BASES PHARMACOCINETIQUES DE L'ANTIBIOTHERAPIE DANS LES INFECTIONS BRONCHOPULMONAIRES BACTERIENNES

Pharmacokinetics is the study of the absorption, distribution and elimination of a drug in the body. Applied to antibiotic therapy, it gives information on the concentrations of antibiotics that reach the bacteria at a given time at their site of multiplication for a given dose and route of administration. The future of an antibiotic within a body is largely related to passive transfer. This can be compared to the dialysis of molecules across a semi-permeable membrane, the passage from one side to the other being a function of the concentration of molecules in the 'upstream' side, the size of the molecules and their own particular transfer speed. The final result is affected by: the partition coefficient itself related to the degree of aqueous and lipid solubility of the molecules; the degree of ionisation of the molecules, non-ionised molecules being the only ones to be transferred; protein binding, as only the unbound fraction is biologically active and capable of diffusing across the membranes; and by the interplay of the combined phenomena of resorption, distribution and elimination. Penicillins and macrolides are the antibiotics of choice in broncho-pulmonary infections. The tetracyclines and the sulfamethoxazole-trimethoprim combination come second. The combination of a β-lactam, an aminoglycoside and/or metronidazole are reserved for the most severe infections. The lung being a particularly well-vascularised organ, the pulmonary concentrations of the antibiotic may equal the serum levels. But the concentration in the bronchial secretions only reaches 55% of the serum levels for clindamycin, 25 to 30% for aminoglycosides, minocycline and bacampicillin, 20% for cephalosporins and doxycycline and less than 10% for ampicillin and erythromycin. Only oleandomycine, spiramycin and trimethoprim are present in concentrations equal to those in the serum.