Metabolism of taxol by human and rat liver in vitro: a screen for drug interactions and interspecies differences

Study on Cytochrome P450 Metabolic Profile of Paclitaxel on Rats using QTOF-MS

BACKGROUND

Paclitaxel (PTX) is a key drug used for chemotherapy for various cancers. The hydroxylation metabolites of paclitaxel are different between humans and rats. Currently, there is little information available on the metabolic profiles of CYP450 enzymes in rats.

OBJECTIVE

This study evaluated the dynamic metabolic profiles of PTX and its metabolites in rats and in vitro.

METHODS

Ultra-performance liquid chromatography coupled with quadrupole time-of-flight mass spectrometry (UHPLC-Q-TOF-MS) and LC-MS/MS were applied to qualitative and quantitative analysis of PTX and its metabolites in rats, liver microsomes and recombinant enzyme CYP3A1/3A2. Ten specific inhibitors [NF (CYP1A1), FFL (CYP1A2), MOP (CYP2A6), OND (CYP2B6), QCT (CYP2C8), SFP (CYP2C9), NKT (CYP2C19), QND (CYP2D6), MPZ (CYP2E1) and KTZ (CYP3A4)] were used to identify the metabolic pathway in vitro.

RESULTS

Four main hydroxylated metabolites of PTX were identified. Among them, 3'-p-OH PTX and 2-OH PTX were monohydroxylated metabolites identified in rats and liver microsome samples, and 6α-2-di-OH PTX and 6α-5"-di-OH PTX were dihydroxylated metabolites identified in rats. CYP3A recombinant enzyme studies showed that the CYP3A1/3A2 in rat liver microsomes was mainly responsible for metabolizing PTX into 3'-p- OH-PTX and 2-OH-PTX. However, 6α-OH PTX was not detected in rat plasma and liver microsome samples.

CONCLUSION

The results indicated that the CYP3A1/3A2 enzyme, metabolizing PTX into 3'-p-OH-PTX and 2- OH-PTX, is responsible for the metabolic of PTX in rats. The CYP2C8 metabolite 6α-OH PTX in humans was not detected in rat plasma in this study, which might account for the interspecies metabolic differences between rats and humans. This study will provide evidence for drug-drug interaction research in rats.

FMO3 deficiency of duck leads to decreased lipid deposition and increased antibacterial activity

Background

Most duck eggs possess a fishy odor, indicating that ducks generally exhibit impaired trimethylamine (TMA) metabolism. TMA accumulation is responsible for this unpleasant odor, and TMA metabolism plays an essential role in trimethylaminuria (TMAU), also known as fish odor syndrome. In this study, we focused on the unusual TMA metabolism mechanism in ducks, and further explored the unclear reasons leading to the debilitating TMA metabolism.

Methods

To achieve this, transcriptome, proteome, and metagenome analyses were first integrated based on the constructed duck populations with high and low TMA metabolism abilities. Additionally, further experiments were conducted to validate the hypothesis regarding the limited flavin-containing monooxygenase 3 (FMO3) metabolism ability of ducks.

Results

The study demonstrated that liver FMO3 and cecal microbes, including Akkermansia and Mucispirillum, participated in TMA metabolism in ducks. The limited oxidation ability of FMO3 explains the weakening of TMA metabolism in ducks. Nevertheless, it decreases lipid deposition and increases antibacterial activity, contributing to its survival and reproduction during the evolutionary adaptation process.

Conclusions

This study demonstrated the function of FMO3 and intestinal microbes in regulating TMA metabolism and illustrated the biological significance of FMO3 impairment in ducks.

Pharmaceutical Excipients and Drug Metabolism: A Mini-Review

Conclusions from previously reported articles have revealed that many commonly used pharmaceutical excipients, known to be pharmacologically inert, show effects on drug transporters and/or metabolic enzymes. Thus, the pharmacokinetics (absorption, distribution, metabolism and elimination) of active pharmaceutical ingredients are possibly altered because of their transport and metabolism modulation from the incorporated excipients. The aim of this review is to present studies on the interaction of various commonly-used excipients on pre-systemic metabolism by CYP450 enzymes. Excipients such as surfactants, polymers, fatty acids and solvents are discussed. Based on all the reported outcomes, the most potent inhibitors were found to be surfactants and the least effective were organic solvents. However, there are many factors that can influence the inhibition of CYP450, for instance type of excipient, concentration of excipient, type of CYP450 isoenzyme, incubation condition, etc. Such evidence will be very useful in dosage form design, so that the right formulation can be designed to maximize drug bioavailability, especially for poorly bioavailable drugs.

Distinguishing Pharmacokinetics of Marketed Nanomedicine Formulations Using a Stable Isotope Tracer Assay

The pharmacokinetics of nanomedicines are complicated by the unique dispositional characteristics of the drug carrier. Most simplistically, the carrier could be a solubilizing platform that allows administration of a hydrophobic drug. Alternatively, the carrier could be stable and release the drug in a controlled manner, allowing for distribution of the carrier to influence distribution of the encapsulated drug. A third potential dispositional mechanism is carriers that are not stably complexed to the drug, but rather bind the drug in a dynamic equilibrium, similar to the binding of unbound drug to protein; since the nanocarrier has distributional and binding characteristics unlike plasma proteins, the equilibrium binding of drug to a nanocarrier can affect pharmacokinetics in unexpected ways, diverging from classical protein binding paradigms. The recently developed stable isotope tracer ultrafiltration assay (SITUA) for nanomedicine fractionation is uniquely suited for distinguishing and comparing these carrier/drug interactions. Here we present the the encapsulated, unencapsulated, and unbound drug fraction pharmacokinetic profiles in rats for marketed nanomedicines, representing examples of controlled release (doxorubicin liposomes, Doxil; and doxorubicin HCl liposome generic), equilibrium binding (paclitaxel cremophor micelle solution, Taxol generic), and solubilizing (paclitaxel albumin nanoparticle, Abraxane; and paclitaxel polylactic acid micelle, Genexol-PM) nanomedicine formulations. The utility of the SITUA method in differentiating these unique pharmacokinetic profiles and its potential for use in establishing generic nanomedicine bioequivalence are discussed.

Comparison of microbial hosts and expression systems for mammalian CYP1A1 catalysis

Mammalian cytochrome P450 enzymes are of special interest as biocatalysts for fine chemical and drug metabolite synthesis. In this study, the potential of different recombinant microorganisms expressing rat and human cyp1a1 genes is evaluated for such applications. The maximum specific activity for 7-ethoxyresorufin O-deethylation and gene expression levels were used as parameters to judge biocatalyst performance. Under comparable conditions, E. coli is shown to be superior over the use of S. cerevisiae and P. putida as hosts for biocatalysis. Of all tested E. coli strains, E. coli DH5α and E. coli JM101 harboring rat CYP1A1 showed the highest activities (0.43 and 0.42 U g⁻¹(CDW), respectively). Detection of active CYP1A1 in cell-free E. coli extracts was found to be difficult and only for E. coli DH5α, expression levels could be determined (41 nmol g⁻¹(CDW)). The presented results show that efficient expression of mammalian cyp1a1 genes in recombinant microorganisms is troublesome and host-dependent and that enhancing expression levels is crucial in order to obtain more efficient biocatalysts. Specific activities currently obtained are not sufficient yet for fine chemical production, but are sufficient for preparative-scale drug metabolite synthesis.

Effect of several compounds on biliary excretion of paclitaxel and its metabolites in guinea-pigs

The objective of this study was to evaluate the in vivo metabolic profile of paclitaxel and to examine the effect of potential co-administered drugs on the biliary secretion of paclitaxel and its metabolites in guinea-pigs. We first investigated in vitro paclitaxel metabolism using liver microsomes obtained from various species to identify the most suitable animal model with a similar metabolism to humans. Then, in vivo paclitaxel metabolism was investigated in male guinea-pigs. The levels of paclitaxel and its metabolites were measured by high-performance liquid chromatography in bile samples from guinea-pigs after paclitaxel i.v. injection (6 mg/kg). We further evaluated the effects of various drugs (quercetin, ketoconazole, dexamethasone, cotrimoxazole) on the biliary secretion of paclitaxel and its metabolites in guinea-pigs. This work demonstrated significant in vitro interspecies differences in paclitaxel metabolism. Our findings showed both in vitro and in vivo similarities between human and guinea-pig biotransformation of paclitaxel. 6alpha-Hydroxypaclitaxel, the main human metabolite of paclitaxel, was found in guinea-pig bile. After paclitaxel combination with ketoconazole or quercetin in guinea-pigs, the cumulative biliary excretion of paclitaxel and its metabolites up to 6 h was significantly decreased by 62 and 76%, respectively. The co-administration of cotrimoxazole or pretreatment with dexamethasone did not alter significantly cumulative biliary excretion. The guinea-pig is a suitable model to study metabolism and biliary excretion of paclitaxel, and to investigate in vivo drug interactions.

Intravenous administration of paclitaxel in Sprague-Dawley rats: what is a safe dose?

Few studies describe the administration of Taxol to rats; however, rats are typically used to study the toxicity of new drugs or novel formulations. A dose finding study was conducted to determine a safe dose of Taxol following intravenous administration in rats. Male Sprague-Dawley rats received a bolus of paclitaxel 5-20 mg/kg i.v. Blood was drawn before administration and at the following times after administration: 0.5, 1, 2, 3, 4, 6, 8, 12, 16, 20 and 24 h. Plasma concentrations were determined using high performance liquid chromatography. Two rats received paclitaxel 20 mg/kg and died immediately. Nine rats received paclitaxel 10 mg/kg; seven of these rats died within 12 h and two rats were killed due to moribund conditions. Ten rats received paclitaxel 5 mg/kg with no morbidity. The following pharmacokinetics for paclitaxel in the plasma were estimated: C0, 8977 ng/ml; AUC(0 --> infinity), 7477 ng*h/ml; CL(s), 668 ml/h*kg; V(ss), 1559 ml/kg; V(z) 2557 ml/kg and t(1/2), 2.6 h. It is concluded that further pharmacokinetic studies that are rationally designed to include appropriate measures of preclinical toxicity associated with paclitaxel are needed to identify formally the safest dose in rats following intravenous administration; however, these data indicate that male Sprague-Dawley rats can safely receive Taxol in a 5 mg/kg i.v. bolus.

Paclitaxel chemotherapy: from empiricism to a mechanism-based formulation strategy

Paclitaxel is an anticancer agent effective for the treatment of breast, ovarian, lung, and head and neck cancer. Because of water insolubility, paclitaxel is formulated with the micelle-forming vehicle Cremophor EL to enhance drug solubility. However, the addition of Cremophor EL results in hypersensitivity reactions, neurotoxicity, and altered pharmacokinetics of paclitaxel. To circumvent these unfavorable effects resulting from the addition of Cremophor EL, efforts have been made to develop new delivery systems for paclitaxel administration. For example, ABI-007 is a Cremophor-free, albumin-stabilized, nanoparticle paclitaxel formulation that was found to have significantly less toxicity than Cremophor-containing paclitaxel in mice. Pharmacokinetic studies indicate that in contrast to Cremophor-containing paclitaxel, ABI-007 displays linear pharmacokinetics over the clinically relevant dose range of 135-300 mg/m(2). In a phase III study conducted in patients with metastatic breast cancer, patients treated with ABI-007 achieved a significantly higher objective response rate and time to progression than those treated with Cremophor-containing paclitaxel. Together these findings suggest that nanoparticle albumin-bound paclitaxel may enable clinicians to administer paclitaxel at higher doses with less toxicity than is seen with Cremophor-containing paclitaxel. The role of this novel paclitaxel formulation in combination therapy with other antineoplastic agents needs to be determined.

Interactions Between Antiretrovirals and Antineoplastic Drug Therapy

Despite the established impact of highly active antiretroviral therapy (HAART) in reducing HIV-related morbidity and mortality, malignancy remains an important cause of death. Patients who receive the combination of cancer chemotherapy and HAART may achieve better response rates and higher rates of survival than patients who receive antineoplastic therapy alone. However, the likelihood of drug interactions with combined therapy is high, since protease inhibitors (PIs) and non-nucleoside reverse transcriptase inhibitors (NNRTIs) are substrates and potent inhibitors or inducers of the cytochrome P450 (CYP) system. Since many antineoplastic drugs are also metabolised by the CYP system, coadministration with HAART could result in either drug accumulation and possible toxicity, or decreased efficacy of one or both classes of drugs. Although formal, prospective pharmacokinetic interaction studies are not available in most instances, it is possible to infer the nature of drug interactions based on the metabolic fates of these agents. Paclitaxel and docetaxel are both metabolised by the CYP system, although differences exist in the nature of the isoenzymes involved. Case reports describing adverse consequences of concomitant taxane-antiretroviral therapy exist. Although other confounding factors may have been present, these cases serve as reminders of the vigilant monitoring necessary when taxanes and HAART are coadministered. Similarly, vinca alkaloids are substrates of CYP3A4 and are, thus, vulnerable to PI- or NNRTI-mediated changes in their pharmacokinetics. Interactions with the alkylating agents cyclophosphamide and ifosfamide are complicated as a result of the involvement of the CYP3A4 and CYP2B6 isoenzymes in both the metabolic activation of these drugs and the generation of potentially neurotoxic metabolites. Existing data regarding the metabolic fate of the anthracyclines doxorubicin and daunorubicin suggest that clinically detrimental interactions would not be expected with coadministered HAART. Commonly used endocrine therapies are largely substrates of the CYP system and may, therefore, be amenable to modulation by concomitant HAART. In addition, tamoxifen itself has been associated with reduced concentrations of both anastrozole and letrozole, raising the concern that similar inducing properties may adversely affect the outcome of PI- or NNRTI-based therapy. Similarly, dexamethasone is both a substrate and concentration-dependent inducer of CYP3A4; enhanced corticosteroid pharmacodynamics may result with CYP3A4 inhibitors, while the efficacy of concomitant HAART may be compromised with prolonged dexamethasone coadministration. Since PIs and NNRTIs may also induce or inhibit the expression of P-glycoprotein, the potential for additional interactions to arise via modulation of this transporter also exists. Further research delineating the combined safety and pharmacokinetics of antiretrovirals and antineoplastic therapy is necessary.

Pharmacokinetics of paclitaxel-containing liposomes in rats

In animal models, liposomal formulations of paclitaxel possess lower toxicity and equal antitumor efficacy compared with the clinical formulation, Taxol. The goal of this study was to determine the formulation dependence of paclitaxel pharmacokinetics in rats, in order to test the hypothesis that altered biodistribution of paclitaxel modifies the exposure of critical normal tissues. Paclitaxel was administered intravenously in either multilamellar (MLV) liposomes composed of phosphatidylglycerol/phosphatidylcholine (L-pac) or in the Cremophor EL/ethanol vehicle used for the Taxol formulation (Cre-pac). The dose was 40 mg/kg, and the infusion time was 8 to 9 minutes. Animals were killed at various times, and pharmacokinetic parameters were determined from the blood and tissue distribution of paclitaxel. The area under the concentration vs time curve (AUC) for blood was similar for the 2 formulations (L-pac: 38.1 +/- 3.32 microg-h/mL; Cre-pac: 34.5 +/- 0.994 microg-h/mL), however, the AUC for various tissues was formulation-dependent. For bone marrow, skin, kidney, brain, adipose, and muscle tissue, the AUC was statistically higher for Cre-pac. For spleen, a tissue of the reticuloendothelial system that is important in the clearance of liposomes, the AUC was statistically higher for L-pac. Apparent tissue partition coefficients (K(p)) also were calculated. For bone marrow, a tissue in which paclitaxel exerts significant toxicity, K(p) was 5-fold greater for paclitaxel in Cre-pac. The data are consistent with paclitaxel release from circulating liposomes, but with efflux delayed sufficiently to retain drug to a greater extent in the central (blood) compartment and reduce penetration into peripheral tissues. These effects may contribute to the reduced toxicity of liposomal formulations of paclitaxel.

High-content profiling of drug-drug interactions: cellular targets involved in the modulation of microtubule drug action by the antifungal ketoconazole

Drug-drug interactions play an important role in the discovery and development of therapeutic agents. High-content profiling was developed to unravel the complexity of these interactions by providing multiparameter measurements of target activity at the cellular and subcellular levels. Two microtubule drugs, vinblastine and curacin A, were shown to modulate multiple cellular processes, including nuclear condensation, the activation of the extracellular signal-regulated kinase pathway as measured by RSK90 phosphorylation, and the regulation of the microtubule cytoskeleton as measured in detergent-extracted cells. The heterogeneity of the response, addressed through population analysis and multiparameter comparisons within single cells, was consistent with vinblastine and curacin A having similar effects on nuclear morphology and 90 kDa ribosomal s6 kinase (RSK90) phosphorylation despite having distinct effects on the microtubule cytoskeleton. Ketoconazole, originally developed as an antifungal agent, exhibited concentration-dependent inhibitory and potentiating effects on both drugs in HeLa and PC-3 cells at concentration ranges near the plasma levels of ketoconazole attained in human subjects. Thus, high-content profiling was used to dissect the cellular and molecular responses to interacting drugs and is therefore a potentially important tool in the selection, characterization, and optimization of lead therapeutic compounds.

Metabolism of paclitaxel in mice

Previous mass balance studies in humans and mice have shown that the fecal and urinary recovery of paclitaxel and known metabolites (3' -hydroxypaclitaxel, 6alpha-hydroxypaclitaxel and 3',6alpha-dihydroxypaclitaxel) was not complete. Obviously this discrepancy is caused by the existence of other yet unknown metabolites. Mdr1a/1b(-/-) mice excrete very low quantities of unchanged paclitaxel. We have therefore used these mice receiving i.v. [3H]paclitaxel to further study the metabolic fate of paclitaxel. The major part of the radiolabel, being 70%, was excreted in the feces. A lipophilic sample, containing about 70% of the radioactivity present in the feces sample, was obtained by diethyl ether extraction. The aqueous residue containing about 30% of the radioactivity was further extracted using methanol. The high-performance liquid chromatography (HPLC) chromatograms of the lipophilic and aqueous sample revealed two and five putative new metabolites of paclitaxel, respectively. The HPLC fractions containing substantial amounts of radioactivity were subjected to tandem mass spectrometry. Two novel monohydroxylated paclitaxel structures were identified, which are probably 2m-hydroxypaclitaxel and 19-hydroxypaclitaxel, structures previously identified in rats. Including these metabolites, about 60% of the mass balance of paclitaxel could be quantified.

Reactivity of atropaldehyde, a felbamate metabolite in human liver tissue in vitro

Antiepileptic therapy with a broad spectrum drug felbamate (FBM) has been limited due to reports of hepatotoxicity and aplastic anemia associated with its use. It was proposed that a bioactivation of FBM leading to formation of alpha,beta-unsaturated aldehyde, atropaldehyde (ATPAL) could be responsible for toxicities associated with the parent drug. Other members of this class of compounds, acrolein and 4-hydroxynonenal (HNE), are known for their reactivity and toxicity. It has been proposed that the bioactivation of FBM to ATPAL proceeds though a more stable cyclized product, 4-hydroxy-5-phenyltetrahydro-1,3-oxazin-2-one (CCMF) whose formation has been shown recently. Aldehyde dehydrogenase (ALDH) and glutathione transferase (GST) are detoxifying enzymes and targets for reactive aldehydes. This study examined effects of ATPAL and its precursor, CCMF on ALDH, GST and cell viability in liver, the target tissue for its metabolism and toxicity. A known toxin, HNE, which is also a substrate for ALDH and GST, was used for comparison. Interspecies difference in metabolism of FBM is well documented, therefore, human tissue was deemed most relevant and used for these studies. ATPAL inhibited ALDH and GST activities and led to a loss of hepatocyte viability. Several fold greater concentrations of CCMF were necessary to demonstrate a similar degree of ALDH inhibition or cytotoxicity as observed with ATPAL. This is consistent with CCMF requiring prior conversion to the more proximate toxin, ATPAL. GSH was shown to protect against ALDH inhibition by ATPAL. In this context, ALDH and GST are detoxifying pathways and their inhibition would lead to an accumulation of reactive species from FBM metabolism and/or metabolism of other endogenous or exogenous compounds and predisposing to or causing toxicity. Therefore, mechanisms of reactive aldehydes toxicity could include direct interaction with critical cellular macromolecules or indirect interference with cellular detoxification mechanisms.

Drug distribution and pharmacokinetic/pharmacodynamic relationship of paclitaxel and gemcitabine in patients with non-small-cell lung cancer

BACKGROUND

Gemcitabine and paclitaxel are two of the most active agents in non-small-cell lung cancer (NSCLC), and pharmacologic investigation of the combination regimens including these drugs may offer a valuable opportunity in treatment optimization. The present study investigates the pharmacokinetics and pharmacodynamics of paclitaxel and gemcitabine in chemotherapy-naive patients with advanced NSCLC within a phase I study.

PATIENTS AND METHODS

Patients were given i.v. paclitaxel 100 mg/m2 by one-hour infusion followed by gemcitabine 1,500, 1,750 and 2,000 mg/m2 by 30-min administration. Plasma levels of paclitaxel, gemcitabine and its metabolite 2',2'-difluorodeoxyuridine (dFdU) were determined by high-performance liquid chromatography (HPLC). Concentration-time curves were modeled by compartmental and non-compartmental methods and pharmacokinetic/pharmacodynamic (PK/PD) relationships were fitted according to a sigmoid maximum effect (Emax) model.

RESULTS

Paclitaxel pharmacokinetics did not change as a result of dosage escalation of gemcitabine from 1,500 to 2,000 mg/m2. A nonproportional increase in gemcitabine peak plasma levels (Cmax, from 18.56 +/- 4.94 to 40.85 +/- 14.85 microg/ml) and area under the plasma concentration-time curve (AUC, from 9.99 +/- 2.75 to 25.01 +/- 9.87 h x microg/ml) at 1,500 and 2,000 mg/m, respectively, was observed, suggesting the occurrence of saturation kinetics at higher doses. A significant relationship between neutropenia and time of paclitaxel plasma levels > or = 0.05 micromol/l was observed, with a predicted time of 10.4 h to decrease cell count by 50%. A correlation was also observed between percentage reduction of platelet count and gemcitabine Cmax, with a predicted effective concentration to induce a 50% decrease of 14.3 microg/ml.

CONCLUSION

This study demonstrates the lack of interaction between drugs, the nonproportional pharmacokinetics of gemcitabine at higher doses and the Emax relationship of paclitaxel and gemcitabine with neutrophil and platelet counts, respectively. In addition, gemcitabine 1,500 mg/m2 is the recommended dosage in combination with paclitaxel 100 mg/m2 for future phase II studies, due to its predictable kinetic behaviour and less severe thrombocytopenia than expected.

Drug interactions with the taxanes: clinical implications

BACKGROUND

The taxanes paclitaxel and docetaxel are among the most active antitumor agents. Clinically important pharmacodynamic interactions have been reported to occur with these agents that are sequence or schedule dependent. Because the taxanes undergo hepatic oxidation via the cytochrome P450 system, pharmacokinetic interactions due to enzyme induction or inhibition can also occur.

METHODS

A comprehensive literature search was conducted using Medline to identify clinically important drug-interactions with the taxanes.

RESULTS

Clinically significant taxane interactions were identified for carboplatin, cisplatin, doxorubicin, docetaxel, epirubicin and anticonvulsants. Doxorubicin and epirubicin should be administered 24 h before paclitaxel, and the cumulative anthracycline dose limited to 360 mg/m(2). This will prevent the enhanced toxicities due to sequence and schedule dependent interactions between anthracyclines and paclitaxel. Conversely, paclitaxel should be administered at least 24 h before cisplatin to avoid a decrease in clearance and increase in myelosuppression. With concurrent anticonvulsant therapy, cytochrome p450 enzyme induction results in decreased paclitaxel plasma steady state concentrations, possibly requiring an increased dose of paclitaxel. A number of other drug interactions have been reported in preliminary studies for which clinical significance has yet to be established.

CONCLUSION

Clinically significant drug interactions have been reported to occur when paclitaxel is administered with doxorubicin, cisplatin, or anticonvulsants (phenytoin, carbamazepine, and phenobarbital).

Inter-species differences in drug properties

There has been a clear trend towards decreased reliance upon animal studies and increased emphasis upon experiments with human-derived tissues. Nonetheless, we continue to need investigations of interspecies differences for two principal reasons: (1) to prospectively design experiments so that the animal species most similar to humans can be chosen, on a case-by-case basis, for each drug; (2) to properly evaluate and interpret data obtained from the experiments ("risk assessment"). Four core examples derived from the work in our FDA laboratory are used to illustrate these points. For paclitaxel, different metabolites were formed in humans and rats, which makes metabolic drug-drug interaction studies in rats irrelevant. For zidovudine (AZT), rapid glucuronidation in humans produced a much shorter half-life than expected from studies in animals, which have negligible glucuronidation. The toxicology and efficacy of both parent drug and metabolite need to be assessed in cases such as iododeoxydoxorubicin, in which the parent molecule is the dominant circulating species in mice, but patients have more than 10-fold greater exposure to the metabolite compared with the parent. While rats have highly-active arylamine N-acetyltransferases, dogs totally lack this enzyme family, and humans have intermediate amounts. For some situations, we've suggested that it can be desirable to inhibit NAT to make the human exposure more similar to dogs. In conclusion, although the ratio of animal:human data is decreasing, our ability to use animal data effectively for drug development has actually increased. Continued focus should be placed upon the application of comparative interspecies data for prospective design of animal experiments and retrospective interpretation of animal findings in terms of the potential for human risk and benefit.

Risks and benefits of taxanes in breast and ovarian cancer

The taxanes are a unique class of agents with a broad spectrum of clinical activity. They act by binding to tubulin, producing unnaturally stable microtubules and subsequent cell death. The distribution and elimination of paclitaxel depend on dose and administration rate. This nonlinearity is much less evident at lower infusion rates (24-hour infusions) and more evident at high plasma concentrations (3-hour infusions). The pharmacokinetics of docetaxel also suggest the presence of nonlinear pathways, but these appear to be clinically insignificant at the current doses utilised (60 to 100 mg/m2). Both agents undergo hepatic metabolism and biliary excretion and require dose adjustment in the setting of liver dysfunction. Drug interactions are quite common with these agents, some of which are sequence-dependent and clinically significant. The optimal dose of paclitaxel is not known at this time, and controversy over possible dose- or schedule-related differences in efficacy still remain. Docetaxel is somewhat more consistent in its dose and scheduling information, but controversy remains regarding a dose-benefit relationship as well as scheduling differences (weekly vs every 3 weeks). Toxicity profiles for these agents are somewhat different. Paclitaxel is more likely to be associated with peripheral neuropathy and myalgias/arthralgias than docetaxel. Docetaxel is more likely to be associated with a cumulative fluid retention syndrome that can be dose limiting. Paclitaxel and docetaxel are both highly active agents against breast cancer, including tumours that are resistant to anthracyclines. Docetaxel tends to have higher response rates overall, but direct comparisons at maximally tolerated doses have not been completed. Combination regimens with many different agents are attempting to improve on the responses seen with single-agent taxanes. The combination of paclitaxel and a platinum compound should be utilised as first-line therapy of advanced ovarian cancer. Controversy lies in the choice of the platinum compound and the dose and administration schedule of paclitaxel. Substitution of docetaxel for paclitaxel in these platinum-containing regimens is also being investigated. The taxanes also exhibit activity against ovarian cancer in patients previously exposed to platinum agents. These agents may also be administered intraperitoneally for local therapy of metastatic ovarian cancer. Although docetaxel and paclitaxel are often considered similar in activity and tolerability, this review emphasises the fact that these agents are indeed different. Clinicians need to be familiar with the benefits and adverse events related to each agent in order to make informed, appropriate clinical decisions.

Clinical pharmacokinetic and in vitro combination studies of nolatrexed dihydrochloride (AG337, Thymitaq) and paclitaxel

A clinical study of nolatrexed dihydrochloride (AG337, Thymitaq) in combination with paclitaxel was performed. The aims were to optimize the schedule of administration and determine any pharmacokinetic (PK) interactions between the two drugs. In vitro combination studies were performed to assist with schedule optimization. Three patients were entered on each of three different schedules of administration of the two drugs: (1) paclitaxel 0-3 h, nolatrexed 24-144 h; (2) nolatrexed 0-120 h, paclitaxel 48-51 h; (3) nolatrexed 0-120 h, paclitaxel 126-129 h. Paclitaxel was administered at a dose of 80 mg m(-2) over 3 h and nolatrexed at a dose of 500 mg m(-2) day(-1) as a 120-h continuous intravenous infusion. Plasma concentrations of both drugs were determined by high performance liquid chromatography. In vitro growth inhibition studies using corresponding schedules were performed using two head and neck cancer cell lines. In both HNX14C and HNX22B cell lines, synergistic growth inhibition was observed on schedule 2, whereas schedules 1 and 3 demonstrated antagonistic effects. In the clinical study, there was no effect of schedule on the pharmacokinetics of nolatrexed. However, patients on schedules 1 and 3 had a higher clearance of paclitaxel (322-520 ml min(-1) m(-2)) than those on schedule 2 (165-238 ml min(-1) m(-2)). Peak plasma concentrations (1.66-1.93 vs. 0.86-1.32 microM) and areas under the curve (392-565 vs. 180-291 microM min(-1)) of paclitaxel were correspondingly higher on schedule 2. The pharmacokinetic interaction was confirmed by studies with human liver microsomes, nolatrexed being an inhibitor of the major routes of metabolism of paclitaxel. Toxicity was not schedule-dependent. Nolatrexed and paclitaxel may be safely given together when administered sequentially at the doses used in this study. Studies in vitro suggest some synergy, however, due to a pharmacokinetic interaction, paclitaxel doses should be reduced when administered during nolatrexed infusion.

Effects of the antifungal agents on oxidative drug metabolism: clinical relevance

This article reviews the metabolic pharmacokinetic drug-drug interactions with the systemic antifungal agents: the azoles ketoconazole, miconazole, itraconazole and fluconazole, the allylamine terbinafine and the sulfonamide sulfamethoxazole. The majority of these interactions are metabolic and are caused by inhibition of cytochrome P450 (CYP)-mediated hepatic and/or small intestinal metabolism of coadministered drugs. Human liver microsomal studies in vitro, clinical case reports and controlled pharmacokinetic interaction studies in patients or healthy volunteers are reviewed. A brief overview of the CYP system and the contrasting effects of the antifungal agents on the different human drug-metabolising CYP isoforms is followed by discussion of the role of P-glycoprotein in presystemic extraction and the modulation of its function by the antifungal agents. Methods used for in vitro drug interaction studies and in vitro-in vivo scaling are then discussed, with specific emphasis on the azole antifungals. Ketoconazole and itraconazole are potent inhibitors of the major drug-metabolising CYP isoform in humans, CYP3A4. Coadministration of these drugs with CYP3A substrates such as cyclosporin, tacrolimus, alprazolam, triazolam, midazolam, nifedipine, felodipine, simvastatin, lovastatin, vincristine, terfenadine or astemizole can result in clinically significant drug interactions, some of which can be life-threatening. The interactions of ketoconazole with cyclosporin and tacrolimus have been applied for therapeutic purposes to allow a lower dosage and cost of the immunosuppressant and a reduced risk of fungal infections. The potency of fluconazole as a CYP3A4 inhibitor is much lower. Thus, clinical interactions of CYP3A substrates with this azole derivative are of lesser magnitude, and are generally observed only with fluconazole dosages of > or =200 mg/day. Fluconazole, miconazole and sulfamethoxazole are potent inhibitors of CYP2C9. Coadministration of phenytoin, warfarin, sulfamethoxazole and losartan with fluconazole results in clinically significant drug interactions. Fluconazole is a potent inhibitor of CYP2C19 in vitro, although the clinical significance of this has not been investigated. No clinically significant drug interactions have been predicted or documented between the azoles and drugs that are primarily metabolised by CYP1A2, 2D6 or 2E1. Terbinafine is a potent inhibitor of CYP2D6 and may cause clinically significant interactions with coadministered substrates of this isoform, such as nortriptyline, desipramine, perphenazine, metoprolol, encainide and propafenone. On the basis of the existing in vitro and in vivo data, drug interactions of terbinafine with substrates of other CYP isoforms are unlikely.

Taxanes: an overview of the pharmacokinetics and pharmacodynamics

Paclitaxel and docetaxel have emerged in the last two decades as effective antitumor agents in a variety of malignancies. Paclitaxel is a semisynthetic taxane isolated from bark of the Pacific yew tree. Docetaxel is a semisynthetic taxane derived from the needles of the European yew (Taxus baccata). These compounds bind to tubulin, leading to microtubule stabilization, mitotic arrest and, subsequently, cell death. Plasma clearance of paclitaxel exhibits nonlinear kinetics, which results in a disproportionate change in plasma concentration and area under the curve (AUC) with dose alterations. In contrast, docetaxel has a linear disposition over the dose ranges used clinically, so its concentration changes linearly with changes in the dosage. Premedicating with corticosteroids and histamine H1 and H2 receptor antagonists is advocated prior to paclitaxel administration; prior to docetaxel administration, premedication with corticosteroids is suggested. The taxanes are metabolized in the liver by the cytochrome P-450 enzymes and are eliminated in the bile. The known metabolites are either inactive or less potent than the parent compounds. The toxic effects associated with paclitaxel therapy are mainly neutropenia, peripheral neuropathy, and, rarely, cardiotoxicity. Docetaxel toxicity produces mainly myelosuppression and a cumulative dose fluid retention syndrome. Paclitaxel demonstrates sequence-dependent interactions with cisplatin, cyclophosphamide, and doxorubicin. Docetaxel has shown increased myelosuppression with preceding ifosfamide in a preliminary study. The future holds increasing indications for taxanes in newer combination regimens; consideration of their pharmacologic characteristics is an important aspect of designing and applying new taxane-based treatment regimens.

Overview of recent topics in clinical pharmacology of anticancer agents

The rationale for studying the clinical pharmacology of antineoplastic agents is that the information obtained will result in enhanced drug development and enhanced or improved clinical use. A great deal of effort has been expended in studying the pharmacokinetics and pharmacodynamics of investigational and noninvestigational antineoplastic agents. More recently, a deeper appreciation has developed regarding the importance of the metabolism of antineoplastic agents and the potential role of metabolites in their activity or toxicity, as well as the potential for drug-drug interactions. Investigators studying the clinical pharmacology of antineoplastic agents face an increasingly challenging task as new agents continue to be developed. Some of these challenges arise from the enhanced potency of new agents, resulting in increased difficulty in measuring such agents in biological matrices. Furthermore, as agents have been developed to affect specific biological targets, the necessity of assessing pharmacodynamics at the biochemical or molecular level has become increasingly important. In addition, development of agents with cytostatic, as opposed to cytotoxic, properties poses a further challenge to assessment of pharmacologic effect. In addressing these challenges, a great deal of effort has been expended to develop increasingly sensitive analytical chemical techniques, in evaluating alternative biological matrices, such as saliva, in which to monitor drug concentrations in a less invasive fashion, and in developing limited sampling strategies to assess both the pharmacokinetics and pharmacodynamics of antineoplastic agents. Similarly, a great deal of effort has been expended in providing suitable means for assessing the numerous novel targets for which antineoplastic agents are being developed. These include the assessment of cell cycle kinetics and specific oncoproteins, definition of cell damage such as cleavable complexes, and formation of drug-macromolecular adducts in suitable target cells. Additional effort is being expended to explore nontraditional means of drug delivery. In this regard, the increasing importance of orally administered agents reflects a fundamental change in the approach to antineoplastic drug delivery. Finally, the increased computational power made available by faster personal computers has facilitated a number of innovative modeling techniques involving population modeling, modeling of combination chemotherapy, and assessment of drug-drug interactions.

Phase II and pharmacokinetic study of paclitaxel therapy for unresectable hepatocellular carcinoma patients

Hepatocellular carcinoma (HCC) is a common lethal disease in Asia and there is no effective chemotherapy. Identification of new effective drugs in the treatment of inoperable HCC is urgently need. This is a phase II clinical study to investigate the efficacy, toxicity and pharmacokinetics of paclitaxel in HCC patients. Twenty patients with measurable, unresectable HCC, normal serum bilirubin, normal bone marrow and renal functions were studied. Paclitaxel 175 mg m(-2) was given intravenously over 3 h every 3 weeks. No complete or partial responses were observed. Five patients had stable disease. Major treatment toxicities (grade 3-4) were neutropenia (25%), thrombocytopenia (15%), infection (10%) and allergy (10%). Treatment-related deaths occurred in two patients. The median survival was 12 weeks (range 1-36). Paclitaxel is metabolized by the liver and the pharmacokinetics of paclitaxel in cancer patients with liver involvement or impairment may be important clinically. Pharmacokinetic study was completed in 13 HCC patients. The paclitaxel area under the curve was significantly increased (P < 0.02), clearance decreased (P < 0.02) and treatment-related deaths increased (P = 0.03) in patients with hepatic impairment. In conclusion, paclitaxel in this dose and schedule has no significant anti-cancer effect in HCC patients. Paclitaxel should be used with caution in cancer patients with liver impairment.

Glucuronidation of 3'-azido-3'-deoxythymidine (zidovudine) by human liver microsomes: relevance to clinical pharmacokinetic interactions with atovaquone, fluconazole, methadone, and valproic acid

Zidovudine (3'-azido-3'-deoxythymidine [AZT]), an antiviral nucleoside analog effective in the treatment of human immunodeficiency virus infection, is primarily metabolized to an inactive glucuronide form, GAZT, via uridine-5'-diphospho-glucuronosyltransferase (UGT) enzymes. UGT enzymes exist as different isoforms, each exhibiting substrate specificity. Published clinical studies have shown that atovaquone, fluconazole, methadone, and valproic acid decreased GAZT formation, presumably due to UGT inhibition. The effect of these drugs on AZT glucuronidation was assessed in vitro by using human hepatic microsomes to begin understanding in vitro-in vivo correlations for UGT metabolism. The concentrations of each drug studied were equal to those reported with the usual clinical doses and at concentrations at least 10 times higher than would be expected with these doses. High-performance liquid chromatography was used to assess the respective metabolism and formation of AZT and GAZT. All four drugs exhibited concentration-dependent inhibition of AZT glucuronidation. The respective concentrations of atovaquone and methadone which caused 50% inhibition of GAZT were > 100 and 8 micrograms/ml, well above their usual clinical concentrations. Fluconazole and valproic acid exhibited 50% inhibition of GAZT at 50 and 100 micrograms/ml, which are within the clinical ranges of 10 to 100 and 50 to 100 micrograms/ml, respectively. These data suggest that inhibition of AZT glucuronidation may be more clinically significant with concomitant fluconazole and valproic acid. Factors such as inter- and intraindividual pharmacokinetic variability and changes in AZT intracellular concentrations should be considered as other mechanisms responsible for changes in AZT pharmacokinetics with concomitant therapies.

Predicting drug interactions in vivo from experiments in vitro. Human studies with paclitaxel and ketoconazole

This study was performed to evaluate whether concomitant treatment with ketoconazole could reduce the clearance of paclitaxel given to ovarian cancer patients. Paclitaxel, 175 mg/m2, was given as a 3-hour continuous intravenous infusion and repeated every 21 days. Initially, ketoconazole, 100 to 1600 mg, was given as a single oral dose 3 hours after paclitaxel. Later, ketoconazole, 200 mg, was given perorally 3 hours before paclitaxel. Plasma drug concentrations were measured by high-pressure liquid chromatography (HPLC), and cytochrome P450 3A (CYP3A) activity was measured with the erythromycin breath test (ERMBT). Ketoconazole did not alter plasma concentrations of paclitaxel or its principal metabolite, 6 alpha-hydroxypaclitaxel. Although there was marked inter- and intrapatient variability in ketoconazole pharmacokinetics, peak plasma concentrations in all but one course were below the 50% inhibitory concentration (IC50) point determined for inhibition of paclitaxel metabolism in vitro. Therefore, paclitaxel and ketoconazole can be coadministered safely without dose adjustments. There was no correlation between ERMBT measurements and serial plasma concentrations of paclitaxel. The erythromycin breath-test measurements did correlate with the corresponding ketoconazole plasma concentrations. The erythromycin breath test is a valuable tool for measuring instantaneous CYP3A activity in vivo. This clinical study confirms the results of prior studies with human-derived materials in vitro, reinforcing the notion that such studies are useful predictors of drug pharmacokinetics and interactions in vivo.

Metabolism of rifabutin in human enterocyte and liver microsomes: kinetic parameters, identification of enzyme systems, and drug interactions with macrolides and antifungal agents

Biotransformation of rifabutin, an antibiotic used for treatment of tuberculosis in patients infected with the human immunodeficiency virus (HIV), and its interactions with some macrolide and antifungal agents were studied in human intestinal and liver microsomes. Both liver and enterocyte microsomes metabolized rifabutin to 25-O-deacetylrifabutin, 27-O-demethylrifabutin, and 20-, 31-, and 32-hydroxyrifabutin. The same products (except 25-O-deacetylrifabutin) were formed by microsomes from lymphoblastoid cells that contained expressed CYP3A4. The apparent Michaelis-Menten constant (Km); approximately 10 to 12 mumol/L) and maximal velocity (Vmax; approximately 100 pmol/min/mg of protein) values for CYP-mediated metabolism were similar in liver and enterocyte microsomes. Deacetylation of rifabutin (Km approximately 16 to 20 mumol/L and Vmax approximately 50 to 100 pmol/min/mg of protein) was catalyzed by microsomal cholinesterase. Clarithromycin, ketoconazole, and fluconazole inhibited CYP-mediated metabolism of rifabutin in enterocyte microsomes equally or more potently than in liver microsomes but had no effect on cholinesterase activity. Azithromycin did not inhibit in vitro metabolism of rifabutin. This study provides evidence that CYP3A4 and cholinesterase are major enzymes that biotransform rifabutin in humans and that intestinal CYP3A4 contributes significantly to rifabutin presystemic first-pass metabolism and drug interactions with macrolide and antifungal agents.

Metabolism of rifabutin and its 25-desacetyl metabolite, LM565, by human liver microsomes and recombinant human cytochrome P-450 3A4: relevance to clinical interaction with fluconazole

Rifabutin and fluconazole are often given concomitantly as therapy to prevent opportunistic infections in individuals infected with the human immunodeficiency virus. Recent reports have shown increased levels of rifabutin and its 25-desacetyl metabolite, LM565, in plasma when rifabutin is administered with fluconazole. Since fluconazole is known to inhibit microsomal enzymes, this study was undertaken to determine if this rifabutin-fluconazole interaction was due to an inhibition of human hepatic enzymes. The metabolism of both rifabutin and LM565 was evaluated in human liver microsomes and recombinant human cytochrome P-450 (CYP) 3A4 in the presence of fluconazole and other probe drugs known to inhibit CYP groups 1A2, 2C9, 2D6, 2E1, and 3A. The concentrations of rifabutin (1 microg/ml), LM565 (1 microg/ml), and fluconazole (10 and 100 microg/ml) used were equal to those observed in plasma after the administration of rifabutin and fluconazole at clinically relevant doses. High-performance liquid chromatography was used to assess the metabolism of rifabutin and LM565. Rifabutin was readily metabolized to LM565 by human microsomes, but the reaction was independent of NADPH and was not affected by the P-450 inhibitors. No rifabutin metabolism by recombinant CYP 3A4 was found to occur. LM565 was also metabolized by human microsomes to two products, but metabolism was dependent on NADPH and was affected by certain P-450 inhibitors. In addition, LM565 was readily metabolized by the recombinant CYP 3A4 to the same two products found with its metabolism by human microsomes. Therefore, rifabutin is metabolized by human microsomes but not via cytochrome P-450 enzymes, whereas LM565 is metabolized by CYP 3A4.

Extrapolation from animals to man. The integration of pharmacokinetics and pharmacodynamics

This paper has focused on the difficulties of extrapolating toxicological or pharmacological data obtained from animals to those expected in man. For some drugs, under certain conditions, there may be no problem, but for many, this is clearly not the case. Differences in apparent activity are impossible to reconcile without "normalizing" the dose for differences in pharmacokinetics and metabolism. The increasing use of artificial intelligence and expert systems in drug investigations may provide a greater insight into why these differences may occur and allow prediction but, in the end, they must be tested in the experiments undertaken. The use of kinetic dynamic relationships in different species will certainly help in this regard and, wherever possible, should be included in experimental design to build up a database of experience since such information is sadly lacking. But we must interpret with caution the data produced by those that continue to extrapolate animal data to humans without some attempt to discuss in detail the validity of their assumptions.