Introduction
P-glycoprotein (P-gp) is a drug transporter recommended by regulatory agencies for in-vitro and in-vivo DDI evaluation. Due to high inter-laboratory variability in in-vitro P-gp data, regulators have recommended using two in-vitro test systems for assessing potential DDIs involving the transporter. As P-gp is expressed in multiple sites in the body (intestine, liver, kidney, blood-brain barrier), a probe substrate whose disposition is dependent on P-gp in a given organ should be selected for clinical DDI studies to evaluate changes in exposure involving that organ. Typically, a probe substrate is chosen to decipher a significant fold change in plasma exposure (AUC, Cmax) with and without inhibitor to assess a potential DDI. Dabigatran Etexilate (DE) and digoxin have been screened as clinical probe substrates for studying DDIs involving intestinal P-gp. Table 1 shows examples of some P-gp inhibitors that were studied with both DE and digoxin as probe substrates.

Although amiodarone and dronedarone showed similar fold change in AUC of dabigatran and digoxin, cobicistat and glecaprevir/pibrentasvir showed higher AUC fold change with dabigatran than with digoxin. Dabigatran is even more sensitive at a subtherapeutic (micro) dose, which results in a gut concentration close to its Km for P-gp. In this commentary, ITC suggested selecting a probe substrate based on the question to be answered.

 Ideal Clinical Probe Substrate Characteristics for P-gp DDI Studies

  • Highly selective – transported by P-gp only
  • Stable – No or minimal biotransformation
  • Sensitive – Low to moderate fraction absorbed
  • Tolerability – Have sufficient safety margin to cover an increase in exposure with a P-gp inhibitor
  • Ease of analysis – Availability of standards and suitable analytical method

Key Takeaways:

  1. Selection of clinical probe substrate should be based on the specific DDI questions to be addressed; e.g., digoxin for renal P-gp inhibition or to determine the safety of a comedication specifically with digoxin, and DE for intestinal P-gp inhibition.
  2. Digoxin clinical studies may still be warranted in spite of its narrow therapeutic index. To also assess the impact on renal P-gp, calculation of renal clearance is recommended.
  3. Microdose of DE (375 µg) more sensitive for assessing intestinal P-gp DDI. Assess a potential perpetrator as an inhibitor of CES beforehand.
  4. Good IVIVC with DE: IC50 values for several P-gp inhibitors based on DE basolateral-to-apical transport predicted the risk of DDIs with no false negatives.