Labeling and distribution of linear peptides identified using in vivo phage display selection for tumors

To develop targeting molecules to be used for vascular targeting of short half-lived α-emitters for radioimmunotherapy, linear peptide phage display libraries were selected in vivo for binding to IC-12 rat tracheal tumors growing in severe combined immune deficient mice. After three rounds of selection, 15 phage clones were analyzed for DNA sequence, and the deduced translation products of cDNA inserts were compared. Three consensus sequences were chosen from three separate experimental selection series and peptides of these sequences with added -gly-gly-tyr were obtained. Peptides were radiolabeled on tyrosine with ¹²⁵I and the biodistribution in tumor-bearing mice was determined. The radioiodinated peptides were stable in vitro and when injected in tumor-bearing mice 3.0 %ID/g accumulated in the tumor; however, much of the ¹²⁵I was found in the gastrointestinal tract and thyroid, indicative of dehalogenation of the labeled peptide. Radiolabeling peptide 2 with N-succinimidyl-3-¹²⁵I-iodobenzoate resulted in faster excretion, which in turn resulted in lower levels in tumor and other organs, especially thyroid and gastrointestinal tract. Peptide 2 was derivatized with the bifunctional isothiocyanates of cyclohexyl-B diethylenetriaminepentaacetic acid (DTPA) or CHX-A″ DTPA by direct conjugation or with a hydroxylamine derivative of 1B4M-DTPA (2-(p-O-(carboxamylmethyl)hydroxylamine]benzyl)-6-methyl-diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid ) coupled at the N-terminus. The primary molecular species in the conjugated products were shown by mass spectrometry to have one DTPA per peptide. Peptide chelate conjugates were radiolabeled with ²¹³Bi and the products tested for biodistribution in tumor-bearing mice. The data show that chelation of ²¹³Bi to peptides was accomplished by both the direct method of DTPA attachment and by the method using the linker at the N-terminus. Only small amounts of peptide accumulated at tumor sites. We conclude that phage display is a powerful tool to select peptides with restricted binding specificity; however, the peptides isolated to date do not bind with high retention to tumor sites in vivo.