Model concentration profiles of PAMAM-RGD polymers as a function of distance from the spheroid center (r/rmax) and time (hours). Concentration profiles were calculated for PAMAM-RGD conjugates displaying 2, 3, 7, and 10 RGD peptides and are reported as unbound concentrations, C, normalized by the initial polymer concentration exposed to the spheroids, Cmax, of 100 nM.