- Email: [email protected]
Drug solubilization in lung surfactant
Journal of Controlled Release 65 (2000) 43–47 www.elsevier.com / locate / jconrel
Drug solubilization in lung surfactant T.S. Wiedmann a , *, R. Bhatia a , L.W. Wattenberg b a b
Department of Pharmaceutics, University of Minnesota, 308 Harvard St. SE, Minneapolis, MN 55455, USA Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, MN 55455, USA Received 17 March 1999; accepted 22 September 1999
Abstract The relative affinity of glucocorticosteroids for lung surfactant was determined for the purpose of identifying chemopreventive agents with a high therapeutic index for lung cancer. The aqueous solubility and the extent of solubilization in SurvantaE, a native extract of bovine lung, of budesonide, triamcinolone acetonide, dexamethasone, and flunisolide were determined as a function of temperature by a dialysis technique. The aqueous solubilites at 378C were 19.6, 35.8, 104 and 120 mg / ml for the above listed compounds, respectively. The temperature dependence of the solubilities was modest consistent with the hydrophobic properties of the steroids. The amount of drug in solution was significantly enhanced in the presence of SurvantaE with solubilization ratios of 0.019, 0.023, 0.014, and 0.02 mg drug dissolved per mg of SurvantaE phospholipid, respectively. In addition, the extent of solubilization also generally increased with temperature, although the phase transition of the surfactant lipid appeared to complicate the functional relation between temperature and solubilization. These results show that there is enhanced solubilization of glucocortosteroids by lung surfactant which is secreted by the cancer susceptible type II cells of the lung. 2000 Elsevier Science B.V. All rights reserved. Keywords: Solubilization; Lung surfactant; Cancer chemoprevention; Glucocortosteroids
1. Introduction Virtually all drugs must be in a dissolved state for therapeutic activity. However, the vast majority of drugs are delivered as solids. Thus, there is tremendous value in understanding and predicting the solubility and solubilization of drugs. In this study, the solubilization of drugs was examined from a general perspective of investigating respiratory drug delivery and with a very specific interest in finding a chemopreventive agent for lung cancer. *Corresponding author. Tel.: 11-612-624-5457; fax: 11-612626-2125. E-mail address: [email protected] (T.S. Wiedmann)
Typically, drugs are delivered to the respiratory tract by metered dose or dry powder inhalers. These devices deliver solid aerosol particles to the respiratory tract for the purpose of maximizing the deposition of drug in the airways. To reach the site of action, the particles must be wetted, and the medium must allow dissolution to occur. Although the lung enjoys a relative humidity near 100%, the volume of solution at the lung lining is extremely small. The question arises how does dissolution take place in this environment of minimum fluid volume. Within the context of the broad topic of respiratory drug delivery, we also have a specific interest in chemoprevention. First, chemoprevention can be defined as the administration of a chemical entity
0168-3659 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0168-3659( 99 )00230-8
44
T.S. Wiedmann et al. / Journal of Controlled Release 65 (2000) 43 – 47
that interferes with the development of cancer. This approach has grown from its roots of largely anecdotal commentaries to reach a point of full justification with the approval of tamoxifen for the prevention of breast cancer. It should be appreciated that drugs given for the purpose of disease prevention must first ‘do no harm’. As such, candidates for chemoprevention must have therapeutic indices approaching infinity until such time as the incidence of cancer can be confidently predicted. In spite of the daunting requirement of the therapeutic index, it should be noted that preventing lung cancer holds special attributes. First, drugs may be directly delivered to the lung and need not pass through the general circulation. This has been shown to provide a therapeutic advantage for prevention [1]. A second aspect that forms the basis of this work is the question does drug association with lung surfactant enhance the therapeutic effectiveness. The rationale arises from the observation that a common form of lung cancer is adenocarcinoma. This occurs in the type II cells of the epithelial lining of the alveoli. Type II cells have a critical role in the production, secretion, and recycling of lung surfactant, the lipid-rich material that allows the formation of the large air / water interface required for gas exchange. Due to the recycling activity, it is hypothesized that drugs associated with lung surfactant will be preferentially delivered to type II cells and thereby have a higher therapeutic index. As the first step to investigate this hypothesis, the solubility and solubilization in lung surfactant of a number of potential chemopreventive steroids have been determined.
2. Materials and methods Budesonide, triamcinolone acetonide, dexamethasone, and flunisolide were purchased from Sigma Chemical Co. (St. Louis, MO) and used as received. SurvantaE, a bovine, native lung extract was obtained from Ross Laboratories. The solubility in normal saline of each compound was determined as a function of temperature by a dialysis method described by Li et al. [2]. Equilibration was complete in 24 h and concentration was determined by diluting an aliquot with ethanol and
measuring the UV absorbance at 244 nm against the appropriate standard curve. The extent of solubilization was determined in a similar manner with the drug powder placed in dialysis bags (MWCO 10 000 Da), and a range of concentrations of SurvantaE were placed in the external solution. The bags were tied with surgical thread and placed into PyrexE screw cap test tubes with TeflonE liners. The tubes were oscillated for 24 h, and the outer solution was assayed by measuring the absorbance. SurvantaE did not interfere with the absorbance measurements since it is solubilized by the addition of ethanol. The water solubility is express in mole fractions calculated with the assumption that the density is unity. The solubilization ratio of drug in SurvantaE, R, was determined from the slope of the observed total molar concentration of drug in solution plotted as a function of the molar SurvantaE lipid concentration, in which it was assumed that the phospholipid had an average molecular weight of 750 Da. The SurvantaE / aqueous distribution coefficient is expressed as the ratio of the mole fraction solubility in SurvantaE, Xs , and the aqueous solubility where the solubilization ratio is related to the SurvantaE solubilization mole fraction by R 5 1 /(1 1 Xs ). The thermodynamic parameters, the change in enthalpy, DH, and the change in entropy, DS, were calculated from the slope and intercept of a plot of the logarithm of the mole fraction solubility or solubilization as a function of the reciprocal of the absolute temperature.
3. Results and discussion The basis of this work was to investigate the general aspects of respiratory drug delivery as well as address the question, ‘does drug association with lung surfactant enhance the therapeutic effectiveness of chemopreventive agents?’ Research to date has shown that steroids, and in particular budesonide, are effective as chemopreventive agents [1]. However, to our knowledge, there are no published reports of the relative affinity of steroids for lung surfactant. The four compounds that were chosen for the focus of this study are also approved for the treatment of asthma. The advantage from a chemoprevention point of view is that they have been shown to be safe
T.S. Wiedmann et al. / Journal of Controlled Release 65 (2000) 43 – 47
Fig. 1. Solubility of steroids as a function of temperature for (d) budesonide, (♦) triamcinolone acetonide, (m) dexamethasone, and (.) flunisolide.
if not free of side effects. With these compounds, the aqueous solubility and the solubilization in Survanta were determined as a function of temperature. The observed solubilities of budesonide, triamcinolone acetonide, dexamethasone, and flunisolide at room temperature agree well with the available literature values [3]. The use of the dialysis method allowed for rapid equilibration and over came the inherent wetting problem of these hydrophobic powders. The rank order of solubility is, in general, consistent with expectations based on structural information and the melting behavior. The aqueous solubility for each compound is given as a function of temperature in Fig. 1. As can be seen in the figure, there is a non-linear increase in
45
solubility with temperature. The data were fit to the Arrhenius equation, and the enthalpy and entropy of solution were determined and are given in Table 1. The enthalpy changes were relatively large and endothermic reflective of the stable crystal structure. The entropy was also large, but negative. The negative entropy change indicates an increase in order. Since the entropic change of the drug should be positive because it is transferred from the solid to the solution, the increase in order is presumably a result of water structuring about the nonpolar steroids, characteristic of the hydrophobic effect. The addition of lung surfactant led to an increase in the amount of steroid solubilized in a linear fashion for budesonide, triamcinolone acetonide, and dexamethasone up to a concentration of 2500 mg lipid / ml. For flunisolide, a plot of the amount solubilized as a function of SurvantaE lipid was initially linear, but at higher concentrations, the slope became smaller. From the linear portions of the curve, the solubilization ratio was calculated for each steroid. The solubilization ratios ranged from 0.019 to 0.026 moles of steroid solubilized per mole of SurvantaE lipid at 378C. From the reciprocal of the value of the solubilization ratio, it can be deduced that about 40–50 lipid molecules are needed to solubilize one steroid molecule. The solubilization was carried out as a function of temperature from which the thermodynamic parameters were obtained. In analyzing the results, the solubilization ratios were first converted to mole fraction solubilities based on the SurvantaE lipid content. Thereafter, the ratio of the mole fraction SurvantaE solubilization and the mole fraction aqueous solubility was defined as the distribution coefficient. The logarithm of the distribution or partition coefficient is given as a function of the reciprocal of the absolute temperature in Fig. 2. Both triamcinolone and flunisolide evidently have
Table 1 Thermal transition temperature, mole fraction solubility, and enthalpy and entropy of solution Steroid Budesonide Triamcinolone acetonide Dexamethasone Flunisolide a
dec, decomposed.
T m (8C) 221–232 (dec) 292 268–271 245
a
Sol (378C)
DH (kJ / mol)
DS (e.u.)
8.18E–07 1.48E–06 4.77E–06 4.97E–06
17.3 20.1 11.7 23.9
260.0 247.3 264.3 225.3
46
T.S. Wiedmann et al. / Journal of Controlled Release 65 (2000) 43 – 47
Fig. 2. Survanta / aqueous distribution coefficient of steroids as a function of the reciprocal of the absolute temperature for (d) budesonide, (♦) triamcinolone acetonide, (m) dexamethasone, and (.) flunisolide.
distribution coefficients that increase with increasing temperature while for budesonide and dexamethasone the distribution appears largely independent of temperature. Given that all the compounds had relatively large enthalpies of solution, the solubilization increase with temperature was comparable or smaller than the increase in solubility with temperature. Although the phospholipid component of the lung surfactant has been emphasized, it is important to recognize that SurvantaE is a complex mixture consisting of an aqueous dispersion of lipids and proteins. SurvantaE is a lung surfactant replacement that is produced by enriching a calf lung extract with synthetic lipids. The product information lists the
composition as 11.0–15.5 mg / ml DPPC, 0.5–1.75 mg / ml triglycerides, 1.4–3.5 mg / ml free fatty acids, 0.1–1.0 mg / ml proteins B / C, and 7.65–10.35 mg / ml sodium chloride. The gel to liquid crystalline phase transition is expected to be broad and lie between 30 and 408C. Moreover, the phase state of the lipids is expected to influence the solubilization capacity. In general, the solubilization is greater for lipids in the liquid crystalline state as compared to the gel state [4]. Finally, at temperatures in the transition region, solubilization may be unique in comparison to both the gel and liquid crystalline state [4]. Examining the thermodynamic parameters, the change in molar free energy of transfer of steroid from saline to lung SurvantaE lipid was large and negative in each case (Table 2). The lipid environment is considerably more favorable relative to water for solubilizing the steroids. The entropy changes are also large but positive. The dramatic increase in disorder in transferring the steroids from water to the surfactant lipid is consistent with the hydrophobic effect where structured water is released from the steroids. The enthalpy changes associated with the transfer ranged from a small exothermic change for flunisolide to a large endothermic change for dexamethasone and even larger change for budesonide. The enthalpy change associated with a classic hydrophobic effect is expected to be small and exothermic. The rationale is that there is an increase in the hydrogen bonding with the structuring of water. Therefore, in its simplest form, the transfer of a steroid from water to lipid should be associated with slight endothermic change. Thus, flunisolide and triamcinolone are unremarkable since the changes are slight. For dexamethasone and budesonide, the large endothermic change suggests the steroids may
Table 2 The change in molar free energy, enthalpy, and entropy for the transfer of steroids from aqueous solution to SurvantaE lipid Steroid
DG transfer (kJ / mol)
DH (kJ / mol)
DS (e.u.)
Budesonide Triamcinolone acetonide Dexamethasone Flunisolide
227.2 226.1 222.2 224.1
14.2 0.484 8.89 21.58
129. 86.3 101 72.6
T.S. Wiedmann et al. / Journal of Controlled Release 65 (2000) 43 – 47
be interfering with the attractive interactions among the SurvantaE lipids. The results of this study can be compared with those described by Arrowsmith et al. [4]. In this study, the thermodynamics of transfer of a number of steroids from water to dimyristoylphosphatidylcholine above and below the gel to liquid crystalline phase transition were determined. The range of the equilibrium distribution coefficients on a weight / weight basis from water to DMPC ranged from 46 to 1609. The corresponding value for budesonide at 378C was 1533. Thus, there is reasonable agreement in comparing the present data with that of Arrowsmith et al. [4]. The enthalpy of transfer at temperatures below the gel to liquid crystalline phase transition noted by Arrowsmith et al. [4] ranged from 12 to 96 kJ / mol. For temperatures above the phase transition, the enthalpies of transfer were lower ranging from 0.36 to 50 kJ / mol. The range of values observed in this study is in reasonable agreement. Within the thermodynamic argument, it would appear that the steroids have a smaller influence on the attractive interactions among the components of SurvantaE than observed with the DMPC. To gain a better appreciation of the practical consequences of this study, the relative amounts of aqueous solution and surfactant in the lung are needed. The liquid lining in the alveoli is estimated to be about 10 mm thick. The amount of lung surfactant has been estimated from recycling studies to be between 8 and 24 mg surfactant / kg body weight [5]. Of this, about half is believed to reside within the type II cells and the other half resides in the alveolar space. Thus, for a 70-kg adult, about 700 mg of surfactant would reside within the 80 ml of fluid assuming the surface area of the lung is 80 m 2 . At these surfactant concentrations, the amount of drug dissolved is greater by a factor of 1.6 for dexamethasone, 1.9 for flunisolide, and 5.5 for budesonide and triamcinolone. This indicates that a much greater extent of solubilization occurs due to the presence of lung surfactant than would be predicted from a consideration of the aqueous solubility. Since about half of the lung surfactant resides
47
within the type II cells of the lung and half in the alveolar space, the distribution of solubilized drug between the intracellular and extracellular space would be expected to be nearly equal distribution of surfactant. The important point is that the site of the development of adenocarcinoma in the lung is in the type II cell and therefore the solubilization of drugs may have implications for chemoprevention. Finally, solubilization of drug in surfactant may affect the residence time of solute within the lung. Without a better description of the rate limiting barriers for clearance, the alveolar residence time can not be estimated. Nevertheless, the results for the compounds of this study suggest that the presence of surfactant at levels found in the lung would increase the total amount of drug in solution by a factor of 3 to over 10. The affinity of the lung surfactant for these compounds must necessarily reduce the driving force for removal from the lung and thereby must be considered in estimating the residence time in the lung. While more studies are needed to delineate the many complicated kinetic aspects of drug distribution in the lung alveoli, there may be an advantage in the prevention of cancer by selecting compounds that are preferentially solubilized in lung surfactant.
References [1] L.W. Wattenberg, T.S. Wiedmann, R.D. Estensen, C.L. Zimmerman, V.E. Steele, G.J. Kelloff, Chemoprevention of pulmonary carcinogenesis by aerosolized budesonide in female A / J mice, Cancer Res. 57 (1997) 5489–5492. [2] C.-Y. Li, C.L. Zimmerman, T.S. Wiedmann, Solubilization of retinoids by bile salt–phospholipid aggregates, Pharm. Res. 13 (1996) 535–541. [3] R. Bratts, B.I. Axelsson, Inhaled glucocorticosteroids in asthma. Mechanisms and clinical actions, in: R.P. Schleima, W.W. Busse, P.M. O’Byrne (Eds.), Lung Biology in Health and Disease, Vol. 97, 1995, pp. 351–379. [4] M. Arrowsmith, J. Hadgraft, I.W. Kellaway, Thermodynamics of steroid partitioning in dimyristoylphosphatidylcholine liposomes, Biochim. Biophys. Acta 750 (1983) 149–156. [5] M. Hallman, Recycling of surfactant: A review of human amniotic fluid as a source of surfactant for treatment of respiratory distress syndrome, Rev. Perinat. Med. 6 (1989) 197–226.
Drug solubilization in lung surfactant T.S. Wiedmann a , *, R. Bhatia a , L.W. Wattenberg b a b
Department of Pharmaceutics, University of Minnesota, 308 Harvard St. SE, Minneapolis, MN 55455, USA Department of Laboratory Medicine and Pathology, University of Minnesota, Minneapolis, MN 55455, USA Received 17 March 1999; accepted 22 September 1999
Abstract The relative affinity of glucocorticosteroids for lung surfactant was determined for the purpose of identifying chemopreventive agents with a high therapeutic index for lung cancer. The aqueous solubility and the extent of solubilization in SurvantaE, a native extract of bovine lung, of budesonide, triamcinolone acetonide, dexamethasone, and flunisolide were determined as a function of temperature by a dialysis technique. The aqueous solubilites at 378C were 19.6, 35.8, 104 and 120 mg / ml for the above listed compounds, respectively. The temperature dependence of the solubilities was modest consistent with the hydrophobic properties of the steroids. The amount of drug in solution was significantly enhanced in the presence of SurvantaE with solubilization ratios of 0.019, 0.023, 0.014, and 0.02 mg drug dissolved per mg of SurvantaE phospholipid, respectively. In addition, the extent of solubilization also generally increased with temperature, although the phase transition of the surfactant lipid appeared to complicate the functional relation between temperature and solubilization. These results show that there is enhanced solubilization of glucocortosteroids by lung surfactant which is secreted by the cancer susceptible type II cells of the lung. 2000 Elsevier Science B.V. All rights reserved. Keywords: Solubilization; Lung surfactant; Cancer chemoprevention; Glucocortosteroids
1. Introduction Virtually all drugs must be in a dissolved state for therapeutic activity. However, the vast majority of drugs are delivered as solids. Thus, there is tremendous value in understanding and predicting the solubility and solubilization of drugs. In this study, the solubilization of drugs was examined from a general perspective of investigating respiratory drug delivery and with a very specific interest in finding a chemopreventive agent for lung cancer. *Corresponding author. Tel.: 11-612-624-5457; fax: 11-612626-2125. E-mail address: [email protected] (T.S. Wiedmann)
Typically, drugs are delivered to the respiratory tract by metered dose or dry powder inhalers. These devices deliver solid aerosol particles to the respiratory tract for the purpose of maximizing the deposition of drug in the airways. To reach the site of action, the particles must be wetted, and the medium must allow dissolution to occur. Although the lung enjoys a relative humidity near 100%, the volume of solution at the lung lining is extremely small. The question arises how does dissolution take place in this environment of minimum fluid volume. Within the context of the broad topic of respiratory drug delivery, we also have a specific interest in chemoprevention. First, chemoprevention can be defined as the administration of a chemical entity
0168-3659 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0168-3659( 99 )00230-8
44
T.S. Wiedmann et al. / Journal of Controlled Release 65 (2000) 43 – 47
that interferes with the development of cancer. This approach has grown from its roots of largely anecdotal commentaries to reach a point of full justification with the approval of tamoxifen for the prevention of breast cancer. It should be appreciated that drugs given for the purpose of disease prevention must first ‘do no harm’. As such, candidates for chemoprevention must have therapeutic indices approaching infinity until such time as the incidence of cancer can be confidently predicted. In spite of the daunting requirement of the therapeutic index, it should be noted that preventing lung cancer holds special attributes. First, drugs may be directly delivered to the lung and need not pass through the general circulation. This has been shown to provide a therapeutic advantage for prevention [1]. A second aspect that forms the basis of this work is the question does drug association with lung surfactant enhance the therapeutic effectiveness. The rationale arises from the observation that a common form of lung cancer is adenocarcinoma. This occurs in the type II cells of the epithelial lining of the alveoli. Type II cells have a critical role in the production, secretion, and recycling of lung surfactant, the lipid-rich material that allows the formation of the large air / water interface required for gas exchange. Due to the recycling activity, it is hypothesized that drugs associated with lung surfactant will be preferentially delivered to type II cells and thereby have a higher therapeutic index. As the first step to investigate this hypothesis, the solubility and solubilization in lung surfactant of a number of potential chemopreventive steroids have been determined.
2. Materials and methods Budesonide, triamcinolone acetonide, dexamethasone, and flunisolide were purchased from Sigma Chemical Co. (St. Louis, MO) and used as received. SurvantaE, a bovine, native lung extract was obtained from Ross Laboratories. The solubility in normal saline of each compound was determined as a function of temperature by a dialysis method described by Li et al. [2]. Equilibration was complete in 24 h and concentration was determined by diluting an aliquot with ethanol and
measuring the UV absorbance at 244 nm against the appropriate standard curve. The extent of solubilization was determined in a similar manner with the drug powder placed in dialysis bags (MWCO 10 000 Da), and a range of concentrations of SurvantaE were placed in the external solution. The bags were tied with surgical thread and placed into PyrexE screw cap test tubes with TeflonE liners. The tubes were oscillated for 24 h, and the outer solution was assayed by measuring the absorbance. SurvantaE did not interfere with the absorbance measurements since it is solubilized by the addition of ethanol. The water solubility is express in mole fractions calculated with the assumption that the density is unity. The solubilization ratio of drug in SurvantaE, R, was determined from the slope of the observed total molar concentration of drug in solution plotted as a function of the molar SurvantaE lipid concentration, in which it was assumed that the phospholipid had an average molecular weight of 750 Da. The SurvantaE / aqueous distribution coefficient is expressed as the ratio of the mole fraction solubility in SurvantaE, Xs , and the aqueous solubility where the solubilization ratio is related to the SurvantaE solubilization mole fraction by R 5 1 /(1 1 Xs ). The thermodynamic parameters, the change in enthalpy, DH, and the change in entropy, DS, were calculated from the slope and intercept of a plot of the logarithm of the mole fraction solubility or solubilization as a function of the reciprocal of the absolute temperature.
3. Results and discussion The basis of this work was to investigate the general aspects of respiratory drug delivery as well as address the question, ‘does drug association with lung surfactant enhance the therapeutic effectiveness of chemopreventive agents?’ Research to date has shown that steroids, and in particular budesonide, are effective as chemopreventive agents [1]. However, to our knowledge, there are no published reports of the relative affinity of steroids for lung surfactant. The four compounds that were chosen for the focus of this study are also approved for the treatment of asthma. The advantage from a chemoprevention point of view is that they have been shown to be safe
T.S. Wiedmann et al. / Journal of Controlled Release 65 (2000) 43 – 47
Fig. 1. Solubility of steroids as a function of temperature for (d) budesonide, (♦) triamcinolone acetonide, (m) dexamethasone, and (.) flunisolide.
if not free of side effects. With these compounds, the aqueous solubility and the solubilization in Survanta were determined as a function of temperature. The observed solubilities of budesonide, triamcinolone acetonide, dexamethasone, and flunisolide at room temperature agree well with the available literature values [3]. The use of the dialysis method allowed for rapid equilibration and over came the inherent wetting problem of these hydrophobic powders. The rank order of solubility is, in general, consistent with expectations based on structural information and the melting behavior. The aqueous solubility for each compound is given as a function of temperature in Fig. 1. As can be seen in the figure, there is a non-linear increase in
45
solubility with temperature. The data were fit to the Arrhenius equation, and the enthalpy and entropy of solution were determined and are given in Table 1. The enthalpy changes were relatively large and endothermic reflective of the stable crystal structure. The entropy was also large, but negative. The negative entropy change indicates an increase in order. Since the entropic change of the drug should be positive because it is transferred from the solid to the solution, the increase in order is presumably a result of water structuring about the nonpolar steroids, characteristic of the hydrophobic effect. The addition of lung surfactant led to an increase in the amount of steroid solubilized in a linear fashion for budesonide, triamcinolone acetonide, and dexamethasone up to a concentration of 2500 mg lipid / ml. For flunisolide, a plot of the amount solubilized as a function of SurvantaE lipid was initially linear, but at higher concentrations, the slope became smaller. From the linear portions of the curve, the solubilization ratio was calculated for each steroid. The solubilization ratios ranged from 0.019 to 0.026 moles of steroid solubilized per mole of SurvantaE lipid at 378C. From the reciprocal of the value of the solubilization ratio, it can be deduced that about 40–50 lipid molecules are needed to solubilize one steroid molecule. The solubilization was carried out as a function of temperature from which the thermodynamic parameters were obtained. In analyzing the results, the solubilization ratios were first converted to mole fraction solubilities based on the SurvantaE lipid content. Thereafter, the ratio of the mole fraction SurvantaE solubilization and the mole fraction aqueous solubility was defined as the distribution coefficient. The logarithm of the distribution or partition coefficient is given as a function of the reciprocal of the absolute temperature in Fig. 2. Both triamcinolone and flunisolide evidently have
Table 1 Thermal transition temperature, mole fraction solubility, and enthalpy and entropy of solution Steroid Budesonide Triamcinolone acetonide Dexamethasone Flunisolide a
dec, decomposed.
T m (8C) 221–232 (dec) 292 268–271 245
a
Sol (378C)
DH (kJ / mol)
DS (e.u.)
8.18E–07 1.48E–06 4.77E–06 4.97E–06
17.3 20.1 11.7 23.9
260.0 247.3 264.3 225.3
46
T.S. Wiedmann et al. / Journal of Controlled Release 65 (2000) 43 – 47
Fig. 2. Survanta / aqueous distribution coefficient of steroids as a function of the reciprocal of the absolute temperature for (d) budesonide, (♦) triamcinolone acetonide, (m) dexamethasone, and (.) flunisolide.
distribution coefficients that increase with increasing temperature while for budesonide and dexamethasone the distribution appears largely independent of temperature. Given that all the compounds had relatively large enthalpies of solution, the solubilization increase with temperature was comparable or smaller than the increase in solubility with temperature. Although the phospholipid component of the lung surfactant has been emphasized, it is important to recognize that SurvantaE is a complex mixture consisting of an aqueous dispersion of lipids and proteins. SurvantaE is a lung surfactant replacement that is produced by enriching a calf lung extract with synthetic lipids. The product information lists the
composition as 11.0–15.5 mg / ml DPPC, 0.5–1.75 mg / ml triglycerides, 1.4–3.5 mg / ml free fatty acids, 0.1–1.0 mg / ml proteins B / C, and 7.65–10.35 mg / ml sodium chloride. The gel to liquid crystalline phase transition is expected to be broad and lie between 30 and 408C. Moreover, the phase state of the lipids is expected to influence the solubilization capacity. In general, the solubilization is greater for lipids in the liquid crystalline state as compared to the gel state [4]. Finally, at temperatures in the transition region, solubilization may be unique in comparison to both the gel and liquid crystalline state [4]. Examining the thermodynamic parameters, the change in molar free energy of transfer of steroid from saline to lung SurvantaE lipid was large and negative in each case (Table 2). The lipid environment is considerably more favorable relative to water for solubilizing the steroids. The entropy changes are also large but positive. The dramatic increase in disorder in transferring the steroids from water to the surfactant lipid is consistent with the hydrophobic effect where structured water is released from the steroids. The enthalpy changes associated with the transfer ranged from a small exothermic change for flunisolide to a large endothermic change for dexamethasone and even larger change for budesonide. The enthalpy change associated with a classic hydrophobic effect is expected to be small and exothermic. The rationale is that there is an increase in the hydrogen bonding with the structuring of water. Therefore, in its simplest form, the transfer of a steroid from water to lipid should be associated with slight endothermic change. Thus, flunisolide and triamcinolone are unremarkable since the changes are slight. For dexamethasone and budesonide, the large endothermic change suggests the steroids may
Table 2 The change in molar free energy, enthalpy, and entropy for the transfer of steroids from aqueous solution to SurvantaE lipid Steroid
DG transfer (kJ / mol)
DH (kJ / mol)
DS (e.u.)
Budesonide Triamcinolone acetonide Dexamethasone Flunisolide
227.2 226.1 222.2 224.1
14.2 0.484 8.89 21.58
129. 86.3 101 72.6
T.S. Wiedmann et al. / Journal of Controlled Release 65 (2000) 43 – 47
be interfering with the attractive interactions among the SurvantaE lipids. The results of this study can be compared with those described by Arrowsmith et al. [4]. In this study, the thermodynamics of transfer of a number of steroids from water to dimyristoylphosphatidylcholine above and below the gel to liquid crystalline phase transition were determined. The range of the equilibrium distribution coefficients on a weight / weight basis from water to DMPC ranged from 46 to 1609. The corresponding value for budesonide at 378C was 1533. Thus, there is reasonable agreement in comparing the present data with that of Arrowsmith et al. [4]. The enthalpy of transfer at temperatures below the gel to liquid crystalline phase transition noted by Arrowsmith et al. [4] ranged from 12 to 96 kJ / mol. For temperatures above the phase transition, the enthalpies of transfer were lower ranging from 0.36 to 50 kJ / mol. The range of values observed in this study is in reasonable agreement. Within the thermodynamic argument, it would appear that the steroids have a smaller influence on the attractive interactions among the components of SurvantaE than observed with the DMPC. To gain a better appreciation of the practical consequences of this study, the relative amounts of aqueous solution and surfactant in the lung are needed. The liquid lining in the alveoli is estimated to be about 10 mm thick. The amount of lung surfactant has been estimated from recycling studies to be between 8 and 24 mg surfactant / kg body weight [5]. Of this, about half is believed to reside within the type II cells and the other half resides in the alveolar space. Thus, for a 70-kg adult, about 700 mg of surfactant would reside within the 80 ml of fluid assuming the surface area of the lung is 80 m 2 . At these surfactant concentrations, the amount of drug dissolved is greater by a factor of 1.6 for dexamethasone, 1.9 for flunisolide, and 5.5 for budesonide and triamcinolone. This indicates that a much greater extent of solubilization occurs due to the presence of lung surfactant than would be predicted from a consideration of the aqueous solubility. Since about half of the lung surfactant resides
47
within the type II cells of the lung and half in the alveolar space, the distribution of solubilized drug between the intracellular and extracellular space would be expected to be nearly equal distribution of surfactant. The important point is that the site of the development of adenocarcinoma in the lung is in the type II cell and therefore the solubilization of drugs may have implications for chemoprevention. Finally, solubilization of drug in surfactant may affect the residence time of solute within the lung. Without a better description of the rate limiting barriers for clearance, the alveolar residence time can not be estimated. Nevertheless, the results for the compounds of this study suggest that the presence of surfactant at levels found in the lung would increase the total amount of drug in solution by a factor of 3 to over 10. The affinity of the lung surfactant for these compounds must necessarily reduce the driving force for removal from the lung and thereby must be considered in estimating the residence time in the lung. While more studies are needed to delineate the many complicated kinetic aspects of drug distribution in the lung alveoli, there may be an advantage in the prevention of cancer by selecting compounds that are preferentially solubilized in lung surfactant.
References [1] L.W. Wattenberg, T.S. Wiedmann, R.D. Estensen, C.L. Zimmerman, V.E. Steele, G.J. Kelloff, Chemoprevention of pulmonary carcinogenesis by aerosolized budesonide in female A / J mice, Cancer Res. 57 (1997) 5489–5492. [2] C.-Y. Li, C.L. Zimmerman, T.S. Wiedmann, Solubilization of retinoids by bile salt–phospholipid aggregates, Pharm. Res. 13 (1996) 535–541. [3] R. Bratts, B.I. Axelsson, Inhaled glucocorticosteroids in asthma. Mechanisms and clinical actions, in: R.P. Schleima, W.W. Busse, P.M. O’Byrne (Eds.), Lung Biology in Health and Disease, Vol. 97, 1995, pp. 351–379. [4] M. Arrowsmith, J. Hadgraft, I.W. Kellaway, Thermodynamics of steroid partitioning in dimyristoylphosphatidylcholine liposomes, Biochim. Biophys. Acta 750 (1983) 149–156. [5] M. Hallman, Recycling of surfactant: A review of human amniotic fluid as a source of surfactant for treatment of respiratory distress syndrome, Rev. Perinat. Med. 6 (1989) 197–226.