A new slow release formulation of metoprolol: in-vitro and in-vivo evaluation in dogs

Journal of Controlled Release, 23 ( 1993) l-12 0 1993 Elsevier Science Publishers B.V. All rights reserved

016%3659/93/$06.00

COREL 00784

A new slow release formulation of metoprolol: in-vitro and invivo evaluation in dogs P. Albin”, A. Markusa, Z. Ben-Zvib and Z. Pelah” “The Institutesfor Applied Research and bDepartment of Clinical Pharmacology, Faculty of Health Sciences, Ben-Gurion Universityof the Negev, Beer-Sheva, Israel (Received

10 October 199 1; accepted in revised form 2 1 July 1992)

Metoprolol, a #I,-adrenergic blocker, was formulated into a number of sustained-release formulations. Some of these were prepared by a spray-drying technique specially developed by us to produce homogeneous spherical microparticles, 5-20 pm, which could then be compressed into tablets. The application of this technique, which is usually employed in the food industry, to pharmaceuticals provides an innovative means of formulating slow-release drugs, specially for highly water-soluble compounds. Our formulations were compared with a commercially available product, Lopresor Divitab (Ciba-Geigy ) and with “nonformulated” metoprolol in in vitro and in vivo (dogs) tests. The kinetics of the rate of release in simulated gastrointestinal juices of the active ingredient from the sustainedrelease formulations were found to fit best to the Hixon-Crowell equation. Statistical treatment of the pharmacokinetic data obtained in the in vivo tests showed that there were no significant differences between our products and the commercially available formulation. These encouraging results open the way for the possible testing of our formulations on human volunteers. Key words: Metoprolol tartrate; Dissolution; Encapsulation; Pharmacokinetics

Introduction Substances that block/T-receptors do so by preventing natural neurotransmitters or adrenomimetic drugs from activating these receptors. & Blockers thus constitute an important group of drugs in the treatment of cardiovascular diseases such as hypertension, angina pectoris, ischemia and cardiac arrhythmias. Metoprolol [ ( 2 )-l-isopropylamino-3 [ 4- ( 2-methoxyethyl )phenoxypropan-2011, a potent selective &blocker [ 11, is often preferred to nonselective /3-blockers, such Correspondence to: P. Albin (c/o Dr. A. Markus), The Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva, P.O. Box 84105, Israel.

as propranolol, which are not recommended for patients suffering from bronchospasm or diabetes (the latter being treated with insulin or oral hypoglycemic drugs [ 2,3 ] ) .

Scheme 1. Molecular structure of metoprolol.

The main drawback of metoprolol is its short half life; tablets have to be taken a few times a day to maintain /I-blockage. The resulting high peak plasma levels might cause adverse effects such as hypotension, dizziness, fatigue or headache [ 5 1. With a slow-release product adminis-

2

tered only once daily, treatment is more efficient [ 61, since plasma concentrations fluctuate less and compliance is improved [6 1. In this study we set out to prepare a series of sustained-release formulations of metoprolol tartrate and compare them with the “nonformulated” drug in in vitro and in vivo tests. The tartrate salt of metopro101 was used because this is the form of the drug used widely throughout the world (metopro101itself is highly insoluble),

mogeneous solution of the core material in the polymer gel, The mixture was then allowed to stand until all the air bubbles had escaped, and the clear solution was spray dried [ 71 through a tiny nozzle that formed a delicate spray. As the solution made contact with the hot air, the water was evaporated off, leaving small drug particles encapsulated in a polymer envelope. The spray dryer (Buchi-190 mini spray dryer, Switzerland) was operated at a flow rate of 800 ml/min, an inlet temperature of 170- 175 oC and an outlet tem~rature of 1lo- 115 oC. The white powder (particle size 5-20 ,um in diameter) obtained by this technique was compressed into tablets, containing 200 mg of active material by applying 2 tons of pressure in the tablet-compressing equipment that has been developed at Ben-Gurion University of the Negev. In some of the formulations, we included only one polymer, but in others we used a synergistic combination of two polymers to obtain better crosslinking which would, in turn, cause slower release of metoprolol through the polymer pores Eg,91. The second method used for preparing microcapsules is based on a phase-separation technique [ lo]. Metoprolol was dissolved in agaragar solution and added dropwise to ethyl acetate to precipitate microspheres 200-500 pm in diameter. The microcapsules so obtained were air dried and filled into hard gelatin capsules, each

materials and Methods Metoprolol tartrate was formulated into a variety of sustained-release products by means of encapsulation either in a polymer that forms a hydrogel or in an agar-agar envelope. In the former method the following polymers were used alone or in combination: hydroxypropylmethyl cellulose (Methocel ) (Dow Chemicals, USA), carboxymethyl-cellulose (CMC) (Fluka, Switzerland), polyalginic acid (Fisher Scientific Co., USA) and polyg~acturonic acid (Sigma, USA), as their sodium salts. The general method for producing microspheres of metoprolol tartrate (core material) in a polymer matrix may be described as follows (Table 1): the polymer was dissolved in water to give a clear solution of suitable viscosity and fluidity. Metoprolol tartrate was added, and the gel was mixed in a high-sheer disperser to give a hoTABLE 1 Fo~ulation Formulation

PA7-180 PA7-181 PA7-182 PA7-183 PA7-186 PA7-187 PA7- 189

of metoprolol tartrate with different polymers Polymer 1

Polymer 2

Type

g

Methocel Methocel Methocel Agar agar Agar agar Methocel Methocel

1.5 4.0 5.0 7.5 3.2 5.0 8.0

PGA = po~y~la~turonic acid Na Methocel = hydrox~ropylme~ylcellulose.

salt;

Al&ate

Type

g

PGA Alginate

6.0 5.0

PGA CMC

5.0 2.5

= alginate

( Na

salt ) ;

Water (ml)

Metoprolol tartrate (g )

450 350 400 60 80 350 750

18.75 12.0 15.0 3.0 2.0 15.0 12.5

CMC = carboxymethylceIlulose

Na

salt;

3

containing 200 mg of active ingredient. The composition of the different formulations prepared by us is given in Table 1. In vitro tests

In vitro dissolution tests were performed on the core material, on our formulations and on the commercially available product, Lopresor Divitab (Ciba Geigy). Tests were carried out in two media prepared according to USP XX [ 111: simulated gastric juice (SGJ) pH 1.5 and simulated intestinal juice (SIJ) pH 7.2. The experiment was performed as follows: six beakers, each containing 900 ml of liquid, SGJ or SIJ, were placed in a pedal dissolution system (80 rpm) at 37 ? 1“C. The tablet or capsule was put into a small polyethylene net bag attached to the wings of the pedal. At fixed intervals, the optical density of the solution was read at 275 nm in duplicate. The aqueous sample was then returned to the beaker so that the volume of the solution would not change. Three dissolution experiments were carried out for each formulation. Optical density readings were converted to milligrams of metoprolol by means of a calibration curve. In vivo experiments

The pharmacokinetic profiles of three of our sustained-release formulation PA7- 182 and PA7187 tablets and PA7-186 capsules were compared with those of Lopresor Divitab and metopro101tartrate. The metoprolol tartrate tablet or capsule containing 200 mg core material was administered to four mongrel dogs, three males and one female, each weighing approximately 30 kg, in a Latin square cross-over system. The washout time was at least seven days. The dogs were fasted for 24 h before the start of the experiment. The tablet or capsule was administered directly into the back of the mouth, and 100 ml of dextrose solution were given immediately after the drug. Blood samples were drawn from the front leg vein by a venflon at zero time (before administration of the drug) and 0.5, 1, 1.5, 2, 2.5, 3, 4, 5, 6, 7, 8, 9, 9.45, 10.5, 11.15

and 12 h after dosing. After each sampling, a solution of heparin was introduced to the dog to prevent coagulation. The blood samples, collected into tubes containing I&-EDTA, were centrifuged for 10 min at 2500 rpm, and the plasma was stored at -70” C pending analysis. Plasma levels of metoprolol were determined by the method of Miller and Greenblatt [ 12 1, as modified by us. The HPLC system used consisted of Spectra-Physics Sp. 8700 x R extended range LC pump equipped with SP. 4290 integrator and an ISCO, V4, Absorbance Detector. The column used was a Merck 5 pm Lichrocart@ RP 18, 250-4 mm. The mobile phase consisted of 650 ml of methanol, 350 ml of doubly distilled water, and 0.15 ml of triethylamine. The pH of the mobile phase mixture was adjusted to 3.703.75 with concentrated phosphoric acid. The flow rate of the mobile phase was 0.85 mlnmin-‘, and optical density was monitored at 220 nm. Samples for HPLC analysis were prepared such that each sample contained 1 ml of plasma, 4 ml of ethyl acetate, 50 ~1 of 1 M NaOH and 50 ~1 of internal standard (propranolol HC120 pg.ml- ’ ). The mixture was vortexed for 30 s and then centrifuged for 20 min at 2900 rpm. The organic phase was transferred to a second tube and was then evaporated to dryness under nitrogen. The residue was dissolved in 150 ~1 of the mobile phase. One hundred microliters of the final solution was injected into the HPLC via a 50 ~1 loop. Plasma levels of metoprolol were calculated from a calibration curve (a fresh calibration curve was prepared each day). The minimal detectable concentration was 20 ng/ml. Pharmacokinetic parameters were determined using multifit computer program (Versie 18-0890 Copyright J.H. Proost ). Statistical analysis

Student’s t-test and ANOVA with repeated measures and multiple comparisons [ 13 ] were used for statistical treatment of the data.

4

Results and Discussion Morphology

The morphological and surface characteristics of metoprolol tartrate, of the sustained release formulations, and of the core materials are shown in the electron micrographs presented in Fig. 1(A-I). These micrographs illustrate the structure of the products (drug-polymer matrix) of small microparticles ( 5-20 pm) with a spherical shape and a smooth surface (Fig. 1F-I ) on one hand, and the active ingredient (Fig. IA) and the different polymers (Fig. 1(B-E) on the other hand. Tablets prepared from our microspherical formulations have the following advantages [ 141: (a) the spherical shape of the product’s particles may enable a constant homogeneous release rate from the tablet, while it is disintegrating in the gastrointestinal tract; (b) during disintegration of the tablet the material is dispersed homogeneously throughout the entire gastrointestinal tract, which facilitates a constant rate of absorption of the active materials; and (c) the small size (5-20 pm) of the microspheres reduces the likelihood of damage and distortion of the shape during tabletting (compression ) . In vitro tests The release rates of metoprolol tartrate, from three of our formulations and from Lopresor in simulated gastrointestinal media, pH 1.5 and 7.2, are shown in Fig. 2. An examination of the figure led us to ask the question whether the release rate of the drug (which, being basic [ 15 1, is highly soluble in acid media) from the formulation is indeed pH dependent. This information would also help us determine whether the nature of the polymer influences the release rate. Fig. 2 shows that the release rate in the highly acidic medium (SGJ) was indeed significantly faster in most cases than at pH 7.2. Such a difference in release rates was also found by Ragnarsson et al. [ 14 ] for a new sustained-release type formulation of metoprolol succinate. Explanation for the faster release rate at pH 1.5 might lie in the influence

Fig. 1 (A-I ) Scanning electron micrographs of metoprolol tartrate and its different formulations. (A) metoprolol tartrate, (B) Methocel (HPMC), (C) polygalacturonic acid (Na salt), (D) alginate (Nasalt), (E) CMC, (F) PA7-182, (G) PA7-187, (H) PA7-189, (I) PA7-186.

5

Time (h)

-

pH=7.2

--c

pH=1.5

Time thf Time (h)

-W-

pH=7.2

--+--

ptlf1.5

Time (h) 0

2

3

4

5

6

Time (h)

100 90 80

Fig. 2 (a-g). In vitro release of met~proIo~ at pK 1.4 and 7.2. (a) metoprolol tartrate (no~fo~~~~ted), (b) Lopresor - Cuba Geigy, (c) PA7-182, (d) PA7-186, (e) PA7-187, (f) 4 formulated products at pH 1S, (g) 4 formulated products at pH 7.2. *Significant difference (P
70 60 $0 40 30 20 $0 0

of the acid medium on the hydrophilic polymer matrix. This might cause an increase in pore size, facilitating the absorption of water and the movement of the drug from the saturated solution (in the polymer matrix ) to the sink (SGJ ) * Walker and Wells f 8 f showed that the nature of the polymer matrix could influence the rate of release of metoprolol, and that the drug was released more slowly from a combination of sodium CMC and a hydrophilic polymer such as

0

2

3

4

5

Time (h)

Methocel. In this synergistic combination crosslinking occurred as a result of hydrogen bonding between the carboxy groups of the CMC and the hydroxyl groups of the Methocel, resulting in an increase of viscosityy, and a consequent decrease in release rate.

7

TABLE 2 Correlation curve parameters for the kinetic treatment of the release rate of metoproiol tartrate at pH 7.2 and 1.5* Kinetics at pH 1.5

Kinetics at pH 7.2 n

Lopresor (Ciba-Geigy I r 4 x coefficient SE of x coefficient PA7-182 r 3 x coefficient SE ofx coefftcient PA%186 r 3 x coefficient SE of x coefficient PA7-187 r 5 x coefficient SE ofx coefficient

Zero order

First order

Higuchi

HixonCrowell

n

Zero order

First order

Higuchi

HixonCrowell

0.9695 23.2722 1.4583

0.9968 0.2717 0.0054

0.9373 0.8195 0.0749

0.9942 0.3952 0.0106

3

0.9843 33.8048 1.5109

0.9090 0.6156 0.0689

0.9460 1.1444 0.0967

0.9875 0.7370 0.0395

0.9727 46.3662 3.4747

0.9702 0.6308 0.0494

0.9266 1.6579 0.2086

0.9921 0.8569 0.0343

4

0.9009 26.64 11 2.9457

0.9968 0.5258 0.0100

0.8464 0.8614 0.1223

0.9900 0.6205 0.0208

0.9922 28.5071 1.1269

0.9953 0.2066 0.062

0.8900 1.2998 0.1616

0.9962 0.3479 0.0087

3

0.9733 30.3089 1.8961

0.9848 0.49 11 0.023 1

0.9440 0.9882 0.0910

0.9980 0.6303 0.0106

0.9146 30.0095 3.0557

0.9857 0.6836 0.0275

0.8601 0.9771 0.1313

0.9976 0.7502 0.0122

3

0.8807 37.7509 5.2521

0.9524 1.1955 0.1010

0.8329 1.2094 1.2048

0.9967 1.1013 0.0238

*In this case, all data from the n experiments were plotted in a single curve and correlation was tested.

Since the thickness of the polymer envelope and the size, surface character and shape of the microspheres are all factors that can influence the rate of release of the active ingredient [ 16 1, it was important to determine the kinetic order of this rate. The question arose whether our multiunit system is essentially a dispersion of small particles or a “big” source of polymer matrix. Gross et al. [ 171 showed that the kinetic behavior of a single particle differs from that of the entire particle population taken as a whole. We attempted to determine whether the rates of release of metoprolol from the different formulations followed zero-order, first-order, Higuchi-model [ 181 or Hixon-Crowell-mode1 [ 191 kinetics. Correlation-curve parameters for the treatment of the kinetic release rate (Fig. 2a-e) are given in Table 2. Table 3 shows that the kinetic order of the different formulations gave the best fit with the Hixon-Crowell equation, also known as the “cubed-root” model. This model provides a mathematical expression of the dissolution of a spherical particle into solution and provides a quantitative kinetic evaluation of the rate at

which tablets dissolve and the surface erodes while maintaining their geometrical shape [ 2023 1, as occurred in our formulations. Similar findings have been reported for the kinetics of the release rate of salicylate from an ethyl cellulose matrix [24] and for indomethacin from a combined ~e~ocel/ethyl cellulose polymer system f 251. Fig. 3 shows the good correlation of two of the formulations according to the HixonCrowell model. In vivo experiments Plasma levels vs time of the five formulations in the four dogs are shown in Fig. 4A-D, and the pharmacokinetic data are presented in Table 4. As can be seen from Fig. 4 and Table 4 the highest plasma levels (Cmax) were obtained with the nonformulated metoprolol in all four dogs (p c 0.05 ). Much more moderate levels were obtained with the sustained-release formulations. Similar results have also been reported in the literature [ 26,27 1. There are also reports of a shift in tmax in human volunteers from l-3 h for the

8 TABLE 3 Kinetic order of release rates of metoproloi tartrate from different formulations at pH 7.2 and 1.5 Product

Order

Lopresor PA7-182 PA7-186 PAl-187

pH 7.2

pH 1.5

Hixon-Crowell + first order Hixon-Crowell Hixon-Crowell + first order Hixon-Crowell

Hixon-CrowellS zero order Hixon-Crowell + first order Hixon-Crowell Hixon-Crowell

4 54-

f

?A7-182

+

LOPRESOR

3.5-

Time (h)

Fig. 3. Hixon release of PA7-182 and Lopresor at pH 7.2.

core material to 3-5 h in sustained-release formulations [ 28-30 J. In contrast we obtained a tmax shift that was not statistically significant. Our in vivo experiments were carried out in dogs because of the similarity in the motility of the gastrointestinal tract in the dog and man [26,31,32]. There are, however, very few reports in the literature concerning metoprolol kinetics in dogs. The results with our fo~ulations

prepared by the spray dryer technique are similar to previously published results for formulations not necessarily prepared as tartrate salts [31,33]. Plasma levels of our formulations were higher at extended times as compared to non-formulated metoprolol, as was previously found [27,28,34,35]. Dayer et al. [36] showed good correlation between the plasma concentration of metoprolol and the pha~acolo~~l effect (inhibition of exercise tachycardia) , at concentrations ranging between 5-200 ngaml-’ in human volunteers. We can assume that the prolongation of the metoprolol release to the plasma by our formulations will also have the effect of extending the duration of the pharmacological activity. In our study there was no statistically significant difference in the pharmacokinetic parameters: AUC, tllz, Kel; (p>O.O5) between the active material and our sustained-release formulations. Similar results were obtained by other groups for their fo~ulations tested on hu-

TABLE 4 Pharmacokinetic data for metoprolol tartrate formulations administered to four dogs

Mean +SE

Parameter

MT

Lopresor

PA7-182

PA7-186

PA7-187

tmax Cmax

lSOf0.31 1126.50+ 134.79 1.4620.15 0.49 rt 0.04 2.9920.30

3.75kO.41 492.75 k48.91 2.12kO.22 0.34 * 0.04 2.19kO.41

3.00 +0.53 558.75 k49.96 4.21 +1.92 0.276rtO.08 3.13 rt0.58

2.25 +0.52 576.002 62.56 2.39f0.44 0.33f0.07 2.98f0.71

2.66 t 0.27 638.33+ 38.96 2.29kO.83 0.4oIto.14 2.81 kO.78

t1/2

Kel AUC(O-cct)

tmax=time Units: 0nax - h; Cmax - ngfml; tl,z - h; Kel- l/h; AuC(0-c~) -pg.h/ml; 1421; Cm~=m~imalplasma~n~ntmtion; for maximal plasma concentration; Kel = elimination rate constant; trj2= terminal half life; AUC=area under the time concentration curve. MT=nonfo~uIated metoprolol tartrate.

9

GRAY

man volunteers [27,28,37,38]. The lack of statistical difference in the AUC of the nonformulated metoprolol tartrate and all the formulated products may indicate that the degree of absorption of the active ingredient from all preparations is similar. From the values presented in Table 4, the following ratios of AUC formulated to AUC nonformulated were obtained: 0.73 t 0.09, 1.0520.23, 1.04+0.30, and 0.9420.07 for Lopresor, PA7-182, PA7-186 and PA7-187, respectively (~~0.05). These ratios show that the amounts of metoprolol tartrate absorbed from the various formulations were not significantly different, as mentioned above. The variability in pharmacokinetic parameters between the subjects, a well-known phenomenon for metoprolol, is probably due to first-pass metabolism and pharmacogenetic variations [ 3,4,36,39-411.

1000 90QA

time(h)

PILI

Conclusions 4

0

12

time(h)

From our results we can conclude that the reduction in plasma levels and the maintenance of moderate plasma levels for long periods of time indicate that our formulations are indeed satisfactory. These encouraging results indicate the feasibility of testing our formulations in human volunteers.

EDUARD

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time(h)

ALPH 2

3

4

8

12

time(h)

Fig. 4. Plasma concentrations vs time of metoprolol products in dogs (A-D).

4

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