What drug‑drug interaction exists between valacyclovir and fezolinetant?

No Clinically Significant Drug-Drug Interaction Between Valacyclovir and Fezolinetant

There is no documented drug-drug interaction between valacyclovir and fezolinetant based on their distinct metabolic pathways and lack of shared enzymatic or transporter mechanisms.

Pharmacokinetic Rationale

Valacyclovir Metabolism and Elimination

Valacyclovir is primarily eliminated renally with minimal hepatic metabolism 1. The drug does not undergo significant CYP450 metabolism and has no clinically significant drug-drug or drug-food interactions documented in its FDA label 1. Dose adjustments are only required based on renal function, with recommendations for creatinine clearance-based dosing in patients with renal impairment 2.

Fezolinetant Metabolism and Elimination

Fezolinetant is primarily metabolized by CYP1A2 and to a lesser extent by CYP2C9 and CYP2C19 3. The drug is eliminated 76.9% in urine (only 1.1% unchanged) and 14.7% in feces 3, 4. Critically, fezolinetant and its major metabolite ES259564:

• Are not inhibitors of CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, or CYP3A4
• Are not inducers of CYP1A2, CYP2B6, or CYP3A4
• Are not substrates or inhibitors of P-glycoprotein (P-gp)
• Are not substrates of BCRP, OATP1B1, OATP1B3, OAT1, OAT3, OCT2, MATE1, or MATE2-K 3

Why No Interaction Exists

The two drugs operate through completely independent pathways:

1. Different elimination routes: Valacyclovir relies on renal excretion without hepatic metabolism, while fezolinetant undergoes hepatic CYP1A2 metabolism
2. No shared enzymes: Valacyclovir does not interact with CYP450 enzymes that metabolize fezolinetant
3. No transporter conflicts: Neither drug affects the other's transporter systems
4. No documented interactions: The FDA label for valacyclovir explicitly states no clinically significant drug interactions are known 1

Clinical Considerations

Renal Function Monitoring

While there is no direct interaction, monitor renal function when using valacyclovir, particularly:

• In patients with pre-existing renal impairment (adjust valacyclovir dose per creatinine clearance) 2
• In elderly patients
• When dehydration risk is present (especially in summer months, when valacyclovir-related acute kidney injury is more common) 5

Avoid Concomitant NSAIDs with Valacyclovir

The primary drug interaction concern with valacyclovir involves NSAIDs, not fezolinetant. Concomitant use of valacyclovir with NSAIDs significantly increases acute kidney injury risk (ROR 26.0 for valacyclovir + loxoprofen) 6. If analgesia is needed with valacyclovir, acetaminophen is safer than NSAIDs 5, 7.

Fezolinetant-Specific Interactions

Fezolinetant's clinically relevant interactions involve CYP1A2 inhibitors only 8:

• Strong inhibitors (fluvoxamine): increase fezolinetant AUC by 840%
• Moderate inhibitors (mexiletine, ciprofloxacin): increase AUC by 199-461%
• Weak inhibitors (cimetidine): increase AUC by 61-101%

Valacyclovir does not affect CYP1A2 and therefore does not alter fezolinetant exposure.