A major reason for the poor durability of tooth-colored resin restorations is that collagenolytic MMPs in the dentin matrix are uncovered and activated during resin bonding procedures (Pashley et al., 2004;Tay et al., 2006b;Nishitani et al., 2006;Mazzoni et al., 2006). This results in gradual but continuous degradation of collagen fibrils that anchor these fillings to underlying mineralized dentin. The recent discovery that chlorhexidine (CHX), a potent antimicrobial agent, also has potent anti-MMP -2, -8 and -9 activity (Gendron et al., 1999), led us to determine if CHX can stabilize resin-bonded collagen and increase the durability of resin- dentin bonds thereby saving the cost of frequent replacement of bonded restorations. Our in vitro success (Pashley et al., 2004;Nishitani et al., 2006, Carrilho et al., 2007a, Breschi et al., 2009a) using CHX to increase the durability of resin-dentin bonds led us to demonstrate CHX's success in increasing the stability of bonds in vivo (Hebling et al., 2005;Carrilho et al., 2007b;Brackett et al., 2007). These results have stimulated this grant application to delve more deeply into the mechanisms responsible for CHX inhibition of MMPs. We noted that CHX contains biguanide groups that form acidic addition salts with a cationic charge delocated over the five neighboring secondary amine nitrogen atoms of the biguanides. These function similarly to quaternary ammonium compounds that also form polycations that are also antibacterial. We have recently discovered several quaternary ammonium-containing monomers that can inhibit Clostridium collagenase. This result may cause a paradigm shift in adhesive formulations. By incorporating "therapeutic monomers" capable of inhibiting MMPs in adhesive blends, we may be able to extend the service of all adhesive resin restorations. Before this goal can be realized, we need to evaluate this new class of adhesive monomers. Although they inhibit bacterial collagenase, will they inhibit true MMPs? We will test the ability of a number of anticollagenolytic ("AC") monomers for their ability to inhibit soluble dentin MMP activity using soluble fluorescein-labeled collagen. Then we will test their ability to inhibit matrix-bound MMPs using the same soluble collagen substrate. Finally, we will determine if the "AC" monitors can inhibit bound-MMPs activity on the dentin matrix itself, as a substrate. We will also determine if "AC" monomers prevent the decrease in mechanical properties of dentin matrices that normally occur over time. Finally, we will learn how to blend these "AC" monomers with adhesive comonomers without reducing their degree of conversion, ability to wet dentin, or lowering their initial (24 hr) bond strength. Once we have proven that we can produce high bond strengths, we will test the durability of resin-dentin bonds made with control comonomers versus comonomers mixed with "AC" monomers using fatigue lifetime studies and initial versus 1 yr interfacial fracture toughness studies. If the results of this work are successful, we will then seek additional support for clinical trials of the best of these "AC" monomers, in our goal to provide more durable resin-dentin bonds.
Our successful use of topical chlorhexidine (CHX) application to acid-etched dentin to inhibit dentin MMPs and maintain the integrity of resin-dentin bonds in vitro and in vivo has been very gratifying. Chlorhexidine contains strongly basic biguanide groups that impart cationic charges to the molecules. Quaternary ammonium compounds (QAC) contain similar cationic charges that are responsible for their antibacterial properties that are responsible both for its antimicrobial activity and for its anti-MMP activity. We have discovered a number of QAC-containing monomers that inhibit collagenases. If we can incorporate them into adhesive resins, we may be able to permanently inhibit dentin MMPs that are thought to degrade resin-dentin bonds, while we are bonding. This new class of MMP-inhibitors is an exciting new discovery with profound clinical potential. Their successful use in adhesive resins could potentially save billions of dollars currently spent on the replacement of tooth-colored resin composites.
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