Efficacy and Safety of Anti-Tuberculosis Therapy: Pharmacokinetic and Pharmacogenetic Studies in Latvian Patient Population

The first studies supporting the use of rifampicin (RIF), pyrazinamide (PZA), isoniazid (INH), and ethambutol (ETB) for tuberculosis (TB) treatment date back to the 1950s and 1960s. Today, this four-drug combination is one of the first-line treatment regimens used for drug-susceptible tuberculosis (DS-TB). Yet, the many aspects of their exposure-response relationship remain unclear, likely hindering efforts to control this persistent global public health problem. Regarding treatment efficacy, pharmacokinetic (PK) studies have reported substantial interpatient variability in drug plasma (or serum) exposure using standard doses, with lower values often linked to suboptimal treatment response and even treatment failure. At the same time, despite increasing interest in using C-reactive protein (CRP) as a biomarker of treatment response, data on the influence of anti-TB drug exposure on TB-associated inflammation, including the dynamics of serum CRP levels, are limited. As for safety, an important concern related to the combined RIF, PZA, INH, and ETB regimen is drug-induced liver injury (DILI), an adverse drug reaction with a complex and poorly understood nature, carrying the potential for fatal outcomes. To date, various patient- and disease-related factors have been described to affect the PK of anti-TB drugs and increase the risk of DILI. By comparison, pharmacogenetic (PGx) studies focusing on RIF-metabolising enzymes, transporters, and associated regulatory proteins have not yet reached a consensus on the impact of genetic variability in this context. Therefore, this Thesis aimed to investigate the effect of variants in RIF pharmacogenes on its plasma exposure and the development of anti-tuberculosis drug-induced liver injury (anti TB DILI), as well as to characterise the early changes in serum CRP levels and their influencing factors in Latvian patients with DS-TB. Towards this aim, two methodological studies and two subsequent prospective observational studies were conducted. The first methodological study focused on developing a liquid chromatography-tandem mass spectrometry method for quantifying RIF, PZA, INH, and ETB in human plasma, whereas the second involved creating a targeted next-generation sequencing protocol for PGx analysis. Both approaches were successfully applied to generate data, which were then interpreted in conjunction with clinically relevant information. Thus, the third study examined whether variants detected in the genes encoding enzymes (arylacetamide deacetylase gene), transporters (solute carrier organic anion transporter family member 1B1 and 1B3 genes, and adenosine triphosphate-binding cassette subfamily B member 1 gene), and nuclear receptors (nuclear receptor subfamily 1 group I member 2 [NR1I2] gene) involved in RIF disposition are related to its plasma exposure and the development of anti-TB DILI. The fourth study investigated the impact of patient- and disease-related factors, including anti-TB drug plasma exposure, on the decline in serum CRP levels during the first 10–12 days of therapy and assessed the predictive value of this biomarker for bacteriological response. In brief, analysis of the variants detected in the set of genes revealed the relationship between one intronic NR1I2 gene variant (rs3732357) and variation in RIF plasma exposure. None of the examined variants were related to anti-TB DILI. When exploring early changes in serum CRP levels after the initiation of anti-TB treatment, several patient characteristics and disease severity at diagnosis were identified as factors affecting the reduction in levels of this biomarker. Despite considerable underexposure rates of RIF, PZA, INH, and ETB among enrolled patients, the plasma exposure of these drugs did not significantly influence the serum CRP levels within the given timeframe. Furthermore, levels of this biomarker on the 10th to 12th day of therapy, along with their change from baseline, were not predictive of bacteriological response. Summarising all the above, this Thesis contributes to the current understanding of PK and PGx variability and their implications for the efficacy and safety of anti-TB treatment. It provides new data on the genotype-phenotype relationships for variants in the RIF pharmacogenes, offers a detailed characterisation of early changes in serum CRP levels, and underscores the clinical significance of the obtained findings. The methodologies developed for PK and PGx data acquisition hold practical value for future TB research and clinical management.