Pharmacological perspectives on periodontal therapy

Introduction

Since the discovery of acetyl salicylic acid by Hoffmann in 1899, it has long been suspected that another way of research was about to be opened up, from which innovative pharmacological treatments would be developed. Initially, these would certainly be laborious but, over the last decades, extremely fruitful. These innovations allow us to understand not only the technology of the development of biologically active molecules, their sensitivity and enzymology, but equally or general knowledge and understanding. A basically clinical concept, the tetrad of Celsus, has been replaced with a cellular and molecular concept, a triad of components involved in inflammation : cells, enzymes and mediators. Guided rationally by the soundness of this concept, pharmacological modulators of cell function, enzymes and inflammatory mediators are being developed. By way of example, one can cite the direct inhibitors of the formation of lipid mediators such as those derived from arachidonic acid, prostaglandins, and leucotrienes, which are obtained by blocking the key enzymes, cyclooxygenase and lipoxygenase respectively. Similarly, one can mention the indirect inhibition of the formation of peptide mediators, in particular certain cytokines (TNF-α) by pharmacological modulators such as type 4 phosphodiesterase by directing this enzyme to the inflammatory cells that produce these cytokines.

In the area of periodontal diseases, when surgery is almost unavoidable, one is led to suggest a non-surgical alternative, developing from a pharmacological approach to the disease. However, even if the pathology is due to a an imbalance between the host an infectious agent, the most promising way forward seems to be towards pharmacological control of the host response, that is to say, towards pharmacological control of the inflammatory components of the pathology. Beside the established strategies of using steroidal and non-steroidal anti-inflammatory agents, there are now innovative approaches that have already been used in the pharmacological control of inflammation but their relevance to periodontal diseases remains to be evaluated.

Pharmacological modulators of inflammary cells

The majority of these products have not been used recently for therapeutic purposes but they continue to be of pharmacological interest for research or clinical use into inflammation. In effect, this is to provide information on the mechanisms of pharmacological activity at a molecular level. This allows us to consider these products not only as real analytical tools in pharmacological research but equally as methods of treatment, the value of which has to be assessed at a clinical level. In order to illustrate this proposition, we will take two types of product, one still at an experimental stage (liposomal clodronate) and the other (colchicine) which is already in use.

Liposomal clodronate

Clodronate (international nomenclature), otherwise known as dichloromethylene biphosphonate (chemical name) is an organo-phosphate of the biphosphonate family. It has a strong affinity not only for hydroxyapatite but equally for numerous divalent cations (calcium, magnesium, etc.) ^{(Fonong et al., 1983)}. It has cytotoxic activity not only against procaryotes such as the amoeba Dictyostelium discoideum ^{(Pelorgeas et al., 1992)} but also against some eucaryototic cells, such as osteoclasts, and therefore leads to inhibition of bone resorption ^{(Flanagan and Chambers, 1989)}. Intravenous injection of clodronate encapsulated in liposomes (liposomal clodronate) into experimental animals causes a reduction of splenic macrophages and of the Küppfer cells of the liver ^{(van Rooijen and van Nieuwmegen, 1984)}. These results demonstrate that in animal experiments, a reduction in macrophages can take place, using liposomes containing clodronate ^{(van Rooijen and Sanders, 1994)}.

This essentially experimental pharmacological approach, using this form of clodronate, is of relevance to the role of the macrophage in the recruitment of polymorphonuclear neutrophils. This inflammatory response is brought about by the formation and secretion of chemotactic factors by macrophages such as chemokines (IL-8, GRO/MGSA, CINC, MIP-2, etc.) ^{(Miller and Krangel, 1992)}, platelet activating factor (PAF) (quoted by ^{Braquet et al., 1987}) and leucotriene B4 ^{(Walsh et al., 1981)}. The clinical application of this pharmacological approach is limited by the means of administration of liposomal clodronate in relationship to the location of the macrophages to be depleted.

Colchicine

This product is currently used in the treatment and prevention of acute attacks of gout. In this inflammatory disease, there is an accumulation of microcrystals of sodium urate (gout) or of calcium pyrophosphate (pseudo-gout) in joints ^{(Spilberg et al., 1980)} which induces the chemotaxis, adhesion and phagocytosis by polymorphonuclear neutrophils. This alkaloid extract of the seeds and bulbs of colchicum, Colchicum autumnale (autumn crocus or meadow saffron) of the Liliaceous family, has a cytotoxic action directed mainly against the locomotor system of polymorphonuclear neutrophils. In effect, colchicine is a poison that acts on the microtubular cytoskeleton. These microtubules consist of a polymerised protein, the tubulin. Colchicine is able to attach or form complexes that attach to these 100 kDa molecules at the site of elongation of these microtubules, inhibiting their growth by polymerisation ^{(Detrich et al., 1981} and ¹⁹⁸²⁾.

It is well known that neutrophils have a part to play in tissue destruction ^{(Weiss, 1989)}. A pharmacological involvement on these polymorphonuclear cells involved in the inflammatory reaction is a more innovative anti-inflammatory approach (though still old !). Steroidal or non-steroidal anti-inflammatory drugs (SAIDs or NSAIDs) may be used. Unlike SAIDs and NSAIDs, colchicine does not have an analgesic effect. However, since tubulin is also found in smooth and striated muscle cells and in sperm cells, the non-specificity of the intracellular target limits the therapeutic margin of safety of colchicine. The side effects of its use that have been described - digestive problems, myopathy, oligo- or aspermia - illustrate this.

Pharmacological modulators of enzyme involved in inflammation

Amongst the enzymatic activity generally recognized as being implicated in the inflammatory response, one cannot fail to recall the cells that lead to the formation of derivatives of arachidonic acid and, more especially, the prostaglandins and thromboxane derived from prostaglandin H2 synthetase, otherwise known as cyclo-oxygenase (COX). Furthermore, other enzymatic activities are also present and their modulation by drugs can lead to a reduction in inflammatory cell response and of the biosynthesis of certain mediators of inflammation. In addition, we will present novel inhibitors of type 2 cyclo-oxygenase (COX-2), metalloprotease inhibitors as well as inhibitors of type 4 phosphodiesterase.

COX-2 inhibitors

We owe the identification of COX-2 to Needleman's group. They detected a protein different from cyclo-oxygenase (COX) until then known to be derived from human monocytes that had been stimulated by IL-1 ^{(Fu et al., 1990)}. Subsequent research into its molecular biology, with the aim of identifying the genes responsible for its induction during the immediate or early inflammatory stimulus, isolated a gene very homologous to that for COX that had already been identified. It was therefore recognized that there were 2 isoforms of COX, one later to be called COX-1 and the other, induced by various pro-inflammatory agents such as LPS, MDP, activated zymosan, IL-1, TNF-α, called COX-2.

Research into the inhibitors of COX-2 have revealed that its main role ^{(Hawkley, 1999)} is to control the inflammatory response, not only in terms of its efficacy and specificity but equally in terms of tolerance. Aspirin as well as all the NSAIDs block COX-2 but they also block COX-1 and it is the effect on the latter which very likely causes the numerous undesirable side effects, especially the digestive problems that have bee described in connection with these drugs. Actually, several inhibitors of COX-2 have ben identified and put onto the market in France. They have a preferential specificity in the case of meloxicam (Mobic^®) and selective in the case of rofecoxib (Vioxx^®). A study of the data sheets indicates that the digestive side effects have been overcome but the other side effects remain.

Matrix metalloprotease inhibitors

The role of matrix metalloprotease (MMP) in the inflammatory process has been demonstrated by the action of proteases in the breakdown of the extracellular components of the matrix ^{(Birkedal-Hansen, 1993)} and by their capacity to activate pro-inflammatory cytokines, such as TNF-α ^{(Gearing et al., 1994)}.

Developments in the area of MMP inhibitors (MMPIs) are attributed to ^Golub and ¹⁹⁸⁴⁾. They have provided evidence for the inhibitory activity of tetracyclines on MMPIs, especially the collagenases MMP-8 and MMP-13 and the gelatinases MMP-2 and MMP-9. Subsequent decisive work to develop chemically modified tetracyclines (CMTs) has met with success. These compounds, which lack antibacterial activity but which otherwise retain improved MMPI activity ^{(Golub et al., 1987)}, have enabled pharmacological control of those biological processes that are dependent on MMPs. Because CMT-3 has also been shown to have a major effect on the division and invasiveness of tumour cells as well as on their potential for metastatic spread in vivo, they are currently under evaluation in oncology ^{(Seftor et al., 1998)}. Doxycycline, in non-antimicrobial doses, has been developed by the company CollaGex Pharmaceutical, Inc. under the name Periostat^®. In October 1998 it received an AMM from the FDA as an indication of its value as an adjuvant in the treatment of periodontal disease.

Another promising way in connection with pharmacological inhibitors of MMPs seems to be with derivatives of hydroxamic acid. In particular, the compound FN-439, a tetrapeptide derived from hydroxamic acid, a selective inhibitor of MMPs (p-NH2-Bz-Gly-Pro-D-Ala-NHOH, Fuji Chemical Industry) has shown an ability to interfere with the recruitment of neutrophils stimulated by LTB₄ ^{(Oda et al., 1995)}.

Inhibitors of type 4 phosphodiesterase

This innovative approach is founded on the capacity of type 4 phosphodiesterase (PDE4) inhibitors to increase the concentration of intracellular cAMP. Type 4 phosphodiesterases are implicated in the cleavage of cAMP to 5'-AMP. Their inhibition allows the phosphorylation cascade to be activated via protein kinases A, thus contributing to the abolition of many of the functions of inflammatory and immunologically active cells ^{(Teixeira et al., 1997)}.

Admittedly, the evaluation of PDE4 inhibitors has focused on allergic reactions having a strong inflammatory component, such as asthma, but the experimental results are very encouraging for septic shock induced by LPS, ischaemia and reperfusion situations and rheumatoid arthritis ^{(Teixeira et al., 1997)}. This opens up a large number of applications for drug therapy in the treatment of human disease. However, the factor limiting clinical trials on PDE4 inhibitors is their potential for inducing undesirable side effects, especially nausea and vomiting.

Pharmacological modulators of imediators of inflammation

The development of new drugs relating to the area of peptidic mediators of inflammation, notably pro-inflammatory cytokines such as TNF-α, IL-1, IL-6 and IL-8 is dependent on progress made in our understanding of the mechanisms of action of various types of drugs that are already known to be appropriate for the treatment of inflammation :

- DPE4 inhibitors. These compounds inhibit the release of certain cytokines (TNF-α and IL-8) ^{(Teixeira et al., 1997);}

- the glucocorticoids especially dexamethasone. These products are capable of interfering with the synthesis of pro-inflammatory cytokines (^{Satoh et al., 1991}) ;

- anti-cytokine antibodies. These experimental products enable characterisation of the pro-inflammatory cytokines implicated in the inflammatory reaction.

Conclusion

One may be surprised by a paucity of pharmacological discoveries, but the domain of pharmacological innovation encompasses both the discovery of new compounds and new indications for the use of existing compounds. Nevertheless, our perception of a strategy for the use of anti-inflammatory agents in the periodontal area is not restricted to pharmacological considerations or by their means of administration, oral or parenteral. The local or topical administration, particularly intrasulcular, thanks to irrigation systems or slow release devices, must not be neglected. However, in order to guarantee the best treatment for the patient, the efficacy and safety of these products must, especially, be taken into consideration.

BIBLIOGRAPHY

• BIRKEDAL-HANSEN H. Matrix metalloproteinases. A review. Crit Rev Oral Biol Med 1993;4:197-250.
• BRAQUET P, TOUQUI L, SHEN TY, VARGAFTIG BB. Perspectives in platelet-activating factor research. Pharmacol Rev 1987;39:97-145.
• DETRICH HW, WILLIAMS RC, MAC DONALD TL, WILSON L, PUETT D. Changes in circular dichroism spectrum of colchicine associated with its binding to tubulin. Biochemistry 1981;20:5999-6005.
• DETRICH HW, WILLIAMS RC, WILSON L. Effects of colchicine binding on the reversible dissociation of the tubulin dimer. Biochemistry 1982;21:2392-2400.
• FLANAGAN AM, CHAMBERS TJ. Dichloromethylene biphosphonate (Cl2MBP) inhibits bone resorption through injury to osteoclast that resorb Cl2-MBP-coated bone. Bone Miner 1989;6:33-43.
• FONONG T, BURTON DJ, PIETRZYK DJ. Determination and formation constants of calcium complexes of difluoromethylenediphosphonic acid and related diphosphonates. Anal Chem 1983;55:1089-1094.
• FU JY, MASFERRER JL, SEIBERT K, RAZ A, NEEDLEMAN P. The induction and the suppression of prostaglandin H2 synthase (cyclooxygenase) in human monocytes. J Biol Chem 1990;265:16737-16740.
• GEARING AJH, BECKETT P, CHRISTODOULOU M, CHURCHILL M, CLEMENTS J, DAVIDSON AH et al. Processing of tumor necrosis factor-aprecursor by metalloproteinases. Nature 1994;370:555-557.
• GOLUB LM, LEE HM, LEHRER G, NEMIROFF A, MC NAMARA TF, KAPLAN R et al. Minocycline reduces gingival collagenolytic activity during diabetes. J Periodont Res 1983;18:516-524.
• GOLUB LM, MCNAMARA TF, D'ANGELO G, GREENWALD RA, RAMAMURTHY N. A non-antibacterial chemically modified tetracycline inhibits mammalian collagenase activity in human crevicular fluid and from other mammalian sources. J Dent Res 1987;66:1310-1314.
• GOLUB LM, RAMAMURTHY N, MCNAMARA TF, GOMES B, WOLFF M, CASINO A et al. Tetracyclines inhibit tissue collagenase activity. J Periodont Res 1984;19:651-655.
• HAWKLEY CJ. COX-2 inhibitors. Lancet 1999;353:307-314.
• MILLER MD, KRANGEL MS. Biology and biochemistry of the chemokines : a family of chemotactic and inflammatory cytokines. Crit Rev Immunol 1992;12:17-46.
• ODA T, KATORI M, HATANAKA K, NAGAI Y. Inhibition of neutrophil migration by a selective inhibitor of matrix metalloproteinase : analysis by intravital microscopy. Mediators of Inflammation 1995;4:133-137.
• PELORGEAS S, MARTIN JB, SATRE M. Cytotoxicity of dichloromethane diphosphonate and of 1-hydroxyethane-1,1-diphosphonate in the amoebae of the slime mould Dictyostelium discoideum. Biochem Pharmacol 1992;11:2157-2163.
• SATOH M, ADACHI K, SUDA T, YAMAZAKI M, MIZUNO D. TNF-driven inflammation during mouse liver regeneration after partial hepatectomy and its role in growth regulation. Mol Biother 1991;14:1045-1050.
• SEFTOR REB, SEFTOR EA, DELARCO JE, KLEINER DE, LEFERSON J, STETLER-STEVENSON WG et al. Chemically modified tetracyclines inhibit human melanoma cell invasion and metastasis. Clin Exp Metastasis 1998;16:217-225.
• SPILBERG I, MC LAIN, SIMCHOWITZ L, BERNEY S. Colchicine and pseudogout. Athritis Rheum 1980;23:1062-1063.
• TEIXEIRA MM, GRISTWOOD RW, COOPER N, HELLEWELL PG. Phosphodiesterase (PDE) inhibitors : anti-inflammatory drugs of the future. TIPS 1997;18:164-170.
• VAN ROOIJEN N, VAN NIEUWMEGEN R. Elimination of phagocytic cells in the spleen after intra-venous injection of liposome-encapsulated dichloromethylene diphosphonate. An enzyme-histochemical study. Cell Tissue Res 1984;238:335-358.
• VAN ROOIJEN N, SANDERS A. Liposome mediated depletion of macrophage : mechanism of action, preparation of liposomes and application. J Immunol Methods 1994;174:83-93.
• WALSH CE, WAITE BM, THOMAS MJ, DE CHALET LR. Release and metabolism of arachidonic acid in human neutrophils. J Biol Chem 1981;256:7228-7231.
• WEISS SJ. Tissue destruction by neutrophils. N Engl J Med 1989;320:365-375.