replication fork collides with the cleavage complexes formed by DNA and CPT. Chemistry The lactone ring in CPT is highly susceptible to hydrolysis. The open ring form is inactive and it must therefore be closed to inhibit topoisomerase I. The closed form is favored in acidic condition, as it is in many cancer cells microenvironment. CPT is transported into the cell by passive diffusion. Cellular uptake is favored by lipophilicity, which enhances intracellular accumulation. Lipophilicity makes compounds more stable because of improved lactone partitioning into red blood cells and consequently less hydrolysis of the lactone. CPT has affinity for human serum albumin (HSA), especially the carboxylate form of CPT. Because of that, the equilibrium between the lactone ring and the carboxylate form is driven toward the carboxylate. Reduced drug-HSA interactions could result in improved activity. Structure-activity relationship Studies have shown that substitution at position 7, 9, 10 and 11 can have positive effect on CPT activity and physical properties, e.g. potency and metabolic stability. Enlargement of the lactone ring by one unit also enhances its abilities, as in homocamptothecin. Substitution at position 12 and 14 leads to inactive derivative. A- and B-ring modification Alkyl substitution Alkyl substitution at position 7 has shown increased cytotoxicity, such as ethyl (C2H5) or chloromethyl (CH2Cl). These groups are able to react with the DNA in the presence of topoisomerase I which leads to more tumor activity. It has also been shown that increasing the length of the carbon chain (in position 7) leads to increased lipophilicity and consequently greater potency and stability in human plasma. Other 7-modified CPT analogues are silatecans and karenitecins. They are potent inhibitors on topoisomerase I and both have alkylsilyl groups in position 7 which make them lipophilic and more stable. Silatecans or 7-silylcampthothecins have shown reduced drug-HSA interactions which contributes to its blood stability and they can also cross the blood brain barrier. DB-67 is a 10-hydroxy derivative and is among the most active silatecans. BNP1350 which belongs to the series of karenitecins exhibits cytotoxic activity and ability to overcome drug resistance. Still another route to make CPT’s lipophilic is to introduce lipophilic substituents, such as iminomethyl or oxyiminomethyl moieties. One of the most potent compounds is the oxyiminomethyl derivative ST1481 that has the advantage to overcome drug resistance caused by transport systems. Basic nitrogen in a carbon chain at position 7 makes the compound more hydrophilic and hence