Development of Controlled Release Captopril Granules Coated with Ethylcellulose and Methylcellulose by Fluid Bed Dryer

Abstract

Captopril granules of controlled release with different polymers as ethylcellulose, ethyl/methylcellulose, and immediate release with polyvinylpyrrolidone (PVP) were developed by fluid bed dryer technique. The formulations were analyzed by scanning electron microscopy, X-ray powder diffraction, and dissolution profiles. To compare the formulations an in vivo setting rat blood pressure assay was performed, using angiotensin I as a vasoconstrictor agent. The scanning electron microscopy of granules showed differences in morphology, and X-ray powder diffraction technique presented some modification in crystalline structure of captopril in granules coated with PVP and ethyl/methylcellulose. The dissolution profile of granules coated with ethylcellulose showed a median time release of 4 hr whereas for granules coated with ethyl/methylcellulose, this time was 3.5 hr. The blockage of angiotensin I-induced hypertensive effect lasted 8 hr in granules coated with PVP and of more than 12 hr in the granules coated with ethylcellulose and ethyl/methylcellulose.

Keywords :

• Captopril
• Granules
• Controlled Release
• Polymers
• Fluid Bed Dryer

The pharmacological and therapeutic benefits of controlled release forms for compounds with short half-lives have been reported in the literature (Rhodes and Portner 1998; Royce, Weaver, and Shah 2004). Captopril (2S-1-[3-mercapto-2-methylpropioniyl]-L-proline) is a highly selective competitive inhibitor of angiotensin converting enzyme (ACE), and used for the treatment of hypertension and heart failure. Given orally, captopril is rapidly absorbed and has a bioavailability of ∼75%. The duration of action after a single oral dose is 6-8 hr (Seta et al. 1988; Franklin et al. 1998). Due to its relative short half-life, a controlled released captopril formulation would bring benefits to patients, including reduction in frequency of administration, increased patient compliance and effectiveness of treatment, reduction in plasma concentration fluctuations, as well as a reduction in side effects (Pastrana et al. 2000; Agnihotri and Aminabhavi 2004). Some efforts targeting these goals have been published. These studies include the use of polymers for coated capsules and tablets and some pharmaceutical formulations like pellets, bioadhesive systems, hard gelatin capsules, pulsed-delivery systems, microcapsules, enteric-coated granules, floating tablets, and oily semisolid matrices. As captopril degrades in the presence of water or moisture, difficulties in either development or stability of these formulations have been reported (Singh and Robinson 1988; Matharu and Shangavi 1992; Wilding et al. 1992; Ho, Wang, and Sheu 1997; Saks and Gardner 1997; Efentakis and Vlachou 2000; Nur and Zhang 2000; Khattab et al. 2001).

Application of different coating materials onto multiparticles allows the improvement/obtention of several desirable functional properties such as resistance to acid, sustained release and targeted release. Fluid bed coating is a popular method of coating multiparticles due to its high throughput and excellent coating (Heng, Chan, and Tang 2006). Our group has described excellent results regarding extended captopril stability by the use of fluid bed coating in association with ethylcellulose and methylcellulose polymers (Stulzer and Silva 2006).Therefore, in the present report we describe a controlled release system for captopril granules coated with ethylcellulose and ethyl/methylcellulose using fluid bed dryer. In addition, we show that polyvinylpyrrolidine (PVP) can be used as a water-soluble polymer to produce granules of immediate release.

MATERIALS AND METHODS

Captopril was purchased from Shenyang Fine Chemical (China). Microcrystalline cellulose 101, ethylcellulose (Ethocel 10STD), methylcellulose (15 PR), monohydrate lactose, and PVP K30 were kindly provided by Colorcon (São Paulo, SP, Brazil). All other materials were of analytical grade.

Granule Formulations

The coated granules of captopril were produced at Colorcon Brazil in a fluid bed dryer (Narong model FBS-1). Single polymer or polymer mixtures for coating were diluted in ethanol 90%. Coating process was carried out in the following conditions: temperature inlet = 56°C; product = 38°C; air volume = 240 m³/h; atomizer pressure = 2.5 bar; and peristaltic pump flow = 4.5 g/ml. Following coating, granules were sifted through 20-mesh sieves. Granule formulations are presented in Table 1.

TABLE 1 Summary of granules composition of the different formulations

Ingredients	Formulation F1 (g)	Formulation F2 (g)	Formulation F3 (g)
Captopril	150	150	150
Monohydrate lactose	25	25	25
Microcrystalline cellulose 101	25	25	25
Ethylcellulose (Ethocel 10STD)	60	50	—
Methylcellulose	—	10	—
(15 PR)
Polyvinylpyrrolidone	—	—	2
Ethanol 90 %	540	540	100

Scanning Electron Microscopy and X-Ray Powder Diffraction (XRPD)

Granule morphology was observed by scanning electron microscopy (Phillips XL30). Samples were mounted onto metal stubs using double-sided adhesive tape, vacuum-coated with gold (350 Å) in a Polaron E-5000, and analyzed. To characterize the influence of the coating process in captopril crystallinity, X-ray diffractions patterns were obtained on a Siemens X-ray diffractor, model D 5000, with tube of Cuk α, voltage of 40 kW, and current of 40 mA, in the range of 3-65 (2θ) and 1 s of pass-time, using the powder XRD method.

In Vitro Drug Release

United States Pharmacopea (USP) dissolution apparatus I (basket method) was used for release studies. A sample equivalent to 45 mg captopril was added to 900 mL phosphate buffer 0.05 N, pH 7.0 at 50 rpm. The temperature was maintained at 37 ± 0.5°C. For assessment of dissolution profiles, samples were removed at 1, 3, 5, 10, 15, and 20 min for granules of immediate release (coated with PVP) and until the release of 80% of the initial dosage for controlled release granules. Samples were centrifuged and analyzed in a UV spectrophotometer at 212 nm. The modifications of some parameters have been validated in accordance with the International Conference of Harmonization Q2A.

Theoretical Models for Drug Release

Drug release mechanism from microparticles was fitted to the following kinetic models: first order (Wagner 1969), zero order (Vudathala and Rogers 1995), Hixson-Crowell cubic root (Hixson and Crowell 1931), Weibull (Lagenbucher 1972), Korsmeyer-Peppas (Korsmeyer et al. 1983), Higuchi (Higuchi 1963), and Baker-Lonsdale (Baker and Lonsdale 1993). The best model to describe our results of captopril release from the microparticles was the Korsmeyer-Peppas model.

In Vivo Studies

Animals

Female Wistar rats (weighing 280–350 g) were housed in a temperature (23 ± 2°C) and light-(12 hr light/dark cycles) controlled room, with free access to water and food. All procedures were approved by our Institutional Ethics Committee and are in accordance with NIH Animal Care Guidelines.

Blood Pressure

Under anesthesia with ketamine and xylazine (90 and 15 mg/kg, respectively, supplemented at 45-to 60-min intervals), rats heparinized PE 20 and PE 50 polyethylene catheters were inserted, respectively, into the left femoral vein for drug injections and into the right carotid artery for recording mean arterial pressure (MAP), and blood withdrawal. To prevent clotting, a bolus dose of heparin (300 I.U.) was injected immediately after vein cannulation. Animals were allowed to breathe spontaneously via tracheal cannula. Body temperature was monitored by a rectal thermometer and maintained at 36°C ± 1°C. Drugs were diluted in sterile Dulbecco's phosphate-buffered saline (PBS, in millimoles: 137 NaCl, 2.7 KCl, 1.5 KH₂ PO₄, and 8.1 NaHPO₄; pH 7.4). At the end of the experiment, animals were sacrificed with pentobarbitone overdose. Blood pressure data were recorded (at a 10-sec sampling rate) with a Digi-Med blood pressure analyzer system (model 190) connected to a Digi-Med system integrator (model 200; Louisville, KY, USA) software, running under Windows 98™ (Microsoft Corporation, Redmond, WA, USA). Results are expressed as means ± SEM of the peak changes, relative to baseline, in MAP (mmHg) recorded following administration of angiotensin I.

Experimental Protocol

Rats were randomly divided at 4 groups of 5 animals each. Group 1 received captopril granules coated with PVP; group 2 received captopril granules coated with ethylcellulose; group 3 received captopril granules coated with ethyl/methylcellulose (all at 10 mg/kg, p.o.) and group 4 (control) received sterile PBS (300 μ l). At different time points after treatment (0.5, 1, 4, 8, 16 hr), animals were instrumented for MAP recording. The vasoconstrictive response to angiotensin I (30 pmol/kg) was obtained.

RESULTS AND DISCUSSION

Scanning Electron Microscopy and X-Ray Powder Diffraction

Substances in solid state can present crystalline or amorphous characteristics, and in some cases both. The crystal has an ordered arrangement of molecules and atoms, maintained in contact through noncovalent interactions. However, amorphous solids are characterized by random state. These characteristics are important in the absorption process. Amorphous solids are, in general, more soluble than the crystalline form, due to free energies involved in the dissolution process. Solids in amorphous state have their molecules arranged randomly and thus low energy is required to separate them. Consequently, their dissolution is faster than when in the crystalline form (Krist et al. 1996; Storpirts et al. 1999; Florence and Attwood 2003). Captopril (Figure 1) has a crystalline aspect that presents in orthorhombic crystal form (Kaul, Nguyen, and Venkataram 1992; Giron 2003).

FIG. 1 Scanning electron microscopy of captopril (panels A and B, magnification × 200 and ×400, respectively).

According to Markovich et al. (1997), the solid-state form (i.e., crystalline polymorphs, solvates, and amorphous solids) of a given drug can have a significant impact on the solubility, dissolution rate, stability in a pharmaceutical formulation, and bioavailability. The influence of coating process by fluid bed dryer on the crystalline form of captopril was investigated using XRPD and the patterns are shown in Figure 2. Granules coated with PVP and ethyl/methylcellulose presented a decrease of characteristic captopril peaks. This finding is of importance since it contributes to dissolution profiles of distinct granules formulations.

FIG. 2 X-ray diffraction spectra of captopril and granules coated with PVP, methylcellulose/ethylcellulose, and ethylcellulose.

Reduction in particle size increases the surface area and accelerates the dissolution rate. Porous amorphous drug/excipients particles with high surface areas present higher dissolution rates (Wei and Thau 1996; Jinno et al. 2006; Purvis et al. 2006). Photomicrographs of granules (Figure 3) show that captopril coated with ethylcellulose yielded spherical granules with a relatively small surface area. Granules coated with ethyl/methylcellulose or with PVP resulted in irregular shaped granules with higher surface area.

FIG. 3 Scanning electron microscopy of captopril granules with different coatings. Coating of PVP (panels A and B, magnification × 50 and × 100); coating of ethyl/methylcellulose (panels C and D, magnification × 50 and × 100, respectively), and coating of ethylcellulose (panels E and F, magnification × 50 and ×100, respectively).

Drug Release

Release rates of the different captopril coating formulation are presented in Figure 4. The results clearly indicate that the formulations had a differentiated pattern of release. Granules coated with ethylcellulose released captopril during 240 min and for granules coated with ethyl/methylcellulose, the time was 180 min. In contrast, granules coated with PVP released almost all captopril in 20 min. Thus, granules coated with celluloses had a longer release profile.

FIG. 4 Release profiles of granules coated with PVP, granules coated with ethylcellulose, and granules coated with ethyl/methylcellulose.

The mechanism of drug release from granules is determined by different physicochemical phenomena. According to Nixon (1983), three steps lead to drug release from microparticles in aqueous medium: inhibition of the release medium into microparticles, dissolution of the drug inside microparticles, and drug release by a diffusion process into the aqueous medium. The Korsmeyer-Peppas model (equation 1), a semiempirical model correlating drug release to time by a simple exponential equation for the fraction of release drug < 0.6, has been used to evaluate drug release from controlled release polymeric devices. It is particularly useful when the drug release mechanism is unknown or when there is more than one release mechanism (Korsmeyer et al. 1983; Costa and Lobo 2001). where Mt/M∞ is the proportion of drug released at time t, k is the kinetic constant, and the exponent n has been proposed as indicative of the release mechanism. In this context, n ≤ 0.43 indicates Fickian release and n = 0.85 indicates a purely relaxation controlled delivery that is referred to as case II transport. Intermediate values 0.43 < n < 0.85 indicate an anomalous behavior (non-Fickian kinetics corresponding to coupled diffusion/polymer relaxation; Ritger and Peppas 1987a, 1987b). Occasionally, values of n > 1 has been observed, which has been regarded as super case II kinetics (Ranga, Devi, and Buri 1988; Munday and Cox 2000). With the linear form of equation 1, plotting of ln Mt/M∞ against ln t (Figure 5) yielded the diffusion exponential (N), the Pearson coefficient (r²), and the diffusion constant (k). The results are summarized in Table 2.

FIG. 5 Linearity of different granules forms using equation 1.

TABLE 2 Analysis of release data from captopril granules

Formulation	N	r^2	k (min^−n)
Granules coated with ethyl/methylcellulose	0.6712	0.9948	1.87
Granules coated with ethylcellulose	0.4123	0.9972	1.06

Inspection of the results shown in Table 2 indicates that granules coated with ethyl/methylcellulose presented an N value of 0.6712 indicative of anomalous behavior. This behavior probably reflects the polymer binary blend used, ethylcellulose presenting a hydrophobic and methylcellulose a hydrophilic behavior. The non-Fickian kinetics corresponds to the coupling of diffusion from ethylcelllulose and polymer relaxation from methylcellulose. However, granules coated only with ethylcellulose presented an N value of 0.4123, indicating that a Fickian release occurred only by diffusion process. The granules coated with PVP presented an immediate solubilization in dissolution medium. The k values were related with the kinetics of captopril release. These values were calculated from equation 2: As shown in Table 2, the smaller k value for granules coated with ethylcellulose indicated a slower release, whereas for granules coated with ethyl/methylcellulose, k value was of 1.87.

In Vivo Studies

Angiotensin I is devoid of any significant vasoconstrictor activity. However, when given intravenously, angiotensin I is rapidly converted to angiotensin II by the action of angiotensin converting enzyme (ACE). Angiotensin II is a potent vasoconstrictor, whose potency is 40-fold higher than epinephrine in eliciting blood pressure increase (Ribeiro and Muscará 2001). ACE inhibitors are highly selective drugs that do not interact directly with other components of the renin-angiotensin system. Their pharmacological and clinical effects result for the blockade of the conversion of angiotensin I to angiotensin II. Captopril, the first ACE inhibitor to be marketed, is a potent ACE inhibitor (Regoli, Park, and Rioux 1974; Petrillo and Ondetti 1982; Romankiewicz et al. 1983).

As can be seen in Figure 6 and Figure 7, 30 min after oral treatment with captopril granules coated with PVP, the increase in the blood pressure following angiotensin I injection was reduced by 80%. This result indicates an efficient inhibition of ACE by captopril. The inhibition carried out by captopril coated with PVP lasted for a period of 8 hr. Captopril granules coated with ethylcellulose and ethyl/methylcellulose retained their potency but the permanence of their inhibitory effects was increased by 50% (from 8 to > 12 hr).

FIG. 6 Effects of captopril granules coated with ethylcellulose and PVP on vasoconstrictive response to angiotensin I. Animals were treated orally with formulations or PBS (control). At indicated times animals were prepared for mean arterial pressure measurement. Thus, the vasoconstrictive response to angiontensin I (30 pmol/kg; panel A) and mean blood pressure (panel B) were recorded. Each point or bar represents the mean of 5 animals and vertical lines are the S.E.M. * p < 0.05 compared with the control group (PBS). Statistical analysis was performed using ANOVA test followed by Bonferroni's posthoc t test.

FIG. 7 Effects of captopril granules coated with ethyl/methylcellulose and PVP on vasoconstrictive response to angiotensin I. Animals were treated orally with formulations or PBS (control). At indicated times animals were prepared for mean arterial pressure measurement. Thus, the vasoconstrictive response to angiontensin I (30 pmol/kg; panel A) and mean blood pressure (panel B) were recorded. Each point or bar represents the mean of 5 animals and vertical lines are the S.E.M. * p < 0.05 compared with the control group (PBS). Statistical analysis was performed using ANOVA test followed by Bonferroni's posthoc t-test.

As for the effects on blood pressure, oral captopril granules coated with PVP administration induced a decrease of ∼30% below baseline levels within 30 min (Figures 6 and 7). The effect lingered for 8 hr and by 12 hr blood pressure had returned to baseline values. Coated granules with ethylcellulose and ethyl/methylcellulose displayed around the same time for the onset of action but their effect on blood pressure lasted 50% longer (8 to 12 hr; Figures 6 and 7).

CONCLUSIONS

Our results show that captopril coated with PVP or as granules coated with ethyl/methylcellulose was rapidly absorbed after oral administration, with a peak of ACE inhibition about 30–60 min after administration. However, cellulose-coated granules provided a significant increase in the duration of the effect. This increase is enough to allow changing posology and hence to reduce the frequency of administrations. Captopril is largely used for chronic diseases such as hypertension and heart failure. Because these conditions usually require life-long treatment, this strategy is important to improve the quality of life and patient adherence, as well as to reduce the costs of the treatment.

We acknowledge the support of Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) for fellowships and Conselho Nacional de Desenvolvimento Científico and Tecnológico (CNPq) for the financial support.