Vitamin D and Calcium Homeostasis

Calcium Homeostasis Overview

• Bone resorption and bone formation constantly occur to maintain healthy bone.
• Calcitonin, parathyroid hormone (PTH), and vitamin D regulate calcium homeostasis along with inorganic phosphate.

Vitamin D Biosynthesis

• Vitamin D comes from cholesterol, first biosynthesized from cholecalciferol via solar UV irradiation of 7-dehydrocholesterol in the skin.
• Vitamin D requires two oxidative metabolic steps for activation.
• Vitamin D 25-hydroxylase in the liver catalyzes the first oxidation to 25-hydroxycholecalciferol (calcifediol).
• Calcifediol is a major circulating and storage form of Vitamin D.
• Vitamin D 1α-hydroxylase in the kidney catalyzes the second oxidation in response to hypocalcemia and PTH secretion.
• This creates 1,25-dihydroxycholecalciferol, the active Vitamin D form.
• Calcium, phosphate, PTH, and active vitamin D serum concentrations tightly regulate Vitamin D biosynthesis.

Vitamin D Activity

• Sterol-specific cytoplasmic receptor proteins, or vitamin D receptors, mediate Vitamin D's biological action.
• The active hormone transports from the cytoplasm to the nucleus via the receptor.
• Interacts with target genes to modulate gene transcription in a similar way to other hormonal receptors.
• Resulting proteins stimulate calcium transport into target tissues.

Drug Therapies for Osteoporosis

• Agents for treatment and prevention of osteoporosis are either antiresorptive or bone-forming agents.
• Antiresorptive drugs prevent calcium loss from bones.

Estrogen Analogs - Estrogen Replacement Therapy

• The mechanism by which estrogen prevents bone resorption is unknown.
• Estrogen binds to estrogen receptor (ER a & b subtypes), a nuclear receptor.
• Activation of ER causes dimerization and binding to other proteins like coactivators or corepressors.
• This can enhance or diminish the transcription of target genes.
• This improves calcium absorption, promotes calcitonin biosynthesis, and increases vitamin D receptors.

Selective Estrogen Receptor Modulators (SERMs)

• Tamoxifen is an adjuvant breast cancer therapy, blocking ER-mediated malignancy.
• Raloxifene, a tamoxifen analog, treats osteoporosis.
• Both drugs have agonist activity on ER in certain tissues and antagonist activity in others.
• Tamoxifen and raloxifene exhibit ER antagonist activity in breast and agonist activity in bone.
• Tamoxifen exhibits agonist activity in the uterus, increasing the risk of uterine cancer with long-term use.

Understanding SERM Effects on Receptors

• Different ER ligands such as SERMs may selectively bind to ER a or ER b subtypes.
• The structure of both the ligand and ER subtype dictates the conformation of the ER-ligand complex.
• The structure of the ER-ligand complex determines its ability to interact with coactivators or corepressors.
• Differential expression or accessibility of coactivators/repressors, as well as the ER subtypes, affect tissue specificity.
• The target gene promoter and differences in uptake and/or the metabolism of SERMs contribute to tissue selectivity.

Raloxifene Structure Activity Relationship

• Phenol group is critical for interaction with ER as it mimics the A ring 3-phenol group found in estrogen.
• The orientation of the three aryl rings should be positioned in a propeller-like shape.
• This is essential for tight receptor binding and biologic activity.

Raloxifene Bioavailability

• Intestinal metabolism of raloxifene results in glucuronide conjugation.
• Intestinal cells efflux glucuronide via P-glycoprotein and multidrug resistance–related protein.
• This rapid metabolism and efflux contributes to low bioavailability.

Bisphosphonates

• They mimic pyrophosphate with a central carbon atom.
• This creates a nonhydrolyzable backbone.
• Pyrophosphate is found in bone.
• These analogs selectively target bones, bind to and stabilize calcium phosphate.
• This decreases bone resorption sites.
• High polarity results in poor tissue absorption and rapid excretion.
• Selective uptake and rapid clearance provides a short half-life and limited exposure of non-target tissues.

Bisphosphonates Structure Activity Relationship

• The precise molecular target of bisphosphonates is still under investigation.
• The central carbon has been substituted with functional groups for various compounds.
• The bisphosphonate core structure, the hydroxyl group at R1, and a hydroxyl substituent (R1) are critical.
• The R2 substituent impacts potency.
• R2 amino-substituted bisphosphonates (pamidronate disodium and alendronate sodium) are more potent than etidronate disodium.
• Linear chains for alendronate sodium is more potent than derivatives of pamidronate disodium at the R2 3-carbon amino.
• Alkylation of the amine functional group improves potency as in ibandronate sodium, agents that feature rings (risedronate sodium and zoledronic acid).

Calcitonin

• The hormone is derived from multiple sources, natural, synthetic, eel, humans, salmon and porcine.
• Calcitonin from salmon is preferred for its greater receptor affinity and longer half-life than the human version.
• Administration is IV or nasal sprays with concomitant oral elemental calcium as therapy.
• Side effects include nausea, vomiting, anorexia, and flushing which are more pronounced with injection.
• May also be immunogenic, which can trigger a systemic allergic reaction.

Teriparatide

• A precursor for parathyroid hormone (PTH).
• It gives rise to proparathyroid hormone through cleavage at site 1, and PTH by cleavage at site 2.
• Teriparatide is the biologically active portion of PTH which is available as a synthetic recombinant human protein (PTH 1-34).
• It is the first bone-forming agent and unlike bisphosphonates.
• It enhances both osteoclast and osteoblast function, but dose exposure determines its skeletal effect.
• Intermittent/daily administration preferentially enhances osteoblastic function and bone formation.
• Black box warning for increased osteosarcoma incidence.
• Treatment should not exceed 2 years.

Inorganic Salts

• Calcium salts are co-administered with osteoporosis agents or for poorly mineralized bone.
• Calcium supplementation alone cannot reverse bone loss or significantly improve mineralization.
• Elemental calcium amounts vary among salts, no specific salt is better than another.
• Calcium absorption from the gastrointestinal tract improves under acidic conditions.
• Medications that change the stomach's acidic environment, such as H2 antagonists and proton pump inhibitors, adversely affect calcium absorption.
• More water-soluble salts (citrate, lactate, gluconate) are less dependent on the acidic environment.
• Calcium carbonate is poorly soluble but inexpensive.
• Doses must be taken with acidic food or beverages like citrus juice.

RANK Inhibitor, Denosumab

• RANK receptor is on osteoclasts and their precursors.
• RANK ligand (RANKL) binds RANK and promotes osteoclast formation and activation.
• Osteoblasts produce osteoprotegerin as an effect to balance RANKL by binding to RANK and preventing it from binding to its receptor.
• Osteoporosis disrupts this balance, leading to significant bone loss due to overwhelmed osteoprotegerin activity from RANKL.
• Denosumab is a monoclonal antibody to RANK.
• It mimics osteoprotegerin's biochemical effects.
• It is given subcutaneously every 6 months.
• Adverse events include back pain, serious infections, hypocalcemia, and osteonecrosis of the jaw.