Combined first line anti-TB drugs: new insights into stability

Abstract

Tuberculosis (TB) is the most prevalent infectious cause of death globally, affecting approximately one-third of the world’s population (Borgdoff et al., 2002). Mycobacterium tuberculosis (M. tuberculosis) is spread through small airborne droplets, generated through coughing, sneezing, or even by talking to a person with pulmonary or laryngeal TB (Knechel, 2009). The treatment of tuberculosis with a multi-drug regimen requires therapy for a long period of time. This is associated with risks such as poor patient compliance, treatment failure and drug resistance. To limit these risks, the World Health Organisation and the International Union Against Tuberculosis and Lung Disease recommend the use of fixed-dose combination (FDC) tablets for the treatment of TB (WHO, 1999). The recommended multi-drug treatment approach of TB includes rifampicin, isoniazid, pyrazinamide and ethambutol, daily for 2 - 3 months. The use of FDCs may hence simplify treatment and encourage patient compliance, especially in patients who already take numerous medications, when co-infected with human immunodeficiency virus (Panchagnula et, al., 2004, WHO, 1999). The four current anti-TB drugs, isoniazid, pyrazinamide, ethambutol hydrochloride and rifampicin, belong to two different classes of the Biopharmaceutical classification system (BCS). Isoniazid, pyrazinamide and ethambutol hydrochloride belong to class I (highly soluble and highly permeable) and rifampicin on the other hand, is the only hydrophobic ingredient of the FDC product (Ellard & Fourie, 1999). It has been postulated that polymorphism of rifampicin may be responsible for its variable bio-availability among its solid oral dosage forms (Agrawal et al., 2004). Rifampicin may react adversely with isoniazid to form isonicotinyl hydrazone (HYD) in formulation according to Singh et al (2000). Singh & Mohan (2003) further reported that pyrazinamide and ethambutol are catalytic towards the reaction between rifampicin and isoniazid, since FDCs that contain four-drug combinations have shown far more chemical instability than two-drug FDCs that only contain rifampicin and isoniazid (Singh & Mohan, 2003). Various hypotheses have been put forward to explain inter-drug interactions that may occur in anti-TB FDC formulations and during oral administration. Therefore, for the purpose of this study, the latest techniques were used to determine whether such reported chemical reactions indeed occur and under which conditions they would occur, if at all. Hydrolysis experiments were done in distilled water to determine the extent of decomposition of RIF and INH using single, two, three and four anti-TB APIs. The aim of the investigation was to test the above hypotheses regarding the stability of especially RIF and INH in combination with EMB and PZA. Assays were done at 2, 3, 6, 12, 24 and 48 hours using solutions that were maintained at three different temperatures (5, 25, and 37°C – each ± 2°C). The results showed that EMB together with RIF and INH showed the greatest rate of degradation. Surprisingly the degradation of the four combination active pharmaceutical ingredients was less than that of the above mentioned three combination. Apart from a clear impact of INH and RIF on each other, the presence or absence of EMB and/or PZA also influences their rate of hydrolysis in water. The microcalorimetry results showed at 40°C that no incompatibility exists with and without humidity. Previous studies have suggested that EMB together with humidity conditions is mainly responsible for the RIF degradation and the so called ‘bleeding’ of the tablets. However, it might be that the deliquescence of EMB masks any interaction or stability. It has been suggested that in the solid-state, HYD may also be formed because of a direct interaction between the imino group of RIF and the hydrazine group of INH. This interaction in the solid-state is exactly what we find with the microcalorimetry results at 50°C. The microcalorimetry results showed that an incompatibility exists between RIF and INH in the solid-state. The moisture sorption results confirmed the hygroscopic nature of EMB, but the question remains is that moisture responsible for the degradation of RIF. The TAM and hydrolysis results were not conclusive about this. From the results it is not clear if the hygroscopic nature of EMB is solely responsible for the instability of four combination anti-TB drugs. The stability of the anti-TB FDC tablets remains a challenge to researchers and in future more analysis need to be proposed to solve this problem.