Interaction of quinine with model lipid membranes of different compositions

Water-Soluble Dioxidovanadium(V) Complexes of Aroylhydrazones: DNA/BSA Interactions, Hydrophobicity, and Cell-Selective Anticancer Potential

Five new anionic aqueous dioxidovanadium(V) complexes, [{VO₂L^1,2}A(H₂O)_n]_α (1-5), with the aroylhydrazone ligands pyridine-4-carboxylic acid (3-ethoxy-2-hydroxybenzylidene)hydrazide (H₂L¹) and furan-2-carboxylic acid (3-ethoxy-2-hydroxybenzylidene)hydrazide (H₂L²) incorporating different alkali metals (A = Na⁺, K⁺, Cs⁺) as countercation were synthesized and characterized by various physicochemical techniques. The solution-phase stabilities of 1-5 were determined by time-dependent NMR and UV-vis, and also the octanol/water partition coefficients were obtained by spectroscopic techniques. X-ray crystallography of 2-4 confirmed the presence of vanadium(V) centers coordinated by two cis-oxido-O atoms and the O, N, and O atoms of a dianionic tridentate ligand. To evaluate the biological behavior, all complexes were screened for their DNA/protein binding propensity through spectroscopic experiments. Finally, a cytotoxicity study of 1-5 was performed against colon (HT-29), breast (MCF-7), and cervical (HeLa) cancer cell lines and a noncancerous NIH-3T3 cell line. The cytotoxicity was cell-selective, being more active against HT-29 than against other cells. In addition, the role of hydrophobicity in the cytotoxicity was explained in that an optimal hydrophobicity is essential for high cytotoxicity. Moreover, the results of wound-healing assays indicated antimigration in case of HT-29 cells. Remarkably, 1 with an IC₅₀ value of 5.42 ± 0.15 μM showed greater activity in comparison to cisplatin against the HT-29 cell line.

Insights into the Membranolytic Activity of Antimalarial Drug-Cell Penetrating Peptide Conjugates

Conjugation of TP10, a cell-penetrating peptide with intrinsic antimalarial activity, to the well-known antimalarial drugs chloroquine and primaquine has been previously shown to enhance the peptide's action against, respectively, blood- and liver-stage malaria parasites. Yet, this was achieved at the cost of a significant increase in haemolytic activity, as fluorescence microscopy and flow cytometry studies showed the conjugates to be more haemolytic for non-infected than for Plasmodium-infected red blood cells. To gain further insight into how these conjugates distinctively bind, and likely disrupt, membranes of both Plasmodium-infected and non-infected erythrocytes, we used dynamic light scattering and surface plasmon resonance to study the interactions of two representative conjugates and their parent compounds with lipid model membranes. Results obtained are herein reported and confirm that a strong membrane-disruptive character underlies the haemolytic properties of these conjugates, thus hampering their ability to exert selective antimalarial action.

Binding of warfarin differently affects the thermal behavior and chain packing of anionic, zwitterionic and cationic lipid membranes

Warfarin is a coumarin derivative drug widely used for its anticoagulant properties. The interaction of warfarin with fully hydrated lipid bilayers has been studied by combining differential scanning calorimetry, spectrophotometry, electron spin resonance of chain-labelled lipids and molecular docking. Bilayers formed by lipids with different chemico-physical properties were considered, namely dimyristoyl-phosphatidylcholine (DMPC), dimyristoyl-phosphatidylglycerol (DMPG), and dioleoyltrimethyl-ammoniumpropane (DOTAP). We observed in all cases the binding of warfarin in proximity of the surface of the bilayers, leading to a variety of distinct effects on key molecular properties of the membranes. The drug associates with the lipid bilayers in the deprotonated open chain form, with an association constant similar for DMPC and DMPG (1.27·10⁴ and 2.82·10⁴ M^-1, respectively) and lower for DOTAP (0.46·10⁴ M^-1). In DMPC bilayers, which are zwitterionic and with saturated symmetrical chains, warfarin at 10 mol% suppresses the pre-transition, slightly stabilizes the fluid state and reduces the cooperativity of the main transition. Moreover, it alters the lateral packing density of the chain segments close to the polar/apolar interface at any temperature through the gel phase. In anionic DMPG bilayers, the drug slightly perturbs the thermotropic phase behavior, and at 10 mol% markedly loosens the compact gel phase packing of the first chain segments. In cationic DOTAP bilayers, possessing unsaturated acyl chains, the drug induces a slightly higher degree of order and motional restriction in the outer hydrocarbon region in the frozen state. In all cases, as a surface adsorbed molecule, warfarin does not affect the segmental chain order and dynamics for temperatures in the fluid phase. The overall results provide an outline of the action of warfarin on membranes formed by lipids of different types.

Interactions between artemisinins and other antimalarial drugs in relation to the cofactor model--a unifying proposal for drug action

Artemisinins are proposed to act in the malaria parasite cytosol by oxidizing dihydroflavin cofactors of redox-active flavoenzymes, and under aerobic conditions by inducing their autoxidation. Perturbation of redox homeostasis coupled with the generation of reactive oxygen species (ROS) ensues. Ascorbic acid-methylene blue (MB), N-benzyl-1,4-dihydronicotinamide (BNAH)-MB, BNAH-lumiflavine, BNAH-riboflavin (RF), and NADPH-FAD-E. coli flavin reductase (Fre) systems at pH 7.4 generate leucomethylene blue (LMB) and reduced flavins that are rapidly oxidized in situ by artemisinins. These oxidations are inhibited by the 4-aminoquinolines piperaquine (PPQ), chloroquine (CQ), and others. In contrast, the arylmethanols lumefantrine, mefloquine (MFQ), and quinine (QN) have little or no effect. Inhibition correlates with the antagonism exerted by 4-aminoquinolines on the antimalarial activities of MB, RF, and artemisinins. Lack of inhibition correlates with the additivity/synergism between the arylmethanols and artemisinins. We propose association via π complex formation between the 4-aminoquinolines and LMB or the dihydroflavins; this hinders hydride transfer from the reduced conjugates to the artemisinins. The arylmethanols have a decreased tendency to form π complexes, and so exert no effect. The parallel between chemical reactivity and antagonism or additivity/synergism draws attention to the mechanism of action of all drugs described herein. CQ and QN inhibit the formation of hemozoin in the parasite digestive vacuole (DV). The buildup of heme-Fe(III) results in an enhanced efflux from the DV into the cytosol. In addition, the lipophilic heme-Fe(III) complexes of CQ and QN that form in the DV are proposed to diffuse across the DV membrane. At the higher pH of the cytosol, the complexes decompose to liberate heme-Fe(III) . The quinoline or arylmethanol reenters the DV, and so transfers more heme-Fe(III) out of the DV. In this way, the 4-aminoquinolines and arylmethanols exert antimalarial activities by enhancing heme-Fe(III) and thence free Fe(III) concentrations in the cytosol. The iron species enter into redox cycles through reduction of Fe(III) to Fe(II) largely mediated by reduced flavin cofactors and likely also by NAD(P)H-Fre. Generation of ROS through oxidation of Fe(II) by oxygen will also result. The cytotoxicities of artemisinins are thereby reinforced by the iron. Other aspects of drug action are emphasized. In the cytosol or DV, association by π complex formation between pairs of lipophilic drugs must adversely influence the pharmacokinetics of each drug. This explains the antagonism between PPQ and MFQ, for example. The basis for the antimalarial activity of RF mirrors that of MB, wherein it participates in redox cycling that involves flavoenzymes or Fre, resulting in attrition of NAD(P)H. The generation of ROS by artemisinins and ensuing Fenton chemistry accommodate the ability of artemisinins to induce membrane damage and to affect the parasite SERCA PfATP6 Ca(2+) transporter. Thus, the effect exerted by artemisinins is more likely a downstream event involving ROS that will also be modulated by mutations in PfATP6. Such mutations attenuate, but cannot abrogate, antimalarial activities of artemisinins. Overall, parasite resistance to artemisinins arises through enhancement of antioxidant defense mechanisms.

Genome wide adaptations of Plasmodium falciparum in response to lumefantrine selective drug pressure

The combination therapy of the Artemisinin-derivative Artemether (ART) with Lumefantrine (LM) (Coartem®) is an important malaria treatment regimen in many endemic countries. Resistance to Artemisinin has already been reported, and it is feared that LM resistance (LMR) could also evolve quickly. Therefore molecular markers which can be used to track Coartem® efficacy are urgently needed. Often, stable resistance arises from initial, unstable phenotypes that can be identified in vitro. Here we have used the Plasmodium falciparum multidrug resistant reference strain V1S to induce LMR in vitro by culturing the parasite under continuous drug pressure for 16 months. The initial IC₅₀ (inhibitory concentration that kills 50% of the parasite population) was 24 nM. The resulting resistant strain V1S_LM, obtained after culture for an estimated 166 cycles under LM pressure, grew steadily in 378 nM of LM, corresponding to 15 times the IC₅₀ of the parental strain. However, after two weeks of culturing V1S_LM in drug-free medium, the IC₅₀ returned to that of the initial, parental strain V1S. This transient drug tolerance was associated with major changes in gene expression profiles: using the PFSANGER Affymetrix custom array, we identified 184 differentially expressed genes in V1S_LM. Among those are 18 known and putative transporters including the multidrug resistance gene 1 (pfmdr1), the multidrug resistance associated protein and the V-type H+ pumping pyrophosphatase 2 (pfvp2) as well as genes associated with fatty acid metabolism. In addition we detected a clear selective advantage provided by two genomic loci in parasites grown under LM drug pressure, suggesting that all, or some of those genes contribute to development of LM tolerance – they may prove useful as molecular markers to monitor P. falciparum LM susceptibility.

Effects of the antimalarial drug primaquine on the dynamic structure of lipid model membranes

Primaquine (PQ) is a potent therapeutic agent used in the treatment of malaria and its mechanism of action still lacks a more detailed understanding at a molecular level. In this context, we used differential scanning calorimetry (DSC), pressure perturbation calorimetry (PPC), and electron spin resonance (ESR) to investigate the effects of PQ on the lipid phase transition, acyl chain dynamics, and on volumetric properties of lipid model membranes. DSC thermograms revealed that PQ stabilizes the fluid phase of the lipid model membranes and interacts mainly with the lipid headgroups. This result was revealed by the great effect on the pretransition of phosphatidylcholines and the destabilization of the inverted hexagonal phase of a phosphatidylethanolamine bilayer. Spin probes located at different positions along the lipid chain were used to monitor different membrane regions. ESR results indicated that PQ is effective in changing the acyl chain ordering and dynamics of the whole chain of dimyristoylphosphatidylcholine (DMPC) phospholipid in the rippled gel phase. The combined ESR and PPC results revealed that the slight DMPC volume changes at the main phase transition induced by the presence of PQ is probably due to a less dense lipid gel phase. At physiological pH, the cationic amphiphilic PQ strongly interacts with the lipid headgroup region of the bilayers, causing considerable disorganization in the hydrophobic core. These results shed light on the molecular mechanism of primaquine-lipid interaction, which may be useful in the understanding of the complex mechanism of action and/or the adverse effects of this antimalarial drug.

Live cell cytotoxicity studies: documentation of the interactions of antitumor active dirhodium compounds with nuclear DNA

The promising antitumor activity of dirhodium complexes has been known for over 30 years. There remains, however, a general lack of understanding of their activity in cellulo. In this study, we report the DNA interactions and activity in living cells of six monosubstituted dirhodium(II,II) complexes of general formula [Rh(2)(mu-O(2)CCH(3))(2)(eta(1)-O(2)CCH(3))(L)(CH(3)OH)](+), where L = bpy (2,2'-bipyridine) (1), phen (1,10-phenanthroline) (2), dpq (dipyrido[3,2-f:2',3'-h]quinoxaline) (3), dppz (dipyrido[3,2-a:2',3'-c]phenazine) (4), dppn (benzo[i]dipyrido[3,2-a:2',3'-c]phenazine) (5), and dap (4,7-dihydrodibenzo[de,gh][1,10]phenanthroline) (6). DNA interactions were investigated by UV/visible spectroscopy, relative viscosity measurements, and electrophoretic mobility shift assay. These measurements indicate that compound 5 exhibits the strongest interaction with DNA. Compound 5 also causes the most damage to DNA after cellular internalization, as evaluated by the alkaline comet assay. Compound 5, however, is not the most effective at inhibiting cell viability of the human cancer cells HeLa and COLO-316. The greater hydrophobicity of 5 as compared to that of 4, which is the most effective compound in the series, hinders its ability to reach its cellular target(s). Data from modulation studies of glutathione using N-acetylcysteine and L-buthionine-sulfoximine indicate that changes in glutathione levels do not affect the activity of these particular dirhodium complexes. These results suggest that glutathione is not the only agent involved in the deactivation of these dirhodium complexes.

Impact of hapten presentation on antibody binding at lipid membrane interfaces

We report the effects of ligand presentation on the binding of aqueous proteins to solid supported lipid bilayers. Specifically, we show that the equilibrium dissociation constant can be strongly affected by ligand lipophilicity and linker length/structure. The apparent equilibrium dissociation constants (K(D)) were compared for two model systems, biotin/anti-biotin and 2,4-dinitrophenyl (DNP)/anti-DNP, in bulk solution and at model membrane surfaces. The binding constants in solution were obtained from fluorescence anisotropy measurements. The surface binding constants were determined by microfluidic techniques in conjunction with total internal reflection fluorescence microscopy. The results showed that the bulk solution equilibrium dissociation constants for anti-biotin and anti-DNP were almost identical, K(D)(bulk) = 1.7 +/- 0.2 nM vs. 2.9 +/- 0.1 nM. By contrast, the dissociation constant for anti-biotin antibody was three orders of magnitude tighter than for anti-DNP at a lipid membrane interface, K(D) = 3.6 +/- 1.1 nM vs. 2.0 +/- 0.2 microM. We postulate that the pronounced difference in surface binding constants for these two similar antibodies is due to differences in the ligands' relative lipophilicity, i.e., the more hydrophobic DNP molecules had a stronger interaction with the lipid bilayers, rendering them less available to incoming anti-DNP antibodies compared with the biotin/anti-biotin system. However, when membrane-bound biotin ligands were well screened by a poly(ethylene glycol) (PEG) polymer brush, the K(D) value for the anti-biotin antibody could also be weakened by three orders of magnitude, 2.4 +/- 1.1 microM. On the other hand, the dissociation constant for anti-DNP antibodies at a lipid interface could be significantly enhanced when DNP haptens were tethered to the end of very long hydrophilic PEG lipopolymers (K(D) = 21 +/- 10 nM) rather than presented on short lipid-conjugated tethers. These results demonstrate that ligand presentation strongly influences protein interactions with membrane-bound ligands.

Dirhodium(II,II) complexes: molecular characteristics that affect in vitro activity

In the series Rh2(O2CR)4 (R=CH3, 1; R=CF3, 2), [Rh2(O2CR)2(phen)2]2+ (R=CH3, 3; R=CF3, 4), and [Rh2(O2CR)2(dppz)2]2+ (R=CH3, 5; R=CF3, 6), 2, 4, and 6 are twice as cytotoxic as 1, 3, and 5, respectively. The substitution reactions of 2 with 9-ethylguanine at various temperatures take place at faster rates than those of 1, and the activation energy Ea(1)=69+/-4 kJ/mol is twice Ea(2)=35+/-2 kJ/mol. The higher cytotoxicities of [Rh2(micro-O2CCH3)2(eta1-O2CCH3)L(MeOH)]+ (L=dppz, 7; L=dppn, 8) relative to [Rh2(micro-O2CCH3)2(bpy)L]2+ (L=dppz, 10; L=dppn, 11) are attributed to the labile equatorial groups in 7 and 8 not present in 10 and 11. The toxicities of complexes 1-8 are not related to their charge or the ease by which they transverse the cellular membrane but to the lability of the ligands on the dirhodium core.

Localization of Multidrug Transporter Substrates within Model Membranes†

Active extrusion of drugs from the cell interior by primary and secondary efflux pumps is an essential mechanism underlying the phenomenon of multidrug resistance. The first discovered and best characterized primary efflux pump found in humans is the ABC transporter P-glycoprotein (PGP), which shows very broad substrate specificity. Many of these molecules are lipophilic, and binding most likely takes place within the membrane. PGP could either translocate them from the inner to the outer leaflet (flippase) or extrude them from the membrane into the extracellular environment (hydrophobic vacuum cleaner). Recognition and binding of such a diverse set of substrates must be associated with a preferred membrane location, determined by molecular properties and lipid interactions. Therefore, a systematic study of the interaction among seven PGP substrates (phenazine, doxorubicin, cephalexin, ampicillin, chloramphenicol, penicillin G, and quercetin) and two modulators (quinidine and nicardipine) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) model membranes is reported here. The location profile of these molecules across the membrane was determined by (1)H NOESY MAS NMR based on (1)H-(1)H cross-peaks between their aromatic fingerprint region and lipid resonances. Although structurally rather diverse, all tested substances are found to have their highest concentration between the phosphate of the lipid headgroup and the upper segments of the lipid hydrocarbon chains. Our findings are consistent with PGP substrate and modulator binding from the membrane interface region.

Interaction of Basic Drugs with Lipid Bilayers Using Liposome Electrokinetic Chromatography

PURPOSE

This study explores factors influencing the interactions of positively charged drugs with liposomes using liposome electrokinetic chromatography (LEKC) for the development of LEKC as a rapid screening method for drug-membrane interactions.

METHODS

Liposomes were prepared and the retention factors were measured for a series of basic drugs under a variety of buffer conditions, including various buffer types, concentrations, and ionic strengths as well as using different phospholipids and liposome compositions. LEKC retention is compared with octanol-water partitioning.

RESULTS

The interaction of ionizable solutes with liposomes decreased with increasing ionic strength of the aqueous buffer. The type of buffer also influences positively charged drug partitioning into liposomes. Varying the surface charge on the liposomes by the selection of phospholipids influences the electrostatic interactions, causing an increase in retention with increasing percentages of anionic lipids in the membrane. Poor correlations are observed between LEKC retention and octanol-water partitioning.

CONCLUSIONS

These studies demonstrate the overall buffer ionic strength at a given pH is more important than buffer type and concentration. The interaction of positively charged drugs with charged lipid bilayer membranes is selectively influenced by the pKa of the drug. Liposomes are more biologically relevant in vitro models for cell membranes than octanol, and LEKC provides a unique combination of advantages for rapid screening of drug-membrane interactions.

Ferriprotoporphyrin IX, phospholipids, and the antimalarial actions of quinoline drugs

Two subclasses of quinoline antimalarial drugs are used clinically. Both act on the endolysosomal system of malaria parasites, but in different ways. Treatment with 4-aminoquinoline drugs, such as chloroquine, causes morphologic changes and hemoglobin accumulation in endocytic vesicles. Treatment with quinoline-4-methanol drugs, such as quinine and mefloquine, also causes morphologic changes, but does not cause hemoglobin accumulation. In addition, chloroquine causes undimerized ferriprotoporphyrin IX (ferric heme) to accumulate whereas quinine and mefloquine do not. On the contrary, treatment with quinine or mefloquine prevents and reverses chloroquine-induced accumulation of hemoglobin and undimerized ferriprotoporphyrin IX. This difference is of particular interest since there is convincing evidence that undimerized ferriprotoporphyrin IX in malaria parasites would interact with and serve as a target for chloroquine. According to the ferriprotoporphyrin IX interaction hypothesis, chloroquine would bind to undimerized ferriprotoporphyrin IX, delay its detoxification, cause it to accumulate, and allow it to exert its intrinsic biological toxicities. The ferriprotoporphyrin IX interaction hypothesis appears to explain the antimalarial action of chloroquine, but a drug target in addition to ferriprotoporphyrin IX is suggested by the antimalarial actions of quinine and mefloquine. This article summarizes current knowledge of the role of ferriprotoporphyrin IX in the antimalarial actions of quinoline drugs and evaluates the currently available evidence in support of phospholipids as a second target for quinine, mefloquine and, possibly, the chloroquine-ferriprotoporphyrin IX complex.

Physiological phenotyping of cholecystokinin-responsive rat taste receptor cells

The recent discovery that subsets of rat taste receptor cells (TRCs) express the peptide cholecystokinin (CCK) and that subsets of TRCs respond to CCK with altered potassium currents or elevated intracellular calcium via CCK-A receptor has lead to the hypothesis that CCK may play a novel signaling role within the taste bud, perhaps modifying tastant responses by co-transmission with a classic transmitter. To better understand this phenomenon, CCK-responsive TRCs were characterized for sensitivity to two bitter stimuli, quinine or caffeine, or to the neurotransmitter ACh using a ratiometric procedure with the calcium sensitive dye fura-2. In characterizing TRC responses to quinine, it was observed that quinine-induced elevations of intracellular calcium were not due to endogenous fluorescence of the quinine molecule. Most (60-70%) CCK-responsive cells were also sensitive to either bitter stimuli or to cholinergic stimulation. These data suggest that TRCs expressing CCK-receptors also express receptors to bitter stimuli and/or muscarinic receptors. They further support the notion of a putative modulatory role of CCK with convergence of multiple inputs occurring at the level of intracellular calcium.