Antimalarial Halofantrine Research Articles

Impact of chemoprophylaxis immunisation under halofantrine (CPS-HF) drug cover in Plasmodium yoelii Swiss mice malaria model.

In the present study, we have investigated the role of antimalarial drug halofantrine (HF) in inducing the sterile protection against challenges with sporozoites of the live infectious Plasmodium yoelii (Killick-Kendrick, 1967) in Swiss mice malaria model. We observed that during the first to third sequential sporozoite inoculation cycles, blood-stage patency remains the same in the control and chemoprophylaxis under HF drug cover (CPS-HF) groups. However, a delayed blood-stage infection was observed during the fourth and fifth sporozoite challenges and complete sterile protection was produced following the sixth sporozoite challenge in CPS-HF mice. We also noticed a steady decline in liver stage parasite load after 3th to 6th sporozoite challenge cycle in CPS-HF mice. CPS-HF immunisation results in a significant up-regulation of pro-inflammatory cytokines (IFN-γ, TNF-α, IL-12 and iNOS) and down-regulation of anti-inflammatory cytokines (IL-10 and TGF-β) mRNA expression in hepatic mononuclear cells (HMNC) and spleen cells in the immunised CPS-HF mice (after 6th sporozoite challenge) compared to control. Overall, our study suggests that the repetitive sporozoite inoculation under HF drug treatment develops a strong immune response that confers protection against subsequent challenges with sporozoites of P. yoelii.

A Differential Study on the Favourable Nanoparticle Formulation of the Antimalarial Drug Halofantrine

A Differential Study on the Favourable Nanoparticle Formulation of the Antimalarial Drug Halofantrine

Cardiac TdP risk stratification modelling of anti-infective compounds including chloroquine and hydroxychloroquine.

Hydroxychloroquine (HCQ), the hydroxyl derivative of chloroquine (CQ), is widely used in the treatment of rheumatological conditions (systemic lupus erythematosus, rheumatoid arthritis) and is being studied for the treatment and prevention of COVID-19. Here, we investigate through mathematical modelling the safety profile of HCQ, CQ and other QT-prolonging anti-infective agents to determine their risk categories for Torsade de Pointes (TdP) arrhythmia. We performed safety modelling with uncertainty quantification using a risk classifier based on the qNet torsade metric score, a measure of the net charge carried by major currents during the action potential under inhibition of multiple ion channels by a compound. Modelling results for HCQ at a maximum free therapeutic plasma concentration (free Cmax) of approximately 1.2 µM (malaria dosing) indicated it is most likely to be in the high-intermediate-risk category for TdP, whereas CQ at a free Cmax of approximately 0.7 µM was predicted to most likely lie in the intermediate-risk category. Combining HCQ with the antibacterial moxifloxacin or the anti-malarial halofantrine (HAL) increased the degree of human ventricular action potential duration prolongation at some or all concentrations investigated, and was predicted to increase risk compared to HCQ alone. The combination of HCQ/HAL was predicted to be the riskiest for the free Cmax values investigated, whereas azithromycin administered individually was predicted to pose the lowest risk. Our simulation approach highlights that the torsadogenic potentials of HCQ, CQ and other QT-prolonging anti-infectives used in COVID-19 prevention and treatment increase with concentration and in combination with other QT-prolonging drugs.

Halofantrine metabolism in microsomes in man: major role of CYP 3A4 and CYP 3A5.

We have clarified the contribution of the different enzymes involved in the N-debutylation of halofantrine in liver microsomes in man. The effect of ketoconazole and cytochrome P450 (CYP) 3A substrates on halofantrine metabolism has also been studied. The antimalarial drug halofantrine is metabolized into one major metabolite, N-debutylhalofantrine. In microsomes from nine livers from man, N-debutylation of halofantrine was highly variable with apparent Michaelis-Menten constant V(max) and K(m) values of 215+/-172 pmol min(-1) mg(-1) and 48+/-26 micromol L(-1), respectively, (mean+/-standard deviation). Formation of N-debutylhalofantrine was cytochrome P450 (CYP)-mediated. Studies using selective inhibitors of individual CYPs revealed the role of CYP 3As in the formation of N-debutylhalofantrine. alpha-Naphthoflavone, a CYP 3A activator, increased metabolite formation. In microsomes from 12 livers from man the rate of N-debutylation of halofantrine correlated strongly with CYP 3A4 relative levels (P = 0.002) and less strongly, but significantly, with CYP 2C8 levels (P = 0.025). To characterize CYP-mediated metabolism of halofantrine further, incubations were performed with yeast microsomes expressing specific CYP 3A4, CYP 3A5, CYP 2D6, CYP 2C8 and CYP 2C19 from man. The rate of formation of N-debutylhalofantrine was six- and twelvefold with CYP 3A4 than with CYP 3A5 and CYP 2C8, respectively. CYP 2D6 and CYP 2C19 did not mediate the N-debutylation of halofantrine, but, because in-vivo CYP 2C8 is present at lower concentrations than CYP 3A in the liver in man, the involvement of CYP 3As would be predominant. Diltiazem, erythromycin, nifedipine and cyclosporin (CYP 3A substrates) inhibited halofantrine metabolism. Similarly, ketoconazole inhibited, non-competitively, formation of N-debutylhalofantrine with an inhibition constant, K(i), of 0.05 microM. The theoretical percentage inhibition of halofantrine metabolism in-vivo by ketoconazole was estimated to be 99%. These results indicate that both CYP 3A4 and CYP 3A5 metabolize halofantrine, with major involvement of CYP 3A4. In-vivo, the other CYPs have a minor role only. Moreover, strong inhibition, and consequently increased halofantrine cardiotoxicity, might occur with the association of ketoconazole or other CYP 3A4 substrates.

Structural analysis of the antimalarial drug halofantrine by means of Raman spectroscopy and density functional theory calculations

The structure of the antimalarial drug halofantrine is analyzed by means of density functional theory (DFT) calculations, IR, and Raman spectroscopy. Strong, selective enhancements of the Raman bands of halofantrine at 1621 and 1590 cm(-1) are discovered by means of UV resonance Raman spectroscopy with excitation wavelength lambda(exc)=244 nm. These signal enhancements can be exploited for a localization of small concentrations of halofantrine in a biological environment. The Raman spectrum of halofantrine is calculated by means of DFT calculations [B3LYP/6-311+G(d,p)]. The calculation is very useful for a thorough mode assignment of the Raman bands of halofantrine. The strong bands at 1621 and 1590 cm(-1) in the UV Raman spectrum are assigned to combined C[Double Bond]C stretching vibrations in the phenanthrene ring of halofantrine. These bands are considered as putative marker bands for pipi interactions with the biological target molecules. The calculation of the electron density demonstrates a strong distribution across the phenanthrene ring of halofantrine, besides the electron withdrawing effect of the Cl and CF(3) substituents. This strong and even electron density distribution supports the hypothesis of pipi stacking as a possible mode of action of halofantrine. Complementary IR spectroscopy is performed for an investigation of vibrations of polar functional groups of the halofantrine molecule.

Effect of fluconazole on the pharmacokinetics of halofantrine in healthy volunteers

Halofantrine, an antimalarial drug used in endemic areas such as the tropics is mainly metabolized by CYP3A4 to the active metabolite N-desbutylhalofantrine. Fluconazole, an antifungal agent and an inhibitor of CYP3A4 is an established part of the therapy in HIV patients, who in turn are prone to malaria in the tropics. This study investigated the effect of fluconazole on the pharmacokinetics of halofantrine after concurrent administration of the two drugs. The effect of fluconazole on the pharmacokinetics of the antimalarial drug halofantrine was evaluated in 15 healthy volunteers in a Latin Square crossover design. The subjects received a single oral dose of 500 mg halofantrine hydrochloride alone or with 50 mg fluconazole after an overnight fast. Venous blood samples were collected during the next 336 h and analysed by HPLC for halofantrine and its active metabolite N-desbutylhalofantrine. Co-administration of fluconazole did not alter the pharmacokinetics of halofantrine significantly with the exception of elimination t(1/2) that was significantly increased by 25% (P < 0.05). In contrast, fluconazole significantly altered the pharmacokinetic parameters of the active metabolite by reducing C(max), AUC and metabolite ratio (N-desbutylhalofantrine/halofantrine) between 35 and 41% (P < 0.05) while increasing t(max) by 50%. The 90% confidence intervals of the ratio of the geometric means (with/without fluconazole) were contained within 80-125% for halofantrine but outside this range for N-desbutylhalofantrine. The decreased plasma concentrations of the metabolite are presumably caused by metabolic inhibition of CYP3A4 by fluconazole. Although the therapeutic consequences of this interaction are not clear caution should be exercised when co-administering both drugs to avoid accumulation and subsequent cardiotoxic effects of halofantrine.

Polymorphisms within PfMDR1 alter the substrate specificity for anti‐malarial drugs in Plasmodium falciparum

Resistance to several anti-malarial drugs has been associated with polymorphisms within the P-glycoprotein homologue (Pgh-1, PfMDR1) of the human malaria parasite Plasmodium falciparum. Pgh-1, coded for by the gene pfmdr1, is predominately located at the membrane of the parasite's digestive vacuole. How polymorphisms within this transporter mediate alter anti-malarial drug responsiveness has remained obscure. Here we have functionally expressed pfmdr1 in Xenopus laevis oocytes. Our data demonstrate that Pgh-1 transports vinblastine, an established substrate of mammalian MDR1, and the anti-malarial drugs halofantrine, quinine and chloroquine. Importantly, polymorphisms within Pgh-1 alter the substrate specificity for the anti-malarial drugs. Wild-type Pgh-1 transports quinine and chloroquine, but not halofantrine, whereas polymorphic Pgh-1 variants, associated with altered drug responsivenesses, transport halofantrine but not quinine and chloroquine. Our data further suggest that quinine acts as an inhibitor of Pgh-1. Our data are discussed in terms of the model that Pgh-1-mediates, in a variant-specific manner, import of certain drugs into the P. falciparum digestive vacuole, and that this contributes to accumulation of, and susceptibility to, the drug in question.

The crystal structure of halofantrine–ferriprotoporphyrin IX and the mechanism of action of arylmethanol antimalarials

The crystal structure of the complex formed between the antimalarial drug halofantrine and ferriprotoporphyrin IX (Fe(III)PPIX) has been determined by single crystal X-ray diffraction. The structure shows that halofantrine coordinates to the Fe(III) center through its alcohol functionality in addition to π-stacking of the phenanthrene ring over the porphyrin. The length of the Fe(III)–O bond is consistent with an alkoxide and not an alcohol coordinating group. The iron porphyrin is five coordinate and monomeric. Changes in the electronic spectrum of Fe(III)PPIX upon addition of halofantrine base in acetonitrile solution are almost identical to those observed upon addition of quinidine free base in the same solvent. This suggests homologous binding. Molecular mechanics modeling of Fe(III)PPIX complexes of quinidine, quinine, 9-epiquinine and 9-epiquinidine based on this homology suggests that the antimalarially active quinidine and quinine can readily adopt conformations that permit formation of an intramolecular salt bridge between the protonated quinuclidine tertiary amino group and unprotonated heme propionate group, while the inactive epimers 9-epiquinidine and 9-epiquinine have to adopt high energy conformations in order to accommodate such salt bridge formation. We propose that salt bridge formation may interrupt formation of the hemozoin precursor dimer formed during the heme detoxification pathway and so account for the strong activity of the two active isomers.

Effect of kolanut on the pharmacokinetics of the antimalarial drug halofantrine

To study the effect of the concomitant consumption of kolanut, a caffeine-containing nut, on the pharmacokinetics of halofantrine. A single dose of 500 mg halofantrine hydrochloride was orally administered either alone or concomitant with 12.5 g kolanut to 15 healthy male volunteers in a Latin-square randomized crossover design with a wash-out period of 6 weeks between treatments. Blood samples were collected and analyzed by HPLC for halofantrine and the active metabolite N-desbutylhalofantrine. Concomitant intake of kolanut with halofantrine significantly decreased C(max) and AUC of both halofantrine and the metabolite desbutylhalofantrine, while no significant effect was observed for t (max) and t(1/2) of the compounds. In the case of halofantrine, C(max) decreased from 179 +/- 119 to 98 +/- 32 ng/ml, and the AUC was reduced from 17,450 +/- 4,611 to 11,821 +/- 4,069 ng x h/ml. C(max) of desbutylhalofantrine decreased from 124 +/- 41 to 62 +/- 23 ng/ml and the AUC from 13,341 +/- 4,749 to 7,359 +/- 3,018 ng x h/ml when kolanut was co-administered. Co-administration of halofantrine and kolanut caused a significant decrease in the plasma concentrations of halofantrine and the active metabolite desbutylhalofantrine probably during adsorption of the drug due to complex formation. This indicates that caution should be exerted when the drug is taken together with caffeine-containing nutrients.

Surface-Modified and Conventional Nanocapsules as Novel Formulations for Parenteral Delivery of Halofantrine

The aim of this work was to develop a stable injectable formulation of the antimalarial drug halofantrine (Hf) based on nanocapsules (NC) prepared from biodegradable polymers with Miglyol 810N as the oily core. Poly(D,L-lactide) PLA and its copolymers with poly(ethyleneglycol) (PLA-PEG) were used together with the surfactants poloxamer 188 and lecithin to yield NC with different surface properties. Highly efficient loading of the free base form of Hf was obtained; zeta potential measurements indicated that a part of the associated Hf was at the NC surface, interacting with the lecithin. NC were 150-250 nm in diameter and more stable on storage than nanoemulsions formed from oil and lecithin without polymer. The most stable NC, showing minimal size changes and flocculation, were those with a high density of 20-kDa PEG chains covalently grafted at the surface. Hf release from NC occurred mainly by partition with the external medium. In PBS, even when Tween 80 was added, release was limited to 20% of the total content, whatever the formulation. Addition of serum to the medium allowed complete and rapid release from PLA NC stabilized with adsorbed poloxamer 188, because of the high affinity of Hf for lipoproteins. However, the presence of covalently grafted PEG chains at the surface limited release by providing a hydrophilic steric barrier at the particle surface. A dense coverage with long PEG chains provided the best reduction of release. Such systems could constitute a long-circulating intravenous formulation of Hf for treating severe malaria.

Analysis of the antimalarial drug halofantrine and its major metabolite N-desbutylhalofantrine in human plasma by high performance liquid chromatography

The determination of halofantrine and its major metabolite N-desbutylhalofantrine in human plasma by reversed phase high-pressure liquid chromatography is described. The method involves protein precipitation of plasma samples by acetonitrile followed by basification with sodium hydroxide and subsequent liquid–liquid extraction using hexane-diethyl ether (1:1, v/v). The chromatographic separation was carried out on a C-18 column with a mobile phase consisting of methanol/0.05 M KH 2PO 4 (78:22, v/v) containing 55 mM perchloric acid. Chlorprothixen was used as internal standard. The relative standard deviations of intraday and interday precision for both compounds were less than 7%, the relative standard deviation of the accuracy did not exceed 7.1% at concentrations of 50 and 300 ng/ml. This method is simple, rapid, sensitive and cost effective and was applied to the determination of the pharmacokinetics of halofantrine and N-desbutylhalofantrine in two healthy male volunteers after an oral administration of 500 mg halofantrine. Moreover, the influence of the frequently consumed kolanut on the pharmacokinetics of halofantrine was investigated.

Polymeric Nanostructures for Drug Delivery: Characterization by Atomic Force Microscopy

Formal definitions of nanotechnological devices for drug delivery typically feature the requirements that the device itself or its essential components be man-made, and in the 1-1000 nm range in at least one dimension [1]. The known nanovectors or nanostructures can be filled with drugs for different therapies and for diagnostical aims. Targeting moieties can also be attached to their surface. Polymeric nanovectors are generally made from biodegradable polymers such as polyesters, for example, poly-e-caprolactone (PCL). The drug delivery system known as nanocapsules (NCs) can be defined as a complex nanovector that is composed by a polymeric wall surrounding an oil core, where lipophilic drugs can be encapsulated. The advantages of NCs compared to other nanovectors are the high entrapment efficiencies of lipophilic drugs, low polymer content and low inherent toxicity. On the other hand, because of its complex blend of components NCs suspension allow several forms of nanovectors to be present at the same time, such as nanospheres, liposomes and nanoemulsions [2]. These ‘contaminants’ would be present in accordance with the type of formulation and method of preparation. Atomic force microscopy (AFM) has been used as a method for imaging the surfaces of liposomes [3] and nanospheres [4] allowing information in nanoscaled dimensions. In the present work, the NCs were prepared loading two different drugs, the antifungal albaconazole (ABZ), showing a crystalline drug structure and the antimalarial halofantrine (Hf) free base, having an amorphous form. These drugs possess high lipophilic character, which favours the association of the drug with the oily core, with drug loadings above 94%. Herein we studied the behavior of ABZ-loaded and Hf-loaded NCs through the AFM technique, searching to analyze and understand possible alterations induced by the drug inclusion in these nanostructures.

Interaction of the antimalarial agents halofantrine and lumefantrine with lipid bilayers.

The effects of antimalarial drugs halofantrine and lumefantrine on the fluoresence anisotropy of diphenylhexatriene (DPH)-containing phospholipid vesicles have been examined. Lumefantrine increases DPH anisotropy, indicating a condensing effect on bilayers of dipalmitoylphosphatidyl choline (DPPC), dioleoylphosphatidylcholine (DOPC), egg lecithin and mouse erythrocyte membranes (including membranes isolated from plasmodial-infected mice). Its condensing effect is more pronounced in bilayers of lower microviscosity. In contrast, increases or decreases in DPH anisotropy are observed with halofantrine, depending on the nature of the lipid. Decreases in anisotropy, which reflect a perturbing effect, are observed in bilayers of high microviscosity (for example, gel state of DPPC bilayers). Increases in anisotropy are observed in bilayers of low microviscosity (such as DOPC and egg lecithin bilayers). The perturbing effect of halofantrine is further confirmed by the increases in permeability of calcein-containing DPPC vesicles in the presence of the drug. However the perturbative effects of halofantrine are observed to the same magnitude in uninfected and plasmodial-infected erythrocyte membranes, and may not be relevant to the antimalarial action of the drug. In contrast, the condensing effect of lumefantrine is significantly greater in infected erythrocyte membranes and may contribute to its antimalarial action.

The anti-malarial drug halofantrine and its metabolite N-desbutylhalofantrine block HERG potassium channels.

The antimalarial drug halofantrine has been associated with QT interval prolongation and with fatal and nonfatal arrhythmias in patients without known underlying cardiac abnormalities. A common target for QT interval-prolonging drugs is the human ether-a-go-go gene (HERG) which encodes the pore forming subunit of the rapidly activating delayed rectifier K(+) current (I(Kr)). We studied the effects of halofantrine (0.1-1000 nM) and its major metabolite N-desbutylhalofantrine (3-1000 nM) on wild type HERG K(+) channels stably expressed in HEK 293 cells, using the whole cell patch-clamp recording technique. Halofantrine and N-desbutylhalofantrine blocked HERG K(+) channels in a concentration-dependent manner with a half-maximal inhibitory concentration of 21.6 nM (n=31 cells) and 71.7 nM (n=18 cells), respectively. The development of drug block for both halofantrine and N-desbutylhalofantrine required channel activation indicative of open and/or inactivated state block. Drug washout or cell hyperpolarization resulted in minimal current recovery consistent with virtually irreversible binding. Using a ventricular action potential voltage clamp protocol, halofantrine and N-desbutylhalofantrine block of HERG current was greatest during phases 2 and 3 of the action potential waveform. We conclude that both halofantrine and N-desbutylhalofantrine cause high affinity block of HERG K(+) channels. Although N-desbutylhalofantrine has been suggested to be a safer antimalarial agent compared to halofantrine, our results suggest that the gain in the safety margin for QT interval prolongation-related cardiotoxicity is minimal.

Effect of the antimalarial drug halofantrine in the long QT syndrome due to a mutation of the cardiac sodium channel gene SCN5A

T congenital long QT syndrome (LQTS) is characterized by prolongation of the QT interval as a consequence of a repolarization defect, risk of ventricular tachyarrhythmias (e.g., torsades de pointes), syncopal attacks, and sudden death.1 A variety of pharmacologic agents that delay ventricular repolarization by blocking the HERG channel responsible for the rapid component of the delayed rectifier potassium current IKr may increase the QT interval duration. The most prevalent forms of inherited LQTS are caused by mutated potassium channels KCNQ1 (type LQT1) or HERG (type LQT2), whereas mutations of the sodium channel SCN5A (type LQT3) as well as potassium channel beta-subunits minK (type LQT5) and MiRP (type LQT6) are rare causes of the syndrome.2 Mutations of SCN5A that lead to its inadequate inactivation result in the LQT3 phenotype.3 We describe here the first disease-causing mutation thus far localized in the S5 segment of the fourth SCN5A channel domain. In 2 of the mutation carriers, therapy with the antimalarial drug halofantrine induced a marked prolongation of the QTc interval and torsades de pointes, demonstrating that specific drug treatment may induce ventricular tachycardia in LQT3. • • • The present study represents an extension of our previous investigations in which we have been able to identify 6 different KCNQ14 (and unpublished observations) and 9 different HERG5,6 mutations in 60 Finnish LQTS families. In the present study, 30 unrelated Finnish probands with an autosomal dominant form of LQTS and with a hitherto undefined mutational background were examined. All of them had QTc values of .440 ms, were symptomatic, and had normal hearing. Genomic DNA was isolated from venous blood using standard methods. All patients gave their consent to the study, which was approved by the Ethical Review Committee of the Department of Medicine, University of Helsinki. Because the previously reported mutations appear to cluster in exons 23, 26, and 28 of the SCN5A gene,2 these exons were selected as targets for initial screening. The exon sequences were amplified by polymerase chain reaction using primers described previously7 and sequenced automatically as also previously described.6 Because of the large size of exon 28, only the first 725 N-terminal nucleotides, including transmembrane domains, were studied. Comparison of the SCN5A sequence to its homologs was performed using the BLAST2 sequence alignment program (http:// www.ncbi.nlm.nih.gov/gorf/wblast2.cgi). Student’s t test was used to compare mean QTc values. A nucleotide transition G4999A, predicted to substitute isoleucine (I) for valine (V) at codon 1667, was detected in 1 of the probands. The V1667I mutation is located in exon 28 of the SCN5A gene, corresponding to the highly conserved aminoterminal part of the IV/S5 domain of the sodium channel. The occurrence of the V1667I mutation was studied in the available family members of the proband and was found to be present in heterozygous form in 8 additional subjects (Figure 1), whereas it was absent in 6 remaining family members and in 50 healthy unrelated controls. The mean QTc interval of the V1667I carriers (450 6 30 ms) was significantly longer than that of noncarriers of the family (Figure 1). No mutations have been found in the KCNQ1, HERG, minK, or MiRP genes in this family. Two of 9 mutation carriers were symptomatic, with symptoms triggered by drug administration. The proband, a 16-year old male, had clinically established LQTS based on a history of syncope and prolonged QT interval, whereas his 40-year old mother had previously been asymptomatic. Clinical data of these 2 individuals have been previously reported.8 Both paFrom the Department of Medicine and Hospital for Children and Adolescents, Helsinki University Hospital, Helsinki, Finland. This study was supported by research grants from the Council for the Health Sciences of the Academy of Finland, the Sigrid Juselius Foundation, and the Finnish Foundation for Cardiovascular Research, Helsinki, Finland; the Research and Science Foundation of Farmos, Espoo, Finland; and the Emil Aaltonen Foundation, Tampere, Finland. Dr. Kontula’s address is: Department of Medicine, University of Helsinki, Haartmaninkatu 4, 00290 Helsinki, Finland. E-mail: Kimmo. Kontula@hus.fi. Manuscript received July 19, 2000; revised manuscript received and accepted October 16, 2000. FIGURE 1. Pedigree of the LQT3 family. Empty circles and/or squares: unaffected women and/or men; filled circles and/or squares: affected women and/or men. Subjects marked by an arrow had halofantrine-induced torsades de pointes. Corrected QT intervals (QTc in milliseconds), when available, are shown under individual symbol. Neither clinical nor genetic data were available from those labeled with a question mark.

Potentiation of halofantrine-induced QTc prolongation by mefloquine: correlation with blood concentrations of halofantrine.

1. The antimalarial drug halofantrine can prolong the QT interval and this may be enhanced by prior use of mefloquine. This possible interaction has been investigated by examining the effects of halofantrine and mefloquine alone and in combination. 2. In anaesthetized rabbits (n=6 per group), halofantrine given as bolus doses of 1, 3, 10, and 30 mg kg(-1) at 25 min intervals dose-dependently prolonged the rate-corrected QT (QTc) interval from 313+/-12 ms pre-drug to 410+/-18 ms after the highest dose. Similar doses of mefloquine did not alter QTc intervals significantly. The highest dose of mefloquine (30 mg kg(-1)) caused cardiac contractile failure. 3. Pretreatment with 3 mg kg(-1) mefloquine 25 min before the first dose of halofantrine potentiated the effects of all doses of halofantrine on QTc intervals. 4. The blood concentrations of halofantrine were two to six times higher in the group pretreated with mefloquine compared to the halofantrine alone group; e.g. 1.03+/-0.17 and 0.16+/-0.02 microM respectively after 1 mg kg(-1) halofantrine. There was a significant correlation between blood halofantrine concentrations and QTc intervals (r=0.673). Even after making allowance for overestimation of the potency of halofantrine that may result from the hypokalaemia that is prevalent in anaesthetized rabbits, these effects occurred with concentrations of halofantrine that are found in clinical use. 5. These data indicate clearly that while mefloquine does not alter QTc intervals itself, it does enhance the effects of halofantrine by increasing the circulating concentration of halofantrine.

Mechanism of cardiotoxicity of halofantrine.

To further evaluate the scope and mechanism of potential cardiotoxicity associated with the antimalarial drug halofantrine, case reports submitted to the US Food and Drug Administration Spontaneous Reporting System were examined. Because halofantrine was associated with electrocardiographic prolongation of the QT interval and ventricular arrhythmias, in vitro cardiac electrophysiologic studies (isolated perfused cardiac model and isolated ventricular myocytes) were conducted to test the hypothesis that halofantrine or its metabolite is responsible for cardiotoxicity. Although it is difficult to ascertain causality and to estimate overall incidence, a significant number of adverse events related to the cardiovascular system were reported, including QT interval prolongation, life-threatening arrhythmias, and sudden death. The effect of halofantrine and its active metabolite (N-desbutylhalofantrine) on repolarization were examined in an isolated perfused heart model. Results indicate that halofantrine was able to prolong the QT interval, whereas N-desbutylhalofantrine had minimal effect on the QT interval relative to baseline. In an attempt to further elucidate the mechanism of QT interval prolongation, the effects of racemic halofantrine, its stereoisomers, and N-desbutylhalofantrine on repolarizing currents in isolated ventricular myocytes were studied with use of patch-clamp techniques. Halofantrine produced a stereoselective block of the delayed rectifier potassium channel in isolated feline myocytes. These results indicate that halofantrine is similar to quinidine and class III antiarrhythmics in its ability to prolong repolarization. We conclude that high plasma concentrations of halofantrine should be avoided, especially in women, and that N-desbutylhalofantrine may have potential as a safer antimalarial drug.

The stereoselective distribution of halofantrine enantiomers within human, dog, and rat plasma lipoproteins.

To study the in vitro distribution of the enantiomers of the antimalarial drug halofantrine in human, dog and rat plasma lipoprotein-fractions. Plasma was spiked with racemic halofantrine (1,000 ng/ml) and incubated for 1 h at 37 degree C. The fractions (high and low density lipoproteins, triglyceride-rich lipoproteins and lipoprotein deficient plasma) were separated using density gradient ultracentrifugation. Fractions were assayed for halofantrine enantiomer using stereospecific high performance liquid chromatography. The (-) enantiomer of halofantrine displayed higher affinity for the lipoprotein-deficient fraction than the (+) enantiomer in all three species. The (+) enantiomer was predominately located in the lipoprotein rich fractions of dog and human plasma (the (+):(-) ratio ranging from 1.2-9.6). In contrast, the (+):(-) ratio was consistently < 1 in lipoprotein-deficient fractions. Dog displayed a large magnitude of stereoselectivity in halofantrine distribution to the plasma fractions tested. There were substantial interspecies differences in the pattern of distribution of halofantrine enantiomers within the different fractions. A significant positive relationship was observed between halofantrine uptake into lipoprotein-rich fractions and the percent of apolar core lipid in those fractions. There was also a strong negative correlation between total protein concentration and the enantiomeric ratio in the lipoprotein-deficient plasma fraction. Distribution of halofantrine enantiomer to plasma lipoprotein-fractions is stereoselective and species specific. This differential binding of halofantrine enantiomers to lipoproteins may need to be considered in viewing pharmacokinetic and pharmacodynamic data involving the drug.

In vitro effects of racemates, separate enantiomers and major metabolites of mefloquine and halofantrine on metoprolol biotransformation by rat liver microsomes

1. The effects of the anti-malarial drugs mefloquine and halofantrine and of their major metabolites on metoprolol metabolism by rat liver microsomes have been investigated. 2. The observed Km and Vmax, and the formation kinetics of alpha-hydroxymetoprolol and O-demethylmetoprolol, two major metoprolol metabolites, were in keeping with published data. 3. In vitro, mefloquine competitively inhibited metoprolol biotransformation, whereas halofantrine did so in a mixed fashion. The mefloquine Ki of metoprolol alpha-hydroxylation and O-demethylation were 3.4 and 5.8 muM respectively, whereas those of halofantrine were 0.15 and 0.32 muM respectively. 4. The main metabolites, N-debutylhalofantrine and carboxymefloquine, were 4-10- fold less inhibitory than the parent drugs. The difference in inhibitory potency of parent drugs and metabolites was higher for halofantrine than for mefloquine. The potency order for metoprolol metabolism inhibition was halofantrine mefloquine N-debutylhalofantrine carboxymefloquine. 5. A preliminary study with anti-malarial enantiomers showed a weak difference, in metoprolol metabolism inhibition between the enantiomers of halofantrine or mefloquine. 6. It is concluded that halofantrine is a potent inhibitor of metoprolol metabolism and that halofantrine metabolites or its enantiomers may have a different inhibitor potency than the parent drug: (1) the inhibition potency of these compounds should be studied in vitro and (2) their in vivo elimination half-life and plasma concentrations should be taken into be account to extrapolate this experimental results to in vivo.

Comparison of the acute cardiotoxicity of the antimalarial drug halofantrine in vitro and in vivo in anaesthetized guinea-pigs.

1. Several unrelated drugs have pro-arrhythmic activity associated with an ability to prolong the QT interval of the ECG. The aim of this work was to examine the effects of the antimalarial drug halofantrine in vivo and in vitro. 2. In anaesthetized guinea-pigs consecutive bolus doses of halofantrine (0.3, 1, 3, 10 and 30 mg kg(-1), i.v.) at 25 min intervals caused dose-dependent prolongation of the rate corrected QTc interval and bradycardia. The change in heart rate became significant after administration of 10 mg kg(-1) halofantrine (-23+/-9 beats min[-1]) whereas the increase in QTc was significant with only 1 mg kg(-1) halofantrine (22+/-10 ms). It was only with the highest dose of halofantrine that the PR interval was increased (from 52+/-3 to 67+/-4 ms) and second degree atrioventricular (AV) block (type 1 Mobitz) occurred in all animals. No changes were observed in any parameters in a separate group of guinea-pigs which received vehicle (dimethylacetamide 60% propylene glycol 40%) at equivalent time points. 3. The blood concentrations of halofantrine ranged from 0.26+/-0.17 microM after administration of 0.3 mg kg(-1) to 2.79+/-0.87 microM after 30 mg kg(-1), i.v. There was a significant correlation between the blood concentrations of halofantrine and the changes in QTc interval. 4 In guinea-pig left papillary muscles the effective refractory period was increased significantly 60 min after addition of halofantrine; from 161+/-4 to 173+/-6 ms with 10 microM, 156+/-8 to 174+/-6 ms with 30 microM and 165+/-6 to 179+/-5 ms with 100 microM halofantrine. However, the vehicle (0.1% Tween 80 in DMSO; final concentration of vehicle in Krebs, 1%) also increased the effective refractory period from 164+/-5 to 173+/-6 ms. Similar results were obtained in right ventricular strips but left atrial effective refractory periods were not altered by either the vehicle or halofantrine. 5. The results of these experiments suggest that any direct effects that halofantrine may have had on the effective refractory period of cardiac muscle cannot be separated from those of the vehicle. The prolongation of QTc and consistent observation of AV block with halofantrine in anaesthetized guinea-pigs suggest that in vivo models may be more useful for further studies investigating the mechanisms underlying the cardiotoxicity of halofantrine.