Utilization of cyclosporine H to elucidate the possible mechanisms of cyclosporine A-induced osteopenia in the rat

Abstract

The immunosuppressive agent cyclosporine A (CsA) produces in vivo in the rat marked osteopenia and elevation of serum bone gla protein (BGP) that reflects the high bone turnover and increased circulating levels of 1,25-dihydroxyvitamin D [1,25(OH) 2D]. Cyclosporine H (CsH), a d- N-MeVal 11 analog of CsA, is not immunosuppressive, and in contrast to CsA, it neither binds to cyclophilin nor alters cytokine activity. This distinction between CsH and CsA provides a means of elucidating whether CsA exerts an effect on bone and 1,25(OH) 2D via immune-mediated mechanisms. We therefore studied three groups of male Sprague-Dawley rats (300 to 350 g body weight [BW]) randomly divided to receive either cyclosporine vehicle, CsA, or CsH 15 mg/kg BW by daily gavage for 28 days. Blood samples were assayed for ionized calcium (Ca 2+), serum BGP, serum parathyroid hormone (PTH), and 1,25(OH) 2D by specific radioimmunoassay on days 0, 14, and 28. Histomorphometric evaluations were performed on the right tibia after tetracycline and calcein labeling and killing of the rats at the end of 28 days of treatment. In CsA-treated rats, serum BGP levels were significantly increased (155.2 ± 30.7 ng/mL v 107.3 ± 16.8 and 111.5 ± 13.1 at day 28, P < .05) compared with control and CsH groups, respectively. Similarly, 1,25(OH) 2D was significantly increased in CsA-treated rats versus control and CsH groups (134.9 ± 35.3 pg/mL v 70.2 ± 16.6 and 69.8 ± 20.6 [ P < .05], respectively). PTH and CA 2+ were unchanged. Histomorphometrically, the CsA-treated group showed a significant loss in bone volume ([BV] P < .05) and an increase in the indices of bone remodeling compared with the control group. The CsH group showed no significant alteration in indices of bone remodeling compared with the control group. These results indicate that the accelerated bone remodeling with osteopenia and the increased production of 1,25(OH) 2D is probably mediated by immune mechanisms.