Penetration of panitumumab into tumor xenograft tissue is dose and time dependent. Panitumumab initially surrounded the afferent blood supply and then penetrated diffusely into the surrounding tumor tissue. A431 tumor xenograft samples of animals receiving 0, 20, 200, or 500 μg panitumumab were collected (A) 24 hours or (B) 96 hours after initiation of dosing and incubated with anti-idiotype IgG2-bound panitumumab to detect administered panitumumab (blue chromagen) and DAKO anti-EGFR to detect EGFR (red chromagen). Micron bars = 10 μm all high magnification images and black arrows identify blood vessels. (C) IHC staining intensities for the anti-idiotype IgG2-bound panitumumab were qualitatively graded and plotted as an estimated percent of total staining intensity.