Comparative Pharmacological Effects of Tetracycline Derivatives on Osteoclast- and Osteoblast-Related Phenotypes and Skeletal Accumulation

Background/Objectives: Tetracycline (TC) derivatives possess biological activities beyond their antimicrobial effects and exhibit affinity for mineralized tissues. However, direct comparisons of clinically established TC derivatives across bone-related experimental systems remain limited. This study comparatively evaluated TC, oxytetracycline (OC), doxycycline (DC), and minocycline (MC) to characterize their compound- and concentration-dependent effects on bone-related cellular phenotypes and skeletal accumulation. Methods: TC, OC, DC, and MC were evaluated using primary human osteoclasts, primary human mesenchymal stem cell (MSC)-derived osteoblasts, MG-63 osteoblast-like cells, and zebrafish embryos. Osteoclast formation and resorption, ALP-positive area, cellular metabolic activity, extracellular matrix mineralization, and compound localization were assessed. Results: All four derivatives were associated with concentration-dependent reductions in osteoclast formation, multinucleation, and resorptive activity, with DC showing pronounced effects. At 10 μg/mL, TC, OC, and MC reduced resorption pit volume to approximately 10–20% of untreated control values, while DC markedly reduced resorption even at 5 μg/mL. In primary MSC-derived osteoblasts, treatment with all four derivatives was associated with concentration-dependent reductions in ALP-positive area. In MG-63 cells, none reduced WST-8 metabolic activity at concentrations up to 20 μg/mL, whereas extracellular matrix mineralization showed compound- and concentration-dependent responses. All four derivatives preferentially localized to mineralized matrices in vitro and accumulated in developing skeletal structures in zebrafish, where increasing concentrations were associated with reduced mineralization intensity. Conclusions: Clinically established TC derivatives exhibit distinct compound- and concentration-dependent pharmacological profiles across bone-related experimental models and preferential affinity for mineralized tissues. These findings provide a comparative framework for future mechanistic studies of the bone-related biological effects and skeletal accumulation of TC derivatives.