Phase 1 Study Evaluating the Pharmacokinetics, Dose Proportionality, Bioavailability, and Tolerability of Subcutaneous Levothyroxine Sodium (<scp>XP</scp>‐8121)

Levothyroxine sodium has been the cornerstone of hypothyroidism management worldwide, with daily oral administration (PO) recognized as standard of care. Oral administration of levothyroxine, however, poses challenges due to variability in pharmacokinetics (PK), influenced by factors such as gastrointestinal absorption, food/drug interactions, and patient adherence. XP-8121 (levothyroxine for subcutaneous administration) is a ready-to-use, subcutaneous (SC) injection formulation of levothyroxine in Phase 3 development. This Phase 1, single-center, 2-part study aimed to characterize the PK and dose proportionality of XP-8121 SC compared to 600 μg oral Ievothyroxine in healthy adults. Additionally, the study evaluated the safety and tolerability of XP-8121 and incorporated population pharmacokinetic (PPK) modeling to support future development. Part 1 was a randomized, open-label, crossover, fixed-sequence study (n = 30). Dose linearity was evaluated by escalating XP-8121 SC doses up to 1200 μg. Part 2 was an open-label, single-period study (n = 30) evaluating PK characteristics of a single dose of XP-8121 SC (1500 μg), potential clinical exposure range, and dose proportionality. After oral levothyroxine administration, baseline-adjusted levothyroxine concentration increased rapidly in plasma (T_max median: 3.1 h); absorption for all XP-8121 SC doses was slower compared to 600 μg oral levothyroxine, and levels remained elevated for 4-5 days before decreasing. Dose proportionality was confirmed, and safety results were similar between all groups. PPK analysis results suggested that weekly doses of XP-8121 SC at four times the daily oral levothyroxine dose provide similar exposure at steady state (AUC_ss). Overall, these data for XP-8121 provide adequate predictive performance to inform future phase 2 studies.