Deep-lung delivery of therapeutic proteins

Taking advantage of the body's ability to transfer large molecules through the lungs is a better way to deliver drugs than sticking people with needles.

John S. Patton

M acromolecules, notably proteins and peptides, are large delicate compounds that usually can be administered only by injection. Innovative research in biotechnology and recombinant gene techniques during the past 20 years has led to the growth of macromolecules as drugs, and more than 30 macromolecules have received regulatory approval for use in the United States alone. More than 130 of these compounds are now in human clinical trials, many for the treatment of chronic disorders that afflict millions of people worldwide. Global sales of genetically engineered protein drugs were $10.7 billion in 1995 (Lehman Brothers); macromolecule drug sales are projected to reach $20 billion/year by 2000.

In this article, I review the pulmonary administration of aerosol dry powder forms of proteins and peptides as an alternative to drug administration by injection. Pulmonary delivery provides a direct route to the circulation, increasing patient compliance with a minimum of discomfort and pain, and is a cost-effective option for pharmaceutical and biotechnology companies.

Injections and some alternatives
There are several problems with the administration of proteins and peptides by injection. First, most patients do not look forward to the pain of injection, particularly when required repeatedly for the treatment of chronic disease. Dislike frequently leads to a lack of compliance and poor disease management. A recent study conducted by the Diabetes Control and Complications Trial Research Group concluded that if diabetics used intensive insulin therapy, they could delay or slow the progression of severe diabetic symptoms (1). The use of intensive therapy could be increased with a noninvasive system.

Second, injectable drugs can be expensive because, to ensure safety and compliance, physicians and nurses often administer the drugs in a hospital or office setting. This method not only increases the cost but also is inconvenient for the patient, thus creating another barrier to obtaining all the necessary injections to properly treat a disorder. To avoid both of these problems, many diabetics have been self-administering insulin by injection for a long time, but even dedicated patients do not take all the medicine they need.

Since more long-term and frequently administered injection drugs have become available--growth factors, anti-infectives, and other chronic disease treatments, for example--healthcare professionals have pressed for alternative, more convenient means of administration.

Traditional noninvasive delivery systems do not work for macromolecules; pills or tablets enter the stomach, where enzymes and hydrochloric acid rapidly degrade the protein or peptide. The oral administration of proteins and peptides is under research, but no sure system is commercially available yet. No acceptable transdermal delivery systems have been found because of proteins' size constraints or inherent physical properties that prohibit these large molecules from crossing the diverse layers of the skin without the addition of irritating enhancers.

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Pulmonary delivery: A logical alternative
If other noninvasive routes have not been successful, why should pulmonary delivery be any different? Most basically, the biology of the lung makes it a favorable environment for noninvasive drug delivery (Figure 1). Studies have shown that most large-molecule agents are absorbed naturally by the lungs, and once absorbed in the deep lung, they pass readily into the bloodstream without the need for enhancers used by other noninvasive routes (2).

Figure 1. The structure of the lung...

Figure 2. The Inhale drug delivery device.

On inhalation, air passes through the trachea, which branches more than 17 times into successively smaller tubes that constitute the bronchial network, eventually reaching the grapelike clusters of tiny air sacs known as alveoli. Each breath of air is distributed deep into the lung tissue, to the alveolar epithelium, the surface area of which measures ~100 m² in adults--roughly equivalent to the surface area of a standard singles tennis court. This large area is made up of about half a billion alveoli, from which oxygen passes into the bloodstream via an extensive capillary network.

The lone barrier to the delivery of compounds via the lungs is the tightly packed, single-cell-thick layer known as the pulmonary epithelium. In the lungs, the epithelium of the airway is very different from that of the alveolus. Thick, ciliated, mucus-covered cells line the surface of the airway, but the epithelial cell layer thins out as it reaches deeper into the lungs, until reaching the tightly packed alveolar epithelium. Most researchers believe that protein absorption occurs in the alveoli, where the body absorbs peptides and proteins into the bloodstream by a natural process known as transcytosis.

Logically, there is no reason to expect safety problems related to the inhalation of a substance any different from those associated with the injection of the same amount of the substance. A growing quantity of safety data indicates that inhaling proteins can be safe for patients with healthy or diseased lungs (3-5). The safety of therapeutic inhalation is further supported by the existence of more than 20 small-molecule and one large-protein drug inhalation products approved by the U.S. Food and Drug Administration (FDA); this group of therapeutic inhalants contains 13 different excipients.

The lung is, by necessity, a durable organ. In an eight-hour work day, for example, the average person inhales about 3000 L of air, including all the dust and particles floating in it. In 1992-93, the American Conference of Governmental Industrial Hygienists (ACGIH) determined the Threshold Limit Values for inhalation (6). Among these threshold limits, ACGIH determined that workers can inhale 30 mg of nuisance dust chronically--that is, eight hours per work day over many years--without risk of damage to the lungs. By comparison, Inhale is developing drugs that will require an average daily lung deposition mass of 1-20 mg.

The challenges of aerosol formulations
Although a natural and safe route, pulmonary delivery has its challenges. The key challenge is getting the drug to reach the deep lung. Historically, aerosol formulations have not been able to move the medication into the deep lung efficiently, and until recently, companies developed pulmonary drug delivery systems to dispense drugs to the airways only for local applications. Metered dose inhalers (MDIs), breath-activated dry powder inhalers (DPIs), liquid jet, and ultrasonic nebulizers have proved useful in the management of asthma, but such devices are not designed to deliver drugs into the deep lung. They are not practical for the delivery of most macromolecules because of their low system efficiency, low drug mass per puff, poor formulation stability for macromolecules, and poor dosing reproducibility.

For optimal deep lung delivery of costly proteins and peptides, it is important to use the correct aerosol particle size. Studies have established that these particles should range from 1 to 3 µm in diameter for optimal deposition efficiency. Most existing aerosol systems can deliver only a small fraction (about 10-20%) of the dispensed drug in the correct particle size for lung deposition (7). Furthermore, the amount of drug deposited from the device is highly dependent on the patient's inhalation technique. Any truly effective delivery device for proteins and peptides needs to optimize a patient's ability to inhale correctly.

Most aerosol systems today deliver a total amount of <100 µg of drug per puff to the deep lung; this amount is too low to enable timely delivery of many macromolecules for the required milligram-level doses (8). Any aerosol system developed for large molecules will have to exhibit a characteristic that we call "payload versatility," that is, the ability to deliver varying amounts of a drug. Payload versatility will be necessary because the new macromolecule drugs vary widely in potency from a few micrograms to tens of milligrams per dose. Traditional inhalation systems have been designed primarily to deliver some of the most potent drugs in use today, the bronchodilators and bronchosteroids to treat asthma. Both types of compounds are bioactive in the lung at 5-20 µg per dose. In contrast, many peptide and protein compounds need to be delivered to the deep lung at much larger doses of 2-20 mg (9).

For inhalation therapies to accomplish their medical goals, macromolecule delivery to the lungs must be precise and consistent at every inspiration. Although the natural bioavailability of the deep lung epithelia appears difficult to change, the efficiency of drug deposition offers some opportunity for adjustments. Deposition efficiency from traditional devices has been low, however, with less than 10% of the total inhaled dose reaching the deep lung.

The issue of device reproducibility has not been important because physicians and patients have accepted the highly variable dosing of aerosol asthma drugs for years because the drugs and disease are relatively benign. In the case of the bronchodilator-type asthma drugs, the quick positive response of easier breathing has enabled rapid biofeedback to patients to tell them whether or not they are using proper device technique. So far, no macromolecule drug appears to induce the rapid biofeedback that would help "teach" the proper technique with the conventional inhalers.

Bioavailability
After the aerosolized drug reaches the deep lung, it must be absorbed with high enough bioavailability to make the system practical. As early as 1925, insulin inhalation for the treatment of diabetes was shown to work in humans, but the bioavailability was low (<3%). More recently, several inhalation studies comparing insulin administration by aerosol inhalation (using cumbersome devices) and by subcutaneous injection for the reproducibility of dosing have shown that the variability in glucose response to the two methods was equivalent (10-13). Bioavailability in more recent studies with aerosol insulin were up to 25%, supporting the use of such a method of administration (14). Insulin administered by oral inhalation effectively normalized diabetic patients' plasma glucose levels without adverse effects. Bioavailability studies in humans of the aerosol administration of lutenizing hormone-releasing hormone (LHRH), a decapeptide, and its analogues also have demonstrated that appropriate bioactive systemic levels can be achieved to treat conditions such as endometriosis and prostate cancer.

The mechanism of macromolecule absorption in the deep lung is thought to occur via normal physiological processes that can deliver active compounds with relatively high bioavailability without requiring the addition of penetration enhancers (2). Studies in animal models of intratracheal bioavailability show that, depending on the proteins or peptides tested, bioavailability can vary widely (9). LHRH analogues (used in treating osteoporosis), composed of 10 amino acids, can reach 95% bioavailability; however, interferon- (used in treating hepatitis B and C), composed of 165 amino acids, attains 29% bioavailability. Some smaller peptides such as glucagon (29 amino acids) and somatostatin (28 amino acids) reach 1% bioavailability. The degree of bioavailability is thought to depend on the peptide or protein susceptibility to certain hydrolytic enzymes in the lung.

Dry powder formulations
How a macromolecular drug is formulated also affects its delivery to the deep lung. To overcome the limitations of MDIs and other conventional lung delivery systems, scientists at Inhale have developed dry powder formulations of proteins and peptides. These formulations have several advantages, including product and formulation stability, high drug volume delivery per puff, low susceptibility to microbial growth, and applicability to both soluble and insoluble drugs. Many macromolecules are formulated as dry powders because they are more stable as solids than as liquids. We have produced dry powders of macromolecules that have remained stable at room temperature from several months to up to 2 years.

Compared with liquid aerosol particles, which are mostly water (97%), dry powder aerosol particles can carry 50-100% of the drug. In general, more puffs would be necessary to deliver the equivalent amount of drug to the alveolar epithelium from a liquid aerosol device.

Liquid formulations also carry the risk of microbial growth; the risk of lung infections due to bacterial and fungal contaminants is greatly reduced with dry powder systems. By greatly lowering the possibility of microbial contamination, dry powder systems offer a safer technology.

Simply being in a powdered state does not guarantee the stability of a particular molecule. The cornerstone of Inhale's dry powder formulation is a technology known as glass stabilization, which is used in the protein drying process to stabilize pharmaceutical formulations.

Stabilizing and packaging dry powders
In the liquid state, individual protein or peptide molecules are extremely mobile. When water is removed, macromolecules usually pack together in an amorphous state, unlike the highly ordered packing that occurs in crystallization. When water is removed from proteins, the protein molecules remain mobile and chemical stability stays low in the initial amorphous powder that forms. When a critical amount of water has been removed, a kind of molecular gridlock occurs, producing a greatly increased chemical stability called the "amorphous glass state." In this state, previously mobile molecules slow down drastically. As long as the glass transition temperature of the powder is higher than any environmental temperatures that may occur during normal human use, the powder will remain in a glass state.

We have modified standard powder processing equipment and developed custom techniques to enable us to produce fine (1-5 µm diam) dry powders consistently without drug degradation or significant loss of expensive bulk drug. The resulting powder exhibits excellent performance characteristics including long-term physical and chemical stability and quick dissolution on the alveolar surface. From a manufacturing standpoint, however, these powders are extremely difficult to handle. The fine particles tend to clump together, thus impeding the easy flow of the powder into the packaging. They also rapidly absorb moisture from the air, moving them out of the glass state and reducing shelf-life and performance in the device as well as in the patient.

Long-term stability of the powders is provided by individual (i.e., unit dose), double-foil packaging that provides a strong barrier against moisture. Unit packaging also enables very precise dosing and the ability to deliver blister-packages of different doses and strengths, especially important for drugs such as insulin. Functionally, the blister package we use must slide easily and securely into a slot on the side of the inhalation device (similar to a bank card sliding into an ATM machine) and also protect its fine powder load from moisture without compromising light weight and portability.

To accurately fill the blister-packed doses with the very fine powders, we have developed proprietary filling technology that integrates the process of blister forming, filling, and sealing. Although blister filling technology is ubiquitous and relatively advanced, there was no existing technology that could fill a packet with nonflowing powders, handle such minute particle sizes, and precisely meter out the small quantities needed for individual therapeutic doses. Inhale now has the equipment to fill sufficient amounts of individual packages for clinical trials and, for some drugs, commercial production. Plans include scale-up for additional filling capacity. Inhale has achieved up to two years of stability with some of its molecules.

Getting the drug to the deep lung
Dry powders alone are not enough to ensure that the powdered drug reaches the deep lung. Inhale's challenge was to create a device that would break apart fine, nonflowing aerosol powders and suspend them in a captured volume of air, a "standing cloud," which then could be inhaled calmly and slowly. Other devices on the market that use the force of a patient's inhaled breath to pull the powder from storage and declump it produced varying results; the energy in a human breath is insufficient for this purpose. Furthermore, as patients inhale harder on the device to better break up the powder, they are increasing the velocity of the particle stream and thereby the amount of drug that is lost by hitting the back of the throat. We created a simple, durable device that is actuated independently of the force of the patient's inhalation (Figure 2, p. 36). One of the many features that makes the Inhale system unique is that it actuates without the use of electronics, batteries, or microchips. The purely mechanical device is less expensive than a powered device to manufacture and maintain, and it has fewer points of potential malfunction such as weak or dead batteries, a bad chip, or a malfunctioning electrical system.

To operate the system, a patient places a unit dose blister pack into a slot in the side of the device, extends and closes the pneumatic handle, thus storing compressed air (similar to cocking an air gun), and presses the fire button to release the compressed air (Figure 3). A standing cloud of aerosolized drug, visible in the clear chamber, reassures the patient that the device is functioning properly. In the next several seconds, the patient calmly opens the chamber cap, places the device to his or her lips, and slowly inhales (Figure 4). The entire volume of air in the chamber (~200 mL) with medicine is inhaled at the front end of an average person's normal deep inhalation (2000-4000 mL of air), which means that persons with varying lung capacity can use the device with consistent results. The remaining inhalation pushes the aerosolized medicine to its destination, the deep lung. The patient then holds his or her breath for five seconds allowing the drug particles to settle on the surface of the alveolar epithelium, at which point absorption begins to take place. The chamber is extended during operation and fits compactly over the body of the device for storage.

Figure 3.Schematic diagram in the Inhale device.

Figure 4.Getting the drug to the lungs.

For three proteins now in human trials, the reproducibility of delivery to the bloodstream is equivalent to that of subcutaneous injection, and the drug delivery efficiency is three to six times greater than that achieved with other approved devices. The percentage of drug exiting the mouthpiece with particle sizes of <5 µm varies with the formulation but ranges between 80 and 100%. The aerosol drug formulation must have an average particle size of about 1-3 µm as measured by cascade impaction mass-median aerodynamic diameter.

The future of pulmonary drug delivery
Pulmonary drug delivery offers the potential for noninvasive administration of a wide variety of macromolecules. Nearly every biotherapeutic product that treats chronic or long-term illness would benefit from noninvasive delivery by providing a competitive advantage with current therapeutics. This advantage could expand the market for each product or enable new indications to be considered. Inhalation delivery of macromolecules can extend the life of a drug, increase patient compliance because of prompt effectiveness, and reduce the total costs of long-term healthcare.

We believe the future of drug administration by inhalation will expand vastly. The development of new macromolecule drugs that can treat diseases that previously were either not treatable or only partially treatable has led to renewed interest in noninvasive drug delivery technology. Many new agents are now under investigation for pulmonary delivery: interleukin-1 receptor (asthma therapy), heparin (blood clotting), human insulin (diabetes), -1 antitrypsin (emphysema and cystic fibrosis), interferons (multiple sclerosis and hepatitis B and C), and calcitonin and other peptides (osteoporosis). Inhalation delivery methods may apply gene therapy via tissue targeting and organ targeting. Inhale's novel dry powder formulation, processing, and filling, combined with aerosol device technology, will provide many patients who previously received injections with the ability to independently and painlessly inhale medicine into the deep lung, where it will be absorbed into the bloodstream naturally and efficiently.

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