The effect of preincubation time with ⁵⁹Fe-Tf on the percentage of ⁵⁹Fe released from SK-N-MC NB cells by either incubation medium alone (control; Con) or PIH analogue 311 (0.1 mmol/L). The difference in ⁵⁹Fe release between 311 and the control (311-Con) has also been graphed to clearly show the change in the 311-mediated ⁵⁹Fe release as a function of labeling time. Cells were incubated with ⁵⁹Fe-Tf (1.25 μmol/L) for 15 minutes to 24 hours, washed, and then reincubated with the chelators for 3 hours at 37°C. Results are means from triplicate determinations in a typical experiment.