**5.2 Sustained drug release by gelatin is safe and effective**

Gelatin microcapsules are one type of sustained drug release mechanism that incorporates a shell of gelatin around a core of oil containing drug. One advantage of gelatin is it has been shown to be a safe means for sustained drug delivery because in vivo there is no inflammatory response (Brown, Leong, Huang, Dalal, Green, Haimes, Jimenez & Bathon, 1998; Tuncay, Calis, Kas, Ercan, Peksoy & Hincal, 2000) consistent with our result in the TMJ of a rat that showed injection of microcapsules did not increase the nociceptive response nor increased IL-1β levels in the TMJ. Studies using a shell of gelatin encapsulating oil containing drug demonstrated that uncrosslinked gelatin was effective when given orally (Jizomoto, Kanaoka, Sugita & Hirano, 1993). A previous pharmacokinetic study injecting gelatin microspheres containing NSAID show that the intra-articular concentration of NSAID was 8-fold higher 8 hours after injection versus the concentration of NSAID in a joint injected with a non-encapsulated solution of NSAID (Lu, Zhang, Sun & Zhong, 2007). Another recent study in a rabbit model indicated that intra-articular delivery of basic fibroblast growth factor or platelets via gelatin microspheres improved knee joint swelling, proteoglycan expression, and histological appearance of arthritic knee tissue (Inoue, Takahashi, Arai, Tonomura, Sakao, Saito, Fujioka, Fujiwara, Tabata & Kubo, 2006; Saito, Takahashi, Arai, Inoue, Sakao, Tonomura, Honjo, Nakagawa, Inoue, Tabata & Kubo, 2009). Our work is consistent with these results demonstrating microcapsule drug release can ameliorate arthritic TMJ pain. Future work would focus on encapsulating alternative drugs shown to be effective in relieving TMJ pain, such as mepivacaine (Zuniga, Ibanez & Kozacko, 2007).

### **5.3 Microcapsule size and thickness for in vivo injection**

Thickness of the microcapsule wall can be varied and the timing of drug release would be based on the capsule cell wall thickness. What is important to keep in mind is that the microcapsules for this application need to be injectable and thus the diameter should not exceed the inner bore of the needle (e.g. 165 µM for a 29 gauge needle). As the wall of the bead becomes thicker the amount of loaded drug decreases thus, you inject less drug when using beads with a thicker wall diameter. Thus, a balance between the amount of time you want before drug release and the total amount of drug administered must be obtained. The diameter of the sphere is also important, with spheres greater than 26 µM remaining in the interstitial fluid of the tissue because they are not phagocytosed by cells (e.g., macrophages) (Horisawa, Kubota, Tuboi, Sato, Yamamoto, Takeuchi & Kawashima, 2002). The optimal size range for intra-articular drug delivery systems is application specific (Horisawa, Hirota, Kawazoe, Yamada, Yamamoto, Takeuchi & Kawashima, 2002; Horisawa, Kubota, Tuboi, Sato, Yamamoto, Takeuchi & Kawashima, 2002; Liggins, Cruz, Min, Liang, Hunter & Burt, 2004) but in this intra-articular paradigm we did not want intracellular drug release thus the bead diameter was approximately 30 µM.

In conclusion drug loaded gelatin microcapsules reduce the nociceptive response of an arthritic TMJ. These microcapsules are expected to be useful not only for the treatment of pain but also to modify the joint environment prior to implantation by delivering proregenerative signals in a spatially and temporally controlled fashion. The limitations of current therapeutic strategies for TMJ disorders have led to increased interest in tissue engineering strategies, which combine cells, bioactive factors, and implantable scaffolds to trigger joint regeneration (Detamore & Athanasiou, 2003). In addition to the added benefit of sustained drug release local administration of drug has the added benefit of avoiding systemic drug release which can result in ectopic effects.
