The purpose of this study is to test the safety of the peptide and antibody and at the same time evaluate the tumor imaging of a two step antibody technique in nuclear imaging.

Colo-rectal cancer has an incidence of 8000 cases per year in The Netherlands. Eventually 50% of them will die as a consequence of this disease.

Treatment consists of resection of the primary tumor, followed by adjuvant chemotherapy in some patients. The chemotherapeutical agents, however, are only moderately effective with regard to increase of survival. New treatment modalities are therefore desirable.

Radioimmunotherapy using radioactive labelled antibodies have proven to be effective in hematologic malignancies. Due to slow absorption in solid tumors however an effective radiation dose is not yet achieved.

The aim of our study is to bypass this slow tumor uptake by the use of pre-targeting. We therefore use an unlabeled bispecific antibody infusion. This will be followed several days later by the infusion of a radiolabeled peptide that binds to the bispecific antibody.

We hypothesise that this small peptide will be readily absorbed by the tumor which is pre-saturated by the bispecific antibody. As a consequence a higher radiation dose can be given with a lower toxicity.

Inclusion Criteria:

• > 18 years of age
• Histologic or cytologic diagnosis of colorectal cancer
• Karnofsky performance status >70%

Exclusion Criteria:

• Pregnant or lactating women
• Severe anorexia
• Active second primary malignancy
• Chemotherapy or radiotherapy within four weeks of study entry