Transmission blocking activity of a standardized neem (Azadirachta indica) seed extract on the rodent malaria parasite Plasmodium berghei in its vector Anopheles stephensi

<p>Abstract</p> <p>Background</p> <p>The wide use of gametocytocidal artemisinin-based combination therapy (ACT) lead to a reduction of <it>Plasmodium falciparum </it>transmission in several African endemic settings. An increased impact on malaria burden may be achieved through the development of improved transmission-blocking formulations, including molecules complementing the gametocytocidal effects of artemisinin derivatives and/or acting on <it>Plasmodium </it>stages developing in the vector. Azadirachtin, a limonoid (tetranortriterpenoid) abundant in neem (<it>Azadirachta indica</it>, Meliaceae) seeds, is a promising candidate, inhibiting <it>Plasmodium </it>exflagellation <it>in vitro </it>at low concentrations. This work aimed at assessing the transmission-blocking potential of NeemAzal^®, an azadirachtin-enriched extract of neem seeds, using the rodent malaria <it>in vivo </it>model <it>Plasmodium berghei</it>/<it>Anopheles stephensi</it>.</p> <p>Methods</p> <p><it>Anopheles stephensi </it>females were offered a blood-meal on <it>P. berghei </it>infected, gametocytaemic BALB/c mice, treated intraperitoneally with NeemAzal, one hour before feeding. The transmission-blocking activity of the product was evaluated by assessing oocyst prevalence, oocyst density and capacity to infect healthy mice. To characterize the anti-plasmodial effects of NeemAzal^®on early midgut stages, i.e. zygotes and ookinetes, Giemsa-stained mosquito midgut smears were examined.</p> <p>Results</p> <p>NeemAzal^®completely blocked <it>P. berghei </it>development in the vector, at an azadirachtin dose of 50 mg/kg mouse body weight. The totally 138 examined, treated mosquitoes (three experimental replications) did not reveal any oocyst and none of the healthy mice exposed to their bites developed parasitaemia. The examination of midgut content smears revealed a reduced number of zygotes and post-zygotic forms and the absence of mature ookinetes in treated mosquitoes. Post-zygotic forms showed several morphological alterations, compatible with the hypothesis of an azadirachtin interference with the functionality of the microtubule organizing centres and with the assembly of cytoskeletal microtubules, which are both fundamental processes in <it>Plasmodium </it>gametogenesis and ookinete formation.</p> <p>Conclusions</p> <p>This work demonstrated <it>in vivo </it>transmission blocking activity of an azadirachtin-enriched neem seed extract at an azadirachtin dose compatible with 'druggability' requisites. These results and evidence of anti-plasmodial activity of neem products accumulated over the last years encourage to convey neem compounds into the drug discovery & development pipeline and to evaluate their potential for the design of novel or improved transmission-blocking remedies.</p

MAGNETIC QUENCHING OF POSITRONIUM IN SOME NONPOLAR SOLUTIONS

The magnetic quenching of positronium appears experimentally in the reduction of the long lifetime component in the lifetime spectra in the presence of an external magnetic field. In previous investigations in some solids (e.g. polymers, alkali halides, etc.) and water, the variation of the o-Ps lifetime with the field strength has been interpreted in terms of a shift of the value of the positronium hyperfine splitting constant, (DELTA)E. In the present study, the magnetic quenching of o-Ps in pure liquids (n-hexane, c-hexane and benzene), nitrobenzene in n-hexane, c-hexane and benzene solutions and carbon tetrachloride/n-hexane solutions is examined for fields up to 14 kG by the positron lifetime technique. The obtained spectra are analyzed into three lifetime components. The long-lived positronium lifetime, o-Ps, is the most sensitive component to the external magnetic field. The interpretation in terms of the hyperfine splitting constant can only distinguish the behaviour of positronium from the free atom. It is observed that in the case of pure liquids, dilute nitrobenzene in n-hexane, c-hexane, and benzene solutions and carbon tetrachloride/n-hexane solutions, positronium behaves normally; i.e., as the free atom. However, for 0.05M and 0.1M nitrobenzene/n-hexane solutions, an anomalously strong magnetic quenching is observed at low field, and at higher magnetic field strength, an anomalously weak magnetic quenching is observed. This unexpected behaviour of positronium in high concentrations of nitrobenzene/n-hexane solutions is interpreted in terms of the field effect on the interaction of triplet molecules and radical ions with positronium. The results depend very much on the solute concentration due to the yield of triplet states and radical ions generated in the solution. This implies a significant role of the interaction between nitrobenzene transient radiolytic species and the Ps atom. The same concentration range of carbon tetrachloride/n-hexane solutions are also investigated, and no anomalies are observed. The different results of these two systems may be due to the differences in spur processes in carbon tetrachloride solutions. The proposed mechanism is based on known effects involving delayed fluorescence and electrogenerated chemiluminescence (ECL) measurements, by assuming that positronium behaves as one of the triplet or singlet excited species which reacts with either triplet molecules or radical ions

Antimalarial qinghaosu/artemisinin: The therapy worthy of a Nobel Prize

Malaria is a major cause of human morbidity and mortality in the tropical endemic countries worldwide. This is largely due to the emergence and spread of resistance to most antimalarial drugs currently available. Based on the World Health Organization recommendation, artemisinin-based combination therapies are now used as first-line treatment for Plasmodium falciparum malaria. Artemisinin or qinghaosu (Chinese name) and its derivatives are highly potent, rapidly acting antimalarial drugs. Artemisinin was discovered in 1971 by a Chinese medical scientist Youyou Tu, who was awarded the Nobel Prize in 2015 on her discovering the antimalarial properties of qinghaosu from the traditional Chinese qinghao plant. Nevertheless, artemisinin resistance in falciparum malaria patients has first emerged on the Thai-Cambodian border in 2009, which is now prevalent across mainland Southeast Asia from Vietnam to Myanmar. Here, we reviewed malaria disease severity, history of artemisinin discovery, chemical structure, mechanism of drug action, artemisinin-based combination therapies, emergence and spread of drug resistance, including the recent findings on mechanism of resistance in the falciparum malaria parasite. This poses a serious threat to global malaria control and prompts renewed efforts for the urgent development of new antimalarial drugs