OR21-6 AZP-3601, a Long-Acting Parathyroid Hormone (PTH) Analog, Normalizes Blood Calcium in Thyroparathyroidectomized (TPTX) Rats Without Causing Deleterious Changes in Bone

: PTH peptides can be effective in normalizing blood calcium (Ca++) levels in cases of hypoparathyroidism (HP), but long-term effects on bone are not well understood. AZP-3601 is a long-acting PTH/PTHrP(1-36) analog currently being investigated as a potential treatment option for HP. As compared to conventional PTH1-34, AZP-3601 binds with higher affinity to the R0 conformation of the PTH receptor and thereby induces more sustained signaling responses in cells as well as more prolonged calcemic responses in vivo, despite having a very short circulating half-life. Using the TPTX rat model of HP, we investigated whether the distinct mode-of-action used by AZP-3601 results in different down-stream effects in bone than those induced by PTH1-34, when the peptides are administered long-term (31 days) either by daily sc injection (AZP-3601 and PTH1-34) or by continuous infusion (PTH1-34), and at doses aimed to optimally normalize blood Ca++ levels. METHODS: Male TPTX rats (strain S-D, age 9 weeks, and 2 weeks post TPTX surgery) were injected daily with either AZP-3601 at a dose of 0.7 nmol/kg or PTH1-34 at a dose of 50 nmol/kg, or were continuously infused via Alzet mini-pump with PTH1-34 at a dose of 3.0 nmol/kg/day. Vehicle treatments and sham surgery rats were used as controls. Tail-vein blood was collected on days 1, 7 14, 21 and 28 at 6 hours post-injection and analyzed for total Ca++. Rats were euthanized on day 31 (24 hours after last injection) and blood and femurs collected for analysis. RESULTS: All peptide-treatments increased serum Ca++ levels on days 7-21 to within or near the normal range (8.8-11.7 mg/dL versus 10.8-11.9 mg/dL in sham-operated rats) and to levels significantly higher than those in vehicle-treated TPTX rats (6.9-7.9 mg/dL, P<0.01). Bone mCT analysis revealed that relative to TPTX-vehicle controls, PTH1-34 continuous infusion significantly decreased (p<0.001) distal femur trabecular bone volume relative to tissue volume (BV/TV), trabecular number (Tb.N) and trabecular thickness (Tb.Th), while PTH1-34 daily injection significantly increased (P<0.001) distal femur BV/TV, Tb.Th and Tb.N, as well as mid-femur cortical thickness (Ct.Th, P<0.001). In contrast, AZP-3601 daily injection caused no change in trabecular or cortical bone parameters. Serum levels of the bone formation marker P1NP were significantly increased by daily injection of PTH1-34 (P<0.001 vs. TPTX-vehicle) but were not changed by other treatments. CONCLUSION: With long-term (31 days) treatment and at doses that similarly normalized serum Ca++ in TPTX rats, effects on bone were catabolic for PTH1-34 by continuous infusion, anabolic for PTH1-34 by daily injection, and neutral for AZP-3601 by daily injection. The distinct MOA used by AZP-3601 may thus result in less of an impact on bone than either daily injection or sustained, continuous delivery of PTH(1-34), when used as a long-term treatment for HP. Presentation: Monday, June 13, 2022 12:15 p.m. - 12:30 p.m.