Department of Biochemistry, Postgraduate Institute of Medical Education & Research, Chandigarh160 012, India
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Abstract |
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Keywords: tuberculosis , liposomes, polymers , nebulization , drug delivery
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Introduction |
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Modes of respiratory drug delivery |
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Inhaled therapy with conventional or unformulated ATDs |
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Aerosol administration of interferon gamma (IFN-), a key cytokine in the immunological response against mycobacteria, has also been attempted. The initial studies were inconclusive as the patients receiving adjunctive aerosol IFN-
became smear-negative after 1 month but continued to be culture-positive and the smear response was not sustained.13
However, when the aerosolized IFN-
therapy was continued for 6 months (thrice weekly), most of the patients showed a definite radiological improvement and a reduction in the size of the cavitary lesions.14
It appears that merely aerosolizing an antimycobacterial compound may be inadequate; for efficient bacterial killing, drugs need to be formulated into suitable delivery systems thereby ensuring their rapid uptake into macrophages which harbour the tubercle bacilli. The dictum holds true for the majority of intracellular infections, and liposomes as well as micro/nanoparticles have emerged as useful drug carriers (Table 1) in this context.15,16
Hence, it is not surprising that these carriers have established their potential for antitubercular inhaled therapy (Table 2).
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Pulmonary delivery of liposome-encapsulated ATDs |
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The specific targeting of liposomes towards the alveolar macrophages can be achieved by coating the liposomes with alveolar macrophage-specific ligands such as O-stearyl amylopectin (O-SAP) and maleylated bovine serum albumin (MBSA). The therapeutic efficacy of O-SAP-coated liposomal ATDs was recently reported, however, the intravenous route was employed for liposomal administration.23 Vyas et al.24 prepared O-SAP- and MBSA-appended inhalable liposomes entrapping rifampicin. In vivo studies in albino rats demonstrated a higher pulmonary delivery and better localization of ligand-appended liposomes to alveolar macrophages compared with conventional liposomes or free rifampicin, from 30 min to 24 h post-nebulization. Subsequently, the alveolar macrophages were isolated, spread as a monolayer and infected with Mycobacterium smegmatis. The percentage viability of the bacilli was significantly reduced to 10.9% in the case of MBSA- and 7.1% in the case of O-SAP-coated liposomes, compared with 69% and 31% for control macrophages and conventional liposome-treated macrophages, respectively. The results were based on a single nebulization of liposomal rifampicin and the authors speculated that an ideal situation of 0% viability may be obtained by repeated dosing. It is therefore clear that nebulization of liposomal ATDs, coupled to the use of alveolar macrophage-specific ligands, may improve the chemotherapy of pulmonary TB especially in view of the fact that liposomes are known to be safe when administered via the respiratory route.25 However, with the use of biodegradable polymers in the arena of drug delivery, more emphasis began to be laid on the use of polymeric systems for antitubercular inhaled therapy.
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Pulmonary delivery of microparticle-encapsulated ATDs |
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Although the results with rifampicin-loaded microspheres proved to be encouraging, it was necessary to incorporate other ATDs because the disease requires multidrug therapy for its cure. Hence, other investigators encapsulated isoniazid with rifampicin in polylactide microparticles for dry powder inhalation to rats.32 Drug concentrations inside the alveolar macrophages were found to be higher than that resulting from systemic delivery of free drugs, an indication of the rapid phagocytic uptake and cytosolic localization of the drug-loaded microparticles. The authors discussed that since alveolar macrophages migrate to secondary lymphoid organs, loading these cells with microparticles might lead to transport of drugs to those very sites where macrophages migrate (mimicking the course of spread of mycobacteria). That is to say, pulmonary delivery of microparticle-encapsulated ATDs has the potential to reach extrapulmonary sites of infection as well. Unfortunately, chemotherapeutic studies were not carried out by the authors.
The rising incidence of multidrug-resistant TB (MDR-TB) is a matter of great concern because the treatment involves the use of second-line ATDs, which are more costly and toxic compared with the first-line drugs used to treat drug-susceptible TB. Furthermore, the treatment schedule is more prolonged with a greater risk of patient non-compliance.33 Some of the second-line drugs, e.g. para-aminosalicylic acid (PAS), need to be administered in very large amounts (up to 12 g daily), which is inconvenient to the patient. In order to reduce the drug dosage, investigators have formulated an inhalable microparticulate system for PAS, based on dipalmitoylglycero-3-phosphocholine.34 The microparticles were produced by spray drying, possessed a 95% drug loading and were administered to rats via insufflation. The drug was maintained at therapeutic concentrations in the lung tissue for at least 3 h (the authors did not monitor the drug levels further) following a single dose of just 5 mg of the dried formulation. Accelerated stability studies indicated that the formulation was stable for up to 4 weeks and the authors suggested that the technology could be extended to include other drugs such as rifampicin, aminoglycosides as well as fluoroquinolones.
Despite the satisfactory results obtained with microparticles, the quest for better drug delivery systems ushered in the era of nanoparticles. The design and development of polymeric nanoparticles for experimental antitubercular inhaled therapy have been the recent focus of interest in our laboratory.
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Pulmonary delivery of nanoparticle-encapsulated ATDs |
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A single nebulization of the formulation to guinea pigs was able to maintain a therapeutic drug concentration in the plasma for 68 days and in the lungs for 911 days. There was a striking improvement in the half-life, mean residence time and relative/absolute bioavailability of encapsulated drugs compared with free drugs. It may be asked that if one is aiming at pulmonary deposition of ATDs, how the improvement in systemic bioavailability would be advantageous following inhaled therapy? The argument was that the enhanced bioavailability would lead to more of the drugs reaching the lungs by way of the circulation, i.e. the systemic spillover could not be considered as a drug wastage.39 Repeated administration of the formulation failed to elicit hepatotoxicity as assessed on a biochemical basis. In M. tuberculosis H37Rv infected guinea pigs, five nebulized doses of the formulation spaced 10 days apart, resulted in undetectable cfu in the lungs replacing 46 conventional doses. This was the first report of PLG-nanoparticles as an inhalable ATD carrier.39 The advantage of the system over inhalable microspheres was clear cut; firstly, it was possible to co-administer multiple ATDs encapsulated in nanoparticles and secondly, a better therapeutic response was elicited in the case of nanoparticles.
The formulation was further refined and improved by coupling it to lectin (wheat germ agglutinin, a commonly occurring plant glycoprotein). With the knowledge that lectin receptors are widely distributed in the respiratory tract,40 it was worthwhile to evaluate the chemotherapeutic potential of lectin-functionalized PLG-nanoparticles,41 a somewhat similar approach to ligand-appended liposomes.24 Upon nebulization to guinea pigs, therapeutic drug concentrations were maintained in the plasma/organs for 615 days. Most of the pharmacokinetic parameters were upgraded compared with uncoated PLG-nanoparticles. Most importantly, when nebulized to TB-infected guinea pigs every fortnight, three doses of the formulation produced undetectable cfu in the lungs as well as spleens.41 The series of experiments proved that 46 conventional doses could be reduced to five nebulized doses of PLG-nanoparticles and further to just three doses with lectin-PLG-nanoparticles.
A new concept in nanotechnology is that of solid lipid nanoparticles (SLNs), i.e. lipid nanocrystals in water possessing a solid core into which drugs are incorporated. The SLNs combine the virtues of more traditional drug carriers such as liposomes or polymeric nanoparticles while eliminating some of their disadvantages, e.g. the issues of burst release and long-term stability in the case of liposomes as well as the problems of residual solvents and bulk production in the case of polymeric nanoparticles.35,42 Furthermore, although PLG is completely biodegradable and biocompatible, the degradation rate is slow and repeated administration of the formulation carries a likelihood of accumulation of the polymer or its degradation products in the respiratory tract. The polymer is known to elicit a mild inflammatory response lasting 23 weeks,43 however, the implications for inhaled therapy and possible influence on lung function have yet to be evaluated.
Although the pulmonary delivery of SLNs is in its infancy,44 our experiments with inhalable ATD-loaded SLNs have produced encouraging results in a guinea pig TB model.45 Seven weekly inhaled doses of the formulation resulted in undetectable bacilli in the lungs of M. tuberculosis infected guinea pigs. Another aspect yet to be explored is that of natural polymer (e.g. alginate, chitosan) based ATD delivery systems. A recent report describing the pulmonary delivery of chitosan-loaded DNA encoding M. tuberculosis T cell epitopes46 might well serve as the starting point in this area. Work is in progress in our laboratory to encapsulate ATDs in chitosan-stabilized alginate nanoparticles for pulmonary delivery.
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Future perspectives |
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Patients suffering from endobronchial TB may be particularly suitable for inhaled therapy in future.47 MDR-TB not responding to conventional treatment is another scenario where inhaled therapy may come to have a significant future role. For patients who do not fit into the categories above, the future role of inhaled therapy is less clear. Potentially, a few inhaled doses at the start of treatment for uncomplicated pulmonary TB could help to significantly reduce the pulmonary bacterial burden and hence improve on the efficacy of conventional oral therapy. However, inhaled therapy will need to fit in with existing National TB programmes, and with initiatives such as the Directly Observed Treatment Shortcourse (DOTS) programme. Increased costs, together with the need for strict control of infection precautions to prevent device-associated cross-infections and/or risk to health personnel, may limit the extent to which such technologies come to be widely available, particularly in developing countries. The large-scale production of stable drug formulations at an affordable cost will be the fundamental and decisive obstacle which will need to be overcome before contemplating human trials. However, the rationale behind antitubercular inhaled therapy is persuasive. Hopefully, current and future research efforts will eventually result in this concept moving from the bench to the bedside.
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References |
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