Cytochrome P450 enzyme-mediated drug metabolism at exposure to acute hypoxia (corresponding to an altitude of 4,500�m)

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.

Intermittent Hypoxia Inhibits Hepatic CYP1a2 Expression and Delays Aminophylline Metabolism

Purpose

In this study, we aimed to determine the effects of intermittent hypoxia (IH) on hepatic cytochrome P450 1A2 (CYP1A2) expression and the pharmacokinetics of CYP1A2-mediated aminophylline and warfarin in vitro and in a rabbit model of obstructive sleep apnea.

Materials

Human normal liver (LO-2) cells were exposed to 30 min each of 1%, 1–21%, 21%, and 21–1% O₂, and then, CYP1A2 expression and drug concentrations were analyzed. We compared the pharmacokinetic parameters of drugs administered to normoxic rabbits and those exposed to 10 min of IH during which the oxygen level fluctuated from 21% to 8%–10% (n = 10 per group).

Result

s. The expression of CYP1A2 protein in vitro was significantly reduced in the IH compared with the normoxic cells (0.56 ± 0.11 vs. 1.27 ± 0.17, p < 0.001). Aminophylline was more abundant in cell culture supernatants after 48 h of IH than in those under normoxia. The T_1/2, AUC_0–24 h, and Ke values for aminophylline were significantly higher in the IH group.

Conclusion

Intermittent hypoxia inhibits hepatic CYP1A2 expression and delays aminophylline metabolism, suggesting that the impact of IH on the expression of CYP enzymes should be closely monitored in clinical practice.

A Mechanistic, Enantioselective, Physiologically Based Pharmacokinetic Model of Verapamil and Norverapamil, Built and Evaluated for Drug-Drug Interaction Studies

The calcium channel blocker and antiarrhythmic agent verapamil is recommended by the FDA for drug–drug interaction (DDI) studies as a moderate clinical CYP3A4 index inhibitor and as a clinical Pgp inhibitor. The purpose of the presented work was to develop a mechanistic whole-body physiologically based pharmacokinetic (PBPK) model to investigate and predict DDIs with verapamil. The model was established in PK-Sim^®, using 45 clinical studies (dosing range 0.1–250 mg), including literature as well as unpublished Boehringer Ingelheim data. The verapamil R- and S-enantiomers and their main metabolites R- and S-norverapamil are represented in the model. The processes implemented to describe the pharmacokinetics of verapamil and norverapamil include enantioselective plasma protein binding, enantioselective metabolism by CYP3A4, non-stereospecific Pgp transport, and passive glomerular filtration. To describe the auto-inhibitory and DDI potential, mechanism-based inactivation of CYP3A4 and non-competitive inhibition of Pgp by the verapamil and norverapamil enantiomers were incorporated based on in vitro literature. The resulting DDI performance was demonstrated by prediction of DDIs with midazolam, digoxin, rifampicin, and cimetidine, with 21/22 predicted DDI AUC ratios or C_trough ratios within 1.5-fold of the observed values. The thoroughly built and qualified model will be freely available in the Open Systems Pharmacology model repository to support model-informed drug discovery and development.

Impact of stressors in the aviation environment on xenobiotic dosimetry in humans: physiologically based prediction of the effect of barometric pressure or altitude

Standard health risks from volatile organic compounds (VOCs) are generally interpreted at ambient environmental conditions. The aim of this study was to develop a strategy for using physiologically based pharmacokinetic (PBPK) modeling to compare known risks in the general population to calculated risks in pilots experiencing pressure-based stressors. PBPK models facilitate these comparisons by prediction of how target-tissue specific doses are altered when a stressor, such as high altitude, produces changes in physiological parameters. Cardiac output, regional blood flow, and alveolar ventilation rate following acute exposure to altitude ranging from moderate to extremely high were estimated from published data from 52 groups of human subjects. Scenarios where pilots might inhale toluene, 1,2,4-trimethylbenzene (1,2,4-TMB), or cyclohexane during routine military flight training were simulated. At the recommended Threshold Limit Values (TLV), arterial blood concentrations were predicted to be higher for exposure at 15000 ft (4572 m) than at sea level. The differences were greater for toluene and TMB, which have higher blood: air and fat: blood partition coefficients than less lipophilic cyclohexane. In summary, quantitative approaches to internal dosimetry prediction that take advantage of existing knowledge of physiological changes induced by occupational stressors possess potential as tools in performing a human health risk assessment.

Effects of Gut Microbiota on Drug Metabolism and Guidance for Rational Drug Use Under Hypoxic Conditions at High Altitudes

BACKGROUND

Modern features of drug development such as low permeability, low solubility, and improved release affect the interplay of the gut microbiota and drug metabolism. In recent years, studies have established the impact of plateau hypoxia on gut microbiota, where drug use by plateau populations is affected by hypoxia- induced changes in intestinal microflora-mediated drug metabolism.

METHODS

In this review, we summarized the effects of gut microbiota on drug metabolism, and of plateau hypoxia on the intestinal flora, with the aim of providing guidance for the rational use of drugs in high-altitude populations.

RESULTS

The evidence clearly shows that alterations in gut microbiota can affect pro-drug activation, drug inactivation, and the biotransformation of xenobiotics. Additionally, plateau hypoxia alters drug metabolism by affecting intestinal flora.

CONCLUSION

This review provides an overview of the effects of gut microbiota on drug metabolism and provides guidance for rational drug use under hypoxic conditions at high altitudes.

A Systematic Review and Meta-Analysis Reveals Altered Drug Pharmacokinetics in Humans During Acute Exposure to Terrestrial High Altitude-Clinical Justification for Dose Adjustment?

Bailey, Damian Miles, Benjamin S. Stacey, and Mark Gumbleton. A systematic review and meta-analysis reveals altered drug pharmacokinetics in humans during acute exposure to terrestrial high altitude-clinical justification for dose adjustment? High Alt Med Biol. 19:141-148, 2018.

OBJECTIVE

While physiological responses during acute ascent to terrestrial high altitude (HA) have the potential to alter the pharmacokinetics (PKs) that define absorption and disposition of medicinal drugs, there have been no systematic reviews and meta-analyses performed to date.

METHODS

We conducted a systematic literature search in June 2017 using NCBI PubMed, EMBASE, Web of Science, and Ovid MEDLINE databases to identify relevant observational studies. Studies were deemed eligible based on the following criteria: (1) participants: healthy, nonacclimatized male or female lowlanders (born and bred at sea level) and (2) environment: exposure to low altitude (LA, ≤600 m), followed by terrestrial high altitude (HA, ≤24 hours to ≥2500 m), the time course specifically selected to avoid interpretive complications associated with erythrocytosis. All PK parameters were standardized to be in the same units and the weighted standardized mean difference (SMD) calculated using a combination of fixed and random effects models with heterogeneity evaluated using χ² and I² statistics.

RESULTS

Of 20,840 studies reviewed, 6 prospective cohort studies (n = 75) qualified for inclusion, with participants exposed to a mean altitude of 4025 (mean) ± 380 (SD) m. We observed increases for absorption half-life (SMD: 0.40, 95% CI: 0.01-0.80, p = 0.04], elimination half-life (SMD: 0.89, 95% CI: 0.30-1.48, p = 0.003), and erythrocyte binding (SMD: 0.52, 95% CI: 0.16-0.88, p = 0.004) and reduction in clearance (SMD: -0.56, 95% CI: -1.13 to 0.00, p = 0.05).

CONCLUSIONS

Collectively, these findings reveal impairments in both oral absorption and corresponding clearance of the, although limited, sample of drugs at HA that may potentially require closer patient monitoring and dose adjustments to maintain therapeutic efficacy and avoid incidental toxicity.

Hepatic Cyp1a2 Expression Reduction during Inflammation Elicited in a Rat Model of Intermittent Hypoxia

BACKGROUND

Intermittent hypoxia (IH) is a key element of obstructive sleep apnea (OSA) that can lead to disorders in the liver. In this study, IH was established in a rat model to examine its effects on the expression of hepatic cytochrome P450 (CYP) and CYP regulators, including nuclear receptors.

METHODS

Hematoxylin and eosin staining was conducted to analyze the general pathology of the liver of rats exposed to IH. The messenger RNA (mRNA) expression levels of inflammatory cytokines, CYPs, nuclear factor-κB (NF-κB), and nuclear factors in the liver were measured by quantitative reverse transcription polymerase chain reaction.

RESULTS

We found inflammatory infiltrates in the liver of rats exposed to IH. The mRNA expression level of interleukin-1beta was increased in the liver of the IH-exposed rats (0.005 ± 0.001 vs. 0.038 ± 0.008, P = 0.042), whereas the mRNA expression level of Cyp1a2 was downregulated (0.022 ± 0.002 vs. 0.0050 ± 0.0002, P = 0.029). The hepatic level of transcription factor NF-κB was also reduced in the IH group relative to that in the control group, but the difference was not statistically significant and was parallel to the expression of the pregnane X receptor and constitutive androstane receptor. However, the decreased expression of the glucocorticoid receptor upon IH treatment was statistically significant (0.056 ± 0.012 vs. 0.032 ± 0.005, P = 0.035).

CONCLUSIONS

These results indicate a decrease in expression of hepatic CYPs and their regulator GR in rats exposed to IH. Therefore, this should be noted for patients on medication, especially those on drugs metabolized via the hepatic system, and close attention should be paid to the liver function of patients with OSA-associated IH.

Travel to High Altitude Following Solid Organ Transplantation

Luks, Andrew M. Clinician's corner: travel to high altitude following solid organ transplantation. High Alt Med Biol. 17:147-156, 2016.-As they regain active lifestyles following successful organ transplantation, transplant recipients may travel to high altitude for a variety of activities, including skiing, climbing, and trekking. This review is intended to provide information for medical providers who may encounter transplant patients seeking advice before planned high altitude travel or care for medical issues that develop during the actual sojourn. There is currently limited information in the literature about outcomes during high-altitude travel following solid organ transplantation, but the available evidence suggests that the physiologic responses to hypobaric hypoxia are comparable to those seen in nontransplanted individuals and well-selected transplant recipients with no evidence of organ rejection can tolerate ascents as high as 6200 m. All transplant recipients planning high-altitude travel should undergo pretravel assessment and counseling with an emphasis on the recognition, prevention, and treatment of altitude illness, as well as the importance of preventing infection and limiting sun exposure. Transplant recipients can use the standard medications for altitude illness prophylaxis and treatment, but the choice and dose of medication should take into account the patient's preexisting medication regimen and current renal function. With careful attention to these and other details, the healthy transplant recipient can safely experience the rewards of traveling in the mountains.

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.

Evaluating the Risks of High Altitude Travel in Chronic Liver Disease Patients

Luks, Andrew M., and Erik R. Swenson. Clinician's Corner: Evaluating the risks of high altitude travel in chronic liver disease patients. High Alt Med Biol 16:80-88, 2015.--With improvements in the quality of health care, people with chronic medical conditions are experiencing better quality of life and increasingly participating in a wider array of activities, including travel to high altitude. Whenever people with chronic diseases travel to this environment, it is important to consider whether the physiologic responses to hypobaric hypoxia will interact with the underlying medical condition such that the risk of acute altitude illness is increased or the medical condition itself may worsen. This review considers these questions as they pertain to patients with chronic liver disease. While the limited available evidence suggests there is no evidence of liver injury or dysfunction in normal individuals traveling as high as 5000 m, there is reason to suspect that two groups of cirrhosis patients are at increased risk for problems, hepatopulmonary syndrome patients, who are at risk for severe hypoxemia following ascent, and portopulmonary hypertension patients who may be at risk for high altitude pulmonary edema and acute right ventricular dysfunction. While liver transplant patients may tolerate high altitude exposure without difficulty, no information is available regarding the risks of long-term residence at altitude with chronic liver disease. All travelers with cirrhosis require careful pre-travel evaluation to identify conditions that might predispose to problems at altitude and develop risk mitigation strategies for these issues. Patients also require detailed counseling about recognition, prevention, and treatment of acute altitude illness and may require different medication regimens to prevent or treat altitude illness than used in healthy individuals.

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.

[Characteristics of mepivacaine axillary brachial plexus block performed at 2800 m of altitude]

OBJECTIVE

We evaluated the feasibility and pharmacodynamic profile of axillary brachial plexus nerve blocks performed in high altitude.

STUDY DESIGN

A prospective study was performed in healthy volunteers.

PATIENTS AND METHODS

Ten male volunteers received 20 ml of 1.5% mepivacaine on radial and median nerves (total 40 ml), first at altitude of 150 then at 2877 m. Onset and offset times for sensory, motor and sympathetic blocks were recorded. Blood was sampled up to 60 min after injection to measure plasma concentrations of mepivacaine.

RESULTS

At 150 and 2877 m of altitude, onset times for blocks were similar. Duration of sensory and motor blocks was not different at low and high altitude (193+/-35 min and 180+/-47 min; and 237+/-32 min and 217+/-44 min, respectively). Plasma mepivacaine concentrations were significantly lower at 20 minutes in high altitude (p<0.05). At higher altitude, one patient showed clinical signs of neurological toxicity, with plasma concentrations of mepivacaine ranging from 0.94 to 1.21 mug/ml.

CONCLUSION

At altitude of 2877 m, axillary brachial plexus block with 1.5% mepivacaine is feasible, with onset and offset times for sensory and motor effects similar to those performed at 150 m.

Risk of Impaired Coagulation in Warfarin Patients Ascending to Altitude (>2400 m)

Approximately 476,000 people on warfarin therapy visit a resort at altitude (>2400 m) annually in Colorado. Clinicians practicing at altitude have expressed concern that ascent to altitude adversely affects coagulation in patients taking warfarin in both high altitude residents and visitors. We sought to determine the effect of ascent to and descent from altitude on coagulation in warfarin patients, as assessed by the international normalized ratio (INR). A retrospective medical chart review was conducted on all warfarin patients treated between August 1998 and October 2003 at a cardiology clinic in which travel to and from altitude was documented in association with each INR measurement in high altitude residents. Of the 1139 INR measurements in 49 patients, 143 were associated with changes in altitude (in 32 of 49 patients). The odds of an INR measurement being below the prescribed range were 2.7 times (95% CI: 1.2-5.8) higher among warfarin patients with recent ascent to altitude, 2.1 times (95% CI: 1.4-3.2) higher among warfarin patients with atrial fibrillation, and 5.6 (95% CI: 2.3-13.7) times higher among warfarin patients with both atrial fibrillation and recent ascent to altitude. Increasing altitude is a risk factor for subtherapeutic INR in warfarin patients and this risk is doubled in atrial fibrillation patients.