By a structural combination of phosphonate and bisphosphonate moieties with the vitamin D skeleton a series of new vitamin D analogs was synthesized. Derivatives with 24β-hydroxy- or 24-keto groups exerted considerable vitamin D activities in vitro while the hypercalcemic potentials were significantly reduced as compared to 1α,25-dihydroxyvitamin D3 (calcitriol). Whereas the 24-hydroxy analogs did not influence bone formation in vivo in dosages below the hypercalcemic threshold, the 24-ketones were found to induce synthesis of new bone matrix in non-hypercalcemic doses. Vitamin D bisphosphonate hybrids, on the other hand, which did not elicit substantial vitamin D activities in vitro and tend to decrease serum calcium levels in vivo clearly induced osteoid formation in rats, indicating a mechanism of action different to calcitriol.

The introduction of oxygen atoms into different positions of the vitamin D side chain is described. By combining the arising 23-oxa and 25-oxa elements with other structural modifications (19-nor, iso-19-nor, 20-methyl, 20-ene, 20,21-cyclo) calcitriol analogs with remarkable levels of dissociation between beneficial acitivities on cell growth regulation and undesired hypercalcemia were identified. Structure-activity relations are elaborated in a very systematic outline of the Schering drug finding program in this particular class of vitamin D compounds.