Panitumumab inhibits growth of established A431 xenografts in a dose dependent manner To determine if tumor penetration, EGFR saturation, and inhibition of EGFR activation and proliferation cor related with many anti tumor activity, mice bearing A431 xenograft tumors of approximately 300 mm3 tumors were injected intraperitoneally twice a week for 50 days with PBS, 500 ug of control IgG2 antibody, or 5, 20, 200 or 500 ug of panitumumab. Treatment with panitumumab resulted in a dose dependent tumor inhibition at the 5 and 20 ug doses and in complete tumor eradication at the 200 and 500 ug doses. Control animals were eutha nized on day 22 whereas animals treated with panitumumab at 5 ug and 20 ug were euthanized on days 44 and 67, respectively, because of uncontrolled tumor growth and consistent with IACUC guidelines.

In animals treated with panitumumab at 200 ug and 500 ug, no tumors were detected by day 28 of treatment. These mice remained disease free for an additional 300 days after the last dose was administered, at which time they were euthanized and no further data were collected. No difference in the body weights Inhibitors,Modulators,Libraries between the control treated and panitumumab treated animals were observed. The observed tumor growth data from the A431 xeno graft study were modeled to calculate the growth and death rates upon treatment with panitumu mab. This model described Inhibitors,Modulators,Libraries a mean A431 tumor cell growth of 3. 73 mL h, which was consistent with the observed results. Maximum EGFR mediated tumor cell death rate was 8. 97 h 1 and the steady state concentra tion at the tumor that elicits 50% of maximum Inhibitors,Modulators,Libraries cell death rate was 0.

Inhibitors,Modulators,Libraries 81 ug mL. In addition, the con centration for tumor eradication, which accounts for both tumor Inhibitors,Modulators,Libraries growth and tumor death was estimated to be 0. 20 ug mL. Discussion The data presented here examined the correlation of panitumumab tumor penetration and EGFR saturation, a potential obstacle in drug delivery of large molecules in treating solid tumors, using pharmacokinetics, pharmacodynamics, and anti tumor activity in an A431 epidermoid carcinoma xenograft model system. One important factor that leads to the clinical efficacy of a therapeutic is its ability to modulate the target for which it is intended. Although A431 cells express ap proximately 1. 2 million EGFRs per cell, there is only a minimal amount of basal phosphorylation of the EGFR in vitro or in vivo.

Therefore, to address pani tumumab target coverage, we employed an inhibition of ligand induced phosphorylation assay. Panitumumab treatment inhibited EGFR autophosphorylation in A431 cells in vitro in a dose dependent manner promotion as well as in vivo in the A431 xenograft model. It has been shown that activation of EGFR by EGF resulted in rapid internalization and degradation of the receptor. Our data demonstrated similar reductions in the total EGFR levels upon EGF stimulation.