Treatment with SRL172 (heat‐killed Mycobacterium vaccae) inhibits progression of established experimental periodontal disease in Wistar rats

Periodontal disease is accompanied by a change in the periodontal bacterial flora, and an increase in Th2 cytokine expression. Therefore, preparations that can down-regulate Th2 responses and increase Th1 responses to bacteria may have therapeutic effects. SRL172, a preparation of heat-killed Mycobacterium vaccae (NCTC11659), has been shown to have these properties in man and animals and, in a previous study, a single subcutaneous injection of this material 13 days before application of ligatures to the right second molars of Wistar rats strikingly reduced subsequent destruction of the tooth-supporting tissues. The same material has therefore been tested in a therapeutic protocol, in rats with ongoing periodontal disease. A silk ligature was placed round the maxillary right second molar in the gingival sulcus on day 0. This was removed after two weeks, and the animals immediately received 0.1 mg SRL172 s.c. or saline. One week later (i.e. at three weeks), a boost of 1.0 mg SRL172 or saline was given. Animals were sacrificed at eight weeks (i.e. five weeks after the booster dose). Treatment of ongoing periodontal disease with SRL172 significantly reduced fibre loss (p = 0.046 by histometry) and bone loss (p = 0.0008 by radiography) on the ligatured side, and reduced fibre loss (p = 0.0086) on the control side. Thus SRL172 is an effective treatment in this model. As SRL172 has undergone extensive safety testing in man, these results justify clinical studies in periodontal disease, and such studies are now planned.