The pH-dependence of chloroquine uptake by phosphatidylcholine vesicles

Interactions of primaquine and chloroquine with PEGylated phosphatidylcholine liposomes

This study aimed to analyze the interaction of primaquine (PQ), chloroquine (CQ), and liposomes to support the design of optimal liposomal delivery for hepatic stage malaria infectious disease. The liposomes were composed of hydrogenated soybean phosphatidylcholine, cholesterol, and distearoyl-sn-glycero-3-phosphoethanolamine-N-(methoxy[polyethyleneglycol]-2000), prepared by thin film method, then evaluated for physicochemical and spectrospic characteristics. The calcein release was further evaluated to determine the effect of drug co-loading on liposomal membrane integrity. The results showed that loading PQ and CQ into liposomes produced changes in the infrared spectra of the diester phosphate and carbonyl ester located in the polar part of the phospholipid, in addition to the alkyl group (CH₂) in the nonpolar portion. Moreover, the thermogram revealed the loss of the endothermic peak of liposomes dually loaded with PQ and CQ at 186.6 °C, which is identical to that of the phospholipid. However, no crystallinity changes were detected through powder X-ray diffraction analysis. Moreover, PQ, with either single or dual loading, produced the higher calcein release profiles from the liposomes than that of CQ. The dual loading of PQ and CQ tends to interact with the polar head group of the phosphatidylcholine bilayer membrane resulted in an increase in water permeability of the liposomes.

Time-dependent variability of chloroquine secretion into human saliva

Chloroquine has been reported to be secreted into human saliva. This is a report of a study designed to underscore the importance of the finding. It involved the administration of 600 mg chloroquine per oral to seven volunteers and 150 mg by intramuscular route to six others. Blood and simultaneous saliva samples were collected and analyzed for the drug by an HPLC method. The results showed that chloroquine reaches peak concentration at the same time in both plasma and saliva after oral administration of the drug. A good correlation was obtained between the AUC values derived from saliva and plasma level data. Saliva to plasma concentration ratios obtained after administration of the drug by both routes were high (mean > 11) and exhibited a time-dependent variability. These results suggest that an active process, among other mechanisms, may be involved in the transfer of chloroquine into human saliva. Caution should be exercised in the use of saliva for therapeutic monitoring of chloroquine.

Quinine uptake by human polymorphonuclear neutrophils

The antimalarial drug quinine has been shown to impair human polymorphonuclear leukocyte (PMN) functions. To gain insight into the mechanism of this phenomenon, we investigated quinine uptake by PMN with a fluorometric assay based on the fluorescence properties of this drug. After 30 min of incubation at 37 degrees C in the presence of 1 and 10 micrograms of quinine per ml, PMN-associated quinine reached 90 +/- 6 and 780 +/- 150 ng/2.5 x 10(6) PMN, respectively, giving a cellular-to-extracellular concentration ratio of 140 to 150. A steady state was reached within 5 min. Uptake was partially dependent on temperature, cell viability, and extracellular pH. Fractionation studies showed that 30 to 40% of the PMN-associated quinine was located in the particulate fraction. The efflux of PMN-associated quinine was rapid and complete when the incubation mixture was replaced by drug-free medium. These data suggest that several mechanisms are involved in the uptake of quinine by PMN, including a viability- and energy-independent process possibly related to reversible association of quinine to cell structures (particularly the membrane). Other mechanisms could involve trapping by protonation and/or active PMN transport systems. Thus, most of the quinine taken up by resting PMN is found in the soluble fraction of disrupted cells. This may partly explain the depressive properties of quinine.

Alkalinization of the food vacuole of malaria parasites by quinoline drugs and alkylamines is not correlated with their antimalarial activity

Quinoline-containing antimalarial drugs accumulate inside the acid food vacuole of the parasite where they inhibit the digestion of ingested host cell cytosol, and consequently, parasite growth. In order to verify whether this inhibition is caused by drug-induced alkalinization of the food vacuole, we investigated the accumulation of acridine orange (AO) as a vacuolar pH probe in intact Plasmodium falciparum-infected human erythrocytes as affected by the drugs chloroquine (CQ), 7H-quinoleine (7HQ), quinine (Q) and mefloquine (MQ). It was established by various criteria that AO accumulates primarily in the acid compartment(s) of the parasite as a function of the pH difference between it and the extracellular medium. This pH gradient was dissipated by the drugs in the rank order MQ greater than CQ greater than Q greater than 7HQ. The kinetics of vacuolar alkalinization and the concentration ranges at which it was observed imply that the monoprotic drugs MQ and Q exerted their effect mostly by translocating protons across the vacuolar membrane, i.e. they could cross the membrane as a protonated species, while the diprotic drugs CQ and 7HQ raised the vacuolar pH mostly by proton trapping. Similarly, hydrophobic alkylamines raised the vacuolar pH by proton translocation, while their relatively more polar congeners and ammonia did so by proton titration. However, the alkalinizing effect of each drug was observed at a concentration which was 1-2 orders of magnitude larger than the IC50 of its antimalarial effect. These results mean that vacuolar alkalinization is not the primary effect of antiparasitic action of quinoline antimalarials.

Protonophoric effects of antimalarial drugs and alkylamines in Escherichia coli membranes

Inside-out vesicles of Escherichia coli whose lumen was acidified by substrate oxidation, were used to study the mode of pH gradient dissipation by quinoline-containing antimalarial drugs and alkylamines. The pH was dissipated by micromolar drug concentrations, the dibasic chloroquine being most potent, followed by the monobasic mefloquine, quinine and the dibasic 7H-quinoline. The time dependence of pH dissipation as a function of membrane potential suggests that the monoprotonated forms of the drugs are able to cross the bacterial membrane. Alkylamines were able to dissipate the pH gradient in the 0.01-5 mM range, their rank order of potency being related to their hydrophobicity. Tertiary amines were less effective than less hydrophobic primary primary amines, implying an effect of molecular volume of their diffusion across the membrane. Both sets of results suggest that amphiphilic weak bases can cross membranes in their free-base form, become protonated in an acid environment and diffuse in this form along their concentration gradient and aided by the membrane potential, thereby dissipating the pH gradient.

pH-dependent uptake and macrofilaricidal effects of chloroquine on adult filarial parasites in vitro

Uptake and macrofilaricidal effects of chloroquine (CQ) and other aminoquinolines were found to be highly pH dependent in Brugia pahangi, Acanthocheilonema viteae, Onchocerca volvulus, and Dirofilaria immitis. Using [3H]CQ, it was found that all of the parasites took up more drug under alkaline conditions (RPMI 1640 at pH 8.4) than in neutral (pH 7.4) or acidic (pH 6.8) media. Differences were seen in the amount of drug taken up among the filariae studied. B. pahangi and A. viteae took up 7 times more chloroquine per milligram of tissue than did O. volvulus, and 30 times more than D. immitis during a 60-min incubation period at pH 8.4. Sensitivity to the aminoquinolines also increased with increasing media pH, and was measured using parasite motility as an indicator of drug efficacy. Potency of chloroquine against B. pahangi increased 100-fold at pH 8.4 compared to pH 7.4. A. viteae and O. volvulus showed similar sensitivity to chloroquine compared to B. pahangi; D. immitis was less sensitive. While uptake of chloroquine was linear from pH 6.8 to 8.4, B. pahangi was unaffected by 32 microM of the drug below pH 7.6; at any pH above this, motility of this parasite was completely inhibited. Calculations of the internal pH of this parasite indicated that it shifted upwards significantly with changes in media pH. It was concluded that these shifts in internal pH may render parasites more sensitive to the effects of chloroquine.

Effects of quinoline-containing antimalarials on the erythrocyte membrane and their significance to drug action on Plasmodium falciparum

Quinoline-containing antimalarials are cationic amphiphiles which accumulate to high levels in lysosomes and are known to interact with membrane phospholipids. It was therefore hypothesized that they could exert their antimalarial effect by compromising the integrity of the parasite's acidic organelles. To test this hypothesis, the effects of chloroquine (CQ), quinine (Q) and mefloquine (MQ) on the osmotic stability of human red blood cells exposed to hypotonic solutions have been investigated. With CQ and Q stabilization was observed at pH 7.8 and destabilization at pH 5, indicating that destabilization is caused by the protonated forms of the drugs. With MQ the pH dependence was reversed, i.e. it destabilized at pH 7.8 and stabilized at pH 5, suggesting that destabilization is caused by the unprotonated drug. MQ caused cell lysis at the tenth millimolar range by a detergent effect. The possible destabilizing effect of drugs on the membranes of Plasmodium falciparum acidic organelles was investigated in metabolically-labelled parasites. We expected an increase in degradation of parasite proteins if drugs did indeed cause the release of acid hydrolases from destabilized organelles to the cytoplasm. No effect of drugs on parasite protein degradation could be observed, but protein synthesis was inhibited at therapeutic drug concentrations. These results imply that quinoline-containing antimalarials do not compromise the integrity of parasite acidic organelles, and that inhibition of protein synthesis results from a limited supply of essential amino acid(s) due to the demonstrable drug-mediated suppression of parasite digestion of host cell cytosol.