Aparajita Mohanty

There is an ongoing search for new alternatives to antibiotics because of the fast evolving pathogenic bacterial strains which are resistant to the existing antibiotics. Plant-based cyclotides have been shown to posses diverse bioactivities including potent antibacterial activity. Cyclotides are extremely stable cyclic peptides of 27–37 amino acids which are highly resistant against thermal, chemical and enzymatic degradation. Keeping in view the potential of cyclotides as antibacterial agents, an in silico analysis of native cyclotides, screened against Gram-negative bacteria was undertaken. Cyclotides with minimum inhibitory concentration (MIC) less than 10 μM and more than 10 μM were considered potent and non-potent, respectively. The amino acid composition/properties (viz. number of basic residues, average charge, isoelectric point) were significantly higher in potent antibacterial cyclotides compared to non-potent cyclotides. Two potent antibacterial cyclotides (circulin B and cyclopsychotride A) with low hemolytic activity were compared to non-potent cyclotide (tricyclon A) at the structural level. Surface models of potent cyclotides revealed a hydrophobic patch with larger number of residues compared to non-potent cyclotide. Studies using PPM server showed higher stability of potent cyclotides during peptide-membrane interaction as indicated by lower ΔG_transfer and lower tilt angle along with higher penetration depth compared to the higher ΔG_transfer and tilt angle with lower penetration depth in non-potent cyclotide, tricyclon A. Molecular dynamics simulations further confirmed the results of cyclotide-membrane interaction, obtained using PPM server. Hence, in silico studies can reveal the potential of the potent cyclotides as antibacterial agents that can be explored further for application in pharmaceutics. Similar studies can also be extended to assess other newly discovered native cyclotides for their potential applications as antibacterial agents.

Enzyme prodrug therapy has gained momentum in recent years due to its ability to improve therapeutic index (benefits versus toxic side-effects) and efficacy of chemotherapy in cancer treatment. Inactive prodrugs used in this system are converted into active anti-cancerous drugs by enzymes, specifically within the tumor cells. This therapy involves three components namely prodrug, enzyme and gene delivery vector. Past reports have clearly indicated that the choice of enzyme used is the major determinant for the success of this therapy. Generally, enzymes from nonhuman sources are employed to avoid off-target toxicity. Exogenous enzymes also give better control to the clinician regarding the calibration of treatment by site-specific initiation. Amongst these exo-enzymes, microbial enzymes are preferred due to their high productivity, stability and ease of manipulation. The present review focuses on the commonly used microbial enzymes, particularly cytosine deaminase, nitroreductase, carboxypeptidase, purine nucleoside phosphorylase in prodrug activation therapy. Various aspects viz. source of the enzymes, types of cancer targeted, mode of action and efficacy of the enzyme/prodrug system, efficient vectors used and recent research developments of each of these enzymes are comprehensively elaborated. Further, the results of the clinical trials and various strategies to improve their clinical applicability are also discussed.