Phthalazole, like other sulfanilamide preparations, similar in structure to PABA (para-aminobenzoic acid), by the principle of competitive interrelation prevents its inclusion in the synthesis of folic acid of the microbial cell. This leads to a violation of the formation of folic acid, involved in the synthesis of purine and pyrimidine bases, on which the growth and development of microorganisms depends. The most pronounced bacteriostatic effect of phthalazole is on vegetative forms of microbes. This effect develops gradually, since in the microbial cell there are some stocks of PABC.In addition, the degree of affinity of phthalazole with dihydrofolic acid synthetase, an enzyme that limits the formation of folic acid, is significantly weaker in comparison with PABA. Therefore, phthalazole must be administered at sufficiently high doses that prevent the use of microorganisms PABA contained in tissues. Otherwise, resistant strains of pathogens can be formed, which can not be further affected by sulfanilamide preparations.
In addition to antibacterial, phthalazole, like other sulfonamide preparations, has an anti-inflammatory effect that is associated with its ability to limit the migration of leukocytes, reduce the total number of migrating cellular elements and partially stimulate the production of glucocorticosteroids.
The spectrum of antimicrobial action of phthalazole, like other sulfonamides, is narrower than antibiotics. Sulfanilamides have a bacteriostatic effect on Streptococcus, Pneumococcus, Staphylococcus, Meningococcus, Gonococcus, Escherichia coli, Pseudomonas aeruginosa, Shigella dysenteriae, Proteus vulgaris and a number of large viruses - causative agents of trachoma and follicular conjunctivitis.