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P G Bray

Publications and source records attributed to P G Bray.

At least 37 records · Page 2Linked to original sources
PubMed1997

Quinoline resistance mechanisms in Plasmodium falciparum: the debate goes on.

Despite considerable therapeutic success with the antimalarial 4-aminoquinolines such as chloroquine, there is serious doubt about the future of this drug class due mainly to the development and spread of parasite resistance throughout endemic areas. In this article we review the possible biochemical and molecular basis of resistance. Based on our current understanding we have considered the possibility of developing strategies which may allow the aminoquinolines to once again be used effectively against P. falciparum. Our conclusions are that drug resistance is the result of a reduced rate of drug uptake which in turn reduces the amount of drug available to bind the target. The basis for this reduced accumulation could be an altered pH gradient making the food vacuole more alkaline or the parasite cytosol more acidic, an efflux pump removing drug directly from the membrane or any other process which will reduce the rate of drug uptake. Central to the effectiveness of this resistance mechanism is the transient availability of a high affinity, low capacity drug binding site (possibly haem) within the parasite. Resistance reversers such as verapamil influence the apparent Ka for this drug binding phenomenon via an increased drug uptake rate. We demonstrate that by chemical modification of the aminoquinolines, producing predictable alterations in their physicochemical properties, that it is possible to minimise the verapamil sensitive component of resistance and reduce significantly cross-resistance patterns without loss in absolute activity. Based on these views we suggest that the aminoquinoline antimalarials still have a role to play in the cheap, safe and effective chemotherapy of falciparum malaria.

Aminoquinolines↗
PubMed1996

In vitro selection of halofantrine resistance in Plasmodium falciparum is not associated with increased expression of Pgh1.

Recent investigations into quinoline and phenanthrene methanol resistance in Plasmodium falciparum have described a linkage between amplification of the mdr homologue pfmdr1 and decreased susceptibility to mefloquine (MQ) and halofantrine (HF). We have examined the current theories on cross-resistance patterns and pfmdr1 gene expression by comparing the chloroquine (CQ) resistant isolate K1 with K1Hf, developed from the K1 isolate by intermittent exposure to halofantrine. Reduced halofantrine susceptibility in K1Hf was accompanied by reduced sensitivity to mefloquine and increased sensitivity to chloroquine. These sensitivity changes were reflected by changes in parasite drug accumulation. The loss of high level chloroquine resistance in K1Hf was associated with an inability of verapamil to enhance chloroquine sensitivity or accumulation. In contrast verapamil retained the chemosensitising potential against quinine in this isolate. The changes in phenotype were achieved without any amplification or increased expression of pfmdr1 or reversion of the Tyr86 mutation in the gene. Our data indicates that acquisition of halofantrine and mefloquine resistance and the loss of high level chloroquine resistance can be achieved without enhanced expression of the pfmdr1 gene product.

ATP-Binding Cassette Transporters↗
PubMed1996

The role of drug accumulation in 4-aminoquinoline antimalarial potency. The influence of structural substitution and physicochemical properties.

We have investigated a series of novel 4-aminoquinoline analogues related to amodiaquine, that possess side chain modifications designed to influence both drug pKa and lipophilicity. These compounds have been used to determine the influence of physicochemical properties on antimalarial activity against, and accumulation by, both chloroquine-susceptible and chloroquine-resistant isolates of Plasmodium falciparum. The compounds tested exhibited a 500-fold range of absolute antimalarial potency. Absolute drug potency and drug accumulation were found to be significantly correlated in each of the four isolates of Plasmodium falciparum studied. The level of accumulation was unrelated to lipophilicity and was significantly greater than the predicted levels of accumulation based on drug pKa, compartmental pH, and Henderson-Hasselbach considerations. Further analysis of the relationship between 4-aminoquinoline accumulation and activity implicated the involvement of additional forces in the accumulation process.

Aminoquinolines↗
PubMed1996

Amodiaquine accumulation in Plasmodium falciparum as a possible explanation for its superior antimalarial activity over chloroquine.

Amodiaquine is a 4-aminoquinoline antimalarial whose structure is similar to chloroquine. In contrast to the wealth of information available about chloroquine accumulation and its relationship to activity, little is known about the uptake characteristics of amodiaquine, a drug that is inherently more active against malaria parasites. In this study we have investigated the accumulation of amodiaquine in Plasmodium falciparum in vitro, in order to gain an insight into the mechanisms responsible for its superior activity over chloroquine. The driving force for parasite accumulation of the 4-aminoquinolines is proposed to be a transmembrane proton gradient maintained by a vacuolar ATPase. In the present study, amodiaquine accumulation was greatly reduced, at steady state, in the absence of glucose and at 0 degrees C indicating a clear energy dependence of uptake. Amodiaquine accumulation in Plasmodium falciparum was shown to be 2- to 3-fold greater than chloroquine accumulation. This observation probably accounts for amodiaquine's greater inherent activity but is surprising given that amodiaquine is a weaker base than chloroquine. With this in mind we present evidence for an intraparasitic binding component in the accumulation of the 4-aminoquinolines. Differences in binding affinity of this 'receptor' for amodiaquine and chloroquine may partially explain the greater accumulation and in vitro potency of amodiaquine compared to chloroquine.

Adenosine Triphosphate↗
PubMed1996

Manipulation of the N-alkyl substituent in amodiaquine to overcome the verapamil-sensitive chloroquine resistance component.

Aminoquinoline resistance correlates with lipid solubility at pH 7.2. Consequently, the in vivo dealkylation of amodiaquine, to the less lipid-soluble desethylamodiaquine, is a likely contributor to therapeutic failure in vivo. Therefore, 4-aminoquinoline drugs with lipid solubilities similar to that of amodiaquine, but which are not subject to side chain modification in vivo, should be superior antimalarial agents. In this study, we have identified amopyroquine and N-tertbutylamodiaquine as two such compounds. The values for the logarithms of the partition coefficients for amopyroquine and N-tertbutylamodiaquine are between those for amodiaquine and its dealkylated metabolite, desethylamodiaquine. Both amopyroquine and N-tertbutylamodiaquine possess levels of antimalarial activity greater than that of desethylamodiaquine and significantly reduced cross-resistance patterns; i.e., the former two compounds are not subject to the verapamil-sensitive resistance mechanism. Simple in vitro markers of direct toxicity and potential reactive metabolite formation suggest that these two compounds are no more toxic than amodiaquine and desethylamodiaquine.

Alkylation↗
PubMed1996

4-Aminoquinoline resistance of Plasmodium falciparum: insights from the study of amodiaquine uptake.

The relationship between antimalarial activity and drug accumulation of chloroquine and amodiaquine was evaluated with four chloroquine-resistant and two chloroquine-susceptible isolates of Plasmodium falciparum. Susceptibility of the strains to amodiaquine was correlated with susceptibility to chloroquine (r2 = 0.96). Similarly, accumulation of amodiaquine was correlated with accumulation of chloroquine (r2 = 0.94). Accumulation of both chloroquine and amodiaquine was significantly reduced in chloroquine-resistant isolates (p < 0.005). For the panel of isolates, the accumulation ratio of both drugs was inversely proportional to drug susceptibility (r2 = 0.963 and 0.994 for amodiaquine and chloroquine, respectively). Time course studies highlighted a reduced initial rate of amodiaquine accumulation in chloroquine-resistant isolates compared with chloroquine-susceptible isolates, with no evidence of an enhanced drug efflux rate. Daunomycin, a modulator of parasite chloroquine transport, significantly increased steady state accumulation of both drugs in chloroquine-resistant isolates and, to a lesser extent, in chloroquine-susceptible isolates. Furthermore, daunomycin increased the initial rate of accumulation of amodiaquine in both chloroquine-resistant and chloroquine-susceptible isolates. Resistance to 4-aminoquinoline drugs is associated with reduced drug permeability rather than enhanced cellular exit of preaccumulated drug, and daunomycin seems to increase the permeability of parasites to aminoquinolines. A new model of 4-aminoquinoline resistance is proposed to take account of these and earlier observations.

Aminoquinolines↗
PubMed1996

Physicochemical properties correlated with drug resistance and the reversal of drug resistance in Plasmodium falciparum.

At high molar excess, verapamil can selectively increase the accumulation and cytotoxicity of structurally dissimilar natural product drugs in many multidrug-resistant tumor cell lines. Such concentrations of verapamil are also capable of increasing the accumulation and activity of chloroquine in chloroquine-resistant strains of the human malaria parasite Plasmodium falciparum. Despite such similarities, it is not clear why chloroquine-resistant P. falciparum is often susceptible to closely related compounds such as amodiaquine, whereas cancer cells are cross-resistant to many structurally unrelated drugs. For 13 aminoquinoline and aminoacridine compounds, relative drug resistance was negatively correlated with lipid solubility at physiological pH (r2 = 0.90, p < 0.0001). The ability of verapamil (5 microM) to reverse drug resistance was also negatively correlated with lipid solubility (r2 = 0.88, p < 0.0001). Furthermore, molar refractivity was weakly correlated with relative drug resistance (r2 = 0.46, p < 0.05) and reversal of drug resistance (r2 = 0.52, p < 0.005). Verapamil increases chloroquine accumulation by resistant parasites, a mechanism suggested to account for its selective chemosensitization effect. We show that the initial rate of chloroquine accumulation by resistant parasites is increased by verapamil. This effect of verapamil is abolished when deoxy-glucose is substituted for glucose. Therefore, verapamil produces an energy-dependent increase in the permeability of resistant parasites to chloroquine. For a panel of four chloroquine-resistant and two chloroquine-susceptible isolates, the effect of verapamil on the accumulation of chloroquine and monodesethyl amodiaquine was found to be correlated (r2 = 0.96, p < 0.001). Verapamil chemosensitization was also correlated for the two drugs (r2 = 0.92, p < 0.005), suggesting a common mechanism. In summary, the degree of drug resistance and the extent of verapamil chemosensitization for a particular drug seem to be dependent on general physical features such as lipid solubility and molar refractivity rather than on closely defined structural parameters. These studies provide insight into this important resistance mechanism of malaria parasites and may provide direction for the development of new drugs that are effective against resistant parasites.

Animals↗
PubMed1995

Current views on the mechanisms of resistance to quinoline-containing drugs in Plasmodium falciparum.

The issue of chloroquine resistance in Plasmodium falciparum and cross-resistance patterns with other related chemotherapeutic agents has been the subject of intense interest for many years. Despite this level of investigation, the picture remains very unclear. Although it is accepted that chloroquine resistance is, at least in part, a function of reduced drug accumulation, the question of reduced drug uptake versus enhanced efflux is yet to be resolved at both the molecular and biochemical levels. Further, the absolute cross-resistance patterns of chloroquine-resistant isolates to closely related analogues is a matter for debate, although there appears to be a reciprocal arrangement between resistance to chloroquine and resistance to mefloquine, halofantrine and possibly quinine. Evidence is presented for the coexistence of two or more chloroquine-resistance mechanisms in isolates of P. falciparum, only one of which is verapamil sensitive. In addition, an analysis of cross-resistance patterns, as measured by the inoculum effect, is presented.

Animals↗
PubMed1995

Parasite viability during treatment of severe falciparum malaria: differential effects of artemether and quinine.

The effect of artemether (AR) and quinine (QN) on parasite viability ex vivo was compared in children being treated for severe Plasmodium falciparum malaria. Parasitized blood taken at intervals during treatment was cultured in vitro, and parasite development was assessed microscopically. Parasite viability (defined as the proportion of circulating rings developing to early schizonts) was 56.8% in the AR group (n = 7) 6 hr after the start of treatment, compared with 93.3% for QN (n = 6; P = 0.015). Even after 24 hr of QN treatment, parasite viability was not significantly reduced in five patients. These ex vivo findings, which confirm previous observations of the stage-specific effects of these drugs against P. falciparum, suggest that AR may be superior to QN in the treatment of severe malaria.

Animals↗
PubMed1994

Relationship of global chloroquine transport and reversal of resistance in Plasmodium falciparum.

Control of falciparum malaria has become almost impossible in many areas due to the development of resistance to chloroquine and other antimalarial drugs. Verapamil and a number of unrelated compounds which chemosensitise multi-drug resistant cancer cells also enhance chloroquine susceptibility in Plasmodium falciparum. Chloroquine is accumulated to lower levels in resistant plasmodia, hence the reversal of chloroquine resistance has been attributed to the ability of chemosensitising agents to increase the amount of chloroquine accumulated by the resistant parasite. We have conducted a detailed examination of the effect of verapamil on chloroquine sensitivity and its relationship to chloroquine accumulation. The ability of verapamil to increase steady-state chloroquine accumulation was found to be totally insufficient to explain the increase in chloroquine sensitivity caused by the drug. In contrast, when chloroquine accumulation was increased by raising the pH gradient, the corresponding shifts in sensitivity to chloroquine could be accurately predicted. These results were confirmed with other classes of chemosensitisers and we conclude that an alternative mechanistic explanation is required to completely explain the reversal of chloroquine resistance in P. falciparum.

Animals↗
PubMed1993

Malaria chemotherapy: resistance to quinoline containing drugs in Plasmodium falciparum.

Resistance to quinoline containing drugs, particularly chloroquine (CQ), is a major impediment to the successful chemotherapy and prophylaxis of malaria. CQ-resistant parasites fail to accumulate as much drug as their sensitive counterparts and two major hypotheses have been proposed to account for this phenomenon. CQ-resistant parasites are thought to maintain lower intracellular drug levels by means of an active efflux system, similar to that found in multi-drug resistant cancer cells, despite major differences in both the genetic and biochemical manifestations of drug resistance in the two cell types. Alternatively, CQ-resistance could be linked to a defective CQ uptake mechanism, possibly an impaired acidification process in the food vacuole of the resistant parasite. These two theories are discussed in detail in the following review. The potential of pharmacological intervention to override these resistance mechanisms is also discussed.

Animals↗
PubMed1993

Enhancement of drug susceptibility in Plasmodium falciparum in vitro and Plasmodium berghei in vivo by mixed-function oxidase inhibitors.

A number of compounds, as exemplified by verapamil and desipramine, have been shown to enhance the susceptibility of resistant malaria parasites to chloroquine. The mechanism by which these agents reverse resistance is still controversial but is though to involve alterations in drug transport causing an increase in steady-state drug concentrations. We have proposed that an alternative resistance mechanism may involve the metabolic deactivation of the drug in some resistant parasites via cytochrome P-450 mixed-function oxidases. If the hypothesis is true, it should be possible to alter drug susceptibility in malaria parasites by the use of agents known to inhibit or induce cytochrome P-450 activities. We have assessed the ability of known inhibitors of cytochrome P-450 enzymes (cimetidine, metyrapone, and alpha-naphthoflavone) to enhance chloroquine susceptibility in Plasmodium falciparum culture-adapted and wild-type isolates in vitro and P. berghei in vivo. In all three systems, the inhibitor cimetidine enhanced parasite susceptibility to chloroquine, and this increase in susceptibility was unrelated to changes in chloroquine steady-state concentrations in vitro or to alterations in host pharmacokinetics in vivo. Additionally, the cytochrome P-450 inducer phenobarbital produced slight decreases in P. falciparum drug susceptibility in vitro. We have compared the ability of the cytochrome P-450 inhibitors cimetidine and metyrapone to enhance drug susceptibility with that of verapamil by using wild-type malaria isolates obtained from Cameroon. Verapamil completely reversed resistance, i.e., to below the cutoff point of 70 nM, in all the resistant isolates. Cimetidine enhanced chloroquine susceptibility in 60% of the isolates and reduced 50% inhibitory concentrations by at least 43% in all the resistant isolates. The compounds tested had little or no effect on the 50% inhibitory concentrations for the susceptible isolates. The data support a possible role for detoxification in chloroquine resistance, and even in the absence of such a process we have observed apparent chemosensitization by agents whose common biological feature is the inhibition of cytochrome P-450 enzymes. Additionally, sensitization has been observed in wild-type isolates of P. falciparum obtained form immune residents of an area of endemicity as well as culture-adapted parasites.

Adolescent↗
PubMed1992

Rapid chloroquine efflux phenotype in both chloroquine-sensitive and chloroquine-resistant Plasmodium falciparum. A correlation of chloroquine sensitivity with energy-dependent drug accumulation.

Recent reports suggest that lower levels of chloroquine accumulation in chloroquine-resistant isolates of Plasmodium falciparum are achieved by energy-dependent chloroquine efflux from resistant parasites. In support of this argument, a rapid chloroquine efflux phenotype has been observed in some chloroquine-resistant isolates of P. falciparum. In this study, no relationship was found between chloroquine sensitivity and the rate of [3H]chloroquine efflux from four isolates of P. falciparum with a greater than 10-fold range in sensitivity to chloroquine. All the isolates tested displayed the rapid efflux phenotype, irrespective of sensitivity. However, chloroquine sensitivity of these isolates was correlated with energy-dependent rate of drug accumulation into these parasites. Verapamil and a variety of other compounds reverse chloroquine resistance. The reversal mechanism is assumed to result from competition between verapamil and chloroquine for efflux protein translocation sites, thus causing an increase in steady-state accumulation of chloroquine and hence a return to sensitivity. Verapamil accumulation at a steady-state is increased by chloroquine, possibly indicating competition for efflux of the two substrates. Increases in steady-state verapamil concentrations caused by chloroquine were identical in sensitive and resistant strains, suggesting that similar capacity efflux pumps may exist in these isolates. These data suggest that differences in steady-state chloroquine accumulation seen in these isolates can be attributed to changes in the chloroquine concentrating mechanism rather than the efflux pump. It seems likely that chloroquine resistance generally in P. falciparum, results at least in part from a change in the drug concentrating mechanism and that changes in efflux rates per se are insufficient to explain chloroquine resistance.

Animals↗
PubMed1992

Vacuolar acidification and chloroquine sensitivity in Plasmodium falciparum.

The antimalarial chloroquine concentrates in the acid vesicles of Plasmodium falciparum partially as a result of its properties as a weak base. Chloroquine-resistant parasites accumulate less drug than sensitive parasites. A simple hypothesis is that the intravacuolar pH of resistant strains is higher than that for sensitive strains, as a consequence of a weakened proton pump in the vacuoles of resistant strains, thereby explaining the resistance mechanism. We have attempted to test this hypothesis by the use of bafilomycin A1, a specific inhibitor of vacuolar proton pumping ATPase systems in plant cells, animal cells and microorganisms. Bafilomycin A1 significantly reduces uptake of [3H]chloroquine into both chloroquine-sensitive and -resistant strains of P. falciparum, at concentrations of inhibitor which have no antimalarial effect. Additionally, chloroquine-resistant strains of P. falciparum are more sensitive to bafilomycin A1 than chloroquine-sensitive strains. The use of bafilomycin A1 in combination with chloroquine in the standard in vitro sensitivity assay, produced an apparent reduction in sensitivity of both strains to chloroquine. The reported data support the hypothesis that chloroquine resistance in P. falciparum is associated with increased vacuolar pH, possibly due to a weakened vacuolar proton pumping ATPase.

Animals↗
PubMed1983

The mechanisms of amino acid uptake by Brugia pahangi in vitro.

The transcuticular uptake of L-glycine and L-arginine by adult female Brugia pahangi has been shown to involve both a saturable carrier transport system and diffusion. The uptake of L-glycine was competitive with regard to L-methionine, L-valine, and L-phenylalanine but noncompetitive with L-arginine. It is suggested that at least two distinct loci are involved in the transcuticular movement of amino acids in this filarial nematode.

Animals↗
PubMed1983

The mode of action of suramin on the filarial worm Brugia pahangi.

The mode of action of suramin upon Brugia pahangi has been investigated in vivo and in vitro. The drug was without effect on the glycolytic activity of worms in vitro at 2 X 10(-4) M. The lack of effect was correlated with the failure of [14C]suramin to penetrate the worms in vitro. Suramin bound to the surface of worms in vitro presumably by virtue of its polyanionic nature. B. pahangi adults ingested [14C]suramin in vivo but no reduction in the rate of lactate production, of glucose utilization or in the rates of uptake of [14C]glucose, [14C]leucine or [14C]adenosine was observed in worms recovered from jirds between weeks 1 and 5 following 4 daily doses of suramin at 50 mg/kg given intraperitoneally. Worm death occurred between weeks 5 and 7 but this delayed drug effect was not the result of a progressive accumulation of suramin in the worms. Ultrastructural changes were observed in the intestinal epithelium of worms from suramin-treated jirds and parallel observations on worms exposed to Trypan blue in vivo suggest that both polyanionic compounds are restricted to the intestinal lumen of the worms. The evidence presented is consistent with the concept that, in B. pahangi, suramin acts at the surface of the intestinal epithelium and not by primarily inhibiting glucose catabolism or inhibiting phagosome and lysosome fusion as previously demonstrated for bloodstream trypanosomes and mammalian macrophages, respectively.

Adenosine↗