Drug Interactions Study

1. Based on the iteration of FDA DDI guidance, enzyme activity has always been the simplest and most frequently used endpoint.

2. According to the genetic central dogma, changes in mRNA levels precede changes in enzyme activity levels. Thus, early screening assays can rely only on assessments of the mRNA levels.

3. A single endpoint is susceptible to interference from different factors. Enzyme activity levels are susceptible to misleading results if the investigational drug itself is an inhibitor of that enzyme, while mRNA levels are susceptible to the compound’s potential for down-regulation of gene expression; thus, both endpoints need to be included in the IND-enabling study.

Drug-drug interactions (DDIs) can be classified into two major categories based on their mechanisms:

1. Pharmacokinetic (PK) Interactions (more extensively studied in drug development):

• Enzyme-mediated: Inhibition or induction of drug-metabolizing enzymes (e.g., CYP450 inhibition/induction, UGT inhibition). A well-known example is grapefruit juice inhibiting CYP3A4 activity, which has been shown to cause adverse effects when co-administered with approximately 85 different medications.

• Transporter-mediated: Inhibition of ABC or SLC transporters affecting drug absorption or excretion (e.g., P-gp, OATP1B1 inhibition).

2. Pharmacodynamic (PD) Interactions:

• Additive or antagonistic effects between co-administered drugs.

Important consideration: Evaluation should also determine whether the drug itself is a substrate of metabolic enzymes or transporters.

According to the ICH M12: Drug Interaction Studies guideline (May 2024), DDI evaluation should be conducted throughout the entire drug development process, consisting of three key stages:

Step 1: Preclinical (In Vitro) Assessment

1. Initial evaluation of DDI potential (object or precipitant drug) through:

1）Metabolic phenotyping to identify primary clearance pathways

2）Enzyme inhibition/induction assessment.

2. Transporter studies using validated systems with probe substrates/inhibitors to determine:

1）Whether the investigational drug is a substrate of key transporters.

2）Inhibitory potential against major transporters (P-gp, BCRP, OATP1B1, OATP1B3, OAT1, OAT3, OCT2, MATE1, MATE2-K)

3）Additional transporters may be evaluated if clinically relevant.

Step 2: Clinical Evaluation

1. Various study designs are available depending on study objectives.

2. May utilize:

1）Dedicated clinical DDI studies (e.g., cocktail studies).

2）Model-based approaches (PBPK or static models) when supported by robust in vitro data.

Step 3: Post-Marketing Surveillance

1. Continuous monitoring of real-world DDI risks.

2. Essential for:

1）Identifying unexpected interactions.

2）Assessing potential increases in adverse effects.

3）Optimizing clinical management recommendations.