136
4 Design Principles and Development of Prodrugs for Multiply …
be likely to improve the applicability of the approach to deeper infection sites in the
host.
Another possible alternative to NO is nitroxyl (HNO), which is well known in
the gas phase. It is a weak acid, with the conjugate base NO
− being a reduced
form of NO and isoelectronic with dioxygen. When HNO is released in the aqueous
medium of the bacterial cell it would also be the major species present rather than
NO
− as the pKa has been assessed as 11.5, rather than 4.7 as thought previously
(Switzer et al. 2009). However, the deprotonation reaction of HNO is very slow due
to intersystem crossing and the kinetic and thermodynamic barriers are high enough to
prevent interconversion between HNO and NO. Both species have different chemical
reactivities with NO being a nucleophile favouring electron transfer, while HNO is a
good electrophile which favours addition reactions to nucleophiles, particularly thiols
with consequences for the biochemical targets attacked (Switzer et al. 2009). Also, if
in sufficient concentration locally, HNO dimerises and the dimer (hyponitrous acid)
then decomposes to give nitrous oxide and water.
4.2.5 Cleavable Type III
A- -B- -C →→ A
+ B
+ C
In this type iii case sequential cleavage of each linking group would lead ultimately
to the release of three separate components A
, B
and C
. One expression of this type
concerns a block copolymer (A) to which the antibiotic gentamicin (B) was attached
via a hydrolysable imine linkage to an aromatic aldehyde group in the polymer
and then an N-diazeniumdiolate (C) attached via the secondary amine group (–NH–
CH 3 ) in the gentamicin unit. On exposure to mildly acidic conditions in the bacteria,
simultaneous release of nitric oxide (C
) (from diazeniumdiolate hydrolysis) and
gentamicin (from imine hydrolysis) resulted in a prolonged antibacterial effect which
reduced the viability of a biofilm of Pseudomonas aeruginosa by greater than 90%
in vitro. There is a synergistic effect from having both gentamicin and nitric oxide
present (Nguyen et al. 2016). Nguyen and co-workers have subsequently reviewed
nano-nitric oxide delivery systems and their applications, including antibacterial ones
(Nguyen et al. 2018).
Another expression of type ii is that seen with a compound (compound 5) in
the results presented by Lee et al. (2004). This compound is a tripartite carrierlinked prodrug with two different antibacterial (nalidixic acid and 5-aminoquinoline)
moieties at each terminus with a linking TAT peptide unit. Phenylacetic ester groups
at each end serve as substrates for bacterial penicillin G amidase (PGA) which on
hydrolysis then triggers eliminative release of the two antibacterials plus quinone
methides attached to the TAT peptide which then undergo further intramolecular reactions. The compound was very potent against an Escherichia coli strain transformed
with a PGA gene (not all strains have PGA). One could envisage simpler in-principle
4 Design Principles and Development of Prodrugs for Multiply …
be likely to improve the applicability of the approach to deeper infection sites in the
host.
Another possible alternative to NO is nitroxyl (HNO), which is well known in
the gas phase. It is a weak acid, with the conjugate base NO
− being a reduced
form of NO and isoelectronic with dioxygen. When HNO is released in the aqueous
medium of the bacterial cell it would also be the major species present rather than
NO
− as the pKa has been assessed as 11.5, rather than 4.7 as thought previously
(Switzer et al. 2009). However, the deprotonation reaction of HNO is very slow due
to intersystem crossing and the kinetic and thermodynamic barriers are high enough to
prevent interconversion between HNO and NO. Both species have different chemical
reactivities with NO being a nucleophile favouring electron transfer, while HNO is a
good electrophile which favours addition reactions to nucleophiles, particularly thiols
with consequences for the biochemical targets attacked (Switzer et al. 2009). Also, if
in sufficient concentration locally, HNO dimerises and the dimer (hyponitrous acid)
then decomposes to give nitrous oxide and water.
4.2.5 Cleavable Type III
A- -B- -C →→ A
+ B
+ C
In this type iii case sequential cleavage of each linking group would lead ultimately
to the release of three separate components A
, B
and C
. One expression of this type
concerns a block copolymer (A) to which the antibiotic gentamicin (B) was attached
via a hydrolysable imine linkage to an aromatic aldehyde group in the polymer
and then an N-diazeniumdiolate (C) attached via the secondary amine group (–NH–
CH 3 ) in the gentamicin unit. On exposure to mildly acidic conditions in the bacteria,
simultaneous release of nitric oxide (C
) (from diazeniumdiolate hydrolysis) and
gentamicin (from imine hydrolysis) resulted in a prolonged antibacterial effect which
reduced the viability of a biofilm of Pseudomonas aeruginosa by greater than 90%
in vitro. There is a synergistic effect from having both gentamicin and nitric oxide
present (Nguyen et al. 2016). Nguyen and co-workers have subsequently reviewed
nano-nitric oxide delivery systems and their applications, including antibacterial ones
(Nguyen et al. 2018).
Another expression of type ii is that seen with a compound (compound 5) in
the results presented by Lee et al. (2004). This compound is a tripartite carrierlinked prodrug with two different antibacterial (nalidixic acid and 5-aminoquinoline)
moieties at each terminus with a linking TAT peptide unit. Phenylacetic ester groups
at each end serve as substrates for bacterial penicillin G amidase (PGA) which on
hydrolysis then triggers eliminative release of the two antibacterials plus quinone
methides attached to the TAT peptide which then undergo further intramolecular reactions. The compound was very potent against an Escherichia coli strain transformed
with a PGA gene (not all strains have PGA). One could envisage simpler in-principle