Structural Characteristics of Chloroquine-Bridged Ferrocenophane Analogues of Ferroquine May Obviate Malaria Drug-Resistance Mechanisms

Five compounds displaying an unprecedented binding mode of chloroquine to ferrocene through the bridging of the cyclopentadienyl rings were studied alongside their monosubstituted ferrocene analogues and organic fragments. The antiplasmodial activity was evaluated against strains of the malaria parasite (Plasmodium falciparum). While the chloroquine-bridged ferrocenyl derivatives were less active than their five monosubstituted ferrocenyl analogues, they retained activity in the drug-resistant strains. The biological and physical properties were correlated to antiplasmodial activity. Intramolecular hydrogen bonding was associated with increased antiplasmodial action, but it is not the determining factor. Instead, balance between lipophilicity and hydrophilicity had a greater influence. It was found that calculated partition coefficient (log P) values of 4.5–5.0 and topological polar surfaces area (tPSA) values of ∼26.0 Å2 give the best balance. The particular conformation, compact size, and lipophilicity/hydrophilicity balance observed in the bridged compounds provide them with the structural characteristics needed to escape the mechanisms responsible for resistance.