Quantitating antibody uptake in vivo: conditional dependence on antigen expression levels.
OBJECTIVE
Antibodies form an important class of cancer therapeutics, and there is intense interest in using them for imaging applications in diagnosis and monitoring of cancer treatment. Despite the expanding body of knowledge describing pharmacokinetic and pharmacodynamic interactions of antibodies in vivo, discrepancies remain over the effect of antigen expression level on tumoral uptake with some reports indicating a relationship between uptake and expression and others showing no correlation.


METHODS
Using a cell line with high epithelial cell adhesion molecule expression and moderate epidermal growth factor receptor expression, fluorescent antibodies with similar plasma clearance were imaged in vivo. A mathematical model and mouse xenograft experiments were used to describe the effect of antigen expression on uptake of these high-affinity antibodies.


RESULTS
As predicted by the theoretical model, under subsaturating conditions, uptake of the antibodies in such tumors is similar because localization of both probes is limited by delivery from the vasculature. In a separate experiment, when the tumor is saturated, the uptake becomes dependent on the number of available binding sites. In addition, targeting of small micrometastases is shown to be higher than larger vascularized tumors.


CONCLUSIONS
These results are consistent with the prediction that high affinity antibody uptake is dependent on antigen expression levels for saturating doses and delivery for subsaturating doses. It is imperative for any probe to understand whether quantitative uptake is a measure of biomarker expression or transport to the region of interest. The data provide support for a predictive theoretical model of antibody uptake, enabling it to be used as a starting point for the design of more efficacious therapies and timely quantitative imaging probes.
Thurber
Greg M
GM
Center for Systems Biology, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114, USA.
Weissleder
Ralph
R
D000818:Animals; D000906:Antibodies / Q000378:metabolism*; D019138:Antibodies, Blocking / Q000378:metabolism; D000941:Antigens / Q000378:metabolism*; D005453:Fluorescence; D019073:HT29 Cells; D006801:Humans; D051379:Mice; D008954:Models, Biological; D023041:Xenograft Model Antitumor Assays
D000906:Antibodies; D019138:Antibodies, Blocking; D000941:Antigens
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