Equilibrium studies on tetracycline-metal ion systems.
Protonation and equilibrium constant for oxytetracycline (OTC) and doxycycline (DOX) with Zn2+, Ca2+ and Mg2+ ions have been determined with Calvin-type pH-metric titrations under physiological conditions (37 degrees C, 0.15 M NaCl ionic strength). Even though OTC and DOX are similar in structure, major differences were found in complex composition with regard to the protonation state: OTC generally formed species with less protons compared to those with DOX. Studies of parent complexes were followed by investigations of ternary complexes where ascorbic acid was used as a secondary ligand. Again, OTC showed a tendency to form complexes in which fewer protons are bound than in those with DOX. This equilibrium difference between OTC and DOX might be because DOX has a better pharmacodynamic effect relative to that of OTC.
Novák-Pékli
M
M
Semmelweis University of Medicine, Department of Pharmaceutical Chemistry, Budapest, Hungary.
el-Hadi Mesbah
M
M
Pethó
G
G
D000136:Acid-Base Equilibrium; D000900:Anti-Bacterial Agents / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry; D002413:Cations, Divalent / Q000737:chemistry; D004318:Doxycycline / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D008274:Magnesium / Q000737:chemistry; D008670:Metals / Q000737:chemistry*; D009994:Osmolar Concentration; D010118:Oxytetracycline / Q000737:chemistry; D011199:Potentiometry; D017186:Titrimetry; D015032:Zinc / Q000737:chemistry
D000900:Anti-Bacterial Agents; D002413:Cations, Divalent; D008670:Metals; D008274:Magnesium; D015032:Zinc; D004318:Doxycycline; D002118:Calcium; D010118:Oxytetracycline