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In present study, a novel minocycline hydrochloride sustained-release capsule was prepared with the new extrusion-spheronization method. The in vitro release studies were performed using marketed sample as a reference and data were analyzed in terms of cumulative release amounts as a function of time. Results demonstrated that the developed analysis method was reliable and convenient for the quantification and dissolution study of minocycline hydrochloride. The release characteristics of different batches of preparations were quite similar with each other, similarity factors f2 of 12 batches were all within 50-100, and our developed sample was similar to reference preparation in release characteristics in vitro. The developed sustained-release preparation may be a promising alternative dosage form for treatment of related diseases.

Introduction

Minocycline exerts a bacteriostatic effect by combining to the site A of 30S subunit in ribosome, to prevent the extension of peptide chain and inhibit protein synthesis process of bacterium and other pathogenic microorganism (Blanchard et al., 2014). Minocycline is the bacteriostatic drugs and has the bactericidal effect under high concentrations condition (Nagpal et al., 2015; Norden et al., 2015).

Minocycline hydrochloride is highly lipophilic, has tissue penetration and a range of pharmacological properties such as anti-inflammatory, anti-enzyme and neuroreparable effects (Freiberg et al., 2004; Shibata et al., 2010).

Nowadays, some new theories and methods for sustained or controlled-release are brought about in drug delivery research. Delayed drug delivery system is now the hot topic of oral controlled-release solid dosage forms for pharmaceutical scientists, for instance, the new sustained-release capsule (Peng et al., 2015; Cui et al., 2015). These systems normally consist of a core and a coating. The core is coated with different barriers by film or compression, and the coating can prevent drug quick-release from the core until the shell is completely swollen or eroded by liquid substance in vivo (Ross et al., 2000; Li and Zhu, 2004). These new pharmaceutical preparations can exhibit drug release constantly at a steady rate, the slow and sustained release of the active compounds is beneficial to patients to maintain sustainable levels in blood, thus will bring out better compliance to those need long-term and continuous therapy (Liu et al., 2012).

The aim of our study is to develop a novel minocycline hydrochloride sustained-release capsule. In vitro dissolution testing is frequently used to evaluate the release characteristics of the pharmaceutical products over time. On this basis, a release assay method was established and validated to the in vitro release study for sustained-release capsules. The dissolution tests are performed in different media, the dissolution profiles of the commercial and self-made ones are compared by similar factors method, to evaluate the drug release performance of the developed formulation.

Materials and Methods

The reference substances of minocycline hydrochloride (purity>99.8%) was obtained from the National Institute for the Control of Pharmaceutical and Biological Products (China). Potassium dihydrogen phosphate and sodium hydroxide were provided by Qinjiuhong Chemical Reagent Co., Ltd (China). Hydroxypropyl methylcellulose, microcrystalline cellulose and ethyl cellulose were provided by Beijing Huajinsheng Technology Co., Ltd. The commercial product (Razadyne ER) was purchased from the market. Chemicals were purchased from Nanjing Chemical Reagent Co., Ltd (China) and were of analytical grade. Distilled water was purified by a Milli-Q System (Millipore, USA).

In brief, an appropriate quantity of minocycline hydrochloride was weighed and mixed with microcrystalline cellulose, followed by adding a solution of hydroxypropyl methylcellulose in water; pellets were prepared by extruding and rolling. Immediate-release pellets were prepared by using spray-coating technique with the solution of hydroxypropyl methylcellulose on fluidized bed bottom to envelop the sealing coat; some small samples were taken for spraying to coat with ethyl cellulose water dispersion on fluidized bed bottom similarly, to produce sustained-release pellets. Finally, the immediate-release and sustained-release ones were encapsulated to capsules proportionally and packed yielded the products (Dong et al, 2014; Oya et al, 2014).

The reference solution of minocycline hydrochloride, blank preparation (including hydroxypropyl methylcellulose, microcrystalline cellulose, ethyl cellulose, lactose and diethyl phthalate) were taken and diluted by HCl solution for UV scanning at 200-400 nm.

The developed samples were 19.07 mg standard in minocycline, the appropriate amounts of minocycline hydrochloride was weighed precisely and dissolved in acid solution, to prepare the testing solutions of different concentrations. Absorbencies were determined at 348 nm for all the samples. The calibration curve samples were assayed in triplicate, using concentration (C) as abscissa and absorbance value (A) as ordinates.

Precision was investigated by determining the replicate QC samples of 100% concentration level in recovery determination experiment on one day and three consecutive days, described as intra-day and inter-day precision, respectively.

Absolute recovery of minocycline hydrochloride was investigated by QC samples, and results were evaluated by comparing the means from the excipients solution spiked with reference solution with that of the standard samples. Three concentration levels of analytes were estimated by analyzing the samples at each level.

The stability of minocycline hydrochloride was investigated using the solution at the concentration of 16 μg/mL. The samples were analyzed at 0, 1, 2, 3, 6 and 8 hours after conditioning at room temperature, respectively, both in acid solution.

The oar method for dissolution test was applied to determine dissolution of minocycline hydrochloride from sustained-release capsules. 900 mL release medium was taken to dissolution glass at predetermined temperature; release medium was agitated by stirring blades at the rotation speed of 100 rpmand sampled at the scheduled time after initiating experiment. 10 mL sample was collected and filtered through a 0.45 μm membrane, filtrate was selected to determine as testing solution.

Minocycline hydrochloride content at each time point was determined by absorbance assay. Meanwhile, the proper amounts of reference substance was dissolved and diluted quantitatively by release medium to the final concentration of 15 μg/mL, which was used as standard solution for the total drug amounts (W). The above solutions were analyzed by external standard method, accumulative release amounts and release percent were calculated according to the formula:

Qn was the accumulative release amounts at each time point, Cn was the measured concentration at each time point, V₀ was the bulk volume of release medium, V_iwas the sampling volume, C_i was the measured concentration at time point i, W was the total drug amounts in capsules

The in vitro release feature of minocycline hydrochloride in developed formulation and marketed product was assessed, both the samples were both taken for dissolution determination in hydrochloric acid solution (0.1M HCl), purified water, pH 4.5 buffer and pH 6.8 medium. The solution samples were taken for dissolution determination at 0.5, 1, 2, 3, 4 hours. Filtrate was taken to determine the accumulative release amounts at each time point and draw the release curve.

According to the sustained-release capsule assay provided by dissolution methods for drug products and the characteristic of product, 0.1 mol/Lhydrochloric acid solution was used as release medium. Sampling time was reset at 0.5, 1, 2, 3 and 4 hours.

According to the guideline for bioavailability and bioequiavailability of orally administered solid drugs, similarity (f₂) measuring was applied to evaluate the closeness between the two dissolution profiles. The f₂ was calculated according to the equations given below:

Where, n is the number of time points, R_t and T_t are the percentages of the reference and testing drug release at each time point t, respectively

In order to consider the release profiles similar, the f₂ values should be close to 100. In general, f₂ value of the two drug release profiles is between 50 and 100, and then these two drug release characteristics are similar, whereas value below 50 indicates differences between the release profiles.

Results

Specificity: It was indicated by UV scanning that no interferences from pharmaceutical necessities and solvent were observed. Thus this method showed good specificity and selectivity for the following study under the selected conditions.

Linearity: The calibration curves were prepared at the concentration levels of 3.814-45.768 µg/mL. The typical curve equations were constructed with a weight of 1/x² and described as A = 0.0341C-0.001, with the correlation coefficients (r) higher than 0.999.

Precision: The results were assessed at low, median and high levels. The mean RSD determined for intra- and inter-day precision were within 1%.

Recovery: Absolute recovery of MH was determined by comparing the samples of three levels that incorporated with excipients to that of the standard solutions which were directly diluted by release medium. The recovery was 99.65 ± 0.54%, 99.36 ± 0.46% and 99.89 ± 0.76% at the three concentrations. All these results showed that the absolute recoveries were high enough for the analysis of in preparation.

Stability: The room temperature stability results of MH with the RSD values lower than 5% showed that the testing samples were stable under storage conditions and routine analysis for release study.

The release curve of the reference formulations minocycline hydrochloride extended-release tablets (batch: 701550, 90 mg, produced by Barr Laboratories, Inc.) was shown in Figure 1. Contrast results of release rates were summarized in Table I. We can conclude that minocycline hydrochloride in different batches of samples was released with nearly the same property, and release rates increased continuously along with the time.

The similarity factors (f2) calculating results for sustained-release capsules in different batches

Batch	f2	Batch	f2
20100104-1	71.19948	20100107-1	72.31736
20100110-1	74.76752	20100104-2	64.19677
20100110-2	66.41798	20100107-2	66.32182
20100104-3	72.33353	20100107-3	72.87776
20100110-3	70.10086	20100104-4	69.17828
20100107-4	68.33718	20100110-4	67.37129

From the above results, it showed that drug release amounts for each capsule were about 30% and 50% at 0.5 and 1 hour, higher than 80% corresponding to 3 hours. Thus the release rates were set as 20-40%, 40-60% and higher than 80% for 0.5, 1 and 3 hours.

The release curves of the developed formulation and marketed product were shown in Figure 2 and Figure 3.

It was indicated that although the release speeds of developed formulation and marketed product were different, almost the total drugs were released from both the preparations at 4 hours. The drug release was delayed about 1 hour and consistent with the gastric emptying time, thus 0.1M HCl was selected as the release medium.

The dissolution curve of developed formulation in 12 batches in hydrochloric acid solution was shown in Figure 4. The similarity factors (f₂) were calculated for the formulations using the release profile with the marketed product as the reference, the results were shown in Table I. From the above results, the f₂ values of 12 batches were all within 50-100, suggesting that their release profiles were quite similar to that of the reference. The time points for sampling were set as 1 and 3 hours finally. The release rates results were summarized in Table II.

Batch	1 hour	3 hours
20100104-1	50.8	98.81
20100107-1	51.21	97.33
20100110-1	50.88	96.88
20100104-2	53.76	100.29
20100107-2	54.45	99.26
20100110-2	54.56	99.24
20100104-3	52.24	99.08
20100107-3	51.65	98.5
20100110-3	53.75	98.54
20100104-4	51.56	99.26
20100107-4	50.59	100.34
20100110-4	50.78	100.47
Reference	50.28	95.04

Discussion

According to the sustained-release capsule assay provided by dissolution methods for drug products and the characteristic of product (Tsao et al., 2014; Chopra et al., 2015), 0.1M HCl solution was selected as release medium, 100 rpm was selected as rotation speed. To make the release curve more precisely, 0.5 and 3 hours were added as the time points for sampling, thus the sampling was defined at 0.5, 1, 2, 3 and 4 hours (Peng et al., 2014; Xue et al., 2014).

For the selection of detection wavelength, the testing solution was taken for UV scanning at 200-600 nm, the result showed that the maximum absorption of MH was at 348 nm, with the absorption value 0.9825. According to the guidelines for UV-spectrophotometric method in Chinese Pharmacopoeia, 2000 edition, the appropriate UV absorption for testing solution was 0.3-0.7 (Simpson et al, 2014; Ifuku et al, 2014), thus 348 nm was set as the detection wavelength. The detection concentration was set as 15 μg/mL(C₂₃H₂₇N₃O₇).

Conclusion

We prepared minocycline hydrochloride sustained-release capsules using the new extrusion-spheronization preparative method, and developed an analysis method for the quantification and dissolution study of MH. The results showed this assay method was reliable and convenient enough for the rapid determination of MH contents quantitatively in high-throughput release characteristic studies. Furthermore, the drugs could be well released from sustained-release carriers within the specified time limit, and the release characteristics of the developed formulation and commercial samples were quite consistent with each other.

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