Structure characterization of terazosin drug using mass spectrometry and thermal analyses techniques in comparison with semi-empirical molecular orbital (MO) calculations

Terazosin (C 19 H 25 N 5 O 4 , MW = 387) drug is a selective alpha 1 antagonist. It is used for lowering the blood pressure. Also, it used for treatment of symptoms of an enlarged prostate and is therefore a drug of choice for men with hypertension and prostate enlargement. In the present study, mass spectrometry (MS) and thermal analyses (TA) were used to investigate the fragmentation decomposition pathways of terazosin and confirmed by semi-empirical molecular orbital (MO) calculation, using PM3 procedure on the neutral and the positively charged species of the drug. These calculations included, bond length, bond order, partial charge distribution, ionization energy and heats of formation (Δ H f ). The mass spectra and TA fragmentation pathways were proposed and compared to each other to select the most suitable scheme representing the correct fragmentation pathway of the drug in both techniques. This selection helps understanding of metabolism of the drug in vivo system. Therefore, comparison between MS and TA helps in selection, the proper pathway representing the fragmentation of this drug. This comparison successfully confirmed by MO calculation. Graphical Abstract In the present study, mass spectrometry (MS) and thermal analyses (TA) were used to investigate the fragmentation decomposition pathways of terazosin and confirmed by semi-empirical molecular orbital (MO) calculations. The mass spectra and thermal analyses fragmentation pathways were proposed and compared to each other to select the most suitable scheme representing the correct fragmentation pathway of the drug in both techniques. This selection helps understanding of metabolism of the drug in vivo system. Terazosin hydrochloride (C 19 H 25 N 5 O 4 ) drug (marketed ITRIN), has an IUPAC name; 2-[4-(2-tetrahydrofuranyl) carbonyl]-1 piperazinyl- 6,7-dimethoxy-4-quinazolinamine monohydrochloride dihydrate, and its stereo structure is given below