By P. Armon. Manhattanville College. 2018.
The clearance of the target drug can be the most significant arbiter of the severity of interaction for systemic interactions order 60mg alli with mastercard. Using the venous equilibrium Human Cytochromes P450 and Metabolism-Based Drug-Drug Interactions 59 Figure 3 Influence of clearance on systemic drug-drug interactions cheap 60 mg alli otc. In this case, a 75% reduction in enzyme activity would result in virtuallynochange(*6%) in blood clearance. For low- clearance drugs (assuming fm is 1), the reduction in clearance exactly reflects the reduction in enzyme activity. Although systemically low-clearance drugs would be expected to be the most sensitive to drug-drug interactions, such compounds frequently have high oral bioavailability. As such, a coadministered inhibitor will cause little alter- ation of the Cmax on a single oral dose but would need to be able to maintain inhibitory levels throughout the dosing interval. At steady state, a large inhibi- tory effect could be mediated, but the maximum initial ‘‘jump’’ in blood levels of the target drug would be twofold, with each subsequent dose adding at most another unit until the steady state was reached. Such a relatively gentle rate of elevation of blood levels might enable, in some circumstances, known tolerated adverse effects to be identified before serious toxicity is encountered. Blood-flow-limited drugs are not only theoretically systemic drug-interaction resistant but also rarely make good drugs (because of a low oral bioavailability and a high likelihood of a short half-life), and there are few drugs marketed, except prodrugs. However, on oral dosing, a putative inhibitor of the metabolism of such drugs need only be effective during the first-pass phase to cause a very significant effect. High levels of inhibitory blockade can be achieved because of the concentrations that can be achieved in the gut and the liver during absorption. Since the target drug has a low bioavailability, changes in blood Cmax can be quite sudden and of an order of magnitude or more. For comparison, the probit plots showing the incidence versus potency of these drugs and approximately 2000 typical pharmaceutical company compounds (ca. To some degree these results reflect the relative importance of the P450 enzymes in drug clearance (Fig. A more interesting comparison is that of marketed drugs and pharma- ceutical company compounds. Human Cytochromes P450 and Metabolism-Based Drug-Drug Interactions 61 Figure 4 Incidence of P450 inhibition. Probit plots generated from in vitro P450-inhibition data in the authors’ laboratories using heterologously expressed P450s in microsomal membranes. The plots represent data from approximately 400 marketed drugs and 2000 pharmaceutical company compounds synthesized in 1998. This observation is similar to what has been described in the context of permeability and absorption and is part of the basis of the ‘‘Lipinski rule of five’’ (40). The differences between marketed drugs and pharmaceutical company compounds are less marked for the other major P450 enzymes. There is no increase in the incidence of very potent inhibitors of these P450s in the contemporary company compounds compared with cur- rently marketed drugs. However, there are now many more ‘‘midrange’’ inhibitors and many less ‘‘clean’’ compounds than have been seen previously, primarily because of the general increase in lipophilicity. However, the interaction profile of the next clinically or commercially important drug will always be of the most immediate significance, even if it concerns an otherwise relatively insignificant P450 enzyme. In addition to the aromatic residues, there are several residues able to form hydrogen bonds with substrate molecules. The domination of the pÀp interactions is also evident in the inhibitor selectivity of the enzyme. The quinolone antibacterial enoxacin is an inhibitor that directly coordinates via the 4 -nitrogen0 atom on the piperazine function to the heme iron. In addition, there are aromatic regions and hydrogen bonding functions within the molecule that could be important in forming interactions with residues in the enzyme active site. Indeed, a comparison of a series of quinolone antibiotics has indicated that the keto group, the carboxylate group, and the core nitrogen at position 1 are able to form a similar pattern of hydrogen bonds with the active site, as has been suggested for caffeine. It is able to metabolize basic compounds such as imipramine, but is inhibited by acidic compounds such as enoxacin. In terms of induction by pharmaceutical agents, probably the most significant example is omeprazole. In vivo at higher omeprazole doses (40 and 120 mg for 7 days) there was a significant increase in caffeine metab- olism, as shown by urinary metabolic ratios, the caffeine breath test, and caffeine clearance (44). Inhibition Furafylline, a structural analogue of theophylline, was produced as a long-acting substitute for theophylline. Early clinical studies showed that the compound 64 Clarke and Jones produced marked inhibition of caffeine metabolism. Detailed mechanistic studies have indicated that metabolic processing of the C-8 methyl group is involved in the inactivation (48). This in vitro inhibition translated into a twofold decrease in caffeine clearance by pefloxacin and a sixfold decrease in clearance by enoxacin (50). Because pefloxacin undergoes N-demethylation to norfloxacin (51) and norfloxacin is much more potent as an inhibitor than pefloxacin (50), the observed in vivo interaction seen for pefloxacin may, in part, be due to norfloxacin. Many other quinolone antibacterial agents have been investigated for their interaction with theophylline, and ciprofloxacin has also been shown to have notable inhibitory effects (52).
Hence buy alli 60 mg mastercard, because M-channels are voltage- sensitive alli 60 mg generic, changes in voltage affect current through M-channels and changes in current through M-channels in turn affect voltage, in such a manner as to stabilise the membrane potential Ð a negative feedback effect. The bottom trace shows a synaptic current recorded under voltage clamp at a preset voltage of À60 mV from a ganglion cell on giving a single shock to the preganglionic fibres. The synaptic current is generated by acetylcholine released from the preganglionic fibres, which opens nicotinic cation channels in the ganglion cell membrane to produce an inward cation current. The top trace shows what happens when the voltage-clamp circuit is switched off, to allow the membrane potential to change. The inward synaptic current now generates a depolarisation (the synaptic potential), which in turn initiates an action potential. This is exactly what synaptic potentials should do, of course, but no Na current is seen under voltage clamp because the membrane potential is held below the threshold for Na channel opening. However, action potentials can still be recorded with extracellular electrodes, by placing the electrode near to the cell (Fig. In this case, the electrode tip picks up the local voltage-drop induced by current passing into or out of the cell. Note that (1) the signal is much smaller than the full (intracellularly recorded) action potential and (2) it is essentially a differential of the action potential (because it reflects the underlying current flow, not the voltage change). Nevertheless, since neural discharges are coded in terms of frequency and pattern of Figure 2. The interval between the stimulus and the postsynaptic response includes the conduction time along the unmyelinated axons of the preganglionic nerve trunk. If these are firing asynchronously, the signals may cancel out so that individual action potentials become lost in the noise. This problem becomes less when the cells are made to discharge synchronously, by (for example) electrical stimulation. This is made use of to record evoked potentials with surface electrodes Ð for example, to measure conduction velocities along peripheral nerve trunks. However, the signals are very small (not surprisingly) so have to be averaged by computer. These are used to assess function of sensory systems or in evaluating the progress of demyelinating diseases. However, as with extracellular recording in general, the strongest signal arises when activity of many neurons is synchronised. Hille, B (1994) Modulation of ion channel function by G protein-coupled receptors. The idea that there are specific receptors for hormones and drugs was developed by Erlich and Langley at the end of the nineteenth century, while Hill, Clark, Gaddum and Schild were pioneers in developing a quantitative understanding of the action of drugs. At that time, there was no evidence regarding the structural nature of receptors, although it was widely supposed they were proteins. The value of receptors to higher animals becomes most obvious in considering the functioning of the central nervous system. The integration of sensory input, past experience and inborn instinct by the central nervous system in the generation of appropriate behavioural activity is only possible because of the specialised properties and diversity of neurotransmitter receptors in the nervous system which mediate signalling between neurons. It has long been recognised that a detailed knowledge of the neurotransmitter receptors in the brain is crucial to developing specific therapeutic approaches to correcting unwanted nervous system activity. The aim of this chapter is to consider the structure, distribution and functional properties of neurotransmitter receptors in the brain in general and discuss the principles of how the action of drugs at these receptors can be studied. Each neurotransmitter acts on its own family of receptors and these receptors show a high degree of specificity for their transmitter. Diversity of neurotransmitter action is provided by the presence of multiple receptor subtypes for each neurotransmitter, all of which still remain specific to that neurotransmitter. This principle is illustrated by the simple observations outlined in Neurotransmitters, Drugs and Brain Function. These simple qualitative observations by Langley and others at the beginning of the twentieth century led to the development of more quantitative pharmacological methods that were subsequently used to identify and classify receptors. These methods were based on the use of both (1) agonist and (2) antagonist drugs: (1) If a series of related chemicals, say noradrenaline, adrenaline, methyladrenaline and isoprenaline, are studied on a range of test responses (e. On the other hand, if, as Ahlquist first found in the 1940s, these compounds give a distinct order of potency in some of the tests, but the reverse (or just a different) order in others, then there must be more than one type of receptor for these agonists. In fact, careful quantitative analysis of the order of activity of the agonists in each test, and of the precise potency of antagonists (see Chapter 5 for quantitative detail) has often successfully indicated, although rarely proved, the presence of subclasses of a receptor type (e. The affinity of receptors for selective antagonists determined using the Schild method was a mainstay of receptor classifica- tion throughout the second half of the twentieth century. Thus, a muscarinic receptor can be defined as a receptor with an affinity for atropine of around 1 nM and the M1 subtype of muscarinic receptor can be identified as having an affinity of around 10 nM for the selective antagonist, pirenzepine while muscarinic receptors in the heart (M2 subtype) are much less sensitive to pirenzepine block (K $ 10À7 M). B Classification of receptors according to agonist potency can be problematic because agonist potency depends partly on the density of receptors in the tissue and therefore use of selective antagonists has become a mainstay of receptor identification and classification. The development of radioligand binding techniques (see Chapter 5 for principles) provided for the first time a means to measure the density of receptors in a tissue in addition to providing a measure of the affinity of drugs for a receptor and allowed the relative proportion of different receptors in a tissue to be estimated. These approaches to receptor identification and classification were, of course, pioneered by studies with peripheral systems and isolated tissues.
Although pregnancy is considered contraindicated in women within 3 months of receiving the rubella vaccine purchase alli 60mg with mastercard, the actual risk of congenital rubella syndrome from maternal vaccination would appear to be extremely small cheap alli 60mg on line, if it exists at all (Preblud and Williams, 1985). Measles and mumps vaccines are also considered contraindicated during pregnancy, although pooled immune globulin (0. Obviously the benefits of rabies vaccination (considering the high mortality of rabies of nearly 100 percent) far outweigh any theoretical risk to the fetus, which is actually unknown. Although influenza vaccines are not routinely recommended for all pregnant women, they may be efficacious in cer- tain pregnant women with significant medical complications. The physician is concerned with whether a specific medication is safe for the fetus, remaining cognizant that most car- diac medications are chronically used to treat life-threatening conditions, and that these therapeutics cannot be discontinued when pregnancy is first diagnosed (Little and Gilstrap, 1989). Hence, embryos/fetuses of women with cardiovascular disease are exposed to these medications during the critical period of organogenesis (i. Since heart disease may be inherited in a multifactorial or polygenic fashion, pregnant women with many forms of heart disease may give birth to a newborn with congenital heart disease, and this mal- formation may in turn be blamed by both the patient and her attorney on specific car- diac medications. Scientific studies regarding the efficacy and safety of most cardiac medications during pregnancy are not conclusive, but the life-threatening nature of cardiovascular disease mandates that treatment be provided, even during pregnancy. The few investigations that are available indicate that dose and timing adjustment may be necessary because of (1) decreased drug serum concentrations (Cmax and steady state); (2) decreased half-life; and (3) increased clearance (Table 3. Cardiovascular medications may be classified into several categories: antiarrhythmic, cardiac glycosides, anticoagulants, diuretics, antihypertensives, and antianginals. This classification may prove useful in predicting both the efficacy and the toxicity of a specific agent (Brown and Wendel, 1989). Antiarrhythmics have been classified into six classes according to their major mode of action or effect (Vaughan Williams, 1984), as shown in Tables 3. Lidocaine Commonly used as an amide local anesthetic, lidocaine is also effective in the treatment of ventricular and supraventricular tachycardias. Lidocaine rapidly crosses the placenta and fetal levels reach about 50 percent of maternal levels within less than an hour 54 Cardiovascular drugs during pregnancy (Rotmensch et al. Lidocaine’s half-life is twice as long in the fetus/neonate (3 h) than in the mother (1. Importantly, most information available regarding pharmacokinetics of lidocaine in pregnant and postpartum women and newborns is from studies of regional or local anesthesia (Rotmensch et al. No published data are available on lidocaine from women who received the drug for cardiac arrhythmias. However, local anesthetics given in toxic doses may result in central nervous system and cardiac side effects in both the mother and the fetus. Lidocaine is not known to be ter- atogenic at acute therapeutic levels in humans or in chronic doses in animals (Fujinaga and Mazze, 1986; Heinonen et al. Toxicity risk is mini- mal when maternal lidocaine levels are maintained at less than 4 mg/mL (Bhagwat and Engel, 1995). Amide-type local anesthetics given for paracervical block are associated with spasm of the uterine arteries, causing decreased uterine blood flow. Procainamide Another amide compound, procainamide, is used to treat ventricular tachycardia. There is little information regarding the pharmacokinetics of this drug during pregnancy. However, it has been estimated that fetal levels are approximately one-fourth the mater- nal levels (Garite and Briggs, 1987). Scientific evidence of the safety of procainamide for use during pregnancy does not address possible human teratogenicity. However, given the safety profile of a closely related drug (lidocaine), procainamide seems to not pose a great risk when used during pregnancy (Little and Gilstrap, 1989). Chronic use of this drug should be avoided, unless necessary for life-threatening conditions, because a lupus-like syndrome may occur (Rotmensch et al. Breastfeeding is not contra- indicated in mothers on procainamide (American Academy of Pediatrics, 1994). Encainide and flecainide Two other lidocaine-related antiarrhythmic medications are encainide and flecainide. Encainide was not teratogenic in rats and rabbits when given at doses up to 9 and 13 times the human dose (data from the manufacturer’s insert). Flecainide has been reported to cause teratogenic and embryotoxic effects in some species of rabbits when given in doses four times the usual adult dose. It was not, however, teratogenic in rats, mice, and other species of rabbits when given in the usual adult dose, according to its manufacturer. Flecainide has been used to treat fetal arrhythmias, but fetal deaths have occurred with this treatment. Given the alternative related medications available, fle- cainide should be avoided, or at least the drug of last resort when others have failed. Tocainide Tocainide is another amide antiarrhythmic agent, closely related to lidocaine.
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