Quinone-amino acid conjugates targeting Leishmania amino acid transporters.

The aim of the present study was to investigate the feasibility of targeting Leishmania transporters via appropriately designed chemical probes. Leishmania donovani, the parasite that causes visceral leishmaniasis, is auxotrophic for arginine and lysine and has specific transporters (LdAAP3 and LdAAP7) to import these nutrients. Probes 1-15 were originated by conjugating cytotoxic quinone fragments (II and III) with amino acids (i.e. arginine and lysine) by means of an amide linkage. The toxicity of the synthesized conjugates against Leishmania extracellular (promastigotes) and intracellular (amastigotes) forms was investigated, as well their inhibition of the relevant amino acid transporters. We observed that some conjugates indeed displayed toxicity against the parasites; in particular, 7 was identified as the most potent derivative (at concentrations of 1 µg/mL and 2.5 µg/mL residual cell viability was reduced to 15% and 48% in promastigotes and amastigotes, respectively). Notably, 6, while retaining the cytotoxic activity of quinone II, displayed no toxicity against mammalian THP1 cells. Transport assays indicated that the novel conjugates inhibited transport activity of lysine, arginine and proline transporters. Furthermore, our analyses suggested that the toxic conjugates might be translocated by the transporters into the cells. The non-toxic probes that inhibited transport competed with the natural substrates for binding to the transporters without being translocated. Thus, it is likely that 6, by exploiting amino acid transporters, can selectively deliver its toxic effects to Leishmania cells. This work provides the first evidence that amino acid transporters of the human pathogen Leishmania might be modulated by small molecules, and warrants their further investigation from drug discovery and chemical biology perspectives.

Conjugation of quinones with natural polyamines: toward an expanded antitrypanosomatid profile.

A combinatorial library of quinone-polyamine conjugates designed to optimize the antitrypanosomatid profile of hit compounds 1 and 2 has been prepared by a solid-phase approach. The conjugates were evaluated against the three most important human trypanosomatid pathogens (Trypanosoma brucei rhodesiense, Trypanosoma cruzi, and Leishmania donovani), and several showed promising activity. A subset also inhibited trypanothione reductase in vitro and induced oxidase activity of the enzyme. A highly potent analogue (7) was identified with activity against T. brucei as low as 70 nM and a selectivity index of 72. Interestingly, the presence of a cadaverine tail confers to 7 the ability to target mitochondrial function in Leishmania. In fact, in L. donovani promastigotes, we verified for 7 a decrease of cytoplasmic ATP and mitochondrial potential. Therefore, the current results support the suitability of the conjugation approach for the development of novel polyamine conjugates with enhanced therapeutic potential.

Multitargeted drugs discovery: balancing anti-amyloid and anticholinesterase capacity in a single chemical entity.

Memoquin (1) is a lead compound multitargeted against Alzheimer's disease (AD). It is an AChE inhibitor, free-radical scavenger, and inhibitor of amyloid-ß (Aß) aggregation. A new series of 1 derivatives was designed and synthesized by linking its 2,5-diamino-benzoquinone core with motifs that are present in the structure of known amyloid binding agents like curcumin, the benzofuran derivative SKF64346, or the benzothiazole bearing compounds KHG21834 and BTA-1. The weaker AChE inhibitory potencies and the concomitant nearly equipotent anti-amyloid activities of the new compounds with respect to 1 resulted in a more balanced biological profile against both targets. Selected compounds turned out to be effective Aß aggregation inhibitors in a cell-based assay. By properly combining two or more distinct pharmacological properties in a molecule, we can achieve greater effectiveness compared to single-targeted drugs for investigating AD.

Complementary medicinal chemistry-driven strategies toward new antitrypanosomal and antileishmanial lead drug candidates.

Trypanosomiases and Leishmaniases are neglected tropical diseases that affect the less developed countries. For this reason, they did not and still do not have high visibility in Western societies. The name neglected diseases also refers to the fact that they often received little interest at the level of public investment, research and development. The drug discovery scenario, however, is changing dramatically. After a period in which different socioeconomic factors have prevented massive research efforts in this field, such efforts have increased considerably in the very recent years, with significant scientific advancements. In this context, we have embarked on a new drug discovery project devoted to identification of new small molecules for the treatment of trypanosomal and leishmanial diseases. Two complementary approaches have been pursued and are reported here. The first deals with a structure-based drug design, and a privileged structure-guided synthesis of quinazoline compounds able to modulate trypanothione reductase activity was accomplished. In the second, a combinatorial library, built on a natural product-based strategy, was synthesized. Using whole parasite assays, different quinones have been identified as promising lead compounds. A combination of both approaches to hopefully overcome some of the challenges of anti-trypanosomatid drug discovery has eventually been proposed.

Privileged structure-guided synthesis of quinazoline derivatives as inhibitors of trypanothione reductase.

Novel quinazoline-type compounds were designed as inhibitors of the parasite specific enzyme trypanothione reductase (TR), and their biological activities were evaluated. Some of our compounds inhibited TR, showed selectivity for TR over human glutathione reductase, and inhibited parasite growth in vitro. We propose that the quinazoline framework is a privileged structure that can be purposely modified to design novel TR inhibitors. Furthermore, the use of privileged motifs might emerge as an innovative approach to antiparasitic lead candidates.

Synthesis of a small library of 2-phenoxy-1,4-naphthoquinone and 2-phenoxy-1,4-anthraquinone derivatives bearing anti-trypanosomal and anti-leishmanial activity.

Taking advantage of the structural features of natural products showing anti-trypanosomatid activity, we designed and synthesized a small library of 2-phenoxy-1,4-naphthoquinone and 2-phenoxy-1,4-anthraquinone derivatives. The library was obtained following a parallel approach and using readily available synthons. All the derivatives showed inhibitory activity toward either Trypanosoma or Leishmania species, with 8, 10, and 16 being the most active compounds against Trypanosoma brucei rhodesiense, Leishmania donovani, and Trypanosoma cruzi cells (IC(50)=50nM, IC(50)=0.28microM, and IC(50)=1.26microM, respectively).