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Evaluation of biodegradable microspheres prepared by a solvent evaporation process using sodium oleate as emulsifier
119
Journal of Controlled Release, 3 (1986) Elsevier
Science
Publishers
B.V.,
119-130 Amsterdam -Printed
in The Netherlands
EVALUATION OF BIODEGRADABLE MICROSPHERES PREPARED BY A SOLVENT EVAPORATION PROCESS USING SODIUM OLEATE AS EMULSIFIER Jones W. Fang”, Josephine P. Nazareno, Jane E. Pearson and Hawkins V. Maulding Pharmaceutical (Received
Development
February
Department,
19, 1985; accepted
Sandoz
Research Institute,
East Hanover,
NJ 07936
(U.S.A.1
in revised form June 11, 1985)
Biodegradable microspheres were prepared by a solvent evaporation (emulsion) process using sodium oleate as the emulsifier. Drugs with aqueous solubility of about 0.02 mg/ml or less were successfully microencapsulated to give reproducible, controlled release. Drug release was found to be affected by drug loading, polymer molecular weight, polymer composition and polymer concentration in the organic phase of the emulsion.
INTRODUCTION
This work presents results obtained from biodegradable microspheres prepared by a solvent evaporation process using sodium oleate as the emulsifier [l] . The preparation of microspheres by evaporation of the organic solvent from an emulsion has been disclosed in the patent literature [2, 31. In recent years these processes have been used to prepare microcapsules from biodegradable polymers for a number of applications [4-g]. The core material, drug, is either dissolved or dispersed together with the dissolved wallforming polymer in a volatile, water-immiscible organic solvent. This organic phase is mixed with an aqueous solution containing an emulsifier to form an o/w dispersion. Microspheres are obtained upon evaporation of the organic solvent, usually under reduced pressure. The emulsifiers commonly employed in these processes are hydrophilic colloids, such as gelatin, polyvinyl alcohol, and methylcellulose. The use of gelatin may result in a low yield of microspheres with diameters less *To whom
correspondence
0168-3659/86/$03.50
should
be addressed.
0 1986
Elsevier
Science
than 150 I.trn [ 1, lo]. This represents the maximum size (150 pm) suitable for injection through a 20 gauge syringe needle. The low yield was due to a coarse powder resulting from aggregation of smaller microspheres. Larger particle sizes and aggregates were sometimes noted with other emulsifier systems [4, 61. Polyvinyl alcohol has been used successfully in several recent investigations [5, 9, 11, 121. A recent publication listed a mixture of polyvinyl alcohol and sodium lauryl sulfate as the emulsifier in this process [13]. A number of other nonionic and anionic emulsifiers were evaluated by this microencapsulation process in these laboratories. The fatty acid salt emulsifiers (soaps) were found to be suitable for the preparation of microspheres from biodegradable polymers. Fatty acids are endogenous lipids and residual amounts of these compounds in the microspheres should be pharmaceutically acceptable. Microspheres prepared by this process, using sodium oleate as the emulsifier, elicited no abnormal tissue response when injected intramuscularly into rats and dogs.
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120
EXPERIMENTAL Materials
Thioridazine base and ketotifen hydrogen fumarate were obtained at Sandoz, Inc., East Hanover, NJ. Steroids were obtained from Sigma Chemical Company, St. Louis, MO. Sodium oleate (Purified Grade) was obtained from Fisher Scientific Company, Springfield, NJ. Polymers were prepared by Sandoz, Ltd. (Basle, Switzerland), Battelle Columbus Laboratories (Columbus; OH), and Southern Research Institute (Birmingham, AL). Polymer molecular weights were determined by gel permeation chromatography (GPC) at Sandoz, Inc., Battelle Columbus Laboratories and Sandoz Ltd.
before adding the microcapsule sample. The top cover was raised for introducing the dissolution sample. Adding the sample through one of the openings of the kettle cover was less satisfactory because some of the microcapsules could remain on the neck of the cover. Periodically, the level of the dissolution media was checked as well as ascertaining that none of the sample was adhering to the wall of the kettle. Aliquots were withdrawn with a 10 mL pipet fitted with a 3 cm length of Tygon tubing packed with glass wool to filter any microcapsules. Half of the withdrawn sample was allowed to drain through the glass wool filter to flush any microcapsules back into the dissolution mixture. After removing the glass wool filter, the remaining aliquot was saved in 15 mL bottles for analysis.
Chemicals and solvents Dissolution medium
These materials were of reagent less otherwise specified.
DISSOLUTION
grade un-
TEST PROCEDURE
A phosphate buffer solution of pH 7.4 was prepared from 1.65 g of NaH2P04-H,O and 16.60 g of Na2HP04- 7H20 dissolved in 1 L of water. The pH was adjusted to 7.4 with concentrated H3P04.
Dissolution test apparatus Thioridazine microspheres
The apparatus used for the in vitro dissolution studies consisted of a two-piece 1 L reaction kettle (Fisher Scientific Co., Cat. No. 11847B). The top cover has four openings, three of which were stoppered and the center one left open for the stirrer shaft. The exteriors of both the top and bottom pieces were painted black to allow use with lightsensitive drugs. Stirring was provided by a three-blade, polyethylene stirrer shaft (Fisher Scientific Co., Cat. No. 14-5X3-75) attached to a variable-speed stirring motor (T-Line Laboratory Stirrer Model 101, Talboys Engineering Corp., Emerson, NJ). The dissolution apparatus was placed in a water bath (Thelco Water Bath Model 83, GCA/Precision Scientific Co., Chicago, IL) heated at 37°C. The dissolution medium was equilibrated at this temperature for one hour
A solution of 1.0 g of thioridazine free base and 1.0 g of poly( D,L-la&de) in 10 mL of methylene chloride was emulsified with an aqueous solution of 0.4 g of sodium oleate in 100 mL of distilled water. Other samples were prepared at different polymer concentrations by dissolving the same amount of polymer in different volumes of methylene chloride. The organic solvent was removed by rotary evaporation, 150 rpm, 375 mmHg, at 40°C for 2 h. The product was filtered, washed with water and vacuum dried, 30°C. The yield was 1.59 g (80%) of discrete microspheres with diameter of 15-85 pm. Drug loading was 44.4% (theoretical 50%). The release of thioridazine from the microspheres was determined by placing a sample containing the equivalent of 5 mg thioridazine
121
in 1 L of pH 7.4 phosphate buffer (solubility, 23 mg/L). The mixture was maintained at 37°C with stirring at 300 rpm. Aliquots were withdrawn at various time points and assayed by measuring at the h,, for thioridazine, 265 nm, with a Cary Model 14 spectrophotometer. Ketotifen microspheres
A dispersion of 1 .O g of ketotifen hydrogen fumarate and 1.0 g of poly( D,L-lactide) in 10 mL of methylene chloride was mixed with 47 mL of 0.1 N NaOH (amount required to convert the drug to its free base). Other samples were prepared at different polymer concentrations by dissolving the same amount of polymer in different volumes of methylene chloride. The neutral mixture was then emulsified with an aqueous solution of 0.4 g of sodium oleate in 53 mL of distilled water. The organic solvent was removed by rotary evaporation, 150 rpm, 375 mmHg, at 40°C for 2 h. The product was cooled, filtered, washed with water and vacuum dried, 30°C. The yield was 1.28 g (74% calculated as free base) of discrete microspheres with diameter of 15-75 pm. Drug content was 33.7% (theoretical 42.1%). The release rate was determined by placing microspheres containing the equivalent of 20 mg ketotifen base in 1 L of pH 7.4 phosphate buffer (solubility, 247 mg/L). The mixture was maintained at 37°C with stirring at 300 rpm. Aliquots were withdrawn at various time points and assayed by measuring at the h max for ketotifen, 300 nm, with a Cary Model 14 UV spectrophotometer. Hydrocortisone acetate microspheres
A dispersion of 0.15 g of hydrocortisone acetate and 0.5 g of 50:50 poly(o,~-lactideco-glycolide) in 5 mL of methylene chloride was magnetically stirred for 0.5 h. Other samples were prepared at different polymer concentrations by dissolving the same amount of polymer in different volumes of methylene
chloride. An aqueous solution of 0.2 g of sodium oleate in 50 mL of distilled water was added to the dispersion with stirring to form an oil-in-water emulsion. The organic solvent was removed by rotary evaporation, 150 rpm, 375 mmHg, at 20°C for 1 h. The product was filtered, washed with water and vacuum dried at 30°C. The yield was 0.58 g (89%) of discrete microspheres with diameter of 15-75 pm. Drug loading was 21.5% (theoretical 23.1%). The release rate of hydrocortisone acetate from the microspheres was determined by placing a sample containing the equivalent of 5 mg drug (solubility, 14 mg/L) in 1 L of pH 7.4 phosphate buffer. The mixture was maintained at 37°C with stirring at 300 rpm. Aliquots were withdrawn at various time points and assayed by measuring at the X,,, for hydrocortisone acetate, 240 nm, using a Cary Model 14 UV spectrophotometer.
RESULTS AND DISCUSSION
The following criteria are useful in evaluating a microencapsulation process: (1) high yields of discrete microspheres with minimal agglomeration, (2) reproducible drug release profiles from batch-to-batch and (3) ability to modify the release rate by varying process parameters. The process described in this paper was used to prepare biodegradable microspheres which were free of agglomeration. High yields (74-96s) with high efficiency of drug encapsulation (80-99%) were routinely obtained. Core loadings as high as 50% were attained along with prolonged in vitro release. Most of the microspheres (>80%) prepared by this process were less than 150 pm in diameter, making them suitable for injectable pharmaceutical applications (20 gauge needle). Figure 1 illustrates typical microcapsules fabricated by the emulsion process. Some are sectioned in order to visualize the interior of the spheres. Reproducibility of the amount of drug released as a function of time was found with
122
Fig. 1. Scanning
electron
micrographs
of hydrocortisone
this 1process. Four separate batches of microan ergot derivative were spher es containing lred under identical conditions. The in prepa vitro dissolution profiles in Fig. 2 show that
acetate
microcapsules.
the first 70% of drug was released in a reproducible manner. Similar reproducibility in drug release (28% from average of two batches) was demonstrated by thioridazine micro-
123
Fig. 2. Reproducibility of drug release from four batches of microspheres. All samples were prepared with DL-PL (M, 51,200) at 10% polymer solution concentration (1 g/10 mL) and contained ca. 46% drug. Curve represents the average of the four batches. Points represent one dissolution study per batch (each ca. 20-110 pm).
Fig. 3. In vitro-in uiuo comparison of thioridazine release. Key: (a) in vitro (average of 2 runs); (0) in vivo (average of 3 dogs). The same batch was used in both studies. The sample (25-75 rm) was prepared with DL-PL (M, 51,200) and contained 53% drug.
spheres prepared from both poly( D,L -1actide) and poly(L-lactide) and also by hydrocortisone acetate microcapsules prepared from poly( D,L-lactide-co-glycolide). In vitro-in uiuo correlation was attempted with thioridazine microspheres. Figure 3 gives the results of one batch of 53% thioridazine microspheres which was studied both by dissolution and in animals. In vitro release was measured in pH 7.4 phosphate buffer, 37°C. In uiuo release was followed by analyzing the plasma level of thioridazine in dogs. The in vitro duration of release was 10 days with 50% released during the first 3 days. Plasma levels were detectable for 6 days, peaking at 2-3 days post injection. The next aspect of this work considers means of altering the release profiles of various microspheres. A number of process parameters were studied to determine their effects on drug release. By varying parameters which modify the in uitro release patterns, it may be possible to prepare drugcontaining microspheres with the desired in uiuo release characteristics. The size distribution of various batches of microspheres prepared by this process was generally in the range of 20-150 E.tm. Most of the microspheres (>70%) obtained in any batch was observed by microscopic examination to be in the more narrow range of 50-120 pm (see example in next paragraph). This size distribution of the microspheres was not greatly influenced by changes in process parameters such as stirring speed, initial polymer solution concentration, and emulsifier concentration. This may be due to the high emulsifier efficiency of sodium oleate used in this process. A batch of 46%loaded microspheres was sieved into three fractions: 13% of 2085 pm, 28% of 85-106 pm, 59% of 106120 pm. No significant difference was observed in the release patterns of the three fractions. It is possible that the high core loading of these microspheres has a greater influence on drug release than the size differences in this narrow size range (see Figs.
124
Fig. 4. Effect of core loading on ketotifen release. Key: (A) 34%, lo-75 pm; (0) 49%, 15-75 pm. Both microsphere samples were prepared with DL-PL (M, 51,200) at 10% polymer solution concentration (1 g/10 mL). Points represent one dissolution study on a single batch.
Fig. 6. Effect of polymer solution concentration (in the organic phase of the emulsion) on the release from 23% hydrocortisone acetate microspheres. Key: (0) 1 g DL-PLG/lO mL CH,Cl,, 15-75 pm; (A) 1 g/5 mL, 20-125 Mm; (0) 1 g/3.3 ml;, 15-200 Mm. All samples were prepared with 50:50 DL-PLG (M, 20,200). Points represent one dissolution study on a single batch.
4-6 for effect of core loading). Therefore, the samples were not sized by sieving for the release rate studies reported in this investigation. Core material
Fig. 5. Effect of polymer solution concentration (in the organic phase of the emulsion) on the release from 40% hydrocortisone acetate microspheres. Key: (0) free drug crystals alone; (0) 1 g DL-PLG/ZO mL CH,Cl,, 15-65 pm; (A) 1 g/13.3 mL, 10-100 firn; (9) 1 g/10 mL, lo-70 pm; (0) 1 g/8.3 mL, 15-110 urn. All samples were prepared with 50:50 DL-PLG (M, 20,200). Points represent the average of two dissolution studies on a single batch.
The size of the encapsulated molecules may be important for large molecules such as the peptides. The drugs employed in this work have molecular weights of about 300400 and the differences in their molecular size would not be expected to have much effect on drug release. Two drugs possessing amino groups were successfully microencapsula~d with biodegradable polymers by this procedure. These were the free bases of Mellaril@ (thioridazine), and Zaditen@ (ketotifen). This technique was used to microencapsulate five steroids with 50:50 poly(D,L-lactideco-glycolide), DL-PLG. Of these, only the release of hydrocortisone acetate microspheres was appreciably retarded. The dissolution profiles obtained from microspheres containing cortisone, cortisone acetate, prednisolone and prednisone were almost identical to those of the nonencapsulated drugs.
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TABLE
1
Drug solubility
in aqueous
media Aqueous
Solubilitya,
Dmiz
medium
Temp., “C
m&L Thioridazine Ketotifen Ketotifen Hydrocortisone acetate Cortisone Cortisone acetate Prednisone Prednisolone aObtained from ultraviolet Cary 14 spectrophotometer
0.023 0.017 0.247 0.014 0.304 0.080 0.256 0.309 absorbances
of filtered
pH pH pH pH pH pH pH pH solutions
7.4 9.2 7.4 7.4 7.4 7.4 7.4 7.4
equilibrated
phosphate phosphate phosphate phosphate phosphate phosphate phosphate phosphate overnight
buffer buffer buffer buffer buffer buffer buffer buffer
37 37 37 37 37 37 37 37
with an excess of drug using a
.
The differing degrees of success in microencapsulating these core materials may be partially related to their solubilities in aqueous media as listed in Table 1. Drugs with solubilities in the range of thioridazine (0.023 mg/ mL or less) were successfully microencapsulated to give retarded release. When steroids which were more water soluble than hydrocortisone acetate (e.g., cortisone, cortisone acetate, prednisone and prednisolone) were microencapsulated, considerable amounts of free drug crystals were present. Droplets of the emulsions (prior to the solvent evaporation step) containing these more soluble steroids were examined with a polarized light microscope. During evaporation of the organic solvent from these droplets an iridescent crystal growth was observed on the surface of the microspheres and in the aqueous continuous phase. These nonencapsulated drug particles probably account for the rapid release of these more soluble steroids. Technically, it is the drug solubility in the aqueous emulsifier solution which should be considered. The solubility data in Table 1 were measured in pH 7.4 buffer to determine “sink conditions” for the in vitro dissolution experiments. These solubility values may serve as approximations of drug solubility in the sodium oleate emulsifier solution, but the actual drug solubility should be somewhat
higher due to the effect of the emulsifer. However, this enhanced solubility due to surfactant could be counteracted (to some extent) by amine-containing drugs, which usually have lower solubilities in alkaline solution (the pH of sodium oleate solution is 10). For example, the solubility of ketotifen at pH 9.2 (0.017 mg/mL) is much less than that at pH 7.4 (0.247 mg/mL), Table 1. The drug solubility in water, or preferably the solubility at the pH of the emulsifier, could serve as one guide for selecting drug candidates for microencapsulation by this solvent evaporation method. As mentioned above, the pH of the sodium oleate emulsifier solution is 10. Since these polymers degrade by hydrolysis, it is possible that the molecular weight of the polymer can be decreased by the alkaline pH of the emulsifier solution during the fabrication step. This in turn could affect the release kinetics of the microspheres. Indeed, drug release was found to be enhanced by the addition of alkali salt to the aqueous phase of the emulsion prior to the solvent evaporation step [14]. The amount of drug released as a function of time was proportional to the amount of alkali salt added. All the microsphere batches in this investigation were prepared under identical
126
encapsulation conditions as given in the Experimental section: same amount and concentration of emulsifier solution, stirring speed, vacuum pressure, solvent evaporation time and temperature (except for the temperature effect study). Therefore, any effect due to the alkaline emulsifier solution should be the same because the exposure time to the emulsifier solution was the same for all the microsphere batches. Also, identical encapsulation conditions ensure that the rate of solvent evaporation would be the same during the fabrication step of the microspheres. This should minimize the effect due to differences in any physical characteristics (such as pore size) of the microspheres. It was found that the use of identical encapsulation conditions was the key factor for obtaining reproducibility of drug release. Core loading
The amount of drug released as a function of time is expected to be greater with an increase in core loading because there would be less polymer to act as a barrier. For example, 49% ketotifen microspheres exhibited a dissolution profile indicating more release at a specific time than those with 34% drug (Fig. 4). Similarly, 40% hydrocortisone acetate microspheres released more than the 23% loaded microspheres at a given time-point (Figs. 5 and 6). Initial polymer solution concentration
The initial concentration of the polymer in solution in the organic phase of the emulsion can have a significant effect on the release profile of a drug. Release from the microspheres was found to decrease when the initial polymer solution concentration was increased. This was accomplished by dissolving a given amount of polymer in decreasing volumes of the organic solvent during processing. The extent of this effect was found to be dependent on the drug employed.
Fig. 7. Effect of polymer solution concentration (in the organic phase of the emulsion) on thioridazine base release. Key: (0) 1 g DL-PL/SO mL CH,Cl,, 15--100 Mm; (0) 1 g/10 mL, 15-90 pm. All microsphere samples were prepared with DL-PL (M, 51,200) and contained ca. 43% drug. Points represent one dissolution study on a single batch.
Drug release from 43% thioridazine microspheres was decreased when the initial polymer solution concentration in the organic phase was increased (Fig. 7). Increasing the polymer (DL-PL) solution concentration in the organic phase of the emulsion from 5 to 10% (1 g/20 mL to 1 g DL-PL/lO mL methylene chloride) caused a decrease in the amount of drug released at specific times. The effect of initial polymer solution concentration on dissolution was minimal with both 40 and 23% hydrocortisone acetate microcapsules (Figs. 5 and 6). The release profiles of 40% hydrocortisone acetate microcapsules (Fig. 5) were almost identical for batches prepared with 5, 7.5 and 10% DLPLG solution concentrations (1 g/20 ml, 1 g/13.3 mL and 1 g DL-PLG/lO mL methylene chloride). Microcapsules prepared with 12% DL-PLG solution concentration (1 g/8.3 mL) showed a slightly slower release. The release patterns of 23% hydrocortisone acetate microcapsules (Fig. 6) were almost identical for those prepared with 10 and 20% DL-PLG solution concentrations (1 g/10 mL
127
and 1 g DL-PLG/5 mL). Microcapsules prepared with 30% DL-PLG solution concentration (1 g/3.3 mL) exhibited a slightly decreased release pattern. The extent of the effect of initial polymer solution concentration on drug release may be due to more than just drug dependence. It may also be due to the difference in the solubility of thioridazone and hydrocortisone Thioridazine was completely disacetate. solved in the polymer solvent and formed homogeneous microspheres. Heterogeneous microcapsules were formed with hydrocortisone acetate because the drug was insoluble in the polymer solution. It may be that the effect of initial polymer solution concentration on drug release is greater for a drug which is soluble in the polymer solution than for one which is insoluble.
Da’.‘5
Fig. 8. Effect of DL-PL molecular-weight on thioridazine base release. Key: (0) M, 51,200, 15-110 pm; (A) M, 170,000, 30-140 urn. Both microsphere samples were prepared with 13% polymer solution concentration (1 g/7.7 mL) and contained ca. 53% drug. Points represent the average of two dissolution studies on a single batch.
Polymer molecular weight
The release profiles of drug-containing microspheres are sometimes influenced by the molecular weight of the polymer. Amounts released were observed to decrease with an increase in polymer molecular weight using chlorpromazine as the drug [13] . For 46% thioridazine microspheres the effect of molecular weight on release was insignificant (Fig. 8). Microspheres were prepared from two batches of DL-PL (M, = 51,000; 170,000). The release patterns were the same for both samples during the first 50% of drug release. The remaining drug was released more slowly from the M, = 170,000 DL-PL. Polymer composition
The composition of the polymer used for preparing the microspheres can obviously affect the release profiles. The influence of polymer composition on release rates may be due to a number of polymer properties such as crystallinity, degradation rate, monomer ratio, permeability, glass transition temperature and solubility of the drug in the polymer [15-171. In assessing the effect of polymer composition, structural configuration of the polymer was not considered in this work. When the polymer has a slow degradation rate, the predominant mode of drug release is theoretically one of diffusion through the polymer matrix. With faster degrading polymers the drug release can occur through diffusion and concomitant release of drug due to the degrading polymer. Examples of this latter phenomenon are copolymers of lactide-glycolide and lactide-caprolactone which have faster degradation rates than poly(D,L-lactide) [17]. In this work, the release of drugs from microspheres made with poly( L-lactide), LPL, was more rapid than those made with poly(D,L-lactide), DL-PL. This was found to be the case with thioridazine (Fig. 9) and ketotifen (Fig. 10) These results appear to
I
I
I
1
,
I
I
I
Fig. 9. Effect of polymer composition on thioridazine base release. Key: (0) L-PL (M, 77,800), 20100 pm; (0) 9O:lO L-PLG (M, 215,000), 15-125 pm; (A) 75:25 DL-PLG (M, 130,000), 20-85 pm; 83,000), 25-125 pm; (0) (A) 75:25 L-PLG (M, urn. All microsphere DL-PL (M, 51,200), 15-110 samples contained ca. 53% drug. Points represent the average of two dissolution studies on a single batch. 1 i) 0
?n
Fig. 10. Effect of polymer composition on ketotifen release. Key: (A) 75:25 DL-PLG (M, 130,000), 15150 Frn; (0) 75:25 L-PLG (M, 83,000), 20-125 burn; (0) L-PL (M, 77,800), 15-85 pm; (n) DL-PL (M, 51,200), lo--75 pm. All microsphere samples contained ca. 37% drug. Points represent one dissolution study on a single batch.
be inconsistent with the discussion in the above paragraph since the degradation of L-PL is slower than that of DLPL [16]. They might also not be expected from consideration of polymer crystallinity. Drug diffusion was reported to be reduced by an increase in the crystallinity of the polymer [15]. The more crystalline L-PL would be expected to be slower releasing than the amorphous DLPL. Evidently there are factors other than degradation and crystallinity of the polymer which can influence the release profiles of the drug under the conditions employed in this work. It is known that other factors, such as channels or pores, can have a dominant effect on drug release. It has also been noted in these laboratories that amino groups of encapsulated basic medicaments can catalyze polyester hydrolysis [ 181. Two types of copolymers were investigated for their effects on release rate. These were the copolymers of lactide/glycolide (PLG) and lactide/e-caprolactone (PLCAP). Both types of copolymers gave microspheres which were faster releasing than DL-PL, as would be expected from consideration of polymer degradation rate were it the key factor. Three PLG copolymers were used to prepare 53% thioridazine microspheres (Fig. 9). The release profiles from both 75:25 DLPLG and 75:25 L-PLG microspheres were faster than those from DL-PL but slower than those from L-PL. Microspheres from the third copolymer, 9O:lO L-PLG, released thioridazine rapidly, almost at the same rate as those from L-PL. Two of the PLG copolymers were also used to prepare 37% ketotifen microspheres (Fig. 10). The release patterns for both 75:25 DL-PLG and 75:25 L-PLG microspheres were faster than those from both DL-PL and L-PL. From the second group of copolymers, two PLCAP copolymers were used to prepare 53% thioridazine microspheres (Fig. 11). The release rates for both 9O:lO DL-PLCAP and 80: 20 L-PLCAP microspheres were faster than those from DL-PL but slower than those from L-PL.
129
using evaporation 40°C.
100
90
70
: 10 2 iz :6
60
50
40
30
20
10
0
of O”, 20” and
CONCLUSIONS
80
0
temperatures
I 2
I 4
I
6
I 8
I 10
I
12
Days
Fig. 11. Effect of polymer composition on thioridazine base release. Key: (0) L-PL (M, 77,800), 20100 pm; (0) SO:20 L-PLCAP (M, 244,700), 30240 urn; (A) 9O:lO DL-PLCAP (M, 246,000), 20150 Grn; (0) DL-PL (M, 51,200),15--110 pm. All microsphere samples contained ca. 53% drug. Points represent the average of two dissolution studies on a single batch.
Solvent evaporation temperature
The effect of temperature on the release of drug from 43% thioridazine microspheres was not pronounced. For thioridazine microspheres prepared at O”, 30” and 4O”C, the plots were coincidental during the first 70% of drug release. Temperature of evaporation did not appear to have any effect on release rates of hydrocortisone acetate microcapsules. These were prepared at both 23% and 40% loading levels
Drugs with solubility of 0.02 mg/mL or less in aqueous media were successfully microencapsulated by a solvent evaporation process using sodium oleate as the emulsifier. High yields (74-96%) of biodegradable microspheres, free of agglomeration, were usually obtained in a size range suitable for injectable applications. In vitro release patterns can be modified by varying a number of process parameters. With thioridazine, release rates were decreased by increasing polymer solution concentration in the organic phase of the emulsion. Polymer molecular weight as well as solvent evaporation temperature did not affect the release rates in the cases reported. Polymer composition obviously has an effect on drug release from biodegradable microcapsules. Drug release was frequently slowest with microspheres made from the amorphous DL-PL and faster with those from the crystalline L-PL. Microspheres prepared from PLG or PLCAP gave drug release faster than those prepared from DL-PL in this work. It is difficult to attribute drug release to specific parameters when a great number of variables are present as was the situation in a number of the systems presented in this work. Each individual system must be carefully investigated to obtain the drug release coupled with the polymeric biodegradation rate required for it. REFERENCES J.W. Fong, Process for preparatlon of microspheres, U.S. Patent 4,384,975, May 24, 1983. M.N. Vrancken and D.A. Claeys, Process for encapsulating water and compounds in aqueous phase by evaporation, U.S. Patent 3,523,906, August 11,197O. M. Morishita, Y. Inaba, M. Fukushima, Y. Hattori, S. Kobari and T. Matsuda, Process for enPatent medicaments, U.S. capsulation of 3,960,757, June 1,1976.
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6 7
8
9
10
11
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17
18
International Symposium on Controlled Release of Bioactive Materials, The Controlled Release Society, 1983, p. 262-265. T.R. Tice and D.H. Lewis, Microencapsulation of pharmaceuticals, U.S. Patent 4,389,330. June 21,1983. R. Suzuki and J.C. Rice, Mircoencapsulation and dissolution properties of a neuroleptic in a biodegradable polymer, poly(d,l-lactide), J. Pharm. Sci., 74 (1985) 21.-24. J.W. Fong, Process for preparation of microspheres and modification of release rate of core material, U.S. Patent 4,479,911, October 30, 1984. C.G. Pitt, A.R. Jeffcoat, R.A. Zweidinger and A. Schindler, Sustained drug delivery systems. I. The permeability of poly(e-caprolactone), poly(D.L-lactic acid), and their copolymers, J. Biomed. Mater. Res., 13 (1979) 497-507. C.G. Pitt, M.M. Gratzl, A.R. Jeffcoat, R. Zweidinger and A. Schindler, Sustained drug delivery systems. II. Factors affecting rates of release from poly(e-caprolactone) and related biodegradable polymers, J. Pharm. Sci., 68 (1979) 1534-1538. A. Schindler, R. Jeffcoat, G.L. Kimmel, C.G. Pitt, M.E. Wall and R. Zweidinger, Biodegradable polymers for sustained drug delivery, in: E.M. Pearce and J.R. Schaefgen (Eds.), Contemporary Topics in Polymer Science, Vol. 2, Plenum Press, New York, NY, 1977, pp. 251-289. H.V. Maulding, T.R. Tice, D.R. Cowsar, J W. Fong, J.E. Pearson and J.P. Nazareno, Biodegradable microcapsules: acceleration of polymeric excipient hydrolytic rate by incorporation of a basic medicament, J. Controlled Release, 3 (1986) 103-117.
Journal of Controlled Release, 3 (1986) Elsevier
Science
Publishers
B.V.,
119-130 Amsterdam -Printed
in The Netherlands
EVALUATION OF BIODEGRADABLE MICROSPHERES PREPARED BY A SOLVENT EVAPORATION PROCESS USING SODIUM OLEATE AS EMULSIFIER Jones W. Fang”, Josephine P. Nazareno, Jane E. Pearson and Hawkins V. Maulding Pharmaceutical (Received
Development
February
Department,
19, 1985; accepted
Sandoz
Research Institute,
East Hanover,
NJ 07936
(U.S.A.1
in revised form June 11, 1985)
Biodegradable microspheres were prepared by a solvent evaporation (emulsion) process using sodium oleate as the emulsifier. Drugs with aqueous solubility of about 0.02 mg/ml or less were successfully microencapsulated to give reproducible, controlled release. Drug release was found to be affected by drug loading, polymer molecular weight, polymer composition and polymer concentration in the organic phase of the emulsion.
INTRODUCTION
This work presents results obtained from biodegradable microspheres prepared by a solvent evaporation process using sodium oleate as the emulsifier [l] . The preparation of microspheres by evaporation of the organic solvent from an emulsion has been disclosed in the patent literature [2, 31. In recent years these processes have been used to prepare microcapsules from biodegradable polymers for a number of applications [4-g]. The core material, drug, is either dissolved or dispersed together with the dissolved wallforming polymer in a volatile, water-immiscible organic solvent. This organic phase is mixed with an aqueous solution containing an emulsifier to form an o/w dispersion. Microspheres are obtained upon evaporation of the organic solvent, usually under reduced pressure. The emulsifiers commonly employed in these processes are hydrophilic colloids, such as gelatin, polyvinyl alcohol, and methylcellulose. The use of gelatin may result in a low yield of microspheres with diameters less *To whom
correspondence
0168-3659/86/$03.50
should
be addressed.
0 1986
Elsevier
Science
than 150 I.trn [ 1, lo]. This represents the maximum size (150 pm) suitable for injection through a 20 gauge syringe needle. The low yield was due to a coarse powder resulting from aggregation of smaller microspheres. Larger particle sizes and aggregates were sometimes noted with other emulsifier systems [4, 61. Polyvinyl alcohol has been used successfully in several recent investigations [5, 9, 11, 121. A recent publication listed a mixture of polyvinyl alcohol and sodium lauryl sulfate as the emulsifier in this process [13]. A number of other nonionic and anionic emulsifiers were evaluated by this microencapsulation process in these laboratories. The fatty acid salt emulsifiers (soaps) were found to be suitable for the preparation of microspheres from biodegradable polymers. Fatty acids are endogenous lipids and residual amounts of these compounds in the microspheres should be pharmaceutically acceptable. Microspheres prepared by this process, using sodium oleate as the emulsifier, elicited no abnormal tissue response when injected intramuscularly into rats and dogs.
Publishers
B.V.
120
EXPERIMENTAL Materials
Thioridazine base and ketotifen hydrogen fumarate were obtained at Sandoz, Inc., East Hanover, NJ. Steroids were obtained from Sigma Chemical Company, St. Louis, MO. Sodium oleate (Purified Grade) was obtained from Fisher Scientific Company, Springfield, NJ. Polymers were prepared by Sandoz, Ltd. (Basle, Switzerland), Battelle Columbus Laboratories (Columbus; OH), and Southern Research Institute (Birmingham, AL). Polymer molecular weights were determined by gel permeation chromatography (GPC) at Sandoz, Inc., Battelle Columbus Laboratories and Sandoz Ltd.
before adding the microcapsule sample. The top cover was raised for introducing the dissolution sample. Adding the sample through one of the openings of the kettle cover was less satisfactory because some of the microcapsules could remain on the neck of the cover. Periodically, the level of the dissolution media was checked as well as ascertaining that none of the sample was adhering to the wall of the kettle. Aliquots were withdrawn with a 10 mL pipet fitted with a 3 cm length of Tygon tubing packed with glass wool to filter any microcapsules. Half of the withdrawn sample was allowed to drain through the glass wool filter to flush any microcapsules back into the dissolution mixture. After removing the glass wool filter, the remaining aliquot was saved in 15 mL bottles for analysis.
Chemicals and solvents Dissolution medium
These materials were of reagent less otherwise specified.
DISSOLUTION
grade un-
TEST PROCEDURE
A phosphate buffer solution of pH 7.4 was prepared from 1.65 g of NaH2P04-H,O and 16.60 g of Na2HP04- 7H20 dissolved in 1 L of water. The pH was adjusted to 7.4 with concentrated H3P04.
Dissolution test apparatus Thioridazine microspheres
The apparatus used for the in vitro dissolution studies consisted of a two-piece 1 L reaction kettle (Fisher Scientific Co., Cat. No. 11847B). The top cover has four openings, three of which were stoppered and the center one left open for the stirrer shaft. The exteriors of both the top and bottom pieces were painted black to allow use with lightsensitive drugs. Stirring was provided by a three-blade, polyethylene stirrer shaft (Fisher Scientific Co., Cat. No. 14-5X3-75) attached to a variable-speed stirring motor (T-Line Laboratory Stirrer Model 101, Talboys Engineering Corp., Emerson, NJ). The dissolution apparatus was placed in a water bath (Thelco Water Bath Model 83, GCA/Precision Scientific Co., Chicago, IL) heated at 37°C. The dissolution medium was equilibrated at this temperature for one hour
A solution of 1.0 g of thioridazine free base and 1.0 g of poly( D,L-la&de) in 10 mL of methylene chloride was emulsified with an aqueous solution of 0.4 g of sodium oleate in 100 mL of distilled water. Other samples were prepared at different polymer concentrations by dissolving the same amount of polymer in different volumes of methylene chloride. The organic solvent was removed by rotary evaporation, 150 rpm, 375 mmHg, at 40°C for 2 h. The product was filtered, washed with water and vacuum dried, 30°C. The yield was 1.59 g (80%) of discrete microspheres with diameter of 15-85 pm. Drug loading was 44.4% (theoretical 50%). The release of thioridazine from the microspheres was determined by placing a sample containing the equivalent of 5 mg thioridazine
121
in 1 L of pH 7.4 phosphate buffer (solubility, 23 mg/L). The mixture was maintained at 37°C with stirring at 300 rpm. Aliquots were withdrawn at various time points and assayed by measuring at the h,, for thioridazine, 265 nm, with a Cary Model 14 spectrophotometer. Ketotifen microspheres
A dispersion of 1 .O g of ketotifen hydrogen fumarate and 1.0 g of poly( D,L-lactide) in 10 mL of methylene chloride was mixed with 47 mL of 0.1 N NaOH (amount required to convert the drug to its free base). Other samples were prepared at different polymer concentrations by dissolving the same amount of polymer in different volumes of methylene chloride. The neutral mixture was then emulsified with an aqueous solution of 0.4 g of sodium oleate in 53 mL of distilled water. The organic solvent was removed by rotary evaporation, 150 rpm, 375 mmHg, at 40°C for 2 h. The product was cooled, filtered, washed with water and vacuum dried, 30°C. The yield was 1.28 g (74% calculated as free base) of discrete microspheres with diameter of 15-75 pm. Drug content was 33.7% (theoretical 42.1%). The release rate was determined by placing microspheres containing the equivalent of 20 mg ketotifen base in 1 L of pH 7.4 phosphate buffer (solubility, 247 mg/L). The mixture was maintained at 37°C with stirring at 300 rpm. Aliquots were withdrawn at various time points and assayed by measuring at the h max for ketotifen, 300 nm, with a Cary Model 14 UV spectrophotometer. Hydrocortisone acetate microspheres
A dispersion of 0.15 g of hydrocortisone acetate and 0.5 g of 50:50 poly(o,~-lactideco-glycolide) in 5 mL of methylene chloride was magnetically stirred for 0.5 h. Other samples were prepared at different polymer concentrations by dissolving the same amount of polymer in different volumes of methylene
chloride. An aqueous solution of 0.2 g of sodium oleate in 50 mL of distilled water was added to the dispersion with stirring to form an oil-in-water emulsion. The organic solvent was removed by rotary evaporation, 150 rpm, 375 mmHg, at 20°C for 1 h. The product was filtered, washed with water and vacuum dried at 30°C. The yield was 0.58 g (89%) of discrete microspheres with diameter of 15-75 pm. Drug loading was 21.5% (theoretical 23.1%). The release rate of hydrocortisone acetate from the microspheres was determined by placing a sample containing the equivalent of 5 mg drug (solubility, 14 mg/L) in 1 L of pH 7.4 phosphate buffer. The mixture was maintained at 37°C with stirring at 300 rpm. Aliquots were withdrawn at various time points and assayed by measuring at the X,,, for hydrocortisone acetate, 240 nm, using a Cary Model 14 UV spectrophotometer.
RESULTS AND DISCUSSION
The following criteria are useful in evaluating a microencapsulation process: (1) high yields of discrete microspheres with minimal agglomeration, (2) reproducible drug release profiles from batch-to-batch and (3) ability to modify the release rate by varying process parameters. The process described in this paper was used to prepare biodegradable microspheres which were free of agglomeration. High yields (74-96s) with high efficiency of drug encapsulation (80-99%) were routinely obtained. Core loadings as high as 50% were attained along with prolonged in vitro release. Most of the microspheres (>80%) prepared by this process were less than 150 pm in diameter, making them suitable for injectable pharmaceutical applications (20 gauge needle). Figure 1 illustrates typical microcapsules fabricated by the emulsion process. Some are sectioned in order to visualize the interior of the spheres. Reproducibility of the amount of drug released as a function of time was found with
122
Fig. 1. Scanning
electron
micrographs
of hydrocortisone
this 1process. Four separate batches of microan ergot derivative were spher es containing lred under identical conditions. The in prepa vitro dissolution profiles in Fig. 2 show that
acetate
microcapsules.
the first 70% of drug was released in a reproducible manner. Similar reproducibility in drug release (28% from average of two batches) was demonstrated by thioridazine micro-
123
Fig. 2. Reproducibility of drug release from four batches of microspheres. All samples were prepared with DL-PL (M, 51,200) at 10% polymer solution concentration (1 g/10 mL) and contained ca. 46% drug. Curve represents the average of the four batches. Points represent one dissolution study per batch (each ca. 20-110 pm).
Fig. 3. In vitro-in uiuo comparison of thioridazine release. Key: (a) in vitro (average of 2 runs); (0) in vivo (average of 3 dogs). The same batch was used in both studies. The sample (25-75 rm) was prepared with DL-PL (M, 51,200) and contained 53% drug.
spheres prepared from both poly( D,L -1actide) and poly(L-lactide) and also by hydrocortisone acetate microcapsules prepared from poly( D,L-lactide-co-glycolide). In vitro-in uiuo correlation was attempted with thioridazine microspheres. Figure 3 gives the results of one batch of 53% thioridazine microspheres which was studied both by dissolution and in animals. In vitro release was measured in pH 7.4 phosphate buffer, 37°C. In uiuo release was followed by analyzing the plasma level of thioridazine in dogs. The in vitro duration of release was 10 days with 50% released during the first 3 days. Plasma levels were detectable for 6 days, peaking at 2-3 days post injection. The next aspect of this work considers means of altering the release profiles of various microspheres. A number of process parameters were studied to determine their effects on drug release. By varying parameters which modify the in uitro release patterns, it may be possible to prepare drugcontaining microspheres with the desired in uiuo release characteristics. The size distribution of various batches of microspheres prepared by this process was generally in the range of 20-150 E.tm. Most of the microspheres (>70%) obtained in any batch was observed by microscopic examination to be in the more narrow range of 50-120 pm (see example in next paragraph). This size distribution of the microspheres was not greatly influenced by changes in process parameters such as stirring speed, initial polymer solution concentration, and emulsifier concentration. This may be due to the high emulsifier efficiency of sodium oleate used in this process. A batch of 46%loaded microspheres was sieved into three fractions: 13% of 2085 pm, 28% of 85-106 pm, 59% of 106120 pm. No significant difference was observed in the release patterns of the three fractions. It is possible that the high core loading of these microspheres has a greater influence on drug release than the size differences in this narrow size range (see Figs.
124
Fig. 4. Effect of core loading on ketotifen release. Key: (A) 34%, lo-75 pm; (0) 49%, 15-75 pm. Both microsphere samples were prepared with DL-PL (M, 51,200) at 10% polymer solution concentration (1 g/10 mL). Points represent one dissolution study on a single batch.
Fig. 6. Effect of polymer solution concentration (in the organic phase of the emulsion) on the release from 23% hydrocortisone acetate microspheres. Key: (0) 1 g DL-PLG/lO mL CH,Cl,, 15-75 pm; (A) 1 g/5 mL, 20-125 Mm; (0) 1 g/3.3 ml;, 15-200 Mm. All samples were prepared with 50:50 DL-PLG (M, 20,200). Points represent one dissolution study on a single batch.
4-6 for effect of core loading). Therefore, the samples were not sized by sieving for the release rate studies reported in this investigation. Core material
Fig. 5. Effect of polymer solution concentration (in the organic phase of the emulsion) on the release from 40% hydrocortisone acetate microspheres. Key: (0) free drug crystals alone; (0) 1 g DL-PLG/ZO mL CH,Cl,, 15-65 pm; (A) 1 g/13.3 mL, 10-100 firn; (9) 1 g/10 mL, lo-70 pm; (0) 1 g/8.3 mL, 15-110 urn. All samples were prepared with 50:50 DL-PLG (M, 20,200). Points represent the average of two dissolution studies on a single batch.
The size of the encapsulated molecules may be important for large molecules such as the peptides. The drugs employed in this work have molecular weights of about 300400 and the differences in their molecular size would not be expected to have much effect on drug release. Two drugs possessing amino groups were successfully microencapsula~d with biodegradable polymers by this procedure. These were the free bases of Mellaril@ (thioridazine), and Zaditen@ (ketotifen). This technique was used to microencapsulate five steroids with 50:50 poly(D,L-lactideco-glycolide), DL-PLG. Of these, only the release of hydrocortisone acetate microspheres was appreciably retarded. The dissolution profiles obtained from microspheres containing cortisone, cortisone acetate, prednisolone and prednisone were almost identical to those of the nonencapsulated drugs.
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TABLE
1
Drug solubility
in aqueous
media Aqueous
Solubilitya,
Dmiz
medium
Temp., “C
m&L Thioridazine Ketotifen Ketotifen Hydrocortisone acetate Cortisone Cortisone acetate Prednisone Prednisolone aObtained from ultraviolet Cary 14 spectrophotometer
0.023 0.017 0.247 0.014 0.304 0.080 0.256 0.309 absorbances
of filtered
pH pH pH pH pH pH pH pH solutions
7.4 9.2 7.4 7.4 7.4 7.4 7.4 7.4
equilibrated
phosphate phosphate phosphate phosphate phosphate phosphate phosphate phosphate overnight
buffer buffer buffer buffer buffer buffer buffer buffer
37 37 37 37 37 37 37 37
with an excess of drug using a
.
The differing degrees of success in microencapsulating these core materials may be partially related to their solubilities in aqueous media as listed in Table 1. Drugs with solubilities in the range of thioridazine (0.023 mg/ mL or less) were successfully microencapsulated to give retarded release. When steroids which were more water soluble than hydrocortisone acetate (e.g., cortisone, cortisone acetate, prednisone and prednisolone) were microencapsulated, considerable amounts of free drug crystals were present. Droplets of the emulsions (prior to the solvent evaporation step) containing these more soluble steroids were examined with a polarized light microscope. During evaporation of the organic solvent from these droplets an iridescent crystal growth was observed on the surface of the microspheres and in the aqueous continuous phase. These nonencapsulated drug particles probably account for the rapid release of these more soluble steroids. Technically, it is the drug solubility in the aqueous emulsifier solution which should be considered. The solubility data in Table 1 were measured in pH 7.4 buffer to determine “sink conditions” for the in vitro dissolution experiments. These solubility values may serve as approximations of drug solubility in the sodium oleate emulsifier solution, but the actual drug solubility should be somewhat
higher due to the effect of the emulsifer. However, this enhanced solubility due to surfactant could be counteracted (to some extent) by amine-containing drugs, which usually have lower solubilities in alkaline solution (the pH of sodium oleate solution is 10). For example, the solubility of ketotifen at pH 9.2 (0.017 mg/mL) is much less than that at pH 7.4 (0.247 mg/mL), Table 1. The drug solubility in water, or preferably the solubility at the pH of the emulsifier, could serve as one guide for selecting drug candidates for microencapsulation by this solvent evaporation method. As mentioned above, the pH of the sodium oleate emulsifier solution is 10. Since these polymers degrade by hydrolysis, it is possible that the molecular weight of the polymer can be decreased by the alkaline pH of the emulsifier solution during the fabrication step. This in turn could affect the release kinetics of the microspheres. Indeed, drug release was found to be enhanced by the addition of alkali salt to the aqueous phase of the emulsion prior to the solvent evaporation step [14]. The amount of drug released as a function of time was proportional to the amount of alkali salt added. All the microsphere batches in this investigation were prepared under identical
126
encapsulation conditions as given in the Experimental section: same amount and concentration of emulsifier solution, stirring speed, vacuum pressure, solvent evaporation time and temperature (except for the temperature effect study). Therefore, any effect due to the alkaline emulsifier solution should be the same because the exposure time to the emulsifier solution was the same for all the microsphere batches. Also, identical encapsulation conditions ensure that the rate of solvent evaporation would be the same during the fabrication step of the microspheres. This should minimize the effect due to differences in any physical characteristics (such as pore size) of the microspheres. It was found that the use of identical encapsulation conditions was the key factor for obtaining reproducibility of drug release. Core loading
The amount of drug released as a function of time is expected to be greater with an increase in core loading because there would be less polymer to act as a barrier. For example, 49% ketotifen microspheres exhibited a dissolution profile indicating more release at a specific time than those with 34% drug (Fig. 4). Similarly, 40% hydrocortisone acetate microspheres released more than the 23% loaded microspheres at a given time-point (Figs. 5 and 6). Initial polymer solution concentration
The initial concentration of the polymer in solution in the organic phase of the emulsion can have a significant effect on the release profile of a drug. Release from the microspheres was found to decrease when the initial polymer solution concentration was increased. This was accomplished by dissolving a given amount of polymer in decreasing volumes of the organic solvent during processing. The extent of this effect was found to be dependent on the drug employed.
Fig. 7. Effect of polymer solution concentration (in the organic phase of the emulsion) on thioridazine base release. Key: (0) 1 g DL-PL/SO mL CH,Cl,, 15--100 Mm; (0) 1 g/10 mL, 15-90 pm. All microsphere samples were prepared with DL-PL (M, 51,200) and contained ca. 43% drug. Points represent one dissolution study on a single batch.
Drug release from 43% thioridazine microspheres was decreased when the initial polymer solution concentration in the organic phase was increased (Fig. 7). Increasing the polymer (DL-PL) solution concentration in the organic phase of the emulsion from 5 to 10% (1 g/20 mL to 1 g DL-PL/lO mL methylene chloride) caused a decrease in the amount of drug released at specific times. The effect of initial polymer solution concentration on dissolution was minimal with both 40 and 23% hydrocortisone acetate microcapsules (Figs. 5 and 6). The release profiles of 40% hydrocortisone acetate microcapsules (Fig. 5) were almost identical for batches prepared with 5, 7.5 and 10% DLPLG solution concentrations (1 g/20 ml, 1 g/13.3 mL and 1 g DL-PLG/lO mL methylene chloride). Microcapsules prepared with 12% DL-PLG solution concentration (1 g/8.3 mL) showed a slightly slower release. The release patterns of 23% hydrocortisone acetate microcapsules (Fig. 6) were almost identical for those prepared with 10 and 20% DL-PLG solution concentrations (1 g/10 mL
127
and 1 g DL-PLG/5 mL). Microcapsules prepared with 30% DL-PLG solution concentration (1 g/3.3 mL) exhibited a slightly decreased release pattern. The extent of the effect of initial polymer solution concentration on drug release may be due to more than just drug dependence. It may also be due to the difference in the solubility of thioridazone and hydrocortisone Thioridazine was completely disacetate. solved in the polymer solvent and formed homogeneous microspheres. Heterogeneous microcapsules were formed with hydrocortisone acetate because the drug was insoluble in the polymer solution. It may be that the effect of initial polymer solution concentration on drug release is greater for a drug which is soluble in the polymer solution than for one which is insoluble.
Da’.‘5
Fig. 8. Effect of DL-PL molecular-weight on thioridazine base release. Key: (0) M, 51,200, 15-110 pm; (A) M, 170,000, 30-140 urn. Both microsphere samples were prepared with 13% polymer solution concentration (1 g/7.7 mL) and contained ca. 53% drug. Points represent the average of two dissolution studies on a single batch.
Polymer molecular weight
The release profiles of drug-containing microspheres are sometimes influenced by the molecular weight of the polymer. Amounts released were observed to decrease with an increase in polymer molecular weight using chlorpromazine as the drug [13] . For 46% thioridazine microspheres the effect of molecular weight on release was insignificant (Fig. 8). Microspheres were prepared from two batches of DL-PL (M, = 51,000; 170,000). The release patterns were the same for both samples during the first 50% of drug release. The remaining drug was released more slowly from the M, = 170,000 DL-PL. Polymer composition
The composition of the polymer used for preparing the microspheres can obviously affect the release profiles. The influence of polymer composition on release rates may be due to a number of polymer properties such as crystallinity, degradation rate, monomer ratio, permeability, glass transition temperature and solubility of the drug in the polymer [15-171. In assessing the effect of polymer composition, structural configuration of the polymer was not considered in this work. When the polymer has a slow degradation rate, the predominant mode of drug release is theoretically one of diffusion through the polymer matrix. With faster degrading polymers the drug release can occur through diffusion and concomitant release of drug due to the degrading polymer. Examples of this latter phenomenon are copolymers of lactide-glycolide and lactide-caprolactone which have faster degradation rates than poly(D,L-lactide) [17]. In this work, the release of drugs from microspheres made with poly( L-lactide), LPL, was more rapid than those made with poly(D,L-lactide), DL-PL. This was found to be the case with thioridazine (Fig. 9) and ketotifen (Fig. 10) These results appear to
I
I
I
1
,
I
I
I
Fig. 9. Effect of polymer composition on thioridazine base release. Key: (0) L-PL (M, 77,800), 20100 pm; (0) 9O:lO L-PLG (M, 215,000), 15-125 pm; (A) 75:25 DL-PLG (M, 130,000), 20-85 pm; 83,000), 25-125 pm; (0) (A) 75:25 L-PLG (M, urn. All microsphere DL-PL (M, 51,200), 15-110 samples contained ca. 53% drug. Points represent the average of two dissolution studies on a single batch. 1 i) 0
?n
Fig. 10. Effect of polymer composition on ketotifen release. Key: (A) 75:25 DL-PLG (M, 130,000), 15150 Frn; (0) 75:25 L-PLG (M, 83,000), 20-125 burn; (0) L-PL (M, 77,800), 15-85 pm; (n) DL-PL (M, 51,200), lo--75 pm. All microsphere samples contained ca. 37% drug. Points represent one dissolution study on a single batch.
be inconsistent with the discussion in the above paragraph since the degradation of L-PL is slower than that of DLPL [16]. They might also not be expected from consideration of polymer crystallinity. Drug diffusion was reported to be reduced by an increase in the crystallinity of the polymer [15]. The more crystalline L-PL would be expected to be slower releasing than the amorphous DLPL. Evidently there are factors other than degradation and crystallinity of the polymer which can influence the release profiles of the drug under the conditions employed in this work. It is known that other factors, such as channels or pores, can have a dominant effect on drug release. It has also been noted in these laboratories that amino groups of encapsulated basic medicaments can catalyze polyester hydrolysis [ 181. Two types of copolymers were investigated for their effects on release rate. These were the copolymers of lactide/glycolide (PLG) and lactide/e-caprolactone (PLCAP). Both types of copolymers gave microspheres which were faster releasing than DL-PL, as would be expected from consideration of polymer degradation rate were it the key factor. Three PLG copolymers were used to prepare 53% thioridazine microspheres (Fig. 9). The release profiles from both 75:25 DLPLG and 75:25 L-PLG microspheres were faster than those from DL-PL but slower than those from L-PL. Microspheres from the third copolymer, 9O:lO L-PLG, released thioridazine rapidly, almost at the same rate as those from L-PL. Two of the PLG copolymers were also used to prepare 37% ketotifen microspheres (Fig. 10). The release patterns for both 75:25 DL-PLG and 75:25 L-PLG microspheres were faster than those from both DL-PL and L-PL. From the second group of copolymers, two PLCAP copolymers were used to prepare 53% thioridazine microspheres (Fig. 11). The release rates for both 9O:lO DL-PLCAP and 80: 20 L-PLCAP microspheres were faster than those from DL-PL but slower than those from L-PL.
129
using evaporation 40°C.
100
90
70
: 10 2 iz :6
60
50
40
30
20
10
0
of O”, 20” and
CONCLUSIONS
80
0
temperatures
I 2
I 4
I
6
I 8
I 10
I
12
Days
Fig. 11. Effect of polymer composition on thioridazine base release. Key: (0) L-PL (M, 77,800), 20100 pm; (0) SO:20 L-PLCAP (M, 244,700), 30240 urn; (A) 9O:lO DL-PLCAP (M, 246,000), 20150 Grn; (0) DL-PL (M, 51,200),15--110 pm. All microsphere samples contained ca. 53% drug. Points represent the average of two dissolution studies on a single batch.
Solvent evaporation temperature
The effect of temperature on the release of drug from 43% thioridazine microspheres was not pronounced. For thioridazine microspheres prepared at O”, 30” and 4O”C, the plots were coincidental during the first 70% of drug release. Temperature of evaporation did not appear to have any effect on release rates of hydrocortisone acetate microcapsules. These were prepared at both 23% and 40% loading levels
Drugs with solubility of 0.02 mg/mL or less in aqueous media were successfully microencapsulated by a solvent evaporation process using sodium oleate as the emulsifier. High yields (74-96%) of biodegradable microspheres, free of agglomeration, were usually obtained in a size range suitable for injectable applications. In vitro release patterns can be modified by varying a number of process parameters. With thioridazine, release rates were decreased by increasing polymer solution concentration in the organic phase of the emulsion. Polymer molecular weight as well as solvent evaporation temperature did not affect the release rates in the cases reported. Polymer composition obviously has an effect on drug release from biodegradable microcapsules. Drug release was frequently slowest with microspheres made from the amorphous DL-PL and faster with those from the crystalline L-PL. Microspheres prepared from PLG or PLCAP gave drug release faster than those prepared from DL-PL in this work. It is difficult to attribute drug release to specific parameters when a great number of variables are present as was the situation in a number of the systems presented in this work. Each individual system must be carefully investigated to obtain the drug release coupled with the polymeric biodegradation rate required for it. REFERENCES J.W. Fong, Process for preparatlon of microspheres, U.S. Patent 4,384,975, May 24, 1983. M.N. Vrancken and D.A. Claeys, Process for encapsulating water and compounds in aqueous phase by evaporation, U.S. Patent 3,523,906, August 11,197O. M. Morishita, Y. Inaba, M. Fukushima, Y. Hattori, S. Kobari and T. Matsuda, Process for enPatent medicaments, U.S. capsulation of 3,960,757, June 1,1976.
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4
5
6 7
8
9
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