TSH-based protocol, tablet instability, and absorption effects on L-T4 bioequivalence.

Area of Science:

• Pharmacology
• Endocrinology
• Drug Development

Background:

• Current FDA guidance for levothyroxine (L-T(4)) pharmacokinetic (PK) bioequivalence testing faces scrutiny due to concerns about efficacy and safety.
• Recent recalls highlight intrabrand manufacturing issues, necessitating refinements in bioequivalence testing protocols for L-T(4).

Purpose of the Study:

• To quantitatively examine the efficacy of a TSH-based bioequivalence protocol versus current PK-based methods for L-T(4).
• To assess the impact of tablet stability and absorption variations on L-T(4) bioequivalence using simulation.
• To enhance the precision and safety of L-T(4) bioequivalence testing.

Main Methods:

• Utilized an updated simulation model of human thyroid hormone regulation, validated across diverse thyroid function states.
• Explored a TSH-based bioequivalence protocol with normal replacement dosing in simulated athyreotic patients, assessing TSH, T(4), and T(3) dynamics under varying potency and absorption.
• Simulated tablet potency decay and lot-to-lot variations, comparing the FDA's 95-105% range with the previous 90-110% range.

Main Results:

• Simulated decreases of 10-15% in L-T(4) or absorption caused TSH to fall outside bioequivalence targets, while T(4) and T(3) remained normal.
• A 25% L-T(4) reduction led to a 300% TSH change, with minimal impact on T(4) and T(3) levels.
• While TSH, T(4), and T(3) stayed within normal ranges during potency decay, inter-lot and fresh-vs-expired tablets showed bioequivalence issues.

Conclusions:

• A TSH-based pharmacodynamic bioequivalence protocol using normal L-T(4) replacement dosing in athyreotic volunteers is proposed as more sensitive and safer than current FDA PK guidance.
• The tightened 95-105% L-T(4) tablet potency range is an improvement, but residual potency variations can still affect patients, particularly those on high-dose therapy for cancer.