Tumor-targeted and activated bioconjugates for improved camptothecin delivery.

Earlier reports from our laboratory described bioconjugates of camptothecin (CPT) for tumor targeting. In the current work, the rate and site of CPT release from the bioconjugates were modulated using increasingly sterically hindered amino acids and cysteine proteinase-sensitive peptide linkers, respectively. Polyethylene glycol served as a spacer/scaffold between CPT and folic acid. The folic acid receptor, overexpressed on many cancer cells, was targeted using folate. The delivery system was tested in vitro for hydrolytic stability, enzyme-mediated cleavage, cytotoxicity and targeting potential. The linkers successfully modulated the hydrolysis rate (around 1--100 h) and potential site (tumor microenvironment) of CPT release. Preliminary molecular modeling approaches were utilized to assess the influence of molecular volume on hydrolysis half-life (i.e. CPT release). There was a clear, but non-linear, relationship between in vitro CPT release and increasing steric hindrance offered by the peptide linker. The efficacy of four conjugates was studied in a syngeneic rat breast cancer model. Histopathological analysis on treated tumors was performed to evaluate disease prognosis. The results demonstrate that programmed bioconjugates may provide superior efficacy and greater control over the rate and site of CPT release, resulting in higher anti-tumor efficacy and lower toxicity.