Pharmacology and toxicology of artelinic acid: preclinical investigations on pharmacokinetics, metabolism, protein and red blood cell binding, and acute and anorectic toxicities

Toxicokinetics and Hydrolysis of Artelinate and Artesunate in Malaria-Infected Rats

Comparative toxicokinetic (TK) and hydrolysis studies of intravenously administered two new antimalarial agents, artelinate (AL) and artesunate (AS), were performed in malaria-infected rats using three daily equimolar doses (96 μmoles/kg). The TK evaluation was related to select one drug for severe malaria treatment in U.S. Army. Drug concentration of AS with daily dose of 36.7 mg/kg was one-third less on day 3 than on day 1, which resembled its active metabolite, dihydroartemisinin (DHA), suggesting an autoinduction of hepatic drug-metabolizing enzymes for AS. The results were similar to other artemisinin drugs, but not for AL. TK parameters of AL were very comparable from day 1 to day 3 at same AS molecular dose at 40.6 mg/kg. AS is the prodrug of DHA with the DHA/AS ratio of 5.26 compared to the ratio of 0.01 for DHA/AL. Other TK parameters revealed that the total AUC1–3 days (84.4 μg · h ml−1) of AL was fivefold higher than that of AS (15.7 mu;g h ml−1 of AS plus DHA). The elimination half-life of AL (7.1 h) was much longer than that of AS (0.36 h) or DHA (0.72 h). The remarkable alteration of the TK shape of AL may be caused by poor conversion rates to DHA and an enterohepatic circulation, which is confirmed by the present TK and tissue distribution studies. Compared to AS, higher drug exposure levels and longer exposure time of AL in the rat blood may be the cause of its increased toxicity.

Risk Assessment and Therapeutic Indices of Artesunate and Artelinate in Plasmodium berghei–Infected and Uninfected Rats

Artesunate (AS) is being developed as a potential agent for the treatment of severe and complicated malaria. A risk assessment of the therapeutic index and related hematological changes of AS and artelinate (AL) following daily intravenous injection for 3 days was conducted in Plasmodium berghei–infected and uninfected rats. The minimum doses of AS and AL for parasitemia suppression were 2.3 and 2.5 mg/kg, respectively, and the suppressive doses for half parasitemia (SD50) were 7.4 and 8.6 mg/kg, respectively. The maximum tolerated dose (MTD) for AS was 240 mg/kg with a therapeutic index of 32.6. The MTD for AL was 80 mg/kg with a therapeutic index of 9.3. Hematological changes were studied on days 1 and 8 after the final dosing. In both AS- and AL-treated rats, dose-dependent and rapidly reversible hematological changes (significant reductions in RBC, HCT, Hb, and reticulocyte levels) were seen in the peripheral blood. Bone marrow evaluation revealed a statistically significant reduction in the myeloid/erythroid ratio only at the highest dose of AS (240 mg/kg), albeit still within the normal ratio range (1.0–1.5:1.0). Looking at the respective therapeutic indices the authors have concluded that AS is much safer than AL. Both drugs induced hematological changes in rats that parallel the dose-dependent, reversible anemia and reticulocytopenia previously reported in animals and humans. However, no significant bone marrow depression was seen for either agent.

Comparative pharmacokinetics and pharmacodynamics of intravenous artelinate versus artesunate in uncomplicated Plasmodium coatneyi-infected rhesus monkey model

Background

The US Army designed artelinate/lysine salt (AL) to overcome the instability of sodium artesunate in aqueous solution (AS). To select the most efficacious artemisinin treatment, direct comparison was performed in an uncomplicated non-human primate malaria model.

Methods

Splenectomized rhesus monkeys were inoculated with Plasmodium coatneyi and on day six, single equimolar loading dose of IV AL (11.8 mg kg⁻¹) or IV AS (8 mg kg⁻¹) were administered followed by 1/2 the first dose once daily for 2 more days. Blood smear were performed twice daily and the number of parasites were counted microscopically. Blood samples were obtained after the first dose within 6 h for pharmacokinetic (PK) and ex vivo pharmacodynamic evaluation by simultaneously measuring plasma drug concentration and anti-malarial activity against Plasmodium falciparum in vitro.

Results

The anti-P. coatneyi in vivo activity of both compounds were comparable, but the ex vivo anti-P. falciparum potency of the IV AS regimen as administered was sevenfold higher than that of IV AL. Comparing in vivo pharmacodynamics of AL and AS, daily assessed parasite counts showed comparable 99 % parasite clearance times (PC99: 2.03, 1.84 day), parasite clearance rates (5.34, 4.13 per min) and clearance half-life (PCt_1/2: 7.79, 10.1 h). This study showed strong and significant inverse correlation between PCt_1/2 and t_1/2 of AS + DHA, and AUC_0–∞ of DHA, and correlated with V_z of AS (r² > 0.7, p ≤ 0.002). Lastly, following IV AL, there was a modest inverse correlation between PCt_1/2 and C_max (r² 0.6, p ≤ 0.04). Although all tested monkeys recrudesced subsequently, two died following AL regimen before parasite clearance. While the aetiology of those deaths could not be definitively determined, pathologic evidence favoured a sepsis-like syndrome and suggested that severe malaria was more likely than drug toxicity.

Conclusion

The model demonstrated that both AS and DHA contributed to the anti-malarial activity of IV AS, while IV AL activity was largely restricted to the parent drug. Parasite clearance was strongly and linearly dependent on drug exposure for both artemisinin regimens. However, IV AS had higher ex vivo potency against P. falciparum, leading to an IND filing for GMP manufactured AS in the United States.

Electronic supplementary material

The online version of this article (doi:10.1186/s12936-016-1456-6) contains supplementary material, which is available to authorized users.

Methylene Homologues of Artemisone: An Unexpected Structure-Activity Relationship and a Possible Implication for the Design of C10-Substituted Artemisinins

We sought to establish if methylene homologues of artemisone are biologically more active and more stable than artemisone. The analogy is drawn with the conversion of natural O- and N-glycosides into more stable C-glycosides that may possess enhanced biological activities and stabilities. Dihydroartemisinin was converted into 10β-cyano-10-deoxyartemisinin that was hydrolyzed to the α-primary amide. Reduction of the β-cyanide and the α-amide provided the respective methylamine epimers that upon treatment with divinyl sulfone gave the β- and α-methylene homologues, respectively, of artemisone. Surprisingly, the compounds were less active in vitro than artemisone against P. falciparum and displayed no appreciable activity against A549, HCT116, and MCF7 tumor cell lines. This loss in activity may be rationalized in terms of one model for the mechanism of action of artemisinins, namely the cofactor model, wherein the presence of a leaving group at C10 assists in driving hydride transfer from reduced flavin cofactors to the peroxide during perturbation of intracellular redox homeostasis by artemisinins. It is noted that the carba analogue of artemether is less active in vitro than the O-glycoside parent toward P. falciparum, although extrapolation of such activity differences to other artemisinins at this stage is not possible. However, literature data coupled with the leaving group rationale suggest that artemisinins bearing an amino group attached directly to C10 are optimal compounds.

Pharmacokinetics and pharmacodynamics of oral artesunate monotherapy in patients with uncomplicated Plasmodium falciparum malaria in western Cambodia

Artemisinin-resistant malaria along the Thailand-Cambodian border is an important public health concern, yet mechanisms of drug action and their contributions to the development of resistance are poorly understood. The pharmacokinetics and pharmacodynamics of oral artesunate monotherapy were explored in a dose-ranging trial in an area of emerging artesunate resistance in western Cambodia. We enrolled 143 evaluable subjects with uncomplicated Plasmodium falciparum malaria in an open label study of directly observed artesunate monotherapy at 3 dose levels (2, 4, and 6 mg/kg of body weight/day) for 7 days at Tasanh Health Center, Tasanh, Cambodia. Clinical outcomes were similar among the 3 groups. Wide variability in artesunate and dihydroartemisinin concentrations in plasma was observed. No significant dose-effect or concentration-effect relationships between pharmacokinetic (PK) and parasite clearance parameters were observed, though baseline parasitemia was modestly correlated with increased parasite clearance times. The overall parasite clearance times were prolonged compared with the clearance times in a previous study at this site in 2006 to 2007, but this did not persist when the evaluation was limited to subjects with a comparable artesunate dose (4 mg/kg/day) and baseline parasitemia from the two studies. Reduced plasma drug levels with higher presentation parasitemias, previously hypothesized to result from partitioning into infected red blood cells, was not observed in this population with uncomplicated malaria. Neither in vitro parasite susceptibility nor plasma drug concentrations appeared to have a direct relationship with the pharmacodynamic (PD) effects of oral artesunate on malaria parasites. While direct concentration-effect relationships were not found, it remains possible that a population PK modeling approach that allows modeling of greater dose separation might discern more-subtle relationships.

Semisynthetic Artemisinin and Synthetic Peroxide Antimalarials

Since the discovery of the endoperoxide sesquiterpene lactone artemisinin, numerous second-generation semisynthetic artemisinins and synthetic peroxides have been prepared and tested for their antimalarial properties. Using a case-study approach, we describe the discovery of the investigational semisynthetic artemisinins artelinic acid (8) and artemisone (9), and the structurally diverse synthetic peroxides arteflene (10), fenozan B07 (11), arterolane (12), PA1103/SAR116242 (13), and RKA182 (14).

Artesunate: developmental toxicity and toxicokinetics in monkeys

BACKGROUND

The developmental toxicity, toxicokinetics, and hematological effects of the antimalarial drug, artesunate, were previously studied in rats and rabbits and have now been studied in cynomolgus monkeys.

METHODS

Groups of up to 15 pregnant females were dosed on Gestation Days (GD) 20-50 or for 3-7-day intervals.

RESULTS

At 30 mg/kg/day, 6 embryos died between GD30 and GD40. Histologic examination of 3 live embryos (GD26-GD36) revealed a marked reduction in embryonic erythroblasts and cardiomyopathy. At 12 mg/kg/day, 6 embryos died between GD30 and GD45. Four surviving fetuses examined on GD100 had no malformations, but long bone lengths were slightly decreased. At the developmental no-adverse-effect-level (4 mg/kg/day), maternal plasma AUC was 3.68 ng.h/mL for artesunate and 6.93 ng.h/ml for its active metabolite, dihydroartemisinin (DHA). No developmental toxicity occurred with administration of 12 mg/kg/day for 3 or 7 days, GD29-31 or GD27-33 (maternal plasma AUC of 9.84 ng.h/mL artesunate and 16.4 ng.h/mL DHA). Exposures at embryotoxic doses were substantially lower than human therapeutic exposures. However, differences in monkey and human Vss for artesunate (0.5 L/kg vs. 0.18 L/kg) confound relying solely on AUC for assessing human risk. Decreases in reticulocyte count occur at therapeutic doses in humans. Changes to reticulocyte counts at embryotoxic doses in monkeys (> or =12 mg/kg/day) were variable and generally minor.

CONCLUSIONS

Artesunate was embryolethal at > or =12 mg/kg/day when dosed for at least 12 days at the beginning of organogenesis, but not when dosed for 3 or 7 days, indicating that developmental toxicity of artesunate is dependent upon duration of dosing in cynomologus monkeys.

Neurotoxicity and toxicokinetics of artelinic acid following repeated oral administration in rats

Neurotoxicity secondary to oil-soluble artemisinins has been reported in various animal species. The onset of neurotoxicity and toxicokinetics of oral artelinic acid (AL), a water-soluble artemisinin, were investigated. After dose range study, rats were dosed at either 160 mg/kg daily for 9 consecutive days or at 288 mg/kg once every other day for five doses, so that the total dose (1440 mg/kg) and duration (9 days) were identical. Neuronal damage of varying severity was identified beginning as early as 1 day after completing dosing and continued for up to 10 days post dosing. Neuronal injury was most severe 7 days after the last treatment in each of the two dosing regimens. The rats dosed with 160 mg/kg of AL daily showed moderate neurotoxicity and lost 22% of their body weight during treatment. Compared with the first dose, the toxicokinetic profile of this regimen changed significantly, with the elimination half-life increasing 3.82-fold and the volume of distribution increasing 5.23-fold on the last day of dosing. In the animals treated with AL at 288 mg/kg every other day for 5 doses, minimal neuronal degeneration (severity score 1.17) was identified and the body weight was only 8% loss. Furthermore, there were no obvious differences in the pharmacokinetic parameters between first and last dosing days with this regimen. Additionally, a progressively drug retention in stomach and drug accretion in blood were only found in rats treated with 160 mg/kg daily for 9 days. These results imply that delayed gastric emptying resulted in AL accumulation in blood and prolonged a neurotoxic exposure time (186 h) in 160 mg/kg rats when compared to that (75 h) in 288 mg/kg animals. Therefore, the drug exposure time is a key factor in the neurotoxicity induced by AL.

Toxicity evaluation of artesunate and artelinate in Plasmodium berghei-infected and uninfected rats

A recent therapeutic index study in rats demonstrated that i.v. artesunate (AS) is safer than artelinate (AL). The present study of acute toxicity illustrated an LD(50) of 177 mg/kg and 488 mg/kg for AL and AS, respectively, following daily i.v. injection for 3 days in Plasmodium berghei-infected rats. In uninfected rats, the LD(50) values were 116 mg/kg and 351 mg/kg after a single dose of AL and AS, respectively. This study showed vascular necrosis in 50% of the animals at 13.5 mg/kg AL and at 42.8 mg/kg AS. Animals also showed moderate signs of renal failure at 40 mg/kg AL and 240 mg/kg AS (100 times higher than the therapeutic dose). Histopathological evaluation demonstrated mild to moderate tubular necrosis in uninfected rats treated with 40 mg/kg AL and 240 mg/kg AS; interestingly, fewer pathological lesions were observed in malaria-infected rats. Renal injury was reversible in all cases by Day 8 after cessation of dosing. No neurotoxicity was seen in any case with all i.v. regimens. In conclusion, AL and AS exhibit less toxic effects in P. berghei-infected rats than in uninfected rats. Both agents caused irreversible vascular irritation, reversible nephrotoxicity and no neurotoxicity at high doses. The data indicate that AS is three times safer than AL in rats.

Antimalarial drugs: current status and new developments

Malaria continues to be a major threat in the developing world, with > 1 million clinical episodes and 3000 deaths every day. In the last century, malaria claimed between 150 and 300 million lives, accounting for 2 - 5% of all deaths. Currently approximately 40% of the world population resides in areas of active malaria transmission. The disease symptoms are most severe in young children and pregnant women. A total of 90% of the disease-associated mortality occurs in Subsaharan Africa, despite the fact that malaria is indigenous to most tropical regions. A licensed vaccine for malaria has not become a reality and antimalarial drugs are the only available method of treatment. Although chloroquine, the first synthetically developed antimalarial, proved to be an almost magical cure for > 30 years, the emergence and spread of chloroquine-resistant parasites has made it virtually ineffective in most parts of the world. Currently, artemisinin, a plant-derived antimalarial, is the only available drug that is globally effective against the parasite. Although several new drugs have been introduced in the past 30 years, widespread or isolated cases of resistance indicate that their window of effectiveness will be limited. Thus, there is an urgent need to develop new therapeutics and regimens for malaria control. This article presents an overview of the currently available antimalarial chemotherapy options and the efforts being undertaken to develop new drugs based on both the recent technological advances and modifications to the old remedies, and on combination therapies.

Convenient Access Both to Highly Antimalaria-Active 10-Arylaminoartemisinins, and to 10-Alkyl Ethers Including Artemether, Arteether, and Artelinate

An economical phase-transfer method is used to prepare 10-arylaminoartemisinins from DHA and arylamines, and artemether, arteether, and artelinate from the corresponding alcohols. In vivo sc screens against Plasmodium berghei and P. yoelii in mice reveal that the p-fluorophenylamino derivative 5 g is some 13 and 70 times, respectively, more active than artesunate; this reflects the very high sc activity of 10-alkylaminoartemisinins. However, through the po route, the compounds are less active than the alkylaminoartemisinins, but still approximately equipotent with artesunate.

Nuclear magnetic resonance and molecular modeling analysis of the interaction of the antimalarial drugs artelinic acid and artesunic acid with β‐cyclodextrin

The artemisinin derivatives artelinic acid and artesunic acid are members of a class of compounds that have shown promise for the treatment of multidrug resistant strains of Plasmodium falciparum. Unfortunately, these compounds exhibit poor solubility and stability in aqueous solution. The research presented herein was conducted to determine whether complexation of artelinic acid or artesunic acid with beta-cyclodextrin would result in complexes with increased aqueous solubility while retaining the potent antimalarial activity of these compounds. Preliminary complexation studies with natural beta-cyclodextrins were conducted as a proof of concept, with a primary focus on understanding the electrostatic interactions that stabilize the resulting complexes. Complex formation was monitored using UV spectroscopy. The structures of the resulting complexes were determined using multidimensional nuclear magnetic resonance spectroscopy (NMR) and molecular modeling. NMR results are most consistent for artelinic acid and beta-cyclodextrin forming complexes in a ratio of 2:1; however, the presence of 1:1, 2:2, and 3:1 complexes in solution cannot be excluded based on the experimental data collected. The NMR data also indicate selective insertion of artelinic acid into the hydrophobic cavity of the beta-cyclodextrin via the primary face. NMR results indicate artesunic acid forms a similar complex with beta-cyclodextrin in a ratio of 1:1; again however, the presence of 1:1, 2:2, and 3:1 complexes in solution cannot be ruled out.

Antimalarial chemotherapeutic peroxides: artemisinin, yingzhaosu A and related compounds

Mechanism-based rational design and gram-scale chemical synthesis have produced some new trioxane and endoperoxide antimalarial drug candidates that are efficacious and safe. This review summarises recent achievements in this area of peroxide drug development for malaria chemotherapy.

In vivo active antimalarial isonitriles

Building on the lead from antimalarial isonitriles 1-4 of marine origin, several easily accessible synthetic isonitriles were assessed for their antimalarial activity against Plasmodium falciparum (in vitro) and multidrug resistant Plasmodium yoelii in Swiss mice model (in vivo). Isonitrile 11 has shown promising activity in both these assays.

Photo-oxygenation of geraniol: synthesis of a novel series of hydroxy-functionalized anti-malarial 1,2,4-trioxanes

Photo-oxygenation of geraniol 2, an abundantly available allylic alcohol, furnished a mixture of mono- and di-hydroperoxy products; the latter have been used for the preparation of a novel series of hydroxy-functionalized anti-malarial 1,2,4-trioxanes (7a-d, 8a-d).

The role of glutathione in the neurotoxicity of artemisinin derivatives in vitro

The role of antioxidants in the neurotoxicity of the antimalarial endoperoxides artemether and dihydroartemisinin was studied in vitro by quantitative image analysis of neurite outgrowth in the neuroblastoma cell line NB2a. Intracellular glutathione concentrations were measured by high performance liquid chromatography with fluorescence detection. Both dihydroartemisinin (1 microM) and a combination of artemether (0.3 microM) plus haemin (2 microM) significantly inhibited neurite outgrowth from differentiating NB2a cells to 11.5 +/- 11.0% (SD) and 19.6 +/- 15.2% of controls, respectively. The inhibition by artemether/haemin was prevented by the antioxidants superoxide dismutase (109.7 +/- 47.8% of control), catalase (107.0 +/- 29.3%) glutathione (123.8 +/- 12.4%), L-cysteine (88.0 +/- 6.3%), N-acetyl-L-cysteine (107.8 +/- 14.9%), and ascorbic acid (104.3 +/- 12.7%). Dihydroartemisinin-induced neurotoxicity was completely or partially prevented by L-cysteine (99.5 +/- 17.7% of control), glutathione (57.9 +/- 23.4% of control), and N-acetyl-L-cysteine (57.3 +/- 9.5%), but was not prevented by superoxide dismutase, catalase, or ascorbic acid. Buthionine sulphoximine, an inhibitor of gamma-glutamylcysteine synthetase, significantly increased the neurotoxic effect of non-toxic concentrations of artemether/haemin (0.1 microM/2 microM) and dihydroartemisinin (0.2 microM), suggesting that endogenous glutathione participates in the prevention of the neurotoxicity of artemether/haemin and dihydroartemisinin. Artemether/haemin completely depleted intracellular glutathione levels, whereas dihydroartemisinin had no effect. We conclude that although glutathione status is an important determinant in the neurotoxicity of endoperoxides, depletion of glutathione is not a prerequisite for their toxicity. This is consistent with their mechanisms of toxicity being free radical-mediated damage to redox-sensitive proteins essential for neurite outgrowth, or alteration of a redox-sensitive signalling system which regulates neurite outgrowth.

Pharmacokinetics of artemisinin-type compounds

Various compounds of the artemisinin family are currently used for the treatment of patients with malaria worldwide. They are characterised by a short half-life and feature the most rapidly acting antimalarial drugs to date. They are increasingly being used, often in combination with other drugs, although our knowledge of their main pharmacological features (including their absorption, distribution, metabolism and excretion) is still incomplete. Such data are particularly important in the case of combinations. Artemisinin derivatives are converted primarily, but to different extents, to the bioactive metabolite artenimol after either parenteral or gastrointestinal administration. The rate of conversion is lowest for artelinic acid (designed to protect the molecule against metabolism) and highest for the water-soluble artesunate. The absolute and relative bioavailability of these compounds has been established in animals, but not in humans, with the exception of artesunate. Oral bioavailability in animals ranges, approximately, between 19 and 35%. A first-pass effect is highly probably for all compounds when administered orally. Artemisinin compounds bind selectively to malaria-infected erythrocytes to yet unidentified targets. They also bind modestly to human plasma proteins, ranging from 43% for artenimol to 81.5% for artelinic acid. Their mode of action is still not completely understood, although different theories have been proposed. The lipid-soluble artemether and artemotil are released slowly when administered intramuscularly because of the 'depot' effect related to the oil formulation. Understanding the pharmacokinetic profile of these 2 drugs helps us to explain the characteristics of the toxicity and neurotoxicity. The water-soluble artesunate is rapidly converted to artenimol at rates that vary with the route of administration, but the processes need to be characterised further, including the relative contribution of pH and enzymes in tissues, blood and liver. This paper intends to summarise contemporary knowledge of the pharmacokinetics of this class of compounds and highlight areas that need further research.

Behavioral and neural toxicity of the artemisinin antimalarial, arteether, but not artesunate and artelinate, in rats

Three artemisinin antimalarials, arteether (AE), artesunate (AS), and artelinate (AL) were evaluated in rats using an auditory discrimination task (ADT) and neurohistology. After rats were trained on the ADT, equimolar doses of AE (25 mg/kg, in sesame oil, n=6), AS (31 mg/kg, in sodium carbonate, n=6), and AL (36 mg/kg, in saline, n=6), or vehicle (sodium carbonate, n=6) were administered (IM) for 7 consecutive days. Behavioral performance was evaluated, during daily sessions, before, during, and after administration. Histological evaluation of the brains was performed using thionine staining, and damaged cells were counted in specific brainstem nuclei of all rats. Behavioral performance was not significantly affected in any rats treated with AS, AL, or vehicle. Furthermore, histological examination of the brains of rats treated with AS, AL, and vehicle did not show damage. In stark contrast, all rats treated with AE showed a progressive and severe decline in performance on the ADT. The deficit was characterized by decreases in accuracy, increases in response time and, eventually, response suppression. When performance on the ADT was suppressed, rats also showed gross behavioral signs of toxicity that included tremor, gait disturbances, and lethargy. Subsequent histological assessment of AE-treated rats revealed marked damage in the brainstem nuclei, ruber, superior olive, trapezoideus, and inferior vestibular. The damage included chromatolysis, necrosis, and gliosis. These results demonstrate distinct differences in the ability of artemisinins to produce neurotoxicity. Further research is needed to uncover pharmacokinetic and metabolic differences in artemisinins that may predict neurotoxic potential.