5.4 New Modes of Action/New Targets
169
quite high molecular weights. These compounds are capable of binding to a number
of sites on the cholera toxin receptor.
Bivalent inhibitors of cholera toxin linked via a 1,2,3-triazole unit are discussed
by Leaver et al. (2011), although from this work the strongest inhibitors of cholera
toxin binding to its cell surface receptor were seen with monovalent analogues of
lower molecular weight. These observations could help with trying to design more
potent smaller molecule equivalents and with other targeting substituents.
Instead of antagonists binding to the toxin cell surface receptor, one might consider
compounds which bind directly to the toxin and thus inhibit effective subsequent
binding to the receptor. Clues to inform the design of such decoy toxin traps might
be expected to come from a detailed study of decoy exosomes released by the host
cell to protect against bacterial toxins as described by Keller et al. (2020). These
decoy exosomes, which bind to the toxins, were shown to be active in vitro and
in vivo. In the latter case they were shown to improve survival in MRSA-infected
mice.
Virulence
Considerable scope for multi-targeting is predicted for antivirulence approaches
to attack pathogenic bacteria and this active area has been covered in a recent
review (Calvert et al. 2018). One example of this approach involves the inhibition of macrophage infectivity potentiators (Mips), which are proteins belonging to
the FK506-binding family. They can be important for bacterial virulence as with
the Mip from Burkholderia pseudomallei, the intracellular bacterial pathogen which
causes meliodiosis (Begley et al. 2014). Mips play a role in the invasion of human
macrophages, the white cell type of the immune system responsible, among other
activities, for detecting, engulfing and digesting pathogens. Mips attenuate the effectiveness of the macrophage in its host protection role. The antibiotic Rapamycin can
inhibit this Mip as can some pipecolic acid derivatives and a future endeavour may
wish to look at ways in which this inhibitory activity could synergise with other
activities.
A further example of antivirulence compounds are those developed to negatively
impact on DsbA, the Gram-negative, periplasmic thiol-disulfide oxido/reduction
system important for bacterial virulence (Heras et al. 2015). In view of this, inhibitors
of this system are being investigated using a fragment based approach to identify a new structural class of benzofuran inhibitors of DsbA from Escherichia coli
(Duncan et al. 2019). The strongest binders to DsbA were found to be 2-(6-phenoxy
or 6-benzylbenzofuran-3-yl)acetic acid derivatives which bind in the hydrophobic
groove of the enzyme close to the catalytic disulfide bond. Further optimization of
these structures is indicated. The strongly oxidising DsbA is a member of the Dsb
(disulfide bond) family of enzymes which catalyse critical intramolecular disulfide
bond formation in peptides as they enter the periplasm of the cell. The disulfide bond
formation occurs in a range of virulence factors and, significantly, without functional
DsbA such bacteria have reduced virulence, heightened sensitivity to antibiotics and
a reduced capacity to initiate infection (Heras et al. 2015).
169
quite high molecular weights. These compounds are capable of binding to a number
of sites on the cholera toxin receptor.
Bivalent inhibitors of cholera toxin linked via a 1,2,3-triazole unit are discussed
by Leaver et al. (2011), although from this work the strongest inhibitors of cholera
toxin binding to its cell surface receptor were seen with monovalent analogues of
lower molecular weight. These observations could help with trying to design more
potent smaller molecule equivalents and with other targeting substituents.
Instead of antagonists binding to the toxin cell surface receptor, one might consider
compounds which bind directly to the toxin and thus inhibit effective subsequent
binding to the receptor. Clues to inform the design of such decoy toxin traps might
be expected to come from a detailed study of decoy exosomes released by the host
cell to protect against bacterial toxins as described by Keller et al. (2020). These
decoy exosomes, which bind to the toxins, were shown to be active in vitro and
in vivo. In the latter case they were shown to improve survival in MRSA-infected
mice.
Virulence
Considerable scope for multi-targeting is predicted for antivirulence approaches
to attack pathogenic bacteria and this active area has been covered in a recent
review (Calvert et al. 2018). One example of this approach involves the inhibition of macrophage infectivity potentiators (Mips), which are proteins belonging to
the FK506-binding family. They can be important for bacterial virulence as with
the Mip from Burkholderia pseudomallei, the intracellular bacterial pathogen which
causes meliodiosis (Begley et al. 2014). Mips play a role in the invasion of human
macrophages, the white cell type of the immune system responsible, among other
activities, for detecting, engulfing and digesting pathogens. Mips attenuate the effectiveness of the macrophage in its host protection role. The antibiotic Rapamycin can
inhibit this Mip as can some pipecolic acid derivatives and a future endeavour may
wish to look at ways in which this inhibitory activity could synergise with other
activities.
A further example of antivirulence compounds are those developed to negatively
impact on DsbA, the Gram-negative, periplasmic thiol-disulfide oxido/reduction
system important for bacterial virulence (Heras et al. 2015). In view of this, inhibitors
of this system are being investigated using a fragment based approach to identify a new structural class of benzofuran inhibitors of DsbA from Escherichia coli
(Duncan et al. 2019). The strongest binders to DsbA were found to be 2-(6-phenoxy
or 6-benzylbenzofuran-3-yl)acetic acid derivatives which bind in the hydrophobic
groove of the enzyme close to the catalytic disulfide bond. Further optimization of
these structures is indicated. The strongly oxidising DsbA is a member of the Dsb
(disulfide bond) family of enzymes which catalyse critical intramolecular disulfide
bond formation in peptides as they enter the periplasm of the cell. The disulfide bond
formation occurs in a range of virulence factors and, significantly, without functional
DsbA such bacteria have reduced virulence, heightened sensitivity to antibiotics and
a reduced capacity to initiate infection (Heras et al. 2015).