Development of carbon paste sensor for low level and selective determination of sitagliptin phosphate in pharmaceutical and biological samples

Sitagliptin phosphate (STA), [(R)-4-oxo-4-[3-(trifluoromethyl)-5,6-dihydro[1,2,4] triazolo [4,3-a]pyrazin-7(8H)-yl]-1-(2,4,5-trifluorophenyl)butan-2-amine] phosphate, is an orally active and selective inhibitor of dipeptidyl peptidase-IV that is used for treatment of type II diabetes. DPP-4 inhibitors represent a new therapeutic approach to the treatment of type 2 diabetes that functions to stimulate glucose dependent insulin release and reduce glucagons levels [1]. Several analytical methods have been reported for the determination of STA such as spectrophotometry [2], spectofluorimetry [3,4] and potentiometry by liquid membrane electrode [1]. However, no potentiometric carbon paste electrode (CPE) for this drug has been reported so far. The most advantages of CPEs are ease of preparation, modification and regeneration and low cost. Thus, the introduction of a carbon paste sensore for fast, simple, selective and sensitive determination of STA is very valuble. In this study a new CPE based on incorporation of the ion-association complex of the sitagliptin phosphate-phosphotungstate were studied. The electrode exhibited Nernstian slope of 59.2 mV/decade to sitagliptin over a wide concentration range from 1.7×10-8 - 2.2×10-5 M with low detection limit of 9.0 ×10-9 M. The potentiometric response of the electrode was studied in buffered solution. The proposed sensor manifested advantages of fast response, long life time and most importantly, good selectivities for sitagliptin relative to a wide variety of common foreign inorganic cations and biological species. The influence of temperature on the potential response of the sensor was tested at different temperature and the temperature coefficient of the sensor was calculated. The sensor was successfully applied as an indicator electrode in potentiometric determination of sitagliptin in sitagliptin tablet and blood serum samples.