A wide array of nanoparticle therapeutics that have been approved for clinical use are PEGylated proteins. PEGylation has been applied to various proteins, including
enzymes, antibodies, affibodies, and cytokines. PEGylation can offer increased protein stability, prevent rapid kidney clearance, and provide enhanced plasma life . Oncospar by Enzon Pharma, a PEG-l-asparaginase, was approved by FDA in 1994 to treat acute lymphoblastic leukemia. Oncospar showed an increased plasma half-life and reduced hypersensitivity reaction induced by the naked enzyme in phase I studies . PEGylated cytokines have shown superior activity in clinical trials of melanoma and renal cell carcinoma. For example, Schering’s “Pegintron” for interferon alpha-2b and Roche’s “Pegasys” have showed superior antiviral activity and have been approved for treating hepatitis C, whereas various PEGylated chemotherapeutics, including PEG-irinotecan, PEG-SN-38, and PEG-paclitaxel, are currently in clinical trials . Nanoparticles with targeting ligands have become more attractive because they are envisioned to provide more efficacious therapy. In this chapter we will focus on the specific ligand poly(lactic-co-glycolic acid) (PLGA) nanoparticles we have developed in our laboratory for targeting breast cancer using in vitro and in vivo models. PLGA has been used for a wide variety of medical applications, from resorbable sutures to bone screws and microspheres for drug delivery. This biodegradable and biocompatible polymer is safe to use in the body and is hydrolyzed into metabolic by-products. Recently, these polyester materials have been widely used to deliver peptide or protein drugs, which have unusual physicochemical properties compared with low-molecular-weight drugs. Degradation of PLGA in vitro as well as in vivo mainly takes place through hydrolysis of the ester linkages and/or enzymatic degradation. Degradation behavior of polymeric microspheres is crucial for the successful application of such devices in controlled drug delivery. Attractive breast cancer therapies for the targeting of PLGA nanoparticles can be based on estrogen/estrogen receptor-mediated actions. Estrogen/estrogen receptor-mediated actions play an important role in the normal development and pathophysiology of the breast. Approximately 75% of breast cancer patients respond well to endocrine therapies at the time of diagnosis. Even though systemic hormone therapies that either block local estrogen production by aromatase inhibitors (AIs) or block actions of estrogen/estrogen receptor (ER) by antiestrogens (AEs) is well tolerated, the development of resistance to these agents is still a major concern. The transition of an ER-positive hormone responsive state to an unresponsive state following AE or AI therapy is associated with molecular adaptation that involves loss of ER (ERa), activation of HER2/Raf/MAPK signaling pathways, epigenetic modifications, and alterations in the levels and functions of ER coregulators. Since multiple signaling pathways are involved in ER activation, combination of endocrine and non-endocrine agents that block these signaling pathways may delay the onset of resistance to hormonal therapy. The current hormone therapies involve blockingof estrogen from binding to the ER using AEsor selective estrogen receptor modulators (SERMS) such as tamoxifen
or by inhibiting estrogen synthesis using AIs. Tamoxifen was a widely accepted therapy, until its estrogen-like activity was revealed to significantlyincrease the risk of endometrial cancer and stroke . To some extent it is still used despite these actions, and the superiority of AIs over tamoxifen has reduced its use. Letrozole is a third-generation AI found to be more active than tamoxifen in neoadjuvant, early-and extended-adjuvant, as well as metastatic disease settings of postmenopausal breast cancer patients . Letrozole is a superior therapy for treating breast cancer; however, some patients may develop acquired resistance during the treatment period. To increase the therapeutic efficiency and reduce side effects, much research has been devoted to the development of a biodegradable delivery system capable of increasing the localized dose of drug to the tumor site. Nanoparticles composed of biodegradable polymers have attracted tremendous interest in recent years for clinical administration of anticancer drugs. A major limitation in cancer chemotherapy is the development of acquired drug resistance. Chemoresistance is mainly generated in two ways: (i) physical impairment of drug into the tumor due to poor absorption or increased metabolism and (ii) development of multidrug resistance (MDR) . On the basis of our preliminary results, resistance to AIs mostly lies in the second category. Development of MDR is multifactorial in which the overexpression of membrane bound ATP-dependent drug effect pump (p-glycoprotein/MDR1) is predominant . By using an established model of letrozole resistance involving LTLT-Ca cells, we have observed in our study that site-specific delivery of letrozole by PLGA nanoparticles can increase the therapeutic benefit by delivering a greater fraction of the dose to the target site, which minimizes resistance (unpublished data). Drug delivery throughout the tumor burden is crucial for the treatment to be effective, since residual cancer cell survival can promote regrowth and often imply the cause for drug resistance . Brodie et al. have shown that in LTLT-Ca letrozole-resistant cells several other adaptive signaling pathways, including HER2 and MAPK, are circuited together to execute tumor regrowth . Receptor-mediated endocytic delivery is one of the more widely accepted approaches for targeted drug delivery. CD44, the cell surface glycoprotein, is a potential molecular target for tumor-selective drug delivery. CD44 is reported to be involved in cell-to-cell interactions, adhesion, and migration. CD44 is highly expressed in a number of epithelial carcinomas, but is present in only a limited number of normal cells . CD44 is the receptor for hyaluronic acid (HA), a unique glycosaminoglycan that is present in extracellular matrices and contributes to tissue hydrodynamics and repair . Targeting anticancer drugs using HA nanoparticles may facilitate the selective cellular uptake by tumor cells through CD44 receptor-mediated endocytosis. In this chapter we are presenting some of our recent research for answering whether HA-bound letrozole nanoparticles would be an alternative to overcome resistance to letrozole in a preclinical mouse model of letrozole-resistant tumors.