DISCUSSION
The PK/PD parameters of β-lactam/β-lactamase inhibitor combos are
challenging to determine. This is because some β-lactamase inhibitors
have antibacterial activity in addition to protecting β-lactams from
β-lactamases, and the antimicrobial effects of various combinations are
interdependent. There is a need to establish accurate analytical methods
to determine the PK/PD parameters of β-lactam/β-lactamase inhibitor
combinations because inappropriate analytical methods or models with low
predictive accuracy risks creating a dissociation between susceptibility
data in vitro and actual clinical efficacy. The current research
shows how the MICi concept may be used in conventional
PK/PD analysis to produce PK/PD parameters that are more practical,
versatile, and accurate than those produced usingf T>CT.
The first generation β-lactamase inhibitor combination
piperacillin/tazobactam has been tested for the MIC of piperacillin in a
fixed concentration of tazobactam of 4 mg L-1, at a
dose ratio of 8:1. On the other hand, amoxicillin/clavulanate is tested
for susceptibility at a fixed 2:1 ratio, although several formulations
with ratios of 2:1, 4:1 and 7:1 are used and may not correlate with
efficacy. This in vitro and in vivo difference in
combination ratios may be due to the fluctuating susceptibility of
β-lactam drugs in response to changing inhibitor concentrations over
time is ignored during in vivo analysis. Optimisation off T>CT is based on the assumption
that the susceptibility of β-lactam drugs is constant in the presence of
β-lactamase inhibitors. However, MICs for clinical strains are not
incorporated into the dosing design and do not adequately reflect
changes in β-lactam drug susceptibility caused by different β-lactamase
inhibitor concentrations.
In the present study, the MIC curve for aztreonam varied significantly
in the therapeutic concentration range of nacubactam (approximately
0.1-10 mg L-1) (Fig. 2). It confirmed the importance
of the PK/PD parameters for β-lactam drugs/β-lactamase inhibitors should
consider changes in susceptibility under different β-lactamase inhibitor
concentration conditions. On the other hand,f T>MICi can reflect changes in
β-lactam susceptibility and changes in β-lactamase inhibitor
concentration. In order to compare the accuracy of these two PK/PD
parameters, an attempt was made to predict clinical efficacy in humans
using each analysis method. The results showed that for K.
pneumoniae ATCC BAA-2473 and K. pneumoniae MSC 21444, the
bactericidal effect simulated byf T>CT tended to be estimated lower
than f T>MICi (orange area of 2
log10-kill in Tables 2 and 3). This may be due to
underestimating the PK/PD predictions based onf T>CT. Higher aztreonam
concentrations are clinically effective even when nacubactam
concentrations are below the CT value. Therefore,f T>MICi is considered a more
accurate PK/PD parameter than f T>CT.
However, the f T>MICi approach
requires time-consuming susceptibility testing andE max modelling analysis.
It is worth noting that analyses based onf T>MICi have practical versatility
because they do not need to consider all factors. The MIC curve reflects
enzyme type, inhibitory activity, antimicrobial activity, and the
additive and synergistic effects of antimicrobial agents. Indeed, the
analysis using f T>MICi showed a good
correlation (R 2=0.868) for three β-lactamase
enzyme-producing strains of different genotypes, confirming the in
vivo validity of the f T>MICi-based
analysis. On the other hand, a similar analysis usingf T>CT confirmed that the
CT values of nacubactam depended on the strain or the
dose of aztreonam used in combination (Fig. 7). These results suggest
that PK/PD analysis using f T>CTlacks practical versatility in determining a single target value for
various clinical isolates.
The novel DBO-based β-lactamase inhibitor, nacubactam, is a potent
inhibitor of Ambler class A (e.g. KPC and ESBL) and C (AmpC)
β-lactamases but weakly inhibits class D (OXA) β-lactamases (Morinaka et
al., 2015),(Morinaka et al., 2016). In addition, nacubactam exhibits
antibacterial activity through binding to the bacterial
penicillin-binding protein (PBP)-2. Furthermore, unlike first-generation
β-lactamase inhibitors (e.g. sulbactam, clavulanic acid, tazobactam),
second-generation DBO-based β-lactamase inhibitors, including
nacubactam, do not have a β-lactam structure and are not subject to
β-lactamase degradation. Therefore, they are expected to be a new
treatment option for carbapenemase (Metallo-β-lactamase)-producing
bacteria, including Class B (NDM and IMP) (Livermore et al.,
2015),(Mushtaq et al., 2018), because no other established treatment is
available. In the current checkerboard MIC study, aztreonam/nacubactam
showed good combination efficacy against Class B and Class D
β-lactamase-producing bacteria such as NDM-1, IMP-6, and OXA-48
β-lactamase producing K. pneumoniae (Table 1). Furthermore, the
MIC of nacubactam against all bacterial species in the presence of 4 mg
L-1 aztreonam, which reflects the minimum nacubactam
concentration to achieve the microbiological breakpoint (Clinical and
Laboratory Standards Institute, 2021), can be easily achieved in
clinical practice, indicating aztreonam/nacubactam in vivo potent
antimicrobial effect is obtainable. Indeed, the following two lines of
evidence in this study support the potent β-lactamase inhibition and
antibacterial activity of nacubactam against β-lactamase-producingK. pneumoniae . Firstly, aztreonam/nacubactam combination therapy
was effective in a mouse model against NDM-1 and IMP-6, OXA-48-producingK. pneumoniae in a nacubactam dose-dependent manner, despite
aztreonam monotherapy being ineffective. Secondly, the dose
fractionation study showed a concentration-dependent bactericidal effect
of nacubactam irrespective of the number of doses administered. Thus,
the present study provides the first evidence of a satisfactory
therapeutic effect of nacubactam against refractory
β-lactamase-producing K. pneumoniae , particularly the in
vivo antibacterial effect of nacubactam against NDM-producing bacteria.
There are two drawbacks to this research. The first is that all PD
trials only measured activity against fixed inoculum levels
(approximately 106 CFU mL-1). The
effect of greater inoculum levels on aztreonam/nacubactam drug efficacy
is unknown. Due to the inoculum effect, several antimicrobials have
lower antimicrobial effectiveness at higher initial inoculum levels
(Brook, 1989). Some β-lactam/β-lactamase inhibitor combinations, on the
other hand, have been less influenced by the inoculum effect.
Ceftazidime/tazobactam, for example, showed decreased action at high
bacterial levels, but ceftazidime/avibactam was said to be unaffected by
inoculum levels (Tam et al., 2021). The causes for this are unknown.
However, the inoculum effect could be related to avibactam’s reversible
inhibitory action and resistance to enzymatic hydrolysis. Because
nacubactam inhibits in the same way that avibactam does, it may be less
vulnerable to the inoculum effect. On this point, more evidence is
required. Finally, clinical target values were predicted using PK data
from healthy adults. Although it would be ideal for simulating more
clinically relevant settings using PK data from patients with
infections, PK data for nacubactam in patients with infections is not
yet publically available. Nacubactam PK data in patients with infections
are awaited.