Formulation And Evaluation of a Bilayer Mucoadhesive Buccal Drug Delivery System for Carvedilol Nanoparticles

Among the various routes of drug delivery, the oral route is perhaps the most preferred by patients. Additionally, transmucosal routes of drug delivery (i.e., through the mucosal linings of the oral, nasal, rectal, vaginal, and ocular cavities) offer distinct advantages for systemic drug delivery.

Buccal delivery involves administering drugs through the buccal mucosal membrane, which lines the inside of the cheek (see Fig. 1). The buccal mucosa is highly permeable, has a rich blood supply, and is more robust than other mucosal tissues. Prolonging the residence time of drugs in the buccal mucosa, combined with controlled drug release, can reduce the frequency of administration.

       
           
       
Fig. 1: Site of buccal tablet adhesion.

In the present study, carvedilol was selected as the model drug. Carvedilol is a lipophilic ?-blocker with low bioavailability due to extensive first-pass metabolism. As a result, a higher daily dose (25 mg twice a day) is required to maintain its therapeutic effect throughout the day. However, carvedilol has several significant drawbacks, including hypotension, bradycardia, dizziness, fatigue, and upper respiratory tract infections. Therefore, sustained oral drug delivery is beneficial for long-term treatment.

To enhance the absorption of drugs across the buccal mucosa, the solubility of carvedilol can be improved through nanoprecipitation technology using the Eudragit RL100 (EU-RL) polymer [as described by Ganesh Nayak et al. (Int. J. of Pharm. Sci., 2024, Vol. 2, Issue 7, 2010-2018)]. These carvedilol nanoparticles were then used to formulate mucoadhesive buccal tablets for the treatment of hypertension^1-4.

MATERIALS AND METHODS

Materials: Carvedilol, along with polymers such as chitosan, lactose, hydroxypropyl methylcellulose E15, and magnesium stearate, were purchased from Loba Chemie Labs, Mumbai. All materials used were of either analytical reagent (AR) or laboratory reagent (LR) grade.

Methods:

Pre-formulation studies:

Pre-formulation studies were conducted, with detailed discussions provided in the work by Ganesh Nayak et al³.

Preparation of bilayer buccal tablets with carvedilol (without nanoparticles):

The buccal tablets consist of two parts: the core and the backing layer. The core contains a mixture of carvedilol and other excipients, as outlined in Tables 1 and 2.

Table 1: Composition of the core containing pure carvedilol.

Table 2: Composition of the backing layer.

Buccal tablets were prepared in three stages as follows^4-6:

Stage I - Preparation of the core:

All the ingredients listed in Table 1 were accurately weighed and sifted through a #30 sieve. Carvedilol, polymers, and excipients, except for the lubricant, were mixed homogeneously for 15 minutes by trituration using a glass mortar and pestle. Magnesium stearate was then added, and the mixture was blended for an additional 2 minutes. This mixture was used for compressing the core tablets.

Stage II - Preparation of the backing layer:

All the ingredients listed in Table 2 were accurately weighed and sifted through a #30 sieve. Ethyl cellulose and amaranth powder were mixed homogeneously for 10 minutes by trituration using a glass mortar and pestle. Magnesium stearate was added and mixed for another 2 minutes.

Stage III - Compression:

The mucoadhesive core and backing layer were sequentially compressed using a rotary tablet compression machine (Proton Press, Proton Engineers, Ahmedabad) with 8 mm round concave punches.

Preparation of bilayer buccal tablets containing carvedilol nanoparticles^6-7:

Based on the evaluation parameters, the F2 formulation was selected for the preparation of bilayer buccal tablets containing carvedilol nanoparticles. The nanoparticle formulations NP1, NP2, and NP3 were further developed into formulations F4, F5, and F6, respectively, and compressed into tablets using the direct compression method. The composition of the core layer is provided in Table 3.

Table 3: Composition of the core containing carvedilol nanoparticles.

Evaluation of Mucoadhesive Tablets^7-10

Pre-compression Studies:

Before proceeding with compression, pre-compression studies were conducted to assess bulk density, tapped density, Carr’s index, Hausner’s ratio, angle of repose, and FT-IR compatibility.

Post-compression Studies:

After compression, the general appearance, hardness, thickness, weight variation, and friability of the tablets were evaluated.

Drug Content:

Ten tablets were randomly selected and triturated using a glass mortar and pestle. An accurately weighed quantity of the triturated powder equivalent to 175 mg was placed in a 100 mL volumetric flask and dissolved in 10 mL of methanol. The volume was then adjusted to 100 mL with phosphate buffer (pH 6.8). One millilitre of this solution was withdrawn and diluted to 10 mL with phosphate buffer (pH 6.8). The corresponding concentration was determined using a UV-visible spectrophotometer at 241.4 nm. The test was performed in triplicate, and the average drug content was calculated.

Surface pH Studies:

A combined glass electrode was used to measure the surface pH of the buccal tablets. The tablet was allowed to swell by maintaining contact with 1 mL of distilled water for 2 hours. The pH was determined by placing the electrode in contact with the surface of the formulation and allowing it to equilibrate for 1 minute.

Measurement of Mucoadhesive Strength:

The mucoadhesive strength of the buccal tablets was measured using a modified physical balance.

Setup:

A double-beam physical balance was used with the left pan removed from the arm. A thick thread of suitable length was tied to the left arm, leaving the bottom end of the thread free. A glass stopper with a uniform surface was attached to the free end of the thread. A glass mortar was then placed below the hanging glass stopper.

Methods:
The cellophane membrane was washed with distilled water and then rinsed with phosphate buffer (pH 6.8). This mucosal membrane was tightly secured over a 50 mL glass beaker filled with phosphate buffer (pH 6.8) using a thread. This beaker was placed in a 500 mL beaker, which was filled with isotonic phosphate buffer (pH 6.8) solution, ensuring that the solution reached the surface of the cellophane membrane to keep it moist. The setup was positioned in a glass mortar beneath the hanging glass stopper.

The temperature was controlled by placing a thermometer in the 500 mL beaker and intermittently adding hot water to the outer mortar. The buccal tablet was adhered to the glass stopper through its backing membrane using adhesive. The balance was adjusted such that the white part of the tablet faced the membrane and was allowed to remain in contact for 2 minutes. A weight of 5 g was then added to the right side, applying 5 g of pressure to the buccal tablet against the moist membrane. The balance was maintained in this position for 3 minutes, after which weights were gradually increased on the right pan until the tablet separated from the cellophane membrane. The total weight on the right pan provided the force required to detach the tablet from the membrane, resulting in the mucoadhesive strength measured in grams.

       
           
       
Fig 2: Laboratory setup for measuring mucoadhesive strength of buccal tablets.

Swelling Index:

For the swelling studies, three tablets from each formulation were weighed individually (W1). Six Petri dishes were labelled F1-F6, and 15 mL of phosphate buffer (pH 6.8) solution was added to each dish. Three coverslips were placed in each Petri dish, and individual tablets were positioned on each coverslip such that the backing layer faced upward and the core of the tablet was completely immersed in the buffer solution.

At regular intervals (0.5, 1, 2, 3, 4, 6, 8, and 10 hours), the buccal tablets were removed from the Petri dishes using the coverslip. Excess surface water was carefully removed using Whatman filter paper, and the tablets were then weighed again (W2). The swelling index was calculated using the following formula:

 Swelling index =W1-W2W1

In vitro diffusion studies: In vitro, diffusion of the drug from the buccal tablet was studied by a simple diffusion cell apparatus. The cellophane membrane was tied to one end of the donor compartment and the tablet was placed over it so that the core layer could face the membrane. In the receiving compartment, 20 ml of pH 6.8 phosphate buffer was added, and the donor compartment was placed over it so that the membrane was just dipped into the pH solution. The medium was stirred using a magnetic stirrer, and the temperature was maintained at 37 ± 0.5 °C. A 1 ml sample was withdrawn at various time intervals. The concentration of carvedilol in the samples was determined using spectrophotometric analysis at 241.4 nm. 

Data Analysis: To analyze the mechanism of drug release and release rate kinetics of the dosage form, the data obtained from the in vitro drug release studies were fitted to zero-order, first-order, Higuchi’s and Peppa’s models. 

RESULTS AND DISCUSSION 

Evaluation of tablets: 

Pre-compression parameters: This result indicates that all formulations possess good % compressibility and flow properties (Table 4). 

Table 4: Results of the pre-compression parameters.

Formulation	Bulk density (gm/cm3)	Tapped density (gm/cm3)	Angle of repose(?)	Carr’s index (%)	Hausner’s ratio
F1	0.152± 0.0015	0.164± 0.002	26o 06’± 1.15	7.32± 0.35	1.07± 0.004
F2	0.149± 0.0015	0.161± 0.0006	25o 3’± 0.4	7.65± 0.75	1.08± 0.008
F3	0.147± 0.0003	0.159± 0.0008	26o 83’± 1.202	7.708± 0.495	1.083± 0.005
F4	0.147± 0.0006	0.159± 0.0011	26o 33’± 1.14	7.27± 0.31	1.078± 0.003
F5	0.151± 0.002	0.163± 0.0012	27o 06’± 1.19	7.35± 0.703	1.079± 0.008
F6	0.15± 0.0011	0.163± 0.0019	260 56’± 2.001	8.38± 0.37	1.091± 0.004

Note: The values presented are the means ± SDs of three determinations.

Post-compression parameters: 

Physical appearance: The appearance of the prepared tablets was inspected visually. The tablets are round, with the flat surface having a white core and pink backing layer.

Hardness, Thickness, Weight Variation, and Friability: The mean hardness, thickness, weight variation, and friability of each formulation were recorded and found to be satisfactory (see Table 5). 

Table 5: Results of thickness, weight variation, hardness, and friability.

Formulations	Hardness (kg/cm3)	Thickness (mm)	Weight variation (mg)	(%) Friability
F1	4.66± 0.15	3.16± 0.13	173.4± 4.19	0.6 ± 0.11
F2	4.73± 0.14	3.13± 0.17	175.1± 4.33	0.26± 0.14
F3	4.56± 0.17	3.00± 0.099	173.2± 3.61	0.49± 0.09
F4	4.33± 0.15	3.16± 0.15	174.6± 4.718	0.23± 0.10
F5	4.7± 0.10	3.13± 0.11	173.6± 4.402	0.60± 0.15
F6	4.5± 0.10	3.09± 0.12	174.1± 3.75	0.50± 0.21

Note: The values presented are the means ± SDs of three determinations.

Drug Content of Tablet: The percent drug content of the tablets was found to be between 90.78±0.45 and 96.83±0.86% that of carvedilol (Table 6). The cumulative percentage of drug released from each tablet in the in vitro release studies was based on the average drug content present in the tablet. 

Surface pH: The surface pH of all formulations ranged from 6.68±0.035 to 6.74±0.032, simulating the pH of saliva; hence, the formulation cannot cause any irritation to the buccal mucosa (Table 6).  

Mucoadhesive strength: All six formulations showed good mucoadhesive strength in the range of 21.326±0.4 gm to 24.58±0.78 gm (Table 6). Thus, the formulation can remain in place for a longer period. 

Table 6: Results of % drug content, surface pH, and mucoadhesive strengths.

Formulation	% Drug content	Surface pH	Mucoadhesive strength (gm)
F1	95.69± 0.65	6.68± 0.035	21.59± 0.74
F2	95.31± 0.56	6.703± 0.041	21.743± 0.61
F3	96.83± 0.86	6.71± 0.0057	23.62± 0.86
F4	91.23± 0.22	6.74± 0.032	21.326± 0.45
F5	92.52± 0.34	6.69± 0.055	24.58± 0.81
F6	90.78± 0.45	6.73± 0.0404	24.58± 0.78

               Note: The values are presented as the means ± SDs of three determinations. 

Swelling index: The swelling indices of the tablets were evaluated for 10 hrs. The greatest amount of swelling was observed with formulation F6 at 10 hrs. The F1 formulation showed rapid swelling of 120.68±3.31% in 6 Hrs. and then broke down (Table 7). Hence, formulations F2 to F6 were considered for further studies.

Table 7: Results of the swelling indices of all formulations over time.

Time (hrs)	Swelling index
	F1	F2	F3	F4	F5	F6
0.5	44.95± 4.99	34.35± 0.86	24.28± 2.91	29.031± 4.13	20.83± 0.11	27.24± 2.55
1	71.28± 3.74	50.57± 1.057	35.83± 4.36	47.87± 0.91	33.604± 0.85	39.45± 2.66
2	94.33± 2.86	62.33± 0.69	45.88± 3.69	59.12± 4.25	50.28± 0.39	54.79± 3.098
3	105.43± 5.17	73.15± 1.407	55.63± 1.76	71.42± 3.58	61.68± 2.59	66.78± 2.011
4	118.89± 4.13	82.84± 0.41	64.21± 1.49	80.25± 1.14	73.63± 2.842	74.72± 0.59
6	120.68± 3.31	90.69± 1.37	71.607± 1.19	89.72± 1.97	79.73± 2.081	84.67± 1.23
8	--	94.609± 1.209	74.57± 2.08	96.33± 2.67	87.22± 0.73	92.58± 2.58
10	--	99.43± 1.27	77.53± 2.13	97.75± 2.33	95.002± 0.37	99.67± 3.62

 Note: The values are presented as the means ± SDs of three determinations.

In vitro diffusion study: In vitro diffusion studies were carried out to assess the permeation of the drug across the buccal mucosa. Among the 3 formulations, F4 showed a maximum drug diffusion of 73.81±0.56% through the cellophane membrane (Table 8); 

Table 8: Cumulative % drug diffusion of carvedilol from formulations F4 to F6

Time (min)	Formulations
	F4	F5	F6
0	0	0	0
5	4.91± 0.22	4.047± 0.21	4.61± 0.34
15	8.98± 0.22	7.14± 0.32	8.53± 0.44
30	10.92± 0.46	13.42± 1.42	10.63± 0.46
60	15.79± 1.13	22.92± 1.071	15.64± 0.93
120	27.02± 1.13	33.63± 0.56	27.024± 0.72
180	37.57± 1.65	42.77± 0.96	36.606± 1.34
240	46.33± 1.81	50.202± 0.89	44.61± 1.061
300	54.49± 1.68	56.55± 1.404	51.35± 1.65
360	61.16± 1.71	61.91± 1.54	57.71± 1.55
420	66.69± 1.25	65.91± 1.09	62.28± 1.74
480	70.74± 0.22	68.34± 0.98	64.82± 1.061
540	72.68± 0.56	70.26± 0.56	66.69± 0.44
600	73.81± 0.56	70.83± 0.32	67.82± 0.44
660	73.21± 0.59	69.98± 0.65	66.62± 0.907

Note: The values are presented as the means ± SDs of three determinations.

       
           
       
Fig. 3: ?R vs. time (zero order kinetics) of formulations F4 to F6 for the diffusion study.

The release kinetic data for the F2 to F6 formulations are tabulated in Tables 9. The highest regression values were obtained for first-order kinetics. The in vitro data were subjected to Higuchi plots, and the regression values for all these formulations indicated that they followed a diffusion mechanism.  

Table 9: Mathematical kinetics models of in vitro diffusion studies.

       
           
       
Fig 4: Log % DR vs. time (first-order kinetics) of formulations F4 to F6 in vitro diffusion study¹⁶.

       
           
       
Fig. 5: ?R vs. SQRT of time (Higuchi kinetic model) of formulation F4 in vitro permeation study¹⁵.

       
           
       
Fig 6: Log ?R vs Log Time (Korsmeyer - Peppas kinetic model) of formulation F4 in vitro permeation study¹⁴