Tuberculosis: New Drug Discovery Pipelines

Review Article

Austin J Anal Pharm Chem. 2014;1(3): 1014.

Tuberculosis: New Drug Discovery Pipelines

Arya N1, Raut MK1, Tekale SG1, Shishoo CJ2 and Jain KS1,3*

1Department of Pharmaceutical Chemistry, Sinhgad Institute of Pharmaceutical Sciences, India

2Department of Pharmaceutical Chemistry, Gujarat University, India

3Department of Pharmaceutical Chemistry, SF Jain College of Pharmacy, India

*Corresponding author: :Jain KS, Department of Pharmaceutical Chemistry, SF Jain College of Pharmacy, D-2 60/61,,Telco Road, Chinchwad, Pune-411019, Maharashtra, India.

Received: August 06, 2014; Accepted: September 15, 2014; Published: September 19, 2014

Abstract

The resurgence of tuberculosis from a forgotten disease to a modern and resurgent pathology is a matter of serious global concern. Development and transmission of Multi Drug- Resistant (MDR) and extensively drug-resistant (XDR) tuberculosis is an important public health problem worldwide. This review covers mainly the therapeutic limitations to treat these resistant disease forms, as well as, the current status of drug discovery and development for anti-tubercular therapy and treatment.

Keywords: Tuberculosis; Multi drug- resistant tuberculosis (MDR- TB); Extensively drug-resistant tuberculosis (XDR-TB); New drug discovery pipelines

Introduction

“Tuberculosis is a Social Disease with a Medical Aspect”. By Sir William Osler (1902)

Tuberculosis (TB) is a contagious but curable infectious disease caused by the pathogen, Mycobacterium tuberculosis (Mtb) and is again becoming a major cause of mortality worldwide. The Mtb and its pathogenic strains cause infection mainly in the oxygen-rich macrophages of the lungs. The main causes for the resurgence of this once nearly eradicated infectious diseases are the reduction in the emphasis on TB control programs, the declined socioeconomic standards and also the emergence of immune deficiency states like, AIDS. Despite the fact that TB has been recognized for thousands of years and its etiological agent has been identified since the earliest days of medical microbiology, TB continues to loom as one of the largest infectious diseases, with enormous global burden of morbidity and mortality. TB thrives in impoverished or malnourished communities; individuals weakened by immunological deficiencies and situations where healthcare delivery is poor.

According to global tuberculosis report by World Health Organization (WHO), in 2012, an estimated 8.6 million people developed TB and 1.3 million died from the disease (including 320, 000 deaths among HIV-positive people). Nearly 20 years after the WHO declaration of TB as a global public health emergency, major progress has been made towards 2015 global targets set within the context of the Millennium Development Goals (MDGs) [1,2]. Today the absence of completely protective TB vaccine, slow development of new anti-TB drugs are issues that highlight the re-emergence of the TB crisis [3,4]. Development of multidrug-resistant (MDR) and extremely drug-resistant (XDR) strains of Mtb to anti-TB agents is an increasing problem worldwide [5,6].

After a gap of nearly 40 years, in December 2012, the anti-tubercular New Drug Discovery Research (NDDR) efforts have seen some success. The US Food and Drug Administration (FDA) approval of two new compounds, delamanid and bedaquiline to treat multidrug-resistant TB (MDR-TB) validates the renewed efforts to develop new, better treatments for TB after decades of stagnation. However, the euphoric mood can turn to pessimism as the road to adequate treatment for people with TB is still a long one and full of many difficulties.

Anti-mycobacterial therapy though available; drugs are often partially effective because of the impermeable nature of the mycobacterium cell wall and the propensity of Mtb to develop resistance to the existing drugs in TB therapy [7]. Additionally, Mtb has the capacity to remain viable within infected hosts for a prolonged time. Notably, MDR-TB and XDR-TB have become obstacles to the effective global TB control and have been recognized by the WHO as major challenges to be addressed in the fight against tuberculosis [8,9]. A key strategy to combat and destroy these drug resistant pathogens is the discovery of novel antitubercular agents bearing newer structural skeletons (scaffolds), acting through novel mechanisms of action and bearing no or minimal cross resistance.

The Biology of Mtb.

The common causative organism for TB, Mycobacterium tuberculosis (Mtb) is a major component of the microbiological history and is also referred as the tubercle bacillus.

While, in humans Mycobacterium tuberculosis is the main cause of the infection, several other Mycobacterium species including Mycobacterium bovis, Mycobacterium africanum, Mycobacterium microti, Mycobacterium canettii, Mycobacterium caprae and Mycobacterium pinnipedii are also known to cause the disease.

Pathogenesis of TB [14]

The steps in TB pathogensis are – exposure, infection, disease and death. Infection with Mtb can be pulmonary or extra pulmonary (affecting other parts of the body). Pulmonary TB [15] (the most common form in developed countries) occurs in the lung and comprises almost 85% of all TB cases. The upper lung lobes are more frequently affected by tuberculosis than the lower ones.

Extra pulmonary TB [16] means tuberculosis spreads outside of the lungs. It may co-exist with pulmonary TB as well. Its sites are; lymph nodes, bones and joints (in Pott’s disease of the spine), intestine, genito-urinary tract (in urogenital tuberculosis), pleura (in tuberculosis pleurisy), central nervous system (in scrofula of the neck), meninges, peritoneum, skin, etc. Though it occurs in approximately 15 to 20% of active non HIV cases, it occurs more commonly in immune-suppressed persons and young children. In those with HIV, it occurs in more than 50% of cases.

Infection of a host is initiated with the inhalation of air droplets containing a small number of the bacilli, which spread from the site of initial infection in the lung through the lymphatic system or blood to other parts of the body, as well as, the apex of the lung and the regional lymph nodes. Once in the lung, bacilli are subjected to phagocytosis by the resident macrophages of the lung, the alveolar macrophages, which in turn are activated by the appropriate stimuli can effectively, transfer the phagocytosised Mtb to the destructive environment of lysosomes. However, some bacilli escape lysosomal delivery and survive within the macrophage. Infected macrophages can then either remain in the lung or are disseminated to other organs in the body. As the immune defense system is sufficient to keep Mtb in a check in healthy individuals, only 10% of infected individuals develop TB. Any deterioration of host immunity results in a potentially life-threatening condition of the individual harboring live Mtb.

Symptoms & transmission of the TB infection

The early symptoms of active tuberculosis include, weight loss (consumption), fever, night sweats, fatigue and loss of appetite. At advanced stages of the disease persistent cough with blood-tinged sputum is an additional symptom. About 25% of patients may not have any symptoms or remain “asymptomatic” (Latent TB). While persons with latent TB are not infectious, they may develop the active form of the disease if their immune system is compromised. Diagnosis of active TB relies on radiology (chest X-rays), as well as microscopic examination and microbiological culture of body fluids. Diagnosis of latent TB relies on the tuberculin skin test (TST) and/or blood tests. Prevention relies on screening programs and vaccination with the bacillus Calmette–Guérin (BCG) vaccine. About 5 to 10% of non-HIV individuals, infected with tuberculosis, develop active disease during their lifetime.

TB is an airborne disease and spreads like the common cold by the circulation of aerosols containing Mtb in the air from the coughing, sneezing, talking or spitting of TB patients. When people with active pulmonary TB cough, sneeze, speak, sing, or spit, they expel infectious aerosol droplets 0.5 to 5.0 µm in diameter. Each one of these droplets may transmit the disease, since the infectious dose of tuberculosis is very low [17,18].

Drug therapy and its limitations

TB chemotherapy was first introduced in 1946, when Streptomycin (STR) was used to treat the disease. In most parts of the world, combination of five drugs is used to treat TB effectively namely rifampicin (RIF), isoniazid (INH), ethambutol (ETH), streptomycin (STR) and pyrazinamide (PZA). Antitubercular chemotherapy problems arise when patients develop bacterial resistance to any of these first-line drugs. However, as many of the second-line drugs; ethionamide, aminosalicylic acid (PASA), cycloserine (CYC), amikacin, kanamycin and capreomycin are very toxic they cannot be employed simultaneously (Tables 1,2).