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A comparative chromatographic study was developed for the simultaneous quantitative resolution of trimethoprim (TMP) and sulphamethoxazole (SMX) in veterinary formulations. Multi-wavelength chromatograms were recorded by using diode array detector (DAD) system at the five-wavelength set consisting of 220, 230, 240, 250 and 260 nm. In the first step, five different calibration equations at the above wavelengths for each drug were obtained by using the relationship between concentration and peak area. These calibration graphs were used for the quantitative evaluation of TMP and SMX in samples. These single-wavelength applications were called traditional LC method. In the second step, principal component regression (PCR) and partial least-squares (PLS) calibrations were applied to the above mentioned multi-wavelength chromatograms. The amount of two investigated drugs in samples was determined by the constructed PCR and PLS calibrations. The experimental results obtained from each single-wavelength calibration graph were compared with those obtained by the chemometric approaches and chromatographic multivariate approaches give successful results more than traditional LC method.

The effect of hydroxypropyl-beta-cyclodextrin (HPbetaCD) on the chemical stability of sulfamethoxazole and trimethoprim (co-trimoxazole) under oxidation stress at 50 +/- 2 degrees C was investigated. The concentrations of sulfamethoxazole and trimethoprim in aqueous solutions (pH 5.4) containing 0, 1%, 2%, 5%, 10% and 15% w/v hydroxypropyl-beta-cyclodextrin were measured by HPLC. Both sulfamethoxazole and trimethoprim degradation appeared to follow pseudo-first order kinetics in the presence and in the absence of hydroxypropyl-beta-cyclodextrin. The observed half-lives for sulfamethoxazole and trimethoprim in 15% w/v hydroxypropyl-beta-cyclodextrin were 910 h and 609 h respectively, 11.8 and 3.4 times greater than in solutions without hydroxypropyl-beta-cyclodextrin. Using a Lineweaver-Burk equation, the half-lives for sulfamethoxazole and trimethoprim outside the complex in a solution containing 15% w/v hydroxypropyl-beta-cyclodextrin were estimated at 77 h and 193 h respectively, whereas inside the complex the half-lives were estimated at 850 h and 821 h. In terms of relative increases in stability under oxidation stress the half-lives for sulfamethoxazole and trimethoprim inside the complex were 11.0 times and 4.2 times greater than their half-lives outside the complex. In conclusion, chemical stability of sulfamethoxazole and trimethoprim in co-trimoxazole aqueous solutions under oxidation stress at 50 +/- 2 degrees C can be increased using hydroxypropyl-beta-cyclodextrin as a molecular inclusion excipient.