the present invention relates to the treatment of lung diseases and prevention of epithelial cancer of the respiratory tract in an at - risk individual by administering by inhalation to the respiratory tract of the individual an aerosol comprising a therapeutically effective amount of at least one retinoid . the method provides delivery of the pharmaceutically active retinoid directly to the affected areas , thus increasing bioavailability and decreasing systemic toxicity . it will be recognized by persons skilled in the art that “ prevention ” of cancer is difficult to prove in the absolute sense because one cannot predict with certitude what will transpire in the future . thus , as used in the specification and claims of this application , the terms “ prevention ” or “ preventing ” refer to a reduction of the risk of contracting epithelial cancer , or to a delay in the onset of epithelial cancer . inhalation of pharmaceutically - active compositions is not a new concept , and various compounds used in asthma therapy and the like have been administered in this manner . the major advantage is site - specific drug delivery which often leads to reduced systemic drug levels . the avoidance of elevated serum and liver drug levels should enable long term therapy for cancer chemoprevention in high risk individuals , as well as persons with fibrotic lung disease . commonly , however , aerosols are formed from the active compounds solubilized in water . however , most retinoids of clinical interest , including all of the “ natural retinoids ” such as all - trans retinoic acid , 13 - cis retinoic acid and 9 - cis retinoic acid , are highly lipophilic and thus very insoluble in water . for this reason , conventional water - based formulations cannot be used for aerosol administration of these compounds . to make it possible to perform inhalation therapy using retinoids , it was therefore necessary to define a solvent system which ( 1 ) solubilized sufficient amounts of the retinoids to provide a pharmaceutically - useful dosage , i . e ., from about 0 . 1 to 5 . 0 mg / ml ; ( 2 ) provided a retinoid solution of sufficient stability to permit distribution of a product ; and ( 3 ) was substantially non - toxic in the amounts administered and thus suitable for administration to living patients . working towards this goal , we first looked at organic solvents . in the course of this investigation , we found that retinoids were only slightly soluble in ethanol or ethyl acetate . methylene chloride or chloroform provided adequate solubilization , but the potential toxicity of these materials argued against their use as carriers in an aerosol for use in inhalation therapy . next , because of a report that the solubility of retinol ( vitamin a ) in water could be increased by addition of modified beta cyclodextrin ( molecusol ®), we next tried to prepare aqueous solutions of all - trans retinoic acid using molecusol ® to enhance the solubility . solutions containing 45 % molecusol ® did in fact enhance the solubility of the all - trans retinoic acid to a useful level , but the resulting solution had a thick , syrupy consistency which was unsuited for use in the generation of an aerosol . similarly , efforts to solubilize all - trans retinoic acid in aqueous solution using phosphatidylcholine and phosphatidylethanolamine produced a viscous colloid which was unsuited for aerosol administration . we next tried to use salts of the retinoids to obtain a water - soluble product for aerosol generation . when all - trans retinoic acid is treated with ammonium hydroxide , a water - soluble ammonium salt is obtained . the ph of solutions of this salt is too high ( ph & gt ; 10 ), however , for direct administration as an aerosol . neutralization of the solution after dissolution of the retinoid led to the formation of a precipitate , both in the presence and absence of added beta cyclodextrin . thus , this approach also failed to produce an acceptable solution for use in generating an aerosol . because of the solubility of all - trans retinoic acid in halogenated hydrocarbon solvents , we next considered the solubility of retinoids in various chlorofluorocarbon propellants which have been used to deliver aerosolized solutions of other pharmaceutically - active compounds . all - trans retinoic acid was found to be only slightly soluble ( about 0 . 1 mg / ml ) in 1 , 1 , 2 - trichlorofluoroethane and only slightly more soluble ( 2 mg / ml ) in 2 , 2 - dichloro - 1 , 1 , 1 - trifluoroethane ( hcfc - 123 ). thus , as an initial matter , it did not appear that these solvents would be useful for producing solutions of retinoids for inhalation use . surprisingly , however , we found that the solubility of retinoids in chlorofluoro - carbon solvents could be significantly increased by the addition of alkylamines , particularly secondary , tertiary and quaternary alkylamines having alkyl groups containing from 2 to 8 carbon atoms such as trioctylamine , spermine , triethylamine or tetramethylammonium bromide , and that the resulting solutions were stable for periods of 5 days or longer , and could be solubilized by shaking . thus , one aspect of the present invention is a solution comprising a retinoid , a chlorofluorocarbon solvent , for example hcfc - 123 , hcfc - 134a or hcfc - 227 , and an alkylamine which is effective to solubilize the retinoid in the chlorofluorocarbon solvent . the solution preferably contains from 0 . 1 to 10 mg of the retinoid and 0 . 1 to 5 mg of the alkylamine , more preferably 1 to 2 mg of the retinoid and 0 . 1 to 0 . 5 mg of the alkylamine , per ml of solution . retinoids useful in the present invention include the “ natural retinoids ” as well as pharmaceutically acceptable salts and esters thereof . retinoids of particular interest in the present invention are all - trans retinoic acid , 13 - cis retinoic acid , 9 - cis retinoic acid , and salts and esters thereof . the alkylamine is suitably a charged or uncharged secondary , tertiary or quaternary amine , having alkyl groups of 2 to 8 carbon atoms . specific examples of suitable alkylamines include trioctylaimie , triethylamine , spermine and tetrabutylanmonium bromide . this solution is packaged in an inhaler effective to provide a metered dosage of from 50 to 500 μg , preferably about 100 μg , of retinoid per inhalation as shown generally in fig2 . such an inhaler is a combination of a container 1 and a dispenser assembly 2 . the dispenser assembly 2 is an open tubular construction which has an actuator portion 3 for receiving the container 1 , an oral tube 4 through which the retinoid is dispensed , and an actuator seat 5 which interacts with a metering valve 6 of the container 1 . when the container 1 is pressed downwards within the actuator portion 3 , the actuator seat 5 opens the metering valve 6 , releasing a dose of retinoid 7 from the container 1 , through an orifice 8 in the actuator seat 5 and out through the oral tube 4 . a suitable inhaler is a nasacort ® metered dose container . additional propellant material , for example butane , may be included within the inhaler . the inhaler is used to administer retinoids directly to the lungs of a patient at risk of epithelial cancer of the respiratory tract . patients in this category can be identified by behavioral characteristics . for example , individuals who are heavy smokers can be categorized as being at high - risk . alternatively , a more quantitative approach may be used . thus , the capacity to metabolize a small test dose of all - trans retinoic acid can be used as an indicator of risk , as described in u . s . patent application no . 07 / 885 , 130 filed may 18 , 1992 , which is incorporated herein by reference . the inhaler provides dosages of from 50 to 500 μg of retinoid per inhalation and is suitably used 1 to 5 times per treatment , with the treatment being repeated 1 to 3 times per day . a further format which can be used in accordance with the invention to administer an air - borne composition comprising a retinoid to an individual involves the use of a dry powder carrier . suitable carriers include those which are known to be useful in dry powder inhaler compositions especially the mono - saccharides such as fructose , manintol , arabinose , xylitol and dextrose ( glucose ) and their monohydrates , disaccharides such as lactose maltose or sucrose and polysaccharides such as starches , dextrins or dextrans . retinoids can be formulated into a dry powder with these carrier materials by coating the retinoid onto the surface of the carrier in a micronizer as described generally in u . s . pat . no . 5 , 376 , 386 which is incorporated herein by reference . dry powders containing retinoids are dispensed using known dry powder inhalers in amounts effective to provide dosages comparable to the solubilized formulations discussed above . the solubility of all - trans retinoic acid in various solvents was tested as shown in table 1 . maximum solubility was determined by weighing the residue remaining after evaporation of the solvent . to test the ability of alkylamines to enhance the solubility of all - trans retinoic acid in chlorofluorocarbons , solutions of the retinoids in various solvents with various amounts of amine stabilizers were prepared as shown in table 2 . to evaluate the bioavailability of retinoids delivered by inhalation , 100 μl of an aqueous solution containing 100 μg of all - trans retinoic acid was administered via intra - tracheal instillation to 3 sprague - dawley rats , each weighing approximately 200 g . one rat was then sacrificed at three time points : 1 , 6 and 24 hours after treatment . at the time of sacrifice , the chest cavity was opened , both lungs and trachea were removed en bloc , and the right and left lungs were separated by dissection . the liver was also removed , along with a sample of blood obtained by cardiac puncture . the blood sample was separated by centrifugation , and plasma plus each of the aforementioned tissues were frozen for later analysis . subsequently , the tissue was homogenized , and the homogenized tissue and plasma were extracted with ethanol . the retinoid content was assayed by hplc , and the results shown in table 3 were obtained . the biodistribution and pharmacokinetics of an inhaled formulation of all - trans retinoic acid were compared to both intra - tracheal and intravenous administration . anesthetized ( i . p . pentobarbitol 50 mg / kg ) male sprague - dawley rats weighing 300 to 700 grams were treated by administration of all - trans retinoic acid by inhalation , iv injection or intra - tracheal injection . inhaled and intra - tracheal injections were given through an endotracheal tube placed under direct vision over a guidewire as described by weksler et al ., j . appl . physiol . 76 : 1823 - 1825 ( 1994 ). iv doses were given by catheter injection via the right external jugular vein . for inhalation , all - trans retinoic acid was solublized in hydrofluorocarbon 123 using tetramethylamunonium hydroxide to a concentration of 5 mg / ml , and combined with hydrofluoroalkane 134a as propellant , for a final concentration of 1 . 5 mg per 1 . 475 grams of propellant solution . 21 grams of this solutions was packaged in a multidose inhaler that delivered 77 . 5 mg of product per dose , which resulted in 80 to 120 mg of all - trans retinoic acid per dose . an identical inhaler containing propellant mixture without drug was used as a control . when sprayed through the endotracheal tube , the test canister supplied 49 μg / dose . however , not all of this dose was absorbed by the rats due to escape through the nasal passages and back through the trachea . the actual amount of drug delivered to the lungs ( defined as the “ effective dose ” ) was calculated by delivering 10 doses through the aerosol delivery apparatus into a flask that contained 100 ml of isopropanol . this procedure was also repeated with an angiocatheter fitted to the apparatus . the catheter was subsequently rinsed with 10 ml of isopropanol , which was also collected . the three solutions were then assayed for all - trans retinoic acid concentrations in order to determine the amount of drug delivered through the apparatus , as well as any residual drug that was bound to the angiocatheter . we defined the area under the concentration times - time curve ( auc ) for animals who received intra - tracheal all - trans retinoic acid as 100 % effective dose delivery to the lung . the auc of the inhaled aerosolized drug was divided by the intra - tracheal auc in order to determine the percent effective dose . total absorption amounted to 1 - 2 % of the intended 250 μg dose . for injection , an aqueous solution of all - trans retinoic acid was prepared in 20 % ethanol , 10 % tween 20 and 1 mm nh 4 oh , for a final concentration of 1 ml all - trans retinoic acid per ml of solution . a mixture of 20 % ethanol and 10 % tween 20 served as a control . animals were grouped into three experimental groups and two control groups . the control groups received no treatment or a control inhalant containing no all - trans retinoic acid . animals in the groups receiving iv or intra - tracheal injections each received a single dose containing 250 μg all - trans retinoic acid . animals in the group treated with the multidose inhaler each received three doses for a total of 147 μg of all - trans retinoic acid . animals were sacrificed at 5 minutes and at 1 , 2 , 4 and 6 hours , and blood and liver and lung tissue were harvested for analysis by high performance liquid chromatography ( hplc ) and histology . in addition , animals receiving the inhalant were sacrificed at 24 hours post - treatment . [ 0034 ] fig3 a and 3b show the levels of all - trans retinoic acid found in the plasma and livers of animals in each of the three treatment groups . as shown , intra - tracheal injection and aerosol inhalation result in much lower levels of the compound in the liver ( fig3 b ). on the other hand , as shown in fig4 all three treatments resulted in prolonged levels of all - trans retinoic acid in the lungs where it is desired for therapeutic efficacy . clearance data from plasma and lung and liver tissue is summarized in table 4 . in animals who received the metered dose inhaler , lung concentrations of all - trans retinoic acid were highest at 5 minutes ( 99 ± 44 ng / gm of tissue ), followed by a slow decrease over the next 24 hours ( fig4 ). the calculated tissue half - life of the aerosolized drug in lung was 5 . 4 hours . animals who received intra - tracheal injections showed peak lung levels at 1 hour ( 5 , 767 ± 419 ng / gm ), which were maintained for approximately 6 hours . the lung tissue half - life ( t 1 / 2 ) was 17 . 7 hours by the intra - tracheal route . peak lung levels by the iv route were seen at 5 minutes ( 6 , 440 ± 1 , 865 ng / gm ) and disappeared with a half - life of 1 . 9 hours . the lung levels by the inhaled route were significantly lower than the intra - tracheal route at each time point ( p & lt ; 0 . 02 ); however , the difference from the iv route was only significant before the 2 hour post dosing time point . the lung auc , an indication of total drug delivery to an organ over time , for the intra - tracheal route was twice that of the iv route and 100 times that of the aerosolized route ( table 4 ). animals receiving aerosolized drug displayed peak plasma levels of 71 ± 31 ng / ml 1 hour after dosing , which was not significantly different from intra - tracheal administration at 1 hour ( 68 ± 44 ng / ml ) ( fig3 a ). by contrast , animals in the iv group displayed a characteristic plasma curve with a peak at 5 minutes ( 838 ± 56 ng / ml ) and a rapid decay ( t 1 / 2 = 0 . 4 hours ). differences between the iv and the other groups were significant up to 4 hours ( p & lt ; 0 . 05 ). animals in the inhaled drug group had peak liver levels of 112 ± 28 ng / gm 2 hours after dosing , whereas intra - tracheal injection produced peak liver levels of 753 ± 350 ng / gm at 5 minutes ( fig3 b ). both groups showed a slow decrease over 24 hours . in contrast , iv administration resulted in peak levels in liver at 5 minutes ( 4 , 258 ± 1 , 006 ng / gm ), which had rapidly decreased by 4 hours ( t 1 / 2 = 1 hour ). compared with iv injection , aerosolization and intra - tracheal injection of all - trans retinoic acid resulted in a significantly longer pulmonary half - life of the drug and lower peak serum concentrations than the same dose administered iv . as shown , the aerosol inhalation provided sustained levels of all - trans retinoic acid in the lungs with lower levels in the plasma compared to iv injections , thus offering the ability to use all - trans retinoic acid as a therapeutic agent with a reduction in systemic toxicity . histologic examination of lungs and traches showed no focal irritation attributable to the drug after single - dose administration . these results suggest that aerosolization of retinoids may offer a practical alternative to systemic oral administration for chemoprevention trials or treatment of lung diseases . the advantages of inhaled drug delivery are well - characterized . site - specific drug delivery is readily accomplished , and the portability of multi - dose inhalers , their ease of use , and reduced adverse effects should improve patient compliance . moreover , the avoidance of elevated serum and liver drug levels should further enable long term therapy for cancer chemoprevention in high risk ( by asymptomatic ) persons , as well as individuals with fibrotic lung disease . although the preferred embodiment of the method and products of the invention has been described above in some detail , it should be appreciated that a variety of embodiments will be readily apparent to one skilled in the art . the description of the method and products of this invention is not intended to be limiting to this invention , but is merely illustrative of the preferred embodiment .
administration of retinoids by inhalation is used to overcome the chronic toxicity problems presented by systemic administration and to make retinoid therapy available as an option for the treatment of fibrotic lung disease , emphysema , and the prevention and treatment of epithelial cancers of the respiratory tract , especially those that are associated with tobacco use . retinoids are administered by inhalation to the respiratory tract of the individual as an air - borne composition with a metered dose aerosol - producing inhaler , in which the retinoid is dissolved in a combination of a pharmaceutically acceptable chlorofluorocarbon propellant and an alkylamine solubilizing agent .