Category: Amino acid/peptide conjugates

The conjugation of different amino acids/peptides to various biologically active compounds has fetched the outstanding results as are very promising drug candidates.

Novel derivatives of abietic and dehydroabietic acid

Abietic and dehydroabietic acid are interesting diterpenes with a highly diverse repertoire of associated bioactivities. They have, among others, shown antibacterial and antifungal activity, potentially valuable in the struggle against the increasing antimicrobial resistance and imminent antibiotic shortage. In this paper, we describe the synthesis of a set of 9 abietic and dehydroabietic acid derivatives containing amino acid side chains and their in vitro antimicrobial profiling against a panel of human pathogenic microbial strains. Furthermore, their in vitro cytotoxicity against mammalian cells was evaluated. The experimental results showed that the most promising compound was 10 [methyl N-(abiet-8,11,13-trien-18-yl)-d-serinate], with an MIC90 of 60 μg/mL against Staphylococcus aureus ATCC 25923, and 8 μg/mL against methicillin-resistant S. aureusStaphylococcus epidermidis and Streptococcus mitis. The IC50 value for compound 10 against Balb/c 3T3 cells was 45 μg/mL. ( 2017 Jan 1;25(1):132-137.)

Galloyl-RGD as a new cosmetic ingredient


The cosmetics market has rapidly increased over the last years. For example, in 2011 it reached 242.8 billion US dollars, which was a 3.9% increase compared to 2010. There have been many recent trials aimed at finding the functional ingredients for new cosmetics. Gallic acid is a phytochemical derived from various herbs, and has anti-fungal, anti-viral, and antioxidant properties. Although phytochemicals are useful as cosmetic ingredients, they have a number of drawbacks, such as thermal stability, residence time in the skin, and permeability through the dermal layer. To overcome these problems, we considered conjugation of gallic acid with a peptide.


We synthesized galloyl-RGD, which represents a conjugate of gallic acid and the peptide RGD, purified it by HPLC and characterized by MALDI-TOF with the aim of using it as a new cosmetic ingredient. Thermal stability of galloyl-RGD was tested at alternating temperatures (consecutive 4°C, 20°C, or 40°C for 8 h each) on days 2, 21, 41, and 61. Galloyl-RGD was relatively safe to HaCaT keratinocytes, as their viability after 48 h incubation with 500 ppm galloyl-RGD was 93.53%. In the group treated with 50 ppm galloyl-RGD, 85.0% of free radicals were removed, whereas 1000 ppm galloyl-RGD suppressed not only L-DOPA formation (43.8%) but also L-DOPA oxidation (54.4%).


Galloyl-RGD is a promising candidate for a cosmetic ingredient. (. 2014; 15: 18.)

Traversal of the Blood–Brain Barrier by Cleavable l-Lysine Conjugates of Apigenin

Apigenin, a flavone abundant in parsley and celery, is known to act on several CNS receptors, but its very poor water solubility (<0.001 mg/mL) impedes its absorption in vivo and prevents clinical use. Herein, apigenin was directly conjugated with glycine, l-phenylalanine, and l-lysine to give the corresponding carbamate derivatives, all of which were much more soluble than apigenin itself (0.017, 0.018, and 0.13 mg/mL, respectively). The Lys-apigenin carbamate 10 had a temporary sedative effect on the mice within 5 min of intraperitoneal administration (single dose of 0.4 mg/g) and could be detected in the mice brain tissues at a concentration of 0.82 μg/g of intact Lys-apigenin carbamate 10 and 0.42 ug/g of apigenin at 1.5 h. This study accomplished the delivery of apigenin across the BBB in a manner that might be applicable to other congeners, which should inform the future development of BBB-crossing flavonoids.(J. Agric. Food Chem., 2018, 66 (30), pp 8124–8131)

Synthesis of doxorubicin cell‐penetrating peptide bioconjugates

Cell‐penetrating peptides, also known as protein transduction domain (PTD), have attracted interest as carriers for intracellular drug delivery. We report the first drug conjugate with a negatively charged amphipathic cell‐penetrating peptide. Furthermore, we compare two different doxorubicin cell‐penetrating peptide conjugates, which are both unique in their properties, due to their net charge at physiological pH, namely the positively charged octaarginine and the negatively charged proline‐rich amphipathic peptide. These conjugates were prepared exploiting a novel heterobifunctional crosslinker to join the N‐terminal cysteine residue of the peptides with the aliphatic ketone of doxorubicin. This small linker contains an activated thiol as well as aminooxy functionality, capable of generating a stable oxime bond with the C‐13 carbonyl group of doxorubicin. The disulfide bond formed between the peptide and doxorubicin enables the release of the drug in the cytosol, as confirmed by drug‐release studies performed in the presence of glutathione. Additionally, the cytotoxicity as well as the cellular uptake and distribution of this tripartite drug delivery system was investigated in MCF‐7 and HT‐29 cell lines.

Heterobifunctional crosslinkers are extensively applied in drug modifications, and their utilization has proven to be very important for the attachment of diverse carriers, such as monoclonal antibodies, proteins, polymers, and peptides, to doxorubicin. Established conjugation technique is applied at the C‐13 keto group by hydrazones, due to their fast hydrolysis in acidic environment existing in biological compartments like endosomes and lysosomes. However, the insufficient stability of the doxorubicin hydrazone conjugates has been reported even at physiological pH (7.4), leading to the release of the free drug in the bloodstream. In order to overcome these difficulties, we have chosen a crosslinker capable of creating an oxime bond on doxorubicin’s ketone, due to the higher hydrolytic stability of the oxime group. Additionally, thiol‐containing carriers, like albumin proteins, have also been conjugated to anthracyclines utilizing functional groups that are highly specific for sulfhydryl groups, e.g. maleimides and pyridyl disulfides. The application of pyridyl disulfides is advantageous, because a disulfide bond between the linker and the cargo is formed, which can be reduced in the cytoplasm by glutathione to deliver the freight. Furthermore, pyridyl disulfides can serve as a protective group during synthesis to avoid undesired dimerization as well as an activating group for the thiol to facilitate disulfide formation. In contrast, maleimides react with a thiol via Michael addition; thus, a covalent bond is created that cannot be cleaved under physiological conditions. Therefore, we have selected a heterobifunctional crosslinker that contains a protected aminooxy group and pyridyl disulfide.

Antimicrobial peptide-fatty acid conjugate

The rise of resistant bacteria has prompted the search for new antimicrobial agents. Antimicrobial membrane lytic peptides have potential as future microbial agents due to their novel mode of action. Recently conjugation of a fatty acid to antimicrobial peptides has been explored as a method to modulate the activity and selectivity of the peptide. Our work further explores these phenomena by testing two peptides, YGAAKKAAKAAKKAAKAA (AKK) and LKKLLKLLKLLKL (LKK), conjugated to fatty acids of varying length for their activity, structure, solution assembly properties and the ability to bind model membranes. We found that increasing the length of fatty acids conjugated to peptide AKK, up to a 16 carbons in length, increases the antimicrobial activity. Peptide AKK appears to lose activity when the minimal active concentration is higher than the critical miscelle concentration (CMC) of the molecule. Thus, if the CMC of the peptide conjugate is too low the activity is lost. Peptide LKK has no activity when conjugated to lauric acid and appears to aggregate at very low concentrations. Conjugation of AKK with a fatty acid increases its affinity to model supported lipid membranes. It appears that the increased hydrophobic interaction imparted by the fatty acid increases the affinity of the peptide to the surface thus increasing its activity. At concentrations above the CMC, solution self-assembly inhibits binding of the peptide to cell membranes.

Cleavable Doxorubicin–Peptide Conjugate

Although recent methods for targeted drug delivery have addressed many of the existing problems of cancer therapy associated with undesirable side effects, significant challenges remain that have to be met before they find significant clinical relevance. One such area is the delicate chemical bond that is applied to connect a cytotoxic drug with targeting moieties like antibodies or peptides. Here we describe a novel platform that can be utilized for the preparation of drug–carrier conjugates in a site-specific manner, which provides excellent versatility and enables triggered release inside cancer cells. Its key feature is a cleavable doxorubicin–octreotide bioconjugate that targets overexpressed somatostatin receptors on tumor cells, where the coupling between the two components was achieved through the first cleavable disulfide-intercalating linker. The tumor targeting ability and suppression of adrenocorticotropic hormone secretion in AtT-20 cells by both octreotide and the doxorubicin hybrid were determined via a specific radioimmunoassay. Both substances reduced the hormone secretion to a similar extent, which demonstrated that the tumor homing peptide is able to interact with the relevant cell surface receptors after the attachment of the drug. Effective drug release was quickly accomplished in the presence of the physiological reducing agent glutathione. We also demonstrate the relevance of this scaffold in biological context in cytotoxicity assays with pituitary, pancreatic, and breast cancer cell lines.(Mol Pharm. 2015 Dec 7;12(12):4290-300. )

Amino Acid Carbamates As Prodrugs Of Resveratrol

Resveratrol (3, 5, 4′-trihydroxy-trans-stilbene), a plant polyphenol, has important drug-like properties, but its pharmacological exploitation in vivo is hindered by its rapid transformation via phase II conjugative metabolism. One approach to bypass this problem relies on prodrugs. We report here the synthesis, characterization, stability and in vivo pharmacokinetic behaviour of prodrugs of resveratrol in which the OH groups are engaged in an N-monosubstituted carbamate ester (-OC(O)NHR) linkage with a natural amino acid (Leu, Ile, Phe, Thr) to prevent conjugation and modulate the physicochemical properties of the molecule. We also report a convenient, high-yield protocol to obtain derivatives of this type. The new carbamate ester derivatives are stable at pH 1, while they undergo slow hydrolysis at physiological pH and hydrolyse with kinetics suitable for use in prodrugs in whole blood. After administration to rats by oral gavage the isoleucine-containing prodrug was significantly absorbed, and was present in the bloodstream as non-metabolized unaltered or partially deprotected species, demonstrating effective shielding from first-pass metabolism. We conclude that prodrugs based on the N-monosubstituted carbamate ester bond have the appropriate stability profile for the systemic delivery of phenolic compounds.

All compounds turned out to be highly stable at pH values close to that of the human stomach, no reaction occurring over 24 hours at 37 °C in 0.1 N HCl, and they underwent slow hydrolysis at near-neutral pH (pH 6.8, representing intestinal pH) thus ensuring protection of the phenolic moieties from first pass metabolism during absorption in the gastrointestinal tract. In contrast all the synthesized prodrugs hydrolyzed in murine whole blood, with kinetics suitable for use as prodrugs. (Sci Rep. 2015 Oct 14;5:15216. )

Synthesis of amino acid conjugates of tetrahydrocurcumin

Tetrahydrocurcumin (THC), the hydrogenated and stable form of curcumin, exhibits physiological and pharmacological activities similar to curcumin. A protocol has been developed for the synthesis of novel conjugates of THC with alanine (2a), isoleucine (2b), proline (2c), valine (2d), phenylalanine (2e), glycine (2f) and leucine (2g) in high yields (43–82%). All the derivatives of THC exhibited more potent anti-microbial activity than THC against Bacillus cereus, Staphylococcus aureus, Escherichia coli and Yersinia enterocolitica. The MIC values of the derivatives were 24–37% of those for THC in case of both Gram-positive and Gram-negative bacteria. Derivatives 2g and 2d exhibited maximum anti-mutagenicity against Salmonella typhimurium TA 98 and TA 1538, respectively at a low concentration of 313 μg/plate, with comparable activity for THC evident only at 3750 μg/plate. These results clearly demonstrated that the conjugation of THC at the phenolic position with amino acids led to significant improvement of its in vitro biological attributes.(Food Chem. 2013 Aug 15;139(1-4):332-8.)