The effect of long-term low-dose diphosphonate treatment on rat bone.

Weaning, male rats were given ethane-1-hydroxy 1,1-diphosphonate (EHDP) or dichloromethylene diphosphonate (Cl2MDP) 0.5 mgP/kg/day for 140 days. Samples prepared after sacrifice were: (1) thin ground (30 micrometers) transverse tibial sections, (2) methacrylate-embedded 5 micrometers sections of the proximal tibial metaphysis, and (3) ultrathin calcified metaphyseal sections for electron microscopy. Cl2MDP reduced the diaphyseal medullary cavity, and EHDP increased the osteoid width. Both drugs increased endosteal bone apposition (possible secondarily to the impaired resorption) and, perhaps as a result, periosteal apposition was increased. Consequently, diaphyseal bone area was unchanged. Metaphyseal bone area was increased and osteoclast numbers reduced, in contrast to the increased osteoclast numbers reported previously in animal experiments with diphosphonates. Acid phosphatase activity within osteoclasts was markedly reduced with EDHP and unchanged with Cl2MDP. The ultrastructural changes in osteoclasts, i.e., a deficiency of ruffled borders, reduced numbers of cytoplasmic vacuoles and the presence of crystals between bone and the osteoclastic plasmalemma, were more marked with EHDP than Cl2MDP.