Lack of effect of smoking on the metabolism and pharmacokinetics of quinidine in patients

Biologic and molecular mechanisms for sex differences in pharmacokinetics, pharmacodynamics, and pharmacogenetics: Part I

There are pharmacological differences between women and men that have important clinical consequences. For several drugs, there is a higher incidence in women of drug-induced QT prolongation and a potentially fatal arrhythmia, torsades de pointes. This may be a reflection of the longer baseline QT interval in women. A difference in cardiovascular disease between women and men is that women have a higher mortality rate after myocardial infarction (MI). Women also have a higher rate of hemorrhagic stroke after receiving thrombolytic therapy for an MI. Differences in effectiveness of analgesics have been demonstrated, with kappa opioids providing pain relief for women but not men. Drugs may have different pharmacokinetics in women and men because of differences in phase I and phase II enzymes that metabolize drugs. Conflicting results about biological sex differences have been reported for the major drug metabolizing enzyme, cytochrome P450 3A4 (3A4) and may be related to a role for P-glycoprotein, a cell membrane transporter, reported as two times higher in male livers than those of females. It has been reported that boys need a higher dose of 6-mercaptopurine, which is metabolized by thiopurine methyltransferase (TPMT). TPMT is reported to be 14% higher in male human liver biopsies than those from females. Verapamil, a drug for angina and hypertension, has different clearance and side effects in men and women. Ethnic/racial variations have also been demonstrated with the drug metabolizing enzymes, CYP2C9, 2C19, and 2D6.

Biologic and molecular mechanisms for sex differences in pharmacokinetics, pharmacodynamics, and pharmacogenetics: Part II

There are specific pharmacology issues related to women's unique physiology, including the hormonal changes that occur throughout their life span. Studies have shown alterations in drug metabolism in relation to phase of menstrual cycle, during pregnancy, or after menopause. In the brain, hormones can alter the response to drugs through various mechanisms. Estrogen and other compounds can bind to the estrogen receptor and modulate a wide range of activities within the cell. In addition, animal studies have demonstrated sexual dimorphism in the brain in terms of both the type of response to estrogen and the response as related to timing of administration. Many normal physiological changes that occur during pregnancy can affect pharmacokinetics and pharmacodynamics. These changes during pregnancy are dramatic rises in levels of estrogen and progesterone, increases in maternal blood volume, altered protein binding resulting from a drop in albumin levels, and a rise in levels of other plasma proteins. The field of chronobiology offers a way to study these changes in biological functions. Chronopharmacology is the study of how biological rhythms, particularly 24-hour, menstrual cycle, and annual rhythms, impact the pharmacokinetics and pharmacodynamics of drugs as a function of their timing. Chronopharmacokinetics is the study of the absorption, distribution, metabolism, and elimination of medicines according to the time of day, menstrual cycle, or year. In addition to applying chronobiology to the study of drugs used in women, new technologies were addressed from computer modeling, pharmacogenetics (genetics of the response to drugs), and in vivo drug metabolism studies.

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.

Metabolism of quinine in man: identification of a major metabolite, and effects of smoking and rifampicin pretreatment

Our previous studies have shown that cigarette smoking and rifampicin pretreatment enhance the elimination of quinine, metabolism assumed, by analogy with quinidine, to be carried out by CYP3A (P450IIIA). This finding is unexpected since it has been shown that smoking induces the CYP1A rather than the CYP3A enzyme family, suggesting that the metabolism of quinine may be catalysed by CYP1A. Therefore, we conducted this study to identify possible quinine metabolites in human urine and to determine which metabolic pathway is induced by cigarette smoking and rifampicin pretreatment. A specific HPLC procedure was employed to identify metabolites of quinine in urine samples collected from healthy volunteers after an oral dose of 600 mg quinine sulphate. The results showed that there were at least seven possible metabolites of quinine detected in human urine. Three of these were identified as 2'-oxoquininone, quinine glucuronide and 3-hydroxyquinine. The 3-hydroxyquinine appeared to be a major metabolite of quinine in urine samples from every subject who took an oral dose of quinine. Although cigarette smoking and rifampicin pretreatment were shown to cause a marked increase in the elimination of quinine there were no significant changes in the formation of 3-hydroxyquinine as measured in the urine samples. This suggests that the effects of smoking and rifampicin are more complicated than we expected and require further investigation.

Cigarette smoking enhances the elimination of quinine

The pharmacokinetics of a single dose (600 mg) of quinine sulphate were examined in a group of non-smokers (n = 10) and in heavy cigarette smokers (n = 10). The mean (+/- s.d.) oral clearance of quinine in smokers (0.189 +/- 0.075 1 h(-1) kg(-1)) was significantly greater than in non-smokers (0.107 +/- 0.045 1 h(-1) kg(-1) , P < 0.01). The unbound clearance of quinine which reflects activity of the drug-metabolizing enzyme, was considerably greater (1.5-fold) in the smokers than in the non-smoker subjects. The mean elimination half-life of quinine in smokers was 7.5 +/- 1.4 (s.d.) h, significantly shorter (P < 0.005) than the mean value in non-smokers (12.0 +/- 3.1 h). These results suggest that cigarette smoking enhances the elimination of quinine. The clinical significance of these findings is unknown but they indicate the need for caution in the administration of quinine to patients who are heavy cigarette smokers.

Pharmacokinetics of quinidine in male patients. A population analysis

Quinidine pharmacokinetic behaviour was evaluated in 139 adult hospitalised men receiving oral quinidine therapy. A total of 391 serum quinidine concentrations were measured by enzyme immunoassay for routine clinical purposes. The NONMEM programme was used to examine the relationship between quinidine pharmacokinetics and several potential covariates. A 1-compartment open model with first-order absorption and elimination was assumed. The mean apparent volume of distribution (Vd) was about 230L. When measured, alpha 1-acid glycoprotein (AAG) levels were not included in the analysis. Oral quinidine clearance (CL) decreased with age, severe congestive heart failure and renal disease, and increased in patients with a history of alcohol abuse. The interpatient variability in CL and the intrapatient residual variability expressed as coefficients of variation (CV) were 28 and 31%, respectively. When AAG values were incorporated into the analysis, the only important covariates of CL were the AAG measurements and the presence of renal dysfunction as indicated by a calculated creatinine clearance of less than 50 ml/min (3 L/h). The interpatient variability in CL and the residual intrapatient CVs decreased to approximately 24 and 26%, respectively. Improvement of the CL model by inclusion of measured AAG strongly suggests that quinidine elimination is dependent on the free concentration of drug in plasma and supports the use of free serum quinidine concentrations when evaluating and monitoring quinidine therapy.

The use of single sample clearance estimates to probe hepatic drug metabolism: handprinting the influence of cigarette smoking on human hepatic drug metabolism

1. Conditions were examined under which estimates of drug clearance made from a single measurement of plasma concentration effectively represented multiple-sample estimates of clearance for quinidine, valproic acid, unbound valproic acid, and lorazepam. When plasma concentrations were measured at various post-dose times, both individual and mean values of single-sample clearance estimates, CL, corresponded closely to multiple-sample clearance estimates. Best post-dose sampling times were: quinidine, 8 h; valproic acid, 24 h; and lorazepam, 24 h. 2. Single-sample clearance estimates, CL, were calculated for seven drugs employed as probes of human hepatic drug-metabolizing enzymes. Valproic acid was used to probe microsomal and peroxisomal beta-oxidase activity; antipyrine, phenytoin, quinidine, carbamazepine, and theophylline were used as probes of hepatic mixed-function oxidases (MFO), and lorazepam as a probe for UDP-glucuronosyltransferase activity. 3. A clearance index (CI, namely probe CL for smokers divided by probe CL for non-smokers) was calculated for each probe. The effect of cigarette smoking (and presumably polycyclic aromatic hydrocarbon exposure) on all probe CL values was consolidated and plotted as the logarithm of the CI to produce a handprint of drug metabolizing enzyme activity for cigarette smokers. 4. Only theophylline CL was significantly faster among smokers than non-smokers (P less than 0.01). 5. We conclude that the use of multiple probes of MFO activity when given in a single-dose, single-sample protocol for structuring handprints represents a minimally invasive and useful approach to characterize xenobiotic-mediated effects on hepatic MFO.

Lack of effect of treatment with human recombinant-tumour necrosis factor (HrTNF) on the binding of quinidine to alpha 1-acid glycoprotein (AGP)

Tumour necrosis factor (TNF) is known to be a key mediator in the acute phase response and its administration has been shown to cause a five fold increase in serum alpha 1-acid glycoprotein (AGP) concentration in the rat. Since, in man, plasma AGP level determines the protein binding of many important drugs (e.g. narcotic analgesics, phenothiazines, antiarrhythmics, calcium channel blockers) likely to be given to patients who will be treated with TNF, it is important to determine if TNF treatment of humans causes a similar increase in AGP concentration and drug binding. Therefore, the plasma protein binding of quinidine and the serum level of AGP were studied over a 4 day period in each of five cancer patients who were treated with human recombinant-tumour necrosis factor (HrTNF) using a dosage schedule of 6-8 x 10(+5) units/m2 daily for 5 days. It was observed that the quinidine binding ratio (the quotient of bound and free concentration in plasma) was highly correlated with the plasma concentration of AGP (r = 0.818) and that the mean pretreatment AGP concentration in the patients was about three times that found in normal subjects. However, no effects of the TNF treatment regime used in the present study could be demonstrated on either plasma AGP concentration or quinidine free fraction. These observations allow the tentative conclusion that HrTNF does not cause a significant increase in serum AGP level in cancer patients whose baseline AGP concentration is high. However, further study of the relationship between TNF treatment and serum AGP level is needed.