Steroid Release via Cellulose Acetate Butyrate Microcapsules

12 Steroid Release via Cellulose Acetate Butyrate

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on January 24, 2017 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0033.ch012

Microcapsules DAVID L. GARDNER, DAVID J. FINK, ALBERT J. PATANUS, WILLIAM C. BAYTOS, and CRAIG R. HASSLER Battelle, Columbus Laboratories, 505 King Ave., Columbus, Ohio 43201

Many novel systems for a controlled drug delivery are now being developed. For instance, during the last few years, several investigators have explored the p o s s i b i l i t y of preventing conception through the use of intrauterine or intravaginal administration of drugs released from reservoirs such as S i l a s t i c vaginal rings and tubes (1-5). These devices are impregnated with progestational steroids, such as medroxyprogesterone acetate (MPA), Norgestral, etc., and then inserted i n the uterus or vagina. Once inserted within the uterus or vagina, the device slowly releases the steroid, sometimes over prolonged periods of time, and i s effective i n preventing conception. An alternative controlled drug delivery system might be a system based upon transcervical or intrauterine migration of particles for either contraceptive or medicative purposes. E g l i and Newton (6) discovered that inert carbon particles, placed i n the posterior fornix of the vagina i n three women about to undergo an abdominal hysterectomy, migrated within 35 minutes into the fallopian tubes i n 2 out of the 3 cases. deBoer (7) found that when a quantity of India ink (a colloidal suspension of carbon) was placed i n the uterine cavity of patients about to undergo an abdominal surgical procedure (hysterectomy, oophorectomy, or tubal ligation), the ink particles had been transported to the fallopian tube i n more than 50 percent of the observed patients and migration of the particles from the cervical canal had occurred i n nearly 30 percent of the patients. Both of these studies indicate that inert particles can migrate from the vagi n a l tract or uterine cavity into the fallopian tube. A drug delivery system based upon the transcervical migration of particles might make use of microcapsules. Microcapsules are spherical membranes ("synthetic cells") which can vary i n diameter from a few microns to several millimeters and which can contain either a drug solution, suspension, or emulsion. This paper describes preliminary results of steroid release from cellulose acetate butyrate microcapsules.

171

Paul and Harris; Controlled Release Polymeric Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

172

CONTROLLED

RELEASE POLYMERIC

FORMULATIONS


Materials and Methods Cellulose acetate butyrate (CAB) microcapsules containing two core preparations for drug delivery were prepared by a phaseseparation technique. The f i r s t core consisted of t r i t i a t e d progesterone dissolved i n a high molecular weight o i l (olive) and emulsified i n a phosphate-saline buffer solution (pH 7.3). In the second core solution, t r i t i a t e d progesterone or estrone was suspended directly i n the aqueous medium (phosphate-saline buffer). Microcapsules were prepared i n which the core solution contained 100 pCi of t r i t i a t e d progesterone (specific activity of 50.3 C i / ramole plus 20 mg of unlabeled progesterone) or 50 yCi of t r i tiated estrone (specific activity of 48 Ci/mmole plus 20 mg of unlabeled estrone). In v i t r o release rate studies were performed on several d i f ferent preparations of progesterone and estrone microcapsules by two procedures. In the f i r s t procedure, a known weigjit of capsules was placed i n a volume of phosphate-saline buffer (pH 7.3) with the addition of Triton X-100 surfactant. The flask containing the capsules and buffer solution was then placed i n a constant-temperature bath at 37 C and agitated. Samples of the suspension were periodically removed and f i l t e r e d , and the concentration of drug i n the supernatant determined by liquid scint i l l a t i o n spectrometry. In this procedure, there was a continual increase of the drug i n the supernatant solution since the supernatant solution was not changed during the period of study. The second procedure involved exchanging the test solution daily. Drug-containing capsules and buffer were placed i n a test tube with a specially f i t t e d rubber stopper. The test tube was agitated i n a thermostatically controlled water bath (37 C). A plastic screen at the base of the stopper permitted removal of the supernatant solution while the capsules were retained i n the tube. The supernatant was withdrawn daily from the capsules and the drug concentration determined as above. Fresh test solution was added back to the tube to restart the test. Results In Vitro Progesterone Release Studies. The f i r s t study involved 200-300-lJ-diameter capsules which contained three d i f ferent concentrations of progesterone i n an oil-in-water emulsion. The progesterone concentrations for each capsule preparation were: Preparation 1 - 440 ng/100 mg capsules, Preparation 2 - 155 ng/ 100 mg, and Preparation 3 - 8 3 ng/100 mg. Release rates from 100 mg of each of these capsule preparations were measured into the following solutions (200 ml). Solution A: Phosphate-saline buffer, pH 7.35 Solution B: Phosphate-saline buffer, pH 7.35, plus 2 drops of surfactant (Triton X-100)


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Release


The presence of surfactant greatly enhanced the rate of pro gesterone transport from the capsules as shown i n Table I· The release rates i n the presence of surfactant (Figure 1) indicate a rapid i n i t i a l release of progesterone from the capsules f o l lowed by a more constant release rate. We attribute the high i n i t i a l rate either to rupturing of incompletely formed capsules or the release of surface-adsorbed progesterone into the test solution. TABLE I. RELEASE OF PROGESTERONE FROM CAPSULES CONTAINING DIFFERENT CONCENTRATIONS OF PROGESTERONE IN AN OIL/WATER EMULSION

Times, hr 1 3 6 24 48 72 144

Cumulative Nanograms of Progesterone Released/100 mg of Capsules Prep. 1 Prep. 2 Prep. 3 Solution Solution Solution Β A Β A Β A 23 24 24 30 40 53 88

56 109 122 159 170 190 210

5 18 18 23 18 28 38

20 41 46 52 70 72 91

7 13 1 9 18 21 30

15 16 36 39 41 48 63

The "constant" release rates from 24 hours to 144 hours for each batch of capsules were: Preparation 1 - 10.2 ng/day, Prep aration 2 - 7 . 8 ng/day, and Preparation 3-4.8 ng/day. The long-term release rate of Preparation 1 was roughly twice the release rate of Preparation 3, although the i n i t i a l progesterone concentration was approximately 5 times greater. This indicates that the progesterone concentration may be increased i n the cap sule without a corresponding linear increase i n release rate. Thus, a combination of factors such as drug s o l u b i l i t y , p a r t i tioning rate, and membrane transport rate may contribute to the control of the drug release rate. The second release rate study involved the release of pro gesterone from CAB microcapsules containing progesterone par t i c l e s suspended i n phosphate-saline buffer containing bovine serum albumin. In this study, 500 mg of 420-500-U capsules and 2 g of 510-700-V capsules were placed i n 5 ml of a phosphatesaline buffer solution containing 1 drop of Triton X-100 surfac tant. The 5-ml test solution was separated daily from the cap sules as described e a r l i e r and fresh test solution (phosphatesaline buffer plus surfactant) was added back to the capsules. Duplicate aliquots of the f i l t e r e d test solution were then counted to determine the progesterone release rate.




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175

Release


The results of this study (Table II) indicate that the pro gesterone release rate was considerably greater and more constant over the four test days than the release rates seen with the smaller capsules (200-300 μ) i n the f i r s t test (Table I ) . We believe that the increased transport reflects the different core preparations, i.e., emulsion (Table I) and suspension plus albu min (Table II). The micrograms of progesterone released per day approximated 2.4 pg/g for the 420-500-μ capsules and 1.6 ug/g for the 510-700-μ capsules. TABLE II. IN VITRO RELEASE RATE OF PROGESTERONE FROM MICROCAPSULES CONTAINING SOLID PROGESTERONE SUSPENDED IN A PHOSPHATE-S ALINE-ALBUMIN CORE

Time, days 1 2 3 4

420-500 μ Net CPM Released/ Day/g Capsule 18,400 13,700 12,500 12,700

Time, days 1 2 3 4

510-700 μ Net CPM Released/ Day/g Capsule 8,275 8,350 8,225 8,200

In Vivo Progesterone Release Rate. To determine the i n vivo release rate of progesterone from the above 420-500-μ-diameter capsules, 100 mg of moist capsules were placed i n the uteri of each of two rabbits v i a a simple laparotomy. 100 mg of moist capsules i s equivalent to 3.21 mg on a dry weight basis and con tained 4049 CPM/mg dry capsules. Dry weight was determined after air drying for 24 hours. The rabbits were sacrificed 4 and 6 hours after insertion of the microcapsules, and the uteri were removed and flushed with a phosphate-s aline buffer solution to recover the remaining capsules. The flushed solution was passed over a screen designed to retain this particular sized capsule. The recovered capsules were then a i r dried for 24 hours. The progesterone remaining i n the dried capsules i s shown i n Table III and a comparison between the release rate obtained i n vitro and in vivo i s shown i n Figure 2. The i n vivo progesterone release rate from these capsules i s at least 17 times greater than the i n vitro rate determined i n the preceding release rate study (Table II) using identical cap sules. We believe this increased rate i s due to the highly l i p o p h i l i c uterine environment. Lipophilic materials which penetrate the capsule wall would probably lead to increased progesterone solubilization and perhaps also to higher membrane transport rates This experiment again points out the necessity for correlation of in vivo and i n vitro transport rates when devising i n vitro testing procedures.


176


CONTROLLED RELEASE POLYMERIC

FORMULATIONS

Time (hours) Figure 2.

Comparison of in vitro and in vivo progesterone release rates from 420—500- μ-diameter microcapsules


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Release


TABLE III. PROGESTERONE-LOADED CAPSULES RECOVERED FROM RABBIT UTERI AT 4 AND 6 HOURS

CPM/mg Dried Capsule

Average CPM/mg Dried Capsule

Rabbit No,

Uterine Horn

Dry Weight of Recovered Capsules, mg

104 (4 hr)

Rt side Lf side

0.30 0.54

890 1568

2967 2904

2935

113 (6 hr)

Rt side Lf side

0.35 0.56

689 1401

1968 2502

2238

Net CPM

In Vitro Estrone Release Rate. The f i r s t estrone release rate study involved two different microcapsule sizes from the same microcapsule preparation, i . e . , 297-420 μ and 420-500 μ diameter. The test solution consisted of 5 ml of phosphatesaline buffer (pH 7.3) plus 1 drop of surfactant (Triton X-100). Two hundred mg of capsules were added to the test solution and the tubes were agitated i n a thermostatically controlled water bath (37 C). Two hundred mg of capsules contained approximately 90 μg of estrone. The results of this study are presented i n Table IV. TABLE IV. ESTRONE RELEASE FROM TWO DIFFERENT MICROCAPSULE SIZES

Sampling Time, hr

297-420 μ yg Released/ 200 mg Capsules

420-500 μ yg Released/ 200 mg Capsules

24 48 72 96

4.6 4.4 3.5 2.9

6.0 4.5 2.7 1.1

The results suggested that the release rate was nonlinear over the time of release monitored. To determine i f a zero-order release rate could be obtained over a longer time period, dupli cate 500-mg capsule samples (A and B) were placed i n 5 ml of phosphate-saline buffer solution containing a surfactant (5 drops of Triton X-100/1000 ml). These capsules (297-420 μ), however, were of a different microcapsule preparation than those used i n Table IV and contained 290 μg estrone/500 mg capsules. The results of the release rate study are presented i n Figure 3 and the single point represents the average between duplicate samples.




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179

It i s clearly evident from this study that a near zero-order release rate may be obtained from cellulose acetate butyrate microcapsules. The reason for the nonlinear release rate over the f i r s t 8 days i s believed to be due either to estrone entrapped in the membrane wall or the rupturing of imperfect microcapsule walls. After the i n i t i a l period, a zero-order release rate i s observed during which the drug inside the capsule i s maintained at a fixed concentration, i . e . , at i t s solubility l i m i t .


Discussion The encapsulation of the oil-in-water emulsion i s s i g n i f i cant since this approach permits encapsulation of hydrophobic steroids by the nonaqueous phase-separation method. Prior to this, the nonaqueous phase-separation method had been limited to hydrophilic materials. T* o release of progesterone was significantly slower than that observed in the single i n vivo study. This difference in release rates i s not surprising i n view of the highly l i p o p h i l i c milieu which exists i n the rabbit uterus. If some l i p o p h i l i c substances penetrate the capsule wall, the solubility and membrane transport rate for progesterone should be increased. The penetration of l i p o p h i l i c materials through the capsule membrane should not prevent the realization of a zero-order release rate since the primary requirement governing zero-order release from a reservoir device is the maintenance of saturation concentrations within the device. This was accomplished in these studies since the solubility of progesterone or estrone i s much lower in an aqueous environment (9 yg/ml for progesterone and 30 yg/ml for estrone) (8) than the total amount of steroid encapsulated. Theoretically, we could maintain the saturated concentration level within the capsule u n t i l the drug i s almost completely voided. Thus, i n a reservoir device such as a microcapsule, a zero-order release rate should be feasible but other parameters may affect the release rate. These parameters might include the total number and size distribution of the microcapsule, the microcapsule wall thickness, the type of polymer used for the microcapsule membrane, the rate of drug solubilization within the microcapsule, and the in vivo rate of drug removed from the drug interface area surrounding the microcapsule. Table V was constructed to determine i f the preliminary release rates from cellulose acetate butyrate microcapsules might be satisfactory for use as a transcervical drug delivery device. This table depicts release rates required of microcapsules and assumes different percentages of transcervical migration and possible effective localized drug dosages. In addition, we have examined three different drug delivery rates which might be required at a localized s i t e . The amount of drug which w i l l be required at a localized site of action i s relatively unknown. However, estimates of the 10

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25 50 100

0.02 0.04 0.09

0.25 0.50 1.00

10

150

100 300 500

0.09 0.27 0.45

1.0 3.0 5.0

150

100

10

10

250

1 1 1

10 10 10

51,282 153,840 256,410 12,820 25,641 51,282

50

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50

51,282

100

1,386,962

100

0.09

0.09

1.0

10

150

50

Number of Capsules Migrating

Assumed Drug Delivery Rate Required at Localized Site, ug/day

Weight of Capsules Migrating, mg

1.0

0.09

Volume of Capsules Migrating, ml

1.0

Assumed Trans cervical Migration, percent

10

Capsules Placed In Vagina, 8

4.0 2.0 1.0

10 3.3 2.0

50

50

50

Release Rate Required from Migrating Capsules, yg/day

RELEASE RATES REQUIRED OF MICROCAPSULES WHEN COMPARING DIFFERENT PERCENTAGES OF TRANSCERVICAL MIGRATION AND EFFECTIVE LOCALIZED DRUG DOSAGES

50

Capsule Diam., y

TABLE V·



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181

amount of progesterone required per day to prevent conception indicate that only micrograms or even nanograms of a more potent drug might be sufficient. Roland (9) has shown that 75 y g of Norgestral orally administered per day to humans effectively pre vented pregnancy by altering the cervical mucosa, thus prevent ing sperm migration into the uterine cavity. In addition, S i l a s t i c rubber implants attached to a modified Lippes intra uterine loop released 31 yg progesterone/mm capsule/day i n studies conducted by Lifchez and Scommegna (3)· At a recent symposium, an IUD developed by the Alza Corporation was described which released 50 yg progesterone/day (10). The f i r s t three capsule diameters i n Table V would require a release rate of 50 yg/day/100 mg of capsules. At the 10 yg/ day level, the rate of release required would be 10 yg/100 mg for 1 percent migration, 3.3 yg/100 mg i f there was 3 percent migra tion, or 2 yg/100 mg i f 5 percent migration occurred. Even smaller release rates would be required i f only 1 yg/day were required at a localized s i t e . In our single i n vivo study, we obtained a progesterone release rate of about 4 yg/day/100 mg of capsules and at this release rate, we would need approximately 3 percent transcervi cal migration i f 10 yg/day would suffice as an effective l o c a l ized dosage. An important point to be brought out i s that i f a drug were 10 times more effective than progesterone, the release rates seen i n these studies might be adequate i f the assumed transcervical migration occurred. Thus, from these estimations, a self-administered delivery system based upon microcapsules and transcervical migration appears feasible i f one considers the small amount of drug which might be needed at a localized s i t e . Literature Cited (1)

Mishell, D. R. and Lunkin, Μ. Ε., F e r t i l . & S t e r i l . , (1970), 21, 99. (2) Scommegna, Α., et a l . , F e r t i l . & S t e r i l . , (1970), 21, 201. (3) Lifchez, A. S. and Scommegna, Α., F e r t i l . & S t e r i l . , (1970), 21, 426. (4) Chang, C. C. and Kincl, F. Α., Steroids, (1968), 12, 689. (5) Chang, C. C. and Kincl, F. Α., F e r t i l . & S t e r i l . , (1970), 21, 134. (6) E g l i , G. E. and Newton, Μ., F e r t i l . & S t e r i l . , (1961), 12, 151. (7) deBoer, D. H., J. Reprod. Fert., (1972), 28, 295. (8) Kabasakalian, P., J. Pharm. S c i . , (1966), 55 (6), 642. (9) Roland, Μ., F e r t i l . & S t e r i l . , (1970), 21 (3), 211. (10) Symposium on Drug Therapy and Novel Delivery Systems Challenges and Responses (1974), Battelle Seattle Research Center, Seattle, Washington.


Steroid Release via Cellulose Acetate Butyrate Microcapsules

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