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].
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Bioequivalence requires that the number of PTH(1-34) and PTH(1-84) molecules should be the same: N
PTH(1-34) = N
PTH(1-84)
The clinical dose is based on molecular weight and bioavailability leading to the equation:
(where M is the molecular weight of PTH, P
abs the bioavailability, and m the mass of PTH).
$$\begin{array}{*{20}l}{\frac{{m_{{\text{PTH}}\left( {{\text{1 - 34}}} \right)} \times P_{{\text{abs}}\left( {{\text{1 - 34}}} \right)} }}{{M_{{\text{PTH}}\left( {{\text{1 - 34}}} \right)} }} = } \hfill & {\frac{{m_{{\text{PTH}}\left( {{\text{1 - 84}}} \right)} \times P_{{\text{abs}}\left( {{\text{1 - 84}}} \right)} }}{{M_{{\text{PTH}}\left( {{\text{1 - 84}}} \right)} }}} \hfill \\\end{array} $$
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.
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This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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Open AccessThis is an open access article distributed under the terms of the Creative Commons Attribution Noncommercial License (https://creativecommons.org/licenses/by-nc/2.0), which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.