CollaGenex Pharmaceuticals Inc. has found an untapped niche in marketing therapeutics to dentists. Based on the anticipated potential for its Periostat doxicycline, which was launched in January to treat adult periodontitis, the company has amassed a 125-person sales force to market the product to about 23,000 dentists. But a force this size does not come cheap, and CGPI's SG&A has ballooned to