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Inhibiting energy metabolism to combat M. tuberculosis resistance
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Inhibiting Energy Metabolism, a New Technique to Combat M. tuberculosisResistance
Abstract
Mycobacterium tuberculosis is a human pathogen and the causative agent for tuberculosis. The disease
is responsible for 1,6 million deaths per year. Over the years an increasing number of cases have been
reported where M. tuberculosis was partially or completely resistant against current drug treatments. In
this review, several new targets for anti-tuberculosis drugs will be discussed in detail. These anti-
tuberculosis drugs have a low frequency of resistance and a high bactericidal efficacy which could
mean the new future of anti-tuberculosis drugs.
Introduction
Mycobacterium tuberculosis is a bacillus responsible for the infectious disease tuberculosis. The disease
is responsible for 1,6 million deaths per year and statistics show that around one-third of the world
population will at one point get infected with the bacteria in their lifetime [1]. Another emerging
problem is the resistance of M. tuberculosis against current antibiotic drugs. There can be multiple
reasons as to why bacteria can become resistant to antibiotic drugs such as adaptation of the bacteria to
other metabolic pathways, modification of target proteins or extrusion [2]. The increasing number of
cases where M. tuberculosis has become (partially) resistant against current drug treatments shows as
well that there is an urgent need for new antitubercular drugs [3].
In the past couple of years, research has shown that energy metabolism, more specifically, oxidative
phosphorylation might be an effective target against M. tuberculosis. Oxidative phosphorylation takes
place in the mitochondria of the bacteria. Through successive protein complexes, ATP is obtained,
which is the essential source for the growth and survival of the bacteria. Inhibition of oxidative
phosphorylation would therefore lead to the death of the bacteria [3].
Promising drug candidates are diarylquinoline R207910 (TMC207), which reduces the ATP
synthesizing, ND-011992 combined with Q203 and Imidazo[1,2-a]pyridine inhibitors which both
inhibit the cytochrome bcc-aa3. In this review paper each drug is discussed and which part of the
oxidative phosphorylation they target and their effectivity. Moreover, a review is given and suggestions
are made for possible future research on antitubercular drugs.
Diarylquinolines targets subunit c of mycobacterial ATP synthase
As mentioned in the introduction, targeting energy metabolism is a new
way of investigating antibacterial drug discovery. For example,
bedaquiline (BDQ), also known as R207910, TMC207 and Sirturo,
which belongs to the chemical class known as diarylquinolines
(DARQs). This drug could play an important role in developing
treatment against tuberculosis because it aims to reduce the ATP
synthesizing capability in M.tuberculosis. As known, the ATP
synthesizing capability is positively affected in mycobacteria such as M.
tuberculosis. BDQ works highly restrictive on the mycobacterial energy
metabolism, which causes an energy shortage and is thus effective
against the survival of M. tuberculosis. BDQ has two chiral centers
which lead to four stereoisomers. Stereoisomer (R,S) is the most active
against M. tuberculosis; on the contrary, the enantiomer (S,R) was less
active. The stereoselective activity is suggestive of specific binding to the target protein. Recent
calculations suggested that out of all four stereoisomers, BDQ would bind best to the ATP synthase
subunit c [2].
BDQ is a species-specific inhibitor of the mycobacterial F 1F0 -type ATP synthase. The mechanism of
action of the drug is inhibition of ATP synthase F0-region, which blocks proton translocation and
rotation of the rotor in the F 0-region, figure 1. ATP hydrolysis activity requires the presence of
dissociated F1 regions or the disjoining of proton translocation from rotation. Inhibition of ATP
hydrolysis by the ATP synthase is a concentration-dependent manner. This inhibition is effective when
BDQ is present in low concentrations. Disrupting the F0-region with high concentrations of BDQ
, disjoins ATP hydrolysis and proton translocation. Thereby, BDQ binding will no longer block ATP
hydrolysis. BDQ at low concentrations appears to deplete the proton motive force by disrupting the
interface between the a-and c-subunits to induce a proton leak [4]. For this reason, BDQ seems to be a
promising drug to combat M. tuberculosis.
Using cytochrome bcc-aa3 as the main target
Another possible target could be the bcc-aa3 complex which is also part of the mycobacterial electron
chain [5]. The bcc-aa3 complex is made up of different subunits, such as the cytochrome bc1 complex.
This subunit complex catalyzes the oxidation of ubihydroquinone and reduces cytochrome c in the
mitochondrial respiratory chain, as seen in figure 2. Since bcc-aa3 is a very important complex for the
production of ATP in the bacterial mitochondria, interfering with this mechanism will result in
mitochondrial myopathies, which is consequently lethal for the survival of M. tuberculosis [6].
As mentioned before, M. tuberculosis is getting more resistant to current antitubercular drugs and the
development of new drugs is needed. The QcrB gene encodes for the protein cytochrome b subunit
which is part of the cytochrome bc1 complex and might be a possible target for new drugs since
inhibition of this gene would result in myopathy of the mitochondrial process in M. tuberculosis[7].
Through research, Imidazo[1,2-a]pyridine inhibitors (IP inhibitors) have been found to inhibit the QcrB
gene. This has been proven through experiments, when researchers overexpressed the QcrB gene, IP
inhibitors were not able to inhibit the QcrB gene which led to the survival of M. tuberculosis. This was
not the case with normal QcrB gene expression. Furthermore, IP inhibitors were also not able to inhibit
the QcrB gene, when there was a mutation in the amino acid sequence (ACC to GCC) in the QcrB gene.
This research also showed that treatment with IP-inhibitors was effective against tuberculosis. These
two studies show that the IP inhibitors are potential new antitubercular drugs [7].
Recently, another drug named Telacebec (Q203) has been found to inhibit cytochrome bcc-aa3. This
drug binds to the cytochrome b subunit of the bcc-aa3 complex[8].
However, a problem arises when only a cytochrome bcc-aa3 oxidase inhibitor is used. In this case, a
second terminal oxidase is used in the mycobacterial electron transport chain, named cytochrome bd
oxidase. Cytochrome bd oxidase is less energetically efficient, however it can facilitate mycobacterial
survival during stresses [5]. This second terminal oxidase limits the bactericidal potency of Q203, thus
an inhibitor of cytochrome bd oxidase is necessary to increase the effectiveness of Q203.
This second terminal oxidase inhibitor appears to be ND-011992, which is ineffective on its own in
killing M. tuberculosis, as is Q203 [9]. That is why the two-terminal oxidase inhibitors, Q203 and ND-
011992, should be used together to inhibit cytochrome bcc-aa3, forming a bactericidal drug
combination that kills M. tuberculosis irrespective of its metabolic state or growth status [9]. Normally,
M. tuberculosis enters a non-replicative, antibiotic-tolerant state when it is exposed to low oxygen
tension, low nutrient availability, or other forms of environmental stresses [10]. When the bactericidal
drug combination inhibits cytochrome bcc-aa3, transcription of respiratory chain genes is inhibited, and
ATP homeostasis is disturbed. These changes lead to less oxygen consumption and cell death [8].
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