MBX 2109, a Once-Weekly Parathyroid Hormone Replacement Therapy Prodrug: Phase 1, First-in-Human, Randomized Trial

Introduction: Hypoparathyroidism denotes parathyroid hormone (PTH) deficiency and impaired mineral metabolism. MBX 2109, a novel prodrug yielding a biologically active PTH peptide agonist (PTH[1-32], extended by a fatty acylated Lys33), is being developed as a long-acting, once-weekly PTH replacement therapy. Here, we report the safety, pharmacokinetics (PK), and pharmacodynamics (PD) of MBX 2109 in healthy volunteers.

Methods: This phase 1, randomized, double-blind, placebo-controlled, multiple ascending-dose study (NCT05158335) enrolled healthy adults, who were randomized 4:1 to receive MBX 2109 (200, 400, 600, and 900 μg; n = 8) or placebo (n = 2) by subcutaneous administration once weekly for 4 doses (days 1, 8, 15, and 22). The primary endpoint was safety and tolerability. Key secondary endpoints were PK and PD.

Results: Overall, 40 participants (MBX 2109 n = 32, placebo n = 8) were randomized (mean age, 43.3 years; 22.5% female). Treatment-emergent adverse events (TEAEs) occurred in 50%-88% of MBX 2109 groups and in 25% of placebo participants. In the MBX 2109 groups, no severe or serious TEAEs were observed. Injection-site reaction was the most common treatment-related TEAE. The half-lives were 79-95 hours for MBX 2109 and 184-213 hours for the fatty-acylated biologically active PTH peptide, which showed dose- and time-dependent exposure increases.

Conclusion: The sustained-action PTH prodrug MBX 2109 was well tolerated with no unexpected, off-target safety issues. The long half-life and flat exposure profile of MBX 2109's biologically active PTH agonist supports once-weekly administration. MBX 2109 doses were identified for future studies.