Pitfalls in testing drug uptake by leukocytes using radiolabeled drugs and an explanation of conflicting results with clindamycin.

Although studies of drug uptake by leukocytes use similar methods, the results reported are sometimes vastly different. To determine the possible reasons for this, we studied neutrophil uptake of [3H]-clindamycin using the most frequently used density gradient centrifugation method and the volume probes 3H2O and [3H]-polyethylene glycol. We found that the [3H]-clindamycin available to investigators had undergone radiolytic decomposition; thus, its microbiologic activity was only 20% of its original potency. By thin-layer chromatography and autoradiography, four extra label-bearing regions were observed with [3H]-clindamycin. Results of neutrophil uptake studies have shown a drop of cellular:extracellular concentration ratios from 40:1 in 1981 to 10:1 in 1982 and 1987.