Commonly, peptide based nanoparticles (pep-NPs) could be prepared by using single peptide or polypeptides via different types of bonding such as non-covalent bonding that is hydrogen bonding, ionic or hydrophobic interactions etc. Specific structures could be attained such as one dimensional (1D) structures like nanotapes, nanoribbons, and nanofibers, two dimensional (2D) structures like hydrogel networks and three dimensional (3D) structures like scaffolds if the peptide sequences are skillfully changed. There are various reasons that pep-NPs are the first choice of scientist to design nanostructures over other inorganic or organic nanostructures for in vivo applications due to their numerous advantages. Some of the advantages are as follows:
(a) Peptide based nanoparticles are biocompatible and biodegradable in nature, so they have minimum toxicity to normal cells; (b) Peptide molecules have the self-assembling capacity in which peptides could assemble into one-dimensional nanostructures via intermolecular hydrogen bonding. Additionally, this 1D Structure could lead to the formation of 2D ?-sheets, or promote to 3D network under inter-sheets interactions (Deming, 2010); (c) Easy to modify the structure of Pep-NPs. The peptide sequence can be change easily or incorporate any extraneous molecules to the peptide sequence as per our requirements. (d) Pep-NPs are able to form bioactive hydrogels which are functioned as imitating agent for local extracellular matrix (ECM) and large proteins. These hydrogels system can also be used as vehicles for drug delivery (Cui et al., 2010; Hutmacher, 2010); (e) Pep-NPs and their derivatives have distinctive characters for the capacity of responding to certain environmental stimulus like thermal differences, pH changes, as well as biological activations (e.g., overexpression of some enzymes on the diseased cells) (Deming, 2005); (f) Pep-NPs systems are prepared for focused drug delivery with specific ligands show to specifically control the architecture of the particle through the biological/physical interface. Scientists have employed pep-NPs as templates for designing other systems (Lamm et al., 2008), such as Pd nanoparticles (Coppage et al., 2012). Hence pep-NPs have been extensively used in the various areas like chemical biology, engineering, material science, etc. due to these properties. Pep-NPs also used in the biomaterials applications along with their uses in cell imaging and drug delivery (Tamerler and Sarikaya, 2009).
3. Pep-NPs for Cancer Therapy and Diagnosis
Recently, the term “theranostics” is introduced, which can defined as both diagnostic and therapeutic properties within one molecule. With this molecule diagnosis and therapy can be accomplished in the same period (Thakare et al., 2010). In cancer disease, at all stages the diagnosis and therapy are very crucial for the effective treatment. Various types of therapeutic agents and methods used for theranostic purpose, showing some basic advantages, but these systems are also facing disadvantages like difficulty in fabrication, biocompatibility, duration of action and other basic requirements. On the other hand Pep-NPs have various surprising advantages as the theranostic agent in cancer treatment, when compared with other materials like, small molecules. Pep-NPs are biocompatible and biodegradable in nature (Koo et al., 2011). Peptide molecules have the intrinsic selective capacity, which can be useful as the targeting molecules for the cancer treatment. Nano size range pep-NPs have tendency to remain for a prolonged period of time in the systemic circulation (Danhier et al., 2010). Due to their nano size the pep-NPs have their effect on tumor cells as enhanced permeation and retention (EPR) effect. EPR effect is present due to neovascularization of tumor cells, which means quick formation of new, fenestrated blood vessels. These newly formed vessels, therefore are more susceptible to pep-NPs than normal cells. Therefore, EPR effect shows passive tumor-targeting for pep-NPs with drugs or imaging agents (Fox et al., 2009). Apart from that, their preparation is economical, highly stable and simple make these systems are the best system into the list of other construct nanosystems for theranostic purpose in cancer.
There are some important criteria which should be followed in the designing of best pep-NPs for cancer diagnosis and therapy which are covered in the following sections. Materials as well as peptides used for the formulations should be
Ø biodegradable and biocompatible;
Ø deliver therapeutic/imaging agents to the targeted sites;
Ø Higher number of drugs can be delivered per receptor/targeted sites;
Ø reduce or avoid peptide sequences which may be immunogenic to the body;
Ø highly specific to the target sites so as to ensure a reduced dose concentration and side-effect (Byrne et al., 2008);
Ø RGD peptide may be inhibit renal filtration because of it reduces glomerular filtration. Hence this may enhance blood circulation times and higher retention of the peptide with the target receptors (Schraa et al., 2002);
Ø maximize the “signal-to-noise” ratio in that amplification techniques. It is useful to increase the local concentration of the imaging/therapeutic agents (Allen and Cullis, 2004);
Ø have prolonged retention time (biological half-lives) and homogenous distribution throughout the body by using long-circulating spacers/modules to amend biologically active agents;
Ø Show enhanced permeability and retention (EPR) process due to high molecular weight of carrier molecules which helps in passive retention to the tumor site (Mitra et al., 2005).
Cancer diagnosis is the preliminary step in the cancer treatment. Diagnosis of cancer can be performed by using various molecular imaging techniques which provide important characteristics of tumor tissues, like their specific position, cell division and growth behaviors, the premalignant molecular abnormalities and presence of specific receptors at the molecular level (Liu et al., 2011; Lee et al., 2010). Peptide-based formulations are now used for the molecular imaging as the diagnostic tool for tumors. There are various techniques which uses different types of imaging agents like, micelles for ultrasound imaging, radioisotopes (18F, 99Tc, 123I, 68Ga, 111In) for position-emission tomography (PET) or single photon emission computed tomography (SPECT), iodide derivatives for computed tomography (CT), magnetic nanoparticles for magnetic resonance imaging (MRI), fluorophores for near-infrared (NIR), and peptide conjugated quantum dots (QDs) via organic linkers for optical imaging. Now a day, different types of tumor-targeted pep-NPs probes, have been developed, they have the ability to cross in vivo barriers and concentrate localized to produce sufficient contrast for imaging.