Competitive binding of anticancer drugs 5-fluorouracil and cyclophosphamide with serum albumin: Calorimetric insights.

BACKGROUND: Isothermal titration calorimetry (ITC) has emerged as an excellent method to characterize drug-protein interactions. 5-Fluorouracil and cyclophosphamide have been used in combination for the treatment of breast carcinoma, though individually these drugs have also been useful in treating other types of cancer. A quantitative understanding of binding of these drugs with the transport protein under different conditions is essential for optimizing recognition by the protein and delivery at the target. METHODS: The values of binding constant, enthalpy, and entropy of binding have been determined by using ITC. Fluorescence and circular dichroism spectroscopies have been used to obtain further support to calorimetric observations, monitor conformational changes in the protein and establishing stoichiometry of association. RESULTS: The thermodynamic parameters have enabled a quantitative understanding of the affinity of 5-fluorouracil and cyclophosphamide with bovine serum albumin. The nature of binding has been unraveled based on effect of ionic strength, tetrabutyl-ammonium bromide, and sucrose which interfere in ionic, hydrophobic, and hydrogen bonding interactions. The binding site has been identified by using site marker warfarin in combination with 5-fluorouracil and cyclophosphamide. Further, the experiments have been done to establish whether both the drugs share the same binding site, and the effect of antibiotic drug carbenecillin and anti-inflammatory drug naproxen on their association. GENERAL SIGNIFICANCE: Tuning optimum association of drugs with the transport vehicles for effective drug delivery requires identification of the nature of interacting groups in terms of energetics of interactions. Such studies employing ITC have direct significance in rational drug design.

Complexation of beta-lactam antibiotic drug carbenicillin to bovine serum albumin: energetics and conformational studies.

Binding of the antibiotic drug carbenicillin to bovine serum albumin (BSA) has been studied using isothermal titration calorimetry (ITC) in combination with fluorescence and circular dichroism (CD) spectroscopies. The thermodynamic parameters of binding have been evaluated as a function of temperature, ionic strength, and in the presence of anionic, cationic and nonionic surfactants, tetrabutylammonium bromide, and sucrose. The values of van't Hoff enthalpy do not agree with the calorimetric enthalpy indicating conformational changes in the protein upon drug binding. These observations are supported by the intrinsic fluorescence and CD spectroscopic measurements. A reduction in the binding affinity of carbenicillin to BSA is observed with increase in ionic strength of the solution, thereby suggesting, prevailing of electrostatic interactions in the binding process. The involvement of hydrophobic interactions in the binding of the drug to the protein is also indicated by a slight reduction in binding constant in the presence of tetrabutylammonium bromide. The experiments in the presence of sucrose suggest that hydrogen bonding is perhaps not dominant in the binding. The anionic surfactant sodium dodecyl sulphate (SDS) is observed to completely interfere in the ionic interactions in addition to its partial denaturing capacity. However, the presence of cationic surfactant hexadecyl trimethylammonium bromide (HTAB) and nonionic surfactant Triton-X 100 induce a slight reduction in the values of binding affinity. These calorimetric and spectroscopic results, provide quantitative information on the binding of carbenicillin to BSA and suggests that the binding is dominated by electrostatic interactions with contribution from hydrophobic interactions.

Equimolar mixture of 2,2,2-trifluoroethanol and 4-chloro-1-butanol is a stronger inducer of molten globule state: isothermal titration calorimetric and spectroscopic studies.

A mixture of 4-chloro-1-butanol and 2,2,2-Trifluoroethanol (TFE) has been used to generate Molten globule (MG) state of structurally homologous but functionally different proteins bovine alpha-lactalbumin and hen egg-white lysozyme. The thermal denaturation was done using UV-Visible spectroscopy. From UV-Visible profile, thermal transition was not observed beyond a particular concentration. There was an indication of molten globule state in case of alpha-lactalbumin from circular dichroism experiments. By intrinsic tryptophan fluorescence, acrylamide and potassium iodide quenching, 8-anilino-naphthalene sulfonic acid (ANS) binding and energy transfer studies the presence of molten globule state was confirmed. Quantitative characterization of MG state and determining the binding thermodynamics of ANS to the MG state was done using Isothermal Titration Calorimetry (ITC). Results show that alpha-lactalbumin exists in MG state at a particular concentration but lysozyme does not show features of MG state.