In vitro hepatic metabolism of mefloquine using microsomes from cats, dogs and the common brush-tailed possum (Trichosurus vulpecula)

Feline infectious peritonitis (FIP) is a systemic, fatal, viral-induced, immune-mediated disease of cats caused by feline infectious peritonitis virus (FIPV). Mefloquine, a human anti-malarial agent, has been shown to inhibit FIPV in vitro. As a first step to evaluate its efficacy and safety profile as a potential FIP treatment for cats, mefloquine underwent incubation in feline, canine and common brush-tailed possum microsomes and phase I metabolism cofactors to determine its rate of phase I depletion. Tramadol was used as a phase I positive control as it undergoes this reaction in both dogs and cats. Using the substrate depletion method, the in vitro intrinsic clearance (mean ± S.D.) of mefloquine by pooled feline and common brush-tailed possum microsomes was 4.5 ± 0.35 and 18.25 ± 3.18 μL/min/mg protein, respectively. However, phase I intrinsic clearance was too slow to determine with canine microsomes. Liquid chromatography-mass spectrometry (LC-MS) identified carboxymefloquine in samples generated by feline microsomes as well as negative controls, suggesting some mefloquine instability. Mefloquine also underwent incubation with feline, canine and common brush-tailed possum microsomes and phase II glucuronidative metabolism cofactors. O-desmethyltramadol (ODMT or M1) was used as a positive control as it undergoes a phase II glucuronidation reaction in these species. The rates of phase II mefloquine depletion by microsomes by all three species were too slow to estimate. Therefore mefloquine likely undergoes phase I hepatic metabolism catalysed by feline and common brush-tailed possum microsomes but not phase II glucuronidative metabolism in all three species and mefloquine is not likely to have delayed elimination in cats with clinically normal, hepatic function.

Study on the biochemical basis of mefloquine resistant Plasmodium falciparum

Increase in drug detoxification and alteration of drug uptake and efflux of Plasmodium falciparum were investigated for their possible association with mefloquine (MQ) resistance in five different clones of P. falciparum from Thailand (T994b(3), K1CB(2), PR70CB(1), PR71CB(2) and TM(4)CB8-2.2.3). Fifty percent inhibitory concentration (IC(50)) values from these five clones varied between 30- and 50-fold. Regarding the detoxification mechanism, the ability of P. falciparum clones to biotransform MQ was shown in vitro by parasite microsomal protein prepared from parasite infected red blood cells protein (30mug), NADPH (1nM) and phosphate buffer pH 7.4, carried out at 37 degrees C with agitation. Radiolabelled unmetabolized MQ and possible metabolite(s) generated from the reaction was extracted into ethylacetate and separated by radiometric-HPLC after 1 h. All clones were capable of converting MQ into carboxymefloquine (CMQ), which is the main metabolite in human plasma. In addition, another unidentified metabolite eluted at 4.2 min on the chromatograph could be detected from the incubation reaction. This metabolite has never been detected in human liver microsomes before. There was no significant difference in the percentages of CMQ formed in the resistant (T994(b3), PR(70)CB(1), PR(71)CB(2)) and sensitive (TM(4)CB8-2.2.3, K1CB(2)) clones. Another possible mechanism, i.e., alteration in the accumulation of MQ in the parasites was investigated in vitro using [(14)C]MQ as a tracer. The time courses of [(14)C]MQ uptake and efflux were generally characterized by two phases. A trend of increased efflux of [(14)C]MQ was observed in the resistant compared with sensitive clones.

Utility of alkylaminoquinolinyl methanols as new antimalarial drugs

Mefloquine has been one of the more valuable antimalarial drugs but has never reached its full clinical potential due to concerns about its neurologic side effects, its greater expense than that of other antimalarials, and the emergence of resistance. The commercial development of mefloquine superseded that of another quinolinyl methanol, WR030090, which was used as an experimental antimalarial drug by the U.S. Army in the 1970s. We evaluated a series of related 2-phenyl-substituted alkylaminoquinolinyl methanols (AAQMs) for their potential as mefloquine replacement drugs based on a series of appropriate in vitro and in vivo efficacy and toxicology screens and the theoretical cost of goods. Generally, the AAQMs were less neurotoxic and exhibited greater antimalarial potency, and they are potentially cheaper than mefloquine, but they showed poorer metabolic stability and pharmacokinetics and the potential for phototoxicity. These differences in physiochemical and biological properties are attributable to the "opening" of the piperidine ring of the 4-position side chain. Modification of the most promising compound, WR069878, by substitution of an appropriate N functionality at the 4 position, optimization of quinoline ring substituents at the 6 and 7 positions, and deconjugation of quinoline and phenyl ring systems is anticipated to yield a valuable new antimalarial drug.

El citocromo P-450 y la respuesta terapéutica a los antimaláricos

OBJECTIVES

To assess the relationship between the genetic and phenotypic factors linked to the cytochrome P-450 enzyme system and the response to the antimalarial drugs chloroquine, amodiaquine, mefloquine, and proguanil, as well as to determine how certain biological and social factors of the host influence the behavior of this enzymatic complex.

METHODS

We performed a systematic review of the medical bibliographic databases PubMed, Excerpta Medica, LILACS, and SciELO by using the following Spanish and English descriptors: "CYP-450" and "citocromo P-450" in combination with "proguanil" (and with "mefloquina," "cloroquina," and "amodiaquina"), "farmacocinética de proguanil" (and the same using "mefloquina," "cloroquina," and "amodiaquina"), "resistencia a proguanil" (and the same using "mefloquina," "cloroquina," and "amodiaquina"), "metabolismo," "farmacogenética," "enfermedad," "inflamación," "infección," "enfermedad hepática," "malaria," "nutrición," and "desnutrición." The same terms were used in English. The search included only articles published in Spanish, English, and Portuguese on or before 30 June 2005 that dealt with only four antimalarial drugs: amodiaquine, chloroquine, mefloquine, and proguanil.

RESULTS

Some genetic factors linked to human cytochrome P-450 (mainly its polymorphism), as well as other biological and social factors (the presence of disease itself, or of inflammation and infection, the use of antimalarials in their various combinations, and the patient's nutritional status) influence the behavior of this complex enzymatic system. It has only been in the last decade that the genetics of the cytochromes has been explored and that the mechanisms underlying some therapeutic interactions and aspects of drug metabolism have been uncovered, making it possible to characterize the biotransformation pathway of amodiaquine and chloroquine. Hopefully new research will help answer the questions that still remain, some of which pertain to the metabolism of other antimalarial drugs, the distribution in the population of the genetic alleles linked to the enzymes involved in their metabolism, the contribution of these genetic mutations to therapeutic failure, and the possibility of predicting the response to antimalarial therapy.

CONCLUSIONS

The therapeutic response to antimalarial drugs is a multifactorial process that is poorly understood, so that it is not possible to ascribe to a specific phenotype or genotype a role in the response to antimalarial therapy. Attention should be given to biological and social factors, such as diet, nutritional status, and inflammatory and infectious processes that are often present in areas where malaria is endemic.

Discovery of novel targets of quinoline drugs in the human purine binding proteome

The quinolines have been used in the treatment of malaria, arthritis, and lupus for many years, yet the precise mechanism of their action remains unclear. In this study, we used a functional proteomics approach that exploited the structural similarities between the quinoline compounds and the purine ring of ATP to identify quinoline-binding proteins. Several quinoline drugs were screened by displacement affinity chromatography against the purine binding proteome captured with gamma-phosphate-linked ATP-Sepharose. Screening of the human red blood cell purine binding proteome identified two human proteins, aldehyde dehydrogenase 1 (ALDH1) and quinone reductase 2 (QR2). In contrast, no proteins were detected upon screening of the Plasmodium falciparum purine binding proteome with the quinolines. In a complementary approach, we passed cell lysates from mice, red blood cells, or P. falciparum over hydroxychloroquine- or primaquine-Sepharose. Consistent with the displacement affinity chromatography screen, ALDH and QR2 were the only proteins recovered from mice and human red blood cell lysate and no proteins were recovered from P. falciparum. Furthermore, the activity of QR2 was potently inhibited by several of the quinolines in vitro. Our results show that ALDH1 and QR2 are selective targets of the quinolines and may provide new insights into the mechanism of action of these drugs.

Interaction of chloroquine and its analogues with heme: An isothermal titration calorimetric study

Quinoline-containing drugs such as chloroquine and quinine have had a long and successful history in antimalarial chemotherapy. Identification of ferriprotoporphyrin IX ([Fe(III)PPIX], haematin) as the drug receptors for these antimalarials called for investigations of the binding affinity, mode of interaction, and the conditions affecting the interaction. The parameters obtained are significant in recent times with the emergence of chloroquine resistant strains of the malaria parasites. This has underlined the need to unravel the molecular mechanism of their action so as to meet the requirement of an alternative to the existing antimalarial drugs. The isothermal titration calorimetric studies on the interaction of chloroquine with haematin lead us to propose an altered mode of binding. The initial recognition is ionic in nature mediated by the propionyl group of haematin with the quaternary nitrogen on CQ. This ionic interaction induces a conformational change, such as to favour binding of subsequent CQ molecules. On the contrary, conditions emulating the cytosolic environment (pH 7.4 and 150 mM salt) reveal the hydrophobic force to be the sole contributor driving the interaction. Interaction of a carefully selected panel of quinoline antimalarial drugs with monomeric ferriprotoporphyrin IX has also been investigated at pH 5.6 mimicking the acidic environment prevalent in the food vacuoles of parasite, the center of drug activity, which are consistent with their antimalarial activity.

Role of cytochrome P450 3A in the metabolism of mefloquine in human and animal hepatocytes

We studied mefloquine metabolism in cells and microsomes isolated from human and animal (monkey, dog, rat) livers. In both hepatocytes and microsomes, mefloquine underwent conversion to two major metabolites, carboxymefloquine and hydroxymefloquine. In human cells and microsomes these metabolites only were formed, as already demonstrated in vivo, while in other species several unidentified metabolites were also detected. After a 48 hr incubation with human and rat hepatocytes, metabolites accounted for 55-65% of the initial drug concentration, whereas in monkey and dog hepatocytes, mefloquine was entirely metabolized after 15 and 39 hrs, respectively. The consumption of mefloquine was less extensive in microsomes, and unchanged drug represented 60% (monkey) to 85-100% (human, dog, rat) of the total radioactivity after 5 hr incubations. The involvement of the cytochrome P450 3A subfamily in mefloquine biotransformation was suggested by several lines of evidence. Firstly, mefloquine metabolism was strongly increased in hepatic microsomes from dexamethasone-pretreated rats, and also in human and rat hepatocytes after prior treatment with a cytochrome P450 3A inducer. Secondly, mefloquine biotransformation in rifampycin-induced human hepatocytes was inhibited in a concentration-dependent manner by the cytochrome P450 3A inhibitor ketoconazole and thirdly, a strong correlation was found between erythromycin-N-demethylase activity (mediated by cytochrome P450 3A) and mefloquine metabolism in human microsomes (r=0.81, P < 0.05, N=13). Collectively, these findings concerning the role of cytochrome P450 3A in mefloquine metabolism may have important in vivo consequences especially with regard to the choice of agents used in multidrug antimalarial regimens.

A common mechanism for blockade of heme polymerization by antimalarial quinolines

The antimalarial quinolines are believed to work by blocking the polymerization of toxic heme released during hemoglobin proteolysis in intraerythrocytic Plasmodium falciparum. In the presence of free heme, chloroquine and quinidine associate with the heme polymer. We have proposed that this association of the quinoline-heme complex with polymer caps the growing heme polymer, preventing further sequestration of additional heme that then accumulates to levels that kill the parasite. In this work results of binding assays demonstrate that the association of quinoline-heme complex with heme polymer is specific, saturable, and high affinity and that diverse quinoline analogs can compete for binding. The relative quinoline binding affinity for heme polymer rather than free heme correlates with disruption of heme polymerization. Mefloquine, another important antimalarial quinoline, associated with polymer in a similar fashion, both in cultured parasites and in the test tube. In parasite culture, blocking heme release with protease inhibitor was antagonistic to mefloquine action, as it is to chloroquine action. These data suggest a common mechanism for quinoline antimalarial action dependent on drug interaction with both heme and heme polymer.

Central role of hemoglobin degradation in mechanisms of action of 4-aminoquinolines, quinoline methanols, and phenanthrene methanols

We have used a specific inhibitor of the malarial aspartic proteinase plasmepsin I and a nonspecific cysteine proteinase inhibitor to investigate the importance of hemoglobin degradation in the mechanism of action of chloroquine, amodiaquine, quinine, mefloquine (MQ), halofantrine, and primaquine. Both proteinase inhibitors antagonized the antiparasitic activity of all drugs tested with the exception of primaquine. An inhibitor of plasmepsin I, Ro40-4388, reduced the incorporation of radiolabelled chloroquine and quinine into malarial pigment by 95%, while causing a 70% reduction in the incorporation of radiolabelled MQ. Cysteine proteinase inhibitor E64 reduced the incorporation of chloroquine and quinine into malarial pigment by 60 and 40%, respectively. This study provides definitive support for the central role of hemoglobin degradation in the mechanism of action of the 4-aminoquinolines and the quinoline and phenanthrene methanol antimalarials.

Food increases the bioavailability of mefloquine

OBJECTIVES

To assess the effect of food on the pharmacokinetics of the antimalarial mefloquine and its major plasma metabolite in healthy volunteers.

METHODS

In an open, two-way cross-over study, 20 healthy male volunteers who had fasted overnight were randomised to receive a single oral dose of 750 mg mefloquine in the absence or presence of a standardised, high-fat breakfast, administered 30 min before drug administration. Blood samples were taken at specific times over an 8-week period. Plasma concentrations of mefloquine and its carboxylic acid metabolite were determined by high-performance liquid chromatography for pharmacokinetic evaluation.

RESULTS

The parameters Cmax and AUC of both mefloquine and its metabolite were significantly (P < 0.05) higher under post-prandial conditions than under fasting conditions (mefloquine: mean Cmax 1500 vs 868 micrograms.l-1, mean AUC 645 vs 461 mg l-1.h; metabolite: Cmax 1662 vs 1231 micrograms.l-1, AUC 1740 vs 1310 mg l-1.h). The intersubject variability in Cmax and AUC of mefloquine was less than 30% (coefficient of variation). The time to peak plasma concentration of mefloquine was significantly shorter after food intake (17 vs 36 h). Compared with absorption in volunteers who had fasted, food did not alter t1/2 (mefloquine and its metabolite) and tmax (metabolite).

CONCLUSION

Under the conditions of this study, food increases the rate and the extent of mefloquine absorption. It is reasonable to recommend that mefloquine be administered with food in travellers receiving chemoprophylaxis and in patients on recovery receiving curative treatment. In acutely ill patients, mefloquine should be taken as soon as possible and administration with or shortly after meals should be attempted as soon as feasible.

Stereoselective passage of mefloquine through the blood-brain barrier in the rat

The pharmacokinetics of the enantiomers of mefloquine were studied in the rat after administration of a racemic mixture and of the separate enantiomers (+)-mefloquine and (-)-mefloquine. When 50 mg kg-1 racemic mixture was administered orally for 22 days, plasma concentrations of the (+) enantiomer were 2-3 times higher than those of the (-) enantiomer whereas the opposite was true in every part of the brain (cerebellum, cortex, hippocampus, hypothalamus and striatum). Different concentrations of mefloquine were found in the different regions of the brain; the lowest concentrations of (+/-)-mefloquine (27.0 nmol g-1) were in the cerebellum and the highest (110.0 nmol g-1) in the hippocampus. The main metabolite, carboxymefloquine, was detected in plasma but not in the brain. The results indicate the mefloquine crosses the blood-brain barrier stereoselectively.

Mutually co-operative interactions between modulators of P-glycoprotein

We measured the effects of combinations of verapamil, vinblastine, mefloquine, and tamoxifen, all being modulators of the multidrug resistance pump, P-glycoprotein, on the accumulation of labelled daunomycin into multidrug-resistant P388 leukemia cells at 37 degrees C. We found that, contrary to our initial expectations (based on Ayesh, Shao and Stein (1996) Biochim. Biophys. Acta 1316, 8), vinblastine, mefloquine, and tamoxifen all appeared to interact with one another synergistically, i.e. by the kinetics of a non-competitive interaction. A simple kinetic analysis showed that pairs of co-operating modulators can give apparent non-competitive behaviour, but refined kinetic analysis enables the two types of interaction to be distinguished. The modulators vinblastine, mefloquine, and tamoxifen thus appear to co-operate with one another in pairs to bring about reversal of P-glycoprotein. This may have important implications for the design of new modulators of P-glycoprotein.

Quinoline antimalarials: mechanisms of action and resistance

The quinoline-containing antimalarial drugs, chloroquine, quinine and mefloquine, are a vital part of our chemotherapeutic armoury against malaria. These drugs are thought to act by interfering with the digestion of haemoglobin in the blood stages of the malaria life cycle. Chloroquine is a dibasic drug which diffuses down the pH gradient to accumulate about a 1000-fold in the acidic vacuole of the parasite. The high intravacuolar concentration of chloroquine is proposed to inhibit the polymerisation of haem. As a result, the haem which is released during haemoglobin breakdown builds up to poisonous levels, thereby killing the parasite with its own toxic waste. The more lipophilic quinolinemethanol drugs, mefloquine and quinine, are not concentrated so extensively in the food vacuole and probably have alternative sites of action. The technique of photoaffinity labelling has been used to identify a series of proteins which interact specifically with mefloquine. These studies have led us to speculate that the quinolinemethanols bind to high density lipoproteins in the serum and are delivered to the erythrocytes where they interact with an erythrocyte membrane protein, known as stomatin, and are then transferred to the intracellular parasite via a pathway used for the uptake of exogenous phospholipid. The final target(s) of quinine and mefloquine action are not yet fully characterised, but may include parasite proteins with apparent molecular weights of 22 kDa and 36 kDa. As resistance to the quinoline antimalarials rises inexorably, there is an urgent need to understand the molecular basis for decreased drug sensitivity. A parasite-encoded homologue of P-glycoprotein has been implicated in the development of drug resistance, possibly by controlling the level of accumulation of the quinoline-containing drugs. As our molecular understanding of these processes increases, it should be possible to design novel antimalarial strategies which circumvent the problem of drug resistance.

Photoaffinity labeling of mefloquine-binding proteins in human serum, uninfected erythrocytes and Plasmodium falciparum-infected erythrocytes

A photoreactive quinolinemethanol analog, N-[4-[1-hydroxy-2-(dibutylamino)ethyl]quinolin-8yl]-4- azido-2-salicylamide (ASA-MQ) has been synthesized which closely mimics the action of mefloquine. ASA-MQ possesses potent antimalarial activity against a mefloquine-sensitive strain of Plasmodium falciparum and shows decreased activity against a mefloquine-resistant parasite strain. Radioiodinated ASA-MQ has been used in photoaffinity labeling studies to identify mefloquine-interacting proteins in serum, uninfected erythrocytes and Plasmodium falciparum-infected erythrocytes. We have shown that mefloquine interacts specifically with apo-A1, the major protein of serum high density lipoproteins. In addition, mefloquine was shown to interact specifically with the erythrocyte membrane protein, band 7.2b (stomatin). A further two high affinity mefloquine-binding proteins with apparent molecular masses of 22 and 36 kDa were identified in three different strains of Plasmodium falciparum. We suggest that these two mefloquine-binding parasite proteins may be involved in the uptake of mefloquine or may represent macromolecular targets of mefloquine action in malaria parasites.

Mefloquine metabolism by human liver microsomes. Effect of other antimalarial drugs

A number of drugs have been studied for their effect on the metabolism of the antimalarial drug mefloquine by human liver microsomes (N = 6) in vitro. The only metabolite generated was identified as carboxymefloquine by co-chromatography with the authentic standard. Ketoconazole caused marked inhibition of carboxymefloquine formation with IC50 and Ki values of 7.5 and 11.2 microM, respectively. The inhibition of ketoconazole, a known inhibitor of cytochrome P450 isozymes, and the dependency of metabolite formation on the presence of NADPH indicated that cytochrome P450 isozyme(s) catalysed metabolite production. Of compounds actually or likely to be coadministered with mefloquine to malaria patients only primaquine and quinine produced marked inhibition (IC50, 17.5 and 122 microM; Ki, 8.6 and 28.5 microM, respectively). However, despite these in vitro data with primaquine, clinical studies have failed to show any significant effect of single dose primaquine on the pharmacokinetics of mefloquine. With quinine, because peak plasma concentrations are very close to the Ki value, there is likely to be inhibition of mefloquine metabolism in patients receiving both drugs. Sulfadoxine, artemether, artesunate and tetracycline did not significantly inhibit carboxymefloquine formation.

Pharmacokinetics of mefloquine in treatment failure

The pharmacokinetics of mefloquine at the therapeutic dose of 750 mg single orally were compared between cured and recrudescent patients with acute uncomplicated falciparum malaria. Mefloquine was well-tolerated during the study. The side-effects found were nausea, vomiting and diarrhea. Five patients showed R-I and two showed R-II types of response. All recrudescent patients came from the eastern border of Thailand. The time taken to clear the parasites (PCT) was significantly longer in patients with recrudescence (99.6 +/- 36.9 and 63.0 +/- 8.9 hours); however, there was no difference regarding fever clearance time (FCT: 39.0 +/- 16.1 and 31.0 +/- 21.3 hours). The maximum concentration (Cmax) and the concentration on the first and second days in cured patients were significantly higher than those of treatment failure patients. Other pharmacokinetic parameters appeared to be similar in both groups. The present study indicates the existence of mefloquine-resistant falciparum malaria in the eastern border of Thailand. Inadequate mefloquine concentration may play an important role in this aspect. In addition, this study also suggests that Cmax or the concentrations on the first or second day of treatment may be used as guidelines to predict the outcome of treatment.

Crystal structure and molecular structure of mefloquine methylsulfonate monohydrate: implications for a malaria receptor

The crystal structure of (+/-)-mefloquine methylsulfonate monohydrate was determined by X-ray diffraction and was compared with the crystal structures of mefloquine hydrochloride and mefloquine free base. The conformation of mefloquine was essentially the same in all three crystalline environments and was not dependent on whether mefloquine was a salt or a free base. In mefloquine methylsulfonate monohydrate, the angle between the average plane of the quinoline ring and the average plane of the piperidine ring was 76.9 degrees. The intramolecular aliphatic N-13...O-1 distance was 2.730 +/- 0.008 A (1 A = 0.1 nm), which is close to the aliphatic N...O distance found in the antimalarial cinchona alkaloids. The hydroxyl group formed a hydrogen bond with the water molecule, and the amine group formed hydrogen bonds with two different methylsulfonate ions. The crystallographic parameters for (+/-)-mefloquine methylsulfonate monohydrate were as follows: C17H17F6N2O(+).CH3SO3(-).H2O; Mr = 492.4; symmetry of unit cell, monoclinic; space group, P2(1)/a; parameters of unit cell, a was 8.678 +/- 0.001 A, b was 28.330 +/- 0.003 A, c was 8.804 +/- 0.001 A, beta was 97.50 +/- 0.01 degrees; the volume of the unit cell was 2145.9 A3; the number of molecules per unit cell was 4; the calculated density was 1.52 g cm(-3); the source of radiation was Cu K alpha (lambda = 1.54178 A); mu (absorption coefficient) was 20.46 cm(-1); F(000) (sum of atomic scattering factors at zero scattering angle) was 1,016; room temperature was used; and the final R (residual index) was 6.58% for 1,740 reflections with magnitude of Fo greater than 3 sigma (F). Since the mechanism of antimalarial action and the mechanism of mefloquine resistance may involve hydrogen bond formation between mefloquine and a cellular effector or transport proteins, the common conformation of mefloquine found in each crystalline environment may define the orientation in which mefloquine forms these potentially critical hydrogen bonds with cellular constituents.

A nuclear magnetic resonance study of the interactions of the antimalarials chloroquine, quinacrine, quinine and mefloquine with lipids extracted from normal human erythrocytes

The interaction of four antimalarials (chloroquine, quinacrine, mefloquine and quinine) with lipid membranes re-formed from lipid extracts of normal human erythrocytes was studied using 2H- and lipid extracts of normal human erythrocytes was studied using 2H- and 31P-nuclear magnetic resonance (NMR). Inclusion of small amounts of chain-perdeuterated dipalmitoylphosphatidylcholine (DPPC) or dipalmitoylphosphatidylethanolamine (DPPE) as an 2H-NMR probe allowed us to study separately the effects of drugs on PC or PE components of the membranes. Only a very small decrease in the order parameters of the DPPE, but not the DPPC probe, was observed in the presence of chloroquine at a molar ratio of 1:5 of drug to lipid. Addition of quinacrine at the same molar ratio resulted in a small but significant decrease in the order parameters of the lipid side chains; identical effects were obtained with DPPC or DPPE perdeuterated probes. The presence of quinacrine did not induce non-bilayer lipid phases. In contrast, mefloquine and quinine produced a significant disordering of the lipid side chains; the effect was considerably larger with the DPPE probe. In addition, both mefloquine and quinine induced non-bilayer phases of the lipids; mefloquine induced formation of hexagonal and micellar lipid conformation, whereas addition of quinine resulted in the formation of lipid micelles only. The lipid polymorphism induced by either of these drugs was more pronounced when the DPPE component was observed, indicating that the non-bilayer phases were enriched in PE. The results suggest the presence of strong interactions between mefloquine and quinine with lipid bilayers, especially with the PE component.(ABSTRACT TRUNCATED AT 250 WORDS)