Synthesis and biological evaluation of hydrophilic derivatives of decoquinate
Abstract
Malaria is a mosquito borne disease that is caused by a parasitic protozoan, belonging
to the genus, Plasmodium. Five Plasmodium species are known to infect humans, of which
Plasmodium falciparium is the most virulent. Malaria poses a global health threat, with 40%
of the world‘s population at risk of contracting the disease. In 2015 alone, 216 million people
were reportedly infected with malaria, of which 438,000 died from the disease. This renders
malaria the third leading cause of deaths, following tuberculosis (TB) and acquired
immunodeficiency syndrome (AIDS). Sub-Saharan Africa accounts for 90% of the total
malarial burden.
Tuberculosis (TB) is caused by mycobacteria, with Mycobacterium tuberculosis (Mtb)
being the most important. In 2015, 9.6 million cases of TB and 1.4 million related deaths
were reported, making this disease the leading cause of human mortalities. Eastern Europe
and the South-East Asia regions carry the highest TB burden and account for 58% of the total
TB load, while sub-Saharan Africa accounts for 28% of all reported TB cases.
The troublesome fact about these two diseases is the development and spread of
pathogenic strains that are resistant towards all drugs currently being used clinically for their
treatment. P. falciparum, for example, had developed resistance towards prominent antimalarial
drugs, like chloroquine, Fansidar and mefloquine. A P. falciparum strain that is
resistant to artemisinin combination therapies (ACTs), which are currently used as frontline
drugs for the treatment of malaria, has been reported in at least six different regions in Asia.
Mtb strains that are resistant towards the first line TB treatment regimens, (i.e. rifampicin
andisoniazid) and towards the last treatment option for TB (i.e. fluoroquinolones and
injectable TB drugs, such as kanamycin, amikacin and capreomycin) have been documented
worldwide.
Such growing development and spread of malaria and TB drug resistant pathogens
emphasise the urgent need for identifying and developing new drugs that would help curb the
spread of these diseases.
Decoquinate (DQ) is a safe and inexpensive drug that has been in use for over 30
years for the treatment of coccidiosis infections in livestock. DQ has also demonstrated
potent anti-malarial activity in vitro against the liver, asexual blood stages and gametocytic
stages of malaria parasites. Interestingly, DQ has a flexible and long alkyl chain, rendering it highly lipophilic. This characteristic is expected to make DQ easily permeable to lipophilic layers, such as the mycolic acid wall that surrounds the Mtb. However, it is in fact its poor solubility that has hampered the development of DQ as a human therapeutic agent. DQ also has a metabolically susceptible ester group, which, if hydrolysed into a carboxylic acid in vivo, may reduce the bio-availability of DQ.
During this study, to address the poor solubility of DQ and its undesirable, metabolically susceptible ester, a total of seventy-seven derivatives of DQ were synthesised and evaluated in vitro for their anti-malarial activities against chloroquine sensitive (NF54) and multi-drug resistant (W2 and K1) strains of P. falciparum, for their anti-tubercular activities against a rifampicin sensitive strain of Mtb (H37RV) and for their cytotoxicities against normal human fetal lung fibroblast (Wl-38) cell lines. These results are presented in chapters 4, 5 and 6. Chapter 4: ―Straightforward conversion of decoquinate into inexpensive tractable new quinolones with significant anti-malarial activities‖. This chapter presents the syntheses, cytotoxicity and anti-malarial evaluations of a series of thirty-five decoquinate derivatives. These compounds were prepared by using either simple aminolysis, during which the ethyl ester in DQ was converted into an amide in the presence of the corresponding amine reagent, and/or through acylation, during which the N-1 nitrogen atom of DQ was converted into an amide, upon reacting it with an acyl chloride. All reactions occurred in a basic medium in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The most active compound among this series was a derivative, bearing an acetyl group attached to N-1 of decoquinate. This compound was found five times more active than decoquinate against the Pf NF54 and K1 strains, having an activity profile that was comparable to those of artemether and artesunate against these strains. The selectivity index for this compound was >6494 with respect to normal human fetal lung fibroblast (Wl-38) cell lines, indicating that this compound was not toxic. Chapter 5: ―Syntheses of new decoquinate derivatives with potent anti-mycobacterial activities‖. This chapter presents the syntheses of twenty-five decoquinate derivatives, their anti-tubercular activities against the H37Rv strain of Mtb and their cytotoxicity profiles against the WI-38 cell line. These compounds were prepared, either through N-alkylation at N-1 of DQ with an alkyl bromide and aminolysis, or through N-acylation and aminolysis. Twenty-three of these compounds showed moderate to good activities against Mtb, with the most active compound having an MIC99 of 1 μM. This compound, that contains an ethyl group attached to N-1, and an N-[2-(2-hydroxyethoxy)ethyl]acetamide group at C-3 of the quinolone nucleus, had a selectivity index higher than 10, indicative of adequate selectively against Mtb. Chapter 6: ―New decoquinate derivatives with improved solubilities and in vitro antimalarial activities‖. This chapter reports on the syntheses and anti-malarial activities of a series of seventeen decoquinate derivatives. The compounds in this series were synthesised in a manner similar to those described in chapters 4 and 5. The biological assessments were conducted, using the same strains and cell lines, as in those previous chapters. The most active compound in this part of the study was found to be the DQ derivative, bearing a sulfonyl containing group attached to N-1 of the quinolone. It possessed a twenty-six-fold higher activity than decoquinate against Pf NF54, with an overall activity profile (IC50 ~1 nM) that was superior to those of artemether and artesunate, regardless of the P. falciparum strain used. This derivative showed no toxicity towards mammalian cells, as evidenced by its high selectivity index (SI) of 71428. In summary, this study has uncovered a cost-effective anti-tubercular hit, synthesised from the non-active parent drug, decoquinate. Furthermore, this study has also led to the discovery of three new derivatives with superior anti-malarial activities in vitro, compared to decoquinate, artemether, artesunate and chloroquine against both chloroquine sensitive and -resistant strains of P. falciparum. It is anticipated that these promising compounds may qualify as potential candidates for further investigation in the search for new and effective anti-malarial and anti-tubercular drugs
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