Background
Each year over half a million people die of malaria, with
Plasmodium falciparum being the primary cause of fatal malaria cases [
1]. As the eradication of malaria is threatened by occurrence of clinical resistance to artemisinin derivatives, new drugs for malaria are sorely needed and so the search for new lead compounds continues [
1].
Based on the observation, that addition of calcium pantothenate to
Plasmodium lophurae cultures increased parasite viability, a selection of analogues of pantothenate (pantothenic acid, vitamin B5), were tested for antiplasmodial activity as early as the 1940s [
2]. These compounds included pantoyltaurine, substituted pantoyltaurylamides, sulphonamides, and pantothenones, according to the nomenclature used in a review on this subject by Spry
et al. [
3]. These and similar compounds were tested in different
in vitro and
in vivo malaria models from the 1960s and 1970s [
4,
5]. In 1976, Trager and Jensen published an article describing the continuous culture of
P. falciparum [
6], allowing Divo
et al. to discover that pantothenate is indeed the only water soluble vitamin that needs to be exogenously available for
P. falciparum survival [
7]. Meanwhile, Clifton
et al. prepared a series of analogues with the general structure N1-(substituted) pantothenamide, and found them to have antibacterial activity due to being antimetabolites of pantothenate [
8].
Recent studies showed that some of the pantothenamides were also active against
P. falciparum in vitro, provided that plasma pantetheinase activity was reduced [
9]. This was discovered due to the observation that ‘aging’
P. falciparum growth media increased the anti-malarial activity of some pantothenamides [
9]. Later, this same effect was achieved with heat inactivation of the parasite growth medium by de Villiers
et al. [
10]. The mechanism of breakdown of pantothenamides by pantetheinases of the vanin family was elucidated in detail by Jansen
et al. who discovered that combining pantothenamides with small molecule vanin inhibitors, protected pantothenamides against breakdown, thereby dramatically increasing their antibacterial activity against both
Staphylococcus aureus and
Escherichia coli [
11-
14]. It has also been shown by de Villiers
et al. that small modifications of the pantothenamide core structure could protect the molecule against pantetheinase-mediated degradation, albeit at a cost of a 100-fold decrease in anti-malarial potency [
10].
Compounds, such as the pantothenamides in
E. coli or the fungal product CJ-15,801 in
S. aureus may hijack Coenzyme A (CoA) biosynthesis, being phosphorylated in the first step of the biosynthesis by pantothenate kinase (PanK) and eventually blocking CoA production or interfering with fatty acid synthesis downstream along the pathway [
15-
17]. Almost a decade ago, the fungal product CJ-15,801, was also discovered to have modest anti-malarial activity against asexual intra-erythrocytic stages of
P. falciparum in vitro, and was demonstrated to inhibit parasite growth by a mechanism related to CoA biosynthesis or utilization [
18].
In this study a selection of novel pantetheine analogues of the pantothenone class were investigated for potential use as anti-malarial chemotherapy. The investigated compounds are shown to be conceptually promising either as a monotherapy or in a combination of drugs.
Discussion
This study underscores the potential of pantothenate derivatives for anti-malarial therapy, and demonstrates that the most potent serum-labile anti-malarial pantothenamide (phenethyl-Pan) can be effectively protected against hydrolysis by serum pantetheinases using the novel vanin inhibitor CXP14.1-060. From a mechanistic point of view, this study indicates that the pantothenone RR8 exerts its anti-malarial effect through competition with pantothenate.
In the 1940s, a number of chemical variations on pantothenate were synthesized and tested for anti-malarial activity [
2]. These included pantothenones and sulphonamides, which were found to be active against avian malaria. This study shows that these compounds are also active against the human parasite
P. falciparum.
Although the new vanin inhibitor CXP14.1-060 effectively protected phenethyl-Pan, such a combination of drugs would be undesirable from a drug development perspective. Clearly, the potency and/or stability of pantothenate derivatives needs to be improved before they can enter a drug development programme as therapeutic agents for human malaria infection. Nevertheless, the recent discovery of phenethyl-Pan with an IC
50 of 20 nM is encouraging [
9]. Although this compound is unstable in plasma, it illustrates that it is realistic and feasible to aim for pantothenate derivatives active in the low nanomolar range. The study by de Villiers
et al. showed that structural modifications of pantothenamides can be introduced to confer resistance to plasma-mediated breakdown [
10]. These novel compounds, although less potent than the original pantothenamides, are a starting point for further lead optimization studies [
10]. Optimization of the potency would be important to maximize the risk-benefit of a novel drug, as side effects may be mediated by low-affinity, off-target effects. In addition, increasing the potency may lead to a lower effective dose in humans, and hence impact the cost of treatment. Availability of affordable medicines is an important driver for success in malaria control, and the goal for development of novel drug therapies is to achieve effective treatment with a total cost of US$1 [
26]. In that respect, the molecules described here provide attractive candidates as their chemistry is simple, which ensures a low cost of goods in a manufacturing process.
Many of the marketed anti-malarials and compounds in the clinical development portfolio originate from whole cell phenotypic screening efforts and exert their actions by inhibiting multiple targets or pathways of the parasite. Although such a polypharmacological profile may be important to their efficacy, it is an undesirable feature in a rational medicinal chemistry approach. The exact target of the anti-malarial pantothenate derivatives has not been identified unequivocally but is it likely that they exert their effects by affecting targets dependent on pantothenate. In theory, the observed effects could still be mediated by effects on red blood cell biology (e.g., red blood cell pantothenate kinases (PANK)) rather than directly on the parasite. However, the recent discovery of a parasite-specific pantothenate transporter [
27] leaves very little doubt that the parasite itself is the target of interfering with pantothenate dependent pathways. Future drug development efforts would benefit from information on the molecular targets of pantothenamides, which would include both biosynthetic pathways (CoA synthesis, lipid synthesis, energy metabolism) and pantothenate transport systems. Structural information, which is available for mammalian and bacterial PANK, could guide medicinal chemistry strategies to achieve specific inhibition of the parasite enzyme and reduce side effects on the host.
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Competing interests
KD and RS hold shares in TropIQ Health Sciences, a spin-off company of the RadboudUMC that aims to develop anti-malarial drugs. JS, FPJTR and PHHH hold shares in Pansynt, a spin-off company of the RadboudUMC that aims to develop pantothenate-based drugs for infectious diseases. Some of the compounds described in the manuscript are covered by a patent application (PCT/NL2011/050385) filed by the RadboudUMC (inventors: PAMJ, JS, RS, PHHH, FPJTR).
Authors’ contributions
HEP carried out the majority of
P. falciparum proliferation assays and the majority of the writing of the manuscript. PAMJ carried out the vanin inhibition assays and wrote some of the manuscript. PHHH is responsible for design of active compounds and for writing of the supplemental material of the manuscript. PNMB and CABS synthesized active compounds. RHB was responsible for conception of chemical synthesis routes. WG and MV-B worked on early experimentation with compounds. KMJK performed the experiment that resulted in Figure
2C. FPJTR is responsible for design of active compounds. KD supervised experimental work of HEP and wrote a substantial part of the discussion section. RS approved work being done during experimental process. JS initiated the project, supervised experimental work of HEP and PAMJ, and wrote a substantial part of this publication. All authors have read and approved final version of the manuscript.