Nanoparticles, due to their smaller sizes and associated unique properties, provide unprecedented opportunities to interrogate cellular and molecular processes with realistic clinical applications.1–10
Specific types of nanoparticles are being utilized as drug delivery vehicles, cellular biomarkers, and cancer imaging and therapy agents.1–10
The multifaceted applications of nanoparticles are the direct result of their ability to deliver high pay loads of drugs or biomarkers to the desired sites within the body.1–10
Design and development of tumor specific nanoparticles could significantly amplify the delivering capacity to a specific target of interest, without affecting healthy cells.1–10
The target specificity in nanoparticles could be imparted by tagging with certain biovectors, which navigate them to desired organ or site under in vivo conditions. The most commonly used target vectors are monoclonal antibodies and receptor-specific peptides.11–13
Although, both biomolecules have shown high targeting abilities, the (in vivo) transport properties of monoclonal antibodies and peptides differ drastically. Monoclonal antibodies, due to their larger sizes show poor in vivo mobility resulting in time delayed, and reduced uptake over the desired target. Moreover, monoclonal antibodies are highly immunogenic, which lead to harmful side effects. In sharp contrast, peptides being smaller in size bring various advantages; namely, rapid blood clearance, ease in penetration of tumor vascular endothelium, increased diffusion rates in tissue, and low immunogenicity. Receptors for peptides are highly expressed on a variety of neoplastic and non-neoplastic cells.14–15
Furthermore, receptor targeting peptides have shown high level of internalization within the tumor cells via receptor mediated endocytosis.16
The ability to internalize probes within tumor cells is important for delivering maximum pay loads to tumor cells.14–16
These attractive physical properties coupled with their smaller size make peptides ideal candidates for developing new target specific nanoparticles. Therefore, we have designed and developed peptide conjugated nanoparticles that may circumvent some of the currently encountered problems.
Nanoparticles of gold continue to play pivotal roles in the design and development of tumor imaging and therapy agents.1–13
In particular, gold nanorods (GNRs) have attracted much interest because of their unique photophysical properties, which make them ideal candidates for both tumor imaging and therapeutic applications.11,17–25
Recent studies are focused on utilizing GNRs as contrast agents for photoacoustic tomography. Indeed, Massoud and coworkers have shown that engineered gold nanorods, under in vivo conditions, exhibit significant optoacoustic contrast and increase the diagnostic power of optoacoustic imaging modality.21
Gold nanorods attached with deltorphin, a ligand with high affinity towards delta opioid receptor, have shown selective absorption towards human colon carcinoma cells, establishing the fact they can serve as contrast agents for molecular imaging.26
On the therapy front, El-Sayed and coworkers have utilized the plasmon absorption of gold nanorods, in photothermal therapy, as an effective tool against cancer cells.3
Oyelere et al. demonstrated that peptide coated gold nanorods can be used as nuclear targeting agents for potential in vivo imaging applications.27
These literature examples suggest that gold nanorods possess the potential to serve as theranostics, wherein, a singular agent can serve as both diagnostic and therapeutic. The theranostic capability of gold nanorods could be realized only when GNRs are selectively localized at tumor sites. The selective delivery of GNRs to tumoral region can be achieved by attaching a target-specific vector. In this context, our studies are focused on utilizing bombesin peptide as a target vector for conjugation with gold nanorods. We hypothesized that bombesin peptide can act as a vehicle to deliver gold nanorods specifically to tumor cells. The general structure of gold nanorod-bombesin (GNR-BBN) peptide conjugate is shown in .
General structure of GNR-BBN conjugates.
The 14-amino acid peptide bombesin (BBN) isolated from the skin of the amphibian Bombina and related gastrin-releasing peptides (GRP) exhibit an enhanced response in a variety of tumor tissues, e.g., in small cell lung, prostate, breast, and colon cancer.28–31
BBN functions as a potent autocrine or paracrine growth factors for cells. 28–31
In the last decade, a wealth of information was generated on BBN/receptor expression and physiological information. BBN shows high affinity towards GRP receptor subtype BB2. GRP receptors are over expressed in many cancers, including prostate, breast, and small cell lung cancer. 28–31
Analogues of bombesin with modified structures exhibited a similar or even higher affinity for these receptors. 28–31
Synthetic peptides can be readily generated through automated solid phase techniques. For our studies, we have synthesized and utilized the seven-amino acid truncated bombesin analogue (BBN) as a vehicle to target GRP receptors ().
The main objective of our investigation is to examine whether the synthetic bombesin peptide conjugated gold nanorods can preferentially locate GRP receptors, which are over expressed in prostate and breast tumor cells for subsequent applications as theranostic agents. As part of our overall goal on developing target specific gold nanoparticles for treatment of cancers,32–34
we carried out a systematic investigation on the design and development of targeted gold nanorods by conjugating with GRP-receptor avid bombesin peptide. Our studies, for the first time to our knowledge, establish that GNR-BBN conjugates have very high binding affinity toward GRP receptors in cancer cells and internalize via receptor-mediated endocytosis pathway. The results reported in this letter include: (i) synthesis and characterization of gold nanorod bombesin (GNR-BBN) conjugates, (ii) in vitro stability studies of GNR-BBN conjugates toward various biomolecules, (iii) evaluation of binding affinity (IC50
) values of GNR-BBN conjugates towards GRP receptors over expressed in prostate and breast tumor cells, and (iv) evaluation of internalization mechanism of GNR-BBN conjugates in cells.