Discussion:
The utility of miR-122 and K18 as exploratory DILI biomarkers has been
demonstrated by several multicentre studies and this has resulted in FDA
support for continued development to full qualification (Food and Drug
Administration Centre for Drug Evaluation and Research, 2016). However,
the properties of these biomarkers have not been robustly studied in
patients with TB in Africa.
Given the potential involvement of the liver in TB infection it was
important to determine if circulating concentrations of miR-122 and K18
differed from healthy individuals in the presence of infection,
including active, latent and non-tuberculous mycobacterial infection and
HIV-TB coinfection. For example, if either biomarker was elevated by
mycobacterial infection per se then it would be de-prioritised as
a biomarker in this important context of use. In this study we have
demonstrated that circulating miR-122 and K18 in healthy volunteers and
patients with active TB, latent TB and non-tuberculous mycobacterial
infection are not substantially different. This suggests that the
presence of mycobacterial infection does not affect circulating miR-122
and K18. Furthermore, circulating K18 in HIV-TB coinfected African
patients was similar to the other groups, which were predominantly
Caucasian. This suggests that the healthy reference interval for K18 in
an African and Caucasian population is likely to be similar. In
addition, we have demonstrated that, in the absence of DILI, neither
miR-122 nor K18 change substantially upon commencing treatment. Both
miR-122 and K18 correlate with ALT, indicating these biomarkers may have
diagnostic utility. In this pilot study, miR-122 distinguished those
patients with an elevated ALT with greater sensitivity and specificity
than K18 but this should be interpreted with caution as there were only
two cases of DILI as predefined in our study protocol. In these DILI
cases the elevations in ALT temporarily correlated with a rise in both
miR-122 and K18. In addition, in one of these patients, miR-122 and K18
rose before ALT, indicating a potential for these novel biomarkers to
predict the development of DILI earlier than ALT. The results of this
study provide initial evidence for the potential use of both miR-122 and
K18 as biomarkers of TB medicine associated DILI.
Further work should focus on determining the diagnostic value of the
biomarkers, whether they correlate with rises in ALT and so can diagnose
DILI within this population. A clear definition of the dynamic range,
sensitivity and specificity of miR-122 and K18 within this population is
needed before they can be used as a biomarker of DILI. Furthermore,
given that evidence suggests miR-122 and K18 both rise earlier than ALT
in paracetamol DILI, it is important to determine if they have the same
predictive value in patients with mycobacterial infections. This
predictive ability of these novel biomarkers may enable early
identification of patients at risk of DILI, leading to prevention of
liver injury through halting or altering treatment regimens before
significant liver injury develops. Specifically, the biomarkers could be
a useful early indicator of the development of DILI in patients being
reintroduced to essential anti-TB medications, a group at elevated risk
of DILI recurrence.
Our study had a limited number of cases of anti-TB DILI. Historical data
suggested approximately 2-5% of patients receiving anti-TB treatment in
the UK will develop DILI. However, within the ALISTER study only 2
patients developed DILI, 1.4% of the patients recruited. Larger
multi-centre studies are required to recruit enough patients to
determine the diagnostic power of miR-122 and K18 in anti-TB DILI. The
majority of values for circulating miR-122 concentrations in the patient
groups fell within the published upper limit of the healthy reference
interval of 45 fM generated from the SAFE-T dataset (Church et
al. , 2018). However, there were two patients with miR-122 increased
above this healthy reference interval (miR-122 = 77 & 77 fM) when ALT
was still normal (ALT = 25 & 26 IU/L). This may reflect a limitation of
miR-122, namely that it has been reported to have relatively high
variability (Church et al. , 2018). In our study, the variability
of the novel biomarkers was higher than ALT, with miR-122 having higher
inter-subject variability than K18. Although the previously published
healthy reference interval provides a valuable comparison, the
circulating concentration of miR-122 in healthy volunteers in this study
fell between 0.21-8.75 fM, considerably lower than the published ULN of
45 fM which was generated from the SAFE-T dataset (Church et al. ,
2018). This healthy reference interval was developed using a larger
sample size than that included in this study. However, it was determined
using different quantification and normalisation methods, therefore a
direct comparison is challenging. There were two patients who had
substantially elevated K18 in the absence of elevated ALT. Firstly, in
ALISTER (K18 = 4207 U/L, ALT = 43 IU/L). Secondly, in the SAEFRIF trial,
where a patient had K18 ranged from 10,000-20,000 U/L, but not
substantially elevated ALT (22-52 IU/L). The reason for these two
outliners is unknown and requires further study with larger patient
numbers.
In summary, the presence of mycobacterial infection does not alter
miR-122 or K18 concentrations in the absence of DILI. African HIV-TB
coinfected patients had similar K18 concentrations to healthy volunteers
and Caucasian TB patients. Patients who experienced elevations in ALT
also demonstrated rises in both miR-122 and K18 indicating the
diagnostic potential of these biomarkers. Future trials of miR-122 and
K18 as biomarkers of anti-TB DILI could be performed using the data
presented in this paper to inform the study design.