Protein binding and kinetics of drugs in liver diseases

Re-discover the value of protein binding assessments in hepatic and renal impairment studies and its contributions in drug labels and dose decisions

One of the key pharmacokinetic properties of most small molecule drugs is their ability to bind to serum proteins. Unbound or free drug is responsible for pharmacological activity while the balance between free and bound drug can impact drug distribution, elimination, and other safety parameters. In the hepatic impairment (HI) and renal impairment (RI) clinical studies, unbound drug concentration is often assessed; however, the relevance and impact of the protein binding (PB) results is largely limited. We analyzed published clinical safety and pharmacokinetic studies in subjects with HI or RI with PB assessment up to October 2022 and summarized the contribution of PB results on their label dose recommendations. Among drugs with HI publication, 32% (17/53) associated product labels include PB results in HI section. Of these, the majority (9/17, 53%) recommend dose adjustments consistent with observed PB change. Among drugs with RI publication, 27% (12/44) of associated product labels include PB results in RI section with the majority (7/12, 58%) recommending no dose adjustment, consistent with the reported absence of PB change. PB results were found to be consistent with a tailored dose recommendation in 53% and 58% of the approved labels for HI and RI section, respectively. We further discussed the interpretation challenges of PB results, explored treatment decision factors including total drug concentration, exposure-response relationships, and safety considerations in these case examples. Collectively, comprehending the alterations in free drug levels in HI and RI informs treatment decision through a risk-based approach.

Toward improved predictions of pharmacokinetics of transported drugs in hepatic impairment: Insights from the extended clearance model

Hepatic impairment (HI) moderately (<5-fold) affects the systemic exposure (i.e., area under the plasma concentration-time curve [AUC]) of drugs that are substrates of the hepatic sinusoidal organic anion transporting polypeptide (OATP) transporters and are excreted unchanged in the bile and/or urine. However, the effect of HI on their AUC is much greater (>10-fold) for drugs that are also substrates of cytochrome P450 (CYP) 3A enzymes. Using the extended clearance model, through simulations, we identified the ratio of sinusoidal efflux clearance (CL) over the sum of metabolic and biliary CLs as important in predicting the impact of HI on the AUC of dual OATP/CYP3A substrates. Because HI may reduce hepatic CYP3A-mediated CL to a greater extent than biliary efflux CL, the greater the contribution of the former versus the latter, the greater the impact of HI on drug AUC ratio (AUCRHI ). Using physiologically-based pharmacokinetic modeling and simulation, we predicted relatively well the AUCRHI of OATP substrates that are not significantly metabolized (pitavastatin, rosuvastatin, valsartan, and gadoxetic acid). However, there was a trend toward underprediction of the AUCRHI of the dual OATP/CYP3A4 substrates fimasartan and atorvastatin. These predictions improved when the sinusoidal efflux CL of these two drugs was increased in healthy volunteers (i.e., before incorporating the effect of HI), and by modifying the directionality of its modulation by HI (i.e., increase or decrease). To accurately predict the effect of HI on AUC of hepatobiliary cleared drugs it is important to accurately predict all hepatobiliary pathways, including sinusoidal efflux CL.

Perioperative Acetaminophen Dosing in Obese Children

Acetaminophen is a commonly used perioperative analgesic drug in children. The use of a preoperative loading dose achieves a target concentration of 10 mg/L associated with a target analgesic effect that is 2.6 pain units (visual analogue scale 1–10). Postoperative maintenance dosing is used to keep this effect at a steady-state concentration. The loading dose in children is commonly prescribed per kilogram. That dose is consistent with the linear relationship between the volume of distribution and total body weight. Total body weight is made up of both fat and fat-free mass. The fat mass has little influence on the volume of distribution of acetaminophen but fat mass should be considered for maintenance dosing that is determined by clearance. The relationship between the pharmacokinetic parameter, clearance, and size is not linear. A number of size metrics (e.g., fat-free and normal fat mass, ideal body weight and lean body weight) have been proposed to scale clearance and all consequent dosing schedules recognize curvilinear relationships between clearance and size. This relationship can be described using allometric theory. Fat mass also has an indirect influence on clearance that is independent of its effects due to increased body mass. Normal fat mass, used in conjunction with allometry, has proven a useful size metric for acetaminophen; it is calculated using fat-free mass and a fraction (Ffat) of the additional mass contributing to total body weight. However, the Ffat for acetaminophen is large (Ffat = 0.82), pharmacokinetic and pharmacodynamic parameter variability high, and the concentration–response slope gentle at the target concentration. Consequently, total body weight with allometry is acceptable for the calculation of maintenance dose. The dose of acetaminophen is tempered by concerns about adverse effects, notably hepatotoxicity associated with use after 2–3 days at doses greater than 90 mg/kg/day.

The Effect of Hepatic Impairment on the Pharmacokinetics of Dacomitinib

Background and Objective

Dacomitinib is a kinase inhibitor indicated for the first-line treatment of patients with metastatic non-small cell lung cancer (NSCLC) with epidermal growth factor receptor (EGFR)-activating mutations. To evaluate the effect of hepatic impairment on the pharmacokinetics of dacomitinib, two dedicated studies were conducted to inform optimal dosing.

Methods

Study 1 (NCT01571388) evaluated the effect of mild and moderate hepatic impairment on the plasma pharmacokinetics, safety, and tolerability after a single oral dose of dacomitinib 30 mg, and Study 2 (NCT03865446) evaluated the same endpoints in a severe hepatic impairment population. Both studies were phase I, open-label, parallel-group studies. A one-way analysis of variance (ANOVA) with unequal variance assumption and hepatic impairment group as a fixed effect was used to compare the natural log of area under the plasma concentration-time curve extrapolated to infinite time (AUC_inf), AUC from time zero to the last quantifiable concentration (AUC_last), and maximum plasma concentration (C_max) for each hepatic impairment group to the respective normal hepatic function group. Since dacomitinib is a cytochrome P450 (CYP) 2D6 substrate, only participants with extensive or intermediate CYP2D6 phenotypes were included in the primary analysis.

Results

The AUC_inf for participants with mild, moderate, or severe hepatic impairment decreased by 6%, decreased by 23%, and increased by 4%, respectively, compared with normal hepatic function, while the C_max for participants with mild, moderate, or severe hepatic impairment increased by 3%, decreased by 20%, and increased by 31%, respectively, compared with normal hepatic function. A single oral dose of dacomitinib 30 mg was well tolerated in all participants.

Conclusion

Based on these pharmacokinetic results, dacomitinib pharmacokinetics of participants with mild, moderate, or severe hepatic impairment were not statistically different relative to participants with normal hepatic function based on the ANOVA analysis. No dacomitinib dose adjustments for patients with hepatic impairment are recommended.

Clinical Trial Registration

ClinicalTrials.gov NCT01571388, registered 5 April 2012; ClinicalTrials.gov NCT03865446, registered 6 March 2019.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40261-022-01125-x.

Hepatic impairment as a risk factor of adverse drug reactions

There are a lot of clinical variants of hepatic impairment ranging from asymptomatic increase in transaminases to acute liver failure and fulminant hepatitis. Hepatic impairment is a polietiologic syndrome. According to the epidemiological study conducted in the United States (19982008), the main causes of hepatic impairment were paracetamol overdose (46%), idiopathic liver dysfunction (14%), other drugs (excluding paracetamol, 11%), viral hepatitis B (7%), other infectious and non-infectious diseases with liver damage (except for viral hepatitis) 7%, autoimmune hepatitis (5%), ischemic hepatitis (syn. hypoxic hepatitis, liver infarction) 4%, viral hepatitis A (3%) and Wilson's disease (2%). Hepatic impairment have a direct impact on the pharmacokinetics and pharmacodynamics of drugs decreasing clearance, elimination and excretion of drugs. Also Transjugular intrahepatic porto-systemic shunts, which are often used to treat portal hypertension in patients with liver cirrhosis, can significantly reduce the presystemic elimination of drugs, thereby increasing their absorption. Moreover, in patients with liver cirrhosis, concomitant renal dysfunction also requires an adjustment of the dose of drugs. Correction of pharmacotherapy in accordance to pharmacokinetic and pharmacodynamic changes of drugs ingested by patients with impaired liver function will improve the quality of medical care and reduce the risks of adverse drug reactions.

Pharmacokinetics and safety of evogliptin in hepatically impaired patients

AIMS

Evogliptin is a potent and selective dipeptidyl peptidase-4 inhibitor for glycaemic control in patients with type 2 diabetes mellitus. Since evogliptin is mainly eliminated through hepatic metabolism, we investigated the pharmacokinetics (PKs) and safety characteristics of evogliptin in Korean patients with mild or moderate hepatic impairment.

METHODS

An open-label, parallel-group study was conducted in patients with mild or moderate hepatic impairment and healthy control subjects matched to each patient for sex, age and body mass index. A single dose (5 mg) of evogliptin was administered orally, and serial blood samples were collected over 120 h to assess the PK profile of evogliptin and its main metabolites (M7 and M8).

RESULTS

Patients with mild hepatic impairment and their matched healthy controls showed similar maximum concentration (C_max ) and area under the concentration-time curve values from 0 to 120 h (AUC_last ); the geometric mean ratio (GMR) and 90% confidence interval (CI) were 1.04 (0.80, 1.35) and 1.01 (0.90, 1.14), respectively. Exposure to evogliptin (C_max and AUC_last ) was increased by about 40% in patients with moderate hepatic impairment-the GMR and 90% CI were 1.37 (1.09, 1.72) and 1.44 (1.18, 1.75), respectively. The metabolic ratios of M7 and M8 were lower in patients with moderate hepatic impairment than in matched healthy controls. Evogliptin was well tolerated by both patients and healthy subjects.

CONCLUSION

Although evogliptin exposure was increased in patients with moderate hepatic impairment, the increase is unlikely to affect safety and efficacy adversely, and no dose adjustment is warranted.

Pharmacokinetics of HIV-Integrase Inhibitors During Pregnancy: Mechanisms, Clinical Implications and Knowledge Gaps

Prevention of mother-to-child transmission of HIV and optimal maternal treatment are the most important goals of antiretroviral therapy in pregnant women with HIV. These goals may be at risk due to possible reduced exposure during pregnancy caused by physiological changes. Limited information is available on the impact of these physiological changes. This is especially true for HIV-integrase inhibitors, a relatively new class of drugs, recommended first-line agents and hence used by a large proportion of HIV-infected patients. Therefore, the objective of this review is to provide a detailed overview of the pharmacokinetics of HIV-integrase inhibitors in pregnancy. Second, this review defines potential causes for the change in pharmacokinetics of HIV-integrase inhibitors during pregnancy. Despite increased clearance, for raltegravir 400 mg twice daily and dolutegravir 50 mg once daily, exposure during pregnancy seems adequate; however, for elvitegravir, the proposed minimal effective concentration is not reached during pregnancy. Lower exposure to these drugs may be caused by increased hormone levels and, subsequently, enhanced drug metabolism during pregnancy. The pharmacokinetics of bictegravir and cabotegravir, which are under development, have not yet been evaluated in pregnant women. New studies need to prospectively assess whether adequate exposure is reached in pregnant women using these new HIV-integrase inhibitors. To further optimize antiretroviral treatment in pregnant women, studies need to unravel the underlying mechanisms behind the changes in the pharmacokinetics of HIV-integrase inhibitors during pregnancy. More knowledge on altered pharmacokinetics during pregnancy and the underlying mechanisms contribute to the development of effective and safe antiretroviral therapy for HIV-infected pregnant women.

Pharmacokinetic and pharmacodynamic considerations of general anesthesia in pediatric subjects

Introduction: The target concentration strategy uses PKPD information for dose determination. Models have also quantified exposure-response relationships, improved understanding of developmental pharmacokinetics, rationalized dose prescription, provided insight into the importance of covariate information, explained drug interactions and driven decision-making and learning during drug development.Areas covered: The prime PKPD consideration is parameter estimation and quantification of variability. The main sources of variability in children are age (maturation) and weight (size). Model use is mostly confined to pharmacokinetics, partly because anesthesia effect models in the young are imprecise. Exploration of PK and PD covariates and their variability hold potential to better individualize treatment.Expert opinion: The ability to model drugs using computer-based technology is hindered because covariate data required to individualize treatment using these programs remain lacking. Target concentration intervention strategies remain incomplete because covariate information that might better predict individualization of dose is absent. Pharmacogenomics appear a valuable area for investigation for pharmacodynamics and pharmacodynamics. Effect measures in the very young are imprecise. Assessment of the analgesic component of anesthesia is crude. While neuromuscular monitoring is satisfactory, depth of anaesthesia EEG interpretation is inadequate. Closed loop anesthesia is possible with better understanding of EEG changes.

Pharmacokinetics of HIV-Integrase Inhibitors During Pregnancy: Mechanisms, Clinical Implications and Knowledge Gaps

Prevention of mother-to-child transmission of HIV and optimal maternal treatment are the most important goals of antiretroviral therapy in pregnant women with HIV. These goals may be at risk due to possible reduced exposure during pregnancy caused by physiological changes. Limited information is available on the impact of these physiological changes. This is especially true for HIV-integrase inhibitors, a relatively new class of drugs, recommended first-line agents and hence used by a large proportion of HIV-infected patients. Therefore, the objective of this review is to provide a detailed overview of the pharmacokinetics of HIV-integrase inhibitors in pregnancy. Second, this review defines potential causes for the change in pharmacokinetics of HIV-integrase inhibitors during pregnancy. Despite increased clearance, for raltegravir 400 mg twice daily and dolutegravir 50 mg once daily, exposure during pregnancy seems adequate; however, for elvitegravir, the proposed minimal effective concentration is not reached during pregnancy. Lower exposure to these drugs may be caused by increased hormone levels and, subsequently, enhanced drug metabolism during pregnancy. The pharmacokinetics of bictegravir and cabotegravir, which are under development, have not yet been evaluated in pregnant women. New studies need to prospectively assess whether adequate exposure is reached in pregnant women using these new HIV-integrase inhibitors. To further optimize antiretroviral treatment in pregnant women, studies need to unravel the underlying mechanisms behind the changes in the pharmacokinetics of HIV-integrase inhibitors during pregnancy. More knowledge on altered pharmacokinetics during pregnancy and the underlying mechanisms contribute to the development of effective and safe antiretroviral therapy for HIV-infected pregnant women.

Population pharmacokinetics of haloperidol in terminally ill adult patients

Purpose

Over 80% of the terminally ill patients experience delirium in their final days. In the treatment of delirium, haloperidol is the drug of choice. Very little is known about the pharmacokinetics of haloperidol in this patient population. We therefore designed a population pharmacokinetic study to gain more insight into the pharmacokinetics of haloperidol in terminally ill patients and to find clinically relevant covariates that may be used in developing an individualised dosing regimen.

Methods

Using non-linear mixed effects modelling (NONMEM 7.2), a population pharmacokinetic analysis was conducted with 87 samples from 28 terminally ill patients who received haloperidol either orally or subcutaneously. The covariates analysed were patient and disease characteristics as well as co-medication.

Results

The data were accurately described by a one-compartment model. The population mean estimates for oral bioavailability, clearance and volume of distribution for an average patient were 0.86 (IIV 55%), 29.3 L/h (IIV 43%) and 1260 L (IIV 70%), respectively. This resulted in an average terminal half-life of haloperidol of around 30 h.

Conclusion

Our study showed that the pharmacokinetics of haloperidol could be adequately described by a one-compartment model. The pharmacokinetics in terminally ill patients was comparable to other patients. We were not able to explain the wide variability using covariates.

Psychotropic drugs and liver disease: A critical review of pharmacokinetics and liver toxicity

The liver is the organ by which the majority of substances are metabolized, including psychotropic drugs. There are several pharmacokinetic changes in end-stage liver disease that can interfere with the metabolization of psychotropic drugs. This fact is particularly true in drugs with extensive first-pass metabolism, highly protein bound drugs and drugs depending on phase I hepatic metabolic reactions. Psychopharmacological agents are also associated with a risk of hepatotoxicity. The evidence is insufficient for definite conclusions regarding the prevalence and severity of psychiatric drug-induced liver injury. High-risk psychotropics are not advised when there is pre-existing liver disease, and after starting a psychotropic agent in a patient with hepatic impairment, frequent liver function/lesion monitoring is advised. The authors carefully review the pharmacokinetic disturbances induced by end-stage liver disease and the potential of psychopharmacological agents for liver toxicity.

Physiologically based pharmacokinetic models for the optimization of antiretroviral therapy: recent progress and future perspective

Anti-HIV therapy is characterized by the chronic administration of antiretrovirals (ARVs), and consequently, several problems can arise during the management of HIV-positive patients. ARV disposition can be simulated by combining system data describing a population of patients and in vitro drug data through physiologically based pharmacokinetic (PBPK) models, which mathematically describe absorption, distribution, metabolism and elimination. PBPK modeling can find application in the investigation of clinically relevant scenarios, while providing the opportunity for a better understanding of the mechanisms regulating drug distribution. In this review, we have analyzed the most recent applications of PBPK models for ARVs and highlighted some of the most interesting areas of use, such as drug–drug interaction, pharmacogenetics, factors regulating absorption and tissue penetration, as well as therapy optimization in special populations. The application of the PBPK modeling approach might not be limited to the investigation of hypothetical clinical issues, but could be used to inform future prospective clinical trials.

The blood concentration and organ distribution of haloperidol at therapeutic and toxic doses in severe fatty liver disease

The aim of this study was to investigate the difference between the pharmacokinetics of haloperidol in normal rats and in rats with fatty liver disease. A therapeutic dosage (0.1 mg/kg) and a toxic dose (15 mg/kg) of haloperidol were administrated to normal 9-week-old male rats or those with severe fatty liver disease, and the blood concentration of haloperidol was determined 15 min, 1, 2, and 3 h following haloperidol administration. The concentration of haloperidol in the organs was determined 1, 2, and 3 h after the haloperidol administration. Additionally, the volume of the portal vein blood flow was measured 3 h after haloperidol administration. When given at the therapeutic dosage, the concentrations of haloperidol in both the blood and organs of the rats with fatty liver disease were significantly higher than those in the normal rats. However, when given at the toxic level, the blood and organ haloperidol concentrations 1 h after administration tended to be lower in the rats with fatty liver disease than those in the normal rats; these lower haloperidol levels returned to be the levels in the normal rats 3 h after the administration of haloperidol. The volume of the portal vein blood flow significantly increased following the toxic haloperidol dose as compared with the volume pre-administration and following the therapeutic haloperidol dose in the normal rats. However, the volume did not change after the toxic or the therapeutic dose of haloperidol compared with pre-administration in rats with severe fatty liver disease, although it was significantly higher than in the normal rats. The pathway for haloperidol metabolism might have been saturated before the administration of haloperidol in rats with fatty liver disease; thus, it is possible that the blood concentration of haloperidol tends to be much higher in individuals with severe fatty liver disease than in those with normal livers in an inverse proportion to the dosage of haloperidol.

A replication study of genome-wide CNV association for hepatic biomarkers identifies nine genes associated with liver function

Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) are biochemical markers used to test for liver diseases. Copy number variation (CNV) plays an important role in determining complex traits and is an emerging area in the study various diseases. We performed a genome-wide association study with liver function biomarkers AST and ALT in 407 unrelated Koreans. We assayed the genome-wide variations on an Affymetrix Genome-Wide 6.0 array, and CNVs were analyzed using HelixTree. Using single linear regression, 32 and 42 CNVs showed significance for AST and ALT, respectively (P value < 0.05). We compared CNV-based genes between the current study (KARE2; AST-140, ALT-172) and KARE1 (AST-1885, ALT-773) using NetBox. Results showed 9 genes (CIDEB, DFFA, PSMA3, PSMC5, PSMC6, PSMD12, PSMF1, SDC4, and SIAH1) were overlapped for AST, but no overlapped genes were found for ALT. Functional gene annotation analysis shown the proteasome pathway, Wnt signaling pathway, programmed cell death, and protein binding.

Influence of mild and moderate hepatic impairment on axitinib pharmacokinetics

OBJECTIVE

To evaluate the effects of hepatic impairment on the pharmacokinetics and safety of a single, oral axitinib dose in subjects with mild or moderate hepatic impairment.

METHODS

In this phase I, open-label, parallel-group study, a total of 24 subjects with either normal hepatic function (n = 8) or with mild (n = 8) or moderate (n = 8) hepatic impairment were administered a single, oral dose of axitinib (5 mg). Blood samples were collected at intervals up to 144 h following dosing, and plasma pharmacokinetics and safety were assessed. Changes in axitinib plasma exposures in subjects with mild or moderate hepatic impairment were predicted using computer simulations and used to guide initial dosing in the clinical study.

RESULTS

Axitinib exposure was similar in subjects with normal hepatic function and those with mild hepatic impairment, but approximately twofold higher in subjects with moderate hepatic impairment. Axitinib exposure weakly correlated with measures of hepatic function but was not affected by smoking status. Axitinib protein binding was similar in the three treatment groups. No significant treatment-related adverse events were reported.

CONCLUSIONS

Compared with subjects with normal hepatic function, moderate hepatic impairment increased axitinib exposure, suggesting that the oral clearance of axitinib is altered in these subjects. In addition, these data indicate a possible need for a dose reduction in subjects who develop moderate or worse hepatic impairment during axitinib treatment. A single 5-mg dose of axitinib was well tolerated in subjects with mild or moderate hepatic impairment.

Role of Haloperidol in Palliative Medicine

Haloperidol is a butyrophenone neuroleptic agent characterized as a high-affinity dopamine antagonist, originally used for the treatment of schizophrenia. Awareness of the role dopamine plays in many symptoms in palliative care, such as nausea, vomiting, and delirium, has led to the use of dopamine antagonists such as haloperidol for the treatment of these symptoms in the palliative care setting. Listed as 1 of the 25 important drugs in palliative care, haloperidol can be administered by multiple routes and can be given without dose alteration in the setting of both renal and hepatic insufficiency. Haloperidol is extensively metabolized in the liver, with CYP3A4 the chief cytochrome oxidase responsible for metabolism. This article will review the pharmacology, pharmacokinetics, and current uses of haloperidol in palliative medicine. There will be an examination of the evidence base for the use of haloperidol in palliative medicine.

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.

Interpretation of drug levels: relevance of plasma protein binding

Many centrally acting drugs bind extensively to plasma proteins, particularly albumin. It is generally the free concentration rather than the total concentration which determines the intensity of pharmacological action and the distribution and rate of elimination of a drug. Measurement of the total plasma concentration may therefore give a false idea of the amount of active drug available. Furthermore, variation in the degree of binding from one subject to another, and displacement of drug molecules by a second drug, may complicate the interpretation of serum levels when both bound and free drug are measured together. The strict use of therapeutic ranges of serum levels may therefore be harmful when a larger than normal proportion of the drug is free. In theory, monitoring the free concentration would have advantages, but on a routine basis this is not practical at present for technical reasons. Cerebrospinal fluid is an ultrafiltrate of plasma but lumbar puncture for routine monitoring purposes cannot be justified. Monitoring salivary concentrations is a practical alternative but for some drugs variation in the degree of ionization of the compound may make salivary levels unreliable.

An overview of psychiatric issues in liver disease for the consultation-liaison psychiatrist

Liver disease is a common cause of morbidity and mortality in the United States and elsewhere. Arising from infectious, hereditary, or toxin-induced sources, the detection of liver disease often requires a high index of suspicion. Clinical presentations are highly variable and are often accompanied by neuropsychiatric symptoms. This fact, along with an increased incidence of liver disease among patients with primary psychiatric disorders and the presence of varied drug use, complicates the tasks of providing care to patients with liver disease. To assist the consultation-liaison psychiatrist, the authors present the first of a two-part series focused on psychiatric issues in liver disease.

14C-Lidocaine Disposition in Serum and Tissues of Normal and Liver Diseased Rats

PURPOSE

This study was designed to investigate the disposition of (14)C-lidocaine in serum and tissues in rats with liver dysfunction.

MATERIALS AND METHODS

Eighteen male rats were randomly divided into 2 groups. Group A was considered as control while group B underwent liver damage by administrating CCl(4) 0.4 mg/kg twice a week for 6 weeks. Both groups received 5 doses of 2.5 mg/kg lidocaine mixture (labeled (14)C-lidocaine and nonlabeled). The rats were killed 2 hours after the last dose. Total lidocaine levels ((14)C-lidocaine and (14)C-lidocaine metabolite concentrations) as well as the percent of total lidocaine-bound fractions in tissues were measured.

RESULTS

(14)C-lidocaine concentrations were significantly increased in the serum (9.4+/-0.4 microg/mL), heart (7.8+/-2 microg/gL), and mandible (0.97+/-0.01 microg/g) in diseased rats as compared with normal rats (serum, 5.3+/-1.7 microg/mL; heart, 4.2+/-0.9 microg/g; mandible, 0.68+/-0.02 microg/g, respectively). (14)C-lidocaine bound fractions in the mandible and heart did not show any significant differences between the 2 groups. Instead, (14)C-lidocaine bound fractions in serum were significantly reduced in diseased animals as opposed to normal ones.

CONCLUSION

We concluded that liver dysfunction can modify (14)C-lidocaine concentrations in the serum and tissues without altering the lidocaine binding properties in the mandible and heart.

Evaluation of phenytoin dosage regimens based on genotyping of CYP2C subfamily in routinely treated Japanese patients

Two research groups have reported the effect of genetic polymorphisms of CYP2C9 and CYP2C19 on the pharmacokinetic parameters of phenytoin in Japanese epileptic patients. We measured the plasma phenytoin concentrations at steady-state in 20 routinely treated Japanese patients, and evaluated the usefulness of genotyping the CYP2C subfamily in predicting plasma concentrations and determining the dosage regimens of phenytoin. The plasma phenytoin concentrations predicted by genotypes of the CYP2C subfamily were well correlated with the observed concentrations in some patients, but not in some patients. The pharmacokinetic parameters (Vmax and Km) in individual patients, which were obtained from population estimates according to Bayes' theorem, showed considerable interindividual variability even among patients with the same genotype. In addition, we assessed the effect of plasma protein binding on the residual interindividual variability in the clearance of phenytoin; however, there was no significant correlation between the unbound fraction and the intrinsic metabolic activity (Vmax/Km). These findings suggested that the mechanism responsible for the large variability in the clearance of phenytoin is not completely resolved, and that we should not overestimate the usefulness of genotyping the CYP2C subfamily in determining the dosage regimens of the drug.

The effect of mild and moderate hepatic impairment on the pharmacokinetics of valdecoxib, a selective COX-2 inhibitor

PURPOSE

This study evaluated the effects of varying degrees of hepatic impairment on the pharmacokinetics and safety of valdecoxib following single and multiple dosing.

METHODS

This was an open-label, randomised, parallel group study in 12 subjects with mild hepatic impairment (Child-Pugh Class A) and in 13 with moderate hepatic impairment (Child-Pugh Class B) matched for age, weight, sex, and smoking status; there were two control groups of 12 healthy volunteers, one for each study group. All subjects received a single dose of valdecoxib 20 mg on day 1 and valdecoxib 20 mg twice daily on days 4-7, followed by a single morning dose on day 8. Plasma concentrations of free (unbound) and total valdecoxib and its active hydroxylated metabolite (SC-66905) were measured following single and multiple dosing (day 1 and day 8). Additionally, all subjects received a single intravenous dose of lidocaine 60 mg during the pretreatment period to determine plasma concentrations of monoethylglycinexylidide (MEGX) as a marker of hepatic CYP3A4 activity.

RESULTS

The mean apparent oral clearance of free valdecoxib in plasma at steady state decreased by 22-25% in those with mild to moderate impairment (corresponding to a 28-33% increase in the AUC of free valdecoxib and a 19-23% decrease in the AUC of total SC-66905). The mean free fraction of valdecoxib in plasma increased by 9-38%, resulting in a mean decrease in apparent oral clearance of total valdecoxib in plasma of 0-15% (corresponding to a 0-17% increase in the AUC of total valdecoxib). Individual AUCs for free valdecoxib and total SC-66905 did not correlate well with AUCs for MEGX, indicating that decreases in intrinsic clearance of valdecoxib in those with hepatic impairment could not solely be explained by decreased CYP3A4 expression in hepatic impairment.

CONCLUSIONS

In our small study sample, mild and moderate hepatic impairment appeared to have only a modest effect on valdecoxib and SC-66905 pharmacokinetics. The adjustment of valdecoxib dose or dosing regimen does not appear mandatory in subjects with mild or moderate hepatic impairment, although caution is necessary during treatment of these patients with valdecoxib.

Intraindividual and interindividual variations in the pharmacokinetics of mycophenolic acid in liver transplant patients

The authors evaluated the intraindividual and interindividual variations in the pharmacokinetics of mycophenolic acid after oral administration of mycophenolate mofetil in 10 liver transplant patients. Mycophenolic acid and its metabolite, mycophenolic acid glucuronide, were measured in plasma and urine by high-pressure liquid chromatography. The plasma protein binding of mycophenolic acid was determined by ultrafiltration. The maximum concentration of mycophenolic acid in plasma increased significantly (P < or = .05) with time from 9.1 +/- 7.2 microg/mL (<1 week) to 36.7 +/- 15.6 microg/mL (1 month). The area under the plasma concentration versus time curve of mycophenolic acid also increased significantly with time, from 50.8 +/- 42.1 microg x h/mL to 118.0 +/- 57.6 microg x h/mL (P < or = .05). The plasma protein binding of mycophenolic acid increased from 92% to 98%, and the apparent oral clearance [CL/F] decreased from 32.9 +/- 21.4 L/h during the first study period to 9.0 +/- 4.4 L/h (P < or = .05) during the third study period. The apparent intrinsic clearance of mycophenolic acid did not change significantly over time. The ratio of the area under the curve of mycophenolic acid glucluronide to mycophenolic acid in plasma decreased with time (25.5 +/- 21.2 vs 8.0 +/- 3.3) but did not reach statistical significance. The increased binding of mycophenolic acid to plasma proteins with time after transplantation appeared to contribute to the intraindividual variation, whereas differences in the ability of the liver to metabolize mycophenolic acid between patients appear to contribute to the large interindividual variation in the pharmacokinetics of mycophenolic acid. The observations in this study support the concept of measuring the unbound concentration of mycophenolic acid to optimize immunosuppressive drug therapy with mycophenolic acid.

A model to study intestinal and hepatic metabolism of propranolol in the dog

A model to investigate hepatic drug uptake and metabolism in the dog was developed for this study. Catheters were placed in the portal and hepatic veins during exploratory laparotomy to collect pre- and posthepatic blood samples at defined intervals. Drug concentrations in the portal vein were taken to reflect intestinal uptake and metabolism of an p.o. administered drug (propranolol), while differences in drug and metabolite concentrations between portal and hepatic veins reflected hepatic uptake and metabolism. A significant difference in propranolol concentration between hepatic and portal veins confirmed a high hepatic extraction of this therapeutic agent in the dog. This technically uncomplicated model may be used experimentally or clinically to determine hepatic function and metabolism of drugs that may be administered during anaesthesia and surgery.

Therapeutic options in patients with lymphoma and severe liver dysfunction

OBJECTIVES

To determine the long-term outcome of patients presenting with synchronous lymphoma and severe liver dysfunction and to describe the outcome of patients treated with initial mechlorethamine-based therapy used as a bridge to more conventional chemotherapy.

PATIENTS AND METHODS

We reviewed the clinical course of all patients diagnosed as having lymphoma who presented with severe liver dysfunction and received intravenous mechlorethamine between September 1988 and February 2003 at the Mayo Clinic in Rochester, Minn.

RESULTS

Forty-one patients were identified, 33 (80%) of whom had newly diagnosed, previously untreated lymphoma. Thirty-seven (90%) had non-Hodgkin lymphoma, and 4 (10%) had Hodgkin disease. Thirty-four patients (83%) had stage IV disease, and 31 (84%) of 37 had an intermediate-high International Prognostic Index. The median total bilirubin level before therapy was 10.7 mg/dL (range, 2.5-30.2 mg/dL), and the median alkaline phosphatase level was 982 U/L (range, 233-3415 U/L). In addition to mechlorethamine, 34 patients (83%) received concomitant corticosteroids, and 12 (29%) received concomitant rituximab. Twenty-two patients (54%) had sufficient improvement in liver function to receive subsequent standard chemotherapy. Nine patients (22%) are alive and disease-free at a median of 31 months (range, 4 to > or = 87 months) after mechlorethamine treatment. Factors associated with improved overall survival included improvement in bilirubin levels (P < .001) and receiving subsequent standard chemotherapy (P = .001).

CONCLUSION

Mechlorethamine, high-dose corticosteroids, and rituximab are useful therapeutic interventions for this unique group of patients with lymphoma and severe liver dysfunction. Substantial clinical improvement and long-term survival are possible.

Protein binding in antiretroviral therapies

There is marked variability in the extent to which the three classes of antiretroviral (ARV) drugs bind to plasma proteins (<5 to >99%). Protease inhibitors (PIs), with the exception of indinavir, are more than 90% protein bound, mainly to alpha1-acid glycoprotein (AAG). Efavirenz, a nonnucleoside reverse transcriptase inhibitor (NNRTI), is more than 99% bound, mainly to albumin. Nucleoside reverse transcriptase inhibitors (NRTIs) are not highly protein bound. The pharmacological activity of ARV drugs is dependent on unbound drug entering cells that harbor the human immunodeficiency virus (HIV). There has been concern that changes in protein binding could impact on antiviral activity and management. However, for PIs and NNRTIs, and for many drugs given orally, altered plasma binding would not be expected to influence the average exposure to unbound (active) drug after chronic oral dosing. Nevertheless, there will be a change in the relationship between total and unbound concentrations that will be important if, as part of therapeutic drug monitoring, the total rather than the unbound drug is measured. Measuring drug concentrations that are needed to inhibit different HIV strains (wild type and drug resistant) in vitro could also cause confusion because most methods employ bovine serum in the assay medium, and unbound concentrations are not directly measured. Estimating unbound drug concentrations in human plasma and in incubation media can be highly method dependent and thus may affect the calculated IC50 (the concentration of drug that results in 50% inhibition of viral replication). Because inhibitory quotients (IQs = C(trough)/IC50) are becoming part of pharmacokinetic/pharmacodynamic (PK/PD) analyses of clinical trial data, the strengths and weaknesses of the methods used for the determination of unbound drug concentration in plasma and in vitro systems--ultracentrifugation, ultrafiltration, and equilibrium dialysis--need to be understood. Consensus on standard procedures must be reached. In June 2002, a panel of experts assembled by the Forum for Collaborative HIV Research met in Washington, DC, to review the basic principles of protein binding of ARV drugs, and to discuss the impact that changes in plasma protein binding may have on the PKs and activity of ARV drugs as well as on therapeutic drug monitoring. The purpose of the meeting was to discuss the following topics: (1) basic principles of protein binding and how changes in binding can impact on drug PKs and drug exposure in vivo, (2) variability in plasma protein binding among patients taking ARV drugs, (3) the impact of HIV infection and concomitant diseases on the extent of plasma protein binding, (4) the likelihood of clinically relevant drug interactions at the level of plasma protein binding, (5) the evidence that measuring unbound concentrations of ARV drugs in the plasma of patients gives more meaningful information than total drug concentration and, therefore, should be considered in routine therapeutic drug monitoring of ARV agents, (6) optimal method(s) for measuring the unbound concentration of drugs in vitro (for IC50 determination) and in vivo, and (7) future studies that need to be considered to fully understand the importance of plasma protein binding in therapeutic drug monitoring. This report summarizes the topics discussed at this meeting. It guides the reader through the discussions that allowed the panel to formulate a series of statements regarding the significance of plasma protein binding of ARV drugs when studied in vitro and in vivo. The roundtable participants also identified research priorities that are important for understanding the sources of inter- and intraindividual variability in protein binding in patients. These include obtaining data on unbound as well as on total concentrations in PK studies; looking at variants of AAG and whether they differ in binding affinity; and emphasizing the importance of developing a standard procedure for drug susceptibility assays used to determine IC50 values.

Effect of glycerol-induced acute renal failure on the pharmacokinetics of lidocaine after transdermal application in rats

In this study, the effect of glycerol-induced acute renal failure (ARF) on the pharmacokinetics of lidocaine after transdermal application was investigated in rats. Microdialysis method was applied in vitro and in vivo to the abdominal skin of rats. After topical application of 1% lidocaine, the cumulative amount of lidocaine permeated through the excised rat abdominal skin showed parallel effect between normal and ARF rats with no significant difference in the in vitro permeability coefficient of lidocaine between them, while area under the plasma concentration versus time curve of lidocaine in ARF rats increased significantly. The protein binding rate of lidocaine in ARF plasma and the blood vessel permeability to muscle tissues, assessed by beta-D-glucopyranosyl fluorescein isothiocyanate-labeled (FITC) albumin, increased significantly. After intravenous infusion of 5 mg/h/kg lidocaine, both of the total body clearance and the volume of distribution of lidocaine in the ARF rats decreased significantly. These results suggested that renal dysfunction did not have any effect on the skin permeability of lidocaine, but might change the plasma protein binding of drug and blood vessel permeability which led to high plasma concentration of lidocaine.

The influence of hemorrhagic shock on etomidate: a pharmacokinetic and pharmacodynamic analysis

We studied the influence of hemorrhagic shock on the pharmacology of etomidate in swine. Sixteen swine were randomly assigned to control and shock groups. The shock group was bled to a mean arterial blood pressure of 50 mm Hg and held there until 30 mL/kg blood was removed. Etomidate 300 micro g x kg(-1) x min(-1) was infused for 10 min to both groups. Fifteen arterial samples were collected until 180 min after the infusion began to determine drug concentration. Pharmacokinetic variables for each group were estimated by using a three-compartment model. The bispectral index scale was used as a measure of drug effect. The pharmacodynamics were characterized by using a sigmoid inhibitory maximal effect model. The raw data revealed a 25% increase in the plasma etomidate concentration at the end of the 10-min infusion which resolved after termination of the infusion in the shock group. The pharmacokinetic analysis revealed subtle changes in the variable estimates between groups. The etomidate infusion produced a similar Bispectral Index Scale change in both groups. These results demonstrated that, unlike the influence of hemorrhagic shock on other sedative hypnotics and opioids, moderate hemorrhagic shock produced minimal changes in the pharmacokinetics and no change in the pharmacodynamics of etomidate.

IMPLICATIONS

Hemorrhagic shock produced minimal changes in the pharmacokinetics and no change in the pharmacodynamics of etomidate in swine. These results suggest that, unlike other sedative hypnotics and opioids, minimal adjustment in the dose of etomidate is required to achieve the same drug effect during hemorrhagic shock.

Pharmacokinetics help optimizing mycophenolate mofetil dosing in kidney transplant patients

BACKGROUND

Mycophenolic acid (MPA), the active metabolite of mycophenolate mofetil (MMF), is now routinely used as immunosuppressant in solid organ transplantation in a fixed daily dose regimen (2 g/d) in association with cyclosporine (CsA) and steroids. However, no correlation has been shown between fixed MMF dose and clinical outcome.

METHODS

Here we examined the possibility of optimizing MMF dosing by drug pharmacokinetic monitoring in 46 stable kidney transplant recipients. MPA plasma concentration profiles were measured by a reverse-phase high-performance liquid chromatography method 6-9 months after transplantation and related with routine laboratory analysis tests. Since MPA is extensively bound to serum albumin and only the free fraction is pharmacologically active, in a subgroup of 23 patients free plasma MPA was also determined.

RESULTS

Despite a comparable MMF dose, a large interindividual variability in both MPA area under the curve (AUC) from 0 to 12 h (range 10.1-99.8 microg/mL. h) and in trough levels (range 0.24-7.04 microg/mL) was found. Patients with AUC >40 microg/mL. h showed a better (p<0.05) renal function than patients with lower AUC (creatinine clearance 85.7+/-23.2 versus 64.5+/-17.5 mL/min), despite no difference in CsA dose, CsA AUC and blood CsA trough level. The percentage of free plasma MPA but not total MPA correlated with the red blood cell and leukocyte count.

CONCLUSIONS

Therapeutic MMF drug monitoring might contribute to a better management of kidney transplant recipient with the goal of optimizing drug dosing and limiting the risk of MMF-related toxicity.

Pharmacokinetics of haloperidol: an update

Haloperidol is commonly used in the therapy of patients with acute and chronic schizophrenia. The enzymes involved in the biotransformation of haloperidol include cytochrome P450 (CYP), carbonyl reductase and uridine diphosphoglucose glucuronosyltransferase. The greatest proportion of the intrinsic hepatic clearance of haloperidol is by glucuronidation, followed by the reduction of haloperidol to reduced haloperidol and by CYP-mediated oxidation. In studies of CYP-mediated disposition in vitro, CYP3A4 appears to be the major isoform responsible for the metabolism of haloperidol in humans. The intrinsic clearances of the back-oxidation of reduced haloperidol to the parent compound, oxidative N-dealkylation and pyridinium formation are of the same order of magnitude, suggesting that the same enzyme system is responsible for the 3 reactions. Large variation in the catalytic activity was observed in the CYP-mediated reactions, whereas there appeared to be only small variations in the glucuronidation and carbonyl reduction pathways. Haloperidol is a substrate of CYP3A4 and an inhibitor, as well as a stimulator, of CYP2D6. Reduced haloperidol is also a substrate of CYP3A4 and inhibitor of CYP2D6. Pharmacokinetic interactions occur between haloperidol and various drugs given concomitantly, for example, carbamazepine, phenytoin, phenobarbital, fluoxetine, fluvoxamine, nefazodone, venlafaxine, buspirone, alprazolam, rifampicin (rifampin), quinidine and carteolol. Overall, drug interaction studies have suggested that CYP3A4 is involved in the biotransformation of haloperidol in humans. Interactions of haloperidol with most drugs lead to only small changes in plasma haloperidol concentrations, suggesting that the interactions have little clinical significance. On the other hand, the coadministration of carbamazepine, phenytoin, phenobarbital, rifampicin or quinidine affects the pharmacokinetics of haloperidol to an extent that alterations in clinical consequences would be expected. In vivo pharmacogenetic studies have indicated that the metabolism and disposition of haloperidol may be regulated by genetically determined polymorphic CYP2D6 activity. However, these findings appear to contradict those from studies in vitro with human liver microsomes and from studies of drug interactions in vivo. Interethnic and pharmacogenetic differences in haloperidol metabolism may explain these observations.

Comparative effects of oxygen supplementation on theophylline and acetaminophen clearance in human cirrhosis

BACKGROUND & AIMS

Sinusoidal capillarization in cirrhosis may impair the transfer of oxygen into hepatocytes; this may contribute to impaired oxidative drug metabolism. The aim of this study was to test this hypothesis by comparing the effects of oxygen supplementation in cirrhotic patients on the clearance of theophylline, which is dependent on hepatic oxidative metabolism, with its effect on the clearance of acetaminophen, which is reliant on hepatic conjugation reactions.

METHODS

Ten cirrhotic patients awaiting liver transplant and 5 control subjects were studied. Oral acetaminophen (1000 mg) and intravenous theophylline (3 mg/kg) were administered simultaneously on two separate occasions, 7 days apart. Subjects were randomized to breathe either room air or oxygen via face mask at 12 L/min for 9 hours of blood sampling.

RESULTS

Theophylline and acetaminophen clearances were significantly reduced by a mean of 54% and 50%, respectively, in cirrhotic patients compared with controls. Oxygen supplementation improved plasma theophylline clearance in cirrhotic patients by a mean of 34% (P = 0. 001), whereas acetaminophen clearance remained unchanged.

CONCLUSIONS

These findings indicate that, in cirrhosis, impaired hepatocyte oxygenation contributes to reduced oxidative drug metabolism and that oxidative drug metabolism can be improved by oxygen supplementation.

Human serum attenuates the activity of protease inhibitors toward wild-type and mutant human immunodeficiency virus

The potency of therapeutic regimens containing human immunodeficiency virus (HIV) protease inhibitors is related to the ability to maintain concentrations of drug in the plasma of patients that are sufficient for blocking viral replication. The estimation of concentrations required for in vivo activity using in vitro assays is complicated by the fact that extensive binding of many protease inhibitors to serum proteins attenuates their antiviral potency. To provide insight into the relative in vivo potency of current protease inhibitors, we assayed their in vitro activity against wild-type and mutant HIV in the presence of human serum (HS). Using this assay, ABT-378, a new protease inhibitor with trough levels in humans far in excess of the EC50 in the presence of 50% HS, was identified. The antiviral activity of ABT-378 was only modestly attenuated by HS, in contrast to ritonavir, saquinavir, and nelfinavir. Examination of the effect of individual serum components suggested that the activity of ABT-378 is affected predominantly by binding to alpha1-acid glycoprotein (AGP) while the activity of ritonavir is modulated by both AGP and albumin. The method described here may provide insight into the in vivo potency of protease inhibitors and be useful for the preclinical evaluation and selection of new protease inhibitors for clinical studies.

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.

The organ extraction and splanchnic haemodynamic effects of octreotide in cirrhotic patients

BACKGROUND

Intravenous octreotide is an established treatment of oesophageal variceal haemorrhage in the cirrhotic patient.

AIM

To examine the organ extraction and splanchnic haemodynamic effects of octreotide in cirrhotic patients with portal hypertension.

METHODS

Thirteen patients with cirrhosis had hepatic venous catheterization performed. Hepatic venous pressure gradient (HVPG), indocyanine green (ICG) clearance and hepatic blood flow (HBF) were determined in the basal state and during 60 min of octreotide infusion by bolus injection (0.75 microg/kg) followed by continuous infusion of 0.75 microg/kg x h. Blood samples were simultaneously drawn from the femoral artery and the hepatic and renal veins.

RESULTS

The extraction fraction of octreotide in the liver was 0.05 (-0.01 - 0.14) (median (interquartile range)) and in the kidneys 0.16 (-0.06 - 0.35). The extraction fraction ratio (E(liver)/E(kidney)) was 0.69 (-0.20 - 1.06). Hepatic clearance was 47 mL/min (3-88) (n = 11). No correlations were found between liver biochemistry or galactose elimination capacity (GEC; a metabolic measure of liver function) and renal extraction fraction or liver clearance. Octreotide had no effect on HVPG or wedged hepatic venous pressure although free hepatic venous pressure increased during octreotide infusion: 6 mmHg (5-9) vs. 7 mmHg (6-10) (P = 0.02). No effect on HBF was observed while ICG clearance decreased significantly.

CONCLUSIONS

Octreotide is extracted in cirrhotic patients by both the liver and the kidney, the latter being the most important organ of elimination. Octreotide decreases liver metabolic activity determined by the ICG clearance technique, but no significant effects of octreotide on HVPG or HBF could be demonstrated.

Pharmacokinetics of rifapentine in patients with varying degrees of hepatic dysfunction

In this open-label investigation, the pharmacokinetics of rifapentine and its active metabolite, 25-desacetyl-rifapentine, were characterized in patients with varying degrees of hepatic dysfunction. Eight patients with mild-to-moderate chronic, stable hepatic dysfunction and seven patients with moderate-to-severe hepatic dysfunction received single oral 600-mg doses of rifapentine. Maximum plasma concentration of rifapentine was lower, time to maximum plasma concentration (tmax) was greater, and elimination half-life (t 1/2) was longer in the patients with moderate-to-severe hepatic dysfunction than in those with mild-to-moderate dysfunction. However, mean area under the concentration-time curve extrapolated to infinity (AUC0-infinity) for the two groups was similar. AUC0-infinity values in patients with hepatic dysfunction were 19% to 25% higher than values previously reported for healthy volunteers. The 25-desacetyl metabolite appeared in plasma slowly after the single oral dose of rifapentine. Similar to findings for the parent drug, comparable plasma exposures of 25-desacetyl-rifapentine based on AUC0-infinity were found in the two groups of patients with mild-to-moderate and moderate-to-severe hepatic dysfunction. Rifapentine was well tolerated in this patient population, irrespective of the etiology or severity of hepatic dysfunction. These safety and pharmacokinetic results suggest that no dosage adjustments for rifapentine are needed in patients with hepatic impairment.

The effect of the acute-phase response on in vitro drug metabolism and plasma protein binding in the horse

The effect of the acute-phase response (APR) on the activity of the hepatic drug-metabolizing system (DMS) and on the binding of phenylbutazone to plasma proteins was investigated in the horse. An APR was induced by intramuscular injections of Freund's complete adjuvant in five horses and, five days later, these horses together with five clinically normal horses were shot and the right ventral lobe of each liver removed. The hepatic microsomal fractions from the liver samples were isolated and significantly lower (p < 0.01) concentrations of cytochromes P450 and b5 and activities of aniline-p-hydroxylase and aminopyrine N-demethylase (43%, 55%, 45% and 30%, respectively) were measured in the livers from the adjuvant-inflamed horses, compared to the controls. Phenylbutazone (PBZ) was administered intravenously (4.4 mg/kg) to a further four horses and plasma protein binding was measured by ultracentrifugation. Five weeks later, these horses were injected with Freund's complete adjuvant and the intravenous administration of PBZ (4.4 mg/kg) was repeated. Inflammation induced a significant increase (p < 0.01) in the unbound fraction of PBZ (5.2 +/- 0.5 as against 1.4 +/- 0.6%). These results suggest that the APR depresses the hepatic DMS and reduces the binding of PBZ to plasma proteins.

The effect of inflammation on the disposition of phenylbutazone in thoroughbred horses

The effect of inflammation on the disposition of phenylbutazone (PBZ) was investigated in Thoroughbred horses. An initial study (n = 5) in which PBZ (8.8 mg/kg) was injected intravenously twice, 5 weeks apart, suggested that the administration of PBZ would not affect the plasma kinetics of a subsequent dose. Two other groups of horses were given PBZ at either 8.8 mg/kg (n = 5) or 4.4 mg/kg (n = 4). Soft tissue inflammation was then induced by the injection of Freud's adjuvant and the administration of PBZ was repeated at a dose level equivalent to, but five weeks later than, the initial dose. Inflammation did not appear to affect the plasma kinetics or the urinary excretion of PBZ and its metabolites, oxyphenbutazone (OPBZ) or hydroxyphenylbutazone (OHPBZ) when PBZ was administered at 8.8 mg/kg. However, small but significant increases (P < 0.05) in total body clearance (CLB; 29.2 +/- 3.9 vs. 43.8 +/- 8.1 mL/ h.kg) and the volume of distribution, calculated by area (Vd(area); 0.18+/- 0.05 vs. 0.25 +/- 0.03 L/kg) or at steady-state (Vd(SS); 0.17 +/- 0.04 vs. 0.25 +/- 0.03 L/ kg), were obtained in horses after adjuvant injection, compared to controls, when PBZ was administered at 4.4 mg/kg which corresponded to relatively higher tissues concentrations and lower plasma concentrations (calculated) at the time of maximum peripheral PBZ concentration. Soft tissue inflammation also induced a significantly (P < 0.05) higher amount of OPBZ in the urine 18 h after PBZ administration but the total urinary excretion of analytes over 48 h was unchanged. These results have possible implications regarding the administration of PBZ to the horse close to race-day.

Serum protein binding of propofol in patients with renal failure or hepatic cirrhosis

BACKGROUND

Serum protein binding is a limiting factor in the access of drugs to the central nervous system. Disease-induced modifications of the degree of binding may influence the effect of anesthetic drugs.

METHODS

The protein binding of propofol, an intravenous anaesthetic agent which is highly bound to serum albumin, has been investigated in serum samples from healthy volunteers, from patients with chronic renal failure not undergoing hemodialysis, from patients with chronic renal failure included in a regular hemodialysis program, and from patients with hepatic cirrhosis. Protein binding was determined by the ultrafiltration technique using an Amicon Micropartition System, MPS-1.

RESULTS

The percentage of unbound propofol (mean(SD)) in healthy volunteers (n = 16) was 0.98 (0.48) % showing a high interindividual variability. Chronic renal failure did not significantly modify serum protein binding of propofol. In the chronic renal failure group not undergoing regular hemodialysis (n = 9), unbound propofol was 0.92 (0.34) %. In addition, patients in periodic dialysis did not show changes in propofol binding either compared before (1.11 (0.33) %; n = 13) or after hemodialysis (0.87 (0.38) %; n = 12). A slight decrease in albumin concentration was found in all renal patients (P < 0.05) in comparison to healthy volunteers. Creatinine and urea concentrations were higher in these patients (P < 0.01) but in the postdialysis group, the differences in urea levels were not significant when compared with those of volunteers. No changes in the degree of propofol binding were observed in patients with hepatic cirrhosis (0.97 (0.30) %; n = 14) when compared with the group of healthy volunteers. Significant differences were observed in albumin (P < 0.01) and bilirubin (P < 0.05) concentrations. Considering all subjects, the degree of binding did not correlate with biomedical data.

CONCLUSION

Due to the absence of significant changes in the protein binding it is unlikely that there will be an exaggerated pharmacological response in patients with renal and hepatic disease following the administration of a standard propofol dose, although due to interpatient variability careful titration can be recommended.

The pharmacokinetics or oral and intravenous allopurinol and intravenous oxypurinol in the horse

The pharmacokinetics of oral and intravenous allopurinol was studied in five horses and compared with intravenous oxypurinol. The plasma concentration vs. time curves, following intravenous administration of 5 mg/kg, were best described by the biexponential equations Cp = 106.58e(-25.14t) + 159.93e(-10.96t) for allopurinol and Cp = 321.09e(-9.72t) + 82.39e(-0.44t) for oxypurinol, with an elimination half-life (t1/2 beta) of 0.09 h and an area under the curve (AUC) of 19.8 mumol.h/L after intravenous administration, while the t1/2 beta and AUC of oxypurinol were 1.09 h and 231 mumol.h/L, respectively. The bioavailability of allopurinol was low (14.3%), although no allopurinol was detected in the plasma of two horses after oral administration of allopurinol was equivalent to that of intravenously injected oxypurinol. The results suggest that allopurinol is rapidly metabolised in vivo and that the majority of the pharmacological activity of allopurinol in the horse may result from the action of the active metabolite, oxypurinol.