Background/Purpose: Apremilast (APR), a small molecule specific inhibitor of phosphodiesterase 4, works intracellularly to modulate pro- and anti-inflammatory mediator production in both immune and non-immune cells. Here, the effects of APR on osteoclasts (OCL), osteoblasts (OBL), and osteocytes (OCY) were examined in vitro.

Methods: Human bone marrow mononuclear cells were differentiated into OCL using 10nM dexamethasone and 10nM vitamin D for 7 days. APR (0.1-10 µM) was added along with fresh medium on day 0 and day 3. OCL were stained for tartrate-resistant acid phosphatase 5 (TRAP5), and OBL were stained for alkaline phosphatase. OBL were differentiated into OCY using hydroxyapatite/tricalcium phosphate biphasic calcium phosphate ceramic particles (Graftys BCP), which was placed into polycarbonate filter well inserts and cell culture media changed every 3 days for a total of 28 days. Gene expression was measured by qRT-PCR for the following: receptor activator of nuclear factor kappa-B (RANK), RANK ligand (RANKL), sclerostin (SOST) and osteoprotegerin (OPG). Protein production was measured by enzyme-linked immunosorbent assay.

Results: In OCL cultures, the number of TRAP-5+ cells was reduced by APR by 21%, 49%, and 73% at 0.1 μM, 1 μM, and 10 μM, respectively. In the OCL cultures, APR significantly reduced the levels of sRANKL protein by 25%, 21%, and 38% at 0.1 μM, 1 μM, and 10 μM, respectively. APR 1 μM and 10 μM significantly inhibited RANK gene expression by 30% and 25%, respectively. Alendronate inhibited RANK gene expression by 77%. In OBL, APR reduced sRANKL protein levels by 25% at both 1 μM and 10 μM. Rolipram, alendronate, and sulfasalazine all had no significant effect on sRANKL protein levels in the OBL supernatants. In addition, APR significantly increased OPG protein levels by 42% at 0.1 µM. Overall, APR decreased the sRANKL/OPG protein ratio by 39%, 32%, and 40% at 0.1 μM, 1 μM, and 10 μM, respectively. By comparison, rolipram, alendronate, and sulfasalazine had no effect on the sRANKL/OPG ratio. In OCY, APR significantly reduced sRANKL production by 18%, 14%, and 17% at 0.1 μM, 1 μM, and 10 μM. APR also significantly reduced SOST protein levels by 16%, 20%, and 14% at 0.1 μM, 1 μM, and 10 μM.

Conclusion: These results demonstrate that APR inhibits osteoclastogenesis in vitro at clinically relevant concentrations (0.1-1 μM). This effect was associated with a decrease in sRANKL protein expression by OBL, but also may involve decreased RANK expression on the OCL. Since the osteoclastogenesis studied in this system was driven in part by dexamethasone, these findings indicate that APR may be useful for counteracting the bone catabolic effects of corticosteroids.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-effects-of-apremilast-on-osteoclasts-osteoblasts-and-osteocytes/