In vivo localization experiments: ex vivo immunofluorescence and quantitative biodistribution studies. (A) Mice bearing sc Ramos lymphoma xenografts were injected with SIP(L19), which was chemically labeled with the fluorophore Cy3. The figure shows microscopic images of a lymphoma section 24 hours after injection, confirming the antibody localization around tumor vascular structures (red, Cy3-labeled L19; green, ex vivo immunofluorescence staining of CD31). Scale bars, 100 μm. Quantitative biodistribution results were obtained 24 and 48 hours after injection of ¹²⁵I-radiolabeled SIP(L19) into SCID mice bearing sc Ramos xenografts (B) and BALB/c mice bearing systemic A20 syngeneic lymphoma (C). Mean targeting results are expressed as % ID/g (± SE) and tumor-to-organ ratios. Forty-eight hours after injection, a selective accumulation and retention of the antibody in the lymphoma tissue could be observed, with tumor-to-normal organ ratios of up to 12.5:1 and 35:1, respectively.