs beyond the description of plasma kinetics. According to the plasma concentration-time course in mice, SH045 is measurable up to 24 h after administration of 20 mg/kg BW (i.v.) and up to 6 h orally. The short plasma half-life and rather low oral bioavailability are contrasted by its reported high potency. Dosage limits were not worked out, but absence of safety concerns for 20 mg/kg BW supports further dose exploration. The disposition of SH045 is described. In particular, a high extravascular distribution, most prominent in lung, and a considerable renal elimination of SH045 were observed. SH045 is a substrate of CYP3A4 and CYP2A6. Hydroxylated and glucuronidated metabolites were identified under optimized LC-MS/MS conditions. The results guide a reasonable selection of dose and application route of SH045 for target-directed preclinical studies in vivo with one of the rare high potent and subtype-selective TRPC6 inhibitors available.

Effects of Absorption Kinetics on the Catabolism of Melatonin Released from CAP-Coated Mesoporous Silica Drug Delivery Vehicles

Melatonin (MLT) is a pineal hormone involved in the regulation of the sleep/wake cycle. The efficacy of exogenous MLT for the treatment of circadian and sleep disorders is variable due to a strong liver metabolism effect. In this work, MLT is encapsulated in mesoporous silica (AMS-6) with a loading capacity of 28.8 wt%, and the mesopores are blocked using a coating of cellulose acetate phthalate (CAP) at 1:1 and 1:2 AMS-6/MLT:CAP ratios. The release kinetics of MLT from the formulations is studied in simulated gastrointestinal fluids. The permeability of the MLT released from the formulations and its 6-hydroxylation are studied in an in vitro model of the intestinal tract (Caco-2 cells monolayer). The release of MLT from AMS-6/MLT:CAP 1:2 is significantly delayed in acidic environments up to 40 min, while remaining unaffected in neutral environments. The presence of CAP decreases the absorption of melatonin and increases its catabolism into 6-hydroxylation by the cytochrome P450 enzyme CYP1A2. The simple confinement of melatonin into AMS-6 pores slightly affects the permeability and significantly decreases melatonin 6-hydroxylation. Measurable amounts of silicon in the basolateral side of the Caco-2 cell monolayer might suggest the dissolution of AMS-6 during the experiment.

Current Treatment Options for HCC: From Pharmacokinetics to Efficacy and Adverse Events in Liver Cirrhosis

BACKGROUND

Hepatocellular carcinoma (HCC) is among the world's most common cancers. For over ten years, the only medical treatment for it has been the multikinase inhibitor Sorafenib. Currently, however, other first or second-line therapeutic options have also shown efficacy against HCC, such as multikinase inhibitors (Regorafenib, Lenvatinib, and Cabozantinib), a monoclonal antibody against the vascular endothelial growth factor receptor 2 (Ramucirumab), and immune-checkpoint inhibitors (Nivolumab, Pembrolizumab, Ipilimumab).

AIM

The aim of this paper is to review the metabolic pathways of drugs that have been tested for the treatment of HCC and the potential influence of liver failure over those pathways.

METHODS

The Food and Drug Administration (FDA)'s and European Medicines Agency (EMA)'s datasheets, results from clinical trials and observational studies have been reviewed.

RESULTS

This review summarizes the current knowledge regarding targets, metabolic pathways, drug interactions, and adverse events of medical treatments for HCC in cirrhotic patients.

CONCLUSION

The new scenario of systemic HCC therapy includes more active drugs with different metabolic pathways and different liver adverse events. Clinical and pharmacological studies providing more data on the safety of these molecules are urgently needed.

Investigating the bioavailabilities of olerciamide A via the rat's hepatic, gastric and intestinal first-pass effect models

Olerciamide A (OA) is a new alkaloid isolated from Portulaca oleracea L. that has been proved to possess a low bioavailability (F) after oral administration in rats in our previous study. Hence, to clarify the reasons for its low bioavailability, hepatic, gastric and intestinal first-pass effect models were established, and a rapid, sensitive UHPLC method was validated and applied for the determination after dosing via the femoral, portal, gastric and intestinal routes. As inhibitors of CYP3A and P-gp, verapamil, midazolam and borneol in low and high dose groups were selected to improve the low bioavailability of olerciamide A. Moreover, a rectal administration method was also carried out to improve the bioavailability of olerciamide A. The results showed that the bioavailability of olerciamide A using hepatic, gastric and intestinal routes were 92.16%, 84.88% and 5.76%, respectively. The areas under the plasma concentration-time curve from zero to infinity (AUC_{0 → ∞} ) were increased a little after being dosed with 10 and 30 mg/kg verapamil (p > 0.05), but markedly increased after being dosed with 0.4 and 1.2 mg/kg midazolam as well as 8 and 24 mg/kg borneol (p < 0.05). Besides, the AUC_0 → ∞ values after the lower and upper rectal administrations were separately similar to the intravenous and intraportal administrations. Our study showed that the intestinal first-pass effect mainly contributed to the low bioavailability of olerciamide A in rats due to it being a substrate of CYP3A and P-gp as well as to its poor intestinal absorption.

Single oral dose pharmacokinetics of decursin and decursinol angelate in healthy adult men and women

The ethanol extract of Angelica gigas Nakai (AGN) root has promising anti-cancer and other bioactivities in rodent models. It is currently believed that the pyranocoumarin isomers decursin (D) and decursinol angelate (DA) contribute to these activities. We and others have documented that D and DA were rapidly converted to decursinol (DOH) in rodents. However, our in vitro metabolism studies suggested that D and DA might be metabolized differently in humans. To test this hypothesis and address a key question for human translatability of animal model studies of D and DA or AGN extract, we conducted a single oral dose human pharmacokinetic study of D and DA delivered through an AGN-based dietary supplement Cogni.Q (purchased from Quality of Life Labs, Purchase, NY) in twenty healthy subjects, i.e., 10 men and 10 women, each consuming 119 mg D and 77 mg DA from 4 vegicaps. Analyses of plasma samples using UHPLC-MS/MS showed mean time to peak concentration (T_max) of 2.1, 2.4 and 3.3 h and mean peak concentration (C_max) of 5.3, 48.1 and 2,480 nmol/L for D, DA and DOH, respectively. The terminal elimination half-life (t_1/2) for D and DA was similar (17.4 and 19.3 h) and each was much longer than that of DOH (7.4 h). The mean area under the curve (AUC_0-48h) for D, DA and DOH was estimated as 37, 335 and 27,579 h∙nmol/L, respectively. Gender-wise, men absorbed the parent compounds faster and took shorter time to reach DOH peak concentration. The human data supported an extensive conversion of D and DA to DOH, even though they metabolized DA slightly slower than rodents. Therefore, the data generated in rodent models concerning anti-cancer efficacy, safety, tissue distribution and pharmacodynamic biomarkers will likely be relevant for human translation.

Therapeutic drug monitoring in a developing nation: a clinical guide

Therapeutic drug monitoring is aimed at using drug concentration measurements to manage a patient's medication requirement and optimise clinical outcome, particularly in respect of drugs with narrow therapeutic index. Typically, immunoassay methods of various techniques are employed with the advantage of rapid turnaround time and ease of operation. The chromatographic methods are specific and cost effective, though more demanding and require technical expertise. The most crucial aspect of any therapeutic drug monitoring service is the expert clinical interpretation of drug concentration measurements taking into consideration individual pharmacokinetic variability in drug disposition across different populations. The setting up of a therapeutic drug monitoring service requires enormous resources, both in terms of equipment and trained personnel. This poses considerable constraints in developing countries due to limited scarce resources, coupled with ignorance among health practitioners on the relevance of therapeutic drug monitoring in clinical practice. Consequently, the need for advocacy, training and encouragement of health practitioners on the usefulness of therapeutic drug monitoring in enhancing patient care and overall clinical outcome in a developing country such as Nigeria can never be over-emphasised.

A cooperative computational and experimental investigation on electrochemical behavior of metoprolol and its voltammetric determination

The electrochemical behavior of metoprolol (MTP) was studied via experimental and computational approaches. Theoretical calculations were performed at the B3LYP/6-31+G(d)//AM1 level whereas experimental studies were carried out on a hanging mercury drop electrode (HMDE) and glassy carbon electrode (GCE). According to the computational results, both HOMO and LUMO of MTP were located at the phenyl ring. Hence, oxidation and reduction are expected to take place at the phenyl ring. Experimental studies on HMDE were based on reversible reduction at approximately –1.4 V and studies on GCE were based on irreversible oxidation at approximately 0.9 V versus Ag/AgCl (3 mol L−1 KCl) in Britton−Robinson buffer. Voltammetric methods with and without adsorptive stripping modes were developed. Proposed methods were successfully applied to tablet solutions and spiked human serum samples. Results are satisfactory with recovery values between 94.5% and 102.5% and a relative standard deviation lower than 6%.

Intestinal absorption, metabolism, and excretion of (-)-epicatechin in healthy humans assessed by using an intestinal perfusion technique

BACKGROUND

(-)-Epicatechin is a dietary flavonoid present in many foods that affects vascular function, but its action is limited by incomplete absorption, conjugation, and metabolism. Factors that influence this activity may be attributed to instability in the gastrointestinal lumen, low permeability across the intestinal wall, or active efflux from enterocytes and extensive conjugation.

OBJECTIVE

With the use of a multilumen perfusion catheter, we investigated the jejunal absorption, systemic availability, metabolism, and intestinal, biliary, and urinary excretion of (-)-epicatechin in humans.

DESIGN

In a single-center, randomized, open, controlled study in 8 healthy volunteers, 50 mg purified (-)-epicatechin was perfused into an isolated jejunal segment together with antipyrine as a marker for absorption. (-)-Epicatechin and conjugates were measured in intestinal perfusates, bile, plasma, and urine.

RESULTS

Forty-six percent of the dose was recovered in the perfusate either as unchanged (-)-epicatechin (22 mg) or conjugates (0.8 mg); with stability taken into account, this result indicates that ∼46% of the dose had apparently been absorbed. The conjugates were predominantly sulfates, which indicated conjugation by sulfotransferases followed by efflux from the enterocytes. In contrast, epicatechin glucuronides were dominant in plasma, bile, and urine.

CONCLUSIONS

Almost one-half of the (-)-epicatechin is apparently absorbed in the jejunum but with substantial interindividual differences in the extent of absorption. The data suggest that the nature and substitution position of (-)-epicatechin conjugation are major determinants of the metabolic fate in the body, influencing whether the compound is effluxed into the lumen or absorbed into the blood and subsequently excreted.

Simultaneous determination of metoprolol and α-hydroxymetoprolol in human plasma using excitation–emission matrix fluorescence coupled with second-order calibration methods

Background: Metoprolol (MET) is a β1-adrenoceptor antagonist, which is widely used in the treatment of cardiovascular diseases, and α-hydroxymetoprolol (α-OHM) is its hydroxylated metabolite. Owing to their similar structures, optimization of the condition for the chromatography approach, which is in common use for determination, is both time consuming and laborious. Results: A new and effective strategy that combines the excitation–emission matrix fluorescence with second-order calibration methods was developed for simultaneous determination of MET and α-OHM in human plasma. Conclusion: Although the fluorescence spectra of MET and α-OHM overlapped and a large number of unknown and uncalibrated fluorescent components coexisted, the developed method enables accurate concentrations together with reasonable resolution of excitation and emission profiles for the analytes of interest. An additional advantage of the proposed method is that there is no need for separation and sample pretreatment, in addition to lower cost than traditional methods.

Loss of orally administered drugs in GI tract

The aim of this review is to provide a broad perspective on intestinal absorption and the impact of intestinal first-pass metabolism on both clearance and drug-drug interaction prediction along with its historical perspectives. The review also considers abilities to bridge the gap between the increasing amount of intestinal in vitro data and the importance of intestinal first-pass metabolism in vivo. The significance of efflux transporters on the intestinal absorption is also discussed.

Comparison of serum bisphenol A concentrations in mice exposed to bisphenol A through the diet versus oral bolus exposure

BACKGROUND

Bisphenol A (BPA) is a widely produced endocrine-disrupting chemical. Diet is a primary route of exposure, but internal exposure (serum concentrations) in animals and humans has been measured only after single oral bolus administration.

OBJECTIVE

We compared serum concentrations of BPA over a 24-hr period after oral bolus administration or ad libitum feeding in mice and assessed for buildup with dietary exposure.

METHODS

Adult female mice were administered [dimethyl-d6]-BPA (BPA-d6) as a single oral bolus (20 mg/kg body weight) or fed a diet containing 100 mg BPA-d6/kg feed weight ad libitum for 1 week. Serum concentrations were analyzed using isotope dilution liquid chromatography coupled with electrospray tandem mass spectrometry and compared between exposure groups over the first 23 hr and after 7 days of dietary exposure.

RESULTS

Maximum concentration (Cmax) for BPA-d6 during the first 24 hr was reached at 1 hr and 6 hr for oral bolus and diet groups, respectively. Relative BPA-d6 bioavailability (unconjugated BPA-d6) was higher in diet-exposed mice than in the bolus group despite a relative lower absorption, a phenomenon consistent with an inhibitory effect of food on first-pass hepatic metabolism. In mice with ongoing dietary exposure, unconjugated BPA-d6 was higher on day 7 than on day 1.

CONCLUSIONS

This is the first report of serum BPA concentrations in an animal model exposed to this chemical via the diet. Although bolus administration of BPA-d6 led to peak concentrations within 1 hr, Cmax for diet-exposed mice was delayed for several hours. However, bolus administration underestimates bioavailable serum BPA concentrations in animals-and presumably humans-than would result from dietary exposure. Exposure via diet is a more natural continuous exposure route than oral bolus exposure and is thus a better predictor of BPA concentrations in chronically exposed animals and humans.

Preparation and evaluation of a novel delayed-onset sustained-release system of propranolol hydrochloride

The objective of this work was to prepare and evaluate a new delayed-onset sustained-release system, comprising a sustained-release core tablet with hydroxypropyl methylcellulose as polymer matrix and an ethylcellulose/Eudragit L coating capable of delaying the drug release. The sustained core containing propranolol hydrochloride as the model drug was prepared by granulate tableting and the polymer coating was applied in a computer-controlled coating pan. The dissolution tests demonstrated that the in-vitro drug release was pH-dependent with sufficient gastric resistance, and the lag time (t10%) could be controlled by adjusting the coating level. Three dosage forms including commercial tablet, sustained-release tablet and the delayed-onset sustained-release tablet were administrated to six beagle dogs and the plasma levels of propranolol hydrochloride were measured with high-performance liquid chromatography. The delayed-onset sustained-release tablet had a lag time of 3.0 h in-vitro and 3.5 h in-vivo, and a tmax of 7.0 h. The relative bioavailability for delayed-onset sustained-release tablet was 96.98% compared with commercial tablets. The results indicate that the new propranolol delayed-onset sustained-release system could achieve a relatively constant drug release followed by a programmed lag time, and this may provide a promising drug delivery form for chronopharmacotherapy of certain cardiovascular diseases.

Pharmacokinetics of 1,8-cineole, a dietary toxin, in the brushtail possum (Trichosurus vulpecula): Significance for feeding

1,8-Cineole (cineole) is a Eucalyptus leaf toxin that defends against predation by herbivores such as the brushtail possum (Trichosurus vulpecula). The aim of the current study was to characterize the pharmacokinetics of cineole in the possum to improve understanding about how possums can avoid cineole toxicity when eating a Eucalyptus diet. Nine male possums were trapped in the wild and acclimated to captivity; a subcutaneous port was then implanted for venous blood sampling. Cineole was administered intravenously (10 and 15 mg kg(-1)) via a lateral tail vein and orally (30, 100 and 300 mg kg(-1)) by gavage, and blood concentrations of cineole and its metabolites were determined by gas chromatography. Cineole had a large terminal volume of distribution (V(z) = 27 l kg(-1)) and a high clearance (43 ml min(-1) kg(-1)), equal to hepatic blood flow. The terminal half-life was approximately 7 h. Oral bioavailability was low (F = 0.05) after low doses, but increased tenfold with dose, probably due to saturable first-pass metabolism. These findings indicate that when possums feed on a cineole diet, they eat until the cineole consumed is sufficient to saturate pre-systemic metabolism, leading to a rapid rise in bioavailability and cineole blood levels, and a cessation of the feeding bout. This is the first report on the pharmacokinetics of a dietary toxin in a wild herbivore, and provides insights into the interactions between the blood concentration of a plant secondary metabolite and the browsing behaviour of a herbivore.

Pharmacokinetics and dosage adjustment in patients with hepatic dysfunction

The liver plays a central role in the pharmacokinetics of the majority of drugs. Liver dysfunction may not only reduce the blood/plasma clearance of drugs eliminated by hepatic metabolism or biliary excretion, it can also affect plasma protein binding, which in turn could influence the processes of distribution and elimination. Portal-systemic shunting, which is common in advanced liver cirrhosis, may substantially decrease the presystemic elimination (i.e., first-pass effect) of high extraction drugs following their oral administration, thus leading to a significant increase in the extent of absorption. Chronic liver diseases are associated with variable and non-uniform reductions in drug-metabolizing activities. For example, the activity of the various CYP450 enzymes seems to be differentially affected in patients with cirrhosis. Glucuronidation is often considered to be affected to a lesser extent than CYP450-mediated reactions in mild to moderate cirrhosis but can also be substantially impaired in patients with advanced cirrhosis. Patients with advanced cirrhosis often have impaired renal function and dose adjustment may, therefore, also be necessary for drugs eliminated by renal exctretion. In addition, patients with liver cirrhosis are more sensitive to the central adverse effects of opioid analgesics and the renal adverse effects of NSAIDs. In contrast, a decreased therapeutic effect has been noted in cirrhotic patients with beta-adrenoceptor antagonists and certain diuretics. Unfortunately, there is no simple endogenous marker to predict hepatic function with respect to the elimination capacity of specific drugs. Several quantitative liver tests that measure the elimination of marker substrates such as galactose, sorbitol, antipyrine, caffeine, erythromycin, and midazolam, have been developed and evaluated, but no single test has gained widespread clinical use to adjust dosage regimens for drugs in patients with hepatic dysfunction. The semi-quantitative Child-Pugh score is frequently used to assess the severity of liver function impairment, but only offers the clinician rough guidance for dosage adjustment because it lacks the sensitivity to quantitate the specific ability of the liver to metabolize individual drugs. The recommendations of the Food and Drug Administration (FDA) and the European Medicines Evaluation Agency (EMEA) to study the effect of liver disease on the pharmacokinetics of drugs under development is clearly aimed at generating, if possible, specific dosage recommendations for patients with hepatic dysfunction. However, the limitations of the Child-Pugh score are acknowledged, and further research is needed to develop more sensitive liver function tests to guide drug dosage adjustment in patients with hepatic dysfunction.

Pharmacokinetics and dosage adjustment in patients with hepatic dysfunction

The liver plays a central role in the pharmacokinetics of the majority of drugs. Liver dysfunction may not only reduce the blood/plasma clearance of drugs eliminated by hepatic metabolism or biliary excretion, it can also affect plasma protein binding, which in turn could influence the processes of distribution and elimination. Portal-systemic shunting, which is common in advanced liver cirrhosis, may substantially decrease the presystemic elimination (i.e., first-pass effect) of high extraction drugs following their oral administration, thus leading to a significant increase in the extent of absorption. Chronic liver diseases are associated with variable and non-uniform reductions in drug-metabolizing activities. For example, the activity of the various CYP450 enzymes seems to be differentially affected in patients with cirrhosis. Glucuronidation is often considered to be affected to a lesser extent than CYP450-mediated reactions in mild to moderate cirrhosis but can also be substantially impaired in patients with advanced cirrhosis. Patients with advanced cirrhosis often have impaired renal function and dose adjustment may, therefore, also be necessary for drugs eliminated by renal exctretion. In addition, patients with liver cirrhosis are more sensitive to the central adverse effects of opioid analgesics and the renal adverse effects of NSAIDs. In contrast, a decreased therapeutic effect has been noted in cirrhotic patients with beta-adrenoceptor antagonists and certain diuretics. Unfortunately, there is no simple endogenous marker to predict hepatic function with respect to the elimination capacity of specific drugs. Several quantitative liver tests that measure the elimination of marker substrates such as galactose, sorbitol, antipyrine, caffeine, erythromycin, and midazolam, have been developed and evaluated, but no single test has gained widespread clinical use to adjust dosage regimens for drugs in patients with hepatic dysfunction. The semi-quantitative Child-Pugh score is frequently used to assess the severity of liver function impairment, but only offers the clinician rough guidance for dosage adjustment because it lacks the sensitivity to quantitate the specific ability of the liver to metabolize individual drugs. The recommendations of the Food and Drug Administration (FDA) and the European Medicines Evaluation Agency (EMEA) to study the effect of liver disease on the pharmacokinetics of drugs under development is clearly aimed at generating, if possible, specific dosage recommendations for patients with hepatic dysfunction. However, the limitations of the Child-Pugh score are acknowledged, and further research is needed to develop more sensitive liver function tests to guide drug dosage adjustment in patients with hepatic dysfunction.

Current methods for predicting human food effect

Food can impact the pharmacokinetics of a drug product through several mechanisms, including but not limited to, enhancement in drug solubility, changes in GI physiology, or direct interaction with the drug. Significant food effects complicate development of new drugs, especially when clinical plans require control and/or monitoring of food intake in relation to dosing. The prediction of whether a drug or drug product will show a human food effect is challenging. In vitro models which consider physical-chemical properties can classify the potential for a compound to demonstrate a positive, negative or no food effect, and may be appropriate for screening compounds at early stages of drug discovery. When comparing various formulations, dissolution tests in biorelevant media can serve as a predictor of human drug performance under fasted and fed conditions. Few in vivo models exist which predict the magnitude of change in pharmacokinetic parameters in humans when dosing in the presence of food, with the dog appearing to be the most studied species for this purpose. Control of gastric pH, as well as the amount and composition of the fed state in dogs are critical parameters to improving the predictability of the dog overall as a food effect model. No single universal model is applicable for all drugs at all stages of drug development. One or more models may be required depending whether the goal is to assess potential for a food effect, determine the magnitude of change in pharmacokinetic parameters in the fed/fasted state, or whether formulation efforts have the ability to mitigate an observed food effect.

Determination of metoprolol, and its four metabolites in dog plasma

Metoprolol and its metabolites alpha-hydroxymetoprolol, O-desmethylmetoprolol, metoprolol acid and deaminated metoprolol were quantified in dog plasma using an isocratic high-performance liquid chromatographic method (with a BetaBasic Cyano column) with fluorescence detection. A solid phase extraction technique (Oasis HLB, Waters) was used to extract metoprolol and its four metabolites from dog plasma with high recovery rates. The method was shown to be sensitive and reproducible and was used to determine the pharmacokinetic profile of metoprolol in dog. To the best of our knowledge, this is the only method that can extract and analyze metoprolol and its four metabolites in a single assay.

Oral vinorelbine pharmacokinetics and absolute bioavailability study in patients with solid tumors

BACKGROUND

Vinorelbine is a vinca alkaloid obtained by hemisynthesis, which makes the molecule more lipophilic than the other vincas. An injectable formulation is already marketed for the treatment of non small cell lung cancer (NSCLC) and advanced breast cancer (ABC). A new oral form has been developed and its file registration is being submitted. As part of its development, a clinical study was conducted to determine the absolute bioavailability and pharmacokinetics of oral vinorelbine administered as softgel capsules, and to evaluate its safety profile compared with intravenous administration.

PATIENTS AND METHODS

Thirty-two patients with solid tumours were included in the study. Patients fasted and were randomised to receive vinorelbine on day 1, either as a 20 minute intravenous (i.v.) infusion of 25 mg/m2 or as softgel capsules at a dose of 80 mg/m2. Patients were treated with the alternate route after a one week wash-out period. Blood and urine samples for pharmacokinetic analysis were collected during each vinorelbine administration. Safety was assessed after each administration using the CALGB/expanded CTC classification.

RESULTS

Twenty-four patients were eligible for pharmacokinetic evaluation. Oral vinorelbine was rapidly absorbed at 80 mg/m2 (Tmax 1.4 +/- 0.7 h) and showed a bioavailability of 43 +/- 14, and close to 40% based on AUC(last) and AUC(inf), respectively. A bioequivalence analysis was conducted on dosage-normalised blood exposures. Equivalence was demonstrated between 80 mg/m2 oral and 30 mg/m2 i.v., and between 60 mg/m2 oral and 25 mg/m2 i.v. The inter-individual variability was equivalent for both routes (CV: 38% and 39% for oral and i.v., respectively). A correlation was found in both methods between AUClast and % nadir variation in white blood cells (WBC) and polymorphonuclears (PMN). More cases of neutropenia (all grades pooled), leucopenia (grades 3-4 only) and nausea (grades 2-3) were induced by 80 mg/m2 oral vinorelbine than by 25 mg/m2 i.v. The greatest intensity of these effects, following oral administration, probably reflects the higher, observed drug exposure.

CONCLUSION

At therapeutic dosage levels, pharmacokinetic behaviour and safety profiles were similar for both routes. The absolute bioavailability of the oral vinorelbine (new, soft gelatine capsule) was close to 40%. Inter-individual variability in drug exposure was equivalent in both routes. The pharmacokinetic/pharmacodynamic (PK/PD) relationship in haematological toxicity was independent of the routes of administration. Reliable, corresponding doses between oral and i.v. vinorelbine were established, which will result in bioequivalent AUC.

Strategies to overcome simultaneous P-glycoprotein mediated efflux and CYP3A4 mediated metabolism of drugs

Cytochrome P450 3A4 (CYP3A4), abundant in both the liver and upper intestinal enterocytes, limits the systemic bioavailability of xenobiotics. P-glycoprotein (P-gp), the MDR1 gene product, is also known to reduce the oral bioavailability of the drug molecules. High cellular expression of P-gp and CYP3A4 in mature intestinal enterocytes and their similar substrate specificity suggest that the function of these proteins may be complementary and may form a co-ordinated intestinal barrier. Various ongoing preclinical and clinical studies have demonstrated that the oral bioavailability of various P-gp and/or CYP3A4 substrates can be increased by simultaneous administration of P-gp and/or CYP3A4 inactivators. The current review describes the background and summarises several proposed hypotheses in modifying oral bioavailability by various drug-inhibitor interactions.

Food does not influence the pharmacokinetics of a new extended release formulation of tolterodine for once daily treatment of patients with overactive bladder

OBJECTIVE

To determine whether food intake influences the pharmacokinetics of a new, once daily, extended release (ER) capsule formulation of tolterodine in healthy volunteers, and to compare its bioavailability with that of the existing immediate release (IR) tablet.

DESIGN

Open, randomised, 3-way crossover trial.

PARTICIPANTS

17 healthy volunteers (3 females, 14 males) aged between 19 and 50 years. With the exception of 1 male volunteer, all participants were classified as extensive metabolisers by cytochrome P450 2D6 genotyping.

METHODS

Volunteers received single oral doses of tolterodine L-tartrate ER 8 mg (2 x 4 mg capsules) on an empty stomach or with a standardised high-fat breakfast. Reference therapy comprised tolterodine L-tartrate IR 4 mg (2 x 2 mg tablets), administered in the fasting state. Serum concentrations of tolterodine, its active 5-hydroxymethyl metabolite (5-HM) and the active moiety (sum of unbound tolterodine + 5-HM) were measured for up to 72 hours post-dose. Safety endpoints were also determined.

RESULTS

No effect of food on the bioavailability of tolterodine ER capsules was apparent and there was no sign of dose-dumping with meals. The geometric mean fed:fasting ratio of area under the serum concentration-time curve to infinity (AUCinfinity) of the active moiety, for all volunteers combined, was 0.95 (90% confidence interval 0.88 to 1.03). Equivalence with respect to AUCinfinity (dose-corrected) was also found for the ER capsule compared with the IR tablet, although uncorrected maximum serum concentrations were around 50% lower despite the fact that the capsule dose was twice as high. Seven volunteers reported adverse events, predominantly headache. No volunteer reported dry mouth. Overall, there were no safety concerns.

CONCLUSIONS

The new ER formulation of tolterodine shows no pharmacokinetic interaction with food. On the basis of these results, patients with overactive bladder may, therefore, be advised to take the drug without regard to the timing of meals, maximising convenience during therapy.

Predicting the impact of physiological and biochemical processes on oral drug bioavailability

Recent advances in computational methods applied to the fields of drug delivery and biopharmaceutics will be reviewed with a focus on prediction of the impact of physiological and biochemical factors on simulation of gastrointestinal absorption and bioavailability. Our application of a gastrointestinal simulation for the prediction of oral drug absorption and bioavailability will be described. First, we collected literature data or we estimated biopharmaceutical properties by application of statistical methods to a set of 2D and 3D molecular descriptors. Second, we integrated the differential equations for an advanced compartmental absorption and transit (ACAT) model in order to determine the rate, extent, and approximate gastrointestinal location of drug liberation (for controlled release), dissolution, passive and carrier-mediated absorption, and saturable metabolism and efflux. We predict fraction absorbed, bioavailability, and C(p) vs. time profiles for common drugs and compare those estimates to literature data. We illustrate the simulated impact of physiological and biochemical processes on oral drug bioavailability.

Active secretion and enterocytic drug metabolism barriers to drug absorption

Intestinal phase I metabolism and active extrusion of absorbed drug have only recently been recognized as major determinants of oral drug bioavailability. Both CYP3A4, the major phase I drug metabolizing enzyme in humans, and the multidrug efflux pump, P-glycoprotein (P-gp), are present at high levels in the villus enterocytes of the small intestine, the primary site of absorption for orally administered drugs. Moreover, these proteins are induced by many of the same compounds and demonstrate a broad overlap in substrate and inhibitor specificities, suggesting that they act as a concerted barrier to drug absorption. Clinical studies have demonstrated that inhibition of CYP3A4-mediated intestinal metabolism can significantly improve the oral bioavailability of a wide range of drugs. Intestinal P-gp is a major route of elimination for both orally and intravenously administered anticancer drugs in animal models, and experiments with the Caco-2 cell line have provided strong evidence that inhibition of intestinal P-gp is another means by which oral drug bioavailability could be enhanced.

Food increases the bioavailability of tolterodine but not effective exposure

The objective of this study was to investigate the influence of food on the pharmacokinetics of tolterodine, its active 5-hydroxymethyl metabolite (5-HM), and exposure to the active moiety (sum of unbound tolterodine + 5-HM) in healthy volunteers. Serum concentrations of tolterodine and 5-HM were measured for up to 12 hours after a single oral dose (2 mg) of tolterodine L-tartrate, administered either on an empty stomach or with a standardized medium-fat breakfast. All 23 subjects completing the study were classified as extensive metabolizers (phenotyped with debrisoquine). Pharmacokinetic data on tolterodine and the active moiety were evaluable for 22 subjects; all completing subjects were evaluable for 5-HM pharmacokinetics. Based on Cmax and AUC(infinity) ratios, relative bioavailability of tolterodine in the presence of food was 1.49 (90% confidence interval [CI], 1.35-1.71) and 1.53 (1.35-1.72), respectively. The pharmacokinetics of 5-HM and the active moiety were unaffected by food, as were the rates of drug absorption and terminal half-lives of tolterodine and 5-HM. Given that bioequivalence was observed for the active moiety underfed and fasting conditions, the authors concluded that coadministration of tolterodine with food is not expected to have any clinically relevant effects.

Tissue distribution and interindividual variation in human UDP-glucuronosyltransferase activity: relationship between UGT1A1 promoter genotype and variability in a liver bank

The variability in a liver bank and tissue distribution of three probe UDP-glucuronosyltransferase (UGT) activities were determined as a means to predict interindividual differences in expression and the contribution of extrahepatic metabolism to presystemic and systemic clearance. Formation rates of acetaminophen-O-glucuronide (APAPG), morphine-3-glucuronide (M3G), and oestradiol-3-glucuronide (E3G) as probes for UGT1A6, 2B7, and 1A1, respectively, were determined in human kidney, liver, and lung microsomes, and in microsomes from intestinal mucosa corresponding to duodenum, jejunum and ileum. While formation of E3G and APAPG were detectable in human kidney microsomes, M3G formation rates from kidney microsomes approached the levels seen in liver, indicating significant expression of UGT2B7. Interestingly, rates of E3G formation in human intestine exceeded the hepatic rates by several fold, while APAPG and M3G formation rates were low. The intestinal apparent Km value for E3G formation was essentially identical to that seen in liver, consistent with intestinal UGT1A1 expression. No UGT activities were observed in lung. Variability in APAPG and M3G activity across a bank of 20 human livers was modest (< or = 7-fold), compared to E3G formation, which varied approximately 30-fold. The E3G formation rates were found to segregate by UGT1A1 promoter genotype, with wild-type (TA)6 rates significantly greater than homozygous mutant (TA)7 individuals. Kinetic analyses were performed to demonstrate that the promoter mutation altered apparent Vmax without significantly affecting apparent Km. These results suggest that glucuronidation, and specifically UGT1A1 activity, can profoundly contribute to intestinal first pass metabolism and interindividual variability due to the expression of common allelic variants.

Pharmacogenetics and psychopharmacotherapy

Response to drugs can vary between individuals and between different ethnic populations. The biological (age, gender, disease and genetics), cultural and environmental factors which contribute to these variations are considered in this review. The most important aspect is the genetic variability between individuals in their ability to metabolize drugs due to expression of 'polymorphic' enzymes. Polymorphism enables division of individuals within a given population into at least two groups, poor metabolisers (PMs) and extensive metabolisers (EMs) of certain drugs. The two most extensively studied genetic polymorphisms are those involving cytochrome P450 2D6 (CYP2D6) and CYP2C19. CYP2D6 metabolizes a number of antidepressants, antipsychotics, beta-adrenoceptor blockers, and antiarrhythmic drugs. About 7% of Caucasians and 1% of Asians are PMs of CYP2D6 substrates. CYP2C19 enzyme participates in the metabolism of omeprazole, propranolol and psychotropic drugs such as hexobarbital, diazepam, citalopram, imipramine, clomipramine and amitriptyline. The incidence of PMs of CYP2C19 substrates is much higher in Asians (15-30%) than in Caucasians (3-6%). Variations in metabolism of psychotropic drugs result in variations in their pharmacokinetic parameters. This may lead to clinically significant intra- and inter-ethnic differences in pharmacological responses. Such variations are discussed in this review. Differential receptor-mediated response may play a role in ethnic differences in responses to antipsychotics and tricyclic antidepressants, but such pharmacodynamic factors remain to be systematically investigated. The results of studies of ethnic differences in response to psychopharmacotherapy appear to be discrepant, most probably due to limitations of study design, small sample size, inadequately defined study sample, and lack of control of confounding factors. The clinical value of understanding pharmacogenetics is in its use to optimize therapeutic efficacy, to prevent toxicity of those drugs whose metabolism is catalysed by polymorphic isoenzymes, and to contribute to the rational design of new drugs. Finally, applications and impact of pharmacogenetics in the field of psychopharmacotherapy are discussed.

Pharmacokinetics of diclofenac sodium in chronic active hepatitis and alcoholic cirrhosis

The objective of this study was to assess the pharmacokinetics of diclofenac sodium and its five metabolites following administration of a 150 mg oral dose to healthy subjects and patients with either chronic active hepatitis of varying morphology or alcoholic cirrhosis. Six healthy subjects, 6 chronic active hepatitis patients, and 6 alcoholic cirrhosis patients were enrolled in this prospective, open-label, parallel study. Blood samples were drawn at 0, 0.25, 0.5, 0.75, 1, 2, 4, 6, 8, 12, 24, 48, 72, 144, 312, and 480 hours, and urine samples were collected for 144 hours after administration of a single oral dose of diclofenac sodium. The mean area under the serum concentration-time curve extrapolated to infinity, oral clearance, half-life, maximal concentration, and time to peak concentration for diclofenac and its metabolites were determined and compared using analysis of variance. Cirrhotics had a mean +/- SD diclofenac AUC value (19,114 +/- 6806 ng.h/ml) significantly different (p < 0.02) from hepatitis patients (6071 +/- 1867 ng.h/ml) and healthy subjects (7008 +/- 2006 ng.h/ml), whereas healthy subjects and hepatitis patients had similar values. Comparable results were found for 4'-hydroxydiclofenac. The AUC values for 3'-hydroxydiclofenac and 3'-hydroxy-4'methoxydiclofeanc were significantly different when healthy subjects were compared to cirrhotics. However, hepatitis subjects were not significantly different from either group. The results indicate that hepatitis does not alter the pharmacokinetics of diclofenac. Alcoholic cirrhosis increased the mean diclofenac AUC approximately three times compared to normal subjects, indicating that one-third of the usual dose in cirrhotics would produce equivalent AUC values in normal subjects and subjects with alcoholic cirrhosis. However, since pharmacodynamic measurements were not made and no increase in untoward or side effects was noted in the alcoholic cirrhosis patients after a single dose, maintenance doses should be titrated to patients response.

Comparison of CYP2D6 content and metoprolol oxidation between microsomes isolated from human livers and small intestines

PURPOSE

To assess the role of intestinal CYP2D6 in oral first-pass drug clearance by comparing the enzyme content and catalytic activity of a prototype CYP2D6 substrate, metoprolol, between microsomes prepared from human intestinal mucosa and from human livers.

METHODS

Microsomes were prepared from a panel of 31 human livers and 19 human intestinal jejunal mucosa. Microsomes were also obtained from the jejunum, duodenum and ileum of four other human intestines to assess regional distribution of intestinal CYP2D6. CYP2D6 content (pmole/mg microsomal protein) was determined by Western blot. CYP2D6 activity was measured by alpha-hydroxylation and O-demethylation of metoprolol.

RESULTS

Kinetic studies with microsomes from select livers (n = 6) and jejunal mucosa (n = 5) yielded K(M) estimates of 26 +/- 9 microM and 44 +/- 17 microM, respectively. The mean Vmax (per mg protein) for total formation of alpha-OH-M and ODM was 14-fold higher for the liver microsomes compared to the jejunal microsomes. Comparisons across intestinal regions showed that CYP2D6 protein content and catalytic activity were in the order ofjejunum > duodenum > ileum. Excluding the poor metabolizer genotype donors, CYP2D6 content varied 13- and 100-fold across the panels of human livers (n = 31) and jejunal mucosa (n = 19), respectively. Metoprolol alpha-hydroxylation activity and CYP2D6 content were highly correlated in the liver microsomes (r = 0.84, p < 0.001) and jejunal microsomes (r = 0.75, p < 0.05). Using the well-stirred model, the mean microsomal intrinsic clearance (i.e., Vmax/K(M)) for the livers and jejunum were scaled to predict their respective in vivo organ intrinsic clearance and first-pass extraction ratio. Hepatic and intestinal first-pass extractions of metoprolol were predicted to be 48% and 0.85%, respectively.

CONCLUSIONS

A much lower abundance and activity of CYP2D6 are present in human intestinal mucosa than in human liver. Intestinal mucosal metabolism contributes minimally to the first-pass effect of orally administered CYP2D6 substrates, unless they have exceptionally high microsomal intrinsic clearances and/or long residence time in the intestinal epithelium.

Coronary heart disease. At the interface of molecular genetics and preventive medicine

BACKGROUND

Susceptibility to common chronic diseases such as coronary heart disease (CHD) appears to be influenced by "context-dependent effects," which include interactions among genes (genetic epistasis) and among genes and environmental factors (gene-environment interactions).

METHODS

A synthesis of current knowledge and research findings demonstrates the importance of integrating genetic research on cardiovascular disease with preventive medicine and public health initiatives.

RESULTS

A variety of candidate genes have been implicated in risk for CHD, but only limited examples of context-dependent effects have been described. Interactions between genetic and environmental factors appear to influence lipid metabolism, plasma homocysteine levels, and pharmacologic response to many commonly prescribed medications. Quantification of genetic effects associated with increased disease risk that are modifiable by interventions such as diet, exercise, and smoking cessation is an important interface between molecular genetics and preventive medicine.

CONCLUSIONS

As a primary focus of preventive medicine expands to encompass early detection and treatment of asymptomatic individuals at risk for disease, the ability to quantify the influence of context-dependent effects on disease risk will be critical for determining drug safety and effectiveness in diverse patient populations and for implementing effective prevention and treatment programs.

Toxicokinetics of organic solvents: a review of modifying factors

This article reviews, with an emphasis on human experimental data, factors known or suspected to cause changes in the toxicokinetics of organic solvents. Such changes in the toxicokinetic pattern alters the relation between external exposure and target dose and thus may explain some of the observed individual variability in susceptibility to toxic effects. Factors shown to modify the uptake, distribution, biotransformation, or excretion of solvent include physical activity (work load), body composition, age, sex, genetic polymorphism of the biotransformation, ethnicity, diet, smoking, drug treatment, and coexposure to ethanol and other solvents. A better understanding of modifying factors is needed for several reasons. First, it may help in identifying important potential confounders and eliminating negligible ones. Second, the risk assessment process may be improved if different sources of variability between external exposures and target doses can be quantitatively assessed. Third, biological exposure monitoring may be also improved for the same reason.

Effect of hepatic insufficiency on pharmacokinetics and drug dosing

The liver plays a central role in the pharmacokinetics of many drugs. Liver dysfunction may not only reduce the plasma clearance of a number of drugs eliminated by biotransformation and/or biliary excretion, but it can also affect plasma protein binding which in turn could influence the processes of distribution and elimination. In addition, reduced liver blood flow in patients with chronic liver disease will decrease the systemic clearance of flow limited (high extraction) drugs and portal-systemic shunting may substantially reduce their presystemic elimination (first-pass effect) following oral administration. When selecting a drug and its dosage regimen for a patient with liver disease additional considerations such as altered pharmacodynamics and impaired renal excretion (hepatorenal syndrome) of drugs and metabolites should also be taken into account. Consequently, dosage reduction is necessary for many drugs administered to patients with chronic liver disease such as liver cirrhosis.

Clinical pharmacokinetics of vasodilators. Part I

Understanding the mechanism of action and the pharmacokinetic properties of vasodilatory drugs facilitates optimal use in clinical practice. It should be kept in mind that a drug belongs to a class but is a distinct entity, sometimes derived from a prototype to achieve a specific effect. The most common pharmacokinetic drug improvement is the development of a drug with a half-life sufficiently long to allow an adequate once-daily dosage. Developing a controlled release preparation can increase the apparent half-life of a drug. Altering the molecular structure may also increase the half-life of a prototype drug. Another desirable improvement is increasing the specificity of a drug, which may result in fewer adverse effects, or more efficacy at the target site. This is especially important for vasodilatory drugs which may be administered over decades for the treatment of hypertension, which usually does not interfere with subjective well-being. Compliance is greatly increased with once-daily dosing. Vasodilatory agents cause relaxation by either a decrease in cytoplasmic calcium, an increase in nitric oxide (NO) or by inhibiting myosin light chain kinase. They are divided into 9 classes: calcium antagonists, potassium channel openers, ACE inhibitors, angiotensin-II receptor antagonists, alpha-adrenergic and imidazole receptor antagonists, beta 1-adrenergic agonist, phosphodiesterase inhibitors, eicosanoids and NO donors. Despite chemical differences, the pharmacokinetic properties of calcium antagonists are similar. Absorption from the gastrointestinal tract is high, with all substances undergoing considerable first-pass metabolism by the liver, resulting in low bioavailability and pronounced individual variation in pharmacokinetics. Renal impairment has little effect on pharmacokinetics since renal elimination of these agents is minimal. Except for the newer drugs of the dihydropyridine type, amlodipine, felodipine, isradipine, nilvadipine, nisoldipine and nitrendipine, the half-life of calcium antagonists is short. Maintaining an effective drug concentration for the remainder of these agents requires multiple daily dosing, in some cases even with controlled release formulations. However, a coat-core preparation of nifedipine has been developed to allow once-daily administration. Adverse effects are directly correlated to the potency of the individual calcium antagonists. Treatment with the potassium channel opener minoxidil is reserved for patients with moderately severe to severe hypertension which is refractory to other treatment. Diazoxide and hydralazine are chiefly used to treat severe hypertensive emergencies, primary pulmonary and malignant hypertension and in severe preeclampsia. ACE inhibitors prevent conversion of angiotensin-I to angiotensin-II and are most effective when renin production is increased. Since ACE is identical to kininase-II, which inactivates the potent endogenous vasodilator bradykinin, ACE inhibition causes a reduction in bradykinin degradation. ACE inhibitors exert cardioprotective and cardioreparative effects by preventing and reversing cardiac fibrosis and ventricular hypertrophy in animal models. The predominant elimination pathway of most ACE inhibitors is via renal excretion. Therefore, renal impairment is associated with reduced elimination and a dosage reduction of 25 to 50% is recommended in patients with moderate to severe renal impairment. Separating angiotensin-II inhibition from bradykinin potentiation has been the goal in developing angiotensin-II receptor antagonists. The incidence of adverse effects of such an agent, losartan, is comparable to that encountered with placebo treatment, and the troublesome cough associated with ACE inhibitors is absent.

A sensitive assay of metoprolol and its major metabolite alpha-hydroxy metoprolol in human plasma and determination of dextromethorphan and its metabolite dextrorphan in urine with high performance liquid chromatography and fluorometric detection

A reverse-phase High Performance Liquid Chromatographic (HPLC) method was developed for the analysis of metoprolol in the large number of human plasma samples obtained in in vitro-in vivo correlations (IVIVC) and bioavailability studies of extended release formulations of metoprolol tartrate. The metabolite, alpha-hydroxy metoprolol (OH-met), could also be quantified. The analytes were extracted from the plasma using solid phase columns, separated on a C-4 analytical column followed by fluorimetric detection. The linearity, precision, accuracy, stability, selectivity and ruggedness were validated for the concentration ranges of 1-400 ng ml-1 for metoprolol and 0.5-200 ng ml-1 for OH-met. The same chromatographic conditions were slightly modified to quantify dextromethorphan and its metabolite dextrorphan in urine in the concentration range 0.052-0.05 microgram ml-1 as a method for screening for fast metabolizers.

An in vitro method for predicting in vivo oral bioavailability of novel immunosuppressive drugs

OBJECTIVE

To evaluate an in vitro method for predicting oral availability of novel immunosuppressive drugs, cyclosporine A (CsA) and rapamycin (RAPA).

METHODS

In this study, we report the development and characterization of an in vitro method to study the influence of vehicle composition on cyclosporine A (CsA) and rapamycin (RAPA) drug efflux across 12 days postconfluent, absorptive human Caco-2 intestinal epithelial cell monolayers. The apical-to-basal (Jab) and the basal-to-apical (Jba) fluxes of 0.5 muCi 3H-CsA or 0.05 muCi 14C-RAPA solubilized in a 10 mg/L final drug concentration in vehicle were measured.

RESULTS

The Jab CsA flux was found to be dose dependent, temperature sensitive, and highly polarized (Jab > Jba). For CsA the vehicles were Neoral, Sandimmune, 95% (v/v) ethanol/fetal bovine serum (ethanol/FBS); and for RAPA these were polyethylene glycol/dimethylacetamide (PEG/DMA), polysorbate/Phosal PEG, ethanol/FBS. When Neoral-CsA was tested, the Jab flux of 3H-CsA was the highest and increased almost linearly even after an incubate time of 240 min. The Jab flux of 3H-CsA when Sandimmune-CsA or ethanol/FBS-CsA were used as vehicle was lower and reached a maximal rate by 120 min. In contrast the Jab flux of 14C-RAPA using either PEG/DMA-RAPA or ethanol/FBS-RAPA as vehicle was highest and reached a maximal rate by 120 min, in contrast to the polysorbate/Phosal PEG-RAPA vehicle, which was significantly lower.

CONCLUSION

These data are consistent with the pharmacokinetics of these ISD reported in vivo in human patients or in rabbits, using the same vehicles in the oral formulation. As an integral part of drug development, the data presented that an in vitro system as described may be useful in predicting the effect of drug vehicle on absorption in vivo.

The site of absorption in the small intestine determines diltiazem bioavailability in the rabbit

PURPOSE

Since the ability of the small intestine to biotransform a drug may decrease in distal segments of the intestine, this study aimed to assess whether the site of administration in the small intestine could affect the systemic bioavailability of diltiazem and its two active metabolites, N-desmethyldiltiazem (MA) and desacetyl-diltiazem (M1).

METHODS

Five mg/kg of diltiazem were administered into the lumen of the proximal (0-30 cm, n = 9) or the distal (150-180 cm) small intestine (n = 7) of anesthetized New Zealand rabbits. Blood samples were drawn from the femoral artery for 6 hours, and diltiazem, MA and M1 were assayed by HPLC.

RESULTS

The area under the curve (AUC0-->infinity) of diltiazem administered into the distal small intestine was larger than that estimated when diltiazem was given in the proximal segment (14.20 +/- 2.82 vs 8.14 +/- 0.88 micrograms.min/ml, p < 0.05), due to a lower diltiazem oral clearance (440 +/- 78 vs 660 +/- 55 ml/min/kg, p < 0.05). The AUC0-->360 of MA was not affected by the site of diltiazem administration, but the AUC0-->360 of M1 was increased when diltiazem was administered in the distal segment of the small intestine. When administered into the distal segment of the intestine, the molar sum of diltiazem and its active metabolites was 48% greater than when delivered into the 0-30 cm segment of the small intestine; as a consequence, absorption of diltiazem in distal segments of the small intestine may enhance its pharmacological response.

CONCLUSIONS

The site of absorption into the intestine modulates the bioavailability of diltiazem and its two active metabolites.

Extrahepatic metabolism of frusemide in anaesthetized rabbits

1. Frusemide is removed from the body by biotransformation and renal secretion, but since frusemide metabolism is not altered in patients with hepatic cirrhosis, the role of the liver may be questioned. The aim of the study was to investigate which organs contribute to the first-pass metabolism and systemic clearance of frusemide. 2. Groups of anaesthetized New Zealand rabbits were administered frusemide proximally (prox) and distally (dist) to different organs, and blood was sampled from the abdominal aorta. The area under frusemide plasma concentrations-time curve (AUC0-infinity) was calculated and frusemide extraction by an organ was estimated from the ratio (AUCdist-AUCprox)/AUCdist. The small intestine extracted 83% of the absorbed dose of frusemide but the first-pass uptake by the liver and lungs was negligible. 3. To assess the contribution of the intestine and the kidneys to the systemic clearance of frusemide, it was injected into the jugular vein and blood was sampled proximal and distal to each organ. The kidneys extracted 24% of frusemide circulating in the renal arteries; on the other hand, the ability of the intestine to extract frusemide from the systemic circulation could not be detected. 4. The lungs did not metabolize frusemide in vitro; the rate of metabolism of frusemide in vitro by kidneys was similar to that estimated in the intestine, and both rates were faster (P < 0.05) than that observed in the liver. 5. It is concluded that in rabbits, presystemic metabolism of frusemide is carried out by the intestine, and that systemic clearance of frusemide is mainly performed by the kidneys, although other organs, such as the intestine and the liver, must contribute to it.

Metabolism of diltiazem in hepatic and extrahepatic tissues of rabbits: in vitro studies

Diltiazem (DTZ) is a calcium channel blocker widely used in the treatment of angina and hypertension. DTZ undergoes extensive metabolism yielding several metabolites, some of which are active like N-desmethyldiltiazem (MA), desacetyldiltiazem (M1) and N-desmethyl,desacetyldiltiazem (M2). Due to the nature of its biotransformation, several organs should have the ability to metabolize DTZ, however it is still assumed that the liver is the only organ implicated in its elimination. In this study, the fate of DTZ, MA and M1 was assessed in several organs that could contribute to their biotransformation. To this purpose, DTZ (48.2 microM) was incubated in the 10,000 x g supernatant of homogenates of rabbit tissues for 60 min at 37 degrees C. Multiple samples were withdrawn, and DTZ and its metabolites were assayed by HPLC. The elimination rate constant of DTZ in 10,000 x g supernatants varied between the organs: liver 334 +/- 45, proximal small intestine 69 +/- 11, distal small intestine 25 +/- 3, lungs 15 +/- 6 and kidneys 8 +/- 6 (10(-4) min-1). The metabolism of DTZ in the liver generated large amounts of MA but no M1, and in the small intestine, modest amounts of both metabolites. When MA (50.0 microM) or M1 (53.7 microM) were incubated in liver homogenates, the estimated elimination rate constant were 166 +/- 23 and 468 +/- 53 (10(-4) min-1), respectively. The rate of degradation of the metabolites in the small intestine was much slower.(ABSTRACT TRUNCATED AT 250 WORDS)