Polymorphic metabolism of the beta-adrenoreceptor blocking drugs and its clinical relevance

Advances in the Pharmacogenomics of Adverse Drug Reactions

Rapid developments in pharmacogenomics have been noticeable in recent years, and much of this knowledge has improved understanding of adverse drug reactions. This improved knowledge has largely been the result of improved sequencing technologies and falling costs in this area, as well as improved statistical techniques to analyse the data derived from studies. While the genetic reasons behind adverse drug reactions are becoming better understood, translation of this knowledge, particularly in terms of biomarkers that might be clinically applicable at the bedside, has been more difficult. Understanding of the technologies and their application is limited among practising clinicians. The cost of some of the technologies available may also be prohibitive in stretched healthcare economies. As education about the potential for applying pharmacogenomics improves and costs fall, understanding of adverse drug reactions and application of this knowledge in a clinical setting should improve.

Polymorphisms of genes CYP2D6, ADRB1 and GNAS1 in pharmacokinetics and systemic effects of ophthalmic timolol. A pilot study

OBJECTIVE

To test the hypotheses that (1) CYP2D6 genotype is associated with pharmacokinetics of ophthalmic timolol and (2) variation in genotypes of ADRB1 (beta(1)-adrenoceptor) and GNAS1 (alpha-subunit of G-protein) modulate heart rate (HR), and systolic (SAP) and diastolic (DAP) arterial pressure responses to timolol.

METHODS

Nineteen glaucoma patients and eighteen healthy volunteers were treated with 0.5% aqueous and 0.1% hydrogel formulations of ophthalmic timolol using a randomised cross-over design. The participants conducted head-up tilt and maximum exercise test at four visits. Plasma concentration of timolol was measured twice for glaucoma patients and ten times for healthy volunteers on each visit. Also, the genotypes for CYP2D6, ADRB1 and GNAS1 were determined.

RESULTS

Among healthy volunteers using aqueous timolol, poor metabolisers (PMs, n=2) of CYP2D6 had higher maximum plasma concentrations (C(max), values 2.63 and 2.94 ng/ml), longer elimination half-lives ( T(1/2), 5.49 and 6.75 h), and higher area-under-curve (AUC, 19.54 and 23.25 ng.h/ml) than intermediate [IMs, n=6, mean+/-SD 1.73+/-0.59 ng/ml (not significant), 3.30+/-0.48 h, 11.32+/-3.72 ng.h/ml], extensive (EMs, n=8, 1.60+/-0.72 ng/ml, 3.24+/-1.24 h, 8.52+/-6.12 ng.h/ml) and ultra-rapid (UMs, n=2, values 1.23 and 1.67 ng/ml, 2.22 and 2.52 h, 6.16 and 6.94 ng.h/ml) metabolisers. The IMs, EMs and UMs did not differ from each other for any of the kinetic variables. Also, the elevation of HR from rest to maximum level tended to differ between PMs and IMs, and between PMs and UMs. The pharmacokinetics and pharmacodynamics between the CYP2D6 groups did not differ with statistical significance when hydrogel timolol was used. Upon head-up tilt, the Ser49 homozygotes (n=26) had higher SAP (P=0.03) and DAP (P<0.01) than the Gly carriers (n=11). The change in DAP from rest to maximum during exercise was lower (P<0.01) in subjects with CC alleles of GNAS1 (n=13) than those with at least one T allele (n=24).

CONCLUSION

The CYP2D6 poor metabolisers may be more prone to systemic adverse events with aqueous timolol than extensive metabolisers. Since CYP2D6 genotyping is not routine clinical practice, using 0.1% timolol hydrogel instead of 0.5% aqueous preparation will increase patient safety.

Modulation of beta-adrenergic receptor subtype activities in perioperative medicine: mechanisms and sites of action

This review focuses on the mechanisms and sites of action underlying beta-adrenergic antagonism in perioperative medicine. A large body of knowledge has recently emerged from basic and clinical research concerning the mechanisms of the life-saving effects of beta-adrenergic antagonists (beta-AAs) in high-risk cardiac patients. This article re-emphasizes the mechanisms underlying beta-adrenergic antagonism and also illuminates novel rationales behind the use of perioperative beta-AAs from a biological point of view. Particularly, it delineates new concepts of beta-adrenergic signal transduction emerging from transgenic animal models. The role of the different characteristics of various beta-AAs is discussed, and evidence will be presented for the selection of one specific agent over another on the basis of individual drug profiles in defined clinical situations. The salutary effects of beta-AAs on the cardiovascular system will be described at the cellular and molecular levels. Beta-AAs exhibit many effects beyond a reduction in heart rate, which are less known by perioperative physicians but equally desirable in the perioperative care of high-risk cardiac patients. These include effects on core components of an anaesthetic regimen, such as analgesia, hypnosis, and memory function. Despite overwhelming evidence of benefit, beta-AAs are currently under-utilized in the perioperative period because of concerns of potential adverse effects and toxicity. The effects of acute administration of beta-AAs on cardiac function in the compromised patient and strategies to counteract potential adverse effects will be discussed in detail. This may help to overcome barriers to the initiation of perioperative treatment with beta-AAs in a larger number of high-risk cardiac patients undergoing surgery.

Soft drugs. 12. Design, synthesis, and evaluation of soft bufuralol analogues

In the search for more potent but still short-acting beta-blockers (BB), the methyl, ethyl, isopropyl, tert-butyl, cyclohexyl, 2-(1-adamantyl)ethyl, and methylthiomethyl esters of the acidic inactive metabolite of bufuralol were synthesized based on the "inactive metabolite" approach. The cleavage of the ester bond by blood and tissue esterases rapidly deactivates these compounds, resulting in an ultrashort duration of action. The beta-antagonist potencies and time courses of actions of the new "soft" BBs were characterized by recording ECG and intra-arterial blood pressure (BP) in rats. In the isoproterenol-induced tachycardia model, while bufuralol at an iv dose of 1 mg/kg (3.8 micromol/kg) diminished heart rate (HR) for at least 2 h, the effects of the soft drugs lasted for only 10-30 min at equimolar dose. The inactive metabolite did not decrease HR significantly. The first four members of this series of compounds showed the highest beta-blocking potencies, ranging between 25% and 50% of that of bufuralol. Next, the effects of these most active compounds on resting HR and BP were evaluated in comparison to esmolol. Infused for 10 min at a rate of 20 micromol/kg/min, esmolol decreased HR and mean arterial pressure (MAP) by 40% and 60%, respectively. The soft drugs at doses ranging only between 2 and 4 micromol/kg/min resulted in a 20-40% decrease in HR and a 30-50% reduction in MAP. However, the time courses of both the bradycardic and hypotensive effects of the soft drugs were superimposable to that of esmolol, diminishing within 60 min after the discontinuation of the infusions.

Location of the hydroxyl functions in hydroxylated metabolites of nebivolol in different animal species and human subjects as determined by on-line high-performance liquid chromatography-diode-array detection

Nebivolol hydrochloride (R067555), is a new antihypertensive drug. Aromatic and alicyclic hydroxylation at the benzopyran ring systems of nebivolol are important metabolic pathways. Generally, NMR is used to unambiguously assign the sites of hydroxylation. Because of the low dose rates and the extensive metabolism of nebivolol in the different species, NMR identification is not always possible, and therefore another spectroscopic technique was searched for to address this problem. UV-chromophore absorption is affected by the kind and arrangement of adjacent atoms and groups (auxochromes). The effect of these auxochromes (e.g. -NH2, -NR2, -SH, -OH, -OR and halogens) can be strongly influenced by the pH. This paper proves that HPLC at high pH combined with on-line diode-array detection is an excellent technique for the location of the hydroxyl functions in hydroxylated metabolites of nebivolol. With this technique it is possible to differentiate between glucuronidation at the automatic and aliphatic or alicyclic hydroxyl functions.

Determination of debrisoquine and metabolites in human urine by gas chromatography-mass spectrometry

A gas chromatographic-mass spectrometric analysis has been developed for the determination of debrisoquine and its metabolites in the urine of healthy individuals (controls) and patients with chronic renal failure. The sensitive and specific assay comprises selected-ion monitoring of the drug and the metabolites 4-hydroxydebrisoquine and 8-hydroxydebrisoquine using guanoxan as the internal standard. The limit of detection is ca. 0.2 microgram/ml. The clinical study shows that the healthy individuals and patients with chronic renal failure can be divided in two groups of extensive metabolizers and poor metabolizers, respectively. The extensive metabolizers excreted large amounts of 4-hydroxydebrisoquine and minor amounts of 8-hydroxydebrisoquine. The poor metabolizers excreted small amounts of 4-hydroxy metabolite, and no 8-hydroxydebrisoquine was detected in the urine.

The genetic polymorphism of debrisoquine/sparteine metabolism--clinical aspects

It has been established that the metabolism of more than twenty drugs, including antiarrhythmics, beta-adrenoceptor antagonists, antidepressants, opiates and neuroleptics is catalyzed by cytochrome P-450dbl. The activity of this P-450 isozyme is under genetic rather than environmental control. This article discusses the therapeutic implications for each of the classes of drugs affected by this genetic polymorphism in drug metabolism. Not only are the problems associated with poor metabolizers who are unable to metabolize the compounds discussed, but it is also emphasized that it is difficult to attain therapeutic plasma concentrations for some drugs in high activity extensive metabolizers.

[Central effects of five beta-adrenergic receptor blockers in healthy volunteers: a quantitative EEG study]

The effects of five beta blockers on the central nervous system of healthy subjects was studied by computerized EEG analysis. All subjects underwent continuous recording with a Holter magnetic type recorder during the experimental period. For 10 consecutive days, five groups of subjects received alternately placebo and the beta blockers acebutolol 600 mg, carteolol 20 mg, metoprolol 200 mg, pindolol 30 mg and sotalol 320 mg. EEG recordings (C4/P4, P4/02 and C3/P3, P3/01) lasting 5 min were made between 8.30 and 9.30 a.m. Subjects were at rest with eyes closed and there was no vigilance control. The signal was recorded on a magnetic tape recorder and then processed by Nicolet MED 80 system. Comparisons of absolute and relative powers and of average frequencies were then made between the different sequences and groups. The possible correlations between the changes observed in the power spectrum and the clinical, pharmacological and pharmacokinetic specific properties of each beta blocker are discussed.

Analysis of alpha-hydroxy metabolites of metoprolol in human urine after phosgene/trimethylsilyl derivatization

Three metoprolol metabolites containing an alpha-hydroxy group were identified in human urine by capillary column gas chromatography/mass spectrometry. After aqueous phase cyclization with phosgene the neutral or acidic derivatives formed were isolated by solvent extraction at pH 10 or 3, respectively. Following silylation the electron impact mass spectra of the metabolites exhibited a characteristic ion at m/z 336 of high abundance which originated from cleavage of the bond adjacent to the alpha-OTMS group. Most probably the identified compounds were formed by further biotransformations of alpha-hydroxy metoprolol, which is a primary metabolite. The analytical method is applicable to detect the metoprolol metabolites reported so far. A quantitative assay for one of the metabolites (H 119/72) with nitrogen selective detection is described. The total amount of this metabolite excreted by one subject within 24 h after dosing was about 0.25% of the given dose.