A combination of subtherapeutic doses of chemically modified doxycycline (CMT-8) and a bisphosphonate (clodronate) inhibits bone loss in the ovariectomized rat: A dynamic histomorphometric and gene expression study

Recent studies have demonstrated that tetracyclines can reduce bone loss in the ovariectomized (OVX) rat model of osteoporosis. In the current study, a non-antimicrobial, chemically modified doxycycline (CMT-8), alone or in c ombination with a bisphosphonate (Clodronate), was evaluated in this model. Forty-two, 6month old. female rats were randomly assigned to the following groups, (6/ group): a) sham/vehicle, b) OVX/vehicle; c) OVX/1 mg/day CMT-8 ; d) OVX/2 mg/day CMT-8, e) OVX/1 mg/week Clodronate; and f) OVX/1 mg/day C MT-8 + 1 mg/week Clodronate, CMT-8 was administered by oral gavage, Clodron ate injected S/C. Following sham surgery or OVX, the rats were treated for 90 days with CMT-8 or vehicle alone, injected at three different times with fluorochrome labels, the rats were sacrificed, and the tibiae excised for analysis by dynamic bone histomorphometry. Femurs were aseptically removed and analyzed for collagen, collagenase and osteopontin mRNAs by Northern an d dot blot analysis. As expected, OVX decreased trabecular bone volume (BV/ TV by 73.8% vs. sham p<.01), and also reduced trabecular thickness, numbers , and increased spacing. Done loss in the OVX animals was partially prevent ed with either 2 mg/day CMT-8 or 1 mg/wk Clodronate (p<.01), while the mg/d ay CMT-8 had no effect. Interestingly, the efficacy of the combination ther apy of CMT-8 and Clodronate was significantly better than either treatment by itself; maintaining bone mass and structural indices at levels identical to sham values. OVX rats mRNA for collagen, collagenase and osteopontin we re elevated indicating high-turnover bone loss. Only COMBO therapy signific antly reduced the collagenase and osteopontin mRNA. In summary, CMT-8 mono- therapy (2 mg) alone partially inhibited bone loss in this animal model of osteoporosis. However, 1mgday (CMT-8) monotherapy had no effect on bone los s or bone mRNA levels and when combined with Clodronate, interacted to incr ease efficacy. Thus, a combination of a suboptimal dose of CMT-8 and a bisp hosphonate appears to increase the amount of bone by suppressing resorption in a model of osteoporosis.