**3.3.2 Nanoparticulate carrier systems**

114 Rheumatoid Arthritis – Treatment

In addition to the potential for passive targeting, the two primary cell types found within the pannus tissue, rheumatoid arthritis synovial fibroblasts (RASFs) and rheumatoid arthritis synovial macrophages (RASMs) selectively express surface receptors, such as CD44 (Haynes et al., 1991; Johnson et al., 1993), folate receptor (Nagayoshi et al., 2005; van der Heijden et al., 2009), and integrin V3 (Wilder, 2002) that are candidates for active targeting. Angiogenic vascular endothelial cells (VECs) are also present as a result of neovascularization, and the E-selectin adhesion molecule has been identified as another

In general, drug delivery systems can be divided into two categories: polymer-drug conjugates and nanoparticulate carrier systems. "Nanoparticles" in this sense include liposomes and micelles, as well as traditional metallic and polymeric nanoparticles. As drug delivery systems have become increasing advanced, the distinction between these two

In 1975, Ringsdorf proposed a model for polymer-based drug delivery wherein discrete sections of a polymer backbone are used for attachment of therapeutics, solubilizers, and targeting moieties (Fig. 2) (Ringsdorf, 1975). Since this seminal manuscript, numerous macromolecular therapeutics have been synthesized and evaluated. Although initial research focused on the use of N-(2-hydroxypropyl) methacrylamide (HPMA) based upon similarity to Ringsdorf's Model, polymers with a variety of architectures and structural elements are currently being explored, including linear mono- and di-functional polymers, star polymers, and dendrimers (Fig. 3) (Kopecek et al., 2000; Peterson et al., 2003). Dendrimers can alternatively be classified as nanoparticulate carriers when drug molecules are entrapped within the interior rather than covalently linked to the surface functional groups (Gillies & Frechet, 2005; M. Liu & Frechet, 1999). Mono- and di-hydroxyl terminated poly(ethylene glycol) (PEG) have proven to be the most versatile polymers for increasing the stability, solubility, and pharmacokinetic properties of associated therapeutics, with several PEG-drug conjugates on the market for a number of indications (Joralemon et al., 2010). Only recently has this research translated to the development of macromolecular therapeutics for the treatment of rheumatoid arthritis, as discussed further in Section 4. Despite demonstrated success, polymer-drug conjugates suffer from the necessity to chemically modify the drug molecule and the potential to reduce therapeutic activity and

Fig. 2. (A) Ringsdorf's model of polymer-drug conjugates for drug delivery. (B) HPMA was

efficacy by such modification (Haag & Kratz, 2006; Kim et al., 2009).

investigated for use due to similarity to Ringsdorf's model.

**3.2 Active targeting** 

**3.3 Carrier systems** 

categories has become less clear.

**3.3.1 Polymer-drug conjugates** 

viable target for drug delivery (Jamar et al., 2002).

Nanoparticulate carrier systems permit entrapment/encapsulation of therapeutics without modification, as is requisite for polymer-drug conjugates. The colloidal particles can range in size from 10 nm to 1 m; however, sizes are more typically between 20 and 300 nm, thereby minimizing uptake by macrophages of the reticuloendothelial system, while permitting passive targeting of tissue with leaky vasculature. Liposomes, micelles, metallic nanoparticles, and polymeric nanoparticles constitute the most commonly used nanoparticulate carrier systems for drug delivery (Fig. 4) (Jain, 2008).

Fig. 4. Various nanoparticulate carrier systems. (A) Micelles, (B) liposomes, and (C) polymeric nanoparticles.

Liposomes are vesicles formed from phospholipid bilayers with aqueous centers. Consequently, liposomes are used to encapsulate both hydrophobic and hydrophilic drugs within the bilayer and the aqueous core, respectively. Liposomal properties are largely controlled through the choice of phospholipids, as well as the addition of sterols, particularly cholesterol, and glycolipids (Jain, 2008). Although conventional liposomes suffer from rapid uptake by the reticuloendothelial system, incorporation of PEG into the bilayer yields so called "stealth" liposomes with enhanced circulation times. Starting with the anticancer compound Doxil (Gabizon, 2001), several PEG-modified liposomes with encapsulated therapeutics have reached commercialization (Joralemon, et al., 2010). As a consequence, liposomal use has been widely studied as a potential carrier system for drug delivery for rheumatoid arthritis (Foong & Green, 1993; Konigsberg et al., 1999; Monkkonen et al., 1993; Monkkonen & Heath, 1993; Monkkonen et al., 1994; Shaw et al., 1979) .

The Development of Targeted Drug Delivery Systems for Rheumatoid Arthritis Treatment 117

opportunity to increase the efficacy of existing rheumatoid arthritis therapeutics while reducing adverse effects. A summary of current drug delivery strategies that encompasses

Several polymer-drug conjugates have been developed to improve the therapeutic efficacy of both conventional DMARDs and biologics. A number of these compounds were only recently applied to rheumatoid arthritis after originally being developed for cancer. For example, methotrexate conjugated to human serum albumin (MTX-HSA) was shown to passively accumulate within the inflamed paws of arthritic mice. Further study revealed a reduction in cellular invasion, a downregulation of proinflammatory cytokine levels, and a decrease in cartilage damage for arthritic mice treated with MTX-HSA relative to untreated, arthritic mice. The conjugates were also useful in preventing the onset of arthritis in mice when administered prior to induction (Fiehn et al., 2004; Wunder et al., 2003). Due to the limitations of exogenous albumin, a methotrexate pro-drug has recently been developed

that will react with endogenous albumin upon administration (Fiehn et al., 2008).

Fig. 5. Cellular uptake of carrier systems occurs by an endocytotic process. Systems can be designed to release their therapeutic payload within the extracellular space, the endosome,

As mentioned in Section 3, PEG has been used extensively in all areas of drug delivery. PEG-dexamethasone conjugates were recently synthesized that reduced joint inflammation when administered intravenously to arthritis rats (Liu et al., 2010). PEGylation has been applied to biologics, in addition to conventional, small molecule therapeutics. To the authors' knowledge, the only polymer-drug conjugate to reach clinical trials for rheumatoid arthritis treatment thus far is certolizumab pegol (CDP870), a PEG conjugated TNF antibody fragment originally developed for treatment of Chron's disease (Barnes & Moots, 2007). Administration to a number of patients who did not respond well to conventional DMARDs, particularly methotrexate, led to a reduction in disease activity and joint damage. Unfortunately, an increase in adverse side effects was also observed (Ruiz Garcia et al.,

**4.1 Polymer-drug conjugates for rheumatoid arthritis treatment** 

Sections 4 and 5 is given in Table 1.

or the lysosome.

Above a certain concentration, referred to as the critical micelle concentration (CMC), molecules that possess both hydrophobic and hydrophilic segments, such as amphipathic block-co-polymers, will self assemble to form colloidal particles with hydrophobic interiors and hydrophilic exteriors. Micelles are typically smaller than liposomes (20-50 nm) and the hydrophobic cores are used to entrap drugs that possess low aqueous solubility (Haag & Kratz, 2006). The CMC provides an indicator of stability, where systems with low CMCs are not easily disrupted or disintegrated (Oerlemans et al., 2010). Only a handful of investigators have used micelle-based drug delivery systems to improve the efficacy of DMARDs that have historically suffered from unpredictable pharmacokinetics resultant from poor solubility (Bader et al., 2011; Koo et al., 2011; Zhang et al., 2007).

Both metallic and polymeric nanoparticles are used to encapsulate drugs within the solid core. Although nanoparticles are defined as any system with a submicron ( 1 m) size (van Vlerken & Amiji, 2006), most typically have sizes below 200 nm (Jain, 2008). Metallic nanoparticles include iron-oxide nanoparticles, silica-gold nanoshells, gold nanoparticles, and Quantum dots (cadmium, selenium, and zinc) (Riehemann et al., 2009; van Vlerken & Amiji, 2006). Although originally developed for cancer treatment, these technologies are currently being translated to rheumatoid arthritis treatment applications (Corthey et al., 2010). The use of metals can yield multifunctional nanoparticles whereby both therapeutic delivery and imaging are facilitated (Riehemann et al., 2009). Polymer-based nanoparticles, are advantageous in that modification permits the ready addition of the following elements: targeting ligands, environment-sensitive drug release, and biologically functional polymers. The carrier systems discussed above can be further modified to optimize disease treatment. For example, co-administration of multiple therapeutics from one convenient platform is feasible. Additional modification with ligands specific for receptors found on diseased cells can facilitate active targeting. Furthermore, surface coating with PEG can be used to tailor circulation time (Riehemann et al., 2009; van Vlerken & Amiji, 2006). As detailed in Section 4, these technologies have only recently been applied in the realm of drug delivery for rheumatoid arthritis treatment.

#### **3.3.3 Therapeutic release from carrier systems**

Most drugs are inactive when bound to/encapsulated within the carrier system; therefore, a method that permits drug release at the diseased site is often requisite. Cellular uptake of therapeutic-loaded carrier systems typically proceeds by fluid-phase endocytosis, adsorptive endocytosis, or receptor-mediated endocytosis (Fig. 5). During each of these endocytotic processes, the pH drops from that within the extracellular space (pH ≈ 7.4 for healthy tissue and pH < 7.4 for diseased tissue) to pHs of ~6.0 and ~4.0 in the endosomes and lysosomes, respectively (Haag & Kratz, 2006; Petrak, 2005). Thus, the conjugate and particulate carriers can be formulated such that release is only permitted at a specified pH. Alternatively, drugs may be released after enzymes cause non-specific hydrolysis (Haag & Kratz, 2006; Kim et al., 2009). An ideal carrier system will only respond to environmental features unique to the diseased tissue, such as elevated levels of a specific enzyme. Stimuliresponsive drug delivery systems that are currently in development for the treatment of rheumatoid arthritis will be discussed in Section 5.

### **4. Current drug delivery systems and strategies for rheumatoid arthritis**

A number of carrier systems have been designed to to improve rheumatoid arthritis treatment based upon the principles described in Section 3. These carrier systems provide an opportunity to increase the efficacy of existing rheumatoid arthritis therapeutics while reducing adverse effects. A summary of current drug delivery strategies that encompasses Sections 4 and 5 is given in Table 1.
