Synthesis and antimalarial evaluation of artesunate-polyamine and trioxolane-polyamine conjugates

A series of artesunate-polyamine and trioxolane-polyamine conjugates have been prepared. The conjugates were evaluated for antimalarial activity towards the K1 dual drug resistant and NF54 chloroquine-sensitive strains of Plasmodium falciparum (Pf) and for cytotoxicity towards the rat myoblast cell line L6. (Bis)-Boc-(bis)-artesunate-polyamine and (tetra)-artesunate-polyamine conjugates exhibited potent in vitro activity towards both strains of Pf, with IC₅₀ values in the range of 0.3-1.1 nM, comparable to the parent artesunate. Cytotoxicity within this series of analogues typically increased with polyamine (PA) chain length, identifying the PA3-4-3 (spermine), and to some extent the PA3-7-3 series, as being highly selective towards the parasite. The corresponding series of (bis)-Boc-(bis)-trioxolane and (tetra)-trioxolane-polyamine conjugates were less active as antimalarials than the parent trioxolane acid, highlighting the limitation of using this warhead for drug-conjugate studies. Preliminary in vivo evaluation of two artesunate-polyamine conjugates 11 and 16 demonstrated 95.5-99.8% reduction in parasitaemia with maximal 30 day survival rates (ip delivery). Oral testing of 11 proved less efficacious, with 95.7% activity and inconsistent survival rates of 16-30 days. In contrast, trioxolane-polyamines were substantially less effective (ip delivery), exhibiting only modest reductions in parasitaemia and modest to no increase in survival rates.