Hypoxia and drug metabolism

New strategy for rational use of antihypertensive drugs in clinical practice in high-altitude hypoxic environments

High-altitude hypoxic environments have critical implications on cardiovascular system function as well as blood pressure regulation. Such environments place patients with hypertension at risk by activating the sympathetic nervous system, which leads to an increase in blood pressure. In addition, the high-altitude hypoxic environment alters the in vivo metabolism and antihypertensive effects of antihypertensive drugs, which changes the activity and expression of drug-metabolizing enzymes and drug transporters. The present study reviewed the pharmacodynamics and pharmacokinetics of antihypertensive drugs and its effects on patients with hypertension in a high-altitude hypoxic environment. It also proposes a new strategy for the rational use of antihypertensive drugs in clinical practice in high-altitude hypoxic environments. The increase in blood pressure on exposure to a high-altitude hypoxic environment was mainly dependent on increased sympathetic nervous system activity. Blood pressure also increased proportionally to altitude, whilst ambulatory blood pressure increased more than conventional blood pressure, especially at night. High-altitude hypoxia can reduce the activities and expression of drug-metabolizing enzymes, such as CYP1A1, CYP1A2, CYP3A1, and CYP2E1, while increasing those of CYP2D1, CYP2D6, and CYP3A6. Drug transporter changes were related to tissue type, hypoxic degree, and hypoxic exposure time. Furthermore, the effects of high-altitude hypoxia on drug-metabolism enzymes and transporters altered drug pharmacokinetics, causing changes in pharmacodynamic responses. These findings suggest that high-altitude hypoxic environments affect the blood pressure, pharmacokinetics, and pharmacodynamics of antihypertensive drugs. The optimal hypertension treatment plan and safe and effective medication strategy should be formulated considering high-altitude hypoxic environments.

Searching for molecular hypoxia sensors among oxygen-dependent enzymes

The ability to sense and respond to changes in cellular oxygen levels is critical for aerobic organisms and requires a molecular oxygen sensor. The prototypical sensor is the oxygen-dependent enzyme PHD: hypoxia inhibits its ability to hydroxylate the transcription factor HIF, causing HIF to accumulate and trigger the classic HIF-dependent hypoxia response. A small handful of other oxygen sensors are known, all of which are oxygen-dependent enzymes. However, hundreds of oxygen-dependent enzymes exist among aerobic organisms, raising the possibility that additional sensors remain to be discovered. This review summarizes known and potential hypoxia sensors among human O2-dependent enzymes and highlights their possible roles in hypoxia-related adaptation and diseases.

The material-enabled oxygen control in thiol-ene microfluidic channels and its feasibility for subcellular drug metabolism assays under hypoxia in vitro

Tissue oxygen levels are known to be critical to regulation of many cellular processes, including the hepatic metabolism of therapeutic drugs, but its impact is often ignored in in vitro assays. In this study, the material-induced oxygen scavenging property of off-stoichiometric thiol-enes (OSTE) was exploited to create physiologically relevant oxygen concentrations in microfluidic immobilized enzyme reactors (IMERs) incorporating human liver microsomes. This could facilitate rapid screening of, for instance, toxic drug metabolites possibly produced in hypoxic conditions typical for many liver injuries. The mechanism of OSTE-induced oxygen scavenging was examined in depth to enable precise adjustment of the on-chip oxygen concentration with the help of microfluidic flow. The oxygen scavenging rate of OSTE was shown to depend on the type and the amount of the thiol monomer used in the bulk composition, and the surface-to-volume ratio of the chip design, but not on the physical or mechanical properties of the bulk. Our data suggest that oxygen scavenging takes place at the polymer-liquid interface, likely via oxidative reactions of the excess thiol monomers released from the bulk with molecular oxygen. Based on the kinetic constants governing the oxygen scavenging rate in OSTE microchannels, a microfluidic device comprising monolithically integrated oxygen depletion and IMER units was designed and its performance validated with the help of oxygen-dependent metabolism of an antiretroviral drug, zidovudine, which yields a cytotoxic metabolite under hypoxic conditions.

Warfarin Dose Requirements in Adults Hospitalized With COVID-19 Infection: A Retrospective Case Series

PURPOSE

To describe the impact of hospitalization with COVID-19 infection on warfarin dose requirements in adult inpatients.

SUMMARY

A retrospective chart review of 8 adults on warfarin admitted to Michigan Medicine with COVID-19 infection was conducted and reported as a case series. Outcomes of interest were difference in average daily dose of warfarin prior to admission (PTA) and while inpatient (IP), warfarin sensitivity, time in therapeutic range (TTR), confirmed or suspected thromboembolic event, any major or clinically significant bleeding episodes, and in-hospital mortality. IP average daily warfarin doses were lower when compared to PTA average daily doses [1.3 mg (1.3) vs. 6.2 mg (4.1)]. The mean percentage decrease in dose was 68.8% (23) and the mean absolute dose difference was 4.8 mg (4.3). Mean IP percentage tests in range was 30.8% (24.6) and mean IP warfarin sensitivity was 4.2 (3.8), both of which differed from PTA TTR and warfarin sensitivity for those with data available (n = 3, n = 6, respectively). One patient was treated for suspected acute pulmonary embolism while on warfarin and one patient experienced clinically relevant bleeding. In-hospital mortality was zero, mean length of stay (LOS) was 17 days (14.4), and mean intensive care unit (ICU) LOS for the 3 patients requiring ICU level care was 14.3 days (4.5).

CONCLUSION

Decreased warfarin dose requirements were evident in this group of adults hospitalized with COVID-19 infection. These findings suggest lower doses of warfarin may be needed to achieve therapeutic anticoagulation while inpatient.

Dabigatran vs. warfarin in relation to the presence of left ventricular hypertrophy in patients with atrial fibrillation- the Randomized Evaluation of Long-term anticoagulation therapY (RE-LY) study

Aim

We tested the hypothesis that left ventricular hypertrophy (LVH) interferes with the antithrombotic effects of dabigatran and warfarin in patients with atrial fibrillation (AF).

Methods and results

This is a post-hoc analysis of the Randomized Evaluation of Long-term anticoagulation therapY (RE-LY) Study. We defined LVH by electrocardiography (ECG) and included patients with AF on the ECG tracing at entry. Hazard ratios (HR) for each dabigatran dose vs. warfarin were calculated in relation to LVH. LVH was present in 2353 (22.7%) out of 10 372 patients. In patients without LVH, the rates of primary outcome were 1.59%/year with warfarin, 1.60% with dabigatran 110 mg (HR vs. warfarin 1.01, 95% confidence interval (CI) 0.75-1.36) and 1.08% with dabigatran 150 mg (HR vs. warfarin 0.68, 95% CI 0.49-0.95). In patients with LVH, the rates of primary outcome were 3.21%/year with warfarin, 1.69% with dabigatran 110 mg (HR vs. warfarin 0.52, 95% CI 0.32-0.84) and 1.55% with 150 mg (HR vs. warfarin 0.48, 95% CI 0.29-0.78). The interaction between LVH status and dabigatran 110 mg vs. warfarin was significant for the primary outcome (P = 0.021) and stroke (P = 0.016). LVH was associated with a higher event rate with warfarin, not with dabigatran. In the warfarin group, the time in therapeutic range was significantly lower in the presence than in the absence of LVH.

Conclusions

LVH was associated with a lower antithrombotic efficacy of warfarin, but not of dabigatran, in patients with AF. Consequently, the relative benefit of the lower dose of dabigatran compared to warfarin was enhanced in patients with LVH. The higher dose of dabigatran was superior to warfarin regardless of LVH status.

Clinical trial registration

http:www.clinicaltrials.gov. Unique identifier: NCT00262600.

Arteriolar oxygen reactivity: where is the sensor and what is the mechanism of action?

Arterioles in the peripheral microcirculation are exquisitely sensitive to changes in PO2 in their environment: increases in PO2 cause vasoconstriction while decreases in PO2 result in vasodilatation. However, the cell type that senses O2 (the O2 sensor) and the signalling pathway that couples changes in PO2 to changes in arteriolar tone (the mechanism of action) remain unclear. Many (but not all) ex vivo studies of isolated cannulated resistance arteries and large, first-order arterioles support the hypothesis that these vessels are intrinsically sensitive to PO2 with the smooth muscle, endothelial cells, or red blood cells serving as the O2 sensor. However, in situ studies testing these hypotheses in downstream arterioles have failed to find evidence of intrinsic O2 sensitivity, and instead have supported the idea that extravascular cells sense O2 . Similarly, ex vivo studies of isolated, cannulated resistance arteries and large first-order arterioles support the hypotheses that O2 -dependent inhibition of production of vasodilator cyclooxygenase products or O2 -dependent destruction of nitric oxide mediates O2 reactivity of these upstream vessels. In contrast, most in vivo studies of downstream arterioles have disproved these hypotheses and instead have provided evidence supporting the idea that O2 -dependent production of vasoconstrictors mediates arteriolar O2 reactivity, with significant regional heterogeneity in the specific vasoconstrictor involved. Oxygen-induced vasoconstriction may serve as a protective mechanism to reduce the oxidative burden to which a tissue is exposed, a process that is superimposed on top of the local mechanisms which regulate tissue blood flow to meet a tissue's metabolic demand.

The activity, protein, and mRNA expression of CYP2E1 and CYP3A1 in rats after exposure to acute and chronic high altitude hypoxia

The effects of exposure to acute and chronic high altitude hypoxia on the activity and expression of CYP2E1 and CYP3A1 were examined in rats. Rats were divided into low altitude (LA, 400 m), acute moderate altitude hypoxia (AMH, 2800 m), chronic moderate altitude hypoxia (CMH, 2800 m), acute high altitude hypoxia (AHH, 4300 m), and chronic high altitude hypoxia groups (CHH, 4300 m). Probe drugs were administrated orally to all five groups. Then the serum concentration of probe drug and its metabolite was determined by RP-HPLC. The activity of CYP2E1 and CYP3A1 was evaluated using the ratio of the metabolite to chlorzoxazone and testosterone, respectively. ELISA and real-time PCR were used to analyze the protein and mRNA expression of CYP2E1 and CYP3A1 in liver microsomes, respectively. Chronic high altitude hypoxia caused significant decreases in the activity and protein and mRNA expression of rat CYP2E1 and CYP3A1 in vivo. Acute high altitude hypoxia was not found to change the activity, protein or mRNA expression of rat CYP2E1 or CYP3A1. This study showed significant changes in the activity and protein and mRNA expression of CYP2E1 or CYP3A1 in rats after exposure to chronic high altitude hypoxia.

Changes in Warfarin Sensitivity During Decompensated Heart Failure and Chronic Obstructive Pulmonary Disease

Background: Heart failure (HF) has been associated with an elevated international normalized ratio (INR) in patients on warfarin. Objectives: Compare warfarin sensitivity during hospital admission for HF exacerbation and chronic obstructive pulmonary disease (COPD) exacerbation with admissions unrelated to HF or COPD (controls) as well as during disease stability. Methods: We conducted a case-controlled observational study. Patients admitted to a tertiary teaching hospital for HF exacerbation (n = 37), COPD exacerbation (n = 26), and admissions unrelated to HF or COPD (controls, n = 60) were included. Warfarin sensitivity (INR per daily mg dose of warfarin) at admission was compared to periods of disease stability and also compared between the 3 groups. Results: The increase in warfarin sensitivity at admission was 94% for HF patients ( P < 0.0001), 59% for COPD ( P = 0.003) patients, and 24% for controls ( P = 0.002). HF patients with New York Heart Association (NYHA) class 3 and 4 and NYHA class 1 and 2 experienced changes in warfarin sensitivity of 125% ( P = 0.006) and 50% ( P = 0.13) at admission. HF patients had higher warfarin sensitivity at admission (mean = 1.62 [SD = 1.27]) compared to the control group (0.91 [0.52], P < 0.0001) and COPD group (1.03 [0.79], P = 0.04). and required greater intervention with vitamin K than controls (14% vs 0%, P = 0.007). Conclusion: HF and COPD patients were more sensitive to warfarin during disease exacerbation, with HF exacerbation having the largest impact, resulting in clinically significant management implications.

Effect of exposure to acute and chronic high-altitude hypoxia on the activity and expression of CYP1A2, CYP2D6, CYP2C9, CYP2C19 and NAT2 in rats

We investigated the effect of exposure to acute and chronic high-altitude hypoxia (AHH and CHH) on the activity and expression of CYP1A2, CYP2D6, CYP2C9, CYP2C19 and NAT2 in rats. The rats were divided into plain (400 m), acute middle-altitude hypoxia (2,800 m), chronic middle-altitude hypoxia (2,800 m), AHH (4,300 m) and CHH (4,300 m). After probe drugs had been orally administered to the rats of the 5 groups, the serum or urine concentration of the probe drug and its metabolite was determined by reversed-phase HPLC. The activity of cytochrome P450 isozyme and NAT2 was evaluated by the ratio of the metabolite to the probe drug. The ELISA and real-time PCR were used to analyze the protein and mRNA expression of cytochrome P450 isozyme and NAT2, respectively. AHH and CHH caused significant decreases in the activity and protein and mRNA expression of rat CYP1A2 in vivo. AHH downregulates the activity and mRNA expression of rat NAT2 in vivo, and CHH upregulates the activity and protein and mRNA expression of rat CYP2D6. AHH and CHH did not change the expression of CYP2C9 and CYP2C19 in rats. This study found significant changes in the activity and protein and mRNA expression of CYP1A2, CYP2D6 and NAT2 in rats in the special environment of high-altitude hypoxia.

Hepatocellular necrosis, fibrosis and microsomal activity determine the hepatic pharmacokinetics of basic drugs in right-heart-failure-induced liver damage

PURPOSE

To explore how liver damage arising from cardio-hepatic syndromes in RHF affect the hepatic pharmacokinetics of basic drugs.

METHODS

The hepatic pharmacokinetics of five selected basic drugs with different physicochemical properties were studied in IPRL from control rats and rats with RHF. Hepatic pharmacokinetic modelling was performed with a two-phase physiologically-based organ pharmacokinetic model with the vascular space and dispersion evaluated with the MID technique. The liver damage arising from RHF was assessed by changes in liver biochemistry and histopathology. The expression of various CYP isoforms was evaluated by real-time RT-PCR analysis.

RESULTS

Four of the five basic drugs had a significantly lower E in RHF rat livers compared to the control rat livers. Hepatic pharmacokinetic analysis showed that both the CL int and PS were significantly decreased in the RHF rat livers. Stepwise regression analysis showed that the alterations in the pharmacokinetic parameters (E, CL int and PS) can be correlated to the observed histopathological changes (NI, CYP concentration and FI) as well as to the lipophilicity of the basic drugs (logP app).

CONCLUSIONS

Serious hepatocellular necrosis and fibrosis induced by RHF affects both hepatic microsomal activity and hepatocyte wall permeability, leading to significant impairment in the hepatic pharmacokinetics of basic drugs.

Aminophylline, administered at usual doses for rodents in pharmacological studies, induces hippocampal neuronal cell injury under low tidal volume hypoxic conditions in guinea-pigs

OBJECTIVES

To establish whether aminophylline, administered at usual doses for rodents in pharmacological studies, induces brain injury in systemic hypoxaemia in guinea-pigs.

METHODS

 A hypoxaemia (partial oxygen tension of arterial blood (PaO₂) = 40-60 mmHg) model was developed by low tidal volume mechanical ventilation in guinea-pigs.

KEY FINDINGS

Under hypoxic conditions, aminophylline significantly increased the concentration of brain-specific creatine kinase in the serum in a dose- and time-dependent manner. A reduced number of hippocampal neuronal cells in the CA1 region, an increase in the concentration of neuron-specific enolase (NSE) in cerebrospinal fluid (CSF), an increase in lipid hydroperoxides and a decrease in the ratio of glutathione to glutathione disulfide in the brain tissues were also observed. These effects were not observed when aminophylline at the same doses was administered under normoxic conditions (PaO₂ = 80-100 mmHg). There was no difference in either serum or CSF concentrations of theophylline between normoxic and hypoxic conditions. Another methylxanthine, caffeine, did not increase the concentration of NSE in CSF.

CONCLUSIONS

Aminophylline potentially induces brain damage under hypoxic conditions. We suggest that aminophylline treatment has adverse effects in patients with hypoxaemia subsequent to respiratory disorders such as asthma.

Effects of hypoxia-inducible factor-1α chemical stabilizer, CoCl(2) and hypoxia on gene expression of CYP3As in human fetal liver cells

Distinctive response patterns of CYP3A4 and CYP3A7 to cobalt chloride (CoCl(2)) in human fetal liver (HFL) cells were observed and compared with those under hypoxic conditions. The expression levels of CYP3A4 and CYP3A7 mRNAs were decreased by CoCl(2) and hypoxia, although significance could not be determined in HFL cells cultured under 3% O(2). The hypoxia-inducible factor-1α (HIF-1α) protein content in HFL cells was significantly increased by CoCl(2) and 3% O(2). Transcriptional activities of CYP3A4 and CYP3A7 were not altered by 3% O(2) when reporter plasmids containing the promoter region ranging up to about 10 kb and 12 kb upstream, respectively, were transfected into HFL cells, although the activity was significantly suppressed by CoCl(2). These results suggested that the mechanisms controlling CYP3A gene expression of HIF-1α chemical stabilizer in fetal hepatocytes might be different from those in adult hepatocytes, and that HIF-1α is not directly involved in regulation of CYP3A4 or CYP3A7 expression.

Hyperbaric oxygen reduces acetaminophen toxicity and increases HIF-1alpha expression

OBJECTIVES

To investigate the effect of hyperbaric oxygen (HBO2) on acetaminophen (APAP)-induced hepatotoxicity. The authors further evaluated the effects of APAP poisoning and HBO2 on the expression and function of hypoxia-inducible factor 1-alpha (HIF-1alpha) in an effort to further describe the mechanisms of APAP-induced hepatotoxicity. In vitro assays were performed to better understand the effects of HBO2 on HIF-1alpha function.

METHODS

In vivo, four groups of C57BL/6 mice were treated as follows: APAP only, APAP followed by HBO2, HBO2 only, and untreated shams. Plasma alanine aminotransferase activity was measured, and hepatic HIF-1alpha induction was determined by Western blot. In vitro, cultured HEP G2 hepatocytes were exposed to HBO2, hypoxia (2.5% O2), or normoxia. HIF-1alpha DNA-binding and transcriptional activity were assessed.

RESULTS

Alanine aminotransferase activity was reduced in the APAP+HBO2 group (2,606 IU/L +/- 4,080; vs. APAP: 6,743 +/- 3,397, p = 0.01 at 6 hours). APAP-only, HBO2-only, and APAP+HBO2 treatments all increased HIF-1alpha expression relative to shams (p = 0.02, p = 0.02, and p < 0.01, respectively). HBO2 increased HIF-1alpha DNA binding 5.7 (+/- 1.2)-fold relative to controls (p < 0.01); however, a parallel increase in HIF functional transcriptional activity did not occur.

CONCLUSIONS

Hyperbaric oxygen reduced early APAP-induced hepatocellular injury. APAP poisoning increases HIF-1alpha protein levels and functional activity. HBO2 increases HIF-1alpha protein levels and DNA binding without a corresponding increase in transcriptional activity.

The bioenergetic role of dioxygen and the terminal oxidase(s) in cyanobacteria

Owing to the release of 13 largely or totally sequenced cyanobacterial genomes (see and ), it is now possible to critically assess and compare the most neglected aspect of cyanobacterial physiology, i.e., cyanobacterial respiration, also on the grounds of pure molecular biology (gene sequences). While there is little doubt that cyanobacteria (blue-green algae) do form the largest, most diversified and in both evolutionary and ecological respects most significant group of (micro)organisms on our earth, and that what renders our blue planet earth to what it is, viz. the O(2)-containing atmosphere, dates back to the oxygenic photosynthetic activity of primordial cyanobacteria about 3.2x10(9) years ago, there is still an amazing lack of knowledge on the second half of bioenergetic oxygen metabolism in cyanobacteria, on (aerobic) respiration. Thus, the purpose of this review is threefold: (1) to point out the unprecedented role of the cyanobacteria for maintaining the delicate steady state of our terrestrial biosphere and atmosphere through a major contribution to the poising of oxygenic photosynthesis against aerobic respiration ("the global biological oxygen cycle"); (2) to briefly highlight the membrane-bound electron-transport assemblies of respiration and photosynthesis in the unique two-membrane system of cyanobacteria (comprising cytoplasmic membrane and intracytoplasmic or thylakoid membranes, without obvious anastomoses between them); and (3) to critically compare the (deduced) amino acid sequences of the multitude of hypothetical terminal oxidases in the nine fully sequenced cyanobacterial species plus four additional species where at least the terminal oxidases were sequenced. These will then be compared with sequences of other proton-pumping haem-copper oxidases, with special emphasis on possible mechanisms of electron and proton transfer.

The Hepatic Sinusoid in Aging and Cirrhosis

The fenestrated sinusoidal endothelium ('liver sieve') and space of Disse in the healthy liver do not impede the transfer of most substrates, including drugs and oxygen, from the sinusoidal lumen to the hepatocyte. Plasma components transfer freely in both directions through the endothelial fenestrations and into the space of Disse. The endothelium is attenuated, there is no basement membrane and there is minimum collagen in the space of Disse, thus minimising any barriers to substrate diffusion. Both cirrhosis and aging are associated with marked structural changes in the sinusoidal endothelium and space of Disse that are likely to influence bulk plasma transfer into the space of Disse, and diffusion through the endothelium and space of Disse. These changes, termed capillarisation and pseudocapillarisation in cirrhosis and aging, respectively, impede the transfer of various substrates. Capillarisation is associated with exclusion of albumin, protein-bound drugs and macromolecules from the space of Disse, and the progressive transformation of flow-limited to barrier-limited distribution of some substrates. There is evidence that the sinusoidal changes in cirrhosis and aging contribute to hepatocyte hypoxia, thus providing a mechanism for the apparent differential reduction of oxygen-dependent phase I metabolic pathways in these conditions. Structural change and subsequent dysfunction of the liver sieve warrant consideration as a significant factor in the impairment of overall substrate handling and hepatic drug metabolism in cirrhosis and aging.

Vascular microenvironment in gliomas

Structural and functional abnormalities of the vascular microenvironment determine pathophysiological characteristics of gliomas, such as loss of blood-brain barrier function, tumor cell invasiveness, or permselectivity for large molecules. Moreover, the effectiveness of various therapeutic strategies critically depends upon the successful transvascular delivery of molecules. In order to shed more light on the vascular microenvironment in gliomas, a variety of experimental and clinical techniques have been applied to study the glioma microvasculature, including histology, vascular corrosion casts, microangiography, autoradiography, tracer washout techniques, magnetic resonance imaging, as well as intravital fluorescence microscopy. This review summarizes the characteristic features of vascular morphology, angio-architecture, tumor perfusion, microvascular permeability, and microvessel-related immunological competence in gliomas. An improved understanding of the vascular microenvironment in gliomas will help in the future to optimize glioma imaging and to improve delivery of vectors for gene therapy or encapsulated drug carriers for pharmacotherapy in patients.

Hypoxia-induced down-regulation of CYP1A1/1A2 and up-regulation of CYP3A6 involves serum mediators

1. Acute moderate hypoxia modifies the catalytic activity and expression of certain isoenzymes of hepatic cytochrome P450 (P450). The aim of this study was to document whether hypoxia affects hepatic P450 directly or through the release of serum mediators. 2. Rabbits were subjected to a FiO(2) of 8% for 48 h, sacrificed, and serum and hepatocytes were isolated; hepatocytes from control and rabbits with hypoxia were incubated with serum from control and hypoxic rabbits for 4 and 24 h, and total P450 content, CYP1A1, 1A2 and 3A6 activities and expressions were assessed. Sera were fractionated by size exclusion chromatography and fractions tested for their ability to modify activity and amount of P450, and serum mediators were identified through neutralization experiments. 3. Total serum and fractions with proteins of 15-23 and 65-94 kDa of M(r) reduced P450 content and expression of CYP1A1, 1A2 and 3A6, as well as CYP1A1, 1A2 and 3A6 mRNA. Total serum and the fraction with 32-44 kDa proteins increased CYP3A6 activity and protein and mRNA. The serum mediators implicated in the decrease in activity and expression of CYP1A1, 1A2 and 3A6 were interferon-gamma (IFN-gamma), interleukin-1beta (IL-1beta) and IL-2. Erythropoietin (Epo) was partly responsible for the increase in P450 content and CYP3A6 expression. 4. In conclusion, acute moderate hypoxia diminishes the activity and expression of CYP1A1, 1A2 and CYP1A1, 1A2 mRNA, and increases CYP3A6 protein, activity and CYP3A6 mRNA. Several mechanisms contribute to these changes in P450, among them the release of cytokines acting as serum mediators.

Effects of acute moderate hypoxemia on kinetics of metoclopramide and its metabolites in chronically instrumented sheep

Hypoxemia is known to induce various physiological changes which can result in alteration in drug pharmacokinetics. To examine the effect of acute moderate hypoxemia on metoclopramide (MCP) pharmacokinetics, a continuous 14-hour infusion of MCP during a normoxemic, hypoxemic and subsequent normoxemic period was conducted in eight adult sheep. Arterial blood and urine samples were collected to examine the effects on the pharmacokinetics of MCP and its deethylated metabolites. MCP and its mono- and di-deethylated metabolites were quantitated using a GC/MS method. Steady-state concentrations of MCP were achieved in each of the three periods. During hypoxemia, MCP plasma steady-state concentration increased significantly from 50.72 +/- 1.06 to 63.62 +/- 1.79 ng/mL, and later decreased to 55.83 +/- 1.15 ng/mL during the post-hypoxemic recovery period. Total body clearance (CL(TB)) of MCP was significantly decreased from 274.2 +/- 48.0 L/h to 205.40 +/- 28.2 L/h during hypoxemia, and later restored to 245.8 +/- 44.2 L/h during the post-hypoxemic period. Plasma mono-deethylated MCP concentration (32.78 +/- 1.73 ng/mL) also increased, compared to the control group (21.20 +/- 1.39 ng/mL), during hypoxemia and subsequent normoxemic period. Renal excretion of MCP and its metabolites was also decreased during hypoxemia, while urine flow was increased with a concomitant decrease in urine osmolality. Thus, the results indicate that acute moderate hypoxemia affects MCP pharmacokinetics.

Reoxygenation after cold hypoxic storage of cultured precision-cut rat liver slices: effects on cellular metabolism and drug biotransformation

Cultured rat precision-cut liver slices (PCLS) were used to study the influence of hypothermic preservation and reoxygenation at 37 degrees C on cellular metabolism and drug biotransformation. Cold hypoxic storage caused a depressed metabolism in rat liver slices, but reoxygenation for 8 h at 37 degrees C partially restored the levels of both ATP and GSH and totally restored the capacity to synthesize proteins. Metabolism of midazolam (CYP3A-dependent oxidation) by cold preserved liver slices was decreased by 30% but no further affected by reoxygenation, showing the same profile as freshly cut slices. Such a reoxygenation at 37 degrees C is accompanied by a dramatic loss of CYP3A2 protein while CYP3A1 protein was unaffected. These results suggest that CYP3A2 did not play a major role in midazolam oxidation. Such results are not consistent with a putative reoxygenation injury but rather with cold hypoxic damage. Since cold preserved liver slices did not respond to bacterial endotoxin stimulation (lipopolysaccharides), a minor role of non-parenchymal cells is suggested as mediators for deleterious effects developed during the cold storage.

Vascular microenvironment in gliomas

Structural and functional abnormalities of the vascular microenvironment determine pathophysiological characteristics of gliomas, such as loss of blood-brain barrier function, tumor cell invasiveness, or permselectivity for large molecules. Moreover, the effectiveness of various therapeutic strategies critically depends upon the successful transvascular delivery of molecules. In order to shed more light on the vascular microenvironment in gliomas, a variety of experimental and clinical techniques have been applied to study the glioma microvasculature, including histology, vascular corrosion casts, microangiography by injection of dyes, blood flow measurements by autoradiography, tracer washout techniques, magnetic resonance imaging, as well as intravital fluorescence microscopy. This review summarizes the characteristic features of vascular morphology, angio-architecture, tumor perfusion, microvascular permeability, as well as microvessel-related immunological competence in gliomas. An improved understanding of the vascular microenvironment in gliomas will help in the future to optimize glioma imaging and delivery of vectors for gene therapy or encapsulated drug carriers in patients.

Intravital fluorescence videomicroscopy to study tumor angiogenesis and microcirculation

Angiogenesis and microcirculation play a central role in growth and metastasis of human neoplasms, and, thus, represent a major target for novel treatment strategies. Mechanistic analysis of processes involved in tumor vascularization, however, requires sophisticated in vivo experimental models and techniques. Intravital microscopy allows direct assessment of tumor angiogenesis, microcirculation and overall perfusion. Its application to the study of tumor-induced neovascularization further provides information on molecular transport and delivery, intra- and extravascular cell-to-cell and cell-to-matrix interaction, as well as tumor oxygenation and metabolism. With the recent advances in the field of bioluminescence and fluorescent reporter genes, appropriate for in vivo imaging, the intravital fluorescent microscopic approach has to be considered a powerful tool to study microvascular, cellular and molecular mechanisms of tumor growth.

Fetal hepatic drug elimination

The majority of studies of fetal hepatic elimination have concentrated on the expression and activity of the metabolizing enzymes, but the unique physiologic milieu of the fetal liver should also be considered. The basic structure of the liver is formed by the end of the first trimester. The fetal hepatic circulation differs substantially from that of the adult in that there is an extra input vessel, the umbilical vein, and there is shunting of 30-70% of hepatic blood flow via the ductus venosus. The left and right lobes of the fetal liver seem to function independently with respect to a variety of biochemical parameters, due at least in part to the lower oxygen supply to the right lobe. The zonation of drug-metabolizing enzymes along the hepatic acinus, which is prominent in the adult liver, is absent in the fetal liver. Unlike rodent species, the human fetal liver has a significant capacity for drug metabolism. Of the oxidative enzymes, CYP3A7 accounts for up to 50% of total fetal hepatic cytochrome P450 content. Expression of this enzyme decreases dramatically after birth. CYP1A1 and CYP2D6 have also been detected in human fetal liver, but whether CYP2E1 is expressed remains controversial. Several other cytochrome P450s have been identified and await characterization. Fetal hepatic drug conjugation may prolong fetal exposure to the metabolites produced, which, being more water soluble, do not readily cross the placenta back to the mother and, if excreted in fetal urine, can be recycled in the fetus via amniotic fluid and fetal swallowing. Limited activity of glucuronidation enzymes has been demonstrated in human fetal liver in contrast to the activity of sulfation enzymes, which is significant. Limited in vivo studies in fetal sheep have demonstrated significant fetal hepatic drug elimination, and this has been confirmed in studies of the isolated perfused fetal sheep liver. Our understanding of fetal hepatic elimination processes has advanced steadily over the years. Future developments, however, should consider more fully the influence of the unique physiological milieu of the fetal liver, in addition to the expression and activity of drug metabolizing enzymes.

Does reductive metabolism predict response to tirapazamine (SR 4233) in human non-small-cell lung cancer cell lines?

The bioreductive drug tirapazamine (TPZ, SR 4233, WIN 59075) is a lead compound in a series of potent cytotoxins that selectively kill hypoxic rodent and human solid tumour cells in vitro and in vivo. Phases II and III trials have demonstrated its efficacy in combination with both fractionated radiotherapy and some chemotherapy. We have evaluated the generality of an enzyme-directed approach to TPZ toxicity by examining the importance of the one-electron reducing enzyme NADPH:cytochrome P450 reductase (P450R) in the metabolism and toxicity of this lead prodrug in a panel of seven human non-small-cell lung cancer cell lines. We relate our findings on TPZ sensitivity in these lung lines with our previously published results on TPZ sensitivity in six human breast cancer cell lines (Patterson et al (1995) Br J Cancer 72: 1144-1150) and with the sensitivity of all these cell types to eight unrelated cancer chemotherapeutic agents with diverse modes of action. Our results demonstrate that P450R plays a significant role in the activation of TPZ in this panel of lung lines, which is consistent with previous observations in a panel of breast cancer cell lines (Patterson et al (1995) Br J Cancer 72: 1144-1150; Patterson et al (1997) Br J Cancer 76: 1338-1347). However, in the lung lines it is likely that it is the inherent ability of these cells to respond to multiple forms of DNA damage, including that arising from P450R-dependent TPZ metabolism, that underlies the ultimate expression of toxicity.

Oxidative injury reproduces age-related impairment of oxygen-dependent drug metabolism

The Oxygen Diffusion Barrier Hypothesis states that aging in the liver is associated with restricted oxygen uptake that explains the age-related impairment of phase I drug clearance observed in vivo with preservation of in vitro phase I enzyme activity and in vivo phase II drug clearance. Aging in the liver may be secondary to oxidative stress. Therefore we examined the effects of oxidative injury on oxygen uptake, and phase I and phase II drug metabolism in the liver. Oxidative stress was induced in the perfused rat liver with hydrogen peroxide. The intrinsic clearances of propranolol and morphine were used as markers of phase I and phase II activity, respectively. Oxidative injury was associated with a 14+/-99% (P=0.03) reduction in oxygen uptake. The decrease in the intrinsic clearance of propranolol was greater than that of morphine (57+/-14% vs 34+/-7% P<0.005). This result supports the concept of a restriction of oxygen supply constraining hepatic drug metabolism following oxidative stress. This has implications for aging and hepatic drug metabolism.

Ethanol attenuates lactate production in hypoxic postnatal day 4 rat cerebella

Ethanol consumption during pregnancy may lead to a low oxygen supply to the brain of the developing fetus. Such a reduction in the oxygen supply will result in changes in intra- and extracellular lactate production, which subsequently may lead to cytoplasmic acidosis, changes in cerebral metabolism, and eventually, cell death. We used a novel application of gas chromatography to measure lactate changes, on a global level, in the cerebellar tissue of postnatal day (PD) 4 and PD 10 rat pups following in vitro exposure of either hypoxia or hypoxia plus ethanol (hypoxia/ethanol). The results showed hypoxia-induced increases in lactate concentrations as a function of treatment time in both PD 4 and PD 10 cerebellar tissue. However, there was a differential response to the additional ethanol treatment between the two age groups assessed, with an attenuation of the time-dependent increase of lactate production following hypoxia treatment in PD 4 cerebellar tissue. The results also indicated that PD 4 cerebellar tissue had increased oxygen utilization when compared with PD 10 tissue exposed to the same conditions. The ethanol-induced reduction in lactate is hypothesized as being due to limitations in glucose transport and utilization under ethanol/hypoxia exposure. It is believed that such limitations in cellular function may initiate a sequence of events that produce at least some of the cerebellar neuronal loss reported in the fetal alcohol literature.

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.

DNA fragmentation during exposure of rat cerebella to ethanol under hypoxia imposed in vitro

To gain a better understanding into the mechanisms of damage incurred by neurons in periods following heavy alcohol exposure during development, we used an in vitro system to monitor the effects of alcohol and hypoxia on cell survival and DNA integrity. Samples representing the first few hours of exposure to alcohol and hypoxia were compared to those resulting from hypoxia alone. Measurements were taken from cell counts using Trypan blue exclusion and TUNEL assays as well as digital scans of the ethidium bromide fluorescence of genomic DNA isolated from the treated tissue. We found that DNA degradation from hypoxia was accelerated by several hours in the presence of 100 mM ethanol. This result depended on age, with adult animals (>8 months) having a similar response to 4-day postnatal animals, while the effect on 10-day postnatal animals and those of intermediate age (45 days postnatal) was increasingly delayed. Different methods of inducing the processive degradation of DNA produced laddering typical of apoptosis, a biphasic degradative process, or patterns usually associated with necrosis.

Chronic hypoxia alters glucose utilization during GSH-dependent detoxication in rat small intestine

We showed that hypoxia alters glutathione (GSH)-dependent detoxication and induces mucosal metabolic instability. To determine the impact of these changes and the role of reductant supply in intestinal lipid peroxide disposition, pair-fed (16 g/day) Sprague-Dawley rats were exposed to air (20.9% O2; n = 6) or 10% O2 (n = 6) for 10 days. Jejunal and ileal everted sacs were exposed to 75 microM peroxidized fish oil with or without 10 mM glucose or 1 mM GSH. Peroxide transport was determined as the abluminal recovery of thiobarbituric acid-reactive substances. Peroxide recovery in hypoxic intestine was twice that in normoxic intestine. Addition of GSH and glucose did not affect peroxide recovery, indicating reduced intracellular GSH-dependent metabolism and enhanced output by the hypoxic intestine. Glucose uptake by normoxic and hypoxic intestine is similar, whereas its utilization for detoxication is decreased in hypoxic cells. Determination of NADPH supply indicates that decreased glucose availability for NADPH production during hypoxia impairs GSH disulfide reduction, compromises hydroperoxide metabolism, and increases peroxide output from hypoxic intestine.

Pharmacokinetics of high-dose thiopental in pediatric patients with increased intracranial pressure

This study was conducted on 10 pediatric patients in whom intracranial hypertension stemming from head injury was reduced by high-dose thiopental administered for a prolonged period. All children showed a favorable recovery of their neurological functions. The total dose normalized to body weight averaged 335 +/- 135 mg/kg, and the mean treatment duration was 93.0 +/- 37.1 h. Data analysis was modeled for each patient to cover all the doses on the whole plasma thiopental concentration-time curve, according to a one-compartment open model. A better fit was obtained using a linear model rather than a Michaelis-Menten elimination model. Model selection was guided by evaluation of the minimum objective function, the weighted residuals, and the Akaike criterion. Thiopental pharmacokinetic parameters in pediatric patients were compared with those determined in an adult control group with similar total doses and durations of treatment. No significant difference was found between the two groups in spite of a 33% decrease of the elimination half-life in children (11.7 +/- 5.7 h) compared with adults (17.5 +/- 9.03 h). The mean values obtained were 2.42 and 2.19 ml/min/kg for total clearance and 2.18 and 2.90 L/kg for Vd in pediatric and adult groups, respectively. The linear regression of pharmacokinetic parameters in terms of age was not significant. When high doses of thiopental were administered over a prolonged period, the pharmacokinetic parameters computed for pediatric patients did not differ from those obtained in adults.

Transient accumulation of heme O (cytochrome o) in the cytoplasmic membrane of semi-anaerobic Anacystis nidulans. Evidence for oxygenase-catalyzed heme O/A transformation

Incubation of obligately photoautotrophic and aerobic cyanobacterium Anacystis nidulans (Synechococcus sp. PCC 6301) in the light in the presence of the photo-system II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea and equilibrated with approximately 1% (v/v) O2 in N2 (10 microM O2 in solution) led to a decrease of the heme A content of isolated cytoplasmic membranes and to the appearance of heme O. The latter was not seen in membranes from fully aerated cells (> 210 microM dissolved O2). Non-covalently bound hemes extracted from the membranes were identified by reversed phase high performance liquid chromatography. Heme A and O contents of the membranes changed in a reversible fashion solely depending on the ambient oxygen regime. Both hemes A and O combine with the same apoprotein as suggested by immunoblotting. CO/reduced-minus-reduced optical difference spectra, photoaction spectra of CO-inhibited O2 uptake by the membranes, and pyridine hemochrome spectra pointed to either heme belonging to a functional form of the terminal oxidase. The NADH:O2 oxidoreductase reaction catalyzed by membranes from both high O2 and low O2 cells was strictly dependent on the addition of catalytic amounts of cytochrome c, fully inhibited by 1.2 microM KCN, and insensitive to 5 microM 2-n-heptyl-4-hydroxyquinoline-N-oxide. O2 uptake by the membranes was effectively catalyzed by N,N,N',N'-tetramethyl-p-phenylenediamine but not 2-methylnaphthoquinol or plastoquinol-1 as artificial substrates. Therefore we conclude that the cyanobacterial respiratory oxidase, irrespective of the type of heme in its O2-reducing center, is a cytochrome c rather than a quinol oxidase.

Occurrence of heme O in photoheterotrophically growing, semi-anaerobic cyanobacterium Synechocystis sp. PCC6803

Extraction and identification of the non-covalently bound heme groups from crude membrane preparations of photoheterotrophically grown Synechocystis sp. PCC 6803 by reversed phase high performance liquid chromatography and optical spectrophotometry led to the detection of heme O in addition to hemes B and A which latter was to be expected from the known presence of aa3-type cytochrome oxidase in cyanobacteria. In fully aerated cells (245 microM dissolved O2 in the medium) besides heme B only heme A was found while in low-oxygen cells (< 10 microM dissolved O2) heme O was present at a concentration even higher than that of heme A. Given the possible role of heme O as a biosynthetic intermediate between heme B and heme A, together with generally much higher Km values of 5-50 microM O2 for oxygenase as compared to Km values of 40-70 nM O2 for typical cytochrome-c oxidase, our findings may permit the conclusion that the conversion of heme O to heme A is an obligately oxygen-requiring process catalyzed by some oxygenase directly introducing oxygen from O2 into the 8-methyl group of heme O. At the same time thus the occurrence of heme O (cytochrome o) in cyanobacteria does of course not imply the existence of an 'alternative oxidase' since according to the well-known 'promiscuity of heme groups' both hemes O and A are likely to combine with one and the same apoprotein.

Oxygen supplementation restores theophylline clearance to normal in cirrhotic rats

BACKGROUND/AIMS

Capillarization associated with hepatic fibrosis may present a functional barrier to oxygen diffusion into the hepatocyte, and restriction on cellular oxygen supply may represent the rate-limiting constraint on hepatic oxidative drug metabolism. The aim of this study was to test this hypothesis by examining the effect of oxygen supplementation on plasma theophylline clearance in 10 control and 10 cirrhotic rats.

METHODS

Theophylline (3 mg/kg) was administered intravenously on two separate occasions, 24 hours apart, during which time the rats breathed either room air or oxygen (95%) from 1 hour before dosing until the end of plasma sampling with a randomized order of gas exposure.

RESULTS

Theophylline clearance was significantly reduced by a mean of 37% (n = 10; P = 0.003) in cirrhotic rats compared with controls. Oxygen supplementation significantly improved plasma theophylline clearance in cirrhotic rats by a mean of 40% (n = 10; P = 0.007), whereas clearance remained unchanged in healthy rats. Clearance in oxygen-supplemented cirrhotic rats was not significantly different from that in controls (P > 0.05).

CONCLUSIONS

These novel findings indicate an important role for hepatic oxygenation in improving drug disposition in cirrhosis, which may have potentially important clinical implications for the management of this disease.

Fetal alcohol syndrome: the vulnerability of the developing brain and possible mechanisms of damage

Fetal alcohol exposure has multiple deleterious effects on brain development, and represents a leading known cause of mental retardation. This review of the effects of alcohol exposure on the developing brain evaluates results from human, animal and in vitro studies, but focuses on key research issues, including possible mechanisms of damage. Factors that affect the risk and severity of fetal alcohol damage also are considered.

Leukotriene B4 formation upon halothane-induced lipid peroxidation in liver membrane fractions under low O2 concentrations in vitro

Lipid peroxidation was induced in rat liver membrane fractions in vitro upon NADPH-dependent metabolic activation of the anesthetic agent halothane at low O2 concentrations. Halothane-induced lipid peroxidation was dependent on time, concentration of halothane, and the calculated O2 concentrations present in the system. Lipid peroxidation was inducible at increasing O2 concentrations up to 12 microM, decreased at higher O2 concentrations up to 48 microM, and was not detectable at normoxic conditions. Leukotriene B4 (LTB4) was identified as a product arising upon lipid peroxidation by reverse-phase high-pressure liquid chromatography combined with a radioimmunoassay. LTB4 formation was maximal under conditions of maximal lipid peroxidation at a calculated O2 concentration of 12 microM. Even at high concentrations, the 5-lipoxygenase inhibitors MK886 (10 microM), ZD2138 (20 microM), and ZM230487 (20 microM) were not inhibitory in halothane-induced lipid peroxidation nor in the associated formation of LTB4. Synthetic LTB4 was transformed into its 20-hydroxy derivative by omega-oxidation in an O2-concentration-dependent manner, being considerably reduced at the low O2 concentrations that maximally promoted lipid peroxidation. The collective evidence of these data raises the possibility that exposure to halothane might lead to peroxidation-associated net synthesis of LTB4 through 5-lipoxygenase-independent escape routes in liver tissue under physiologically or pathophysiologically low O2 concentrations.

Pharmacokinetics and pharmacodynamics in the critically ill patient

1. Until recently, when drugs were used in critically ill patients they were expected to behave in the same way as in less seriously ill patients. Now the unpredictability of even the most reliable drugs has been recognized. With this there is an awareness of the adverse effects drugs may have on organs other than the ones the drug was intended to act on. In patients with multiorgan dysfunction, poly-pharmacy is usually needed. The drugs may not only interfere with the action of each other at the receptor and enzyme level, but may also change protein binding and elimination. All these effects may be unimportant in less seriously ill patients, but may affect outcome in the critically ill. A high degree of awareness and suspicion of unknown drug-induced adverse reaction is needed by clinicians and pharmacologists alike.

Effect of hypoxia on the hepatic metabolism of lidocaine in the isolated perfused pig liver

The metabolism of lidocaine to monoethylglycinexylidide has been found useful as an indicator of liver function in association with liver transplantation. It has been postulated that this is due to the common effect of hypoxic damage on liver function and lidocaine metabolism. The effects of hypoxia on the elimination of lidocaine and the formation of monoethylglycinexylidide and on indexes of liver function were investigated with the isolated perfused pig liver preparation. This study was performed at similar hepatic effluent lidocaine concentrations of approximately 5 micrograms.ml-1 in normoxic (n = 7) and hypoxic (n = 8) livers of similar mass harvested from male Landrace x Large White pigs and perfused at standard unit hepatic flow rates. Whole blood lidocaine extraction ratio was 0.63 +/- 0.02 in normoxic livers (30% O2 at oxygenator inflow). It was significantly less (0.23 +/- 0.03) in livers subjected to hypoxia (2% O2 at oxygenator inflow), as were hepatic clearance (57.1 +/- 2.1 vs. 20.3 +/- 3.1 ml.min-1.100 gm-1), intrinsic clearance (1,706 +/- 182 vs. 284 +/- 53 ml.min-1.100 gm-1) and monoethylglycinexylidide formation as indicated by monoethylglycinexylidide/lidocaine ratios in the hepatic venous effluent (0.379 +/- 0.061 vs. 0.073 +/- 0.014) (p < 0.01). Hepatic oxygen consumption, adenine nucleotide status and bile flow were significantly impaired by hypoxia. Whereas perfusate potassium concentration increased early, AST levels showed delayed increases and ALT levels showed no changes. These changes correlated strongly with hepatic lidocaine elimination (p < 0.01). We conclude that lidocaine metabolism may be an early indicator of severe hepatic hypoxia.

Hypoxia and oxygen dependence of cytotoxicity in renal proximal tubular and distal tubular cells

Ischemia and hypoxia are major causes of renal failure and altered oxygen supply may affect renal responses to toxic chemicals. In vitro experiments were designed to evaluate the susceptibility of isolated proximal tubular (PT) and distal tubular (DT) cells from rat kidney to brief periods of oxygen deprivation and to assess how variations in oxygen supply affect chemical-induced cytotoxicity. Isolated cells were incubated for 1 hr in either oxygen (95% O2/5% CO2), air (21% O2), or nitrogen (95% N2/5% CO2) atmosphere. PT cells exhibited no injury due to brief oxygen deprivation whereas DT cells exhibited moderate, but significant injury, indicating that DT cells are more susceptible than PT cells to hypoxic injury. The cytotoxicity of chemicals that alter cellular redox status [i.e. tert-butyl hydroperoxide (tBH), menadione, methyl vinyl ketone] and the cytotoxicity of "chemical hypoxia" [i.e. KCN + iodoacetic acid] were greatest in air, intermediate in oxygen, and lowest in nitrogen. In contrast, the cytotoxicity of the alkylating agent N-dimethylnitrosamine was independent of oxygen concentration and the cytotoxicity of p-aminophenol was related directly to oxygen concentration. The mechanism of the oxygen dependence of chemical injury was investigated further, employing tBH as a model toxicant. tBH metabolism was oxygen independent in both PT and DT cells. Depletion of cellular protein sulfhydryl groups by tBH increased with increasing oxygen concentration and lipid peroxidation due to tBH was inhibited in nitrogen but was not different in air as compared with oxygen. Although these processes may contribute to the much lower toxicity in nitrogen as compared with oxygen, it does not explain the higher toxicity in air as compared with that in oxygen. Other processes that predominate at lower oxygen concentrations but that only produce injury if enough oxygen is present are likely to be responsible for the enhanced susceptibility of both PT and DT cells to oxidants in air as compared with oxygen.

Sensitivity of propranolol elimination to hypoxia in the isolated perfused rat liver preparation

1. The relationship between the hepatic elimination of propranolol and hepatic oxygen delivery was examined in the single-pass isolated perfused rat liver preparation. Varying rates of oxygen delivery were produced (1.35-8.10 mumol/min per g liver) by equilibrating the perfusate with O2/N2 mixtures. 2. In two experiments, in which the rate of oxygen delivery was increased or decreased within the hypoxic range (< 4-5 mumol/min per g liver) every 5 min for 120 min, propranolol clearance responded very rapidly in the same direction as the change in oxygen delivery. 3. In five experiments, propranolol clearance, measured at steady state during an initial 30 min normoxia phase and three subsequent 30 min hypoxia phases (oxygen delivery in the range 1.35-5.89 mumol/min per g liver), was linearly related to hepatic oxygen delivery and consumption (r = 0.92 +/- 0.07). 4. These data, combined with those from six further experiments that used one normoxia phase followed by one hypoxia phase, showed that there was a threshold for oxygen delivery of about 6 mumol/min per g liver, below which propranolol clearance decreased with decreasing oxygenation. 5. This study shows that in the intact liver propranolol elimination is very sensitive to hepatic oxygen supply, with impairment in clearance occurring at the lower limit of what is considered normal hepatic oxygenation in the rat.

Fasting increases the sensitivity of hepatic harmol glucuronidation to hypoxia

Livers from fasted (N = 16) and fed (N = 22) rats were perfused with harmol (50 microM) for an initial 30 min with normal oxygen delivery (6-10 mumol/min/g liver), then for 45 min with perfusate equilibrated with O2/N2 mixtures, which reduced hepatic oxygen delivery to 0.9-6 mumol/min/g liver, and finally for a further 30 min period of normal oxygenation. Seventy per cent of the harmol eliminated was accounted for as the glucuronide conjugate and approximately 5% as the sulphate conjugate. During the hypoxia phase with fed preparations, decreasing oxygenation did not reduce harmol clearance or harmol glucuronide formation clearance until oxygen delivery was less than 2.5 mumol/min/g liver, whereas with fasted preparations this hypoxic threshold was much higher (5 mumol/min/g liver). Below the hypoxic threshold, harmol clearance was linearly related to oxygen delivery in both groups. Hepatic tissue concentrations of unchanged harmol at the end of the hypoxia phase were double those after the same period of normal oxygenation, whereas tissue harmol glucuronide concentrations were similar. By establishing a hypoxic threshold for reduced oxygen availability this study shows that harmol glucuronidation is relatively insensitive to hypoxia, but sensitivity increases markedly in fasted animals.

Effects of oxygen concentration on the metabolism of anisole homologues by rat liver microsomes

1. The effects of oxygen concentration were studied on the metabolic pathways of anisole homologues (anisole, phenetole and isopropoxybenzene) catalysed by liver microsomes from phenobarbital-treated rats. 2. With increase of oxygen concentration, the rate of anisole o-hydroxylation reached a plateau at about 35 microM O2, while the rates of O-demethylation and aromatic p-hydroxylation were still increasing at 223 microM O2 (air). 3. The rates of all three metabolic reactions of phenetole reached plateau levels at about 80 microM O2. 4. The rates of all three metabolic reactions of iso-propoxybenzene were still increasing as 223 microM O2 (air). 5. The ratio of aromatic p-hydroxylation or O-dealkylation to aromatic o-hydroxylation decreased in anisole metabolism, and showed no uniform change in phenetole and isopropoxybenzene metabolism with decreasing oxygen concentration. 6. The ratio of aromatic p-hydroxylation to O-dealkylation was essentially constant over the range of oxygen concentration studied in anisole and phenetole metabolism, while in iso-propoxybenzene metabolism the ratio was different between higher and lower oxygen concentrations than 60 microM. 7. This series of compounds with increasing chain length did not show homologous changes in rates of product formation or O2 dependent of product formation.

Effects of inspired oxygen concentration on in vivo redox reaction of nitroxide radicals in whole mice

Effects of inspired oxygen concentration on metabolism of nitroxide radical were measured in whole mice by using in vivo ESR. Oxygen concentration influenced spin-clearance of nitroxide differently in abdomen and head, suggesting that either O2 concentrations or metabolic mechanisms of nitroxide in the two domains were different.

Effect of chronic hypoxia on acetaminophen metabolism in the rat

The effect of chronic hypoxia (10.5% O2 for 8-9 days) on acetaminophen metabolism was studied in vivo or in isolated cell or microsomal systems. Results from in vivo studies with oral administration of acetaminophen showed that in hypoxic rats, the plasma appearance of the drug was delayed and the plasma half-life was increased. Analyses of the area under the curve (AUCoral) showed that this value was higher in hypoxic rats, whereas the rate constants for elimination (kelim) and absorption (kabs) were lower in these animals. Formation of the glucuronide and sulfate conjugates was decreased significantly (P less than 0.05) in hypoxic animals. The calculated volume of distribution (Vd) after an intravenous dose was not different in either group but total clearance (CL) was 35% lower in hypoxic rats. Studies with isolated hepatocytes from both groups revealed that glucuronidation and sulfation were inhibited markedly at low O2 concentrations. The O2 concentrations required for half-maximal production (P50 values) of glucuronide (2.3 microM O2) and sulfate (1.8 microM O2) conjugates in cells from hypoxic animals were lower than for control cells (5.3 microM and 3.9 microM O2 for glucuronide and sulfate conjugates, respectively). Maximal rates of conjugation in cells from hypoxic rats were 60-70% of control rates. Similar decreases in microsomal UDP-glucuronosyltransferase and cytosolic sulfotransferase activities were found in livers of animals exposed to chronic hypoxia. These lower P50 values are consistent with a lower P50 for oxidation of mitochondrial cytochromes in hypoxic cells. In comparison, the P50 for glutathione conjugation (4.1 microM O2) was not statistically different from control (4.6 microM O2), but the maximal rate was 65% higher. The results show that chronic hypoxia causes a change of absorptive processes and decreased glucuronidation and sulfation reactions which affects the disposition of acetaminophen and potentially the disposition of a variety of other exogenous and endogenous compounds.

The effect of respiratory disorders on clinical pharmacokinetic variables

Respiratory disorders induce several pathophysiological changes involving gas exchange and acid-base balance, regional haemodynamics, and alterations of the alveolocapillary membrane. The consequences for the absorption, distribution and elimination of drugs are evaluated. Drug absorption after inhalation is not significantly impaired in patients. With drugs administered by this route, an average of 10% of the dose reaches the lungs. It is not completely clear whether changes in pulmonary endothelium in respiratory failure enhance lung absorption. The effects of changes in blood pH on plasma protein binding and volume of distribution are discussed, but relevant data are not available to explain the distribution changes observed in acutely ill patients. Lung diffusion of some antimicrobial agents is enhanced in patients with pulmonary infections. Decreased cardiac output and hepatic blood flow in patients under mechanical ventilation cause an increase in the plasma concentration of drugs with a high hepatic extraction ratio, such as lidocaine (lignocaine). On a theoretical basis, hypoxia should lead to decreased biotransformation of drugs with a low hepatic extraction ratio, but in vivo data with phenazone (antipyrine) or theophylline are conflicting. The effects of disease on the lung clearance of drugs are discussed but clinically relevant data are lacking. The pharmacokinetics of drugs in patients with asthma or chronic obstructive pulmonary disease are reviewed. Stable asthma and chronic obstructive pulmonary disease do not appear to affect the disposition of theophylline or beta 2-agonists such as salbutamol (albuterol) or terbutaline. Important variations in theophylline pharmacokinetics have been reported in critically ill patients, the causes of which are more likely to be linked to the poor condition of the patients than to a direct effect of hypoxia or hypercapnia. Little is known regarding the pharmacokinetics of cromoglycate, ipratropium, corticoids or antimicrobial agents in pulmonary disease. In patients under mechanical ventilation, the half-life of midazolam, a new benzodiazepine used as a sedative, has been found to be lengthened but the underlying mechanism is not well understood. Pulmonary absorption of pentamidine was found to be increased in patients under mechanical ventilation. Pharmacokinetic impairment does occur in patients with severe pulmonary disease but more work is needed to understand the exact mechanisms and to propose proper dosage regimens.