Michael D. Edstein

First Assessment in Humans of the Safety, Tolerability, Pharmacokinetics, and Ex Vivo Pharmacodynamic Antimalarial Activity of the New Artemisinin Derivative Artemisone

ABSTRACT In preclinical studies, artemisone (BAY 44-9585), a new artemisinin derivative, was shown to possess enhanced efficacy over artesunate, and it does not possess the neurotoxicity characteristic of the current artemisinins. In a phase I program with double-blind, randomized, placebo-controlled, single and multiple ascending oral-dose studies, we evaluated the safety, tolerability, pharmacokinetics, and ex vivo pharmacodynamic antimalarial activity of artemisone. Single doses (10, 20, 30, 40, and 80 mg) and multiple doses (40 and 80 mg daily for 3 days) of artemisone were administered orally to healthy subjects. Plasma concentrations of artemisone and its metabolites were measured by liquid chromatography/tandem mass spectrometry (LC/MS-MS). Artemisone was well tolerated, with no serious adverse events and no clinically relevant changes in laboratory and vital parameters. The pharmacokinetics of artemisone over the 10- to 80-mg range demonstrated dose linearity. After the single 80-mg dose, artemisone had a geometric mean maximum concentration of 140.2 ng/ml (range, 86.6 to 391.0), a short elimination half-life (t1/2) of 2.79 h (range, 1.56 to 4.88), a high oral clearance of 284.1 liters/h (range, 106.7 to 546.7), and a large volume of distribution of 14.50 liters/kg (range, 3.21 to 51.58). Due to artemisones short t1/2, its pharmacokinetics were comparable after single and multiple dosing. Plasma samples taken after multiple dosing showed marked ex vivo pharmacodynamic antimalarial activities against two multidrug-resistant Plasmodium falciparum lines. Artemisone equivalent concentrations measured by bioassay revealed higher activity than artemisone measured by LC/MS-MS, confirming the presence of active metabolites. Comparable to those of other artemisinins, artemisones t1/2 is well suited for artemisinin-based combination therapy for the treatment of P. falciparum malaria.

Efficacy of monthly tafenoquine for prophylaxis of Plasmodium vivax and multidrug-resistant P-falciparum malaria

We assessed monthly doses of tafenoquine for preventing Plasmodium vivax and multidrug-resistant P. falciparum malaria. In a randomized, double-blind, placebo-controlled study, 205 Thai soldiers received either a loading dose of tafenoquine 400 mg (base) daily for 3 days, followed by single monthly 400-mg doses (n = 104), or placebo (n = 101), for up to 5 consecutive months. In volunteers completing follow-up (96 tafenoquine and 91 placebo recipients), there were 22 P. vivax, 8 P. falciparum, and 1 mixed infection. All infections except 1 P. vivax occurred in placebo recipients, giving tafenoquine a protective efficacy of 97% for all malaria (95% confidence interval [CI], 82%-99%), 96% for P. vivax malaria (95% CI, 76%-99%), and 100% for P. falciparum malaria (95% CI, 60%-100%). Monthly tafenoquine was safe, well tolerated, and highly effective in preventing P. vivax and multidrug-resistant P. falciparum malaria in Thai soldiers during 6 months of prophylaxis.

Lengthy antimalarial activity of atovaquone in human plasma following atovaquone-proguanil administration

Recently, it was reported that sera from healthy volunteers given atovaquone-proguanil (Malarone) inhibited parasite transmission and asexual blood stage development for up to 6 weeks after treatment (1). The lengthy persistence of drug activity was quite unexpected because earlier studies had shown that proguanil and atovaquone had elimination half-lives of about 14 to 20 h (2-4, 10, 11) and 2 to 3 days, (5, 9-11), respectively. This drug combination acts synergistically against malaria parasites and avoids the rapid selection of atovaquone-resistant parasites whenever parasites are exposed to the action of atovaquone alone (7). The present study was designed to quantitate the persistence of atovaquone in three Caucasian volunteers who had participated in a study to assess the effects of repeated subclinical infections on the development of immunity to Plasmodium falciparum (8). The three volunteers (mean weight, 84.8 ± 21.9 kg) had been inoculated intravenously with about 30 parasitized erythrocytes of P. falciparum (3D7 strain) on three occasions at 35-day intervals, and on each occasion, treatment with atovaquone-proguanil (1,000 mg atovaquone-400 mg proguanil daily for 3 days) was started 8 days later. Blood samples were collected at 6, 20, and 35 days after the onset of treatment (Fig. ​(Fig.1).1). None of the volunteers developed symptoms of malaria. FIG. 1.  Mean plasma atovaquone (ATQ) concentrations versus time after repeated atovaquone-proguanil treatment for uncomplicated Plasmodium falciparum malaria in three volunteers infected on three occasions. Plasma ATQ concentrations were measured by HPLC (•) ... Mean plasma atovaquone concentrations-versus-time curves measured by high-pressure liquid chromatography (HPLC) (9) were similar after each treatment with atovaquone-proguanil (Fig. ​(Fig.1).1). Comparable values were obtained by bioassay of samples using the 3D7 atovaquone-sensitive strain (6). The average elimination half-life of atovaquone in the three volunteers was 5.9 days by HPLC and 4.9 days by bioassay. The prolonged persistence of atovaquone was further illustrated by the complete suppression of schizont formation when plasma collected up to 35 days after treatment was incubated with parasites of the 3D7 strain (50% inhibitory concentration [IC50], ∼2 ng/ml). In contrast, plasma collected 6 days after treatment failed to inhibit the maturation of parasites of three atovaquone-resistant strains (TM90-C2b, TM91-C32b, TM93-C1088), with mean IC50 values exceeding 4,000 ng/ml (7). These findings suggest that residual atovaquone may have contributed to the suppression of malaria in the volunteers challenged with the 3D7 strain 35 days after treatment (8). The lengthy 5- to 6-day elimination half-life of atovaquone in these three Caucasian volunteers is twice as long as observed previously in African and Asian patients treated with atovaquone (5, 10, 11). Although proguanil concentrations were not measured in this study, numerous previous studies have shown that proguanil and its cycloguanil metabolite are eliminated much more quickly than atovaquone (2-4, 10, 11), and none would be present within a week after atovaquone-proguanil treatment to potentiate the antimalarial activity of atovaquone. The prolonged presence of atovaquone is of little concern as long as this expensive drug continues to be used mainly by travelers to malarious areas. However, if atovaquone-proguanil were used more widely by expatriate residents living in areas of endemicity, long-lasting low atovaquone concentrations in the absence of any residual proguanil may facilitate the rapid selection of atovaquone-resistant parasites.

Pharmacokinetics and Ex Vivo Pharmacodynamic Antimalarial Activity of Dihydroartemisinin-Piperaquine in Patients with Uncomplicated Falciparum Malaria in Vietnam

ABSTRACT Compared to healthy subjects, malaria patients show a reduction in the mean oral clearance (1.19 versus 5.87 liters/h/kg of body weight) and apparent volume of distribution (1.47 versus 8.02 liters/kg) of dihydroartemisinin in Vietnamese patients following treatment with dihydroartemisinin-piperaquine (Artekin) for uncomplicated Plasmodium falciparum. Dihydroartemisinin is responsible for most of the ex vivo antimalarial activity of dihydroartemisinin-piperaquine.

Dinuclear ruthenium(II) antimicrobial agents that selectively target polysomes in vivo

Wide-field fluorescence microscopy at high magnification was used to study the intracellular binding site of Rubb16 in Escherichia coli. Upon incubation of E. coli cells at the minimum inhibitory concentration, Rubb16 localised at ribosomes with no significant DNA binding observed. Furthermore, Rubb16 condensed the ribosomes when they existed as polysomes. It is postulated that the condensation of polysomes would halt protein production, and thereby inhibit bacterial growth. The results of this study indicate that the family of inert dinuclear ruthenium complexes Rubbn selectively target RNA over DNA in vivo. Selective RNA targeting could be advantageous for the development of therapeutic agents, and because of differences in ribosome structure between bacteria and eukaryotic cells, the Rubbn complexes could be selectively toxic to bacteria. In support of this hypothesis, the toxicity of Rubb16 was found to be significantly less to liver and kidney cell lines than against a range of bacteria.

Modern malaria chemoprophylaxis

Currently available medications for malaria chemoprophylaxis are efficacious but the problems of patient compliance, the advance of parasite drug resistance, and real or perceived serious adverse effects mean that new chemical compounds are needed.Primaquine, which has been widely used to treat relapsing malaria since the 1950s, has been shown to prevent malaria when taken daily. Tafenoquine is a new 8-aminoquinoline with a much longer half-life than primaquine. Field trials to date indicate that tafenoquine is efficacious and can be taken weekly or perhaps even less frequently. Both primaquine and tafenoquine require exact knowledge of a person’s glucose 6-phosphate dehydrogenase status in order to prevent drug-induced haemolysis. Other potential malaria chemoprophylactic drugs such as third-generation antifol compounds and Mannich bases have reached advanced preclinical testing. Mefloquine has been seen to cause serious neuropsychiatric adverse effects on rare occasions. Recent public controversy regarding reputedly common serious adverse effects has made many Western travellers unwilling to take mefloquine.Special risk groups exposed to malaria, such as long-term travellers, children, pregnant women, aircrew and those requiring unimpeded psychomotor reactions, migrants returning to visit malarious countries of origin and febrile persons who have returned from malaria endemic areas, all require a nuanced approach to the use of drugs to prevent malaria. The carrying of therapeutic courses of antimalarial drugs to be taken only if febrile illness develops is indicated in very few travellers despite its appeal to some who fear adverse effects more than they fear potentially lethal malaria infection. Travellers with a significant exposure to malaria require a comprehensive plan for prevention that includes anti-mosquito measures but which is still primarily be based on the regular use of efficacious antimalarial medications.

Simultaneous Measurement of Quinine and Quinidine in Human Plasma, Whole Blood, and Erythrocytes by High-performance Liquid Chromatography with Fluorescence Detection

A high-performance liquid chromatographic method with fluorescence detection is described for the simultaneous measurement of quinine and quinidine in plasma, whole blood, and erythrocytes. The compounds were separated on an Ultrasphere C18 reversed-phase column (25 cm x 4.6 mm inside diameter, 5 μm particle size) using a mobile phase of acetonitrile/ water/triethylamine (11:88:1, vol/vol) at pH 2.5. The method, simple, accurate, and selective, requires only a single-step liquid-liquid extraction and uses the structurally similar alkaloid, cinchonine, as the internal standard. The commercial impurities, dihydroquinine and dihydroquinidine, and unknown metabolites were well resolved from the parent drugs. The assay is precise, with interassay coefficients of variation 7.0% and an accuracy of 7.3% over a concentration range of 0.125 to 4.0 μg/0.25 ml. The extraction recoveries of the two drugs were similar, averaging 82.9% for quinine and 79.3% for quinidine from the three biological fluids. The clinical application of the method for routine drug monitoring and for estimating the pharmacokinetics of quinine and quinidine in man are discussed.

Sex Affects the Steady-State Pharmacokinetics of Primaquine but Not Doxycycline in Healthy Subjects

We evaluated whether sex affects the steady-state pharmacokinetics of the antimalarial drugs, primaquine and doxycycline, in healthy subjects. Seventeen male and 17 female healthy Vietnamese subjects were administered 30 mg (base) of primaquine daily for 14 days. After a 2-week washout period, 14 male and 14 female subjects were administered 100 mg (base) of doxycycline daily for 14 days. Women had significantly higher median values of C(max) (212 versus 122 ng/mL, P< 0.001) and AUC(0-24) (1,909 versus 917 ng . h/mL, P < 0.001) of primaquine compared with men. Other than a longer t(max) in women, no sex-related differences were seen in the pharmacokinetics of doxycycline. The primaquine pharmacokinetic data suggest that women have increased exposure to primaquine, which may put them at increased risk for toxicity when administered the same maintenance dose as men. The similar pharmacokinetics of doxycycline between the two sexes justifies the same maintenance dose.

Comparison of tafenoquine (WR238605) and primaquine in the post-exposure (terminal) prophylaxis of vivax malaria in Australian Defence Force personnel.

On return from duty in North Solomons Province (including Bougainville Island), Papua New Guinea, 586 Australian Defence Force personnel received either primaquine (14-d) or tafenoquine (3-d) post-exposure malaria prophylaxis. Within 12 months, 6 of the 214 volunteers receiving primaquine and 7 of 378 receiving tafenoquine had developed vivax malaria. Overall, volunteers preferred the shorter course of tafenoquine.

Aza-acyclic Nucleoside Phosphonates Containing a Second Phosphonate Group As Inhibitors of the Human, Plasmodium falciparum and vivax 6-Oxopurine Phosphoribosyltransferases and Their Prodrugs As Antimalarial Agents.

Hypoxanthine-guanine-[xanthine] phosphoribosyltransferase (HG[X]PRT) is considered an important target for antimalarial chemotherapy as it is the only pathway for the synthesis of the purine nucleoside monophosphates required for DNA/RNA production. Thus, inhibition of this enzyme should result in cessation of replication. The aza-acyclic nucleoside phosphonates (aza-ANPs) are good inhibitors of Plasmodium falciparum HGXPRT (PfHGXPRT), with Ki values as low as 0.08 and 0.01 μM for Plasmodium vivax HGPRT (PvHGPRT). Prodrugs of these aza-ANPs exhibit antimalarial activity against Pf lines with IC50 values (0.8-6.0 μM) and have low cytotoxicity against human cells. Crystal structures of six of these compounds in complex with human HGPRT have been determined. These suggest that the different affinities of these aza-ANPs could be due to the flexibility of the loops surrounding the active site as well as the flexibility of the inhibitors, allowing them to adapt to fit into three binding pockets of the enzyme(s).