On the mechanisms for the selective action of vitamin D analogs

A variety of analogs of 1,25-(OH)₂D₃ with less calcemic activity and lower receptor binding affinity than 1,25-(OH)₂D₃ have been developed. However, these compounds have equal or greater ability to differentiate leukemia cells and psoriatic fibroblasts and to suppress PTH synthesis and secretion. The mechanism for this selectivity has not been elucidated. Because the lower potency of ergocalciferol compared to cholecalciferol in preventing or curing rickets in chicks was associated with a lower affinity of the avian vitamin D binding protein (DBP) for vitamin D₂, we tested five analogs with low calcemic activity including 22-oxa-1,25-(OH)₂D₃ (OCT), MC903, 1,25-(OH)₂-16 ene-23-yne D₃, 1,25-(OH)₂-26,27 dihomo-22-ene-D₃, and 1,25-(OH)₂-24-trihomo-22-ene-D₃ for their affinity for rat serurn DBP. All analogs had a low affinity for DBP, ranging from 50-3000 times less than that of 1,25-(OH)₂D₃. OCT also bound with low affinity to dog and human serum DBP. We tested with OCT the possible consequences of its low affinity for serum DBP. One of the functions of DBP is to prolong the lifetime of 1,25-(OH)₂D₃ in circulation. Quantification of the metabolic clearance rate (MCR) of OCT in 8 normal dogs using a single bolus injection technique showed that OCT was cleared at a rate of 48.2 ± 7.5 ml/min, approximately 6-7 times more rapidly than 1,25-(OH)₂D₃ (6.8 ± 0.4 ml/min). The estimated half-life of OCT in the circulation was 2.5 ± 0.3 h compared to 7.0 ± 0.6; n = 7 for 1,25-(OH)₂D₃. As our primary interest is the potential of OCT in treating the secondary hyperparathyroidism of CRF, we also measured the MCR of OCT in 5/6 nephrectomized dogs. Uremia does not affect the rate of clearance of OCT from the circulation (MCR: 56.8 ± 4.5; t_1/2 = 2.1 ± 0.2 n = 4). Despite its shorter half-life, OCT suppressed PTH secretion in vivo in uremic dogs. The effects of low binding to DBP on the percentage of free sterol were determined using an ultrafiltration procedure. We compared the proportion of free (unbound) OCT and 1,25-(OH)₂D₃ in 0.1% BSA-PBS with concentrations of human serum ranging from 0-25%. The proportion of OCT in the free form was significantly higher than that of 1,25-(OH)₂D₃ for every serum concentration tested. The physiological relevance of a higher percentage of free OCT was tested in normal human macrophages. Both 1,25-(OH)₂D₃ and OCT (100 pg/ml) significantly stimulated vitamin D catabolism. The presence of serum in the incubation media decreased this stimulation in a dose-dependent manner for both compounds, suggesting that the free form of the sterol was responsible for this effect. Due to a higher proportion of free OCT, the analog was more effective than 1,25-(OH)₂D₃ in stimulating vitamin D catabolism for every serum concentration tested. A higher proportion of OCT in the unbound active form may offset its lower receptor binding affinity and its more rapid clearance. These experiments demonstrate that the low affinity of the noncalcemic analogs for DBP results in a higher proportion of the free, active form in serum and more rapid clearance from the circulation. Therefore, alternative mechanisms for the selective action of vitamin D analogs may be: 1) the requirement of specific carrier proteins for the delivery of the sterol to various target tissues; 2) pharmacokinetic differences between the analog and the parent hormone; 3) preferential uptake of either the free or bound form by different target cells; and 4) different intracellular signaling in different target tissues.