Novel Polypeptide-Based Biomaterials for Prostate Cancer Therapies
Prostate cancer is the second leading cause of cancer-related death among American men, and current therapies are nonspecific with many negative side effects. As described in Chapter 1, these side effects are generally due to inadvertent harm to healthy cells, and therefore, the development of targeted therapies can improve patient welfare, as well as improve the efficacy of current therapies. Furthermore, the materials used for these therapies should be biocompatible and avoid undesired immune responses. One type of novel biomaterial is the polypeptide, which is comprised of amino acids that have a wide variety of chemical and physical properties. Polypeptides can also be designed to adopt secondary structures, such as alpha-helices, as well as self-assemble into macromolecular structures, such as nano-sized vesicles. Using polypeptides as the “core” material in this thesis, we have developed and investigated (i) novel cell penetrating peptides comprised of galactosylated poly(methionine), which can efficiently internalize into prostate cancer cells, and (ii) photoresponsive polypeptide-based gold nanoshells, where gold-coated polypeptide vesicles generate heat in response to near infrared light to kill cancer cells.
Naturally-occurring cell penetrating peptides, such as HIV-Tat, are generally cationic, with an abundance of the positively-charged amino acids arginine and lysine. Researchers have shown that synthetic polypeptides can be created to mimic the primary and secondary structures of natural cell penetrating peptides. Furthermore, these synthetic polypeptides can be modified to increase their internalization efficiency to outperform the natural peptides, for example, through incorporation of alpha-helical structures, cationic and functional side chains, and hydrophobic moieties. In Chapter 2, we investigated a novel polypeptide material synthesized by Dr. Timothy Deming’s lab at UCLA for use as a cell penetrating peptide, galactosylated poly(methionine). To optimize the polypeptide, we performed cytotoxicity and fluorescent cellular uptake studies using polypeptides with a variety of properties, such as chain length and percent functionalization with galactose groups. We determined that the 50% functionalized galactosylated poly(methionine) was non-toxic to cells, and demonstrated significantly greater cellular uptake compared to nona-arginine (R9), which is known to be efficiently internalized by cells. This novel polypeptide represents a promising biomaterial for delivering anti-cancer therapies to prostate cancer cells.
The second focus of this thesis has been the use of self-assembled vesicles comprised of the block copolypeptide, poly(L-lysine)60-b-poly(L-leucine)20 (K60L20), as the core material for photothermal therapy using near infrared light. Researchers have developed gold nanostructures, which demonstrate enhanced absorption of light due to surface plasmon resonance (SPR). The SPR of the gold nanostructures can be tuned to a variety of wavelengths, ranging from ultraviolet to visible to the near infrared. In order to non-invasively treat tumors beneath the skin, near infrared light is desired, where light at this wavelength can penetrate a few centimeters into the tissue. Gold nanoshells, where a core material is coated with a thin layer of gold, have been shown to generate heat in response to irradiation with near infrared light. The core materials used for gold nanoshells include solid silica nanoparticles, polymeric nanoparticles, and more recently, liposomes. In Chapter 3, we report the first ever use of polypeptide materials as the core material for a gold nanoshell. Our polypeptide based K60L20 gold nanoshells have demonstrated low toxicity in the absence of laser exposure, significant heat generation upon exposure to near infrared light, and as a result, localized cytotoxicity within the region of laser irradiation. In Chapter 4, we demonstrate the ability to reduce the overall size of the polypeptide-based gold nanoshells, so that they are in the size range to take advantage of the enhanced permeability and retention (EPR) effect, which leads to the collection of nanoparticles at a solid tumor site. These smaller gold nanoshells similarly demonstrated significant heat generation in response to near infrared irradiation and localized laser-induced cytotoxicity. To gain a better understanding of our gold nanoshells in the context of photothermal therapy, we developed a comprehensive mathematical model for heat transfer in Chapter 5 to predict the temperature as a function of time and position in our in vitro experimental set-up. Our mathematical model was validated by comparing our predictions with our experimental results. This model can therefore be used in the future to determine which parameters in our gold nanoshells can be manipulated to improve heat generation, and therefore, potentially improve the destruction of tumors.
In Chapter 6, we conjugated a targeting ligand, the A11 minibody, to the polypeptide-based K60L20 gold nanoshells to improve their efficacy. Researchers have shown that active targeting agents, such as proteins and antibodies, can improve the specificity and effectiveness of photothermal therapy for cancer cells. The A11 minibody, which binds specifically to the prostate stem cell antigen (PSCA) overexpressed on the surfaces of local and metastatic prostate cancer cells, has successfully been demonstrated by Dr. Anna Wu’s group at UCLA as a positron emission tomography (PET) imaging agent for prostate cancer with in vivo models. Here, we demonstrate the first use of the A11 minibody to target a therapeutic agent to prostate cancer cells. Specifically, we were able to generate A11-conjugated nanoshells with the appropriate size and heat generation properties. These PSCA-targeted polypeptide-based gold nanoshells also showed promise by exhibiting greater efficacy as a photothermal therapy agent compared to non-targeted gold nanoshells.