SAT-LB72 Design of the PaTH Forward Phase 2 Trial of TransCon PTH, a Long-Acting PTH, in Patients With Hypoparathyroidism

Background: Hypoparathyroidism (HP) is caused by a deficiency in parathyroid hormone (PTH), which leads to hypocalcemia and hyperphosphatemia. Standard of care (SoC), i.e. large doses of calcium (Ca) and active vitamin D, worsens hypercalciuria and increases the serum Ca (sCa) x serum phosphate (sP) product. Studies have shown that continuous subcutaneous infusion of PTH(1-34) normalizes sCa, sP, serum magnesium, urine Ca (uCa) and bone turnover better than SoC or once- or twice-daily injections of PTH in HP patients. TransCon PTH, an investigational long-acting prodrug of PTH (1-34) transiently bound to an inert carrier via a linker, is under development as a potential once-daily replacement therapy for HP. Under physiological conditions, linker auto-cleavage occurs, releasing active PTH at a controlled rate with ~60 hour half-life. Phase 1 trial results demonstrated that once-daily TransCon PTH provided a flat, infusion like profile within the normal range of PTH 24 hours per day, increasing sCa while controlling uCa and decreasing sP, with no evidence of bone anabolic activity. Phase 2 Design: PaTH Forward is a global phase 2 trial evaluating the safety, tolerability, and efficacy of TransCon PTH in adult subjects with HP. Patients with HP treated with SoC were randomized at sites worldwide to daily TransCon PTH 15, 18 or 21 µg PTH(1-34) or daily blinded placebo via pen-injector for 4 weeks. The primary composite endpoint requires 1) normal albumin-adjusted sCa, and 2) normal Fractional Excretion of Ca (or ≥50% decrease from baseline), and 3) not taking active D, and 4) taking ≤1000 mg/d of Ca. After four weeks, all subjects enter an open-label extension period with the opportunity to receive TransCon PTH individually optimized to doses of 6 to 30 µg daily to evaluate long-term safety and efficacy. Other endpoints include the impact of treatment on patient experience, CaxP product, bone turnover markers, and bone mass by DXA and TBS to confirm the lack of anabolic effect. Preliminary Results: Approximately 55 subjects are expected to be randomized and dosed. Preliminary diary data on the initial 8 subjects completing 4 weeks of follow-up in the extension trial shows that all have discontinued SoC. Additional information will be presented at ENDO 2020. The Phase 2 trial is designed to inform the starting dose for a global pivotal phase 3 trial and evaluate TransCon PTH as a “true” PTH replacement therapy.