Hepatic Drug Extraction Research Articles

Phase I Trial of Hepatic Arterial Infusion of Nanoparticle Albumin–Bound Paclitaxel: Toxicity, Pharmacokinetics, and Activity

Because liver involvement in patients with metastatic cancer has limited options and poor outcomes, we conducted a phase I study to determine the safety, activity, and pharmacokinetic characteristics of hepatic arterial infusion of nanoparticle albumin-bound paclitaxel (HAI nab-paclitaxel). Cohorts of three patients having predominant hepatic metastases received HAI nab-paclitaxel at three dose levels (180, 220, and 260 mg/m(2), respectively) infused for more than 1 hour every 3 weeks (3 + 3 design). Some patients participated in comparative pharmacokinetic studies (i.v. vs. HAI), receiving their first course i.v., to determine peak concentrations and effect of first-pass hepatic extraction compared with subsequent courses administered by HAI. The highest dose level was expanded to determine the safety and activity of HAI nab-paclitaxel. Thirty-eight patients were treated. There were no dose-limiting toxicities at doses up to 260 mg/m(2). Common adverse events included alopecia, fatigue, myelosuppresion, nausea, and vomiting. Three patients had stable disease for 4 or more months and 2 patients (1 of 12 with breast cancer and 1 of 1 with cervical cancer) achieved a partial response lasting for 5 and 15 months, respectively. Peak concentrations were lower (∼50%) with greater hepatic extraction of drug (∼42%) following HAI than i.v. infusion based on area under the curve comparison of drug exposure. HAI nab-paclitaxel showed partial hepatic extraction. At doses 260 mg/m(2) or less given for 1 hour every 3 weeks, the treatment was well-tolerated and showed activity in advanced cancer patients with predominant liver metastases.

A phase I study of gemcitabine given via intrahepatic pump for primary or metastatic hepatic malignancies

To determine the maximum tolerated dose and duration of hepatic arterial infusion (HAI) gemcitabine in patients with unresectable hepatic metastases from colorectal cancer or primary hepatic malignancies. Patients received weekly gemcitabine via the side-port of an implantable HAI pump for 3 weeks in a 28-day cycle. During the dose escalation phase, increasing doses of HAI gemcitabine (800, 1,000, 1,200, and 1,500 mg/m(2)) were given at a fixed dose-rate of 10 mg/(m(2) min). This was followed by the infusion duration escalation (IDE) phase, in which HAI gemcitabine at 1,000 mg/m(2) was given over increasing lengths of time (200, 300, and 400 min). To estimate hepatic drug extraction, the pharmacokinetics of HAI gemcitabine was compared with those of intravenous gemcitabine given at the same dose-rate to the same patient in the IDE phase. Twenty-eight of 30 patients were evaluable. HAI gemcitabine was well tolerated up to 1,500 mg/m(2) given at 10 mg/(m(2) min) and up to 1,000 mg/m(2) infused over 400 min. There were no protocol-defined dose-limiting toxicities. One patient with cholangiocarcinoma had a partial response. Hepatic extraction of gemcitabine seems highly variable among patients and does not correlate with the length of HAI infusion. Hepatic arterial infusion of gemcitabine given at doses higher or longer than the recommended systemic dose of 1,000 mg/m(2) over 30 min is well tolerated. For future studies, we recommend an infusion of 1,500 mg/m(2) at a fixed dose-rate of 10 mg/(m(2) min).

Presentation of a modified dispersion model (MDM) for hepatic drug extraction and a new methodology for the prediction of the rate-limiting step in hepatic metabolic clearance

The main objectives were to investigate the roles of and interplay between determinants of hepatic clearance (CLH) in humans, to develop a methodology and reference system for the evaluation and prediction of the rate-limiting step in CLH, and to update the dispersion model and compare it with traditionally used liver extraction models.The new methodology enables predictions of the hepatic uptake and CLH, dissociation, and rate-limiting step. In general, absorption, dissociation and diffusion are comparably rapid processes, and metabolism is rate-limiting. The liver appears to have a high passive uptake capacity. The Modified Dispersion Model (MDM) has a dispersion number of 0.5 and a distribution factor (dƒ = 0.87) for the correction of a longer hepatic transit time of unbound molecules and the exclusion of the hidden unbound fraction within erythrocytes. Liver models are functionally equivalent at low CLH, but differ for highly extracted compounds. Well-stirred and parallel-tube models demonstrate the greatest difference in performance, for example, 6- and 800 000-fold differences in the estimated in vivo intrinsic CLH and predicted oral bioavailability of the high CLH drug naloxone, respectively.The roles of and interplay between determinants of CLH have been further clarified and can now be better predicted. Apparent advantages with the MDM include its scientific rationale and intermediate/ balanced performance.

An integrated approach to model hepatic drug clearance

It has been well accepted that hepatic drug extraction depends on the blood flow, vascular binding, transmembrane barriers, transporters, enzymes and cosubstrate and their zonal heterogeneity. Models of hepatic drug clearances have been appraised with respect to their utility in predicting drug removal by the liver. Among these models, the “well-stirred” model is the simplest since it assumes venous equilibration, with drug emerging from the outflow being in equilibrium with drug within the liver, and the concentration is the same throughout. The “parallel tube” and dispersion models, and distributed model of Goresky and co-workers have been used to account for the observed sinusoidal concentration gradient from the inlet and outlet. Departure from these models exists to include heterogeneity in flow, enzymes, and transporters. This article utilized the physiologically based pharmacokinetic (PBPK) liver model and its extension that include heterogeneity in enzymes and transporters to illustrate how in vitro uptake and metabolic data from zonal hepatocytes on transport and enzymes may be used to predict the kinetics of removal in the intact liver; binding data were also necessary. In doing so, an integrative platform was provided to examine determinants of hepatic drug clearance. We used enalapril and digoxin as examples, and described a simple liver PBPK model that included transmembrane transport and metabolism occurring behind the membrane, and a zonal model in which the PBPK model was expanded three sets of sub-compartments that are arranged sequentially to represent zones 1, 2, and 3 along the flow path. The latter model readily accommodated the heterogeneous distribution of hepatic enzymes and transporters. Transport and metabolic data, piecewise information that served as initial estimates, allowed for the unknown efflux and other intrinsic clearances to be estimated. The simple or zonal PBPK model provides predictive views on the hepatic removal of drugs and metabolites.

Ion-trapping, microsomal binding, and unbound drug distribution in the hepatic retention of basic drugs.

This study investigated the relative contribution of ion-trapping, microsomal binding, and distribution of unbound drug as determinants in the hepatic retention of basic drugs in the isolated perfused rat liver. The ionophore monensin was used to abolish the vesicular proton gradient and thus allow an estimation of ion-trapping by acidic hepatic vesicles of cationic drugs. In vitro microsomal studies were used to independently estimate microsomal binding and metabolism. Hepatic vesicular ion-trapping, intrinsic elimination clearance, permeability-surface area product, and intracellular binding were derived using a physiologically based pharmacokinetic model. Modeling showed that the ion-trapping was significantly lower after monensin treatment for atenolol and propranolol, but not for antipyrine. However, no changes induced by monensin treatment were observed in intrinsic clearance, permeability, or binding for the three model drugs. Monensin did not affect binding or metabolic activity in vitro for the drugs. The observed ion-trapping was similar to theoretical values estimated using the pHs and fractional volumes of the acidic vesicles and the pKa values of drugs. Lipophilicity and pKa determined hepatic drug retention: a drug with low pKa and low lipophilicity (e.g., antipyrine) distributes as unbound drug, a drug with high pKa and low lipophilicity (e.g., atenolol) by ion-trapping, and a drug with a high pKa and high lipophilicity (e.g., propranolol) is retained by ion-trapping and intracellular binding. In conclusion, monensin inhibits the ion-trapping of high pKa basic drugs, leading to a reduction in hepatic retention but with no effect on hepatic drug extraction.

Absorption, first-pass metabolism, and disposition of itraconazole in rats.

This study examined the pharmacokinetic disposition, oral absorption and hepatic extraction of itraconazole and its active metabolite, hydroxyitraconazole, in rats. After i.v. injection, serum itraconazole concentrations decreased biexponentially, with an average terminal elimination half-life, volume of distribution and systemic clearance of 4.9 h, 6.0 l/kg and 14.2 ml/min/kg, respectively. When given orally, its absorption was low, with a mean absolute bioavailability of 16.6%. The metabolite to parent drug area under the curve (AUC) ratio was higher after oral administration compared with i.v. injection (mean ratio, 2.7 vs. 0.9). The hepatic drug extraction ratio determined after femoral and portal vein administration averaged 18.5%. When hydroxyitraconazole was injected i.v., the elimination half-life, volume of distribution and systemic clearance of itraconazole averaged 10.0 h, 2.4 l/kg and 3.4 ml/min/kg, respectively. The fraction of the systemically available itraconazole that was metabolized to hydroxyitraconazole was 21.0% and 76.0% after i.v. and oral administration, respectively. In summary, this study is the first reporting the hepatic extraction of itraconazole and the i.v. disposition characteristics of hydroxyitraconazole in rats. Itraconazole is a drug with a low hepatic extraction ratio and its systemic clearance appears to be largely accounted for by hepatic metabolism.

Theoretic and Experimental Aspects of Regional Liver Infusion

Pharmacokinetic theory of hepatic arterial infusion chemotherapy predicts that parameters, such as hepatic blood flow, hepatic drug extraction, and extrahepatic drug clearance, may potentially be manipulated to enhance the selectivity of hepatic artery drug administration. In practice, diminution of hepatic blood flow through the use of degradable starch microspheres or gelatin sponge embolization has been accomplished and has improved drug selectivity. Likewise, augmentation of hepatic drug extraction improves the pharmacokinetic advantages of regional chemotherapy for liver tumors. Development of optimal hepatic infusion chemotherapy approaches can be undertaken rationally and in an expeditious fashion by applying pharmacokinetic principles to create innovative treatment regimens and by testing them in preclinical and phase I investigations.

Recovery of Hepatic Drug Extraction After Hypothermic Preservation

Comment: This is an extremely well done study, and the authors are to be congratulated for using the model they did and performing the study as well as they have. But perhaps they come up with their own best comment about the article in their closing lines, when they state, “Future work may indicate whether compromised drug metabolism is a sensitive and/or specific marker of dysfunction in the transplanted liver.” It has always been a mystery why the 5 or 10% of livers or greater percentage of kidneys shut down function soon after being in place when they appear to be adequately perfused and preserved. We need early markers of this destruction if we are to understand and prevent it. One marker that has been tried and continues to be tried is to look at perfusion, especially with sophisticated techniques such as perfusion of air microspheres with ultrasound. Whether the perfusion to the small areas of the liver is a good marker or whether compromised drug metabolism is a better marker, meaning more specific and more sensitive, remains to be determined.

Recovery of Hepatic Drug Extraction after Hypothermic Preservation

To determine whether liver preservation before transplantation impairs hepatic drug metabolism, hepatic extraction of drugs with different metabolic pathways (fentanyl, morphine, and vecuronium) in isolated rat livers was measured either immediately or after 24 h of hypothermia at 4 degrees C using a standard preservation-reperfusion sequence. Isolated rat livers were perfused via the portal vein for 30 min to document initial viability. Test livers (n = 5) were perfused with iced Belzer solution, stored for 24 h at 4 degrees C, and flushed with 6% hetastarch. After hypothermic preservation for 24 h, or in control livers (n = 5) immediately after the 30-min perfusion, livers were perfused single-pass at a constant flow rate with solutions containing fentanyl, morphine, and vecuronium at 37 degrees C. Perfusate and bile samples were obtained at regular intervals for 64 min, after which liver tissue was harvested for analysis. Drug concentrations were measured using radioimmunoassay and gas chromatography. Metabolic capacity of the liver was estimated from the extraction fraction of each drug at steady-state. After warming to 37 degrees C, preserved livers consumed oxygen and produced bile at rates similar to that of control livers. Hypothermic preservation did not affect extraction of fentanyl and morphine. Vecuronium extraction was initially less in preserved livers, but this difference disappeared as the preserved livers returned to 37 degrees C (< 16 min). Biliary excretion and tissue concentrations of vecuronium were similar in each group. Hypothermic preservation does not significantly impair extraction of these drugs in this liver preservation model. If these results apply to human liver transplantation, little danger of drug accumulation exists during the early postoperative period if hepatic function is normal.

Diuretic use in the elderly: Potential for diuretic-induced hypokalemia

The more potent “loop” diuretics are being used with increasing frequency. The elderly constitute a growing portion of the population undergoing treatment with diuretics. The alterations in renal function and pharmacokinetics in the elderly (over 60 years of age) may result in the development of certain adverse effects. In patients over 70 years old, there is a progressive decline in overall renal function, resulting in a more than 50% decrease in glomerular filtration rate. Most of the pharmacokinetic changes in the elderly consist of alterations resulting in enhanced plasma levels of any given drug; diminished hepatic drug extraction, detoxification/metabolism or prodrug conversion; decreased renal excretion of drug; and diminished volume of distribution of drug. Adverse reactions to diuretics may be grouped into metabolic changes (e.g., hypokalemia), physiologic alteration (e.g., volume contraction), toxic manifestation (e.g., interstitial nephritis) and allergic or idiosyncranic phenomena (e.g., rash or thrombocytopenia). There is general agreement that significant hypokalemia, particularly among elderly patients receiving digitalis glycosides, is significant and requires therapy. Diuretic-associated hypokalemia reflects the potency and duration of action of a diuretic, factors modulating potassium balance including dietary intake and concurrent medical processes. The short duration of action and greater natriuresis relative to kaliuresis characteristics of loop diuretics may result in a lesser degree of hypokalemia than that seen with traditional thiazide diuretics.

Perfusion-limited effects of plasma drug binding on hepatic drug extraction

The hepatic elimination of phenytoin has been studied in the isolated rat liver perfused at constant flow with Krebs solution alone and in the presence of albumin. At an albumin concentration of 0.5 g/dl, 46.6% of the phenytoin was bound in the perfusate and the comparable value at 5.0 g/dl was 87.4%. The increase in binding resulted in a reduction in the hepatic extraction ratio from 0.67 in Krebs to 0.54 and 0.28 at the two albumin concentrations, respectively. Analysis of this data together with that from the literature on propranolol and warfarin indicated that they were consistent with the perfusion-limited model of hepatic clearance. Accordingly, the general relationship between the extraction ratio and the free fraction of drug in the blood is hyperbolic with the precise shape being determined by the ratio of the clearance of the drug from liver water to the hepatic blood flow rate.

Intravenous thiobarbiturate anesthesia for cesarean section.

A strictly controlled thiobarbiturate–succinylcholine–oxygen sequence, with thiobarbiturate dosage 4-7 mg/kg, seems safe for elective cesarean section. Two hundred and forty-eight patients were divided into four groups, which received 4, 5⅓ 6⅓ or 8 mg/kg of thiobarbiturate, respectively. Blood samples for barbiturate analysis were drawn at birth from the maternal antecubital vein and the umbilical artery and vein. Apgar scores were good (7-0) in 75-90 per cent, and fair (4-6) in 6-19 per cent, of infants in the first three groups, but only 60 per cent good, 34 per cent fair, after 8 mg/kg. Amniotic or other fluid had accumulated in the oropharynxes of most infants with low scores. Factors combining to protect the fetal brain against massive invasion by thiobarbiturale from the mother include: swift decline of drug concentrations in maternal blood; nomhomogeneity of blood in the intervillous space; absence of a maternalfetal capillary bed; hepatic extraction of drug from umbilical venous blood; progressive dilution; extensive shunting in the fetal circulation.