Various controlled release techniques for pulmonary delivery of proteins have been studied (Table 6.1). The major successful ones include the following:
1. Use polymeric materials 2. Microcrystallization 3. Liposomes
2.1 Use of Polymeric Materials
Some of the most promising systems for the controlled release of proteins and peptides involve encapsulation or entrapment in biocompatible polymeric materials. The most widely used polymers to date are poly(ethylene glycol) (PEG), (d,l)-poly(lactic glycolic acid) (PLGA), poly(lactic acid) (PLA) and chitosan. Polymers could be attached to the protein to increase the overall molecular weight of the system and so reduce the rate of absorption across the epithelium of the alveoli, or the protein could be encapsulated in the polymeric system and slowly released into systemic circulation.
2.1.1 Attaching PEG to Proteins
Attaching PEG to a protein such as insulin first involves understanding where the reac- tive functional groups are and where the active centre of the protein is. This is impor- tant so as not to block the biological activity of the protein by sterically hindering
Table 6.1Table showing the effect of excipient/delivery vehicle on the release pattern of protein/peptide via the inhalation route Controlled release strategyMethod of designMean particle diameter (mm)Molecule of interestExcipients/delivery vehicleEvidence of controlled releaseReference Polymeric entrapment/ encapsulationSolvent evaporation8.20InsulinPLGA96-h systemic glucose level suppression, compared with 4 h for uncoated insulin 6 Solvent evaporation2.53InsulinCyclodextrin/PLGAProlonged release in diabetic rats, compared with other insulin formulations
7 Gas anti-solvent (GAS)13.8DeslorelinPLGA7-day in vivo drug release8 Solvent evaporationInsulinPLGA48-h systemic glucose level reduction in guinea pigs, compared with 6 h for uncoated 9 Polymeric conjugationSonication3InsulinCalcium phosphate/ PEG12-h presence in serum, compared with 6 h for unconjugated
10 Spray drying3.3*InsulinPEG12-h presence in serum, compared with 2 h for unconjugated 11 MicrocrystalSeed zone3.0InsulinRhombohedral? Rhombus microcrystals
Increased Tmax from 2 h (solu- tion) to 5 h (microcrystal) with prolonged hypogly- caemic effect
12 LiposomesThin film hydration<200 nm**DetirelixDSPC/DSPG/ cholesterolIncreased half-life of Detirelix from 8.2 (free) to 21.6 h (liposomes) 13 PLGA (d,l)-poly(lactic glycolic acid), PEG poly(ethylene glycol), DSPC distearoyl-L-a-phosphatidylcholine, DSPG distearoyl-L-a-phosphatidylglycerol) *Aerodynamic diameter, ** Liposome vesicle size
access to the active site by the polymer. Once this has been identified, the reactive functional groups close to the active site are protected by attaching t-Boc to them in a suitable organic solvent such as dimethyl sulphoxide-triethylamine (DMSO-TEA) mixture. The reaction mixture is then extensively dialyzed and lyophilized (see Note 1).
The t-Boc protein can then be attached to methoxy PEG-succinimidyl propion- ate in DMSO-TEA mixture. The mPEG-Boc-protein solution can then be exten- sively dialyzed and lyophilized [14]. Quantitative deprotection of the lyophilized product can then be achieved by reaction with trifluoroacetic acid (TFA) at 0°C.
The conjugate can now be purified by using reverse phase-high performance liquid chromatography (RP-HPLC).
Following the production of the PEG-protein conjugate, there is a need to pre- pare the powder for inhalation. This can be done by spray drying the solution of the conjugate containing appropriate stabilizing excipients (such as surfactants and polyols). High-quality particles for inhalation can be achieved by using the appropriate parameters (inlet temperature, outlet temperature, pump rate, flow rate, etc.).
2.1.2 Preparation of Polymeric Microspheres
PLGA is the most widely used polymer for encapsulating proteins for pulmonary delivery. The most commonly used method for preparing protein-encapsulated PLGA microspheres is the solvent evaporation technique based on the formation of a double emulsion (w/o/w). Incorporation of protein into the microspheres could be done by two methods [15, 16].
The first method involves preparing the microspheres first by solvent evapora- tion and then loading the protein into the microspheres. The emulsion is prepared by adding about 1–2 ml of deionised water to about 5 ml of methylene chloride containing certain amount of PLGA which forms the oil phase. This mixture is sonicated for about 60 s to form a water-in-oil (w/o) emulsion. The resulting w/o emulsion is then quickly added to about 200 ml of deionised water containing about 0.5% (w/o) of poly(vinyl alcohol) and stirred at about 1,500 rpm for about 2 min to allow evaporation of the methylene chloride and hardening of the microspheres.
The hardened microspheres can now be washed three times with excess deionised water and freeze dried. The porous microspheres formed can then be loaded with the protein of choice by suspending a specific amount of the microspheres in a solution of the protein in buffer and gently shaken for about 2 h. The protein-loaded microspheres can now be separated by centrifugation and freeze dried.
The second method involves incorporating the protein in the microspheres dur- ing the preparation of the double emulsion by dissolving the protein in the initial water phase before mixing with the oil phase. Other processes as earlier-described are then performed.
A more recent approach is the formation of a solid-in-oil-in-water emulsion (s/o/w) before solvent evaporation [18]. This involves the formation of solid protein particles by either spray drying or spray freeze-drying, followed by the dispersion of
certain amount of the particles formed in about 5 ml of PLGA methylene chloride solution by ultrasonication for about 1 min. The PLGA/protein mixture is then dispersed into about 0.5% PVA aqueous solution with agitation using a stirrer at 250 rpm for about 4 h to allow the methylene chloride to evaporate. The solid particles formed can then be collected by filtration, rinsed three times with deionised water and then freeze dried.
2.2 Microcrystallization
Microcrystals of proteins with mean diameter <3 mm can be prepared for sus- tained release by the “seed zone” method. The sustained release effect could be attributed to the decreased solubility of the microcrystals [12]. This method has been used in the making of insulin microcrystals of rhombohedra shape without aggregates. Following intratracheal instillation of the insulin microcrystal suspension (32 U/kg) to rats, the blood glucose levels were reduced and hypogly- caemia was prolonged for 13 h when compared with the unmodified insulin solution [12].
Apart from the seed zone method, a conventional seeding technique can also be used for preparing microcrystals for inhalation. This method involves the creation of seeds by suspending protein particles in a suitable buffer solution and centrifug- ing this suspension at about 10,000 rpm for about 10 min. The supernatant can then be stored at 4°C, to be used as a seed solution. Protein particles can then be dis- solved in another buffer solution but at a much lower pH to facilitate the dissolution of the protein. The amount of protein is then slowly increased to achieve supersatu- ration. The solution is then filtered and the initially prepared seed solution is added to the filtrate. The mixture is sealed and incubated at 37°C. The only issue with this conventional seeding method is that crystals above 5 mm could be formed, which are not suitable for pulmonary delivery (see Note 2).
Although microcrystals of proteins seem to be a promising sustained release technique, the large molecular weight and flexibility of most proteins could mean that not all therapeutic proteins would be amenable to this technique.
2.3 Liposomes
Liposomes are artificial, spherical vesicles consisting of amphiphilic lipids (mostly phospholipids), enclosing an aqueous core. Depending on the processing condi- tions and the chemical composition, liposomes can either be unilamellar or multilamellar.
Liposomes are mostly prepared by the thin film hydration method in which a thin film is produced by dissolving the phospholipids in suitable organic solvent
(mostly chloroform or ethanol) and then evaporating the solvent in a rotary evaporator under vacuum. Hydration of this thin lipid film with an aqueous solution of the protein, followed by physical shaking, leads to formation of the liposomes [13].
The size of the vesicles can be reduced by sonication. Unencapsulated protein can be removed by centrifugation and separation of the supernatant. The liposomes formed are then dried by either lyophilisation or spray drying following their sus- pension in an isotonic aqueous solution.
Liposomes can be classified according to the number of lamellae and size:
1. Small unilamellar vesicle (SUV) 2. Large unilamellar vesicle (LUV) 3. Multilamellar vesicle (MLV) 4. Multivesicular vesicle (MVV)
SUVs have a diameter of 20 to ∼100 nm while MLVs, LUVs and MVVs range in size from a few hundred nanometres to several micrometers. An average membrane of a liposome (phospholipid bilayer) measures about 7 nm.
Large liposomes are formed when phospholipids are hydrated at temperature above their phase transition temperature (Tc). Although MLVs are normally formed when lipid films are hydrated below Tc, these can be transformed into small vesicles by using high pressure homogenisation.
The fact that liposomes can be formed from a variety of lipids makes them quite versatile, having a wide range of physicochemical properties depending on the types of lipids used. These physicochemical properties such as liposomes size, surface charge, method of preparation and bilayer fluidity affect their drug release properties. It has been observed that the vesicle size and the number of bilayers are major factors in determining the circulating half-life and extent of drug encapsulation [17]. Liposomes less than 0.1 mm are generally less rapidly opsonised than are larger liposomes (>0.1 mm), which translates to having a longer half-life (see Note 3).
Niven et al. [18] have demonstrated that small liposomes also have a slower release rate than do large multilamellar vesicles following nebulisation. It has also been suggested that liposomes of 50–200 nm diameter are optimal for clinical applications, as they tend to avoid phagocytosis by macrophages and still trap use- ful drug loads [19].
The Tc of lipids used in the preparation of the liposomes also has significant effect on the release rate of encapsulated drugs. Lipids have a characteristic Tc which depends on the length and saturation of the fatty acid chains and can vary from 20 to 90°C [20]. Below the Tc, lipids are in a rigid, well-ordered arrangement (gel phase) and above the Tc, in a liquid crystalline state (fluid phase) (see Note 4).
Incorporation of lipids with high Tc (Tc > 37°C) makes the bilayer of liposomes less fluid at the physiological temperature and less leaky. Tc also appears to influence uptake of liposomes by macrophages, with lipids with high Tc having lower uptake [17]. Cholesterol is an example of lipids with high Tc and is mostly incorporated into the lipid bilayer to increase stability of the liposomes (see Note 5).