Novel amidines and analogues as promising agents against intracellular parasites: a systematic review

Abstract

Parasitic protozoa comprise diverse aetiological agents responsible for important diseases in humans and animals including sleeping sickness, Chagas disease, leishmaniasis, malaria, toxoplasmosis and others. They are major causes of mortality and morbidity in tropical and subtropical countries, and are also responsible for important economic losses. However, up to now, for most of these parasitic diseases, effective vaccines are lacking and the approved chemotherapeutic compounds present high toxicity, increasing resistance, limited efficacy and require long periods of treatment. Many of these parasitic illnesses predominantly affect low-income populations of developing countries for which new pharmaceutical alternatives are urgently needed. Thus, very low research funding is available. Amidine-containing compounds such as pentamidine are DNA minor groove binders with a broad spectrum of activities against human and veterinary pathogens. Due to their promising microbicidal activity but their rather poor bioavailability and high toxicity, many analogues and derivatives, including pro-drugs, have been synthesized and screened in vitro and in vivo in order to improve their selectivity and pharmacological properties. This review summarizes the knowledge on amidines and analogues with respect to their synthesis, pharmacological profile, mechanistic and biological effects upon a range of intracellular protozoan parasites. The bulk of these data may contribute to the future design and structure optimization of new aromatic dicationic compounds as novel antiparasitic drug candidates.

Fig. 1

Chemical structures of aromatic diamidines and arylimidamides.

Fig. 2

Fluorescent microscopy of aromatic diamidine DB1582 (A) and DB1651 (B) staining of bloodstream trypomastigotes (A) and intracellular forms (B) of T. cruzi (Y strain). Parasites and infected-cardiac cell cultures were incubated for 60 min with10 µg mL⁻¹ DB1582 and DB1651. Note the dicationic compound in the parasite KDNA (arrow), nuclei (star) and within several non-DNA-containing organelles (arrowhead) distributed within the cytoplasm of intracellular amastigotes (acidocalcisomes-like structures). See host cell nuclei also labelled (N).

Fig. 3

Representative examples of diamidines, amidoximes, methamidoximes with some known pharmacokinetic properties.

Fig. 4

Representative examples of AIAs tested against intracellular Leishmania. Panel A – Compounds possessing potent antileishmanial activity (IC₅₀ values <1 µm). Panel B – Compounds that are inactive (IC₅₀ values >10 µm).

Fig. 5

Representative examples of amidines tested against Trypanosoma cruzi in in vitro and in vivo studies.

Fig. 6

Transmission electron microscopy of Neospora caninum tachyzoites cultured in the absence (A, B) and presence (C–E) of the arylimidamide DB745. (A) and (B) show control cultures at 48 h post-infection. Tachyzoites are situated within the cytoplasm and proliferate within a parasitophorous vacuole, surrounded by a parasitophorous vacuole membrane (arrows in (B)). Rhoptries (rop), micronemes (mic), dense granules (dg), mitochondria (mito) and the nucleus (nuc) are visible. Arrows in (A) pointed to the conoid at the apical end of the parasites, while arrows in (B) indicate the parasitophorous vacuole membrane. The nuclei (nuc) contain electron-dense chromatin, and white arrows point to the intact nuclear membrane. Hcmito = host cell mitochondria, hcnuc = host cell nucleus, pvtn = parasitophorous vacuole tubular network forming the matrix of the vacuole. Bar in (A) = 0·84 µm; bar in (B) = 0·55 µm. (C–E) represent images of tachyzoites after treatment with 1 µm DB745 for 24 h (C) and 48 h (D–E). In (C), separated nuclear membranes are pointed out with white arrows. Cytoplasmic vacuoles (v) are partially filled with membranous and/or electron-dense material of unknown origin. Bar = 0·65 µm. (D) Tachyzoites after 48 h of culture but still fused together, showing increased vacuolization (v) and large numbers of dense granules (dg). (E) Higher magnification view of the fused contact sites shown in (D). Parasites are clearly separated by two distinct inner membranes, but held together by the outer plasma membrane. The black arrow indicates the blind ending of the inner membrane of the upper tachyzoite, the white arrow shows the contact site with the outer membrane fusing the parasites together. Bars in (D) = 0·4 µm; (E) = 0·25 µm.

Scheme 1

Reagents and Conditions: a) HCl(g), EtOH b)NH₃ or RNH₂, EtOh Synthesis of furamidine by the Pinner method.

Scheme 2

Reagents and Conditions: a) NH₂OH-HCl, KO-t-Bu, DMSO, rt b) AcOH, Ac₂O, rt c) H₂, Pd-C, HOAc, rt Synthesis of an aromatic diamidine via a diamidoxime.

Scheme 3

Reagents and Conditions: a) LiN(TMS)₂, THF, rt overnight then HCl, EtOh rt 12h Synthesis of an aromatic diamidine using LiN(TMS)₂.

Scheme 4

Reagents and Conditions: a) H₂, Pd/C, EtOH, rt b) SnCl₂ dihydrate, EtOH, DMSO, 80°C c)MeCN, EtOH, rt Synthesis of the arylimidamide DB766.