Osteoporose bei rheumatoider Arthritis – Bedeutung von Alfacalcidol in Prävention und Therapie

Abstract

Besides localised osteopenia, patients with rheumatoid arthritis (RA) with or without corticosteroids develop in 30-50% osteoporosis induced by several factors and thus a higher risk of fractures. Bone loss appears very early and correlates directly with disease activity and also later with the negative effects of restrictive mobility. Corticosteroids reduce as a pathogenetic co-factor intestinal calcium absorption and increase renal calcium excretion resulting in compensatory increased PTH-release and increased sensitivity of bone to PTH. In addition, corticosteroids inhibit osteoblast function as well as the favourable effects of growth factors and sex hormones on bone. It has recently been recognised that the expression of D-hormone receptors (VDRs) is suppressed by these medications and that corticosteroids probably induce VDR disorders. The negative influence of corticosteroids on muscle strength (indirectly--via increased PTH-levels, lowered IGF-1-levels or reduced D-hormone activity) is a feature which has been underestimated. The demonstrated drop in 1,25(OH)2D3 (D-hormone) levels in patients with RA in correlation with C-reactive protein (CRP) is of significance in the pathogenesis of RA-induced osteoporosis and could further promote the process of inflammation. There is a general consensus that cytokines (e.g. IL-1, IL-6, IL-12, TNF-alpha) induce bone resorption in inflammatory rheumatic diseases. There are, however, new findings which show that cytokines like TNF-alpha also interfere with bone formation by promoting apoptosis of osteoblasts and reduce the muscle strength, too. D-hormone preparations (alfacalcidol, calcitriol) possess immunoregulatory effects in vitro and in vivo by inhibiting the cytokines IL-1, IL-6, TNF-alpha and particularly IL-12. At the cellular level, D-hormone reduces the expression of Th1 helper cells directly or indirectly by inhibition of IL-12 from monocytes. Therapy with alfacalcidol or calcitriol results in increased production of Th2 helper cells which produce bone protective cytokines like IL-4 and IL-10. It is important to know that D-hormone protects osteoblasts against TNF-alpha-induced cell death. After conversion to D-hormone in the liver and bone, alfacalcidol antagonises the above described pathogenetic factors of the corticosteroids. D-hormone is one of the body's own immunoregulators, which is produced in macrophages in cases of need to reduce immunological overreactions in a feed-back loop. Improved understanding of the pathogenesis of corticosteroid-induced osteoporosis and of the pharmacological effects of alfacalcidol in this type of iatrogenic bone loss as well as the results of specific animal models simulating bone loss in inflammatory diseases explain the favourable effects of alfacalcidol in this indication. Various clinical studies have demonstrated clearly that alfacalcidol retards corticosteroid-induced bone loss in contrast to plain vitamin D. Due to its immunomodulating properties, alfacalcidol is particularly suitable for RA-induced bone loss and for the prevention of transplantation osteoporosis, and an adjuvant contribution to the disease-modifying therapy of RA and to the immunosuppressive therapy after transplantation can not be excluded.