MBX Biosciences Inc. (NASDAQ:MBX) shares dropped 19.6% in premarket trading on Friday despite the company reporting encouraging one-year clinical data for its investigational treatment canvuparatide in patients with chronic hypoparathyroidism.
The clinical-stage biopharmaceutical company released updated findings from its 12-week Avail Phase 2 study alongside one-year results from an ongoing open-label extension trial evaluating once-weekly canvuparatide in adult patients with the rare endocrine disorder.
Long-Term Data Demonstrates Sustained Clinical Benefit
The open-label extension study showed that 57% of patients met the responder criteria after one year of treatment.
While slightly below the 63% responder rate recorded at the 12-week stage of the Phase 2 trial, the data continued to demonstrate meaningful long-term efficacy.
During the Phase 2 study, 63% of patients receiving canvuparatide achieved the primary composite endpoint, compared with 31% of patients treated with placebo.
The primary endpoint required patients to maintain albumin-adjusted serum calcium levels within the normal range while remaining independent from conventional therapy.
Strong Patient Retention Supports Results
MBX reported a 90% retention rate among participants who entered the open-label extension study, with the vast majority remaining on treatment through the one-year mark.
According to the company, the data suggest that canvuparatide successfully restores systemic parathyroid hormone (PTH) activity, supporting multiple markers of disease control.
These included normalization of serum calcium levels, reduced urinary calcium excretion, improvements in bone metabolism, and enhanced kidney function.
Improvements Seen in Kidney Function and Calcium Control
Mean serum calcium levels remained within the normal range throughout the one-year treatment period.
At the same time, average 24-hour urine calcium levels declined from baseline measurements, indicating improved calcium regulation.
Researchers also observed improvements in estimated glomerular filtration rate (eGFR), a key indicator of kidney function.
The gains in eGFR first became apparent by Week 12 and were maintained throughout the one-year follow-up period.
Safety Profile Remains Favorable
Canvuparatide continued to demonstrate a favorable safety profile during the extension study.
The company reported no new safety concerns and no treatment-related serious adverse events.
Most treatment-emergent adverse events were classified as mild or moderate in severity.
Injection-site reactions were reported in approximately 10% of patients receiving the therapy.
Phase III Study Remains on Schedule
MBX Biosciences said development of canvuparatide remains on track, with the company continuing preparations for a pivotal Phase III study.
Management expects to initiate the Phase III trial during the third quarter of 2026.
Despite the positive efficacy and safety findings, investors appeared to focus on the modest decline in responder rates compared with the earlier Phase II results, contributing to the sharp decline in the stock following the announcement.