FLB was a gift sample from FDC Limited, Mumbai, India. Crosspovidone was gift samples from Matrix laboratories, Hyderabad, India. All other chemicals used were of analytical grade.

Powder mixtures of different formulations were evaluated for angle of repose, bulk density, tapped density and Carr’s compressibility index. The fixed funnel method was employed to measure the angle of repose (θ) and it was calculated using the following formula:

Tan θ = h/r

In which, θ is the angle of repose, h is the height of the cone and r is radius of the cone base. To measure the angle of repose, a funnel was fixed to a stand so that the lower tip of the funnel is 2.5 cm above the surface. A graph paper was placed on a flat surface. The powder blend was allowed to fall freely on the graph paper through the funnel, till the tip of the heap formed just touches the funnel. The radius of the heap was noted and from this angle of repose was determined. Angle of repose less than 30^o suggests free flowing properties of the material.

The bulk density of a powder is determined by measuring the volume of a known mass of powder sample that may have been passed through a screen, into a 50 ml graduated cylinder. Tapped densities of powder samples were determined by a tap density apparatus (Intelli, Kshitij Innovations, India). The apparatus was set for 500 tappings for 5 min at a stroke height of 20 mm [15]. The Carr’s compressibility index is determined from the bulk and tapped densities and is calculated using the following formulas:

Carr’s Index = ((ρ_{tap -} ρ_b) / ρ_tap) / ×100

In which, ρ_b is bulk density and ρ_tap is tapped density.

FDTs were prepared by sublimation method. Flurbiprofen, sublimating agents, crosspovidone and other tabletting excipients were passed through a mesh no 60. The drug was mixed with proper portion of sublimating agent and superdisintegrant. Care should be taken to confirm the proper mixing with drug. Then excipients other than glidant and lubricant were added and mixed in a poly bag for 5-10 min. The obtained blend was lubricated with talc and magnesium stearate for another 5 min and the resultant mixture was directly compressed into tablets with 8 mm round flat punches using 16 station rotary tabletting machine, Cad mach, Ahmadabad, India. Then these tablets were subjected to sublimation, by placing in a hot air oven at 60°C for 2 h to generate a porous matrix, due to removal of volatilable component. The final weight of the tablet was adjusted to 200 mg and the compositions of the tablets were given in Table 1.

The prepared tablets were studied for their physical properties like weight variation, hardness and friability. For estimating weight variation, 20 tablets of each formulation were weighed using an electronic weighing balance (AW 120, Shimadzu Corporation, Japan). The strength of tablet is expressed by measuring hardness and friability. The hardness of six tablets was measured using Monsanto tablet hardness tester. Friability was determined on ten tablets in a Roche friabilator (Electrolab, Mumbai, India) for 4 min at 25 rpm.

For estimation of drug content, ten tablets were crushed in mortar to get powder; this powder was dissolved in 0.1N HCl buffer by sonication for 30 min and filtered through filter paper. The drug content was analyzed spectrophotometrically at 247 nm using UV spectrophotometer. Each measurement was carried out in triplicate and the average drug content was calculated.

In vitro disintegration time of FDT’s was determined by following the procedure described by Gohel et al. Briefly, 10 ml of water at room temperature was taken in a petridish of 10 cm in diameter. The tablet was then carefully placed in the centre of petridish and the time required for the tablet to completely disintegrate into fine particles was noted. Measurements were carried out in triplicates [16]. In vitro dispersion time was measured by dropping a tablet in a measuring cylinder containing 6ml of pH 6.8 (simulated saliva fluid). Three tablets from each formulation were randomly selected and in vitro dispersion time is expressed in sec.

Wetting time was determined as described in the literature elsewhere.Briefly, two circular tissue papers were placed in a Petri dish of 10 cm diameter. Ten ml of water containing 0.5 (% w/v) of phenol red was added to the petridish. The dye solution was used to identify the complete wetting of the tablet surface. A tablet was carefully placed on the surface of the paper in the petridish at room temperature. The time required for water to reach the upper surface of tablet and to completely wet them was noted as wetting time. Wetting time was recorded using stop watch and the measurements were carried out in triplicates.

The weight of the tablet prior to placement in the petridish was noted (W_b) using digital balance (Shimadzu, Japan). The wetted tablet was removed and reweighed (W_a). Water absorption ratio (R), was then calculated according to the following equation.

The release of FLB from FDT’s was carried out using USP XXIV Type II (paddle method) dissolution apparatus (Lab India) at a rotation speed of 100 rpm, and a temperature of 37±0.5°C. The drug release studies were carried out in 0.1N HCl buffer. An aliquot of 5 ml was collected at predetermined time intervals and replaced with fresh dissolution medium. The samples were filtered, by passing through 0.45 µm membrane filters (Millipore, USA) and analyzed spectrophotometrically at 247 nm.

Cumulative percent drug release was plotted as a function of time and percent drug release in 15 min (Q₁₅) was calculated. Initial dissolution rate (IDR) was calculated as percentage dissolved of drug over the first 15 min per min. Dissolution efficiency (DE) was calculated from the area under the dissolution curve at time t and expressed as a percentage of the area of the rectangle described by 100% dissolution in the same time (15). Relative dissolution rate (RDR) is the ratio between amounts of drug dissolved from optimized conventional formulations at 15 min [12].

To study the possible interaction between FLB and excipients, DSC study was carried out on pure FLB, camphor and best formulation. Differential thermal analysis thermograms were obtained using DSC (Perkin-Elmer, Shelton, U.S). The analyses were performed under nitrogen (nitrogen flow rate 50 ml/min) in order to eliminate oxidative and pyrrolytic effects at a standard heating rate of 15ºC/minute over a temperature range of 50ºC - 350ºC. The FTIR spectra of FLB, camphor and best formulation recorded between 400 to 4000 cm^-1 on FTIR to detect the drug-excipients interactions. The FTIR spectra for the test samples were obtained using KBr disk method using an FTIR spectrometer (Perkin Elmer FT-IR, Perkin Elmer Inst. USA). The resultant spectra were compared for any possible changes in the peaks of the spectra.

To assess the drug and formulation stability, stability studies were done according to ICH guidelines. Best formulation was sealed in aluminum packaging coated inside with polyethylene, and three replicates were kept in the humidity chamber maintained at 40±2^oC and 75±5% RH for six months [17]. Samples were collected after six months of storage and analyzed for the drug content and in vitro dissolution rateand they were subjected to statistical analysis using paired t-test to test the significance of difference at 0.05 level of significance (LS). Then the similarity factor (f₂) was calculated between dissolution rates of optimized tablets before and after storage to prove the stability of the dosage form [18].

The institutional animal ethical committee (Approval No. SCP/2013/8/2) approved the protocol of the in vivo study of FLB sublimated fast dissolving tablets. Twelve male albino rabbits weighing 2.0±0.3 kg were used for this study. In the present study a crossover study was followed in which twelve male albino rabbits were participated and divided into two equal groups (group I and group II). In the first phase of study group I (n=6) received an immediate release tablet (dose 50 mg) whereas group II (n=6) volunteers received fast dissolving tablet (dose 50 mg). The animals were fasted and had free access to tap water from twelve hours before the experiment. The rabbit’s mouth was opened; tongue was elevated and placed the tablet. Small amount of water was added to surface of the tablet before administering. The mouth was closed for 2 min to avoid chewing or swallowing of the tablet. 2 ml of water was administered after dosing. In the second phase of the study, after 15 days washout period, group I received fast dissolving tablet and group II received immediate release tablet. Blood samples were collected at 0, 0.5, 1, 2, 3, 4, 5, 6, 8, 10, 12, and 24 hr after dosing from marginal vein.

The collected blood samples were centrifuged at 4000 rpm for 15 min and the serum was separated and transferred to 5 ml micro centrifuge tubes. To the 1 ml of above serum 1 ml of acetonitrile was added and centrifuged for 10 min at 3000 rpm and the supernatant liquid was separated and stored at -40^oC until the analysis of sample for unchanged drug. An established HPLC method was used to measure the FLB serum concentration [19]. The quantitative determination of FLB in plasma was performed using HPLC method by injecting the supernatant liquid into the HPLC column (loop volume 20 μl and flow rate 1 ml/min). The analysis was performed at ambient temperature and the run time was set to 8 min and the eluents were monitored at 254 nm using UV detector [15, 19].

To elucidate the FLB behavior in plasma, the required pharmacokinetic parameters were calculated using FLB plasma concentration-time data. Pharmacokinetic parameters from plasma data were estimated using Kinetica software (Kinetica 20000 version 3.0, Inna Phase Corporation, 2000). Non compartmental analysis was used. From the plot of time versus plasma concentration, the peak plasma concentration (C_max) and the time to reach peak plasma levels (T_max) were obtained. From linear part in the elimination phase of a semi-log plot of concentration versus time, the elimination rate constant (K_e) was calculated. Finally the absorption rate constant (K_a) was calculated from the linear part of residual line using residual method. The area under the curve (AUC) was calculated using the trapezoidal rule [20, 21].

The calculated pharmacokinetic parameters of both immediate release and fast dissolving tablets of FLB were subjected to statistical analysis using analysis of variance (ANOVA) to test the significance of difference. A value of P<0.05 was considered statistically significant.

The powder mixtures of different formulations were evaluated for angle of repose, bulk density, tapped density and compressibility index and their values were shown in Table 2. Bulk density and tapped density values ranged from 0.321 to 0.343 and 0.382 to 0.414 respectively. The results of angle of repose and compressibility index (%) ranged from 27.66±3.95 to 31.76±0.70 and 13.21 to 21.31 respectively.

The physical evaluation parameters of FLB fast dissolving tablets were shown in Table 3 and 4. In weight variation test, the pharmacopoeial limit for the tablets of not more than 7.5% of the average weight. The hardness of the tablets was found to be in the range of 2.8 to 3.4 kg/cm². Another measure of tablets strength is friability. Conventional compressed tablets that loss less than 1% of their weight are generally considered acceptable. The percentage friability for all formulations was below 1% i.e., 0.28 to 0.36, indicating that the friability is within the prescribed limits. The tablets were found to contain 97.24±1.54% to 99.98±0.34% of the labeled amount indicating uniformity of drug content. The disintegration time of all formulations was found in the range of 34.42±0.74 to 61.32±0.14 sec. The dispersion time and wetting time of formulated tablets was found in the range of 62.76±0.62 to 79.46±0.88 and 30.12±1.14 to 44.28±1.68 sec respectively. Water absorption ratio was 40.28±1.37 to 48.22±1.61. From the results it has been found that FDTs containing camphor showed better results than the others.

From the preliminary studies to optimize the suitable super disintegrating agent and concentration used in the fast dissolving tablets, different formulations were prepared and evaluated for drug in various proportions Fig. (1). From the dissolution studies, the formulation containing 10 mg of crosspovidone showed fast dissolution rate with rapid disintegration time (data is not presented). Among all the formulations, F2 tablets showed complete drug release within 30 min and rapid dissolution when compared to other formulations i.e., 99.94±0.26% in 30 min. While in the similar conditions, the conventional tablets of same dose showed 56.24±0.92% drug release in 60 min Fig. (2).

Fig. (1). Release profile of FLB from sublimated fast dissolving tablets (n=3).

Fig. (2). Comparison of FLB release from F2 sublimated fast dissolving tablets and control conventional tablets (n=3).

Fig. (3). DSC thermograms of 1) FLB, 2) F2 formulation.

Fig. (4). Fourier transform infrared spectra of 1) FLB, 2) F2 formulation.

Fig. (5). In vivo plasma concentrations of FLB from control conventional and F2 sublimated fast dissolving tablets (n=6).

The percent drug release in 15 min (Q₁₅) and initial dissolution rate for formulation F2 was 91.46±1.42%, 6.10%/min. These were very much higher compared to conventional tablets (22.92±0.47%, 1.53%/min). The improvement in the dissolution characteristics of a drug described in terms of dissolution efficiency (DE) and relative dissolution rate

(RDR). The dissolution efficiency was found to be 53.44 and it is increased by 4.5 fold with F2 FDT tablets compared to conventional tablets i.e., 10.96 and the relative dissolution rate was found to be 3.98 (Table 5).

DSC studies were performed to understand the nature of the drug in the formulated tablets. DSC curves obtained for pure FLB and F2 formulation were shown in Fig. (3). A sharp endothermic peak corresponding to the melting point of Flurbiprofen was found at 116^oC whereas during scanning of Camphor and crosspovidone at 217°C, 89.25°C were observed respectively. An endothermic peak corresponding to the melting point of Flurbiprofen in F2 formulation was observed at 115.4^oC. Thermogram of the powder mixture did not show any significant shift in the endothermic peak, indicating that there was no physical change in drug in the FDT’s. FTIR studies were performed to understand the nature of the drug in the formulated tablets. The FTIR spectral analysis of pure FLB and formulation F2 were showed the main peaks at comparable wave numbers (Fig. 4). From the pure FLB FTIR spectra gave the main peaks at of 1701.22, 1415.75, 1217.06, 923.9, 765.7and 696.23 cm^-1 wave numbers. In case of F2 formulation, the spectra showed peaks at 1701.22, 1419.61, 1217.06, 925.83, 765.7 and 696.23 cm^-1. The excipients used in the tablets responsible for some additional peaks which were absent in pure drug spectra.

To manifest the prospective utility of the formulation, stability studies were carried out at 40±2°C and 75±5% RH for six months. After storage of six months, the formulation

F2 was subjected to a drug assay and in vitro dissolution studies (Table 6) and from the statistical analysis there was no significant difference between before and after storage (P<0.05). The similarity factor value between dissolution profiles of optimized formulation before and after storage was found to be 83.42, which is more than 50 indicates similarity between the dissolution profile before and after storage [18, 19].

In the present study, pharmacokinetic evaluation was carried out for both control tablets and F2 sublimated fast dissolving tablets. The mean FLB plasma concentrations following the oral administration of both tablets were showed in Fig. (5) and the mean pharmacokinetic parameters from the in vivo experiments of both tablets were given in Table 7.