3.2. Methods

The saturation solubility test was performed in various media to investigate the pH-dependent solubility of TEL. Excess TEL powder was placed into microtubes (Eppendorf, Westbury, NY, USA) containing 1 mL of test media: enzyme-free simulated gastric fluid (pH 1.2), acetate buffer (pH 4.0), enzyme-free simulated intestinal fluid (pH 6.8), phosphate buffer (pH 10) adjusted by 1 N NaOH solution, or deionized water. The samples were vortexed and then placed under mechanical stirring at 25 ± 0.5 °C at 100 rpm for 5 days, followed by centrifugation at 10,000× g for 10 min. The supernatant was collected, filtered through a 0.45 µm PTFE syringe filter, and then diluted with mobile phase to quantify the TEL concentration using HPLC (Agilent 1260 Infinity, Agilent technologies, Santa Clara, CA, USA). The HPLC system had a pump (Agilent 1260 Quat pump), a Capcell Pak C18 column (Shiseido, 250 × 4.6 mm I.D., 5 μm) maintained at a 35 °C oven temperature. The mobile phase consisted of a 40:60 (v/v) mixture of potassium dihydrogen phosphate and acetonitrile (pH 3.7) eluted at 1 mL/min and 10 μL volume of injection. The signals were monitored with an Agilent 1260 VWD detector at λ = 296 nm.

In order to select the appropriate solid carriers for SD, various hydrophilic polymer and alkalizers were evaluated for their aqueous solubility with the drug. An excess of TEL powder was added to 1 mL aqueous solution of each polymer or alkalizer and the suspension was vortexed. Then, the samples were mechanically stirred (water bath) for 5 days at 25 °C and 100 rpm. The resulting suspension was centrifuged at 10,000× g for 10 min, subsequently filtered using a 0.45 µm PTFE syringe filter. The filtrate was sufficiently diluted with the mobile phase and assessed for TEL concentration by HPLC method, as described above. All experiments were carried out in triplicate (n = 3).

The polymer that showed the highest aqueous drug solubility and an alkalizing agent (chosen specifically) from the screening result were selected as appropriate carriers. Then, the mixture of drug and carriers was blended roughly 10 min using a V-shell blender and extruded using a co-rotating twin-screw mini HME (Haake MiniLab II, Thermo Fisher Scientific, Karlsruhe, Germany) to obtain the TEL-loaded pH_M-SDs. To determine the optimum drug-carriers ratio, 12 formulations were prepared by varying the weight ratio of the drug (telmisartan, TEL), polymer (Soluplus^®, SOL), and alkalizer (sodium carbonate, SC) (Table 4). The HME processing conditions were barrel temperature 150–160 °C, screw speed 100 rpm, and feed rate 3 g/min, respectively. Thus, generated extrudate was milled into a fine powder by means of a laboratory grinder and sieved through a USP #30-mesh to produce a fine dry powder. Furthermore, all the prepared formulations were evaluated for their aqueous solubility test as per the method described above. The formulation showing the highest aqueous drug solubility was chosen as the most appropriate and used for further studies.

Compositions of TEL-loaded solid dispersion (% w/w).

We prepared the tablets for dissolution and stability studies using direct compression via a single-punch tablet press machine (MCTMI, GlobePharma Inc., New Brunswick, NJ, USA). Before direct tablet compression, Flowlac 90/Ac-Di-Sol/magnesium stearate was mixed at a weight ratio of 90/9/1. Then, the excipient mixture was introduced to the drug powder (40 mg) or optimized formulation (100 mg). Subsequently, the final powder mixture (200 mg) was manually fed into the die (10 mm) and pressed with a compression force of 500 psi to obtain flat-faced tablets of 200 mg total weight, each containing 40 mg of equivalent TEL. The hardness of the tablets was manually evaluated and compared to the commercial tablets.

The drug release profiles of pure TEL tablets, in-house-developed tablets, and the marketed tablets were computed using a USP type II dissolution apparatus (ERWEKA; DT 620, Heusenstamm, Germany). The samples were placed into the dissolution tester filled with 900 mL of distilled water at 37 ± 0.5 °C with a paddle stirring speed of 75 rpm. The powder sample was loaded in a size “0” capsule and the capsules were placed inside sinkers and subjected to dissolution testing. One milliliter of sample was collected at different time intervals (0, 5, 10, 15, 20, 30, and 60 min), and an equivalent volume of fresh medium was subsequently replenished into the vessel to compensate for the media loss. The samples were immediately filtered using a 0.45 µm PTFE syringe filter and sufficiently diluted with mobile phase. After sufficient dilution of the filtrate, the content of TEL was quantified by the HPLC, as detailed above. All experiments were repeated in triplicate.