Optimising pyrazinamide for the treatment of tuberculosis

Discussion

Pyrazinamide is a standard component of first-line tuberculosis treatment, yet the “right dose” is not established. In this study using data from three international phase 2 clinical trials, higher concentrations of pyrazinamide were associated with higher culture conversion rates at 2 and 3 months of treatment. These analyses suggest that current dosing may be insufficient to maximise efficacy [17]. In the trials that enrolled from geographically diverse settings, parametric survival modelling revealed that PK−PD relationships differed for participants from African versus non-African sites. To achieve targets associated with treatment-shortening, drug doses that are beyond the range of tolerability would likely be needed in African patients in the absence of other new drugs. Modelling and simulation showed that increasing doses of both rifampicin and pyrazinamide appears to be a more promising strategy. The range of pyrazinamide concentrations was broad, yet elevations in liver enzymes were rare, and there was not an association between pyrazinamide levels and hepatotoxicity.

Pyrazinamide is an important sterilising agent. Early on, it was highly effective in two-drug combinations with isoniazid, provided it was given at a high enough dose and for sufficient duration [8, 18–22]. Following the discovery of rifampicin, adding pyrazinamide to rifampicin-containing regimens reduced relapses [2, 23–25], and pyrazinamide became an essential part of current “short-course” treatment. Giving it during the first 8 weeks allows for the shortening of treatment from nine to 6 months. In the trials demonstrating its treatment shortening activity, though, the doses given were 30–40 mg·kg⁻¹ daily, not the currently recommended 20–25 mg·kg⁻¹ for adults [26–28].

In our study, culture conversion rates increased with increasing pyrazinamide exposure both when pyrazinamide was combined with standard-dose rifampicin or higher-dose rifampicin, and the best activity was seen when exposures of both drugs were high, demonstrating that there was an observable exposure–response relationship even when the companion drug was a potent sterilising agent given at a high dose. Interestingly, in the TBTC studies, PK–PD relationships were different for non-African and African participants. African participants tended to have higher baseline extent of disease (higher likelihood of three or more sputum smear grade or large lung cavities) than non-Africans, but these factors were not significant in our multivariate models, and there are likely other unobserved factors contributing to lower treatment response. This same phenomenon of lower treatment response, even after adjusting for known risk factors, was seen in the larger S28 study population and in TBTC Studies 29 and 29X and remains unexplained [29, 30]. While optimising pyrazinamide in the context of first-line therapy is important, pyrazinamide also has a role in multidrug-resistant tuberculosis treatment; treatment outcomes are significantly worse if the multi-drug resistant tuberculosis strain is pyrazinamide-resistant [28]. Pyrazinamide also enhances the activity of new and investigational drugs, namely bedaquiline, delamanid and pretomanid, so optimisation of pyrazinamide may be valuable in multiple contexts [31–33].

Our study was not the first modern study to find that pyrazinamide pharmacokinetic influenced treatment outcomes. In our study, C_max was the covariate identified in the final PK–PD model as the most informative. However, we note that C_max and AUC_0–24 h are highly correlated and each has a strong association with outcomes; depending on the sampling strategy of a given study, which influences how well that parameter is estimated, one might have a modestly stronger correlation or better precision. For example, single samples can sometimes fail to capture C_max well. In the TBTC studies, coefficient of variation % for C_max and AUC_0–24 h were similar at 23.1% and 26.1%, respectively, so variability in these estimates was similar. In a study in Botswana, after adjusting for HIV infection and CD4 cell count, patients with pyrazinamide C_max less than 35 μg·mL⁻¹ (a putative pyrazinamide PK target) had a 3.4-fold higher risk of poor treatment outcome [6]. In South Africa, pyrazinamide AUC_0–24 h was a top predictor of poor long-term outcomes [7]. In children with and without HIV in India, low pyrazinamide and rifampicin C_max were associated with unfavourable outcomes [34]. In a recent meta-analysis, low pyrazinamide concentrations were shown to increase the risk of poor outcomes with relative risk of 1.73 [35]. At currently recommended doses, a high proportion of patients do not have drug concentrations that reach 35 μg·mL⁻¹, let alone a proposed alternative target of 58 μg·mL⁻¹ [10], and the evidence base for selection of a 20–25 mg·kg⁻¹ dose is limited. Likely, the optimal dose for this drug lies somewhere between 35 and 45 mg·kg⁻¹. Higher doses would produce exposures exceeding 5000 μg·h·mL⁻¹ (see below) in some patients [29, 36]. At the current dose, we are undertreating many.

Pyrazinamide commonly causes arthralgias, but its most dreaded toxicity is liver injury. In early studies, doses of at least 40–50 mg·kg⁻¹ given for 24 weeks or longer caused unacceptable rates of liver toxicity (5–10%), while rates were substantially lower (2–5%) if the duration or dose was reduced [37]. Currently, doses of 30–40 mg·kg⁻¹ daily are well-tolerated as part of MDR-TB treatment [38]. In a meta-analysis involving 4490 individuals, risk of liver toxicity did not appear to increase as a function of drug exposure until exposures were quite high (weekly AUC of >5000  μg·h·mL⁻¹) [39]. Pyrazinamide toxicity appears to be idiosyncratic up until a point, after which dose-related increases in liver toxicity are seen [10]. The mechanism for and contributing factors to pyrazinamide-associated liver injury are not clearly understood and may differ for different companion drugs [40]. Consistent with previous reports, in our study, median weekly AUC were 2100 (TBTC) and 2310 (PanACEA MAMS-TB) μg·h·mL⁻¹, and liver injury was rare and not related to exposure in the ranges seen.

Our study has limitations. Because of differences in culture methodology and covariates collected between TBTC and PanACEA trials, we could not combine pharmacokinetic-efficacy data into a single model. Model-predicted doses assumed proportional dose effects at doses higher than those observed, which may not be the case. In our parametric survival analyses, we did not consider interval censoring. The concentration that increases risk of liver toxicity could not be determined, as liver toxicity was rare and the dose range limited. Adjusting pyrazinamide pharmacokinetic parameters for isolates’ MIC values did not improve model fit; likely this is because the MIC range was narrow, MICs were not measured precisely, and many participants did not have MIC data. In one TBTC trial, some patients had intermittent dosing-sensitivity analyses suggested removing those patients did not change model parameters or fit. Lastly, predicting an effective treatment-shortening regimen using microbiologic data from phase 2 trials is an uncertain science; while 90–95% culture conversion on solid media by 8 weeks of treatment has been proposed, there are no well-validated prediction models using liquid culture [41].

It is important that the dose of each drug in a tuberculosis treatment regimen be optimised. Higher-dose rifampicin and pyrazinamide have the potential to shorten tuberculosis treatment. Simply prolonging the duration over which pyrazinamide is given was not sufficient to reduce treatment duration from 6 to 4 months in historical trials [4]. Using parametric survival modelling and trial simulations, we discovered that increasing just the pyrazinamide dose does not seem as though it will improve outcomes in the hardest-to-treat patients [29]. Indeed, the predicted doses of pyrazinamide that would be required as part of the standard regimen to produce rapid and sustained culture conversion (90% conversion by 2 months, for example) for all patients were high and would not be safe. Whether or not an enhanced multidrug regimen containing high-dose rifampicin (e.g. ≥35 mg·kg⁻¹) and higher-dose pyrazinamide (e.g. 30–40 mg·kg⁻¹) will be sufficient to meaningfully reduce tuberculosis treatment duration must be explored prospectively, with attention to safety and tolerability.