This invention relates to a method of measuring the mass of an aerosol for¬ mulation delivered from a container after activation of a metered dose valve.

BACKGROUND OF THE INVENTION

Drugs for treating respiratory and nasal disorders are frequently administered in aerosol formulations through the mouth or nose. One widely used method for dispensing such aerosol drug formulations involves making a suspension formula¬ tion of the drug as a finely divided powder in a liquefied gas known as a propellant. The suspension is stored in a sealed container capable of withstanding the pres- sure required to maintain the propellant as a liquid. The suspension is dispensed by activation of a dose metering valve affixed to the container.

A metering valve may be designed to consistently release a fixed, predeter¬ mined mass of the drug formulation upon each activation. As the suspension is forced from the container through the dose metering valve by the high vapor pres- sure of the propellant, the propellant rapidly vaporizes, i.e., boils, leaving a fast moving cloud of very fine particles of the drug formulation. This cloud is usually di¬ rected into the nose or mouth of the patient by a channeling device, e.g., a cylinder like or cone-like passage, with one of its ends attached to the outlet of the pressur¬ ized container, and the other end inserted in the mouth or nose of the patient. Concurrently with the activation of the aerosol dose metering valve, the patient in¬ hales the drug formulation particles into the lungs or nasal cavity. Systems for dispensing drugs in this way are known as "metered dose inhalers" (MDI's). See Peter Byron, Respiratory Drug Delivery, CRC Press, Boca Raton, FL 1990 for a general background on this form of therapy.

Patients often rely on medication delivered by MDI's for rapid treatment of respiratory disorders which are debilitating and, in some cases, even life threaten¬ ing. Therefore, it is essential that the prescribed dose of aerosol medication deliv¬ ered to the patient consistently meet the specifications claimed by the manufac¬ turer and comply with the requirements set forth by drug regulatory authorities such as the FDA. Thus, testing of MDI units for proper drug delivery by the meter¬ ing dose valve is a part of the manufacturer's quality assurance procedure.

One conventional method to test delivery of MDI's consists of taring each MDI and measuring the weight lost after the delivery of one dose. This method is

accurate and adequate for testing a small number of samples. However, it is poorly suited for high speed production and packaging of MDI's.

A common method of testing drug delivery utilizes indirect pressure decay after activation of each MDI. This method does not render a direct mass meas- urement, but rather an approximation based upon the force exerted indirectly by the superheated vapor on a pressure transducer. As a consequence, this method is unable to detect MDI's which are marginally out of tolerance. Further, unac¬ ceptable inaccuracies result unless the production rate is less than about one and a half MDI's per second which is significantly below the optimal production rate, because at higher rates internal resonances tend to be induced. Therefore, multiple testing stations are used in the MDI production line leading to an increased maintenance burden and validation complications.

Thus, the manufacturer of MDI's is faced with a choice of i) testing each MDI and reducing the production speed to less than 1 and a half per second or ii) in- stalling multiple testing machines in the production line. This dilemma has lead to a search for a method of accurately measuring the delivery of each MDI without sacrifice of production speed.

SUMMARY OF THE INVENTION

The present invention comprises a method for measuring the amount, i.e., mass, of a volatile liquid which comprises passing the mass over a heat loss measuring device, calculating the amount of heat loss accompanying the vapori¬ zation of the mass and correlating the heat loss to measurement of heat loss of known liquid aerosol mass made with the same device.

A particularly useful application of this method is the measurement of the mass of an aerosol propellant, delivered upon activation of a metering valve at¬ tached to a vessel containing an aerosol product. This method is well suited as a quality assurance procedure in the production of metered dose inhalers (MDI's) because it can be used to measure delivery from a metering valve for each MDI with acceptable accuracy and precision without slowing production of the MDI's because the heat loss measurement can be taken very quickly. The second aspect of the present invention is a method of quality assurance in the manufactur¬ ing of metered dose inhalers using the present method of measuring the mass of a liquid aerosol formulation.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1a depicts a perspective view of a testing apparatus for carrying out the method of the invention.

Figure 1b is an elevational cross section of the apparatus with a schematic diagram of the electrical connection for the generated signal.

Figures 2a, 2b and 2c are oscilloscope wave traces from testing mass ac¬ cording to the invention where the masses are within tolerable limits, too low and too high, respectively.

DETAILED DESCRIPTION OF THE INVENTION

The transition from liquid state to gaseous state is achieved through the isothermal process known as "evaporation", or "boiling" if the process is rapid. An isothermal process is one that occurs at a constant temperature. As heat is applied to a liquid the temperature will rise proportionately until it reaches the boiling point. Once at the boiling point, as heat is applied the liquid changes to the gaseous state but the temperature remains constant. Thus, the heat energy being applied is the energy required to transform the liquid into a gas or vapor and is known as the "latent heat of vaporization." In effect, for a liquid to change to a gas at its boiling point, it must extract heat from its environment. A discussion of the phenomenon of latent heat of vaporization can be found in standard text books on physical chemistry, e.g. W. Moore, Physical Chemistry, 3rd Ed., Chap. 4, Prentice-Hall (1963).

When a liquid is released from a sealed container, the liquid extracts heat from its environment, e.g., the surrounding air, to evaporate. The extraction of heat from the liquid's surrounding results in a lowering of the temperature of those surroundings. When a volatile liquid, i.e., a liquid with a boiling point below or slightly above ambient temperature, is released into the air as a fine spray at am¬ bient temperature, the evaporation is very rapid and the consequential cooling of the air is observable even to the touch.

The propellants used in aerosol dispensers are highly volatile liquids with boiling points well below ambient temperature. For example, at atmospheric pres- sure, dichlorodifluoromethane, also known as "propellant 12" and "P12," has a boiling point of -29.8°C and 1,1,1,2-tetrafluoroethane, also known as "propellant

134a" and "P134a," has a boiling point of -26.5°C. When the valve of a propelled

aerosol dispenser is activated, the released propellant very rapidly evaporates to produce the aerosol, i.e., a fine suspension of the aerosol formulation in air and vaporized propellant. The rapid evaporation of the propellant extracts heat from the surrounding air producing a significant cooling effect. The specific heat of the ambient air is relatively low compared to the latent heat of vaporization of the propellant and so a large drop in the temperature of the air occurs even if only a small amount of propellant is present

In the method of the present invention, a mass of a volatile liquid to be meas¬ ured is discharged into a constant temperature and constant flow air stream and onto the probe of a heat loss measuring device placed downstream from the point of discharge. The cooling effect of the vaporization of the liquid is sensed as a transient loss of heat from the area surrounding the probe and recorded as a tran¬ sient reduction of temperature of the air stream. Two or more different, known masses of the same liquid are discharged under identical conditions and the cor- responding reduction in temperature recorded. The mass of the sample is deter¬ mined by correlating the reduction in temperature resulting from its discharge with reduction in temperature caused by discharge of the samples of known mass. Thus, the mass of the discharged volatile liquid is calculated as a function of the latent heat of vaporization of the liquid.

If the volatile liquid is a propellant being used to disperse a drug formulation from an MDI, the amount of formulation discharged with each valve activation is di¬ rectly and consistently proportional to the mass of the propellant discharged. That is, knowledge of the mass of propellant discharged from an aerosol dispenser, an MDI, can be directly interpreted as knowledge of the amount of drug formulation dispensed. This knowledge is particularly important for quality assurance proce¬ dures in the manufacturing of MDI's and is discussed in detail hereinbeiow.

It is preferable in the present method of measuring mass that temperature and flow rate of the air stream be kept constant from measurement to measure¬ ment. Therefore, it is convenient to practice the present method within a partially enclosed chamber, i.e., a test chamber, wherein temperature and flow rate can be controlled and isolated from extraneous air currents, e.g., by means of enclosure within a tube having uniform cross section.

In Figures 1a and 1b an apparatus 100 is provided to measure the mass of an aerosol. A constant air flow 1 is maintained through the test chamber 2. The dimensions of the test chamber are not critical, but must be sufficient to allow a

constant air flow through the chamber while preventing extraneous air flow from the surroundings of the chamber. The constant air flow may be generated by an air moving means such as a motorized fan 4 placed i) in close proximity to the exit of the test chamber as depicted in Figure 1a and 1b, ii) close proximity to the en- trance of the test chamber or iii) remotely located and connected by a duct to either the entrance or exit.

With a constant flow of air through the test chamber a baseline condition is established across sensing portion 6 of the probe of a heat loss measuring device 8, e.g., a hot film or hot wire anemometer, and is set as the reference condition, e.g., by a signal conditioner (SC) 10. The SC 10 converts minute changes in the current required to maintain a predetermined temperature at the probe sensing portion 6 to an analog voltage (0-5 VDC). A decrease in temperature at the probe sensing portion 6 results in an increase in current required to maintain the wire or metallic film at the predetermined temperature. The SC 10 also acts as a filter by averaging input signal to essentially eliminate electrical "noise" and provide a "clean," linear signal.

A vessel 12 equipped with a metering dose valve 14 and containing a liquid aerosol propellant 16 is mounted in the test chamber 2 in an orientation whereby i) the discharge of propellant upon activation of the metering valve 14 is in the direc- tion of the air flow 1 and ii) the discharge 18 directly impinges on the sensing por¬ tion 6 of the probe of a heat loss measuring device 8 as shown in Figure 1b. The discharge tube 20 which extends from the metering dose valve 14, is positioned by the vessel holder 22 in a fixed position against the opening in support ring 24 of the vessel holder 22. The metering valve 14 may be activated by placing pressure against the end of the vessel 12 opposite from the metering valve 14. The pres¬ sure forces the discharge tube 20 into the vessel 12 activating the metering dose valve 14. Such pressure may be applied manually or mechanically, e.g. by r^ans of a plunger 26 connected to an electrical, pneumatic or other means to mova the plunger against the vessel 12. After the valve 14 is activated, the air with propellant can be passed through a water bath filter, for example, to remove powder from the gas.

The signal generated by the probe of the heat loss measuring device 8 is electronically and mathematically processed and compared to the cooling resulting from the discharges of the same liquid aerosol propellant under the same condi- tions by the waveform analyzer 28. The processed signal is transmitted to a printer or recorder 30 where it is graphically recorded, and simultaneously trans-

mitted to an oscilloscope 32 for rapid visualization. The processed signal may also be further processed electronically and mathematically, e.g., by a program¬ mable logic controller, 34, to actuate devices which can reject from the production run those aerosol vessels with valves delivering drug formulation above or below the tolerated range.

The heat loss measuring device suitable for use in this invention should pref¬ erably: i) respond within 10 milliseconds to a temperature change, ii) be sensitive enough to sense changes of less than about .01 °C, iii) have an operational range from about 10°C to about 40°C with recovery to a baseline setting within 50 milli- seconds and iv) be capable of producing an electrical signal which can be ampli¬ fied, processed, recorded and viewed with conventional electronic components available to control and monitor manufacturing processes. The speed of such a heat loss measuring device means that MDI's can be tested without a reduction in the optimal production rate of between 2 and 3 MDI's per second.

The temperature sensing probe and related signal processing equipment used in the art of hot wire and hot film anemometry fulfills these requirements and is well suited to the present invention. Anemometry employs a probe tipped by a heated, fine wire which is connected to a bridge circuit. In turn, the bridge circuit is connected to an electronic amplifier which is connected to a device for visualizing the change of electrical signal, such as a recorder or oscilloscope. A change in temperature of the hot wire alters its electrical resistance which unbalances the bridge circuit to generate an electrical signal proportional to the change of tem¬ perature of the wire. The electrical signal may then be amplified, electronically and mathematically processed and presented in a useful, graphic form. In some applications a very small, heated, insulated cylinder bearing a thin metallic film is used in lieu of the hot wire. For the method of this invention either a hot wire or hot film probe is acceptable and any reference hereinafter to either should be taken as including both.

In the art of anemometry, the hot wire probe and the related circuitry is used to measure the flow of air or gases by measuring the cooling of the hot wire when it is placed in the flow. That is, the cooling effect is correlated to the rate of flow. Thus, the hot wire anemometry system is essentially used as a sophisticated, electronic thermometer. The theory and practice of hot wire/hot film anemometry is taught in Experimental Methods for Engineers, J. Holman and W. Gajda, Jr., § 7-7, McGraw-Hill, New York (1978). Further information, including specifications anemometry equipment offered for sale, is found in the brochure, "Hot Wire/Hot

Film Anemometry Probes & Accessories," (1988) by TSI Incorporated, St. Paul, of Mfv

Aerosol vessels containing drug formulations such as MDI's may be sequen¬ tially subjected to the method of this invention by conventional mechanical means known in the art. In particular, MDI's can be subjected to the method using conventional mechanical handling means as they come off the production line after filling at a rate of 2 or more per second. Further, those skilled in the art of manufacturing aerosol formulations will appreciate that some variation in the method taught above may be required to optimize the results.

The magnitude of temperature reduction and the duration of such reduction at the sensing part of the probe is directly proportional to the mass of the propel¬ lant discharged. Therefore, the electronic signal from the probe, processed by the signal conditioner and waveform analyzer and graphically displayed by the printer or recorder (or displayed on the oscilloscope) is also directly proportional to the mass of the propellant discharged.

Figures 2a, 2b and 2c depict graphs of the level of the electronic signal (voltage 201) in relation to duration (time 202) as would be displayed by the printer or recorder on the oscilloscope screen. The area under the curve in each of the three graphs is directly proportional to the mass of the propellant discharged and, hence, the transient temperature reduction at the probe. Figure 2a shows the pattern produced by discharge of propellant through a normal valve. Figure 2b is a corresponding graph produced by a valve which discharges an unacceptably low amount of propellant while Figure 2c is produced by a valve which does not close properly after activation and, thus, discharges an excessive amount of propellant.

Two or more known masses of propellant can be sequentially discharged into a constant air flow onto the probe in the test chamber to establish a series of cor¬ responding standardized graphs. An unknown mass of propellant may then be discharged and the area under the curve in the resulting graph is compared and correlated to the area under the curves produced by the discharges of the stan- dard masses.

The present method of measuring mass of aerosol propellant may be adapted as a method of quality assurance in the manufacture of MDI's which com¬ prise the following steps:

a) determining the mass of the output of an MDI by the method described above,

b) rejecting those MDI's which have output below or above a predetermined range of tolerance.

in particular when the above method of quality assurance is adapted for con¬ tinuous production run the following detailed steps would be appropriate:

a. Placing each MDI sample from a production run into a test cham¬ ber as depicted in Figures 1a and 1b.,

b. Activating the metering valve of the sample MDI from a position fixed in relation to the sensing portion of a hot wire or hot film ane¬ mometer probe as depicted in Figures 1a. and 1b, _.

c. Recording the temperature reduction and the duration of the re¬ duction,

d. Comparing the temperature reduction recorded in step c. with that produced under the same conditions by an essentially identical

MDI containing the same formulation, but having a known mass of output upon activation, i.e., a "standardized MDI".

e. Removing the sample MDI from the test chamber and,

1. Placing Ft back on the production line if the output mass is within the predetermined tolerances in relation to the output mass of the standardized MDI, or

2. Discarding the sample MDI if the output mass is above or below the predetermined tolerances in relation to the output mass of the standardized MDI.

The method of this invention may be adapted to test a wide variety of metered valve aerosol dispensers which use liquid propellants. It is particularly useful in the quality assurance of MDI's employed to deliver respiratory drugs. For exam¬ ple, this method may be used for quality assurance in the production of MDI's containing the respiratory drugs albuterol (sulbutamol), saimeterol, amiloride,

fluticasone propionate and beclomethasone dipropionate.

Suitable liquids which can be measured by the present invention as aerosols include, but are not limited to, trichloromonofluoromethane, dichlorodifluoro¬ methane, 1,1,1,2-tetrafluoroethane, heptafluoropropane, tetrachlorofluoroethane, butane, isobutane and propane.

EXAMPLE

The following example illustrates this invention but should not be construed as a limitation thereof. The symbols and conventions used in these examples are consistent with those used in contemporary engineering literature.

A standardized MDI containing a suspension of albuterol in P12 is placed into an apparatus essentially as depicted in Figures 1a and 1b with the tip of the dis¬ charge tube about 30 mm from the sensing portion of a TSI model 1210-T1.5 hot wire anemometer probe supplied by TSI Incorporated. A constant air flow of about 20 cfm to about 30 cfm is maintained through the test chamber by a tubeaxial fan supplied by Radio Shack.

The signal from the anemometer is transmitted by coaxial, shielded cable (CSC) to a model 157 internally installed signal conditioner supplied by TSI, Inc. The conditioned signal is then transmitted by CSC to a model 170 Gould waveform analyzer supplied by General Electric Instruments, inc. The waveform analyzer transmits a signal by CSC to a chart recorder or a high speed printer and simulta¬ neously by CSC to a model 4073 Gould digital storage oscilloscope supplied by General Electric Instruments, Inc. The signal from the waveform analyzer is also transmitted to an Allen-Bradley, Family 5, programmable logic controller. With the constant air flow through the test chamber the signal from the anemometer is set as the baseline condition.

A standard MDI, the output of which has been previously measured by taring and measuring the weight lost after a discharge, is activated by a downward thrust of the plunger. The resulting temperature drop sensed by the anemometer is re¬ corded electronically in the signal conditioner as the standard. Preferably the out- puts of two or more standardized MDI's are recorded and electronically and mathe¬ matically averaged in the signal conditioner to set the standard.

Alternatively, the outputs of two or more MDI's (previously found to have dif-

ferent outputs) may be correlated with the corresponding drop in temperature upon discharge. Such correlation may conveniently be done electronically and mathe¬ matically in the signal conditioner to yield a standard curve relating mass of output to resulting temperature drop. The mass of the output of an MDI being tested can be determined by relating the temperature drop resulting from its discharge to the corresponding output mass on the standard curve.

The standardized MDI is removed and MDI's from the production run to be tested are sequentially placed in the test chamber and activated in the same way as was the standardized MDI. The signal generated by the anemometer as a re- suit of the temperature reduction about the probe upon discharge of each MDI is electronically and mathematically compared with the signal previously generated by one or more standardized MDI's. If the signal generated by discharge of a sample MDI is outside the preset tolerance, a signal is transmitted to the Allen- Bradley programmable logic controller which, in turn, activates a means to remove the out-of-tolerance MDI from the production run.

The output of an MDI being tested can be observed within 300 miliseconds on the oscilloscope. This rapid, visual indication of the outputs of the production run MDI's is useful to the production line operator in spotting defect trends and detect¬ ing substandard lot of MDI valves. A permanent record can be made with the re- corder or printer.