The effect of linker type and recognition peptide conjugation chemistry on tissue affinity and cytotoxicity of charged polyacrylamide

To increase colonoscopy competence in ambiguous situations (e.g. the existence of flat polyps), an explicit in situ (at real time) diagnosis at the molecular level is required. We have previously shown that the affinity of fluorescent cationic polyacrylamide (Flu-CPAA) to malignant regions in the colon mucosa can be improved by conjugating the recognition peptide EPPT1 to the polymer backbone (to form Flu-CPAA-Pep). Using another recognition peptide, namely VRPMPLQ, we elucidated in the present study the effect of linker type and conjugating methods on Flu-CPAA-VRPMPLQ cytotoxicity and on its affinity to cell lines as well as human colorectal cancer (CRC) biopsies. In order to derive the relationship between the response variable and the experimental factors in a minimal set of experiments, a computerized statistical design of experiment (DoE) strategy was implemented. Data were collected in a six-factor factorial design to study the effect of experimental factors (independent variables) on the ability of the Flu-CPAA polymers to bind specifically to the colon cancer cell lines or the human biopsies (the response). It was found that the presence of VRPMPLQ on the Flu-CPAA improved the polymer's affinity to the human CRC biopsies and to the colon cancer cell lines representing stage B in the Duke severity staging system. The cytotoxicity of Flu-CPAA with high charge density was reduced after conjugated with VRPMPLQ. The replacement of Ahx linker by PEG linker of similar length did not affect the affinity to the human biopsies, nor did it affect cytotoxicity. However, elongating the PEG linker reduced the in vitro affinity to the colon cancer cell lines and to human CRC biopsies. Changing the conjugation method from condensation (amide bond formation) to the click conjugation method did not affect the affinity properties of the polymers. It did reduce, however, the polymer cytotoxicity. We suggest that Flu-CPAA-Pep, with the VRPMPLQ peptide as a recognition moiety, could serve for early diagnosis and screening of CRC patients during endoscopic procedures.