Abstract
Since bacteria which survive within phagocytes may produce serious infection, antibiotics which inactivate these intracellular organisms are needed. To establish those factors which mediate entry of antimicrobial agents into human phagocytes, we studied the uptake of 13 radiolabeled antibiotics by peripheral blood polymorphonuclear leukocytes (PMN). At intervals during a 2-h incubation period, antibiotic uptake by PMN was determined by means of velocity gradient centrifugation, which separates the cell-associated antibiotic from the extracellular antibiotic. Penicillin G and three cephalosporin antibiotics penetrated PMN poorly. The ratio of cellular concentration to extracellular concentration (C/E) of these drugs was less than 0.01 to 0.5. For gentamicin and isoniazid, the C/E values were approximately 0.8 to 1.0. Chloramphenicol, rifampin, and lincomycin, antibiotics with good lipid solubility, were concentrated twofold (C/E = 2) in PMN. Ethambutol (C/E = 5), clindamycin (C/E = 11), and two erythromycin preparations (C/E = 10 to 13) were markedly concentrated within PMN. Clindamycin uptake was rapid: greater than 70% of the total drug entry occurred within the first minute. Accumulation of clindamycin and erythromycin was an active, energy-requiring process, dependent at least in part upon glycolysis. Clindamycin entered PMN by means of an active membrane transport system which was saturable and had a high binding affinity (Km = 2 mM) and maximum velocity of uptake (Vmax = 5 nmol/45 s per 10(6) cells). These observations, together with studies of the biological consequences of intracellular antibiotics, should lead to more effective therapy for infection due to intracellular pathogens..