Vedagopuram Sreekanth

Design, Synthesis, and Mechanistic Investigations of Bile Acid–Tamoxifen Conjugates for Breast Cancer Therapy

We have synthesized two series of bile acid tamoxifen conjugates using three bile acids lithocholic acid (LCA), deoxycholic acid (DCA), and cholic acid (CA). These bile acid-tamoxifen conjugates possess 1, 2, and 3 tamoxifen molecules attached to hydroxyl groups of bile acids having free acid and amine functionalities at the tail region of bile acids. The in vitro anticancer activities of these bile acid-tamoxifen conjugates show that the free amine headgroup based cholic acid-tamoxifen conjugate (CA-Tam3-Am) is the most potent anticancer conjugate as compared to the parent drug tamoxifen and other acid and amine headgroup based bile acid-tamoxifen conjugates. The cholic acid-tamoxifen conjugate (CA-Tam3-Am) bearing three tamoxifen molecules shows enhanced anticancer activities in both estrogen receptor +ve and estrogen receptor -ve breast cancer cell lines. The enhanced anticancer activity of CA-Tam3-Am is due to more favorable irreversible electrostatic interactions followed by intercalation of these conjugates in hydrophobic core of membrane lipids causing increase in membrane fluidity. Annexin-FITC based FACS analysis showed that cells undergo apoptosis, and cell cycle analysis showed the arrest of cells in sub G0 phase. ROS assays showed a high amount of generation of ROS independent of ER status of the cell line indicating changes in mitochondrial membrane fluidity upon the uptake of the conjugate that further leads to the release of cytochrome c, a direct and indirect regulator of ROS. The mechanistic studies for apoptosis using PCR and western analysis showed apoptotsis by intrinsic and extrinsic pathways in ER +ve MCF-7 cells and by only an intrinsic pathway in ER -ve cells. In vivo studies in the 4T1 tumor model showed that CA-Tam3-Am is more potent than tamoxifen. These studies showed that bile acids provide a new scaffold for high drug loading and that their anticancer activities strongly depend on charge and hydrophobicity of lipid-drug conjugates.

Synthesis, structure–activity relationship, and mechanistic investigation of lithocholic acid amphiphiles for colon cancer therapy

We report a structure-activity relationship of lithocholic acid amphiphiles for their anticancer activities against colon cancer. We synthesized ten cationic amphiphiles differing in nature of cationic charged head groups using lithocholic acid. We observed that anticancer activities of these amphiphiles against colon cancer cell lines are contingent on nature of charged head group. Lithocholic acid based amphiphile possessing piperidine head group (LCA-PIP1 ) is ~10 times more cytotoxic as compared to its precursor. Biochemical studies revealed that enhanced activity of LCA-PIP1 as compared to lithocholic acid is due to greater activation of apoptosis.LCA-PIP1 induces sub G0 arrest and causes cleavage of caspases. A single dose of lithocholic acid-piperidine derivative is enough to reduce the tumor burden by 75% in tumor xenograft model.

Bile acid amphiphiles with tunable head groups as highly selective antitubercular agents

Tuberculosis faces major challenges for its cure due to (a) long treatment period, (b) emergence of drug resistance bacteria, and (c) poor patient compliance. Disrupting the membrane integrity of mycobacteria as a therapeutic strategy has not been explored well as the rigid, waxy, and hydrophobic nature of mycobacterial lipids does not allow binding and penetration of charged amtimicrobial amphiphiles and peptides. Here, we present a new concept that fine-tuning of the charged head group modulates the specificity of amphiphiles against bacterial membranes. We show that hard-charged amphiphiles interact with mycobacterial trehalose dimycolates and penetrate through rigid mycobacterial membranes. In contrast, soft-charged amphiphiles specifically inhibit the growth of both E. coli and S. aureus via electrostatic interactions. These subtle variations between interactions of amphiphiles and bacterial membranes could be explored further to design more specific and potent antimycobacterial agents.

Design, regioselective synthesis and cytotoxic evaluation of 2-aminoimidazole-quinoline hybrids against cancer and primary endothelial cells.

In search of new selective anti-cancer agents, a series of sixteen novel 2-aminoimidazole-quinoline hybrid compounds (5a-5p) have been designed and synthesized regioselectively. We have characterized the compounds extensively using IR, 1D and 2D NMR Spectroscopy and mass spectrometry. The cytotoxicity studies against different cancer cell lines showed that the compound 5a (Imd-Ph) emerged as a potent cytotoxic scaffold. Imd-Ph (5a) exhibited a selective anticancer activity against human colon cancer cell line (HCT-116, DLD-1) and was found relatively non-toxic to breast cancer cells (MDA-MB-231) as well as to normal primary endothelial cells (HUVEC). Structure-activity relationship of imidazole-quinoline hybrid scaffolds revealed differential and selective toxicities exerted by the different derivatives against cancer and normal cells. Structural modification of the scaffold with library of a wide variety of substituents may lead to the development of novel selective anti-cancer agents in the future.

Fluorescence (fluidity/hydration) and calorimetric studies of interactions of bile acid-drug conjugates with model membranes.

We have studied the interactions of three bile acid-tamoxifen conjugates, lithocholic acid-tamoxifen (LA-Tam(1)-Am), deoxycholic acid-tamoxifen (DCA-Tam(2)-Am), and cholic acid-tamoxifen (CA-Tam(3)-Am), possessing 1-3 tamoxifen molecules having an amine headgroup with model DPPC membranes and compared with N-desmethylated tamoxifen (TamNHMe) using DPH based fluorescence anisotropy, Prodan based hydration, and differential scanning calorimetry studies. DPH based anisotropy studies showed that bile acid-tamoxifen conjugates increase membrane fluidity, which strongly depends on the number of tamoxifen molecules conjugated to bile acid and the percentage of doping of bile acid-tamoxifen conjugates in the DPPC membranes. The order of membrane fluidity of the coliposomes from bile acid-tamoxifen conjugates and DPPC lipids in gel phase was found to be CA-Tam(3)-Am > DCA-Tam(2)-Am > LA-Tam(1)-Am > TamNHMe. Incorporation of bile acid-tamoxifen conjugates showed an unusual complex behavior of membrane hydration, as evident from Prodan based hydration studies. Temperature dependent study showed incorporation of LA-Tam(1)-Am and DCA-Tam(2)-Am conjugates decreases membrane hydration with an increase in temperature up to the phase transition temperature (T(m)). Differential scanning calorimetry studies showed a decrease in phase transition temperature (T(m)) upon an increase in the percentage of doping of TamNHMe and CA-Tam(3)-Am, whereas LA-Tam(1)-Am and DCA-Tam(2)-Am do not cause a major change in the phase transition temperature (T(m)) of DPPC liposomes. These studies showed the differential behavior of bile acid-tamoxifen conjugates regulating the membrane fluidity, hydration, and phase transition of model membranes depending upon the percentage of doping and tamoxifen conjugation to bile acids.

Number of Free Hydroxyl Groups on Bile Acid Phospholipids Determines the Fluidity and Hydration of Model Membranes

Interactions of synthetic phospholipids with model membranes determines the drug release capabilities of phospholipid vesicles at diseased sites. We performed 1,6-diphenyl-1,3,5-hexatriene (DPH)-based fluorescence anisotropy, Laurdan-based membrane hydration, and differential scanning calorimetry (DSC) studies to cognize the interactions of three bile acid phospholipids, lithocholic acid-phosphocholine (LCA-PC), deoxycholic acid-phosphocholine (DCA-PC), and cholic acid-phosphocholine (CA-PC) with model membranes. These studies revealed that bile acid phospholipids increases membrane fluidity in DCA-PC > CA-PC > LCA-PC order, indicating that induction of membrane fluidity is contingent on the number and positioning of free hydroxyl groups on bile acids. Similarly, DCA-PC causes maximum membrane perturbations due to the presence of a free hydroxyl group, whereas LCA-PC induces gel phase in membranes due to hydrophobic bile acid acyl chain interactions. These DCA-PC-induced membrane perturbations induce a drastic decrease in phase transition temperature (Tm) as determined by calorimetric studies, whereas doping of LCA-PC causes phase transition broadening without change in Tm. Doping of CA-PC induces membrane perturbations and membrane hydration like DCA-PC but sharpening of phase transition at higher doping suggests self-association of CA-PC molecules. Therefore these differential mode of interactions between bile acid phospholipids and model membranes would help in the future for their use in drug delivery.

Nature of the charged head group dictates the anticancer potential of lithocholic acid-tamoxifen conjugates for breast cancer therapy

Modulation of existing drugs is required to achieve enhanced activity for cancer therapy by lowering their effective dose. Strategies for introduction of cationic charge and hydrophobicity have been proposed and explored to enhance the therapeutic effects of anticancer drugs. In this manuscript, we planned modulation of tamoxifen and synthesized eight tamoxifen (Tam) conjugated lithocholic acid (LCA) amphiphiles with variable cationic charged head groups. We determined the anticancer potential of these amphiphiles against different breast cancer cell lines. Activity of these amphiphiles is contingent on nature of the charged head group, as hard-charged amphiphiles exhibit strong membrane interactions and enhanced anticancer activity compared to soft-charged amphiphiles. In-depth mechanistic studies concluded that conjugation of a dimethyl amino pyridine (DMAP) charged head group in case of LCA-Tam-DMAP enhances therapeutic effect of Tam in breast cancer cells, and makes it highly effective even against ER negative cells. Amphiphilic character of these lipid-drug conjugates can be further explored for engineering nanotherapeutics for targeting tumors. Therefore, fine-tuning the interactions of drugs with cell membranes can help in engineering future lipid-drug conjugates for effective cancer therapy.

Design, synthesis, and physico-chemical interactions of bile acid derived dimeric phospholipid amphiphiles with model membranes

Understanding of amphiphile-membrane interactions is crucial in design and development of novel amphiphiles for drug delivery, gene therapy, and biomedical applications. Structure and physico-chemical properties of amphiphiles determine their interactions with biomembranes thereby influencing their drug delivery efficacies. Here, we unravel the interactions of bile acid derived dimeric phospholipid amphiphiles with model membranes using Laurdan-based hydration, DPH-based membrane fluidity, and differential scanning calorimetry studies. We synthesized three dimeric bile acid amphiphiles where lithocholic acid, deoxycholic acid, and cholic acid are conjugated to cholic acid phospholipid using click chemistry. Interactions of these dimeric amphiphiles with model membranes showed that these amphiphiles form different structural assemblies and molecular packing in model membranes depending on the number and position of free hydroxyl groups on bile acids. We discovered that cholic acid-cholic acid dimeric phospholipid form self-assembled aggregates in model membranes without changing membrane fluidity; whereas cholic acid-deoxycholic acid derived amphiphile induces membranes fluidity and hydration of model membranes.

Molecular Self-Assembly of Bile Acid-Phospholipids Controls the Delivery of Doxorubicin and Mice Survivability

Lipid composition in general determines the drug encapsulation efficacy and release kinetics from liposomes that impact the clinical outcomes of cancer therapy. We synthesized three bile acid phospholipids by conjugating the phosphocholine headgroup to the 3-hydroxyl group of benzylated lithocholic acid (LCA), deoxycholic acid (DCA), and cholic acid (CA); and investigated the impact of membrane rigidity on drug encapsulation efficacy, drug release kinetics, anticancer effects, and mice survival. Liposomes with a hydrodynamic diameter of 100-110 nm were subsequently developed using these phospholipids. Fluorescence-probe based quantification revealed a more fluidic nature of DCA-PC- and CA-PC-derived liposomes, whereas the LCA-PC-derived ones are rigid in nature. Doxorubicin encapsulation studies showed ∼75% encapsulation and ∼38% entrapment efficacy of doxorubicin using more fluidic DCA-PC and CA-PC derived liposomes as compared to ∼58% encapsulation and ∼18% entrapment efficacy in the case of LCA-PC derived liposomes. In vivo anticancer studies in the murine model confirmed that doxorubicin entrapped CA-PC liposomes compromise mice survival, whereas rigid drug entrapped LCA-PC-derived-liposomes increased mice survival with ∼2-fold decrease in tumor volume. Pharmacokinetic and biodistribution studies revealed an ∼1.5-fold increase in plasma drug concentration and an ∼4.0-fold rise in tumor accumulation of doxorubicin on treatment with drug entrapped LCA-PC liposomes as compared to doxorubicin alone. In summary, this study presents the impact of bile acid derived liposomes with different rigidities on drug delivery and mice survivability.

Tethering of Chemotherapeutic Drug/Imaging Agent to Bile Acid-Phospholipid Increases the Efficacy and Bioavailability with Reduced Hepatotoxicity

Weakly basic drugs display poor solubility and tend to precipitate in the stomachs acidic environment causing reduced oral bioavailability. Tracing of these orally delivered therapeutic agents using molecular probes is challenged due to their poor absorption in the gastrointestinal tract (GIT). Therefore, we designed a gastric pH stable bile acid derived amphiphile where Tamoxifen (as a model anticancer drug) is conjugated to lithocholic acid derived phospholipid (LCA-Tam-PC). In vitro studies suggested the selective nature of LCA-Tam-PC for cancer cells over normal cells as compared to the parent drug. Fluorescent labeled version of the conjugate (LCA-Tam-NBD-PC) displayed an increased intracellular uptake compared to Tamoxifen. We then investigated the antitumor potential, toxicity, and median survival in 4T1 tumor bearing BALB/c mice upon LCA-Tam-PC treatment. Our studies confirmed a significant reduction in the tumor volume, tumor weight, and reduced hepatotoxicity with a significant increase in median survival on LCA-Tam-PC treatment as compared to the parent drug. Pharmacokinetic and biodistribution studies using LCA-Tam-NBD-PC witnessed the enhanced gut absorption, blood circulation, and tumor site accumulation of phospholipid-drug conjugate leading to improved antitumor activity. Therefore, our studies revealed that conjugation of chemotherapeutic/imaging agents to bile acid phospholipid can provide a new platform for oral delivery and tracing of chemotherapeutic drugs.