Bovine carbonic anhydrase B (CA) inhibitory effect of furosemide was established in two pH values at 26 degress. FFBu [N-furfuryl-4-chloro-5-(butoxymethylsulfamoyl)anthranilic acid] and FFMe [N-furfuryl-4-chloro-5-(methoxy-methylsulfamoyl)anthranilic acid], two of its alkoxymethyl derivatives, did not exert any CA inhibitory effect at those conditions but were found to inhibit the CA activity after their hydrolysis, which yielded the furosemide molecule. The CA inhibitory effect of furosemide was utilized for determining the kinetic rate constants for the hydrolysis of FFBu and FFMe at various pH and temperature levels. The hydrolysis rate constants of FFBu and FFMe were pH-independent in the pH range tested, and the temperature dependence for FFBu yielded an activation energy of 18 kcal/mol. It is pointed out that the hydrolysis rates of FFBu may be important for the explanation of its possible delayed diuretic effect.

The structure activity correlation of several groups of anthranilic acid derivatives was studied. 59 compounds, most of them possessing the anthranilic acid moiety, were synthesized and tested for diuretic and saluretic activities. Equations correlating the biological activities of these compounds with their physicochemical constants suggest positive dependence of the diuretic activity on log P (octanol: water partition coefficient). It is concluded that, within limits, the variation in biological activity is primarily governed by the lipophilicity of the molecule, and further increase in log P value will not enhance this activity.