The pharmacokinetics of quinine in patients with hepatitis

Interspecies allometric scaling of antimalarial drugs and potential application to pediatric dosing

Pharmacopeial recommendations for administration of antimalarial drugs are the same weight-based (mg/kg of body weight) doses for children and adults. However, linear calculations are known to underestimate pediatric doses; therefore, interspecies allometric scaling data may have a role in predicting doses in children. We investigated the allometric scaling relationships of antimalarial drugs using data from pharmacokinetic studies in mammalian species. Simple allometry (Y = a × W(b)) was utilized and compared to maximum life span potential (MLP) correction. All drugs showed a strong correlation with clearance (CL) in healthy controls. Insufficient data from malaria-infected species other than humans were available for allometric scaling. The allometric exponents (b) for CL of artesunate, dihydroartemisinin (from intravenous artesunate), artemether, artemisinin, clindamycin, piperaquine, mefloquine, and quinine were 0.71, 0.85, 0.66, 0.83, 0.62, 0.96, 0.52, and 0.40, respectively. Clearance was significantly lower in malaria infection than in healthy (adult) humans for quinine (0.07 versus 0.17 liter/h/kg; P = 0.0002) and dihydroartemisinin (0.81 versus 1.11 liters/h/kg; P = 0.04; power = 0.6). Interpolation of simple allometry provided better estimates of CL for children than MLP correction, which generally underestimated CL values. Pediatric dose calculations based on simple allometric exponents were 10 to 70% higher than pharmacopeial (mg/kg) recommendations. Interpolation of interspecies allometric scaling could provide better estimates than linear scaling of adult to pediatric doses of antimalarial drugs; however, the use of a fixed exponent for CL was not supported in the present study. The variability in allometric exponents for antimalarial drugs also has implications for scaling of fixed-dose combinations.

Cardiac complications of unwitting co-injection of quinine/quinidine with heroin in an intravenous drug user

Adulterants "cut into" street heroin are common and often not detected by standard urine toxicology screening; however, their unwitting co-injection may have clinical consequences. We report a case of accelerated atrioventricular junctional arrhythmia that we determined to have been caused by quinine/quinidine cut into heroin. While identification and discontinuation of the offending agent helps confirm the diagnosis and is the treatment of choice, this is often complicated by the individual's dependence on the street drug in which the adulterant is mixed. This case highlights the need for clinicians to be aware of common adulterants, to know how to test for them, and to consider them as possible causes of medical complications in individuals who use drugs.

Irreversible CYP3A inhibition accompanied by plasma protein-binding displacement: a comparative analysis in subjects with normal and impaired liver function

In this study, quinine was used as a probe substrate and erythromycin as a prototypical irreversible inhibitor of CYP3A to ascertain whether, like reversible CYP inhibition, the magnitude of irreversible inhibition is also strictly dependent on the status of liver function. The effect of erythromycin on oral quinine disposition was studied in 10 healthy subjects and in 20 patients with cirrhosis of the liver who had varying degrees of liver dysfunction. This effect was shown to be the result of two types of interaction: (i) irreversible inhibition of CYP3A-mediated quinine metabolism, the extent of which proved to be independent of liver function, and (ii) displacement of quinine from plasma protein-binding sites, the magnitude of the displacement increasing dramatically as liver function worsened. Such an interaction causes limited increases in the total concentration of the displaced drug but disproportionate increases in its free concentration; the latter increases are magnified by liver dysfunction, thereby requiring that the monitoring of free drug concentrations be made mandatory.

Pharmacokinetic-pharmacodynamic modeling in the data analysis and interpretation of drug-induced QT/QTc prolongation

In this review, factors affecting the QT interval and the methods that are currently in use in the analysis of drug effects on the QT interval duration are overviewed with the emphasis on (population) pharmacokinetic-pharmacodynamic (PK-PD) modeling. Among which the heart rate (HR) and the circadian rhythm are most important since they may interfere with the drug effect and need to be taken into account in the data analysis. The HR effect or the RR interval (the distance between 2 consecutive R peaks) effect is commonly eliminated before any further analysis, and many formulae have been suggested to correct QT intervals for changes in RR intervals. The most often used are Bazett and Fridericia formulae introduced in 1920. They are both based on the power function and differ in the exponent parameter. However, both assume the same exponent for different individuals. More recent findings do not confirm this assumption, and individualized correction is necessary to avoid under- or overcorrection that may lead to artificial observations of drug-induced QT interval prolongation. Despite the fact that circadian rhythm in QT and QTc intervals is a well-documented phenomenon, it is usually overlooked when drug effects are evaluated. This may result in a false-positive outcome of the analysis as the QTc peak due to the circadian rhythm may coincide with the peak of the drug plasma concentration. In view of these effects interfering with a potential drug effect on the QTc interval and having in mind low precision of QT interval measurements, a preferable way to evaluate the drug effect is to apply a population PK-PD modeling. In the literature, however, there are only a few publications in which population PK-PD modeling is applied to QT interval prolongation data, and they all refer to antiarrhythmic agents. In this review, after the most important sources of variability are outlined, a comprehensive population PK-PD model is presented that incorporates an individualized QT interval correction, a circadian rhythm in the individually corrected QT intervals, and a drug effect. The model application is illustrated using real data obtained with 2 compounds differing in their QT interval prolongation potential. The usefulness of combining data of several studies is stressed. Finally, the standard approach based on the raw observations and formal statistics, as described in the Preliminary Concept paper of the International Conference on Harmonization, is briefly compared with the method based on population PK-PD modeling, and the advantages of the latter are outlined.

Jaundice in malaria

Jaundice is not an unusual accompaniment of malaria. It can occur due to intravascular hemolysis, disseminated intravascular coagulation, and, rarely, 'malarial hepatitis'. Although the primary schizogony of the malarial parasite always leads to the rupture of the infected hepatocyte, alteration of the hepatic functions is uncommonly recorded due to this event. Histologically, the hepatitis or the actual inflammation in the liver has never been demonstrated. Nonetheless, the term 'malarial hepatitis' (MH) has been used in the literature to describe the occurrence of hepatocellular jaundice in patients with Plasmodium falciparum infection. The authors' own data and review of the literature indicate that it is not an uncommon entity. In endemic areas, jaundice is seen in approximately 2.5% of patients with falciparum malaria. It also appears to be a heterogeneous syndrome and one can recognize two clinical subsets. In one group there was an acute, virulent presentation with coma, renal failure and in some cases even hemorrhagic manifestations. It is only in this setting that jaundice signified a 'severe' disease as noted by the World Health Organization action program. This presentation is often confused with acute viral hepatitis and acute hepatic failure in non-endemic areas, but can be clinically differentiated.

Stereoselectivity in the pharmacodynamics and pharmacokinetics of the chiral antimalarial drugs

Several of the antimalarial drugs are chiral and administered as the racemate. These drugs include chloroquine, hydroxychloroquine, quinacrine, primaquine, mefloquine, halofantrine, lumefantrine and tafenoquine. Quinine and quinidine are also stereoisomers, although they are given separately rather than in combination. From the perspective of antimalarial activity, most of these agents demonstrate little stereoselectivity in their effects in vitro. Mefloquine, on the other hand, displays in vitro stereoselectivity against some strains of P. falciparum, with a eudismic ratio of almost 2 : 1 in favour of the (+)-enantiomer. Additionally, for some of these agents (e.g. halofantrine, primaquine, chloroquine), stereoselectivity has been noted in the ability of the enantiomers to cause certain adverse effects. In recent years, stereospecific analytical methods capable of measuring the individual enantiomers after the administration of racemic drugs have been reported for a number of chiral antimalarial drugs. These assays have revealed that almost all the studied antimalarial drugs display stereoselectivity in their pharmacokinetics, leading to enantioselectivity in their plasma concentrations. Whereas the oral absorption of these agents appears to be non-stereoselective, stereoselectivity is often seen in their volume of distribution and/or clearance. With regard to distribution, plasma protein binding of some chiral antimalarial drugs exhibits a significant degree of stereoselectivity, leading to stereoselective distribution to blood cells and other tissues. Because of their low hepatic extraction ratios, stereoselective plasma protein binding also contributes to the stereoselectivity in the metabolism of these drugs. Chiral metabolites are formed from some parent antimalarial drugs, although stereoselective aspects of the pharmacokinetics of the metabolites are not well understood. It is concluded that knowledge of the stereoselective aspects of these agents may be helpful in better understanding their mechanisms of action and possibly optimising their clinical safety and/or effectiveness.

Quinine distribution in mice withplasmodium berghei malaria

The disposition of a single 80 mg/kg injection of quinine base was compared in control and Plasmodium berghei-infected mice. Pharmacokinetic parameters were determined on repeated whole blood samples from caudal vein (experiment 1) and quinine distribution was evaluated in tissues and blood fractions from mice sacrificed two hours post dosing (experiment 2). Quinine concentrations were assessed by high performance liquid chromatography with fluorometric detection. Whole blood concentrations and AUC(0 - infinity) of quinine increased in a parasitaemia-dependent manner. Quinine blood clearance and peak blood concentrations of metabolites negatively correlated with the parasitaemia. The apparent distribution volume of quinine only decreased in severely ill mice. Quinine concentrations rise in a parasitaemia-dependent manner in homogenates of spleen, lungs and kidney and in erythrocyte pellets. The negative relationship, observed between the parasitaemia and the tissue-to-whole blood ratio for muscle, heart, liver and brain, contributes to the reduction of the blood distribution volume. Quinine uptake by muscle and heart was dependent on the free fraction of plasma quinine. The liver and brain concentrations of quinine were similar in control and infected mice. The tissue-to-plasma free fraction ratios decrease when the parasitaemia rises suggesting a restrictive uptake of quinine by these tissues. In conclusion. P. berghei malaria decreases both total clearance and apparent volume of distribution with a heterogeneous redistribution of quinine between the tissues.

Pharmacokinetic interactions of antimalarial agents

Combination of antimalarial agents has been introduced as a response to widespread drug resistance. The higher number of mutations required to express complete resistance against combinations may retard the further development of resistance. Combination of drugs, especially with the artemisinin drugs, may also offer complete and rapid eradication of the parasite load in symptomatic patients and thus reduce the chance of survival of resistant strains. The advantages of combination therapy should be balanced against the increased chance of drug interactions. During the last decade, much of the pharmacokinetics and metabolic pathways of antimalarial drugs have been elucidated, including the role of the cytochrome P450 (CYP) enzyme complex. Change in protein binding is not a significant cause of interactions between antimalarial agents. CYP3A4 and CYP2C19 are frequently involved in the metabolism of antimalarial agents. Quinidine is a potent inhibitor of CYP2D6, but it appears that this enzyme does not mediate the metabolism of any other antimalarial agent. The new combinations proguanil-atovaquone and chlorproguanil-dapsone do not show significant interactions. CYP2B6 and CYP3A4 are involved in the metabolism of artemisinin and derivatives, but further studies may reveal involvement of more enzymes. Artemisinin may induce CYP2C19. Several artemisinin drugs suffer from auto-induction of the first-pass effect, resulting in a decline of bioavailability after repeated doses. The mechanism of this effect is not yet clear, but induction by other agents cannot be excluded. The combination of artemisinin drugs with mefloquine and the fixed combination artemether-lumefantrine have been studied widely, and no significant drug interactions have been found. The artemisinin drugs will be used at an increasing rate, particularly in combination with other agents. Although clinical studies have so far not shown any significant interactions, drug interactions should be given appropriate attention when other combinations are used.

Familial and acquired long qt syndrome and the cardiac rapid delayed rectifier potassium current

1. Long QT syndrome (LQTS) is a cardiac disorder characterized by syncope, seizures and sudden death; it can be congenital, idiopathic, or iatrogenic. 2. Long QT syndrome is so-named because of the connection observed between the distinctive polymorphic ventricular tachycardia torsade de pointes and prolongation of the QT interval of the electrocardiogram, reflecting abnormally slowed ventricular action potential (AP) repolarization. Acquired LQTS has many similar clinical features to congenital LQTS, but typically affects older individuals and is often associated with specific pharmacological agents. 3. A growing number of drugs associated with QT prolongation and its concomitant risks of arrhythmia and sudden death have been shown to block the 'rapid' cardiac delayed rectifier potassium current (IKr) or cloned channels encoded by the human ether-a-go-go-related gene (HERG; the gene believed to encode native IKr). Because IKr plays an important role in ventricular AP repolarization, its inhibition would be expected to result in prolongation of both the AP and QT interval of the electrocardiogram. 4. The drugs that produce acquired LQTS are structurally heterogeneous, including anti-arrhythmics, such as quinidine, non-sedating antihistamines, such as terfenadine, and psychiatric drugs, such as haloperidol. In addition to heterogeneity in their structure, the electrophysiological characteristics of HERG/IKr inhibition differ between agents. 5. Here, clinical observations are associated with cellular data to correlate acquired LQTS with the IKr/HERG potassium (K+) channel. One strategy for developing improved compounds in those drug classes that are currently associated with LQTS could be to design drug structures that preserve clinical efficacy but are modified to avoid pharmacological interactions with IKr. Until such time, awareness of the QT-prolongation risk of particular agents is important for the clinician.

Should people with nocturnal leg cramps drink tonic water and bitter lemon?

A literature search from 1993 to 1997 using MEDLINE and key-words beverages, muscle cramp, and quinine was performed. Three beverages containing quinine were examined in grocery stores. Analysis indicate that leg cramps are a common phenomenon associated with many comorbid disorders especially peripheral vascular and neurologic disorders. Thus, evaluation of a patient's complaining of leg cramps appropriately includes vascular, neurologic, and musculoskeletal examinations. Laboratory investigation of the symptom of leg cramps warrants as a minimum, assessment of thyroid function and determination of platelet counts and serum levels of electrolytes, calcium, and magnesium. A few small studies suggest that quinine is effective in decreasing the frequency of nocturnal leg cramps but not their severity or duration. Quinine consumed in commercial beverages has been reported to cause potentially fatal immunologically mediated hypersensitivity reactions. The concentration of quinine in commercial beverages varies greatly. Although commercial beverages containing quinine generally are labeled "Contains quinine," they typically lack both nutritional information about the amount of quinine and warnings of the health risks. It appears that 325 milligrams of quinine taken by mouth at bedtime typically relieves nocturnal leg cramps, but lower starting doses are appropriate for senior citizens and individuals with impaired renal function. In general, quinine in any form should be avoided by pregnant women and people with hepatic failure. Quinine consumed for the treatment of leg cramps should be prescribed and monitored by physicians, and people who consume quinine in commercial beverages must be warned of the health risks.

Pharmacokinetics of quinine, chloroquine and amodiaquine. Clinical implications

Malaria is associated with a reduction in the systemic clearance and apparent volume of distribution of the cinchona alkaloids; this reduction is proportional to the disease severity. There is increased plasma protein binding, predominantly to alpha 1-acid glycoprotein, and elimination half-lives (in healthy adults quinine t1/2z = 11 hours, quinidine t1/2z = 8 hours) are prolonged by 50%. Systemic clearance is predominantly by hepatic biotransformation to more polar metabolites (quinine 80%, quinidine 65%) and the remaining drug is eliminated unchanged by the kidney. Quinine is well absorbed by mouth or following intramuscular injection even in severe cases of malaria (estimated bioavailability more than 85%). Quinine and chloroquine may cause potentially lethal hypotension if given by intravenous injection. Chloroquine is extensively distributed with an enormous total apparent volume of distribution (Vd) more than 100 L/kg, and a terminal elimination half-life of 1 to 2 months. As a consequence, distribution rather than elimination processes determine the blood concentration profile of chloroquine in patients with acute malaria. Parenteral chloroquine should be given either by continuous intravenous infusion, or by frequent intramuscular or subcutaneous injections of relatively small doses. Oral bioavailability exceeds 75%. Amodiaquine is a pro-drug for the active antimalarial metabolite desethylamodiaquine. Its pharmacokinetic properties are similar to these of chloroquine although the Vd is smaller (17 to 34 L/kg) and the terminal elimination half-life is 1 to 3 weeks.

Pharmacokinetics of quinine in patients with chronic renal failure

METHODS. We investigated the pharmacokinetics of quinine (Qn) following administration of a single oral dose of 600 mg Qn sulphate in six male Thai patients with a moderate degree of chronic renal failure (CRF), and six male Thai subjects with normal renal function. RESULTS. The drug was well tolerated in both groups of subjects; no major adverse reactions were observed. A marked alteration in the pharmacokinetics of Qn was found in patients with CRF compared to healthy subjects; there were six significant changes in the pharmacokinetic parameters. Absorption was delayed, but increased in CRF (tmax 4.5 vs 1.6 h, Cmax 6.17 vs 3.45 micrograms.ml-1). Total clearance was significantly reduced (0.94 vs 2.84 ml.min-1.kg-1, whereas Vz/f remained unchanged (1.82 vs 2.78 l. kg-1). This resulted in the increased values of AUC and prolongation of the t1/2z and MRT in the patients (AUC 181.5 vs 61.8 micrograms.min-1.ml-1, t1/2z 26 vs 9.7 h, MRT 36.4 vs 11.3 h). Median concentrations of plasma unbound fraction of Qn collected at 4 h after drug administration in patients and healthy subjects were 7.3 vs 9.8%, respectively.

Pharmacokinetics of quinine in chronic liver disease

The pharmacokinetics of quinine were investigated in a) six healthy male Thai subjects, and b) nine male Thai patients with a moderate degree of chronic liver disease, after a single oral dose of 600 mg quinine sulphate. tmax and t1/2.2 were significantly prolonged in patients (median [range] tmax 2 [1-5] vs 1.6 [0.8-2] h; t1/2,z 23.4 [17.4-41.7] vs 9.7 [7.8-17.2] h), and Vz/F was significantly larger (median [range] 4.21 [2.33-15.87] vs 2.78 [1.49-3.38] 1 kg-1). Median (range) concentration of the plasma unbound Qn fraction collected from the patients at 4 h after drug administration was 17 (8.4-17.8)% of total drug concentration.