4.2 Introduction to Prodrugs for Triple or Higher Action …
141
have one or more activities, would then be released in a potentially time controlled
manner which could improve potency and final antibacterial outcomes. It is of note
that nature can encapsulate enzymes in nanometric compartments to increase and
modulate the activity of the enzyme (Zhang et al. 2019).
4.3 Release Mechanisms and Prodrug Design
Prodrug release mechanisms, which are crucial considerations in prodrug design,
can involve enzymatic or non-enzymatic (chemical or physical) processes or combinations of these. In the following sub-sections these mechanisms are discussed in
the three main categories: biological, chemical and physical processes.
4.3.1 Biological
A number of bacterial enzymes have been used for prodrug activation and drug
release at intracellular, cell wall, and extracellular bioactivation sites with the last
being close to or contiguous with the bacterium. Most commonly, bacterially specific
β-lactamases (Ehmann et al. 2013) or PBP-penicillin binding proteins have been
utilised for the bioactivation process. A range of others have been studied from the
bioactivation viewpoint including peptide deformylase, an Fe
2+ dependent metallohydrolase which mediates hydrolysis of a terminal N-formyl group from peptides
(Sangshetti et al. 2015; Yuan et al. 2001), azoreductase, bacterial esterases, and
bacterial amidases (for example penicillin G amidase (PGA). Prodrugs need to be
substrates for the bacterial enzymes and preferably not interacting irreversibly at
the enzyme active site so that activation can be continued with release of the active
products. While enzymes only occurring in bacteria are preferential targets for the
activation process, differences between bacterial and human enzymes with a similar
function, for example the recently identified human mitochondrial peptide deformylase, can possibly still be exploited to achieve bacterial specificity. Other issues
which need to be considered in the prodrug design and activation are the differences
in enzymes between Gram-positive and Gram-negative bacteria.
A further consideration is the potential problem of the site of release of the active
compound (s) relative to the antibacterial target sites as in the case of release intracellularly while the target(s) may be in the cell wall or vice versa. Anatomical sites
can also perhaps be used to advantage for specific release. For example, azo group
reducing bacteria are present in the colon (Abet et al. 2017) and it is suggested this
enzymatic reducing ability could potentially be utilised for local release and subsequent absorption of a suitable antibacterial or for localised selective treatment of a
bacterially-mediated colon infection. The ability of the azo group reducing bacteria
to release an active anti-inflammatory drug (5-aminosalicylic acid, which exerts its
effect locally) has been realised with the prodrug balsalazide (Fig. 4.5) (Abet et al.
141
have one or more activities, would then be released in a potentially time controlled
manner which could improve potency and final antibacterial outcomes. It is of note
that nature can encapsulate enzymes in nanometric compartments to increase and
modulate the activity of the enzyme (Zhang et al. 2019).
4.3 Release Mechanisms and Prodrug Design
Prodrug release mechanisms, which are crucial considerations in prodrug design,
can involve enzymatic or non-enzymatic (chemical or physical) processes or combinations of these. In the following sub-sections these mechanisms are discussed in
the three main categories: biological, chemical and physical processes.
4.3.1 Biological
A number of bacterial enzymes have been used for prodrug activation and drug
release at intracellular, cell wall, and extracellular bioactivation sites with the last
being close to or contiguous with the bacterium. Most commonly, bacterially specific
β-lactamases (Ehmann et al. 2013) or PBP-penicillin binding proteins have been
utilised for the bioactivation process. A range of others have been studied from the
bioactivation viewpoint including peptide deformylase, an Fe
2+ dependent metallohydrolase which mediates hydrolysis of a terminal N-formyl group from peptides
(Sangshetti et al. 2015; Yuan et al. 2001), azoreductase, bacterial esterases, and
bacterial amidases (for example penicillin G amidase (PGA). Prodrugs need to be
substrates for the bacterial enzymes and preferably not interacting irreversibly at
the enzyme active site so that activation can be continued with release of the active
products. While enzymes only occurring in bacteria are preferential targets for the
activation process, differences between bacterial and human enzymes with a similar
function, for example the recently identified human mitochondrial peptide deformylase, can possibly still be exploited to achieve bacterial specificity. Other issues
which need to be considered in the prodrug design and activation are the differences
in enzymes between Gram-positive and Gram-negative bacteria.
A further consideration is the potential problem of the site of release of the active
compound (s) relative to the antibacterial target sites as in the case of release intracellularly while the target(s) may be in the cell wall or vice versa. Anatomical sites
can also perhaps be used to advantage for specific release. For example, azo group
reducing bacteria are present in the colon (Abet et al. 2017) and it is suggested this
enzymatic reducing ability could potentially be utilised for local release and subsequent absorption of a suitable antibacterial or for localised selective treatment of a
bacterially-mediated colon infection. The ability of the azo group reducing bacteria
to release an active anti-inflammatory drug (5-aminosalicylic acid, which exerts its
effect locally) has been realised with the prodrug balsalazide (Fig. 4.5) (Abet et al.