Metereddose inhalers

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The pressurized metered-dose inhaler (pMDI or MDI) is the most commonly used inhaler, and is an inexpensive, quick and convenient way to take asthma medicine, and the device may also be used in conjunction with a spacer. The MDI, often simply called an inhaler, is a small device incorporating a pressurized canister that contains aerosol medicine to be inhaled. Many asthma medications are taken with an inhaler. Every day more than 500 million patents in the world carry easy-to-use MDI devices to deliver medicines to their pulmonary airways (Colthorpe, 2003). MDIs are also used by people who suffer from other respiratory ailments such as emphysema, chronic lung disease, and bronchitis.

There are five parts to an MDI: the medication, the propellant, the canister, the metering valve, and the mouthpiece. In addition, surfactants are

Metering Valve

Actuator

Metering Valve

Actuator

Spray Orifice

Figure 9.4 Design of primary functional parts of a classic metered-dose inhaler. Within the canister is the drug formulation, which is typically comprised of submicron-sized drug particles;the formulation is suspended in the propellant and stabilized by a surfactant.

Spray Orifice

Plastic Mouthpiece

Figure 9.4 Design of primary functional parts of a classic metered-dose inhaler. Within the canister is the drug formulation, which is typically comprised of submicron-sized drug particles;the formulation is suspended in the propellant and stabilized by a surfactant.

used to aid dispersion or dissolution of partially soluble drug and to lubricate the metering valve mechanism. The propellant can be either a chlorofluo-rocarbon (CFC) or hydrofluoroalkane (HFA). Figure 9.4 shows the design of the primary functional parts of a classic MDI.

Each time a patient uses an MDI, a precisely measured, or ''metered,'' amount of medicine is delivered. Inhalers can be used by all asthma patients aged 5 and older, according to the American Medical Association. Through inhalation, it takes only 5—15 minutes for short-acting bronchodilators (quick-relief medicine) to have an effect, compared to oral asthma medicines, which can take 1—3 hours. With an inhaler, there are also fewer side-effects because the medicine goes directly to the lungs and not to other parts of the body. Even though MDIs are the most widely prescribed product for the treatment of asthma, bronchitis, allergies, and COPD, it is estimated that up to 70% of patients have difficulty in coordinating their inspiratory flow rate with pressing of the canister (Larsen et al., 1994; Plaza et al., 1998). The inspiratory flow rate can affect the dose emitted from an inhaler, amount inhaled, OPL deposition, and regional lung deposition (Smyth, 2003).

9.3.1.1 Valve-holding chamber or spacer

It is important to administer an MDI in an appropriate way to get the necessary amount of medication into diseased lungs, such as proper coordination between the actuation of the device and the patient's inhalation. To ease this coordination, tubes are attached to the inhalers that act as a reservoir or valve-holding chamber (VHC). These tubes are also called spacers (Figure 9.5a). They serve to hold the medication that is sprayed by

Holding ChamberAerochamber Spacer With Medication

Figure 9.5 (a) Diagram of an AeroChamber Plus® Z STAT® anti-static valve-holding chamber (spacer). (b) Schematic of the proper administration of MDI with the spacer. The spacer has valves that prevent medication loss during exhalation by allowing air to flow through the chamber only during inspiration. Courtesy of Monaghan Medical Corporation, NY, USA;reproduced with permission.

Figure 9.5 (a) Diagram of an AeroChamber Plus® Z STAT® anti-static valve-holding chamber (spacer). (b) Schematic of the proper administration of MDI with the spacer. The spacer has valves that prevent medication loss during exhalation by allowing air to flow through the chamber only during inspiration. Courtesy of Monaghan Medical Corporation, NY, USA;reproduced with permission.

the inhaler. This makes it easier to use the inhaler and helps patients to get more of the medication into the lungs instead of just into the mouth. With proper use, a spacer can make an inhaler 20% more effective in delivering medicine to the lungs. Furthermore, it decreases deposition of medication in the mouth and throat, and eliminates the need to coordinate activating the MDI with inhaling the medication. Spacers can be especially helpful to adults and children who find a regular inhaler hard to use. People who use corticosteroid inhalers should use a spacer to prevent getting the medicine in their mouth, which can cause an oral yeast infection. Therefore, physicians recommend the use of a spacer because it improves delivery of medication to the lungs and also reduces the risk ofmouth infections (yeast).

A reservoir is sometimes used for actuation of the MDI prior to initiating an inhalation. Also, spacers are used to start inhaling prior to actuating the MDI. These auxiliary devices help patients with coordination problems. When the reservoir and spacer are smaller they facilitate larger drug particle deposition in the components due to inertial impaction and gravitational sedimentation. The delays between actuation and inhalation reduce drug delivery efficiency. MDIs may also generate some carcinogenic compounds in low concentrations extracted from the valve system. Clinical improvements in lung function for a given dose of inhaler are markedly increased when a spacer device is used correctly in conjunction with an MDI. Studies showed that the deposition values for terbutaline and budesonide ranged between 8.2% and 16.7% without a spacer (Pauwels et al., 1997). The deposition of terbutaline was determined by both gamma scintigraphy and the charcoal block pharmacokinetic method, and it was shown that when the Nebuhaler spacer was used (inspiratory flow rate 15 L/min), values were 31.6% and 33.8% respectively, approximately three times those obtained when a spacer was not used (Newman et al., 1995). Therefore, it is suggested that the correct use of a spacer with an MDI may result in substantially greater lung deposition.

The comparative advantages and disadvantages of various commonly marketed spacers have not been firmly established with substantial evidence; varying opinions have been expressed. In vitro aerosol deposition from a beclomethasone dipropionate MDI containing HFA propellant compared with that of the MDI in combination with two commonly marketed valve-holding chambers, namely OptiChamber VHC (Cardinal Health, Dublin, OH) and AeroChamber-Plus VHC (Invacare Corp., Elyria, OH), did not demonstrate equivalent performance (Asmus et al., 2003). In order to investigate the Proventil MDI (Schering-Plough Corp., Kenilworth, NJ) aerosol particle size distributions leaving the MediSpacer (Allegiance Healthcare Corp., McGaw Park, IL) and AeroChamber-Plus, Foss et al. (1997) found that both devices released particles of similar size distributions.

Young children in the age range of 5 months to 2 years form a special treatment group, as factors such as cooperation, acceptance, and the use of face masks may determine the success or failure of the inhalation therapy. In addition, when using metal or plastic spacers differences in results can be interpreted as being due to spacer design or to the presence or absence of electrostatic charge. Several studies have shown that plastic spacers can become electrostatically charged, which decreases drug delivery (Barry and O'Callaghan, 1995; Wildhaber et al., 1996). With metal spacers there is less possibility of holding static charges. Electrostatic charge can be minimized by coating the plastic spacer with a household detergent (Wildhaber et al., 1997). In an open crossover study of budesonide pMDI (Pulmicort, AstraZeneca, Lund, Sweden) drug delivery in 25 wheezy infants aged 5-26 months, a metal spacer (Nebuchamber, AstraZeneca, Lund, Sweden), a detergent-coated (DC) and a non-detergent-coated (non-DC) plastic spacer (Babyhaler of Glaxo Wellcome, London, UK)

were tested at home for 7 days each (Janssens et al., 2000). The results showed that the DC spacer provided increased dose delivery compared to the metal and non-DC spacers as it reduced static charges in the aerosols. However, the electrostatic charge had no influence on the dose variability; rather, considerable within-subject dose variability was found for the metal spacer and it was speculated to be due to suboptimal fit of the face mask of the Nebuchamber to the infants. Another in vitro study of the electrostatic charge properties of aerosols leaving new and detergent-coated AeroChamber-Plus spacers showed that the former type releases much higher charge per mass (charge-to-mass ratio) drug particles than the latter (Kwok et al., 2006). However, in all cases it was clear that patient cooperation during administration of inhaled drugs leads to more effective medicinal aerosol treatment. Therefore, it is very helpful for clinicians to prescribe a spacer that will physically fit and be comfortable on the face of the patient.

9.3.1.2 Chlorofluorocarbon to hydrofluoroalkane propellants

A liquefied or compressed gas is used in the MDI as propellant. Most pulmonary drugs are of low propellant solubility and they are frequently formulated as micronized suspensions. Chlorofluorocarbons (CFCs) have been the propellant of choice because of their low pulmonary toxicity, high chemical stability, purity, nonflammability, and compatibility with other packaging materials. After the 1985 discovery of a hole in the ozone layer above Antarctica, governments around the world combined to address the growing problem of ozone depletion. The result was the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer. Although the Montreal Protocol has been amended and updated over the years, the overall goal still remains the same: to protect the ozone layer by controlling the emission of harmful substances, such as CFCs (US FDA, 2006a). The ban on CFCs caused concern for many people with asthma and lung disease, since CFCs have long been an important component of MDIs. To comply with the Montreal Protocol, the US Food and Drug Administration (FDA) began a reformulation effort to find and approve non-CFC medical products. In March 2005, the FDA announced that it would ban the production and sale of all CFC albuterol MDIs by 31 December 2008 (US FDA, 2006b). Ironically, the most significant recent changes in MDI technology have not been the drug molecule characteristics, formulation chemistry or patient compliance factors, but the drive to change propellants triggered by environmental concerns.

The CFC propellants in MDIs are being replaced by hydrofluoroalkanes (HFAs). Two approved propellants of this group are HFA-134a and

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