NATPARA
Clinical safety rating: caution
Comprehensive clinical and safety monograph for NATPARA (NATPARA).
Recombinant human parathyroid hormone (PTH 1-84) that binds to PTH1 receptors, increasing serum calcium by enhancing renal calcium reabsorption, intestinal calcium absorption, and bone resorption.
| Metabolism | Metabolized in the liver via proteolytic cleavage, primarily by cathepsin D and other proteases. |
| Excretion | Primarily renal (≥95% as intact parathyroid hormone and metabolites); biliary/fecal elimination minimal (<5%) |
| Half-life | Terminal half-life approximately 2–5 minutes (subcutaneous); rapid clearance with clinical context: requires twice-daily dosing due to short half-life |
| Protein binding | Approximately 55–60% bound to plasma proteins, primarily albumin |
| Volume of Distribution | Approximately 0.1–0.2 L/kg; reflects limited extravascular distribution, primarily in plasma and interstitial space |
| Bioavailability | Subcutaneous: approximately 55% (relative to intravenous injection) |
| Onset of Action | Subcutaneous: serum calcium elevation begins within 15–30 minutes |
| Duration of Action | Subcutaneous: up to 6–8 hours; clinical note: serum calcium peaks at 2–4 hours post-dose, declines to baseline by 8 hours |
Initial dose: 50 mcg subcutaneously once daily, titrate in 25 mcg increments every 2-4 weeks based on serum calcium and symptoms, maintenance dose range: 25-100 mcg once daily.
| Dosage form | INJECTABLE |
| Renal impairment | eGFR <30 mL/min/1.73 m2: initiate at 25 mcg daily, titrate cautiously; eGFR 30-59: no specific adjustment but monitor calcium; eGFR ≥60: no adjustment. |
| Liver impairment | No formal studies; use with caution in severe hepatic impairment (Child-Pugh C) with increased monitoring. |
| Pediatric use | Not approved for patients <18 years; safety and efficacy not established. |
| Geriatric use | No specific dose adjustment; consider age-related renal decline and lower starting dose (25 mcg). |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for NATPARA (NATPARA).
| Breastfeeding | It is unknown if parathyroid hormone is excreted in human milk. No human lactation studies are available. The molecular weight (4117 Da) suggests minimal excretion, but due to potential for adverse effects in the nursing infant, caution is advised. The M/P ratio is unknown. Consider the importance of the drug to the mother and decide whether to discontinue nursing or discontinue the drug. |
| Teratogenic Risk | NATPARA (parathyroid hormone) is classified as Pregnancy Category C. In animal studies, parathyroid hormone has been associated with reduced fetal weight and skeletal abnormalities when administered during organogenesis. There are no adequate and well-controlled studies in pregnant women. The risk is likely highest during the first trimester due to skeletal development. Exposure in the second and third trimesters may affect fetal calcium homeostasis, but specific human data are lacking. Use only if potential benefit justifies potential risk to the fetus. |
■ FDA Black Box Warning
None.
| Serious Effects |
["Hypersensitivity to recombinant human PTH or any component","Pre-existing hypercalcemia","Metabolic bone diseases (e.g., Paget's disease)","Radiation therapy to skeleton (increased osteosarcoma risk)","Skeletal malignancies or bone metastases","Pediatric patients with open epiphyses"]
| Precautions | ["Risk of osteosarcoma (increased with duration of use; avoid in patients with increased baseline risk)","Digitalis toxicity","Hypocalcemia exacerbation upon discontinuation","Hypercalcemia and hypercalciuria requiring monitoring","Hypomagnesemia","Hypotension with rapid IV administration (not approved IV)","Laboratory test interference (unlikely)"] |
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| Fetal Monitoring | Monitor serum calcium, phosphate, and parathyroid hormone levels at least monthly during pregnancy. Perform fetal ultrasound for skeletal development and growth monitoring. Assess maternal renal function and blood pressure regularly. Consider amniotic fluid assessment for polyhydramnios if high doses are used. |
| Fertility Effects | No human data on fertility effects. In animal studies, high-dose parathyroid hormone has demonstrated adverse effects on reproductive indices including decreased fertility and impaired embryonic survival. Clinical significance in humans is unknown but caution is warranted in women of childbearing potential. |