Study of XP-8121 For the Treatment of Adult Subjects With Hypothyroidism

A Phase 2, Multicenter, Non-Randomized, Open-Label, Single Arm, Self-controlled Study of XP-8121 For the Treatment of Adult Subjects With Hypothyroidism

This is a non-randomized, open-label, single arm, self-controlled study of XP-8121 (levothyroxine sodium) to determine a target dose conversion factor from stably dosed oral levothyroxine to XP-8121 (levothyroxine sodium) in patients with hypothyroidism and to assess the safety and tolerability of XP-8121(levothyroxine sodium) after once weekly subcutaneous injections. This study includes the following periods: Screening, Titration Period, and Maintenance Period. The study will conclude with an End of Maintenance Visit or Early Termination (ET) Visit.

This is a non-randomized, open-label, single arm, self-controlled study of XP-8121 (levothyroxine sodium) to determine a target dose conversion factor from stably dosed oral levothyroxine to XP-8121 (levothyroxine sodium) in patients with hypothyroidism and to assess the safety and tolerability of XP-8121 (levothyroxine sodium) after once weekly subcutaneous injections. This study includes the following periods: Screening, Titration Period (2 to 8 weeks), and Maintenance Period (4 weeks). A pharmacokinetic substudy will be conducted for a subset of participants during the Maintenance Period. The study will conclude with an End of Maintenance Visit or Early Termination (ET) Visit.

Inclusion Criteria:

• Provide written informed consent.
• Male or female between the ages of 18 and 65 years (inclusive) at Screening, with chronic hypothyroidism and on a stable dose of oral levothyroxine (Synthroid or an FDA-approved generic equivalent to Synthroid) or, if in the pharmacokinetic substudy, Synthroid only for at least 3 months.
• Thyroid stimulating hormone (TSH) within the normal range at Screening (central laboratory) and at least 3 months prior to Screening (documented by local laboratory).
• Free thyroxine within the normal range at Screening (central laboratory).

Exclusion Criteria:

• History of hypersensitivity to levothyroxine (any formulation).
• Current dose of oral levothyroxine, based on body weight >2 μg/kg/day.
• Current levothyroxine total daily dose either <50 μg or >375 μg.
• Current use of combined thyroid treatment (i.e., Synthroid plus liothyronine or Synthroid plus Armour Thyroid) and/or any other levothyroxine aside from those used in the study.