An overview of new antitubercular drugs,drug candidates,and their targets

The causative agent of tuberculosis (TB), Mycobacterium tuberculosis and more recently totally drug-resistant strains of M. tuberculosis, display unique mechanisms to survive in the host. A four-drug treatment regimen was introduced 40 years ago but the emergence of multidrug-resistance and more recently TDR necessitates the identification of new targets and drugs for the cure of M. tuberculosis infection. The current efforts in the drug development process are insufficient to completely eradicate the TB epidemic. For almost five decades the TB drug development process remained stagnant. The last 10 years have made sudden progress giving some new and highly promising drugs including bedaquiline, delamanid, and pretomanid. Many of the candidates are repurposed compounds, which were developed to treat other infections but later, exhibited anti-TB properties also. Each class of drug has a specific target and a definite mode of action. These targets are either involved in cell wall biosynthesis, protein synthesis, DNA/RNA synthesis, or metabolism. This review discusses recent progress in the discovery of newly developed and Food and Drug Administration approved drugs as well as repurposed drugs, their targets, mode of action, drug-target interactions, and their structure-activity relationship.     

Novel approaches to the treatment of chronic graft-versus-host disease

Chronic graft-versus-host disease (cGvHD) continues to be the major problem in long-term survivors of allogeneic haematopoietic stem cell transplants and is the principal cause of morbidity and non-relapse mortality. Over the past twenty years, diagnosis, prophylaxis and treatment of cGvHD have slowly evolved. An effective therapy for cGvHD is designed to prevent complications through targeting the disease mechanisms. None of the present therapies for cGvHD are successful in the majority of patients. Conventional drugs in different combinations can control the disease in approximately 50% of patients. Attempts to improve survival have led to evaluation of several alternative approaches in the treatment of refractory cGvHD with varying degrees of success. Clinical trials are needed to establish the role of these new approaches in the treatment of cGvHD as first line or salvage therapy without causing significant side effects. This review summarises the currently available knowledge on conventional and new treatment approaches for cGvHD.     

结核分枝杆菌贝达喹啉和氯法齐明耐药及其交叉耐药机制研究进展

目前,全球结核病耐药现象严重,给结核病的防控工作带来了极大的困难与挑战,并亟需新型抗结核药物来阻止耐药结核病的蔓延.贝达喹啉和氯法齐明分别是世界卫生组织推荐的A组和B组核心抗结核药物,主要用于耐多药结核病的治疗且疗效较好.但近年来贝达喹啉和氯法齐明耐药的现象也时有报道,且二者存在交叉耐药的情况.本文综述了与结核分枝杆菌...     

Melkersson-Rosenthal Syndrome in India: Experience with Six Cases

Six patients with Melkersson-Rosenthal syndrome (M-R syndrome) were presented. Their ages varied from 20 to 60 years; age at onset was between 15 and 50 years. In all 6 patients, swelling started first over the lower lip. Associated facial palsy, facial edema, and scrotal tongue were found in three, four and one patients, respectively. Lip biopsies from 5 patients failed to show any granuloma. Intralesional corticosteroids produced mild regression of labial swelling in two patients after 6 injections. In one patient, near total reduction of labial swelling and facial edema was achieved after 5 months of clofazimine treatment. However, there was no improvement of facial palsy. To the best of our knowledge, this is the largest series of M-R syndrome reported from India.     

Micellar Solubilization of Clofazimine Analogues in Aqueous Solutions of Ionic and Nonionic Surfactants

Pharmaceutical Research -     

Antimicrobial treatment with the fixed-dose antibiotic combination RHB-104 for Mycobacterium avium subspecies paratuberculosis in Crohn’s disease: pharmacological and clinical implications

Introduction: Crohn’s disease (CD) is an inflammatory bowel disease of unknown etiology. However, increasing evidence suggests Mycobacterium avium subspecies paratuberculosis (MAP) as a putative causative agent: 1) MAP is the etiological agent of Johne’s disease, a granulomatous enteritis affecting ruminants, which shares clinical and pathological features with CD; 2) MAP has been detected in tissues and blood samples from CD patients; 3) case reports have documented a favorable therapeutic response to anti-MAP antibiotics.

Area covered: This review provides an appraisal of current information on MAP characteristics, diagnostic methodologies and emerging drug treatments. The authors focus on RHB-104, a novel oral formulation containing a fixed-dose combination of clarithromycin, clofazimine and rifabutin, endowed with synergistic inhibitory activity on MAP strains isolated from CD patients.

Expert opinion: Based on encouraging in vitro data, RHB-104 has entered recently the clinical phase of its development, and is being investigated in a randomized, placebo-controlled phase III trial aimed at evaluating its efficacy and safety in CD. Provided that the overall clinical development will support the suitability of RHB-104 for inducing disease remission in CD patients with documented MAP infection, this novel antibiotic combination will likely take a relevant position in the therapeutic armamentarium for CD management.     

Disseminated Mycobacterium chelonae cutaneous infection: recalcitrant to combined antibiotic therapy

A 25-year-old Thai housewife had a history of tuberculosis of the lymph nodes for six years that had been successfully treated with a course of anti-TB drugs. She developed several red, circumscribed, infiltrative plaques composed of umbilicated papules and pustules on her face and upper part of the body with cervical lymphadenopathy six months later. A pus smear from the lesion grew acid fast bacilli (AFB). Histopathological examination showed a mixed cell granuloma suggestive of infection. A T cell study showed a low CD4 count, and multi skin tests indicated cutaneous anergy. Culture from a biopsy specimen taken from the skin lesion grew M.chelonae; the cultures from blood, urine, and bone marrow. The lesions were not responsive to an anti TB drug given for 2 months based upon the results of the AFB positive pus smear before the culture and sensitivity reports were obtained. Since then the patient was treated with antibiotics according to the results of the sensitivity tests. A combination of amikacin and clarithromycin was started and hyperthermic therapy was later added with a partial response. Based upon the sensitivity test, kanamycin was introduced but had to be stopped because of ototoxicity. Sparfloxacin was used with an effective result but was discontinued for economic reasons. Finally, clarithromycin in combination with clofazimine and cryotherapy were given for a year before the lesions healed completely. It took a three years duration for the total course of treatment for this patient. She is still in remission after two years of follow-up period. This extensive cutaneous M.chelonae infection needed a prolonged combination of antibiotics with the addition of cryotherapy for the non-responsive lesions.     

Clofazimine-induced enteropathy in treatment-resistant nodular vasculitis

We report a chronic case of nodular vasculitis that responded to oral clofazimine 300 mg daily. The condition had previously responded to moderate dose oral prednisolone, 50 mg daily, but would recur with weaning. Multiple corticosteroid-sparing agents were trialled, however these were either ineffective or poorly tolerated. The introduction of clofazimine enabled prednisolone dose reduction, not achieved with other agents, to 22.5 mg daily, and was associated with complete suppression of disease activity. Unfortunately the patient developed a clofazimine-induced enteropathy and the treatment was ceased after almost 2 years of therapy. Cessation of clofazimine was associated with a flare of the condition. Clofazimine should be considered as a corticosteroid-sparing agent in resistant cases of nodular vasculitis. Clinicians should be aware of clofazimine-induced enteropathy as a potentially serious complication of the therapy.     

Antiinfectives targeting enzymes and the proton motive force

There is a growing need for new antibiotics. Compounds that target the proton motive force (PMF), uncouplers, represent one possible class of compounds that might be developed because they are already used to treat parasitic infections, and there is interest in their use for the treatment of other diseases, such as diabetes. Here, we tested a series of compounds, most with known antiinfective activity, for uncoupler activity. Many cationic amphiphiles tested positive, and some targeted isoprenoid biosynthesis or affected lipid bilayer structure. As an example, we found that clomiphene, a recently discovered undecaprenyl diphosphate synthase inhibitor active against Staphylococcus aureus, is an uncoupler. Using in silico screening, we then found that the anti-glioblastoma multiforme drug lead vacquinol is an inhibitor of Mycobacterium tuberculosis tuberculosinyl adenosine synthase, as well as being an uncoupler. Because vacquinol is also an inhibitor of M. tuberculosis cell growth, we used similarity searches based on the vacquinol structure, finding analogs with potent (∼0.5–2 μg/mL) activity against M. tuberculosis and S. aureus. Our results give a logical explanation of the observation that most new tuberculosis drug leads discovered by phenotypic screens and genome sequencing are highly lipophilic (logP ∼5.7) bases with membrane targets because such species are expected to partition into hydrophobic membranes, inhibiting membrane proteins, in addition to collapsing the PMF. This multiple targeting is expected to be of importance in overcoming the development of drug resistance because targeting membrane physical properties is expected to be less susceptible to the development of resistance.There is a need for new antibiotics, due to the increase in drug resistance (1, 2). For example, some studies report that by 2050, absent major improvements in drug discovery and use, more individuals will die from drug-resistant bacterial infections than from cancer, resulting in a cumulative effect on global gross domestic product of as much as 100 trillion dollars (3, 4). To discover new drugs, new targets, leads, concepts, and implementations are needed (5, 6).Currently, one major cause of death from bacterial infections is tuberculosis (TB) (7), where very highly drug-resistant strains have been found (8). Therapy is lengthy, even with drug-sensitive strains, and requires combination therapies with four drugs. Two recent TB drugs/drug leads (9–11) are TMC207 [bedaquiline (1); Sirturo] and SQ109 (2) (Fig. 1). Bedaquiline (1) targets the Mycobacterium tuberculosis ATP synthase (9) whereas SQ109 (2) has been proposed to target MmpL3 (mycobacterial membrane protein large 3), a trehalose monomycolate transporter essential for cell wall biosynthesis (12). SQ109 (2) is a lipophilic base containing an adamantyl “headgroup” connected via an ethylene diamine “linker” to a geranyl (C₁₀) “side chain,” and in recent work (13), we synthesized a series of 11 analogs of SQ109 (2) finding that the ethanolamine (3) was more potent than was SQ109 (2) against M. tuberculosis H37Rv [0.063 vs. 0.25 μg/mL minimal inhibitory concentration (MIC)], and that at least one protonatable nitrogen in the linker was essential for activity. The latter observation suggested to us that SQ109 (2) and ethanolamine (3) might have activity as uncouplers, collapsing the proton motive force (PMF; ∆P) used to drive ATP synthesis, because we had observed similar uncoupling effects for lipophilic bases, US Food and Drug Administration (FDA)-approved drugs, in trypanosomatid parasites (14, 15). The PMF is given by Mitchell (16, 17): ∆P = ∆ψ − Z∆pH, where ∆ψ is the electrical or membrane potential component of ∆P, ∆pH is the transmembrane pH gradient, and Z is 2.303RT/F where R is the gas constant, T is temperature (in kelvins), and F is the Faraday constant.Open in a separate windowFig. 1.Structures of inhibitors/uncouplers and other compounds of interest. Common or previously used names are indicated.We found with SQ109 and its analogs that the most potent M. tuberculosis cell growth inhibitors investigated did indeed collapse pH gradients and ∆ψ, as also observed with the lipophilic bases amiodarone (4) (14) and dronedarone (5) (15), antiarrhythmia drugs, in trypanosomatid parasites (18), and SQ109 (2) also acts as an uncoupler in Trypanosoma cruzi (19). Amiodarone (4) and dronedarone (5) had little uncoupling activity against host cells. In related work, Li et al. (20) found that other TB drug leads, BM212 (6), THPP-2 (7), Ro 48-8071 (8), the urea AU1235 (9), and the indolecarboxamide 2418 (10), most of which had been proposed to target MmpL3, likewise had activity as uncouplers, collapsing pH gradients, and in some cases were active against the nonreplicative bacteria found under hypoxic conditions. Several of these compounds also have enzyme targets. For example, SQ109 (2), ethanolamine (3), and Ro 48-8071 (8) have been found (13, 20) to inhibit enzymes involved in menaquinone biosynthesis, particularly the prenyl transferase 1,4-dihydroxy-2-naphthoate octaprenyltransferase (MenA) and human oxidosqualene cyclase (OSC) (21), and bedaquiline (1) is a potent ATP synthase inhibitor, indicating the possibility of multitarget activity for such compounds. These results are of interest because they show that several recently discovered M. tuberculosis drug leads can act as uncouplers in addition to targeting one or more enzymes that are essential for bacterial cell growth, with membrane targeting being of particular interest because it might be expected to be less susceptible to the development of resistance than is purely enzyme targeting, and SQ109 (2) does indeed have a low frequency of resistance in M. tuberculosis (∼2.55 × 10⁻¹¹) (22). Targeting membrane lipids is also a reason for the low frequency of resistance found with, for example, amphotericin [which binds to ergosterol in fungi and protozoa (23)], as well as the recently discovered teixobactin, which binds to lipid II/III (24).In other work by Goldman (25), it has been pointed out that most of the new TB drug leads that have been discovered by phenotypic screens and genome sequencing are highly lipophilic (logP ∼ 5.7) bases with membrane targets, which suggested to us the possibility that these drug leads might function by targeting the PMF, as well as membrane proteins. Although targeting the PMF might be expected to be purely mitotoxic, Stock et al. (26) have shown that compounds with logP > 6 have generally low mitotoxicity, which is due, they proposed, to low membrane permeability attributable to accumulation in lipophilic membranes.Perhaps the most well-known uncoupler is 2,4-dinitrophenol (DNP; 11). DNP functions as a protonophore, a proton-translocating molecule, and analogs such as niclosamide (12) and nitazoxanide (13) [active form, tizoxanide (14)] are used clinically: niclosamide (12) to treat tapeworm infections (27) and nitazoxanide (13) to treat infections due to Giardia lamblia (28) and Cryptosporidium parvum. Nitazoxanide (13) has also been in clinical trials for the treatment of Helicobacter pylori and Clostridium difficile infections. Interestingly, SQ109 (2) has similar activity against both organisms (29), and with H. pylori, SQ109 (2) once again has a very low (≈10⁻¹²) frequency of resistance (29). In addition, nitazoxanide (13) has been found to kill both replicating and nonreplicating M. tuberculosis (30–33), and Nathan and coworkers (30, 31) were unable to develop resistant colonies using up to 10¹² cfu, proposing a dual “PMF + unknown target” mechanism of action. Niclosamide (12) has been proposed as a lead for the treatment of type II diabetes (34), and it is also an inhibitor of breast cancer stem-like cells (35) and an inhibitor of Pseudomonas aeruginosa quorum sensing (36). There has also been very recent interest in the development of DNP analogs such as DNP methyl ether (37), for treating diabetes, and of controlled-release DNP formulations (38) as mild hepatic mitochondrial uncouplers for treating hypertriglyceridemia, insulin resistance, hepatic steatosis, and diabetes. Niclosamide (12) and tizoxanide (14) are both FDA-approved, and closantel (15) is an anthelmintic uncoupler in veterinary use, and all could provide leads for new and improved inhibitors that target other pathogens. There has also been considerable renewed interest (39) in the use of pyrazinoic acid (16), which functions, at least in part, as a protonophore uncoupler, for treating TB (39, 40), stimulating our interest in discovering new TB drug leads with uncoupler activity.In this work, we carried out three main types of investigation. First, we investigated the uncoupling effects of 21 compounds (primarily known drugs or drug leads) on uncoupling (∆pH/∆ψ collapse) in bacterial inverted membrane vesicles (IMVs) and in porcine mitochondria. Second, we investigated drug–membrane interactions using differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) spectroscopy. Third, we used molecular dynamics (MD) structure-based in silico screening and structure-similarity searches to find prenyl synthase inhibitors with uncoupler activity, leading finally to a consideration of the future prospects for discovering new “enzyme + uncoupler” antiinfective drug leads.