Researchers at Hosokawa Micron review the applications of biodegradeable poly lactide-co-glycolide nanospheres in pulmonary drug delivery
Key to the efforts to the advancement of drug delivery systems (DDSs) are new applications that utilise particle design and new processing technologies. Various research activities are being conducted with DDSs primarily to 1) better deliver the drug to the targeted sites of human body; 2) discharge the minimum amount of drug at an appropriate rate for controlled release; and 3) improve the safety, reliability and efficacy of absorbed drugs.
Site-specific drug delivery can be achieved by controlling and modifying the particle size and surface characteristics of the drugs, encapsulating the drugs to control their breakdown rates for controlled release and DDS applications, or by making drugs and their carriers extremely small to improve their absorption efficiency.
Nanospheres have recently been the subject of considerable interest, because their small size means they can permeate body tissues and have excellent mucous membrane absorbability. In addition, surface-modified nanospheres can have remarkable effectiveness due to their incredibly large specific surface areas. As a result, nanospheres promise superior effectiveness in DDS applications because of their excellent permeability, better bio-affinity, and longer residence time in body tissues compared with conventional micron particles.
However, nanospheres are typically cohesive and difficult to handle. They can form hard agglomerates and also cause drug stability problems. To fully utilise the superior features of nanospheres within commercialisation processes and overcome their application barriers, processing technology and particle design must play a key role.
Hosokawa Micron has undertaken r&d on The Development of DDSs Using Nanocomposite Particles of Biocompatible Macromolecules since 2003, under a commission from the NEDO Fundamental Technology Research Promotion Project. pesented here are the research results and application examples of nanospheres in dry powder inhalation (DPI) for pulmonary drug delivery.
PLGA nanospheres
Almost all drugs are supplied in either liquid or solid form. Generally, solid form drugs have greater stability and are easier to handle. However, there has been an increase in hard-to-dissolve drugs in recent years. The dissolution rate of these drugs can be improved by making their particle sizes smaller in order to achieve larger specific surface area. Small particles can have better bio-membrane permeability, particularly those that are nano-sized. Methods of producing small drug particles include breakdown processes, such as milling to produce fine particles, and build-up processes such as chemical synthesis, and crystallization.
To explore the applications of nanospheres for DDSs, several novel nanoparticle technologies with physiochemical treatment were used to prepare drug encapsulated biocompatible nanoparticles. Additives were employed to make nanocomposite particles, giving the drug-loaded nanospheres improved absorbability, stability, and long-term pharmacological effects.
The carrier used was poly lactide-co-glycolide (PLGA), a biocompatible material that breaks down into biological components in the body by hydrolysis. Drug loaded PLGA nanospheres usually have an average particle size of 200 to 400 nm. (Figure 1 shows the d particle size distribution of the PLGA nanospheres). By using PLGA feedstock with different molecular weights and different ratios of lactic acid and glycolic acid, it is possible to change the drug dissolution rate and thereby control the sustained release of the drug.
inhalation improvements
To prolong the pharmacological effects of the drug, improve its bioavailability, and protect it from enzyme degradation, many types of sub-micron particles were investigated as the drug carriers. Nanoparticle applications can be divided into two categories. The first category is for intravenous injection, which allows surface modified nanoparticles to accumulate in the lung after intravenous or subcutaneous injection. The second is pulmonary administration of nanoparticles with a nebulizer or dry powder inhaler.
In the early stage of nanoparticle research for pulmonary drug delivery, nanoparticles were used to prolong the pharmacological effects of respiratory disease or to deliver peptides. Today, nanoparticles for pulmonary drug delivery extend their applications to gene delivery - plasmid DNA and oligonucleotide, for example. These colloidal drug carriers are typically administered in the suspension form by a nebulizer. Masinde et al prepared poly (L-lactic acid) microspheres with a mean diameter of 4.2 µm for pulmonary drug delivery. Their aqueous solutions were successfully jet-nebulized to generate aerosols suitable for pulmonary administration and lung deposition.
However, Yamamoto et al indicate that micron-sized particles cannot be homogenously aerosolised as the particles condense in the nebulizer. Alternatively, nanoparticles could be aerosolised in the same way as the drug solution and be delivered deep into the lung. Nanoparticle encapsulating calcitonin showed prolonged pharmacological effects compared with calcitonin solution in pulmonary administration. Furthermore, modifying the surfaces of nanoparticles with chitosan could significantly improve their lung tissue adhesion giving a longer retention time.
particle design
Dry powder inhalation devices are one example where PLGA nano composite particles can be used to deliver drug loaded nanospheres directly to the lungs. Commercial inhalation aerosols generated by pressurised metered dose inhalers (pMDIs) or nebulizers deliver drugs by dissolving them in the propellants, or by atomising their suspensions and solutions. In recent years there has been increased interest in dry powder inhalation, due to restrictions imposed upon the use of fluorocarbon gases and the problems of synchronizing drug atomisation and inhalation.
Aerodynamically, the optimum particle sizes for pulmonary drug delivery are said to be 0.5-7 µm. Hosokawa Micron's study found that particles larger than 7 µm collide with and adhere to the throat and trachea, whilst fine particles smaller than 0.5 µm have a low lung deposition rate because they are expelled with the exhaled air.
Further, very few of the nanoparticles can actually make their way to the lung cells due to their strong adhesiveness and cohesiveness, which causes them to adhere to the inhalation devices or form large agglomerates that are unable to reach the alveoli. A need emerged to develop processes to treat the drug-loaded nanospheres and prevent them from adhering to the inhalation device and from forming undesirable agglomerates.
The nanospheres were processed to build composite particles tens of µm in size by the wet or dry granulation methods. The composite particles had good flow properties and were easy to disperse during inhalation. To ensure their efficacy in the lung, the composite particles were designed to be dispersed to primary agglomerates several µm in size by the inhalation device, and then quickly restored to nanospheres when absorbing moisture in the lung.
particle preparation
There are various conceivable composite particle structures depending on the particle size of the feedstock and the presence or absence of carriers and binders. Two representative types of particle structures are often obtained; the ordered mixture type, in which fine drug particles adhere to the surfaces of host carrier particles; and the matrix agglomerate type, in which granules consist of fine drug particles and binder, or fine drug particles are dispersed in the matrix diluent (excipient).
The choice of particle structure is often dependant on the solubility of the drug, the required drug amount in each inhalation dosage, and the encapsulated percentage (content) of drug in the composite particles. In the study, both wet and dry processing methods were investigated. The wet process applied fluidised bed granulation, through the Agglomaster system, and the dry process applied compression and shear forces to bind particles together using the MechanoFusion system.
inhalation characteristics
Several methods have been proposed to assess the inhalation characteristics of pulmonary drugs. Hosokawa Micron's study used an Andersen cascade impactor (figure 2) and twin impinger for in vitro examination. The Andersen cascade impactor is a human lung model used to determine the "respirable fraction" based on the amount of 0.5-7 µm particles recovered after the test. The drug output efficiency of an inhalation device was determined by calculating the amountof drug left in the device and capsule after inhalation administration.
The inhalation characteristics of PLGA nanosphere composite particles depended on their manufacturing processes. With the MechanoFusion system, the bonding force between particles in the ordered mixture changed depending on the clearance, rotation speed and processing time, which affected the dispersibility of composite particles in the inhalation devices.
The experiment also found that setting clearances to soft bonding conditions with an optimum processing time could increase the respirable fraction of nanospheres. Administering PLGA nanospheres alone yielded a respirable fraction of approximately 12%, but optimised nanospheres composite particles made by the MechanoFusion system could improve the respirable fraction to over 44%. The portion of PLGA nanospheres deposited on the trachea and alveoli could be higher than 65%.
Similarly, it was found that freeze-dried PLGA nanospheres yielded a respirable fraction of approximately 14%, while those made by the Agglomaster system could nearly double the respirable fraction (as shown in figure 3) and further increase to 45% with an improved inhalation device. The study also found that it is possible to re-disperse 10% PLGA nanosphere composite particles several µm in size to nanospheres by using mannitol as the composite diluent (as shown in figure 4).
Regarding the in vivo assessment of inhalation characteristics, insulin for diabetes treatment was used as the model drug. The insulin-loaded PLGA nanosphere composite particles made by the Agglomaster system were intratracheally administered to the rats. Changes in the blood glucose level of the rats were monitored over time (see figure 5). Nanosphere composite particles were not only much better than administering insulin solution intratracheally and reducing the blood glucose level, but also more effective than intravenous insulin administration. In addition, its therapeutic effect was sustained for much longer. This can be explained by the observation that PLGA nanosphere composite particles were able to reach deep into lungs and achieved a lengthened residence time there, which would enhance the drug's controlled release effect and suppress enzyme breakdown. The results demonstrated the efficacy of the intratracheal administration of PLGA nanospheres.
future research
Hosokawa Micron's study demonstrated various applications of particle design and processing technology with drug-loaded PLGA nanospheres (see figure 6). The new technology can encapsulate peptide drugs such as insulin and calcitonin for oral dosage applications, which hitherto was not possible. Other potential applications, such as using PLGA nanospheres to encapsulate fluorescent substances and test reagents for biokinetics modelling in medical research, are also being investigated.
PLGA nanospheres are valuable carriers for a range of applications. However, to put them into practical use, it is vitally important to apply the correct particle design and processing technologies to create the desired particle structure and characteristics. Also, it is necessary to control the sizes and surface properties of the nanospheres according to the characteristics of the drug in question, in order to raise the encapsulation ratio, improve drug stability and dispersibility, and make other enhancements. Continuous efforts to improve the manufacturing processes and to reduce production costs are essential if nanosphere DDSs technology is to achieve real commercial success and develop many new applications.