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

Abstract

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 thought 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 at-naphthoflavone) to enhance chloroquine susceptibility in Plasmodiumfalciparum 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%o inhibitory concentrations by at least 43% in all the resistant isolates. The compounds tested had little or no effect on the 50%o 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. fakciparum obtained from immune residents of an area of endemicity as well as culture-adapted parasites.The malaria status in most areas of the world in which malaria is endemic has failed to improve over the last decade, despite predictions and speculations to the contrary, particularly those fuelled by hopes of a vaccine remedy. Resistance to antimalarial drugs has become a progressive and persistent obstacle to chemotherapy. Chloroquine (CQ) resistance, in particular, has continued to spread quite dramatically, while knowledge of the mechanism of development of resistance to this drug still eludes researchers (36).CQ resistance has now been reported in almost all subSaharan African countries and is firmly established in a large number of them (8), placing a considerable bur...

Drug-Regulated Expression ofPlasmodium falciparumP-Glycoprotein Homologue 1: a Putative Role for Nuclear Receptors

“…This observation is unique and unexpected considering that the drug-selected parasite lines had lost the VP-sensitive CQR phenotype on selection for resistance to amantadine and halofantrine, respectively (14). The observation that pretreatment with PB results in an increased resistance to CQ confirms the observations made by Ndifor et al (21,22). The effect of PB on the growth rate of the parasite lines was determined by microscopic analysis.…”

“…It is well known that PB induces the expression of CYPs and ABC transporter proteins in mammals via nuclear receptor-dependent mechanisms (10, 11, 13, 16, 34-36, 42, 48). The changes in CQ susceptibility observed after PB treatment could feasibly be controlled by an increased metabolism of CQ by CYPs, a hypothesis originally put forward by Ndifor et al (21,22). However, it has been shown that CQ is not metabolized by malarial parasites (29,43).…”

Clinical Pharmacokinetics and Metabolism of Chloroquine

“…These preliminary data suggest that chloroquine deactivation may be partly responsible for the observed drug resistance. [138] 8. Stereoselective Pharmacokinetics and Pharmacodynamics…”

Poly ICLC Enhances the Antimalarial Activity of Chloroquine Against Multidrug-Resistant Plasmodium yoelii nigeriensis in Mice

“…In the CQ (4 mg/kg dose)-treated group, a slight prolongation of survival time was observed compared with control, as all the mice in this group died by day 11 postin¬ fection. In contrast, in the group of mice treated with a low dose of Poly ICLC (1.77 mg/kg dose) in combination with CQ (4 mg/kg dose), the initial onset of patent parasitemia was slowed, followed by a gradual increase in parasitemia, reaching a max¬ imum of 20%-25% on day [12][13][14][15][16][17][18] fNw nitro-L-arginine 13.2 mg/kg/day for 6 days.…”