Discussion
This study demonstrated a sex difference in alfaxalone PK parameters
using a 2 stage deterministic approach for male and female Lewis rats.
Sex differences were observed for both CL and Vd, where the former
influences steady-state concentrations and the latter loading dose. As
steady-state conditions are of the most interest only differences in CL
will be discussed. Alfaxalone CL for female Lewis rats was significantly
lower than for male animals. Separate studies by Visser et al. (2002)
and Lau et al. (2013) have shown alfaxalone clearance to be 158 ± 29 ml
min-1 kg-1 and 54.3 ± 6.8 ml
min-1 kg-1 for male and female
Wistar rats respectively, and White et al. (2017) also identified a sex
difference for alfaxalone CL in SD rats. The stepping down of the
infusion rates in the Lewis rats was for purposes of creating dynamic
change towards the end of the experiment, however, no significant change
in concentration was apparent. For this reason, the stepping down was
omitted for the SD animals and the reduced CRI is in fact the
maintenance CRI designed to achieve maintenance levels faster to
replicate the male model.
NLME PK models offer the advantage of having a single model that
describes a population and therefore shares data interpretation between
animals unlike the standard deterministic PK approach. Moreover, sub
populations such as the sex and strain can be described by co-variates
that adjust the fixed effects (typical values). NLME models for
populations 1 and 2 exemplify the sex difference in alfaxalone clearance
where in both cases sex is a highly significant covariate. However, the
deterministic estimated clearance parameter for Lewis rats for both
sexes in the current study were in the lower range compared to the
reported values by the Visser and Lau groups for Wistar rats as well as
the reported values by White et al. for SD rats (White, et al. 2017).
This is consistent with strain being a significant covariate on
clearance in the population 1 NLME PK model.
However, the conclusion of the larger population 2 model suggests that
LCBW is the more significant covariate source for clearance. This may be
partly due to body weight having a wider range in population 2 compared
to the narrower range in population 1 as the models were based on total
dose. However, LCBW contains an allometric transformation for bodyweight
and therefore Lewis rats, which have a lower bodyweight compared to age
matched SD animals, have an enhanced lower clearance compared to SD
animals. This is reflected in Table 4 where the estimated post hoc
clearance in SD female rats is significantly higher than female Lewis
rats when normalised per kg bodyweight. However, the allometric
relationship between clearance and bodyweight is an inverse one, which
is unusual. Alternatively, as there is a correlation between weight and
strain, it may be the case that strain is the real reason for the
difference, as was the case for model 1, and population 2 is biased by
the majority SD data leading to LCBW being the most statistically
significant co-variate.
The present study used arterial pressure measurements as a clinical
biomarker for the PD investigation. Cardiovascular effects were chosen
as a biomarker because alfaxalone exerts a dose dependent depression of
the cardiorespiratory system i.e. a decrease in blood pressure (Khan et
al., 2014; Muir et al., 2009; Sear, 1996) but to a lesser extent
compared to other anaesthetics such as thiopental and propofol
(Goodchild et al., 2015; Visser et al., 2002). As such, a dose reduction
(33%) of alfaxalone near the end of infusion for Lewis rats was
designed to observe a dynamic change in the cardiovascular effect to
determine an IC50. However, there was no noticeable
change in the cardiovascular response as a result of the 30% reduction
phase. One explanation of the blood pressure discrepancies might be the
female Lewis were most responsive being impacted by the higher
alfaxalone plasma concentrations and this was manifest as a relative
hypotension compared to the male animals where a stable blood pressure
was maintained during anaesthesia. The population 1 NLME PK model was
used to simulate a dosing regimen for female SD rats that would give a
similar alfaxalone plasma profile to that of male SD whilst
simultaneously minimising cardiopulmonary depression enhancing the
profile particularly for prolonged anaesthesia.
There are some limitations in the present study for the PD measurements.
All the rats had a sampling cannula secured in the left carotid artery
which required surgery and the use of gaseous anaesthesia for
instrumentation. Isoflurane can cause significant cardiovascular
depression in a dose dependent manner (Yang, et al., 2014), as such, no
arterial pressure baselines in conscious rats were available.
Furthermore, after discontinuing the gases, most rats’ arterial pressure
rebounded rapidly to a higher blood pressure. However, in view of the
concurrent cessation of the volatile agent and a loading dose of
alfaxalone infusion, it is not clear what the value of the real baseline
is. Subsequent work evaluating the suitability of the alfaxalone model
for surgical anaesthesia and compatibility with other adjuncts and
analgesics is advised to move away from reliance on the volatile agents,
thereby minimising their detrimental environmental impact.
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