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Comparative bioavailability of two sildenafil tablet formulations after single-dose administration in healthy Thai male volunteers.

OBJECTIVE
To compare the rate and extent of absorption of two sildenafil tablet formulations (Tonafil®, T.O. Chemicals (1979) Ltd., Thailand as a test formulation and Viagra®, Pfizer Pty Limited., Australia as a reference formulation) in healthy Thai male volunteers after single-dose administration under fasting condition.


METHODS
A randomized crossover study with a washout period of 2 weeks was conducted in 20 healthy Thai male volunteers. The volunteers received either 100 mg of the reference or test formulation. Blood samples were collected at 0, 0.16, 0.33, 0.67, 1, 1.5, 2, 2.5, 4, 6, 9 and 12 h after drug administration. The plasma sildenafil concentrations were determined using a validated high performance liquid chromatography method. The pharmacokinetic parameters of sildenafil were calculated from the observed plasma concentration-time profiles by using a non-compartmental model.


RESULTS AND CONCLUSIONS
The geometric means, C(max), of the reference and the test formulations were 696.42 and 734.06 ng/ml. The mean values of the AUC(0-t) and AUC(0-inf) of the test formulation were 2,073.03 and 2,237.37 ng × h/ml, while those of the reference formulation were 1,950.26 and 2,078.06 ng × h/ml. The geometric mean ratios (%) and 90% confidence intervals (CI) of the test and reference products for the log transformed C(max), AUC(0-t) and AUC(0-inf) of sildenafil were 105.40% (95.0 - 116.95%), 106.30% (99.74 - 113.28%) and 107.67% (100.83 - 116.48%). These values were within 80 to 125% of the US-FDA and the Thai-FDA criteria and therefore it can be concluded that the test formulation was bioequivalent to the reference formulation both in terms of rate and extent of absorption after single-dose administration under fasting condition.