The multiple dose pharmacokinetics of proguanil

Assessing the Roles of Molecular Markers of Antimalarial Drug Resistance and the Host Pharmacogenetics in Drug-Resistant Malaria

Malaria caused by the Plasmodium parasites is a major public health concern in malaria-endemic regions with P. falciparum causing the most severe form of the disease. The use of antimalarial drugs for the management of the disease proves to be one of the best methods to manage the disease. Unfortunately, P. falciparum has developed resistance to almost all the current in-use antimalarial drugs. Parasite development of resistance is primarily caused by both parasite and host genetic factors. The parasite genetic factors involve undergoing mutation in the drug target sites or increasing the drug target gene copy number to prevent the intended action of the antimalarial drugs. The host pharmacogenetic factors which determine how a particular antimalarial drug is metabolized could result in variations of drug plasma concentration and consequently contribute to variable treatment outcomes and the emergence or propagation of resistant parasites. Since both host and parasite genomes play a role in antimalarial drug action, a key question often asked is, “which of the two strongly drives or controls antimalarial drug resistance?” A major finding in our recent study published in the Malaria Journal indicates that the parasite's genetic factors rather than the host are likely to energize resistance to an antimalarial drug. However, others have reported contrary findings suggesting that the host genetic factors are the force behind resistance to antimalarial drugs. To bring clarity to these observations, there is the need for deciphering the major driving force behind antimalarial drug resistance through optimized strategies aimed at alleviating the phenomenon. In this direction, literature was systematically reviewed to establish the role and importance of each of the two factors aforementioned in the etiology of drug-resistant malaria. Using Internet search engines such as Pubmed and Google, we looked for terms likely to give the desired information which we herein present. We then went ahead to leverage the obtained information to discuss the globally avid aim of combating antimalarial drug resistance.

Formation of the diuretic chlorazanil from the antimalarial drug proguanil--implications for sports drug testing

Chlorazanil (Ordipan, N-(4-chlorophenyl)-1,3,5-triazine-2,4-diamine) is a diuretic agent and as such prohibited in sport according to the regulations of the World Anti-Doping Agency (WADA). Despite its introduction into clinical practice in the late 1950s, the worldwide very first two adverse analytical findings were registered only in 2014, being motive for an in-depth investigation of these cases. Both individuals denied the intake of the drug; however, the athletes did declare the use of the antimalarial prophylactic agent proguanil due to temporary residences in African countries. A structural similarity between chlorazanil and proguanil is given but no direct metabolic relation has been reported in the scientific literature. Moreover, chlorazanil has not been confirmed as a drug impurity of proguanil. Proguanil however is metabolized in humans to N-(4-chlorophenyl)-biguanide, which represents a chemical precursor in the synthesis of chlorazanil. In the presence of formic acid, formaldehyde, or formic acid esters, N-(4-chlorophenyl)-biguanide converts to chlorazanil. In order to probe for potential sources of the chlorazanil detected in the doping control samples, drug formulations containing proguanil and urine samples of individuals using proguanil as antimalarial drug were subjected to liquid chromatography-high resolution/high accuracy mass spectrometry. In addition, in vitro simulations with 4-chlorophenyl-biguanide and respective reactants were conducted in urine and resulting specimens analyzed for the presence of chlorazanil. While no chlorazanil was found in drug formulations, the urine samples of 2 out of 4 proguanil users returned findings for chlorazanil at low ng/mL levels, similar to the adverse analytical findings in the doping control samples. Further, in the presence of formaldehyde, formic acid and related esters, 4-chlorophenyl-biguanide was found to produce chlorazanil in human urine, suggesting that the detection of the obsolete diuretic agent was indeed the result of artefact formation and not of the illicit use of a prohibited substance.

Effects of metformin and other biguanides on oxidative phosphorylation in mitochondria

The biguanide metformin is widely prescribed for Type II diabetes and has anti-neoplastic activity in laboratory models. Despite evidence that inhibition of mitochondrial respiratory complex I by metformin is the primary cause of its cell-lineage-specific actions and therapeutic effects, the molecular interaction(s) between metformin and complex I remain uncharacterized. In the present paper, we describe the effects of five pharmacologically relevant biguanides on oxidative phosphorylation in mammalian mitochondria. We report that biguanides inhibit complex I by inhibiting ubiquinone reduction (but not competitively) and, independently, stimulate reactive oxygen species production by the complex I flavin. Biguanides also inhibit mitochondrial ATP synthase, and two of them inhibit only ATP hydrolysis, not synthesis. Thus we identify biguanides as a new class of complex I and ATP synthase inhibitor. By comparing biguanide effects on isolated complex I and cultured cells, we distinguish three anti-diabetic and potentially anti-neoplastic biguanides (metformin, buformin and phenformin) from two anti-malarial biguanides (cycloguanil and proguanil): the former are accumulated into mammalian mitochondria and affect oxidative phosphorylation, whereas the latter are excluded so act only on the parasite. Our mechanistic and pharmacokinetic insights are relevant to understanding and developing the role of biguanides in new and existing therapeutic applications, including cancer, diabetes and malaria.

In vitro effect of the antimalarial drug proguanil hydrochloride on viability and DNA damage in human peripheral blood lymphocytes

This study aimed to evaluate the effect of proguanil, a chemical substance used for treatment and prevention of malaria on viability and DNA integrity in human lymphocytes in vitro. Two different concentrations of proguanil obtained from the plasma concentrations were used: 130ng/ml used for prophylactic treatment and 520ng/ml used in treatment of malaria. Testing was done with and without metabolic activation. Viability of lymphocytes decreased in time and dose dependent manner. Comet assay parameters showed similar effects, indicating that some damage to DNA molecule can occur. Frequency of sister chromatid exchanges did not show significant deviation from the control samples. As for the proliferation kinetics no significant changes were noticed. Since majority of DNA damaging effect is induced after metabolic activation it is to be concluded that activity of proguanil is dependent upon the active metabolite cycloguanil and that monitoring should be conducted especially among frequent travellers.

Computational models to assign biopharmaceutics drug disposition classification from molecular structure

PURPOSE

We applied in silico methods to automatically classify drugs according to the Biopharmaceutics Drug Disposition Classification System (BDDCS).

MATERIALS AND METHODS

Models were developed using machine learning methods including recursive partitioning (RP), random forest (RF) and support vector machine (SVM) algorithms with ChemDraw, clogP, polar surface area, VolSurf and MolConnZ descriptors. The dataset consisted of 165 training and 56 test set molecules.

RESULTS

RF model 3, RP model 1, and SVM model 1 can correctly predict 73.1, 63.6 and 78.6% test compounds in classes 1, 2 and 3, respectively. Both RP and SVM models can be used for class 4 prediction. The inclusion of consensus analysis resulted in improved test set predictions for class 2 and 4 drugs.

CONCLUSIONS

The models can be used to predict BDDCS class for new compounds from molecular structure using readily available molecular descriptors and software, representing an area where in silico approaches could aid the pharmaceutical industry in speeding drugs to the patient and reducing costs. This could have significant applications in drug discovery to identify molecules that may have future developability issues.

Pharmacogenetics, drug-metabolizing enzymes, and clinical practice

The application of pharmacogenetics holds great promise for individualized therapy. However, it has little clinical reality at present, despite many claims. The main problem is that the evidence base supporting genetic testing before therapy is weak. The pharmacology of the drugs subject to inherited variability in metabolism is often complex. Few have simple or single pathways of elimination. Some have active metabolites or enantiomers with different activities and pathways of elimination. Drug dosing is likely to be influenced only if the aggregate molar activity of all active moieties at the site of action is predictably affected by genotype or phenotype. Variation in drug concentration must be significant enough to provide "signal" over and above normal variation, and there must be a genuine concentration-effect relationship. The therapeutic index of the drug will also influence test utility. After considering all of these factors, the benefits of prospective testing need to be weighed against the costs and against other endpoints of effect. It is not surprising that few drugs satisfy these requirements. Drugs (and enzymes) for which there is a reasonable evidence base supporting genotyping or phenotyping include suxamethonium/mivacurium (butyrylcholinesterase), and azathioprine/6-mercaptopurine (thiopurine methyltransferase). Drugs for which there is a potential case for prospective testing include warfarin (CYP2C9), perhexiline (CYP2D6), and perhaps the proton pump inhibitors (CYP2C19). No other drugs have an evidence base that is sufficient to justify prospective testing at present, although some warrant further evaluation. In this review we summarize the current evidence base for pharmacogenetics in relation to drug-metabolizing enzymes.

Atovaquone/proguanil: a review of its use for the prophylaxis of Plasmodium falciparum malaria

Atovaquone/proguanil is a fixed-dose combination tablet of two antimalarial agents and is highly effective for the prevention of Plasmodium falciparum malaria. In combination with proguanil, the ability of atovaquone to inhibit parasitic mitochondrial electron transport is markedly enhanced. Both atovaquone and proguanil are active against hepatic (pre-erythrocytic) stages of P. falciparum, thereby providing causal prophylaxis and eliminating the need to continue post-travel treatment beyond 7 days. Both agents are also active against erythrocytic stages of P. falciparum, thereby providing suppressive prophylaxis. Atovaquone/proguanil is highly effective against drug-resistant strains of P. falciparum, and cross-resistance has not been observed between atovaquone and other antimalarial agents. In comparative, randomised clinical trials, there were no cases of P. falciparum malaria in nonimmune adults, adolescents and children (>/=11 kg) visiting malaria-endemic regions for </=28 days and receiving atovaquone/proguanil (250/100 mg in adults and dosage based on bodyweight in children <40 kg) once daily. The efficacy for the prevention of P. falciparum malaria was estimated at 100% for atovaquone/proguanil and for mefloquine, and 70% for chloroquine plus proguanil. In individuals (>/=11 kg) from endemic regions who may carry some immunity to malaria (semi-immune), the prophylactic efficacy rating for atovaquone/proguanil based on placebo-controlled trials was 95-100%. Atovaquone/proguanil is generally well tolerated by both adults and children. The most common treatment-related adverse events in placebo-controlled trials were headache and abdominal pain, which occurred at a rate similar to that observed with placebo. Atovaquone/proguanil therapy was associated with significantly fewer gastrointestinal adverse events than chloroquine plus proguanil, and significantly fewer neuropsychiatric adverse events than mefloquine in nonimmune individuals. Significantly fewer recipients of atovaquone/proguanil discontinued treatment because of adverse events than individuals receiving chloroquine plus proguanil or mefloquine (p < 0.05).

CONCLUSION

Atovaquone/proguanil is a fixed-dose combination antimalarial tablet that provides effective prophylaxis of P. falciparum malaria, including drug-resistant strains. Both atovaquone and proguanil are effective against hepatic stages of P. falciparum, which means that treatment need only continue for 7 days after leaving a malaria-endemic region. Atovaquone/proguanil was generally well tolerated and was associated with fewer gastrointestinal adverse events than chloroquine plus proguanil, and fewer neuropsychiatric adverse events than mefloquine. Thus, atovaquone/proguanil provides effective prophylaxis of P. falciparum malaria and compared with other commonly used antimalarial agents has an improved tolerability profile, and, overall, a more convenient dosage regimen, particularly in the post-travel period.

Pharmacokinetic interactions of antimalarial agents

Combination of antimalarial agents has been introduced as a response to widespread drug resistance. The higher number of mutations required to express complete resistance against combinations may retard the further development of resistance. Combination of drugs, especially with the artemisinin drugs, may also offer complete and rapid eradication of the parasite load in symptomatic patients and thus reduce the chance of survival of resistant strains. The advantages of combination therapy should be balanced against the increased chance of drug interactions. During the last decade, much of the pharmacokinetics and metabolic pathways of antimalarial drugs have been elucidated, including the role of the cytochrome P450 (CYP) enzyme complex. Change in protein binding is not a significant cause of interactions between antimalarial agents. CYP3A4 and CYP2C19 are frequently involved in the metabolism of antimalarial agents. Quinidine is a potent inhibitor of CYP2D6, but it appears that this enzyme does not mediate the metabolism of any other antimalarial agent. The new combinations proguanil-atovaquone and chlorproguanil-dapsone do not show significant interactions. CYP2B6 and CYP3A4 are involved in the metabolism of artemisinin and derivatives, but further studies may reveal involvement of more enzymes. Artemisinin may induce CYP2C19. Several artemisinin drugs suffer from auto-induction of the first-pass effect, resulting in a decline of bioavailability after repeated doses. The mechanism of this effect is not yet clear, but induction by other agents cannot be excluded. The combination of artemisinin drugs with mefloquine and the fixed combination artemether-lumefantrine have been studied widely, and no significant drug interactions have been found. The artemisinin drugs will be used at an increasing rate, particularly in combination with other agents. Although clinical studies have so far not shown any significant interactions, drug interactions should be given appropriate attention when other combinations are used.

Molecular epidemiology of Plasmodium falciparum antifolate resistance in Vietnam: genotyping for resistance variants of dihydropteroate synthase and dihydrofolate reductase

Using PCR techniques, we analysed the dihydropteroate synthase (DHPS) mutations associated with sulphonamide resistance and the dihydrofolate reductase (DHFR) mutations associated with resistance to pyrimethamine and cycloguanil in samples from Plasmodium falciparum-infected Vietnamese patients. Of the 40 samples analysed, 39 had DHFR mutations associated with high level resistance to pyrimethamine, whereas only three had mutations at position 164, which is linked to cross resistance to both DHFR inhibitors. The DHPS, 437Gly variant associated with very mild resistance to sulphadoxine was found in 38 out of the 40 samples. Of seven samples resistant to Fansidar in vivo, only two were fully explained by the currently documented DHPS mutations. The treatment failure could be due to a high level of pyrimethamine resistance caused by the detected mutations. Most patients, however, were cured with a single dose of Fansidar in spite of the high number of resistance mutations found.

A mechanism for the synergistic antimalarial action of atovaquone and proguanil

A combination of atovaquone and proguanil has been found to be quite effective in treating malaria, with little evidence of the emergence of resistance when atovaquone was used as a single agent. We have examined possible mechanisms for the synergy between these two drugs. While proguanil by itself had no effect on electron transport or mitochondrial membrane potential (DeltaPsim), it significantly enhanced the ability of atovaquone to collapse DeltaPsim when used in combination. This enhancement was observed at pharmacologically achievable doses. Proguanil acted as a biguanide rather than as its metabolite cycloguanil (a parasite dihydrofolate reductase [DHFR] inhibitor) to enhance the atovaquone effect; another DHFR inhibitor, pyrimethamine, also had no enhancing effect. Proguanil-mediated enhancement was specific for atovaquone, since the effects of other mitochondrial electron transport inhibitors, such as myxothiazole and antimycin, were not altered by inclusion of proguanil. Surprisingly, proguanil did not enhance the ability of atovaquone to inhibit mitochondrial electron transport in malaria parasites. These results suggest that proguanil in its prodrug form acts in synergy with atovaquone by lowering the effective concentration at which atovaquone collapses DeltaPsim in malaria parasites. This could explain the paradoxical success of the atovaquone-proguanil combination even in regions where proguanil alone is ineffective due to resistance. The results also suggest that the atovaquone-proguanil combination may act as a site-specific uncoupler of parasite mitochondria in a selective manner.

Hepatitis during chloroguanide prophylaxis

OBJECTIVE

To describe a case of severe hepatitis that we attribute to the use of chloroguanide.

CASE SUMMARY

A patient was admitted with fever and jaundice. This man had recently returned from Indonesia and was still using chloroguanide 200 mg once daily for 6 weeks. Evaluation of the liver function test results was consistent with cholestatic liver disease (alkaline phosphatase 158 U/L, aspartate aminotransferase 33 U/L, alanine aminotransferase 58 U/L increasing up to 183 U/L, gamma-glutamyl transferase 96 U/L). This condition was thought to be caused by chloroguanide. A biopsy of the liver indicated drug-induced hepatitis.

DISCUSSION

To our knowledge, this is the first report in the literature of cholestatic hepatitis associated with chloroguanide prophylaxis. In our patient, chloroguanide-induced hepatotoxicity can be considered as a combined allergic and idiosyncratic manifestation.

CONCLUSIONS

This case shows that it is possible to develop drug-induced hepatitis with considerable morbidity while taking chloroguanide prophylaxis.

Efficacy and pharmacokinetics of atovaquone and proguanil in children with multidrug-resistant Plasmodium falciparum malaria

A trial was conducted in 32 Thai children with uncomplicated multidrug-resistant falciparum malaria to assess the efficacy, safety and pharmacokinetics of atovaquone and proguanil; plasma concentrations of atovaquone, proguanil and its metabolite, cycloguanil, were measured in a subset of 9 children. The children received atovaquone (17 mg/kg/d for 3 d) plus proguanil (7 mg/kg/d for 3 d). Twenty-six children who had only Plasmodium falciparum infection and remained in hospital for 28 d were assessed for drug efficacy. The combination regimen produced a cure rate of 100%. Parasite and fever clearance times were 47 h (range 8-75) and 50 h (range 7-111), respectively. Atovaquone and proguanil were rapidly absorbed, with median time to peak concentrations of 6 h (range 6-24) and 6 h (range 6-12), respectively. Peak concentrations of cycloguanil were achieved between 6 and 12 h (median 6) after administration of proguanil. Mean peak plasma concentration of atovaquone on day 3 was 5.1 micrograms/mL (SD = 2.1). The day 3 mean peak plasma concentration of proguanil was 306 ng/mL (SD = 108) compared with 44.3 ng/mL (SD = 27.3) for cycloguanil. Mean values for the AUC (area under plasma concentration-time curve) were 161.8 micrograms/mL.h (SD = 126.9) for atovaquone, 4646 ng/mL.h (SD = 1226) for proguanil, and 787 ng/mL.h (SD = 397) for cycloguanil. Terminal elimination half-lives of atovaquone, proguanil and cycloguanil were estimated as 31.8 h (SD = 8.9), 14.9 h (SD = 3.3) and 14.6 h (SD = 2.6), respectively. No major adverse effect was attributable to the study drugs. Atovaquone/proguanil combination is safe and highly effective, and should be especially valuable for treatment of multidrug-resistant falciparum malaria.

Ex vivo antimalarial activity of proguanil combined with dapsone against cycloguanil-resistant Plasmodium falciparum isolates

The ex vivo antimalarial activity of plasma samples obtained from 20 healthy Caucasian volunteers following daily proguanil (200 mg) plus dapsone (8 mg) for malaria chemoprophylaxis inhibited five cycloguanil-resistant Thai isolates of Plasmodium falciparum. All volunteers were phenotyped as extensive metabolisers (EMs) of proguanil. Three of the five isolates were obtained from Thai soldiers who had failed malaria prophylaxis on daily proguanil (200 mg) plus dapsone (4.0 or 12.5 mg). The Thai soldiers were also classified as EMs, but had relatively lower plasma cycloguanil concentrations compared to values reported in the literature for Caucasians and black Kenyans. Although the high level of parasite resistance to cycloguanil was the most likely explanation for the Thai soldiers failing prophylaxis on proguanil plus dapsone, their low cycloguanil concentrations may have also contributed to their lack of protection. However, in areas where parasites are more susceptible to cycloguanil, such as in certain regions of Africa, proguanil plus dapsone may still be an effective chemoprophylactic drug combination.

Population pharmacokinetics of proguanil in patients with acute P. falciparum malaria after combined therapy with atovaquone

1. The pharmacokinetics of proguanil were evaluated in patients with acute P. falciparum malaria receiving concomitantly proguanil hydrochloride and atovaquone. The population consisted of 203 Blacks, 112 Orientals and 55 Malays; 274 males and 96 females. Of the 370 patients, 114 and 256 patients were classified as 'poor' and 'extensive' metabolizers of proguanil, respectively. Body weight and age ranged between 11-110 kg and 3-65 years, respectively. 2. A one compartment model with first-order absorption and elimination was fitted to proguanil plasma concentration-time profiles, using non-linear mixed effect modelling (NONMEM). 3. Oral clearance (CLo) showed a 0.785 power relationship with body weight and was 13% higher in Orientals than Blacks and Malays and 17% lower in 'poor' than 'extensive' metabolizers. According to the mean weight of each population, the final population estimates of CLo in Blacks, Orientals and Malays who are 'extensive' metabolizers were 54.0, 61.5 and 64.3 l h-1, respectively. Age, gender and dose had no significant effects on CLo. 4. Apparent volume of distribution (V/F) showed a 0.88 power relationship with body weight. The final population estimates were 562 and 1629 l in children (< or = 15 years) and patients aged > 15 years, respectively, who had a mean body weight of 22.6 and 54.8 kg, respectively. The effect of other covariates on V/F was not examined. 5. The final magnitudes of interpatient variability in CLo and V/F were relatively low at 22.5 and 17.0%, respectively. 6. Population pharmacokinetic parameter estimates in Black, Oriental and Malay patients with acute P. falciparum malaria are in good agreement with results of pharmacokinetic studies in healthy Caucasian volunteers. In view of the 30-50% residual variability in proguanil plasma concentrations, the slight effects of Orientals and 'poor' metabolizers on CLo are unlikely to be clinically significant. Hence, dose recommendation will be solely based on body weight.

Proguanil polymorphism does not affect the antimalarial activity of proguanil combined with atovaquone in vitro

Clinical studies have shown proguanil (PROG) combined with atovaquone (ATQ) to be an effective and safe antimalarial combination for the treatment of multidrug-resistant falciparum malaria. PROG is a prodrug, which undergoes hepatic metabolism to its pharmacologically active metabolite cycloguanil (CYC). Individuals exhibit genetic polymorphism with respect to PROG, and can be phenotyped as either extensive metabolizers (EMs) or poor metabolizers (PMs) by measuring their PROG/CYC concentration ratio in plasma following PROG/ATQ administration. PMs produce lower plasma concentrations of CYC than EMs and thus may be more susceptible to prophylaxis or treatment failure. Both PROG and CYC potentiate the activity of ATQ in vitro. The antimalarial activity ex vivo of Thai patients' plasma samples obtained from EMs and PMs given concurrent PROG and ATQ was studied using the K1 isolate of Plasmodium falciparum. This isolate is resistant to PROG and CYC, but sensitive to ATQ. Maximum inhibitory dilution profiles of the patients' plasma samples containing PROG and ATQ from EMs and PMs were similar. These findings indicate that differences in plasma drug concentrations between EMs and PMs did not alter the antimalarial activity in vitro against the K1 isolate. The phenotypic status of individuals is not an important issue in the treatment of patients with PROG/ATQ.

Clinical pharmacokinetics in the treatment of tropical diseases. Some applications and limitations

In recent years major advances have been made in the clinical pharmacology of many drugs used for the treatment of tropical diseases, particularly in the design and development of dosage regimens for the treatment of severe malaria. For example, by careful manipulation of its rate of administration, chloroquine has been shown to be well tolerated when used for treatment of severe disease caused by susceptible parasites. Similarly, important advances have been made in the rational design of quinine dosage regimens for patients in South East Asia and Africa. Investigation of the pharmacokinetics of mefloquine has drawn attention to the problems associated with its administration as combination therapy with pyrimethamine and sulfadoxine in Thailand. Similarly, evaluation of the pharmacokinetic properties of halofantrine has led to the demonstration that poor and erratic absorption could be just as likely to explain therapeutic failure as resistance of the parasite to effects of this drug. Disposition of the antimalarial biguanides has highlighted the role of host-related effects in the determination of drug response. For example, a small percentage of individuals are unable to convert proguanil (chloroguanide) to its active triazine metabolite, cycloguanil. Finally, agents that reverse chloroquine resistance are currently under development for the treatment of malaria. The importance of assessing the clinical pharmacokinetic properties of potential resistance reversers must be recognised. While limited success has been achieved in antifilarial chemotherapy, other parasitic diseases have been largely neglected with advances in the laboratory still awaiting full recognition of their clinical application. For example, clinical pharmacokinetic concepts may be used to improve the therapy of human hydatid disease. We believe that clinical management of tropical diseases can be improved by the application of clinical pharmacokinetic principles. However, this may not be universally advantageous. For example, the artemisinin (qinghaosu) derivatives are among the most recently developed antimalarials that have great therapeutic promise. Recent evidence suggests that pharmacokinetic data would be of limited value in the design and optimisation of dosage regimens because of its chemical reactivity and the unusual mechanism by which these drugs kill parasites. Similar limitations may apply to the microfilaricidal drug, ivermectin.