The influence of alpha 1-acid glycoprotein on quinine and quinidine disposition in the rat isolated perfused liver preparation

Effects of ritonavir-boosted lopinavir on the pharmacokinetics of quinine

The centuries-old antimalarial drug, quinine, continues to play a critical role in the treatment of severe falciparum malaria and uncomplicated malaria in pregnant women. It shares cytochrome P450 (CYP )-mediated metabolic pathways with several commonly used antiretroviral drugs, raising the potential for clinically important drug–drug interactions. A phase I pharmacokinetic study was conducted to assess the impact of long-term use of ritonavir-boosted lopinavir (LPV/r) on quinine pharmacokinetics in healthy volunteers. LP V/r significantly decreased the exposure of quinine and its major active metabolite, 3-hydroxyquinine, in both total and free (unbound) forms. These findings highlight the complex nature of the influence exerted by LPV/r on several of the drug-metabolizing enzymes involved in quinine disposition,including CYP 3A4, UDP-glucuronosyltransferase (UG T), and P-glycoprotein (P-gp). A decline in quinine exposure may compromise clinical efficacy. Further studies are warranted to assess changes in quinine pharmacokinetics and treatment outcomes in patients with acute malaria receiving antiretroviral therapy that includes LPV/r.

Prediction of human pharmacokinetics – evaluation of methods for prediction of volume of distribution

The aim was to evaluate and review methods for prediction of the steady-state volume of distribution (VD,ss) of xenobiotics in man. For allometry, ˜30–40% of predictions are classified as incorrect, humans and animals belong to different VD,ss categories for ˜30% of the compounds, maximum prediction errors are large (>10-fold), the b-exponent ranges between −0.2 and 2.2 (averaging ˜0.8–0.9), and >2-fold prediction errors are found for 35% of the substances. The performance is consistent with species differences of binding in and outside the vasculature. The largest errors could potentially lead to very poor prediction of exposure profile and failure in clinical studies. A re-evaluation of allometric scaling of unbound tissue volume of distribution demonstrates that this method is less accurate (27% of predictions >2-fold errors) than a previous evaluation demonstrated. By adding molecular descriptor information, predictions based on animal VD,ss data can be improved. Improved predictions (˜1/10 of allometric errors) can also be obtained by using the relationship between unbound fraction in plasma (fu,pl) and VD,ss for each substance (method suggested by the author). A physiologically-based 4-compartment model (plasma, red blood cells, interstitial fluid and cell volume) together with measured tissue-plasma partitioning coefficients in rats, fu,pl, interstitial-plasma concentration ratio of albumin, organ weight and blood flow data has been successfully applied. Prediction errors for one basic and one neutral drug are only 3–5%. The data obtained with this comparably laboratory-intensive method are limited to these two compounds. A similar approach where predicted tissue partitioning is used, and a computational model, give prediction errors similar to that of allometry. Advantages with these are the suitability for screening and avoidance of animal experiments. The evaluated methods do not account for potential active transport and slow dissociation rates.

In vitro synergy and enhanced murine brain penetration of saquinavir coadministered with mefloquine

Highly active antiretroviral therapy has substantially improved prognosis in human immunodeficiency virus (HIV). However, the integration of proviral DNA, development of viral resistance, and lack of permeability of drugs into sanctuary sites (e.g., brain and lymphocyte) are major limitations to current regimens. Previous studies have indicated that the antimalarial drug chloroquine (CQ) has antiviral efficacy and a synergism with HIV protease inhibitors. We have screened a panel of antimalarial compounds for activity against HIV-1 in vitro. A limited efficacy was observed for CQ, mefloquine (MQ), and mepacrine (MC). However, marked synergy was observed between MQ and saquinavir (SQV), but not CQ in U937 cells. Furthermore, enhancement of the antiviral activity of SQV and four other protease inhibitors (PIs) by MQ was observed in MT4 cells, indicating a class specific rather than a drug-specific phenomenon. We demonstrate that these observations are a result of inhibition of multiple drug efflux proteins by MQ and that MQ also displaces SQV from orosomucoid in vitro. Finally, coadministration of MQ and SQV in CD-1 mice dramatically altered the tissue distribution of SQV, resulting in a >3-fold and >2-fold increase in the tissue/blood ratio for brain and testis, respectively. This pharmacological enhancement of in vitro antiviral activity of PIs by MQ now warrants further examination in vivo.

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.

Effects of altered plasma alpha-1-acid glycoprotein levels on pharmacokinetics of some basic antibiotics in pigs: simulation analysis

Effects of altered plasma alpha-1-acid glycoprotein (AGP) levels on pharmacokinetic parameters of basic antimicrobials, erythromycin (EM), lincomycin (LM) and clindamycin (CM) were evaluated in pigs by simulation analysis. Intravenous (i.v.) injections of EM, LM and CM were performed to obtain pharmacokinetic parameters in healthy conditions. Binding parameters were obtained from an in vitro study using ultrafiltration. Simulation studies indicated that an increase of plasma AGP levels resulted in a decrease of both volume of distribution at steady state (Vdss) and total body clearance (Cltot) for all the drugs. Elimination rate constant for LM was almost unchanged by an increase of plasma AGP levels, whereas those for EM and CM were increased. Plasma concentration-time profiles at a high AGP level (often observed in pathophysiological conditions) were also simulated. All of the total plasma concentration-time profiles were different from those at normal AGP level. The differences were characterized by a higher initial concentration with faster or similar elimination. Unbound plasma concentration-time profile of LM was unaffected by AGP levels, whereas EM and CM were eliminated from plasma more rapidly at high AGP level. These results suggested that adjustment of dosage regimen of EM and CM is required in pathophysiological conditions, but that of LM is not required.

Pharmacokinetics of tamsulosin hydrochloride in patients with renal impairment: effects of alpha 1-acid glycoprotein

The pharmacokinetics of tamsulosin hydrochloride in patients with renal impairment were compared with those in healthy volunteers, and the factors that influenced plasma levels of tamsulosin were elucidated. A single oral dose of 0.2 mg of tamsulosin was given and blood and urine samples were obtained for 36 hours after administration. Unbound plasma concentration of tamsulosin was measured by a combination of equilibrium dialysis and liquid chromatography tandem mass spectrometry methods to examine the effect of protein binding on the pharmacokinetics of tamsulosin. Mean values for maximum concentration (Cmax) and area under the concentration-time curve (AUC) of total drug (Cmax,t and AUC1) in patients with renal impairment were 73% and 211% greater, respectively, than those in healthy volunteers. Mean Cmax and AUC of unbound drug (Cmax,u and AUCu), however, were almost the same in the two groups. A high correlation was found between alpha 1-acid glycoprotein (alpha 1-AGP) concentration and AUCt, but no correlation was found between alpha 1-AGP concentration and AUCu,0-36) or between creatinine clearance (ClCR) and AUCu,0-36). These results show that in patients with renal impairment, the pharmacokinetics of tamsulosin are affected by the change in protein binding that is associated with alteration of plasma alpha 1-AGP concentration, but are not largely affected by the decrease in the renal excretion. Although total tamsulosin levels increased as plasma protein binding increased, unbound tamsulosin levels (which are directly associated with the pharmacologic effects) remained unchanged in these patients.

Quinine impairs quinidine clearance in rat perfused liver

We have examined the disposition of the cinchona alkaloids quinine and quinidine in the rat recirculating isolated perfused liver preparation. When administered as separate 1 mg doses, the hepatic clearances of quinine and quinidine were similar to the hepatic perfusate rate of 10 mL min-1. When 1 mg of each was administered simultaneously, mean hepatic clearance of quinine was unchanged (9.00 +/- 2.20 mL min-1 separate dosage, n = 7; 6.87 +/- 1.77 mL min-1 simultaneous dosage, n = 7; P > 0.05). By contrast, mean hepatic clearance of quinidine was reduced significantly by concomitant quinine (10.6 +/- 1.72 mL min-1 separate dosage, n = 7; 4.82 +/- 1.25 mL min-1 simultaneous dosage, n = 7; P < 0.05). There was no significant difference in volumes of distribution when each alkaloid was administered separately (131 +/- 46 mL quinine, 129 +/- 21 mL quinidine; P > 0.05) but concomitant quinine administration increased quinidine volume of distribution to 169 +/- 30 mL (P < 0.05). Four further experiments with simultaneous dosages of 0.5 mg of each alkaloid produced similar findings, indicating that the interactions did not derive from nonlinear drug disposition.