J. Phys. Chem. 1995,99, 16144-16148
16144
A New Thermodynamic Model To Predict Protein Encapsulation Efficiency in Poly(1actide) Microspheres B. Gander* and H. P. Merkle Department of Pharmacy, ETH, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
V. P. Nguy&nand Nam-TrAn Hi3 School of Pharmacy, BEP, Institute of Pharmaceutical Analysis, University of Lausanne, CH-1015 Lausanne, Switzerland Received: January 19, 1995; In Final Form: April 27, 1995@
Entrapment efficiency in protein microencapsulation into biodegradable polymer microspheres depends on the nature of the solvents used for dissolving the polymer. In such a system, three main interactions are present, i.e., polymer-water (aqueous protein solution), polymer-solvent, and solvent-water (aqueous protein solution). These interactions are quantified by the three interaction energies, AintEp, AintE,, and AintE2, respectively. For a given polymer, Ai&, is constant, but AintEland AintEZvary as a function of the nature of the solvent used. In this contribution, the model protein bovine serum albumin was microencapsulated into poly(D,L-lactic acid) by spray-drying. It was demonstrated that increasing absolute values of the sum AintEl-t AintE2leads to decreasing encapsulation efficiencies. AintE1was estimated from the heat of dissolution of the polymer in the selected solvents and the energy of cavity formation, and Ai& from 6 d and 6, and Drago's parameters E and C. A linear fit between the sum of interaction energies, equivalent to AintEl iAintE2,and the actual microencapsulation efficiency gave a reasonable correlation with a correlation coefficient r = 0.954. This represents an acceptable correlation considering enthalpies were used to predict interaction energies. Moreover, if microencapsulation efficiency is correlated directly with AintEl (expressed by its equivalent calculated value), and the parameters a d , 6,, E, and C, instead of Aint&, an even better correlation with a multiple r = 0.9995 is observed. This rational approach in microencapsulation is of high importance as it is based on thermodynamic parameters.
Introduction The potential and growing availability of peptide drugs (hormones and their analogues, antigens, enzymes) require delivery sytems for optimal therapeutic use. One type of drug delivery system for such compounds are biodegradable injectable microspheres based on homo- and copolymers of lactic and glycolic acid (PLA, PLGA). These polymers have a long safety record and are highly biocompatible. Depending on the copolymer composition and molecular weight, the biodegradation time can vary from about one to several months or even years. Biodegradable microspheres based on these polymers have been studied for more than 15 years, and their usefulness for delivering hormone agonists and antagonists (leuprorelin, triptorelin, bromocriptin) has been commerically expoited for more than 5 years. The entrapment of drugs into these polymers is preferably performed by so-called microencapsulation techniques. In these processes, the three major components involved are the active compound to be encapsulated, the polymer forming the microspheres, and the solvent used to dissolve the polymer. A major problem of all the techniques lies in the fact that the encapsulation efficiency of highly water soluble (and very expensive) material is often unsatisfactory. The approaches taken thus far for optimizing drug encapsulation efficiency were very much empirical, using particular additives, changing process temperature or using more hydrophobic analogues of the hydrophilic active compounds.' In a recent study,2 we proposed a thermodynamic approach which is based on interaction energies between drug-polymer-
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Ahsrracrs, October I , 1995.
0022-3654/95/2099- 16144$09.00/0
solvent. We have assumed that encapsulation efficiency for a given drug and a given polymer is a function of the sum of interactions between polymer and solvent, AintEI,on one side, and between water (aqueous protein solution) and solvent, A,,,Ez, on the other side. This sum of interaction energies can be minimized by selecting an appropriate solvent. Recently, Hb3 proposed a new thermodynamic model for liquid-liquid systems. It offers two possibilities for calculating the interaction energy. The first one is based on the sum of the heat of mixing, (AmlXQ~)v, the nonatmospheric work, P@A - VA), the vaporization energy, AvEA,and the cavity term, AcEB. The second possibility is to use four molecular interaction capacity parameters, which are the two Hansen partial cohesion parameters, &(dispersive) and 6 , (dipole-dipole), and the two Drago parameters E (electrostatic) and C (covalent). For a polymer dissolved in a solvent, these quantities either cannot be determined (vaporization energy of the polymer PLA) or are not available in ther literature. Therefore, in this work, we would like to solve this difficulty by using, instead of A,,,E,, the sum of two new terms, namely, the partial heat of dissolution, (hdisHA)p,of the polymer in different solvents and the energy of cavity formation, AcEB. The sum of these two terms represents a new comparative scale for different solvents.
Interpretative Model Interaction Diagram of the Ternary System PolymerSolvent-Water. In a system containing a polymer, a solvent for the polymer, and an aqueous phase containing a drug (=water), three major interactions are prominent: polymersolvent (AintEl), solvent-water (AintE2),and polymer-water (AintEp).Here, the drug-containing aqueous phase is considered 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 43, 1995 16145
Protein Encapsulation Efficiency
measured directly but must be replaced by the enthalpy of dissolution (hdisHA)P and enthalpy of fusion &HA, according to the Hess cycle:
Protein t Water = Water
=
(AmiXQ.Jp
- A$/,
(4)
Combining eqs 3 and 4 gives = (AdisHA)p - A f H A - V A ~ ; - &EB
L-_ _ _ _ _ _ _ _ - _ _ Solvent \:
Polymer
Introducing on both sides of eq 5 the term AintE2yields the expression
*int € 1
Figure 1. Interaction triangle of the components involved in drug
microencapsulation.
AintEl
as pure water for simplifying the quantitative treatment of these interactions. The three interactions can be presented schematically by an interaction triangle (Figure 1). When the polymer and water remain unchanged while the solvent is varied (case studied here), the microsphere properties in terms of drug loading efficiency, LE, must vary as a function of the two interactions AlntEland AIntE2,because the polymerwater interaction, AlntEp, is constant. Hereafter, the term loading efficiency will be used interchangeably with microencapsulation or entrapment efficiency. It is defined as the percentage of the actual loading with respect to theoretical loading. Construction of the Thermodynamic Model. We have put forward the hypothesis that, for a set of solvents, highest protein entrapment efficiency in microspheres is achieved if the absolute value of the sum of interactions AlntE1 and AlntE2 is a minimum:
+
IAlntE, AlntE21= minimum
(1)
For estimating the two interaction energies AlntEl and AlntE2, a recently developed interaction energy model3 was used as the starting point and extended to the present system. Assyming that the difference between the partial molar volume, V, and the molar volume, V, of the solute A is negligible, the proposed equation becomes
(AmlxQ,Jp- vAd,2 -
= -2vA(dd,Add,B EAEB
+ dp,Adp,B) - c,A.cB
(2)
where (Am,,Q~)pis the partial mixing heat (kJ mol-') of solute A at infinite dilution in the solvent B, VA is the molar volume of the solute A, 6~ (MPalI2) is the total solubility parameter; AcEB(kJ mol-]) is the energy of cavity formation for 1 mol of the solute A in the solvent B; dd and 6, are the Hansen partial solubility parameters for dispersive and dipole-dipole contributions, respectively; and E and C are the electrostatic and covalent Drago parameters, respectively. According to H6,3 both sides of eq 2 express the interaction energy between solute A and solvent B. We would like to point out that for the compounds A and B the parameter 6, is exclusively a function of the dipole moment p. Therefore, the product d p , ~ d prepresents ,~ the dipole-dipole interaction energy which, according to Fowkes? is independent of hydrogen bonding. Furthermore, belonging to the van der Waals energy (physical nature), the law of geometric mean is valid. As the parameters d d , 6,, E, and C are not available for the polymer used, the left-hand side of eq 2 will be applied for calculating the polymer- solvent interaction Aln,El:
AlntEl= (Aml,QA), - VAdA2- ACEB
(5)
(3)
Furthermore, as the polymer used is not in the liquid state at working temperature, the heat of mixing (A,,,QA)P cannot be
+ AintE2
= AintE2
+
-
- vAdA2 ACEB(6)
For numeric applications, eq 6 can be transformed into IAintE1
+ AintE2 + A&A + vAd,21
= IAintE2
+ (AdisHA)P
-
&EBI (7) The left-hand side of eq 7 is an extension of eq 1 and, consequently, may allow us to establish a new scale for the suitability of different solvents for protein microencapsulation. For the polymer used, the sum &HA V A ~ isA constant, ~ but cannot be determined as the polymer shows no melting (noncrystalline polymer) nor heat of vaporization. Therefore, the sum 1AintEl Ain1E2 &HA VA~A*I varies in the same sense and by the same order of magnitude as the sum I AintEl A&I, i.e.
+
+
+
+
+
IAlntEl AintE21 = minimum
-
+
+
+
lAintEl AintE2 A&,
+
VA6,21 = minimum (8) For numeric applications, we can therefore use, for the new scale, the right-hand side of eq 7 and put
C E = IAintE2+ (AdisHA)P- A,EBI
(9)
partial molar heat of dissolution (kJ mol-') and can be measured by calorimetry at infinite dilution, and ACEB can be calculated by two different ways, depending on the value of the dipole moment. If the solvent is nonassociated (dipole moment < 2 D), the cavity term is purely d i s p e r s i ~ e : ~ > ~ - ~ (AdisHA)p is the
A , E ~= ~ , d , , , ~
(10)
For solvents with a dipole moment > 2 D, the following expression must be used:3
ACEB = v A d p , B 2
+ '/2VB(dp,B2 +
(1 1)
Here, the indexes A and B refer to the polymer and solvent, respectively. The interaction energy between solvent and water (aqueous protein solution), AintE2,can be calculated by using the aforementioned four interaction capacity parameters. If the solvent is soluble in water, eq 12 must be used, whereas eq 13 applies for the solvents only sparingly soluble or insoluble in water: = - ~ V A ( ~ , , A ~+ , , Bd p , ~ a p-, ~EAEB ) AintE2
= -VA(dd,Add,B
- CACB (12)
+ dp,Adp,B) - EAEB - CACB
(l3)
Among the solvents used, 1P-dioxane exhibited very peculiar characteristics. Being capable to interact with water by
Gander et al.
16146 J. Phys. Chem., Vol. 99, No. 43, 1995 TABLE 1: Thermodynamic Characteristics of the Solvents Used V
solvent dichloromethane ethyl acetate ethyl formate nitromethane 1,4-dioxane tetrahydrofuran acetone water (acidic) water (basic) a
(cm3mol-') 63.9 98.5 80.2 54.3 85.7 81.7 74.0 18.0 18.0
p (D)
1.58 1.82 1.93
3.46 0.45 1.76 2.69 1.84 1.84
6d (MPaO5, 18.2
6,
dh
(MPaO5 ,
(MPaO5 ,
6.3
6.1 9.2
5.3 7.2
15.1
15.5 15.8 19.0 16.8
18.8
1.8
15.5 15.6
15.6
7.6 5.1 7.4
E (kJo m0l-O
C
(kJo m0l-O
5,
S'
5,
1.76 3.31 3.23
0.23 2.00 1.35
2.08
2.64"
0.98" 2.63 4.46 2.58 1.59
3.12 1.93
5.7
8.0
3.80 3.35
10.4 16.0 16.0
7.0 42.3 42.3
3.56 2.68 4.66
2.15
2.08 2.47
0.20
Value for nitrobenzene.
hydrogen bonding, 1,4-dioxane probably adsorbs on water droplets solely by one oxygen. For this reason, the interaction energy between 1P-dioxane and water is set at 0.5AintE2. Incidentally, this phenomenon that a solute can adsorb on a surface by different (horizontal or vertical) projections is wellknown in the field of gas-solid and liquid-solid adsorption.' Correlations between Actual Loading Efficiency, LE, and Interpretative Thermodynamic Models. The usefulness of eq 9 has been examined first by comparing the order of solvents when arranged according to decreasing X E and increasing actual loading efficiency. Theoretically, the solvent with the lowest absolute value of ZE should produce highest encapsulation efficiency. In a second step, the actual loading efficiency was correlated with ZE, according to the expression
The coefficients a and b, estimated by least-squares linear regression, have no physical meaning. To explore further our model (eq 9) and to avoid calculating AintE2,we have examined the following multiple regression, in which AintE2 is replaced by the four parameters &.B, d p . ~EB , and CB of the various solvents:
sulated into the biodegradable poly(D,L-lactide) (Resomer R202 from Boehringer, D-Ingelheim) by a spray-drying t e c h n i q ~ e . ~ Typically, 900 pg of BSA were dissolved in 2.16 mL of pure, ultrdiltered water (W phase) and, separately, 3.0 g polymer was dissolved in 57.0 g of the solvents given in Table 1 (0phase). The W phase was then finely dispersed in the 0-phase by ultrasonication (Vibra cell, VC375, Sonics & Materials, Danbury, CT), and the W/O dispersion spray-dried in a laboratory spray-dryer (Model 190, Buchi, CH-Flawil) under the following conditions: Tjnlet 55 "C, Toutlet 45 "C, air flow 40 m3 h-I, spray flow 450 NL h-I, product feed 2.5 mL min-I. The obtained microspheres were washed with a 0.1% (w/w) Synperonic-F68 solution (ICI, Middlesborough, UK), collected on a 0.2 pm cellulose acetate filter and dried under vacuum for 24 h. On the basis of the hypothesis of the proposed model, the entrapment process of BSA in the PLA microspheres by spraydrying may be illustrated schematically as follows:
+ solvent
A,,J?,(variableJ
PLA
+ solvent
A,,,E2(variableJ
[BSA@H,O]
PLksolvent [BSA@H,O]*solvent
+ [BSA@H20]*solvent
A,,,Ep(constantJ
PLA-solvent
[BSA@H,O]*PLA(solvent) (c) where, al, a2, a3, a4, a5, and b* are the coefficients of the leastsquares multiple regression analysis (SPSS program) and have no other physical meaning. In correspondence to eqs 12 and 13, n = 2 for the solvents miscible with water and n = 1 for the solvents sparingly soluble or insoluble in water. Recently, Drago8 introduced a new unified scale of solvent polarities for specific and nonspecific interactions. This new scale is characterized by the three parameters E, C , and S'. For examining the relevance of these parameters in our model, we propose the following correlation: LE = -al*[-(Adi,HA),
+ ACEB]- a,*S' - a,*& Q~*CB + b**
(16)
where a ' * , a2*, a3*, a4*, and b** are the coefficients of the least-squares multiple regression analysis (SSPS program) and have no further physical meaning. In this analysis, 1,Cdioxane was also included for the calculation, but not ethyl formate, because S' is not reported for this latter solvent.
Experimental Section As a model for an active proteinaceous compound, the highly water soluble bovine serum ablumin (BSA) was microencap-
[BSA@H,O]*PLA(solvent)
AVH vaporizatio;
BSkPLA(s) (solvent
+
+ H,O)(g)
(d)
In our hypothesis we assume that it is not the protein (BSA) itself which interacts with the polymer and the solvent, but the aqueous BSA solution in the form of a strong complex [BSAewater]. For simplicity of calculation, however, pure water was considered to express interaction energies. The diagram suggests that the dissolution of PLA in a solvent creates the interaction between PLA and solvent (AIntEl, step a). Introducing the aqueous BSA solution into the solution of PLA and solvent gives rise to the interaction between [BSAewater] and the solvent (AlntE2,step b). Depending on the type of solvent, both interaction energies, A&' and AlntE2,vary and are treated by the model. In step (c), [BSAewater] undergoes the interaction with PLA; the interaction energy, AlntE,,involved in this process is constant. In the final step (d), the solvent evaporates through spray-drying carrying along some of the water associated with BSA. Finally, the heat of vaporization, AvH, is not considered essential for protein entrapment. The molar volume and the molecular weight of the polymer are 55.72 mL mol-' and 72.06 g mol-', respectively. All solvents used for the polymer were of analytical grade from
J. Phys. Chem., Vol. 99, No. 43, 1995 16147
Protein Encapsulation Efficiency
TABLE 2: Values of the Partial Molar Heat of Dissolution, ( b h H & , Cavitation Energy, AcEB, Interaction Energy, AiotEz,and of the Sum of the Three Energy Terms, EE (Eq 9), for the Various Solvents Studied
(kT mol-')
AintE2 (kT mol-')
ZE (kT mol-')
18.456 12.704 13.387 24.212 20.1 15 15.726 19.201
-32.831 -43.606 -39.434 -38.351 -35.052 -73.795 -74.056
-49.228 -56.956 -52.339 -62.198 -56.082 -86.287 -101.136
(Adiadp
&EB
solvent
(kT mol-')
DCM ethyl acetate ethyl formate nitromethane 1,4-dioxane
2.059 -0.646 0.482 0.365 -0.915 -3.234 -7.879
THF acetone
0
100
6 6 c
80
I
V
f
60
0
-ma
c)
m n
40
8c
W
20
-30
Fluka (CH-Buchs). Table 1 summarizes the physicochemical properties of the solvents used.8*lo-'* The partial molar heat of dissolution of the polymer in the solvents (Adi&TA)p was determined by a heat-flow calorimeter Setaram C80 (Lyon, France), as described el~ewhere.~ For determining the encapsulation efficiency, the albumin was extracted from the microspheres by dissolving the latter in dichloromethane, collecting the protein on a 0.2 pm membrane filter of regenerated cellulose (RC 58, Schleicher & Schuell, D-Dassel), and eluting the protein from the membrane with phosphate buffer pH 7.4. The filters were washed three times with buffer, and the combined solution assayed by HPLC, as specified el~ewhere.~
Results and Discussion The results of the calorimetric measurements (AdisH~)p, as well as the calculated values for the cavitation energy, AcEB, the interaction energy between solvent and water, AintE2,and the sum of interaction energies, CE (eg 9), are summarized in Table 2. The precision of the calculated parameters cannot be given since the precision is not known for the interaction parameters, but it is estimated at approximatively 2% for (Ad,sHA)p. On the basis of the hypothesis that the absolute value of CE should be minimum for efficient protein entrapment, the solvents can be put into an order of decreasing suitability for microencapsulation: DCM, ethyl formate > 1,4-dioxane, ethyl acetate > nitromethane > THF > acetone The data also reveal that the contribution of the molar heat of dissolution to the value of ZE is negligible for many solvents. Only for acetone, THF and DCM represents about 4-8% of the value CE. For the interaction polymer-solvent, the dominant term is the energy of cavity formation of the solvent AcEB. As only the solvents were varied, but not the polymer, it must be the thermodynamic properties of the solvent that are crucial. Therefore, for selecting an appropriate solvent, its dipole moment should be ( 2 D and the dispersive energy density should be low. From the third term, AintE2,it becomes evident that the most appropriate solvents are those interacting the least with water. The choice of an optimum solvent for efficient microencapsulationof aqueous protein solution remains nonetheless difficult as DCM appears thermodynamically superior to all the other solvents tested, neighboring though very closely ethyl formate. Here, we have for the first time demonstrated why DCM has been so far the most commonly and widely used solvent for microencapsulation with PLA and PLGA polymers. For verifying the results of the theoretical study, the encapsulation efficiency for BSA in microspheres was determined experimentally (Table 3). From the loading effciency,
-70
-50
-DO
-110
I E (kJ mol-')
Figure 2. Linear fit of the actual encapsulation efficiency, LE, versus the sum of the interaction parameter XE. The point marked with 1) corresponds to 1,4-dioxane showing a peculiar characteristics.
TABLE 3: Actual BSX Loading Efficiency (LE) in PLA Microspheres solvent
LE (9%)
ethyl formate DCM ethyl acetate nitromethane
79.3 f 3.1 78.5 f 3.5 77.3 f 3.3 72.0 f 7.5
solvent
LE (9%)
1,4-dioxane
59.5 f 2.5 37.7 f 3.2 25.0 f 1.7
THF acetone
the solvents can be put into the following order of decreasing suitability: DCM, ethyl acetate, ethyl formate > nitromethane > 1,4-dioxane > THF > acetone Comparing this range order with that from the thermodynamic interaction value ZE, an agreement is observed for DCM, ethyl formate, THF, and acetone. This agreement between theoretical prevision and the experimental results is satisfactory, despite the fact that the values of E and C for nitromethane are only approximations and the nonatmospheric work is neglected. Moreover, the theoretical approach appears valid for all solvents, independently of their degree of miscibility with water. Finally, we have examined to what extent the values of 2 E can be correlated to the experimentally determined loading efficiency (Figure 2). The linear correlation coefficient r gives a value of 0.954 confirming the relatively good agreement between theory and experiment, particularly if one considers that enthalpies of dissolution were used to predict interaction energies. For this particular analysis, the peculiar 1,Cdioxane was not included in the calculation. Considering the calculated F value (Snedecor) of 49.6, this correlation is highly significant; the tabulated F ( 1 3 values (Snedecor) are 6.61 for a = 5%, and 16.3 for a = 1%. The corresponding correlation constants a and b (eq 14) are as follows: a = 132.17; b = -1.06. A somewhat abnormal behavior shows 1P-dioxane (point especially marked with number 1) in Figure 2. For this solvent we have made the assumption that the molecule adsorbs in a vertical projection on water droplets, whereby the value of the interaction energy AintEzwas divided by 2. If this assumption is not made for 1,4-dioxane, a CE value of -91.134 kJ mol-' results, which would give rise to a much lower expected encapsulation efficiency. It appears that from the encapsulation efficiency, the model for calculating the interaction energies for 1,Cdioxane lies in between the two extreme cases of horizontal and vertical adsorption. Moreover, the suitability of the low molecular weight ester solvents, ethyl acetate, and in particular ethyl formate, is clearly demonstrated by both the theoretical model and by the practical experiments. This represents an important
16148 J. Phys. Chem., Vol. 99, No. 43, 1995
step in the development of new microencapsulation processes which respect toxicological and environmental issues. Incidentally, for safety and environmental reasons, we decided not to study some very toxic solvents, such as thiophenol, for which an even lower EE was estimated. The direct use of the parameters dd,B, d p , ~EB, , and CB (eq 15), instead of Ain&, results in an improvement of the correlation coefficient, multiple r = 0.9995, and gives a highly significant F value of 204, corresponding to a calculated significance of a = 0.053. The regression constants of eq 15 are al = 0.99; a2 = 28.47; a3 = 16.53; a4 = 0.76; a5 = -4.02; b* = 134.75. It may be noteworthy that the first three terms (a!, a2, and u3) dominate and influence mainly the loading efficiency. Therefore, low energy of cavity formation and low values of VB, b d , B , and d p , will ~ greatly improve good encapsulation efficiencies. This confirms our comments made above in the context of theoretical considerations about the order of suitability of solvents for microencapsulation. The use of the parameter S’(Drago) (eq 16), instead of dd,B, and d p , ~results , in an acceptable coefficient r, multiple r = 0.9456, but in a not significant F value of 2.1 1, corresponding to a calculated “significance” of a = 0.476. The correlation constants of eq 16 are a,* = 3.50; a2* = -13.47; a3* = -7.10; a d * = 10.79; b** = 98.96. In the present system, substitution of S’ for d d , ~and d p , has ~ not led to an improved correlation. Moreover, for the type of polymer (donor) and solvents (mostly donors) used here, the unified scale should not be applied because the energy of cavity formation is a dominating term for the polymer-solvent interaction and cannot be substituted by S‘. Finally, our model using the heat of dissolution and the energy of cavity formation for the polymer-solvent interaction, and the four parameter equation with d d and d,, E and C, as proposed by H6,3 for the solvent-aqueous phase interaction appears to fit best the experimental data of microencapsulation efficiency of the model protein BSA.
Conclusions The three thermodynamic terms describing the interaction energy between the drug-containing water phase and the
Gander et al. polymer solvent, on one side, and the heat of dissolution and energy of cavity formation of the polymer, on the other side, provide meaningful tools to improve drug encapsulation efficiency into polymers. This will allow us to predict appropriate solvents for a given water-insoluble polymer in which a watersoluble drug should be embedded. However, for using solvents in the process of drug microencapsulation, two most important boundary conditions have to be taken into consideration, which are the vapor pressure with a minimal value of about 20-30 P a , preferably though higher than 100 Wa, and the toxicological and environmental safety. In light of these restraining parameters, the model also demonstrates the limits of using single-solvent systems. Further work will therefore focus on possibilities and limits of binary solvent mixtures.
References and Notes (1) Jalil, R.; Nixon, J. R. J. Microencapsulation 1990, 7, 297.
(2) Gander, B.; Johansen, P.; H8 N.-T.; Merkle, H. P. Int. J. Pharm., in press. (3) H6 N.-T. J. Phys. Chem. 1994, 98, 5362. (4) Fowkes, F. M.; Kaczinski, M. B; Dwight, D. W. Langmuir 1991, 7, 2464.
(5) Dack, M. R. J. Aust. J. Chem. 1975, 28, 1643. (6) Kamlet, M. J.; Doherty, R. M.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W. J. Pharm. Sci. 1986, 75, 338. (7) H6, N.-T.; Vo, V. N.; Buchmann, M.; Sabra, F.; Ruelle, P.; Kesselring, U. W. Drug Dev. Ind. Pharm. 1993, 19, 1413.
(8) Drago, R. S.; Hirsch, M. S.; Ferris, D. C.; Chronister, C. W. J. Chem. Soc., Perkin Trans. 2 1994, 219. (9) Gander, B.; Wehrli, E.; Alder, R.; Merkle, H. P. J. Microencapsulation 1995, 12, 83. (IO) Barton, A. F. M. Handbook of Solubilify Parameteres and Other Cohesion Parameters; CRC Press: Boca Raton FL, 1982. ( I 1) Drago, R. S.; Dadmun, A. P.; Vogel, G.C. 2norg.Chem. 1993,32, 2473. (12) Drago, R. S. J. Chem. Soc., Perkin Trans. 2 1992, 1827
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