advances-in-protein-chemistry

used for therapeutic purposes. Recently, peptides (melectin, lasioglossins, halictines and macropin) and their analogs were isolated
from the venom reservoirs of wild bees and characterized. These peptides showed high antimicrobial activity against Gram-positive and
-negative bacteria, antifungal activity and low or moderate hemolytic activity. These peptides showed various degree of cell toxicity with
normal and cancerous cell line [103]. Gomesin, a cationic antimicrobial peptide purified from haemocytes of the spider Acanthoscurria
gomesiana, treatment effectively reduced Candida albicans in the kidneys, spleen, liver and vagina of infected mice [104].
Cell penetrating peptides, generally categorized as amphipathic or cationic peptides, are of increasing attention as a non-invasive
delivery technology for macromolecules. Delivery of large numbers of diverse nature of therapeutic peptides has been attempted using
different types of cell penetrating peptides (CPPs) in vitro and in vivo. The SynB peptides (RGGRLSYSRRRFSTSTGR), a family of
cell-penetrating peptides showed charge-mediated blood–brain barrier selectivity [105] and has been used extensively in cationic cellpenetrating
peptide vector-mediated strategies to deliver a large number of small molecules as well as proteins across cell membranes
in vitro and across the blood–brain barrier in vivo [106,107]. Moreover, SynB-PEG-GS (Gelatin-siloxane) nanoparticles had shown to
be a good biocompatibility with brain capillary endothelial cells and higher cellular uptake than that for PEG-GS nanoparticles [108].
CADY-1, an amphipathic peptide, is capable of forming complexes by self-assembly, and possesses cell-penetrating activity, and form
a stable complex with doxorubicin. CADY-1 and doxorubicin complex extended the blood residence time of doxorubicin in a similar
fashion to that of liposomal doxorubicin. In addition, the complex was capable of carrying doxorubicin across the cell membrane,
which increased the therapeutic index of doxorubicin. Also, CADY-1/doxorubicin complex exhibited better tolerance and anti-tumour
activity in experimental animals [109]. Recently, a hepatocarcinoma-binding peptide (A54 peptide) with PEGylated stearic acid grafted
chitosan (A54-PEG-CS-SA) micelles directed doxorubicin into human hepatoma cells (BEL-7402) in vitro, and high distribution ability
to liver and hepatoma tissue in vivo. A54-PEG-CS-SA micelles mediated targeted doxorubicin delivery to hepatoma cells suppressed
tumor growth more effectively and reduced toxicity compared to commercial adriamycin injection [110]. Recently, EAK16-II, an ioniccomplementary,
self-assembling peptide, has been found to stabilize ellipticine in aqueous solution and enhance antitumor activity
of ellipticine encapsulated in EAK16-II (EAK-EPT) was evaluated in vitro and in vivo without causing any side effects [111]. Cellpenetrating
peptides (CPP) have been used to enhance cellular uptake of Morpholinos, a potential anti-bioterrorism agent for inhibiting
replication of deadly Marburg viral infection [112].
Advances and Challenges in Peptide Based Therapeutic Approaches
Challenges
Despite increasing rate of approval and several advantages over small molecules, there are still some significant challenges associated
with the peptides which limit its applicability as drug candidate and finally affect its commercial status in drug market. One of the major
challenges is that most of the peptides cannot be administered orally due to the poor oral availability of the peptides, attributed mainly
to its rapidly inactivation and poor assimilation through the intestinal mucosa. These characteristics of peptides substantially restricts
the use of the most convenient and comfortable way of drug administration i.e. oral delivery, thus leaving option for alternative delivery
routes such as subcutaneous or intravenous, nasal, sublingual, buccal, transdermal, pulmonary etc. as well as novel delivery systems like
using liposomes [111-116]. However, there are a number of challenges when considering other routes of administration as different
routes have been found to be linked to changes in the pharmacokinetic profile as well as the biological activity of the peptides [117].
The liposomal formulation technology frequently suffers from the active pharmaceutical ingredient (API) encapsulation efficiency into
liposomes. Improving the stability and half-life of therapeutic peptides are other important issues as the gastric acid in the stomach
and proteases (peptidases) in the intestinal, tissue and serum could easily degrade peptides. Likewise, peptide antigenicity can result
in severe immune responses; therefore, there are also significant challenges in screening and development of novel peptides. Another
challenge associated with manufacturing of peptide-based drug is the inability of regulatory authorities from different regions to agree
on a common guideline defining the minimal level of peptide impurities present in peptide therapeutics. It directly affects the process
designing meant for producing high quality products and reduces the economic feasibility by increasing the cost of the final products as
compared to the traditional molecules. It is evident from these facts that there is still a long way to go to find novel and effective peptides
with high commercial potential.
Advances
In order to face the peptide-associated challenges and to sort out the problems related to their commercialization, several advances
have been made in the field of peptide based drugs to develop novel peptide therapeutics. The phage display is the recently developed
technology which may pave an improved way for identification and discovery of potential peptides. Additionally, bioinformatics and
systematic biological based strategies are increasingly being supported in boosting search for novel peptide drug candidates and are solely
based on the available information and comprehensive data facts. Discovery of new generation peptide-based vaccine is another feather
in the development of peptide therapeutics and may offer a wide product range in global market [118].
Poor bioavailability of peptides is one of the major concerns in development of an effective and robust therapeutics [119,120].
Attachment of polyethylene glycol (PEG) moieties to the peptides is a common strategy that have been used to improve the bioavailability
of peptides with the additional benefits of improved stability from proteolytic degradation and protection from recognition by the
immune system [121]. PEG-Intron and Pegasys, PEGylated interferon alpha-2b and alpha-2a, respectively, are examples of such peptides
therapeutic currently being used in therapy of hepatitis C infections [122]. However, the use of an efficient targeting method (PEG and
ligands (e.g. antibodies, mannose) attached to peptide–liposome formulation) or slow releasing drug delivery system (such as liposome
encapsulation of peptides) has markedly reduced the requirement of such bulky modifications [115,116].
Stability is another major consideration in peptide therapeutics and several advances have been made in this field in order to
modulate the in vivo stability. With the recent advances in technologies, the stability and selectivity of the peptides can be enhanced by
modifying its structure and/or naturally occurring ligands to make it as long-acting analogs with high binding affinity and high receptor
subtype-specific selectivity. For example, natural somatostatin has the ability to target all the five somatostatin receptor (SSTR) subtypes,
but its synthetic analogs such as L797 &591, octreotide & lanreotide, and BIM23268 show preference for SSTR1, SSTR2, and SSTR5-
subtypes, respectively [123]. To improve the half-life of peptides and to reduce the immunogenicity, their respective moieties have been
successfully fused to human albumin (ex-albiglutide) or an engineered human IgG4 Fc heavy chain (ex-dulaglutide). Furthermore,
via binding to the receptor FcRn, the albumin and Fc-containing molecules were made completely protected from degradation in
cells. Beside using traditional methods of backbone modification (involving the modification of the N-terminus and C-terminus of
OMICS Group eBooks
010