Background. Camptothecin (CPT), an alkaloid isolated from the Chinese tree Camptotheca acuminata,exhibits very high anticancer activity. CPT is a fluorescent compound and this is a very useful property. Using methods of fluorescence spectroscopy, one can determine or predict the biophysical propertiesof this promising compound. The cellular target of CPT is topoisomerase I. CPT interacts only with thosecells which are in the S-phase. CPT molecules bind to topoisomerase I – DNA complex and prevent thereplication process. This means that CPT is toxic to those cells that are undergoing DNA synthesis. Cancerouscells are rapidly replicating and they spend more time in the S-phase in relation to healthy tissues,and therefore they are killed with much higher efficiency than the healthy host tissues. Such selectivity ofcytotoxicity is a promising property of CPT. Due to the S-phase specificity of CPT, a continuous exposureto this drug must be maintained in order to achieve optimum therapeutic efficacy.

Materials and methods. We used fluorescence anisotropy measurements to determine the properties ofbinding to membranes and proteins of two anticancer agents: 7-tert-butyldimethylsilyl-camptothecin and7-tert-butyldimethylsilyl- 10-[4-(1-piperidino)-1-piperidino]-carbonyloxy-camptothecin. The samples of CPT,1Si-CPT and 2Si-CPT were obtained from the laboratory of biotechnology, College of Pharmacy, Universityof Kentucky, Lexington, USA. 2 mM stock solutions of CPT, 1Si-CPT and 2Si-CPT were prepared in DMSO.As model membranes, small unilamelar liposomes formed by DMPC and DMPG lipids were used.

Results. Fluorescence anisotropy measurements prove that this new camptothecin analogue exhibitsdesirable properties: high affinity of lactone form to membranes and low affinity of carboxylate form toHSA. Such properties should ensure high stability of this drug in physiological fluids, including blood.The second considered compound, 7-tert-butyldimethylsilyl- camptothecin, exhibits very high affinity of itslactone form to membranes, but its carboxylate form exhibits also high affinity to HSA. The latter propertyexcludes this compound as a candidate to be a good anticancer drug.

Conclusions. Fluorescence anisotropy measurements show that the lactone forms of 1Si-CPT and 2Si-CPTexhibit a high affinity to membranes but only the carboxylate form of 2Si-CPT behaves in a desirable way inHSA solution i.e. it exhibits a low affinity to this protein. This means that 2Si-CPT could be an excellent candidatefor further in vivo pharmacological studies, and most probably for clinical trials in cancer chemotherapy.