The creation of a new universal TB drug regimen, is complex, compounded by the existence of successful drugs that already treat drug sensitive tuberculosis but are ineffective against drug resistance tuberculosis. New pipeline drugs can however treat both types of tuberculosis, so where does one begin?

One solution to this conundrum is to have a two drug backbone that forms the  basis of regimens for both drug sensitive and drug resistance TB therapy. There are two agents capable of being used in this interchangeable role, being  pyrazinoic acid and pretomanid, that are importantly both therapy virgins. The use of POA esters referred to here, is intended for use by pulmonary delivery the only way in which POA is bioavailable therapeutically, that requires a significant level of expertise both in their formulation, where there are choices that can effect efficacy, and delivery method that may also be important.

I have used extracts from the paper; Synthesis and Evaluation of POA in TB by Fernandes et, al., 2014 who have significant experience with POA esters, and the full publication is linked below.

The basis for the use of POA is; POA is poorly active in antimycobacterial tests, while it cannot pass through mycobacterial cell walls due to its high hydrophilic and ionizable characteristics.Cynamon et al, proposed the esterification of POA to obtain more lipophilic compounds of the active agent, able to cross the cell wall. Several compounds exhibiting activity were synthesized, 5-chloropyrazinoates and 5-methylpyrazinoates being the most active analogs. (Fernandes et, al.,)

"In the MIC assay performed in this work, we obtained activity of 3.96 g/mL to the 2-chloroethyl pyrazinoate in pH 6.6 against M. tuberculosis H37Rv. It is possible to infer that 2-chloroethyl pyrazinoate may show better activity in pH 5.8, and comparing with other compounds reported in the literature, it is considered a highly active compound (Fernandes et al., 2010). It is observed a 2 to 8-fold increase in the activity of POA esters when the pH is lowered from 6.6 to 5.8 (Cynamon et al., 1992). Thus, it can be estimated the activity of 2-chloroethyl pyrazinoate can be even lower" (Fernandes et, al.,)

“The results obtained in the MIC assay showed a significant increase in the POA activity after esterification. The obtained MIC value (3.96 g/mL) is comparable to the MIC of ciprofloxacin, a second-line agent with high activity, and used in the treatment of MDR-TB and other mycobacteriosis) and better than PZA (reported MIC 50–100 g/mL).” (Fernandes et, al.,)

POA esters shows activity against MTB at pH 6.6, but the suggestion is that because POA is capable of lowering further the pH in TB lesions that already has a low pH, there is no reason why it should not be able to reduce the pH of liquefied caseum having neutral pH. Should POA succeed, it will then have a bacteriostatic effect on MTB within caseum once pH 6.6 has been achieved.

The proposal is that aerosolised POA esters in combination with pretominid will have synergistic activity against TB bacilli, both in activated macrophages and in liquefied caseum. Should this assumption prove correct then a replacement for PZA will become available in X/MDR-TB therapy and the same combination will have a further role to play against actively replicating MTB within caseum, where PZA could not.

Pretomanid has a known synergy with pyrazinamide against internalised bacilli, and the expectation is that if used in combination with aerosolised POA esters, because POA can itself lower pH and not have to rely on local acidity for activity against MTB, there will be an increased cidal effect, much more so than when pretomanid is used with pyrazinamide, which runs out of puff when acidity subsides.

The therapeutic importance of targeting TB bacilli within liquefied caseum where a high pH allows MTB to replicate unrestricted, fuelled by available cholesterol, is to inhibit bacterial growth to prevent or reduce cavity formation where extracellular growth of tubercle bacilli in cavities cause lung damage and bronchial spread of the disease. Any favourable result is likely to be associated with ability for POA to penetrate liquefied caseum, and concentration amounts of POA to reduce local pH. 

Aerosolised POA esters have now shown efficacy in the animal model, and this is an important step forward in its use in TB therapy.

POA has been shown to have the same mechanism of action as bedaquiline so apart from the bacteriostatic effects as a weak acid it also has antimycobacterial activity. The promise of synergistic activity with pretomanid is realistic.

The projected example regimens using this backbone are POA+PA+RIF for drug sensitive TB and POA+PA+B+ for both MDR-TB and XDR-TB. In this way, drug choice selection increases considerably allowing for the exclusion of pipeline drugs with unresolvable safety issues

While inhalation therapy is unknown in TB treatment, this is also the case for the use of pyrazinoic acid, and what should be of great interest to the clinician is POA has the same mechanism of action as bedaquiline, as demonstrated by Kirk Bald in the enclosed paper.

Everything runs through the TB Alliance these days, and a trawl of their drug regimens currently in trials are;

Bedaquiline + Pretomanid + Linezolid

Bedaquiline + Pretomanid + Moxifloxacin + Pyrazinamide

Bedaquiline + Pretomanid + Pyrazinamide

Bedaquiline + Clofazimine + Pyrazinamide

Bedaquiline + Clofazimine + Pyrazinamide + PA-824

Pretomanid + Moxifloxacin + Pyrazinamide

These combinations are potential treatments for X/MDR-TB and the expectation is that one of these regimens will emerge able to treat MDR-TB in three months, that is only achievable by including pyrazinamide. Paradoxically the weakness of these regimens is the inclusion of pyrazinamide, because in the event of PZA resistance, which statistically is 60% of all newly diagnosed MDR cases, then these regimens will collapse, based on the level of expectation PZA inclusion provides. In the real world It has been shown that TB therapy does improve using pyrazinamide against PZA resistant strains of Mtb, but the target of three months therapy duration is unobtainable and is likely to become two years plus duration, with an increased incidence of recurrent TB and resistance that is commensurate with treatment longevity. This is a true representation of the dire state that faces X/MDR-TB treatment candidates that apparently everybody knows about, but fail themselves in their failure to resolve this issue.

Of general concern, and not wanting to be critical in any way, is that both bedaquiline and moxifloxacin have FDA black box warnings, and that is the serendipity element associated with drug discovery that is nobody’s fault. However, the use of the suggested backbone of inhaled POA+PA, allows certain suspect drugs to be avoided, or alternatively, used at a lower dose to reduce toxicity, but still able to improve overall treatment efficacy.

There are initiatives underway to obtain evidence that different forms of  pyrazinoic acid when used by inhalation, can be used both as adjunctive treatment and as a replacement for oral PZA. In both situations it is likely that POA can accumulate at a much greater concentration in TB lesions than if obtained from oral PZA alone that should improve therapy outcome in a number of different ways, including improvement in eliminating persister cells.

Aerosolised pyrazinoic acid esters have recently been tested for efficacy in a first animal study. PAE’s were used as a supplement to oral therapy and significantly reduced the organ bacterial burden in comparison to infected, untreated control animals. The team suggest that PAE aerosol therapy is a potentially significant addition to the regimen for PZA resistant MDR-TB and XDR-TB treatment. This group have direct links with my original collaborator and the results are shown in the links below.

This study is a promising indicator for the use of POA+PA as a two drug backbone in TB therapy as suggested here, and without the need for pyrazinamide. TB therapy may well be able to be completed in a much shorter duration, that is largely dependent on the formulation of the therapeutic version of POA esters to be used.

First animal study for Aerosolised pyrazinoic acid esters

Latest on POA product development for experimental use

Inhalation therapy for tuberculosis

 Improving the activity of rifamycins and pyrazinamide. Mitchison.

POA mechanism of action study. Kirk Bald.

Synthesis and Evaluation of POA in TB. J.P-D Santos Fernandes et, al 2014

WHO TB drug pipeline list

November 21st 2016

Copyright © Gino Francesco 2016