The structural basis of camptothecin interactions with human serum albumin: impact on drug stability

Abstract

The intense intrinsic fluorescence emissions from several clinically relevant camptothecin drugs have been exploited in order to study the structural basis of drug binding to human serum albumin. Both HPLC and time-resolved fluorescence spectroscopic methodologies were employed to characterize the associations of camptothecins with HSA in phosphate-buffered saline (pH 7.4) at 37 degrees C. The alpha-hydroxy delta-lactone ring moiety of camptothecin (C), 10-hydroxycamptothecin (HC), 10,11-(methylenedioxy)camptothecin (MC) and 9-chloro-10,11-(methylenedioxy)camptothecin (CMC) was in each case observed to hydrolyze more rapidly and completely in the presence of HSA than in the protein's absence. Binding isotherms constructed by the method of fluorescence lifetime titration showed that HSA bound preferentially the carboxylate forms of C, HC, MC, and CMC over their lactone forms, thereby providing an explanation for the shift to the right in the lactone-carboxylate equilibrium observed for each compound upon HSA addition. In marked contrast, three analogues (SN-38, CPT-11, and topotecan) all displayed enhanced stabilities in the presence of HSA. While the lifetimes of CPT-11, topotecan, and the carboxylate forms of both drugs were insensitive to the addition of HSA, the lifetimes of both SN-38 and its carboxylate form did titrate upon HSA addition. Analysis of binding isotherms constructed for the albumin interactions of SN-38 and its carboxylate form demonstrated a higher overall association constant for the lactone form [640 (M amino acid (aa) residues)-1] relative to the carboxylate form [150 (M aa)-1]. Our studies indicate that specific modifications at the 7- and 9-positions of the quinoline nucleus, such as those contained in CPT-11, topotecan, and SN-38, enhance drug stability in the presence of HSA. In the case of SN-38, the enhanced stability was shown to be due to preferential associations between the drug's lactone form and the blood protein.

Correlating preclinical animal studies and human clinical trials of a multifunctional, polymeric nanoparticle

“…We have modeled these dynamics (SI Appendix, Supplementary Materials and Methods), and the results suggest the different human dynamics are mainly attributable to the much higher binding affinity of CPT to human albumin than the albumin in other species (19). This high binding to human albumin is not observed with the active metabolic of irinotecan, SN-38 (30,31). When CPT was conjugated to linear PEG, the PK profiles of the released CPT in animals (32) and humans (33,34) showed the same form of the dynamics as observed for CRLX101.…”

Rapid deconjugation of SN-38 glucuronide and adsorption of released free SN-38 by intestinal microorganisms in rat

“…Unlike other camptothecin drugs, the SN-38 lactone form preferentially binds to blood proteins as compared with the corresponding carboxylate form, resulting in significant stability in the presence of human plasma (30). Furthermore, since the lactone form of SN-38 is highly water-insoluble, SN-38 is rapidly and tightly bound to hydrophobic bacterial cell walls or dietary fibers ( Fig.…”

Drug resistance against gemcitabine and topotecan mediated by constitutive hsp70 overexpression in vitro: implication of quercetin as sensitiser in chemotherapy

“…This drug was shown to be active in a lactone form only (Burke and Mi, 1994). The equilibrium of lactone and carboxylate form is dependent on pH, protein concentration and other factors.…”