Tumor-targeted and activated bioconjugates for improved camptothecin delivery

Camptothecin (CPT) presents numerous drug delivery challenges because of its low aqueous solubility, chemical instability, dose limiting toxicity and clinically observed variability in pharmacologic effect. Thus, the overall goal of the present research was to enhance the delivery of CPT to tumors.; To overcome these challenges, a two-tier targeted delivery system was developed and evaluated. A first-tier of the bioconjugate delivery system was designed to specifically target cancer cells expressing endogenous transporters/receptors and once internalized, would deliver pharmacologically active CPT only to the target cell. Biotin and folic acid were used as the targeting moieties for biotin transporter and folate receptor respectively. The rationale behind using endogenous transport systems for targeting to tumors is based on the observation that cancer cells over express many nutrient transport systems, including biotin transporter and folate receptor in comparison to healthy cells. In the second tier the rate and site of release of pharmacologically active CPT was programmed using amino acid/short peptide linkers designed to be stable in plasma but susceptible to enzymatic cleavage by enzymes. Many enzymes are present in abundance near/inside cancer cells including cathepsin B, the enzyme targeted in this research. Polyethylene glycol (PEG) was utilized to improve solubility and act as drug delivery carrier/scaffold.; Conjugation of CPT to biotin or folic acid increased its cytotoxicity and/or ability to induce apoptosis by the activation of the caspase-dependent cell death signaling pathway and simultaneous suppression of anti-apoptotic cellular defense. The amino acid linkers were successfully used to modulate the hydrolysis rates (from ∼1--100 hours) of CPT esters. The bioconjugate with tetrapeptide linker was found to be specifically and rapidly cleaved by human cathepsin B. Four bioconjugates were selected for further study in a syngeneic breast cancer model in rats. The results of the in vivo study indicate enhanced tumor regression and a better prognosis in animals treated with the bioconjugates in comparison with animals treated with unconjugated CPT.; The present research demonstrates that the use of two-tiered targeting approach using endogenous transporters/receptors coupled with rational design of linkers to achieve site-specific controlled release is an attractive strategy for CPT delivery.