Drug Targeting Organ-Specific Strategies

64 3 Pulmonary Drug Delivery: Delivery To and Through the Lung

Pressurized metered dose inhalers are still the most frequently used systems and they have proven their value in therapy. However, their application in early phases of biopharmaceutical research and further development of dosage forms seems less convenient, since they require special components including propellants, special containers, metering valves, and controlled filling conditions (pressure-filling or cold-filling).

Nebulizers and dry powder inhalers seem more appropriate systems to be used in the early stages of development of drug products for pulmonary drug delivery. However, it should not be concluded from this that the development of formulations for nebulizers or DPIs is easier and exhibits fewer theoretical and practical problems.

Which system is the most suitable for a particular drug or therapy is determined by both the physicochemical properties of the drug as well as by patient condition in relation to the chosen therapy. Asthma and COPD treatment using drugs such as β2-agonists or corticosteroids is carried out with MDIs and DPIs. For children, nebulizers seem to be preferred, but MDIs with spacers can also be used. For antibiotic (e.g. tobramycin or colistin) therapy in cystic fibrosis patients nebulizers still seem the device of choice. Probably the patient population in this case is too small to make the development of DPIs or MDIs containing antibiotic drugs economically feasible.When peptide or protein delivery is considered, newer and more advanced systems such as the ‘AERx™ system’ or dry powder generators such as the ‘Inhale Therapeutic System (Innova™)’ have been developed [40,41].

3.5.1 Nebulizers

Nebulizers are applied to aerosolize drug solutions or suspensions.There are two basic types: the air jet and ultrasonic nebulizer [42]. Jet nebulizers have a two-fluid nozzle for atomizing the drug solution. Compressed air passes through a narrow hole and entrains the drug solution from one or more capillaries mainly by momentum transfer.The liquid break-up process depends on the design of the nozzle, the air pressure and the physical properties of the drug solution. Droplets in the required size range are entrained by the airflow from the nozzle. Larger droplets impact on a baffle and are returned to the reservoir. Auxiliary airflows, generated by the patient, may pass through special vents to the nebulization cup in order to improve droplet entrainment from the nozzle area. In an ultrasonic nebulizer, droplets are produced by a piezoelectric crystal vibrating at a high frequency. The frequency and again the properties of the drug solution determine the droplet size distribution of the mist.

Many reviews on the relevant technical aspects for drug nebulization are available (e.g. [43–45]. The greatest disadvantages of nebulizers are their poor deposition efficiency (see Section 3.11) and low output rate (e.g. [46]). Several developments have been reported to improve their efficacy, like the use of open vents or breath-assisted open vents [47] and adapted aerosol delivery [48]. A renewed interest in nebulizer therapy may also come from the generation of special aerosols, such as liposomes [49].

The AERx™ pulmonary delivery system [40,41] can be regarded as a combination of a MDI and a nebulizer. This system forms an aerosol by extrusion of an aqueous drug-con- taining solution through a disposable nozzle containing an array of precisely micromachined holes.The droplets are entrained by the airflow passing over the blister. Control over the size distribution of the holes enables the formation of droplets having a narrow size distribution.

Moreover, the system will release the aerosol cloud only when the pre-programmed optimal inhalation flow is generated by the patient. These features enable a controlled and targeted delivery to the lung.

3.5.2 Metered Dose Inhalers

The metered dose inhalers consist of four basic functional elements, container, metering valve, actuator and mouthpiece.

The drug is dissolved or suspended in the liquefied propellant which might contain other excipients. The energy for atomization of the drug suspension (or solution) from a metered dose inhaler is supplied by a liquefied propellant.When after actuation a small amount of the suspension or solution is released from the metering valve connected to the pressurized container, the propellant starts evaporating rapidly, thereby disrupting the liquid into small droplets. Initial droplet size and droplet speed are too high for effective deposition in the lower respiratory tract (the target area), however. Evaporation and deceleration in the upper respiratory tract (mouth and throat) is essential. Consequently, the inhalation manoeuvre is extremely relevant for deposition efficacy (particularly the co-ordination between firing and inhalation of a dose), in spite of the fact that no energy from the inspiratory air (except heat for evaporation of the propellant) is required for fine droplet generation. If spacers are used, the inhalation manoeuvre becomes less critical. For the 3M Autohaler, no firing of a dose is necessary, because dose release is breath triggered.

With respect to the formulations used in MDIs, the development over recent years has focused on the replacement of chlorofluorocarbon (CFC) propellants by hydrofluoroalkane propellants. Recently, new developments have been reviewed in a number of papers [50–52].

3.5.3 Dry Powder Inhalers

Dry powder inhalers have initially found their application in inhalation therapy as a CFCfree alternative for the older MDIs. However, nowadays they seem to have a much larger potential [14,53], because of the high lung deposition that can be attained and also because they are suitable for the pulmonary delivery of therapeutic peptides and proteins [2,10,16].

Dry powder inhalers are generally described as ‘breath actuated’ devices, because the inspiratory airstream releases the dose from the dose system and supplies the energy for the generation of fine drug particles from the powder formulation. Because the efficiency of dose release and powder disintegration increases with increasing inspiratory flow rate for most DPIs, these devices would be better described as ‘breath controlled’ devices. In Section 3.9, the effect of resistance and clinical conditions on the flow curve and relevant flow parameters for DPIs are discussed.

Basically, devices used as dry powder inhalers contain four basic functional elements, i.e.

•Powder container. Dry powder inhalers may contain the dry powder formulation in many different forms. The first DPI, the Spinhaler™ contained single doses in capsules. Other systems, like the Diskus™ or Diskhaler™ may contain the metered dose in blisters, whereas systems like the Turbohaler™, or Novolizer™, have multi-dose containers.

70 3 Pulmonary Drug Delivery: Delivery To and Through the Lung

Table 3.5. Different issues to consider for peptide or protein inhalation formulations.

•Particle size morphology and surface characteristics

•Moisture sorption behaviour

•Stability in dry state and dissolved

•Tendency to form aggregates

•Charge of the molecule, isoelectric point

•Solubility and dissolution behaviour

•Crystallinity and crystal form

the formulation of peptide and protein powders for inhalation requires more advanced techniques and a wide variety of excipients and production processes [16,65,74]. The reason for this difference is found in the more complex nature of the problems and requirements related to peptide and protein formulations.Table 3.5 summarizes a number of issues that need to be considered when peptide or protein formulations for inhalation therapy are developed. Many of the characteristics mentioned in Table 3.5 can be affected by the processes used to prepare the protein or by the composition of the formulation used. Major formulation problems connected to peptides and proteins are their low stability, hygroscopic nature, and tendency to form aggregates, which are too large to cross the alveolar membrane.

If possible, adhesive mixtures or spherical pellets, prepared using simple excipients such as sugars are also preferred for protein formulations. For the preparation of dry peptide-con- taining formulations the most important techniques are lyophilization, spray freeze-drying, spray-drying, co-precipitation and super critical fluid extraction. When lyophilization is used as the drying method, milling to obtain the desired particle size can be used. For spray-drying or supercritical fluid extraction the desired particle size can be obtained immediately from the drying process. Lucas et al. [75] investigated different micronized bovine serum albu- min–maltodextrin (50 : 50) mixtures. Improved aerosolization behaviour was found for adhesive mixtures based on carrier lactoses with surfaces that were modified by micronized lactose or micronized polyethylene glycol 6000. Maa et al. [76] compared particles prepared by spray freeze-drying with particles prepared by spray-drying. The particles contained recombinant human deoxyribonuclease-1, or anti-IgE monoclonal antibody and different sugars as excipient. The large size of the spray freeze-dried particles (about 8–10 m) in combination with their high porosity, turned out to result in improved aerosol performance compared to the denser and smaller spray-dried particles. The lyophilization of proteins was recently reviewed by Wang [77].

Protein instability can either be of a physical or chemical nature. The major mechanisms underlying the degradation of proteins were recently extensively reviewed [78] Unfolding of the protein is the main cause of physical instability and may lead to denaturation, aggregation or surface adsorption. Excipients that preserve the protein in its preferred state of hydration may be used to stabilize the protein. Several studies described the role of different excipients (often in combination with production processes) in the stabilization of proteins [65,79–82]. The major excipients used for stabilization of proteins are classified in Table 3.6. The incorporation of the proteins in amorphous solid matrices of sugar (often referred to as sugar glasses), seems an effective method to stabilize the solid protein [83–85]. The stabiliza-

Table 3.6. Excipients used for protein formulations for inhalation.

tion is explained by the fact that in these amorphous sugar matrices hydrogen bonds between water and the protein in an aqueous environment are replaced by hydrogen bonds between the sugar and protein. This allows the protein to maintain its conformation and provides mechanical protection. Furthermore, inclusion of the protein in the matrix protects it from the environment thereby preventing degradation processes such as hydrolysis or oxidation. It is essential that the sugar in these systems remains amorphous and has a glass transition temperature above storage temperature. In the rubbery state, the glasses are not stable; crystallization may occur and the protection from environmental influences disappears. The glass transition temperatures of many sugars is above 50°C when the sugars are pure and completely free of water. However, both moisture and the included protein may reduce the glass transition temperature, which makes many sugars unsuitable for the formation of sugar glasses. The moisture content of the products is not only important because of the plasticizing effect, but also for their aerosol performance, since a high moisture content may increase powder cohesiveness. Compatibility of the sugar with the protein is necessary to obtain stable formulations. In this respect the use of reducing sugars such as sucrose or glucose is less satisfactory.

72 3 Pulmonary Drug Delivery: Delivery To and Through the Lung

Trehalose is often referred to as the sugar of choice for preparing sugar glasses. It is a nonreducing disaccharide with a glass transition temperature of about 120°C in the anhydrous state. However, its glass transition temperature is rapidly decreased when the moisture content in the sugar increases. Considering the hygroscopic nature of trehalose this is a potential hazard and adequate moisture protection is essential. Furthermore, crystallization to the trehalose dihydrate occurs easily at a relative humidity above 60%. From this perspective, the use of a sugar polymer such as inulin (which is a fructose polymer terminating with a glucose unit) seems much more suitable. Inulin is also a non-reducing sugar. By changing the chain length (number of fructose units) of the molecule, physical characteristics like the glass transition temperature can be changed. Moreover, due to the polymeric character of inulin, crystallization is less likely to occur.

Sugar glasses are prepared by spray-drying, freeze-drying or vacuum-drying. Freeze-dry- ing produces the lowest change in the sugar glass of degradation, whereas spray-drying may result in altering a large proportion of the particles to the preferred size range of 2 to 5 m. In contrast to most other sugars or polyols that yield amorphous materials on spray-drying, mannitol was found to crystallize during spray-drying [73].

A final advantage of the use of sugar glasses is the fact that they include the proteins in a mono-molecular form. In the glassy state, mobility in the systems is insufficient to allow aggregation of the proteins. Upon dissolution of the sugar matrix the protein is released in its mono-molecular form which might enhance its passage through absorptive membranes.

Small amounts of surfactants may be used to prevent aggregation of proteins and may enhance the refolding process when the dried protein dissolves. Buffers may also help to prevent aggregation of the dissolved drug. Similarly, polymers may be used as aggregation inhibitors or to form matrices. Chan et al. [86] prepared crystalline powders of recombinant human deoxyribonuclease with high fractions of sodium chloride. These powders were formulated as adhesive mixtures on lactose and mannitol and showed improved aerosolization behaviour compared to the pure protein.

Preparation of high porosity particles may require special excipients, such as dipalmitoyl phosphatidylcholine or special drying techniques such as spray freeze-drying [76,87,88]. These large porous particles may combine the advantages that larger particle sizes contribute to the properties of powders with an improved aerosol performance. Furthermore, these large porous particles may be used to obtain sustained release of the incorporated drug [89,90].

Other techniques that have been used to obtain sustained release inhalation products are: the coating of the aerosol particles with paraffin wax or encapsulation or incorporation in biodegradable polymers such as poly(L-lactic acid) or poly(DL-lactide-co-glycolide) [91,92]. Talton et al. [93] described a new spray coating technique for applying ultra-thin coating layers on particles. Finally, some authors describe the use of liposomes or other phospholipidcontaining systems to prolong drug release or lung retention [94]. Liposome vaccine formulations have also been used for immunization via the pulmonary route. These developments will not be discussed in detail here.

One of the major questions in relation to absorption enhancers such as surfactants or sustained release products is their safety. Whether damage to lung tissue is caused by the different excipients is not yet clear. The results obtained so far are not very promising for substances like surfactants [39]. What the effects of repetitive administration of insoluble or slowly (bio)degrading particles might be, remains to be established.