Comparison of tumor targeting by escort aptamer and antibody. ^99mTc-radiolabeled aptamer and ¹²⁵I-radiolabeled antibody against the same target protein were administered by intravenous bolus injection into tumor-bearing mice. Concentration of aptamer was measured at 1 hour in tumor (filled circles) and blood (open circles), and concentration of antibody was measured at 24 hours in tumor (filled square) and blood (open square). Three doses of aptamer are compared with each other and with a single dose of antibody. Signal/noise ratios of antibody and aptamer at equivalent tumor loading are represented by the lengths of the double-headed arrows. Note: for both aptamer and antibody, the time of measurement was before optimal tumor/blood ratios were achieved (see Figure 2).