Comment on Kanis et al.: “European guidance for the diagnosis and management of osteoporosis in postmenopausal women”

Abstract

Dear Editors, Kanis et al. erroneously state in a recent paper about the diagnosis and management of osteoporosis in postmenopausal women that 100 μg of PTH(1-84) is equivalent to 40 μg of teriparatide, PTH(1-34) [1]. This equivalence was calculated from their respective molecular weights (4,115 for teriparatide [2], 9,426 for full-length PTH [3]) but does not consider bioavailability. The bioavailability of PTH(1-34) and PTH(1-84) are 95% and 55%, respectively [4, 5]. Bioequivalence requires that the number of PTH(1-34) and PTH(1-84) molecules should be the same: NPTH(1-34) = NPTH(1-84) The clinical dose is based on molecular weight and bioavailability leading to the equation: (where M is the molecular weight of PTH, Pabs the bioavailability, and m the mass of PTH). Using this calculation, 100 µg of PTH(1-84) is equivalent to 25 μg of teriparatide {100 μg × (55/95) × 4,115/9,426 = 25 μg} and these are the approximate doses used in the treatment of postmenopausal osteoporosis.