Pharmacokinetics of first‐line antitubercular drugs in plasma and PBMCs

Table of Links

LIGANDS
RIF (rifampicin)
PZA (pyrazinamide)

This Table lists key ligands in this article that are hyperlinked to corresponding entries in http://www.guidetopharmacology.org, the common portal for data from the IUPHAR/BPS Guide to PHARMACOLOGY 1.

Therapeutic drug monitoring (TDM) permits investigation of the relationship between drug concentrations and pharmacological effects, variability between patients and optimization of dosing 2. Since efficacy against slow‐replicating mycobacteria is crucial for shortening the duration of antitubercular treatment, intracellular concentrations might be relevant. Peripheral blood mononuclear cells (PBMCs) may be useful for estimating drug distribution inside mononuclear cells and, possibly, for predicting compartmental efficacy (alveolar macrophages), but data is lacking on penetration of first‐line drugs in PBMCs.

The aim of this research is to describe plasma and intra‐PBMC concentrations of first‐line drugs in patients with Mycobacterium tuberculosis infection. A prospective observational study was conducted between 2013 and 2015 in patients with Mycobacterium tuberculosis infection at Amedeo di Savoia Hospital, Infectious Diseases Unit, Torino, Italy. Ethics Committee approval (Comitato Etico Interaziendale, Ospedale San Luigi Gonzaga, Orbassano‐Torino) was obtained and written informed consent was given by all patients.

Enrolled patients received once‐daily anti‐tubercular drugs weight‐adjusted according to international guidelines, intravenously for 2 weeks and then switched to the oral route under direct observation.

Post‐dose (C₂, collected at the end of the three infusions for the intravenous route and 2 h post‐dose for the oral administration) and trough plasma concentrations (C_trough, collected before the next dose) were measured at weeks 2 and 4 using liquid chromatography coupled with tandem mass‐spectroscopy. The method for measuring PBMC concentrations was previously developed, validated and published by our group 3. Normal plasma ranges published by Peloquin were considered for our target plasma levels 4.

The limits of detection (LOD) for plasma and PBMC were 117 and 0.976 ng ml⁻¹ for RIF (rifampicin), 58 and 0.391 ng ml⁻¹ for INH (isoniazid), 58 and 2.93 ng ml⁻¹ for ETA (ethambutol), and 68 and 0.391 ng ml⁻¹ for PZA (pyrazinamide). For statistical analysis, values below the LOD were replaced with the value LOD/√2.

Data are expressed as medians (IQR, interquartile range). Nonparametric tests were used for all analyses. C_trough and C₂ were analysed both separately and overall. Plasma and PBMCs concentrations were compared with Spearman's test and correlation indices were analysed. Wilcoxon's test was used to compare plasma concentrations following intravenous and oral routes. All analyses were performed with SPSS version 21 (IBM Corporation, Armonk, NY, USA).

Twenty‐four patients were included, 19 of whom were males (79.25%), with median age of 41.8 (30.5–50.9) years. Four patients (16.7%) dropped out of the study after week 2.

Median doses kg⁻¹ and plasma, PBMC C₂ and ratios are reported in Table 1. Significant correlations were found between plasma and PBMC C₂ and C_trough for RIF (r = 0.563, P < 0.001 and r = 0.872, P < 0.001, respectively) and between plasma and PBMC C₂ for ETA (r = 0.356, P = 0.018). Oral administration was associated with lower maximum plasma concentrations of INH (3782 vs. 2219 ng ml⁻¹, P = 0.03); no other significant association was observed.

Table 1.

Weight‐adjusted doses and post‐dose plasma, PBMCs concentrations and ratios

Drug	Doses(mg kg−1)	Post‐dose plasma concentrations(ng ml−1)	Post‐dose PBMC concentrations(ng ml−1)	Post‐dose ratioI/P
RIF n = 44	9.9 (9.5–10.4)	6650 (3721–7632)	7107 (4706–11 589)	1.1 (0.8–1.8)
INH n = 44	5 (4.7–5.2)	3416 (1859–4841)	<LOD (<LOD‐194)	0 (0–0.05)
ETA n = 44	20.1 (18.6–23.4)	3523 (2168–6685)	89 169 (55 855–222 211)	33.1 (10.8–55.8)
PZA n = 44	23.2 (21.4–24.9)	27 397 (23 311–33 821)	1190 (<LOD‐3743)	0 (0–0.1)

All values are presented as medians (interquartile ranges). RIF, rifampicin; INH, isoniazid; ETA, ethambutol; PZA, pyrazinamide; Ratio_I/P, intracellular:plasma ratio; LOD, limit of detection. LODs for plasma and PBMC were 117 and 0.976 ng ml⁻¹ (RIF), 58 and 0.391 ng ml⁻¹ (INH), 58 and 2.93 ng ml⁻¹ (ETA), 68 and 0.391 ng ml⁻¹ (PZA)

We observed higher intracellular concentrations of ETA and RIF compared to INH and PZA.

Data on intra‐PBMC concentrations in vivo are scantily available; Hartkoorn and colleagues 5 observed RIF intracellular to plasma ratios of 1.6, similar to our results [1.1 (0.8–1.8)]. The potential role of very high intracellular ETA accumulation warrants evaluation in prospective studies. Data on ETA intracellular penetration in animal models were published in the past, but the role in clinical setting has not yet been investigated 6.

The observed correlation between PBMCs and plasmatic concentrations supports the use of plasma drug monitoring for assessing the adequacy of drug concentrations. Plasma target levels are based on data from healthy volunteers and are calculated as maximum concentrations. A high proportion of patients showed post‐dose concentrations below the proposed targets by Peloquin 4: the prevalence was higher for RIF (77.3%, 34) followed by INH (43.2%, 19), ETA (20.4%, 9) and PZA (9%, 4). This confirms that TDM might be useful for tailoring drug dosage and, possibly, for improving drug response in selected patients and for preventing drug toxicity, if confirmed.

Limits that should be acknowledged are the small sample size, the need to demonstrate PBMCs as adequate surrogate of alveolar macrophages, the lack of full PK curves and data on MIC values to calculate AUC/MIC. In conclusion, the good correlations between PBMCs and plasma concentrations support the use of plasma drug monitoring and warrant prospective studies for understanding the relevance of intracellular concentrations.

Competing Interests

There are no competing interests to declare.

Contributors

A.C., S.B., A.D., I.M. and L.B. contributed to study design, interpretation of data and statistical analysis. I.M., P.B. and C.C. contributed to collecting data. L.B., K.B. and G.F. contributed to laboratory analysis. A.C., S.B. and I.M. drafted the first version of the manuscript and finalized the manuscript. G.D.P. contributed to study design, supervision and critical revision of the manuscript for intellectual content.

Motta, I. , Calcagno, A. , Baietto, L. , Bigliano, P. , Costa, C. , Baruffi, K. , Fatiguso, G. , D'Avolio, A. , Di Perri, G. , and Bonora, S. (2017) Pharmacokinetics of first‐line antitubercular drugs in plasma and PBMCs. Br J Clin Pharmacol, 83: 1146–1148. doi: 10.1111/bcp.13196.