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Derivatives of 1,3,4-thiadiazino[2,3-b]quinazoline 7, 9, 9a, 12, 12a, and 13 were prepared from the 3-amino-6-bromo-2,3-dihydro-2-thioxo-4-(1H)-quinazolinone (2) and its analogue without bromine. A series of the title derivatives with or without bromine was tested and the results of pharmacological screening are discussed.

3-Substituted-5-(4-pyridylcarboxamido)tetrahydro-2H-1,3,5-thiad iazine-2- thione derivatives 5a-e were synthesized as prodrugs for isoniazid (INH) to overcome the resistance developed with its therapeutic use. These prodrugs revealed higher lipophilicity compared with INH. Their degradation kinetics were studied in vitro using aqueous buffer solutions of pH values 1.2 and 7.4 was well as biological media of human plasma and rat liver homogenate at 37 degrees C. They were more stable toward enzymatic degradation in biological than in chemical media. Release of INH from these derivatives was detected as a result of both chemical and enzymatic hydrolysis by HPLC. The antimycobacterial activity of the synthesized compounds and INH was tested in vitro against human type of Mycobacterium tuberculosis. They exhibited a greater antitubercular activity than the parent drug. This result is considered as an indicator for an improved permeation of the synthesized prodrugs through mycobacterial cell membranes relative to INH.

The successful synthesis of 17 nitrosimines of 2 H-1,3,4-thiadiazines is reported. They are best characterized by their electronic spectra (lambda max = 504-522 nm). Some of the compounds were able to inhibit the aggregation of blood platelets in the Born-test (inducer collagen). The most active compound was the 3-nonyl-5-phenyl-derivative 10c which showed an IC50 = 6.5 mumol.l-1. In mesenteric arterioles of rats after p.o. administration (60 mg/kg) a 13% inhibition of thrombus formation is observed. The reason for the rather small effects is the high chemical stability of the title compounds and the fact that decomposition occurs mainly by the evolution of molecular nitrogen. Only small yields of NO are formed.