Interaction of clinically important human DNA topoisomerase I poison, topotecan, with double-stranded DNA

Controlling anticancer drug mediated G-quadruplex formation and stabilization by a molecular container

Controlling of ligand mediated G-quadruplex DNA (GQ-DNA) formation and stabilization is an important and challenging aspect due to its active involvement in many biologically important processes such as DNA replication, transcription, etc. Here, we have demonstrated that topotecan (TPT), a potential anticancer drug, can instigate the formation and stabilization of GQ-DNA (H24 → GQ-DNA) in the absence of Na⁺/K⁺ ions via circular dichroism, fluorescence, NMR, UV melting and molecular dynamics (MD) simulation studies. The primary binding mode of TPT to GQ was found to be stacking at the terminal rather than binding to the groove. We have also reverted this conformational transition (GQ-DNA → H24) using a molecular container, cucurbit[7]uril (CB7), by means of the translocation of the drug (TPT) from GQ-DNA to its nanocavity. Importantly, we have carried out the detection of these conformational transitions using the fluorescence color switch of the drug, which is more direct and simple than some of the other methods that involve sophisticated and complex detection techniques.

Ion-Pairing Contribution to the Liposomal Transport of Topotecan as Revealed by Mechanistic Modeling

Actively loaded liposomal formulations of anticancer agents have been widely explored due to their high drug encapsulation efficiencies and prolonged drug retention. Mathematical models to predict and optimize drug loading and release kinetics from these nanoparticle formulations would be useful in their development and may allow researchers to tune release profiles. Such models must account for the driving forces as influenced by the physicochemical properties of the drug and the microenvironment, and the liposomal barrier properties. This study employed mechanistic modeling to describe the active liposomal loading and release kinetics of the anticancer agent topotecan (TPT). The model incorporates ammonia transport resulting in generation of a pH gradient, TPT dimerization, TPT lactone ring-opening and -closing interconversion kinetics, chloride transport, and transport of TPT-chloride ion-pairs to describe the active loading and release kinetics of TPT in the presence of varying chloride concentrations. Model-based predictions of the kinetics of active loading at varying loading concentrations of TPT and release under dynamic dialysis conditions were in reasonable agreement with experiments. These findings identify key attributes to consider in optimizing and predicting loading and release of liposomal TPT that may also be applicable to liposomal formulations of other weakly basic pharmaceuticals.

Insights into accelerated liposomal release of topotecan in plasma monitored by a non-invasive fluorescence spectroscopic method

A non-invasive fluorescence method was developed to monitor liposomal release kinetics of the anticancer agent topotecan (TPT) in physiological fluids and subsequently used to explore the cause of accelerated release in plasma. Analyses of fluorescence excitation spectra confirmed that unencapsulated TPT exhibits a red shift in its spectrum as pH is increased. This property was used to monitor TPT release from actively loaded liposomal formulations having a low intravesicular pH. Mathematical release models were developed to extract reliable rate constants for TPT release in aqueous solutions monitored by fluorescence and release kinetics obtained by HPLC. Using the fluorescence method, accelerated TPT release was observed in plasma as previously reported in the literature. Simulations to estimate the intravesicular pH were conducted to demonstrate that accelerated release correlated with alterations in the low intravesicular pH. This was attributed to the presence of ammonia in plasma samples rather than proteins and other plasma components generally believed to alter release kinetics in physiological samples. These findings shed light on the critical role that ammonia may play in contributing to the preclinical/clinical variability and performance seen with actively-loaded liposomal formulations of TPT and other weakly-basic anticancer agents.

Dynamic, nonsink method for the simultaneous determination of drug permeability and binding coefficients in liposomes

Drug release from liposomal formulations is governed by a complex interplay of kinetic (i.e., drug permeability) and thermodynamic factors (i.e., drug partitioning to the bilayer surface). Release studies under sink conditions that attempt to mimic physiological conditions are insufficient to decipher these separate contributions. The present study explores release studies performed under nonsink conditions coupled with appropriate mathematical models to describe both the release kinetics and the conditions in which equilibrium is established. Liposomal release profiles for a model anticancer agent, topotecan, under nonsink conditions provided values for both the first-order rate constant for drug release and the bilayer/water partition coefficient. These findings were validated by conducting release studies under sink conditions via dynamic dialysis at the same temperature and buffer pH. A nearly identical rate constant for drug release could be obtained from dynamic dialysis data when appropriate volume corrections were applied and a mechanism-based mathematical model was employed to account for lipid bilayer binding and dialysis membrane transport. The usefulness of the nonsink method combined with mathematical modeling was further explored by demonstrating the effects of topotecan dimerization and bilayer surface charge potential on the bilayer/water partition coefficient at varying suspension concentrations of lipid and drug.

Topotecan dynamics, tautomerism and reactivity--1H/13C NMR and ESI MS study

Topotecan (TPT) is in clinical use as an antitumor agent, hycamtin. Because of this, it requires both biologically and chemically useful information to be available. TPT acts by binding to the covalent complex formed by nicked DNA and topoisomerase I. This has a poisonous effect since inserted into the single-strand nick and TPT inhibits its religation. We used NMR to trace TPT dynamics, tautomerism and solvolysis products in various solvents and conditions. Chemical stability was assessed in methanol and DMSO as compared to water, and the regioselectivity of the N- and O-methylation was studied using various alkylating agents. The reaction products of quaternization of the nitrogen atom and methylation of the oxygen atom were characterized by means of ESI MS, (1)H/(13)C-HMBC and -HSQCAD NMR. We have focused on the NMR characterization of TPT with an anticipation that its aggregation, tumbling properties and the intramolecular dipolar interactions will be a common feature for other compounds described in this article. These features can also be useful in tracing the interactions of this class of topoisomerase I (TopoI) poisons with DNA. Moreover, the results explained shed light on the recently disclosed problem of lack of stability of TPT in the heart tissue homogenate samples using the analytical assays developed for this class of compounds carried out in the presence of methanol.

Surface-enhanced Raman spectroscopic studies of coactivator-associated arginine methyltransferase 1

We report, for the first time, the surface-enhanced Raman spectra of an important enzyme, coactivator-associated arginine methyltransferase 1 (CARM1), involved in various biological activities such as tumor suppressor function and stem cell differentiation. We have employed surface-enhanced Raman scattering (SERS) to obtain insight into the structural details of CARM1 by adsorbing it to silver (Ag) nanoparticles. The enzyme retains its activity even after its adsorption onto Ag nanoparticles. We observe strong SERS modes arising from amide vibrations and aromatic ring amino acids. The SERS spectra revealed amide I bands at 1637 cm(-1) and 1666 cm(-1), which arise as a result of the alpha helix of the protein and the polypeptide backbone vibration of a random coil, respectively. In order to confirm the amide vibrations, we have performed SERS on deuterated CARM1, which exhibits a clear red shift in amide band positions. The SERS spectra may provide useful information, which could be harnessed to study the functional interactions of CARM1 with small molecule modulators.

Surface-Enhanced Raman Scattering Studies of Human Transcriptional Coactivator p300

We report for the first time the surface-enhanced Raman scattering (SERS) studies on p300, a large multidomain transcriptional coactivator protein. Vibration spectral analysis has been performed in an attempt to understand the structure of the p300 in the absence of its crystal structure. Strong Raman bands associated with amides I-III have been observed in the protein spectra. This has been confirmed by performing SERS on deuterated p300. We also observe Raman bands associated with the alpha-helix, tryptophan, phenylalanine, tyrosine, and histidine. These bands will provide an ideal tool to study the drug-protein interactions in therapeutics using SERS. We have successfully demonstrated the chloride ion effect on the SERS of p300. The Raman intensity increases in the SERS spectra upon addition of chloride ion along with appearance of new modes. We have developed a new method, namely, the "sandwich technique", which could be used to perform SERS experiments on proteins in dry conditions.

Copper–topotecan complexation mediates drug accumulation into liposomes

These studies describe the role of transition metal ions in the liposomal encapsulation of topotecan. Liposomes (1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) and cholesterol (CH) (55:45, mole ratio)) were prepared with manganese (Mn), copper (Cu), zinc (Zn) or cobalt (Co) ion gradients (metal inside). Subsequently, topotecan was added to the liposome exterior (final drug-to-lipid ratio (mol/mol) of 0.2) and drug encapsulation was measured as a function of time and temperature. No drug loading was achieved with liposomes containing Co or Zn. Topotecan could be encapsulated into Mn-containing liposomes only in the presence of the ionophore, A23187 suggesting that a transmembrane pH gradient was necessary. However, Cu-containing liposomes, in the presence or absence of an imposed pH gradient, efficiently encapsulated topotecan. It has been reported that Cu(II) can form transition metal complexes with camptothecin; therefore, the Cu-topotecan interaction was characterized in solution as a function of pH. These investigations demonstrated that topotecan inhibited formation of an insoluble Cu hydroxide precipitate. Cryo-TEM analysis of the topotecan-loaded Cu liposomes showed electron-dense intravesicular precipitates. Further studies demonstrated that only the active lactone form of the drug was encapsulated and this form predominated in Cu-containing liposomes. Copper complexation reactions define a viable methodology to prepare liposomal camptothecin formulations.

Multiple binding modes of the camptothecin family to DNA oligomers

The binding constants of camptothecin, topotecan and its lactone ring-opened carboxylate derivative to DNA octamers were measured by UV and NMR spectroscopy. The self-association of topotecan (TPT) was also measured. The carboxylate form of TPT binds in the same way as the lactone, but more weakly. Titration of TPT into d(GCGATCGC)2 shows a preferred location stacked onto the terminal G1 base. However, the intermolecular NOEs cannot be reconciled with a single conformation of the complex, and suggest a model of a limited number of conformations in fast exchange. MD calculations on four pairs of starting structures with TPT stacked onto the G1-C8 base pair in different orientations were therefore performed. The use of selected experimental "docking" restraints yielded ten MD trajectories covering a wide conformational space. From a combination of calculated free energies, NOEs and chemical shifts, some of the structures produced could be eliminated, and it is concluded that the data are consistent with two major families of conformations in fast exchange. One of these is the conformation found in a crystal of a TPT/DNA/topoisomerase I ternary complex [Proc. Natl. Acad. Sci. USA 2002, 99, 15 387-15 392].