Secosteroid

Secosteroid is a type of steroid in which one of the four fused rings of the steroid nucleus has been cleaved, resulting in a partially open framework. The opening most commonly involves the B ring, though other ring openings are possible. The term derives from the Latin secare, meaning to cut.

This structural change distinguishes secosteroids from classical closed-ring steroids and alters their three-dimensional shape and chemical reactivity. The remaining rings and substituents determine the molecule’s biological activity, receptor affinity, and metabolic stability. Many secosteroids retain features of the steroid core but exhibit unique conformations and functions due to the open ring.

The most widely known secosteroids are the vitamin D family. Ultraviolet light converts 7-dehydrocholesterol in the skin to previtamin D3, which rearranges to cholecalciferol (vitamin D3); both are secosteroids because the B ring is open. Plants and fungi produce ergocalciferol (vitamin D2) via similar processes. In addition to vitamins, several synthetic secosteroids and their analogs are used therapeutically, notably calcipotriol and related compounds that mimic vitamin D activity with tissue-selective effects.

Biological and clinical relevance: vitamin D secosteroids act as precursors to the hormonally active calcitriol, regulating calcium and phosphate homeostasis and influencing bone metabolism. Structural modifications in secosteroids yield analogs with varied receptor affinity and therapeutic profiles, useful in dermatology and other fields, while requiring careful dose management to avoid hypercalcemia and toxicity.