Effect of raw materials on the formulation of norfloxacin tablets

Norfloxacin is a synthetic antibacterial fluoroquinolone. It is related to nalidixic acid, but its relative potency has been increased by a fluorine atom and a piperazine moiety.¹ Smith et al. reviewed the role of fluoroquinolones, particularly ciprofloxacin, norfloxacin, ofloxacin and lomefloxacin, to treat various eye infections.²

Batch-to-batch variation in the raw material properties and variation between lots from different suppliers can lead to processing difficulties and variability in the properties of the finished product. Such variability may be because of a change in raw material properties arising from modification to the raw material manufacturing process.³ Changes in raw material may occur during the early drug development stage, or when a supplier identifies a cheaper route to prepare the raw material. Apart from polymorphism and solvate formation, there are few reports on interbatch variation in active ingredients, and the effects of this variation on the subsequent product properties and performance. More subtle variability in materials can occur, resulting in problems for formulators. Subtle process changes may have marked effects on active ingredients, as indicated by studies performed on ibuprofen.³ Mechanical property changes in ibuprofen compacts could be produced simply by alteration in the crystallization rate of raw material.

Batch-to-batch variability in the particle size distribution and particle shape of the drug raw material can result in problems during compression. It has been shown that the variability in particle size of raw materials affects their compaction properties.³ Granulation techniques are expected to partly overcome batch-to-batch variability in particle size distribution and the particle shape of the raw material, but this does not rule out problems during compression. Furthermore, variation in drug release once disintegration of granules is complete may be anticipated.⁴

Although physicochemical preformulation screening is practised universally within the pharmaceutical industry, physicomechanical screening is applied to a lesser extent and often only where a problem exists. For example, a physicomechanical quality control test may be introduced during the early development of a formulation to avoid processing or quality problems, such as flow tests for direct compression formulations and surface area tests for drugs where the dissolution rate may be a problem.

Key points

The aim of the present work is to develop norfloxacin tablet formulations by either slugging of RM1 or pregranulation of the noncompressible RM2 with nonaqueous fluid. This will minimize water sorption encountered through wet granulation of RM2 during preparation of the commercial F4 tablets. Two types of commercial norfloxacin raw material will be investigated, a coarse and a fine noncompressible raw material. A third granular form will be developed from the fine commercial drug powder RM2 and further formulated into tablets. Both the physicochemical and physicomechanical characterization of the three materials will be performed. The norfloxacin tablets prepared in this study will be coated to mask the bitter taste. Evaluation will include assessing physical and chemical stability in comparison with tablets prepared by wet granulation, shortly after production and during storage for 12 months under different storage conditions.

Table 1 Norfloxacin raw material characteristics.

Experimental

Materials.

• Norfloxacin from two different suppliers was used: coarse (RM1 [Merck AG, Germany]); fine (RM2 [Siegfried AG, Switzerland])

• PVP K-25 (ISP, Switzerland) as a wet granulation binder

• Aerosil 200 fumed silica (Degussa AG, Germany) as a powder flow aid

• Avicel PH102 microcrystalline cellulose (Lehmann & Voss & Co., Germany) and lactose monohydrate (DMV Campina BV, Holland) as fillers

• talc powder (Tardy, France) and magnesium stearate (Union Deriva, Spain) as lubricants

• Ac-Di-Sol croscarmellose sodium (FMC International, Ireland) as a disintegrant

• starch (Cerestar, Belgium) paste (1%) as a wet binder

• Eudragit E-100 polymeric methacrylate (Rohm Pharma GmbH, Germany) and Methocel E5 LV hydroxypropyl methylcellulose (Colorcon Ltd, UK) as coating agents

• isopropyl alcohol as solvent for coating polymers, methanol as a mobile phase and 1-hexanesulfonic acid sodium salt (BDH Chemicals Ltd, UK) as ion-pairing agent

• monobasic potassium phosphate, glacial acetic acid (El-Nasr Pharmaceuticals Co., Egypt) and sodium hydroxide (Kirsch, Germany) as acetate buffer components

• ethyl alcohol (El-Nasr Pharmaceuticals Co.) as solvent for PVP

• ethylene diamine analogue of norfloxacin, a gift from Merck Sharp & Dohme (Rhahway, NJ, USA) as a degradate of norfloxacin.

Methods. Raw material preparation and testing:

Granulated raw material (RM3) was prepared in a Torrmat (Type FH-6; Drias, Germany) machine by kneading RM2 with a 2% w/v alcoholic solution of PVP K-25, followed by drying in a hot air oven at 60 °C for 5 min. The particle size of the wet granules was then reduced by passing through an oscillating granulator (Type F.G.S.; Erweka, Germany) fitted with a 1 mm sieve.

Table 2 Composition of norfloxacin tablet formulations.

All three norfloxacin raw materials (RM1, RM2 and RM3) were evaluated using relevant physicochemical and physicomechanical tests (Table 1).

Materials were ground with Nujol mull and placed between two potassium bromide disks, and their infrared spectra were recorded using spectrophotometer Model 1430 (Perkin Elmer, USA).

Differential scanning calorimetry (DSC) was performed on 8–10 mg samples that were heated at a rate of 10°C/min between 20–280°C using a Mettler TA 4000 thermal analysis system (Mettler Instrument Corp., Hightstown, NJ, USA). Aluminium pans and lids were used for all samples. Instrument calibration was performed periodically using indium as a standard.

Moisture content of the raw materials was determined by the Karl Fischer method as described in the USP 25.⁵

Moisture uptake was determined by placing 2.0 g sample of each raw material in open, previously weighed petri dish. Petri dishes were then put into a desiccator containing a saturated solution of sodium chloride in purified water (with excess salt) to give a relative humidity of about 75%.⁶ The desiccators were stored at two different temperatures 25 ±1 °C and 40 ±1 °C. These conditions were considered representative of storage in temperate and tropical regions. Petri dishes were then removed from the two desiccators at predetermined time intervals (7, 14, 30, 45, 60 and 90 days) and rapidly weighed. The weight gain, representing the amount of moisture absorbed, and the percentage moisture gain were calculated and plotted against time. Monitoring the weight gain was continued until there was no further change in weight.

Figure 1 SEM of norfloxacin raw materials; (a) RM1 and (b) RM2.

Drug release testing was performed on 0.4 g samples of raw materials in a USP Apparatus II paddles (Pharma Test PTWS III, Germany) employing 750 mL of acetate buffer pH 4, at 37 °C and 50 rpm. 5 mL dissolution media samples, replaced with fresh medium, were withdrawn at a predetermined time, filtered, diluted and assayed spectrophotometerically at 313 nm.⁵ All dissolution experiments were run in duplicates.

The compactability of the raw materials was investigated on a single punch, noninstrumented tablet machine Type AR400 (Erweka GmbH, Germany). 0.6 g of norfloxacin powder were compressed into tablets employing 11 mm concave punches. Punches and dies were lubricated by direct application of a film of magnesium stearate powder. Three different arbitrary compression forces were applied, designated as low, medium and high.

Figure 2 Particle size distribution of tested norfloxacin raw materials.

Powder particle size was determined using laser diffraction (LD) analyses. Samples were dispersed in water using 1% w/v Tween 80 as a wetting agent. An ultrasonic probe was applied to the suspension for varying time periods up to a maximum of 1 min. At various time intervals, samples were pumped through the flow cell of the LD analyser Sald-1100 (Shimadzu, Japan) for sizing.

Figure 3 Moisture uptake of norfloxacin raw materials at 75% RH at (a) 25 °C and (b) 40 °C.

Scanning electron microscopy (SEM) was performed on RM1 and RM2 samples using SEM JSM–5300 (Jeol, Japan).

Powder flow was determined using angle of repose and Carr's compressibility index.^7,8

Tablet preparation and testing

The four tablet formulations (567 mg target weight) used in this study are shown in Table 2. The compositions of the three developed tablets were kept as close as possible to the commercial F4 tablets to allow reliable comparison of all formulations. Formulations F1 and F2 were prepared by slugging using the same tablet machine described above and 25 mm punches. The breaking force of the slugs range was 26–33 KN. The obtained slugs were crushed in an oscillating granulator Type FGS (Erweka, Germany) fitted with 1 mm sieve, mixed with other additives in a double-cone mixer Type DKM (Erweka) and finally compressed. F3 tablets were prepared by direct compression after mixing of all ingredients for 10 min followed by another 5 min after adding the lubricants in a double-cone mixer. F4 tablets were prepared by wet granulation (WG) of norfloxacin RM2 and other ingredients (except lubricants) with starch paste as a binder. WG was performed in a Torrmat kneader and the mass was subsequently passed through a 2.5 mm sieve and dried in a hot air oven at 60 °C for 6 h. Fumed silica, talc and magnesium stearate were added to dry granules and mixed for 5 min in the double-cone mixer. All tablets were prepared using 11 mm concave punches at the arbitrary set medium compression force.

Figure 4 Effect of storage at 40 °C and 75% RH on breaking force (KN) of uncoated (F1–F3) and coated (F1–F4) norfloxacin tablet formulations.

The tablets were film coated in a coating pan (class - E; Manesty, UK) using a 5% solution of either HPMC or Eudragit E-100 in isopropyl alcohol.

The breaking force of the compacts was determined on six tablets from each batch using a hardness tester (PTB 3H, Pharma Test, Germany).

Friability testing was performed on 10 weighed tablets using a Pharma Test machine (Type TAD, Erweka GmbH, Germany) at a fixed speed (25 rpm) for 15 min. The friability testing procedure described above is the same adopted by the manufacturer of the commercial F4 tablets and is applied in this study to enable satisfactory comparison with the experimental formulations (F1–F3).

Disintegration testing was performed on six tablets (Pharma Test) according to the specifications of USP 25 disintegration test for plain (non-enteric) coated tablets using water maintained at 37±2 °C.⁵

Dissolution studies were performed according to USP 25 monograph for norfloxacin tablets.

Assay of drug content was conducted on 20 powdered tablets using 1M sodium hydroxide solution. The absorbance of a standard and test solutions were measured at 335 nm against a blank.⁹

Figure 5 Effect of storage at 25 °C and 40 ° C and 75% relative humidity (RH) and uncontrolled humidity (Un) on disintegration time of F4 norfloxacin tablets.

Stability testing

Coated and uncoated tablets from the four formulations were stored in unstoppered plastic bottles away from light and under each of the following conditions:

• Room temperature (RT, 25 °C) and uncontrolled relative humidity (RH).

• 40 °C and uncontrolled RH.

• RT and 75% RH.

• 40 °C and 75% RH.

Tablets were tested after 3, 6, 9 and 12 months of storage. Any changes in physical parameters including appearance, breaking force, friability, disintegration time and dissolution rate were recorded.

The stored tablets were monitored for any formed ethylene diamine analogue of norfloxacin using a stability-indicating HPLC procedure.¹⁰ The mobile phase consisted of 300 mL methanol and 700 mL of distilled water, to which 1.74 g of monobasic potassium phosphate and 20 mg of ion-pairing reagent (1- hexanesulfonic acid, sodium salt) were added. The pH of the mobile phase was adjusted to 3 with 85% phosphoric acid. Flow rate was 1.5 mL/min, sample volume was 30 µL, UV detector was set at 280 nm.

Results and discussion

Evaluation of raw materials: The infrared spectra (data not shown) of the three raw materials were in accordance with reported spectra of reference 9 with no well-defined carboxylic C=O stretch at 1750 cm^-1 . This indicates that all three raw materials are not anhydrous norfloxacin but exist as norfloxacin dihydrate.

The DSC traces (data not shown) revealed identical drug transition peak (223–225 °C) and more or less the same transition peaks at 105–127 °C, corresponding to the evaporation of water and indicating the presence of either the dihydrate or sesquihydrate forms for the studied raw materials.⁹

SEMs of RM1 and RM2 indicated that the latter material consisted of fine crystalline material; whereas large hexagonal crystals were observed in RM1 mounts (Figure 1). Table 1 and Figure 2 show results for particle size analysis of the raw materials. The average particle size was found to be 198, 26 and 730 μm for RM1, RM2 and RM3, respectively. This variation had direct effects on the flow and compression properties of the materials. Particle size distribution, as well as bulk density changes would influence die filling and hence compression properties of dry raw materials as shown previously for ibuprofen.³

Figure 6 Moisture uptake at 25 °C and 75% RH of developed F1, F2 and F3 Figures (a), (b), and (c) respectively and commercial F4-tablets Figure (d) along with their respective raw materials (RM): U, uncoated tablets; H, coated with HPMC and E, coated with Eudragit E-100.

The values of the moisture content of the three raw materials (RM1– RM3) were 3.13%, 8.76% and 9.53% w/w, respectively. RM2 exhibited higher moisture content than RM1 most probably because of its higher surface area available for moisture sorption.

The moisture uptake profiles for the three raw materials (RM1–RM3) stored at different conditions exhibited similar trend (Figure 3). RM2 material showed the highest moisture uptake rate, followed by RM1 while RM3 exhibited the lowest rate. This is in agreement with the particle size analysis, as RM2 particles were the finest with expectedly the highest surface area available for moisture adsorption. The moisture uptake at 25 °C was slightly higher as compared with 40 °C condition. This may be explained by decreased sorption of gases including water vapour attributed to higher kinetic energy at higher temperature.¹¹ The moisture uptake for the three raw materials progressed at a rapid rate at first because of moisture adsorption by the available surface of the upper layer of the powders. This was followed by slower rate of moisture uptake by the deeper layers. A plateau region was attained when the surface of the powder became almost saturated with water, resulting in blockage of the surface preventing further sorption.

The dissolution profiles of the raw materials were similar (data not shown). All tested raw materials released almost 100% in less than 10 min. The granular RM3 showed the fastest initial dissolution rate, which could be attributed to the hydrophilic nature of PVP, followed by coarse RM1 and finally the fine RM2.

RM3 was readily compacted at all applied compaction levels, with the production of satisfactory compacts (breaking force 6–11 KN). RM1 could be compacted only at the high compaction level (breaking force 5–7 KN) and failed to produce compacts at low and medium compaction levels. However, RM2 was not compressible at any of the studied compaction forces.

Angles of repose were 51 °, 58 ° and 39 ° for RM1, RM 2 and RM3 respectively (Table 1). The coarse norfloxacin RM1 exhibited better flow than the finer raw material RM2. Granular RM3 showed the best flow. As a general guide, powders with angles of repose greater than 50 ° have unsatisfactory flow, whereas angles close to 25 ° correspond to good flow properties.⁸

Table 1 shows bulk, tapped densities and the resulting compressibility index. A decrease in bulk density may be associated with a reduction in particle size (RM1 compared with RM2) and production of a loose packed powder bed, which although being porous is unlikely to flow because of the inherent cohesiveness. The results of percentage compressibility of the three raw materials were 19.6, 38.0 and 13.8, respectively (Table 1). These results indicate that the powder flow is fair for RM1 and very poor for RM2, while it is good for RM3.⁸

Evaluation of tablets

Coated and uncoated tablets from the four formulations had breaking force between 6 and 12 KN (Figure 4) and acceptable friability (0.7–0.89); as confirmed by subsequent successful coating process. The observed relatively high friability is because of the adoption of a longer testing period by the manufacturer of F4 tablets, compared with the USP specifications (100 revolutions at 25 rpm). The mean disintegration times of the uncoated tablets were 10, 8, 4 and 11 min for F1, F2, F3 and F4 tablets, respectively. Both film-coating polymers did not affect tablet disintegration.

Figure 7 Effect of storage at 25 °C and 75% RH on the dissolution rate of F4 tablets.

Both the coated and uncoated tablets passed the USP dissolution test (data not shown), as the amounts of norfloxacin released were more than 80% in 30 min. However, F2 tablets exhibited initial slower dissolution, which may be attributed to the effect of slugging. Moreover, no marked differences were observed in the dissolution behaviour of the tablets coated with either Eudragit or HPMC.

The results of moisture uptake by the various tablet formulations are shown in Figure 6. The uncoated F1, F2 and HPMC-coated F4 tablets picked up moisture, while F3 tablets showed essentially a desorption phase, which may be explained by evaporation of residual isopropanol. The rank order for sorption of uncoated tablets (F2>F1>F4) was in accordance with the moisture uptake profiles of the powder raw materials (Figure 3).

The characteristics of raw materials induced different levels of moisture uptake by their respective tablets. The highest level was observed with F2 and to a lesser extent with F1. However, the desorption process occurring with F3 tablets prepared by pregranulation was not noted with RM3, while F4 tablets prepared by wet granulation showed only a small increase (up to 2%) of its weight. This may be because of the inherently high initial moisture level following the wet granulation adopted in preparation of F4 tablets. The moisture uptake levels of the tablets stored at 25 °C were higher than those obtained upon storage at 40 °C, in accordance with those obtained with norfloxacin raw material.

The profiles in Figure 6 indicate that the increase in moisture uptake of tablet was linear in the initial stage due to the moisture absorption by the outer layers of tablets, followed by slower phase in the deeper layers. A plateau region followed this initial phase after the surfaces of the tablets became saturated with water vapour and no more water vapour could further penetrate the surface. The rank order of moisture sorption for the tablets of each formula was: uncoated tablets > tablets coated with HPMC > tablets coated with Eudragit E-100. Thus, Eudragit E-100 coat provided better protection against moisture sorption.

Tablet stability

Insignificant changes were observed in the appearance, breaking force and the friability of norfloxacin tablets during the stability study for all tested formulations. However, F4 tablets showed the highest increase in breaking force (Figure 4).

Minor changes were observed in the disintegration time during the stability study for all tested tablets except for F4 tablets (Figure 5), which failed to disintegrate completely in less than 15 min following storage for only 3 months at the different storage conditions.

Although freshly prepared F4 tablets passed the USP 25 dissolution test, they failed to pass the test after storage, for example, after 12 months at 25 °C and 40 °C (Figure 7). Statistically significant differences (P=0.001; ANOVA) in dissolution results were evident for freshly prepared F4 tablets, compared with those stored for 3, 6, 9 and 12 months at the same storage temperature.

All other stored tablets (F1, F2 and F3) passed the dissolution test after storage under different storage conditions. Dissolution patterns of tablets aged under uncontrolled humidity were in accordance with those aged at 75% RH. Furthermore, no statistically significant differences (P=0.001) were noticed between dissolution data of the freshly prepared tablets (F1–F3) and those stored for 12 months at the same storage temperature, as evidenced by ANOVA test.

The integrity of norfloxacin in stored tablets was assessed by a stability-indicating HPLC procedure,¹⁰ which distinguished norfloxacin from its degradation product (the ethylene diamine analogue). No traces of ethylene diamine analogue could be detected in any of the tested tablets under different storage condition during the whole period of the stability study.

Conclusions

The granular form of norfloxacin (RM3) prepared in the present work showed better overall physicomechanical properties compared with RM1 and RM2, including:

• enhanced flow properties

• higher bulk and tapped density

• better compatibility

• no change in polymorphic form and dissolution profile

• minimal moisture uptake.

Deterioration in drug release characteristics was marked in F4 tablets stored under different conditions and resulted in failing the USP 25 dissolution test for norfloxacin tablets.

The results obtained with F3 tablets suggest that the moisture sorption tendency of norfloxacin raw material could be controlled by pregranulation with nonaqueous liquid followed by compression and coating the tablets with Eudragit E-100. The resulting coated F3 tablets maintained their good performance characteristics when challenged with high temperature and humidity.

Therefore, it was decided to further study the bioavailability of this formula (F3), coated with Eudragit E-100, compared with a local commercial tablet (Neofloxin) using the innovator tablet (Noroxin) as standard.

Aly H. Nada is associate professor and chair at the Department of Pharmaceutics, Faculty of Pharmacy, Kuwait University, Kuwait.

Mohamed A. Sharaf is director of research and development at Alkan Pharmaceuticals, Egypt.

Zeinab A. El Gholmy is professor of pharmaceutics at the Department of Pharmaceutics, Faculty of Pharmacy, Alexandria, Egypt.

Nawal M. Khalafallah is professor of pharmaceutics at the Department of Pharmaceutics, Faculty of Pharmacy, Alexandria, Egypt.

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