Introduction

Parkinson’s disease (PD) is one of the most common neurodegenerative motor diseases due to the loss of dopaminergic neurons in the substantia nigra pars compacta (SNpc) [1, 2]. PD has many symptoms, which are mainly divided into two groups of motor and non-motor symptoms. Parkinson’s symptoms usually begin gradually and become worse over time. As the disease progresses, people may have difficulty walking and talking. They may also have mental and behavioral changes, sleep problems, and depression. Tremor, rigidity, bradykinesia/akinesia, and postural instability are the most important motor diagnostic symptoms [2].

Prami is an intrinsic full dopamine receptor agonist having very high selectivity for D2 receptors [3]. It is one of the most widely used drugs in the treatment of PD, which is prescribed both alone and in combination with other drugs, including levodopa. Prami can reduce the dose of levodopa up to 30%; therefore, it can reduce levodopa’s side effects [4, 5]. In addition, Prami has neuroprotective, mitochondrial antioxidant, and mitochondrial hydrogen peroxide release inhibition effects [6,7,8,9,10].

In recent years, nanofiber has sprung up a new class of materials with high surface area and a variety of biomedical applications such as wound coating, tissue engineering, implants, and drug delivery. Electrospinning, as a simple and efficient method, is one of the best techniques to produce nanofibers with a diameter of 1 to 1000 nm. In addition, it has received considerable attention owing to its unique properties and wide application in pharmaceutical sciences. In this method, various biocompatible and biodegradable polymers can be used to produce nanofibers having a very high surface-to-volume ratio, high porosity, and small cavities. According to this method, a certain amount of polymer solution is removed through a syringe and converted into nanofibers under the influence of high voltage. Produced nanofibers are collected on fixed or rotating collectors [11]. Nanofibers with desirable mechanical properties can be used in cases, such as tissue engineering, wound healing, and controlled drug release systems [12,13,14].

Carboxymethyl cellulose (CMC) is a natural polymer used to produce nanofibers due to its good biocompatibility, biodegradability, and cost-effectiveness [15, 16]. Owing to some features, such as chain structure and polyanion repulsive forces, the use of it alone for electrospinning is a challenging task; however, this problem can be solved by blending polysaccharide polymers with other synthetic, non-toxic, and biocompatible polymers [17, 18]. Polyvinyl alcohol (PVA), which is a semi-crystalline hydrophilic, synthetic polymer, has good thermal stability and good physical and mechanical properties [19]. This polymer can reduce the repulsive force in the polymer solution under electrospinning, and by improving the mechanical properties, it can play a supporting role in the formation of nanofibers [15].

Recently, researchers have drawn their attention to oral nanofiber delivery systems. In oral drug delivery systems (DDSs), nanofibers can be delivered as nanofiber scaffolds, electrospun nanofibers as an oral fast delivery system, multilayered nanofiber loaded mashes, and surface-modified cross-linked electrospun nanofibers [20]. Enteric-coated formulations are designed to remain intact in the acidic medium of the stomach and then to release the active drug in the upper intestine [21]. The polymers commonly used to achieve enteric properties are anionic polymethacrylates (copolymerization of methacrylic acid and methyl-methacrylate or ethyl acrylate) (Eudragit®) [22].

This study aimed to prepare slow-release nanofibers of Prami to reduce the release rate of the drug from the formulation. Moreover, since Prami is absorbed in the small intestine, the preparation and evaluation of enteric-coated capsules containing Prami nanofibers were investigated.

Materials and Methods

Materials

Pramipexole dihydrochloride monohydrate was kindly gifted by Nikan Exir Bakhtar API manufacturer (Kermanshah, Iran), and polycaprolactone (PCL, average Mn: 80,000) and glutaraldehyde (GA, 50% aqueous solution) were purchased from Sigma-Aldrich (St. Louis MO, USA). CMC, PVA (average Mn: 72,000 kDa), methanol gradient grade for liquid chromatography, and chloroform were purchased from Merck (Darmstadt, Germany).

Fabrication of Nanofibers

PVA was dissolved in hot distilled water (7 wt%). CMC was dissolved in distilled water (10 wt%). Both solutions were stirred for 24 h at room temperature. Prepared solutions with the weight ratio of 80:20 and 90:10 (PVA:CMC) were mixed and stirred for 2 h. One of the nozzles of a dual pump commercial electrospinning system (Fanavaran Nano Meghyas Ltd., Co., Tehran, Iran) was used to fabricate nanofibers. The feeding rate, applied voltage, and nozzle to collector distance were 0.25 mL/h, 20 kV, and 20 cm, respectively.

Hybrid nanofibers were electrospun using the PVA/CMC mixed solution and PCL. For this purpose, a proper amount of PCL was dissolved in chloroform for 12 h at ambient temperature to obtain 10 wt% PCL solution. A dual pump commercial electrospinning system (Fanavaran Nano Meghyas Ltd., Co., Tehran, Iran) was used to fabricate hybrid nanofibers. The feeding rate, applied voltage, and nozzle to collector distance for PVA/CMC solution were adjusted to 0.25 mL/h, 20 kV, and 20 cm, respectively. The PCL solution was fed simultaneously using the other pump with the feeding rate of 0.5 mL/h and converted to nanofibers under 20 kV applied voltage at the nozzle to a collector distance of 20 cm.

Drug Loading on Nanofibers

Prami (20% wt, to the dry weight of PVA) was added to PVA/CMC 80:20 solution and electrospinning at optimal feeding rate and voltage.

Preparation of Cross-Linked Nanofibers

The fabricated nanofibers were cross-linked using 50% glutaraldehyde vapor for up to 12 h at 37 ℃ to promote physical stability and control the drug release. Then, the mats were placed in a vacuum oven for 48 h to remove unreacted glutaraldehyde residue from the system. Table 1 exhibits the composition of fabricated nanofibers.

Table 1 Composition of fabricated nanofibers
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Physical and Chemical Characterization

The morphology of the nanofibers was evaluated using scanning electron microscopy (SEM; FEI Model Quanta 450 FEG, Hillsboro, OR, USA). The nanofibers were coated with a thin layer of gold using a sputter coater and observed at 20 kV accelerating voltage. The fiber diameter was measured by the Digimizer software at 20 random locations for each mat. The probable interaction between drug and polymers in prepared nanofibers was characterized by Fourier transform infrared spectroscopy (FTIR) (Shimadzu IR-prestige 21). To prepare the FTIR spectrum of the nanofibrous membrane, 2 mg of the samples was mixed with 10 mg KBr and compressed into a tablet form. The IR spectra of these pellets were obtained in a transition mode and the spectral region of 400 to 4000 cm−1.

Swelling Test

A swelling test was done for F7 nanofibers. A certain amount of the nanofibers was weighed and placed in phosphate buffer 0.2 M, pH: 7.4. The nanofibers were weighed at 15, 30, 45, 60, 90, 120, 480, and 1440 min after placing at buffer. SEM pictures from immersed nanofibers were prepared after 2, 8, and 24 h.

HPLC Analysis

High-performance liquid chromatography (HPLC) method was applied to determine the drug amount. The HPLC system consisted of Shimadzu (Japan) equipped with a LC-10AD pump, SCL-10AD controller, and a Waters 2996 photodiode array detector. Data acquisition was performed by the Empower software operated on a Pentium® IV microprocessor. Analysis was carried out at 263 nm with a Kromasil 100 C18 reversed-phase column of 250 mm × 4.6 mm i.d. 5 μm dimensions (VDS Optilab, Chromatographie Technik GmbH, Germany). The column temperature was kept at 45 °C and the injection volume was 20 μL. The mobile phase was an 80/20 mixture (v/v) of methanol and deionized trimethylamine buffer (pH = 4, 0.1% v/v), and the flow rate was 1.3 mL/min.

In Vitro Drug Release

The drug release from Prami loaded nanofibers (F2, F3, F4, F5, and F7) was measured to determine the release profile of the drugs. In vitro release of the drug was investigated by the immersion method. About 40 mg of the nanofibers was placed in a dialysis bag (SERVA, MWCO, 12,400 Da) containing 2 mL PBS (pH = 7) that was added to the dialysis bag to act as the donor solution. The bag was immersed in 25 mL of medium (PBS at pH = 7) and incubated at 25 °C under magnetic stirring (about 400 rpm). At the same intervals, 1 mL of medium was taken and replaced with the same volume of fresh PBS solution. All nanofibers released were investigated in triplicate [23].

Kinetic Study

To investigate the release kinetics of the drug from the formulation, various methods can be used, including analysis of variance, model-independent, and model-dependent approaches.

In this study, a model-dependent method was used to investigate the drug release kinetics. In model-dependent approaches, release data were fitted to kinetic models including the zero-order (Eq. 1), first-order (Eq. 2), Higuchi matrix (Eq. 3), and Korsmeyer-Peppas (Eq. 4) release equations to find the equation with the best fit.

$$C={k}_{0}t$$
(1)
$$\mathrm{Log}\;C=Log\;{C}_{0}-kt/2.303$$
(2)
$$Q={kt}^{1/2}$$
(3)
$$F=\left({M}_{t}/M\right)={k}_{m}{t}^{n}$$
(4)

In Eqs. (1) and (2), C0 is the initial concentration of a drug, k is the first-order rate constant, and t is the time. In Eq. (3), Q is the amount of drug released in time (t) per unit area and k is the Higuchi dissolution constant. In Eq. (4), F is a fraction of drug released at the time, Mt amount of drug released at the time, M is the total amount of drug in dosage form, km is kinetic constant, and n is diffusion or release exponent [24].

MTT Assay

MTT assay was employed to investigate the probability of the nanofiber’s toxicity. One hundred microliters of SFIF-PI44 cell suspension with a concentration of 1 × 106/mL was plated in each well of 96-well plates, and 100 μL of DMEM culture medium with 10% FBS was added to each well and kept at 37 °C temperature, 90% humidity, and 5% carbon dioxide and was incubated for 1 day. F7 fibers were first irradiated with UV for 3 h to sterilize and then immersed for 24 h at 37 °C in a sterile DMEM culture medium. In the next step, the nanofibers were removed from the culture medium and the remaining culture medium was diluted with 0, 6.25, 12.5, and 25 µg/mL Prami content.

After aspiration of the culture medium of each well, 300 μL of each dilution was repeated three times inside the plate wells and incubated at 37 °C, 90% moisture, and 5% carbon dioxide. After 24 h, the culture medium was removed from the medium and replaced with 200 μL of culture medium and 20 μL of substance 3-(4,5-dimethylthiazole-2-il)-2,5-d-phenyltetrahydramide or MTT for 4 h. After incubation, the culture medium was removed and 200 μL of dimethyl sulfoxide was added to stop the reaction. Optical plate absorption was read by the plate reader device at a wavelength of 570 nm. The higher the absorption rate, the higher the number of living cells.

Preparation of Enteric-Coated Capsule of Nanofibers

Eudragit® L30D-55 20% w/w in ethanol was prepared. Twenty-five milligrams of the F8 nanofibers was placed inside the capsule (No. 0), then the capsules were immersed in the Eudragit solution to form a uniform layer on it. The coated capsules were used for dissolution and disintegration tests.

Disintegration Test

Six coated capsules were placed inside the Erweka® disintegration device. The medium was HCL 0.1 N at 37 °C for 1 h. Then the medium was changed to phosphate buffer 0.2 M, pH 6.8, at 37 °C for 30 min. The time of disintegration was noted when the capsules were destroyed.

Dissolution Test

Six coated capsules were placed in Erweka® DT-700 dissolution device. The dissolution medium in the first 2 h was 900 mL HCL 0.1 N at 37 °C and then the medium was changed to 900 mL phosphate buffer 0.2 M, pH 6.8, at 37 °C. Samples were taken at specified times (20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 220, 240, 360, 480, and 600 min) and replaced with fresh medium. The drug concentration in the samples was determined according to the calibration curve obtained by HPLC.

Statistical Analyses

All quantitative results were obtained from triplicate samples. Every data point was expressed as mean ± SD. Statistical analyses were carried out by using an unpaired Student’s t test. A value of p < 0.05 was considered to be statistically significant.

Results and Discussion

Morphology of Electrospun Nanofibers

The morphology of the prepared nanofibers was evaluated using SEM. Figure 1 presents the results. The results showed that the nanofibers were uniform, straight, and without beads. Fiber’s diameter measurement showed that the diameters of nanofibers obtained from F1 and F2 were 348 ± 157 nm and 273 ± 87 nm, respectively (Fig. 1a, b). This could be related to the reduction of the polymer solution’s viscosity upon increasing the weight ratio of CMC. Alipour et al. [25] demonstrated that the addition of CMC to PVA reduced the diameter of the nanofibers. Moreover, in another study, Allafchian et al. [26] reported that increasing the weight ratio of CMC reduced the diameter of the nanofibers.

Fig. 1
figure 1

SEM micrograph of the prepared nanofibers a F1 (PVA/CMC; 90/10); b F2 (PVA/CMC; 80/20); c F6 (hybrid PVA/CMC/PCL; d F5 (cross-linked PVA/CMC); and e F7 (cross-linked PVA/CMC/PCL)

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Figure 1c depicts the SEM image of the hybrid nanofibers of F6. In this nanofibrous mat, two kinds of fibers are presented: the first one, fibers with a diameter smaller than 500 nm (which is related to PVA/CMC/Prami fibers), and the other one, fibers with a diameter larger than 500 nm (which is related to PCL nanofibers); the average diameter of fibers in this hybrid nanofibers is 931 ± 618 nm. Figure 1d, e displays the SEM images of cross-linked F5 and F7 nanofibers, respectively. More entanglement of the nanofibers is apparent in these images, and boundaries of nanofibers are indistinct.

FTIR Analysis

Infrared spectroscopy (FTIR) was used to detect and investigate any interaction between drug molecules and polymers in the structure of nanofibers.

Figure 2 shows the results of FTIR analysis of the following cases: pure PVA, CMC, Prami powder, drug-free PVA/CMC nanofibers, Prami loaded PVA/CMC nanofibers. The Prami characteristic absorption peaks of the pure Prami were found at the wavenumbers of 3321–3417, 2962, 1627, 1587, 1438, and 713 representing the functional groups of N–H stretching, C–H stretching, C=N stretching, C=C stretching, C–N stretching, and C–S stretching vibration, respectively [27].

Fig. 2
figure 2

FTIR results of PVA powder, CMC powder, Prami powder, PVA/CMC combination, and PVA/CMC/Prami nanofibers

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In the infrared spectroscopy spectrum of polyvinyl alcohol (PVA) powder, a relatively wide peak corresponding to symmetrical stretching of the O–H group appeared at 3414 cm−1. The peaks appearing in 2924 cm−1 are related to stretching aliphatic C–H. The peak of stretching carbonyl ketones due to incomplete conversion of polyvinyl acetate to polyvinyl alcohol also appeared in the 1732 cm−1 region. The bands in the regions of 1265 cm−1 and 1091 cm−1 can be attributed to the C–O–C and C–O stretching, respectively [28].

The CMC spectrum displays the bands related to the symmetrical stretching of OH bonds in hydroxyl groups at 3417 cm−1; asymmetrical stretching of C–H bonds of the hydroxymethyl groups (R–CH2OH) at 2924 cm−1; asymmetrical and symmetrical stretching of COO in the carboxymethyl groups (R–CH2OCOO–) at 1616 and 1423 cm−1, respectively; in-plane bending of the C–H and OCH in the R–CH2OCOO– groups at 1326 cm−1; and stretching in the C–O and C–O–C in the groups R–CH2OCOO– at 1118 and 1053 cm−1, respectively [29].

By comparing the infrared spectrum of electrospun nanofibers from PVA and CMC, it was found that the peaks of 1735 cm−1, 1249 cm−1, and 1093 cm−1 showed the stretching of carbonyl, stretching of C–O–C, and C=O, respectively. This indicates that they are related to PVA and have appeared in the nanofiber structure. In the present spectrum, 1616 cm−1 peak asymmetric stretching of COO–carboxymethylcellulose salts are removed after the electrospinning process and due to the similarity of CMC and PVA functional groups, most stretching appears in similar areas, such as CH stretching. Alkanes appear in the 2924 cm−1 region. Finally, due to the higher ratio of PVA polymer to CMC, the intensity of PVA peaks is higher.

In the spectrum related to the Prami loaded PVA/CMC nanofibers, peaks appeared in the areas of 1637 cm−1, 1595 cm−1, 1435 cm−1, and 709 cm−1 related to the stretching vibration of Prami C=N, C=C, C–N, and C–S groups. In the final structure, the presence of stretching PVA carbonyl groups in the region of 1734 cm−1, stretching aliphatic and aromatic CH groups in the region of 2922–2852 cm−1, C–O–C stretching in the area of 1249 cm−1, and C–O stretching in the area of 1091 cm−1 are evident.

Analytical Characterization

Calibration graphs were constructed in the range of 4.3–500.0 μg/mL for Prami. The regression equations of these curves and their coefficients of determination (R2) were calculated as follows:

$$y=5841.4x+1.852\left({R}^{2}=0.9996\right)$$

It should be noted that the calibration curve was outside LOD (the lowest concentration of a substance in a sample that can be detected by the device but not accurately measured) and LOQ (the lowest concentration of a substance in a sample that can be measured by the device with acceptable accuracy) ranges.

LOD and LOQ were calculated by standard deviation and slope of the calibration curve (LOD = 6.795 μg/mL, LOQ = 20.589 μg/mL), indicating the high sensitivity of the proposed method to detect small amounts of Prami. The recovery percentage of Prami in this method of analysis was 96.8%, demonstrating the accepted accuracy of the results.

Intra-day accuracy was calculated as the RSD% of the response of three different concentrations, each of which was injected three times into the device. The average of these 3% standard deviations was 1.09, which was an acceptable amount. The inter-day accuracy was calculated by evaluating the response of three different concentrations of the drug injected into the device for three consecutive days and repeated three times per day. The mean RSD of these three responses was 2.79%, which was an acceptable amount.

Swelling Test

Figure 3 shows the weight gain of F7 nanofibers in the swelling test. As the diagram shows, nanofibers could absorb phosphate buffer 2.5 times as much as the dry weight of nanofibers in approximately 30 min. The weight gain was reduced slightly after about 2 h and then rose again.

Fig. 3
figure 3

The weight gain of nanofibers in buffer phosphate pH 7.4

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Figure 4 shows SEM images of fibers during the swelling test (after 2, 8, and 24 h). It can be seen that compared to the dry fiber (273 ± 87 nm), a noticeable change has been occurred in diameter after 2 h (334 ± 75 nm in Fig. 4e). In 8 h, somewhat reducing can be seen in diameter size of nanofibers (301 ± 74 nm), and at 24 h, some of the fibers are expanding (324 ± 59) (Fig. 5f, g).

Fig. 4
figure 4

SEM images of F2 nanofibers (a), F7 nanofibers after 2-h (b), 8-h (c), and 24-h (d) immersion in phosphate buffer at pH 7.4, and F5 nanofibers after 2-h (e), 8-h (f), and 24-h (g) immersion in phosphate buffer at pH 7.4

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Fig. 5
figure 5

The drug release profile of the following: a PVA/CMC nanofibers (F2), PVA/CMC nanofibers exposed to glutaraldehyde vapor for 4 h (F3), PVA/CMC nanofibers exposed to glutaraldehyde vapor for 8 h (F4), and PVA/CMC nanofibers exposed to glutaraldehyde vapor for 12 h (F5); b PVA/CMC nanofibers 12 h exposed to glutaraldehyde vapors (F5) and PCL/PVA/CMC nanofibers 12 h exposed to glutaraldehyde vapors (F7); and c F7 nanofibers in capsules with Eudragit enteric-coated

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Drug Release from Nanofibers

Effect of Cross-Linking

Figure 5a shows a diagram of drug release from different nanofibers: the PVA/CMC nanofibers (F2) and cross-linked PVA/CMC nanofibers that have been exposed to glutaraldehyde vapors at different times (F3, F4, and F5). As the diagram shows, the release profile of Prami from F2 nanofibers, 60% of the loaded drug was released in the first 45 min. This significant slope of drug release continued up to 1 h after the start of the release. Afterward, the drug release rate decreased significantly, so that in monitoring the release of the drug from this nanofiber in 8 h, approximately 75% of the total drug loaded in the formulation was released. The release profile of PVA/CMC nanofiber 4-h exposure to glutaraldehyde vapors (F3) had a significant 2-h release, and approximately 54% of the nanofiber loaded drug was released. In the monitoring of the release within 8 h, a maximum of 58% of the loaded drug was released, which was not significantly different from the release rate of the first 2 h; however, in 24 h, 60% of the drug was released.

The release profile PVA/CMC nanofibers were exposed to glutaraldehyde vapor for 8 h (F4), indicating a significant 4-h release slope during which approximately 45% of the drug loaded into the nanofibers was released. During the 8-h monitoring of the release of this nanofiber, 48% of the total loaded drug was finally released. In addition, at 24 h, the drug release rate did not change significantly.

As shown in the release diagram of Prami from PVA/CMC nanofibers exposed to glutaraldehyde vapor for 12 h (F5), during the first 2 h, the drug release from the nanofibers had a significant slope, which 42% of the drug loaded was released from nanofibers. After this period, the release rate was decreased, so that only 45% of the drug was released in 8 h, and the release rate was increased to 58% in 24 h.

The effect of cross-links on the control of drug release from nanofibers is well known [29]. According to Fig. 5a, cross-linked nanofibers with glutaraldehyde had a slower release, confirmed by Dash et al. [30] research. The best duration of exposure of nanofibers to glutaraldehyde vapors was 12 h.

Given the behavior of nanofibers, the drug delivery occurs in the PVA/CMC matrix based on the following mechanism: water diffusion into the environment and penetration into the top layers of the matrix, swelling the fibers, dissolution of the matrix and creation of a gel layer on the polymer network, dissolution of the drug inside this layer in water, and transport to the bulk media.

By this stage, the rapid delivery of the surface drug is observed. The gel formed on the surface of the fibers makes it difficult for solvent molecules to penetrate and reach the underlying polymeric matrix. Hence, the drug delivery stops after a while, and the re-delivery will be interrupted. This causes the delivery chart to become a plateau. In other words, fluctuations will be observed in drug delivery. The higher the entanglement of the polymer network (the more cross-links due to prolonged exposure to glutaraldehyde vapors), the greater these fluctuations will be. This is confirmed by the diagram for drug delivery from cross-linked PVA/CMC fibers.

Results of the swelling test can confirm this result. The rapid increase in weight and diameter of nanofibers indicates the absorption of water and the formation of a gel layer at the surface of nanofibers. This degree of swelling of the fibers can be considered the main reason for the rapid release of the drug in the first 2 h. The weight gain was reduced slightly after about 2 h, which seems to be due to gel-sol conversion and the removal of some water and polymer from the surface of nanofibers (Fig. 4c, f). The dissolving of this layer caused to somewhat reduced in size and weight of the nanofibers. By absorbing water in the following layers, the gel will re-create, and the nanofiber’s size and weight would be relatively increased (Fig. 4d, g).

Effect of PCL: Hybrid Nanofiber

Figure 5b depicts the effect of PCL hybrid nanofibers on drug release profiles of cross-linked PVA/CMC nanofibers. According to the results, 44% of the drug loaded in F7 was released during the first 4 h. It then hit a 4-h plateau, and after 8 h, 48% of the drug release was obtained. The drug release rate reached approximately 60% in 24 h. By comparing this nanofiber to F5 nanofibers, it can be concluded that F7 can reduce drug release.

In hybrid nanofibers, the presence of PCL regulates the drug delivery and the uniform exit of the drug from the polymer matrix. In other words, this hydrophobic polymer limits the mobility of solvent molecules and their penetration into hydrophilic fibers. Furthermore, the part of the drug rapidly released from the surface of the PVA / CMC, trapped in a polymer PCL network, and is not able to be released at once. Thus, the slope of the initial delivery rate of the drug decreases in terms of PVA/CMC nanofibers. Also, in the presence of PCL, as the drug released from the surface is gradually released into the environment, we will not have any release fluctuations. PCL acts as a control agent for drug release from the polymer matrix system. Briefly, it seems that the PCL nanofibers can create relative coverage on PVA/CMC nanofibers, thereby reducing the rate of solvent absorption onto the PVA/CMC fibers and consequently reducing the drug release rate.

Based on the obtained results, the release kinetics of Prami in the F7 nanofibers was investigated. For this purpose, the drug release in the nanofibers was fitted with different models of release kinetics: zero-degree, first-degree, Higuchi, and Korsmeyer-Peppas models. Table 2 shows the linear and regression equations of each of the Prami release models in nanofibers. Based on obtained R2, it seems that the Korsmeyer-Peppas model is the most compatible with drug release. This model is designed to investigate the release of water-soluble drugs from polymeric drug delivery systems [30, 31]. Results of the release and swelling test confirm the release kinetics with this model. Taken together, based on this kinetic and the other results, it can be said that diffusion is probably the mechanism of drug release, and its control is done by cross-linking as primary and PCL as secondary controls. This theory has been investigated in other studies [32].

Table 2 Kinetic models of drug release from hybrid PCL/PVA/CMC nanofibers exposed 12 h to glutaraldehyde vapors
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Disintegration Test of Capsules

The capsules did not change much in an acidic environment and were able to maintain their structure. However, when they were placed in a phosphate buffer medium, it took about 15 min for them to be destroyed. These results indicate that the coating prepared from Eudragit was suitable for the capsules as an enteric-coated capsule. Since the capsules should not be opened in the acidic environment of the stomach, in this test, the capsule shells were not opened in an acidic environment. In addition, the capsules should be disintegrated in pH 6.8 which is an intestinal environment, so that the drug can be released from the formulation [33].

Dissolution Test of Capsules

Figure 5c shows the release rate of the drug from F7 nanofibers placed inside capsules coated with Eudragit. As Fig. 5c shows, the first 2 h are an acidic environment in which the drug is not released. When the capsules were placed in a phosphate buffer medium of 0.2 M with pH 7.4, the drug began to be released from the formulation. As the diagram shows, according to the release of F7 nanofibers, approximately 50% of the drug was released in 8 h.

Since the drug is not released in an acidic environment, and according to the results of the disintegration test, the coated capsule is resistant in an acidic environment and prevents the drug from being released. In enteric-coated formulations, it is highly important not to release the drug in an acidic environment [34].

MTT Assay

Because the fiber as oral drug delivery stays in the gastrointestinal tract for up to 24 h and the drug is released in the intestine, its potential toxicity to intestinal cells was evaluated. Cytotoxicity of F2, F6, and F7 nanofibers on SFIF-PI 44 cell lines were investigated. According to Fig. 6, results showed that based on 50% maximal inhibitory concentration (IC50), the nanofibers had no cytotoxic effects in used concentrations of drug and polymers.

Fig. 6
figure 6

MTT results of Pramipexole (1), F7/Pramipexole nanofibers (2), F6/Pramipexole nanofibers (3), and F2/Pramipexole nanofibers (4)

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Conclusion

In this study, we attempted to fabricate and investigate the release profile of Prami in nanofibers produced under different conditions. The drug was loaded and electrospun in two types of the mixture of PVA and CMC polymers, and its release was investigated by the effect of variables, such as PCL hybrid nanofiber and cross-linking glutaraldehyde, alone or in combination with each other. This study aimed to find Prami controlled release nanofibers so that the release of the drug takes between 8 and 10 h, as long as the food passes through the small intestine. The release profile of 8 nanofibers indicated that the nanofiber PCL/PVA/CMC exposed 12 h to glutaraldehyde vapors (F7) was the best choice since it had the longest release time of the drug during the first 8 h of release and it had an acceptable cytotoxicity profile. The results confirm the effect of cross-linking the hydrophile polymeric matrix and PCL as a hydrophobic polymer in control of drug release in hybrid electrospun nanofibers. As it is possible to achieve 8 h of uninterrupted release with the changes of the variables mentioned in this study, it can be used to prepare drug enteric-coated capsules containing nanofibers. The optimal nanofibers were placed in gelatin shell capsules, and the dissolution profile of the drug in the capsules was investigated. The nanofibers can be focused as a new formulation in capsule dosage form to reduce the excipients used in tablet form and ease of production.