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Difficulty: MediumPharmacologic Interventions

A 33-year-old woman comes to the clinic to discuss her medication regimen. She was recently diagnosed with estrogen receptor-positive breast cancer and underwent a lumpectomy with sentinel lymph node biopsy two weeks ago. Her oncologist plans to initiate adjuvant tamoxifen therapy next week. The patient has a 5-year history of major depressive disorder, which has been well controlled for the past two years with daily paroxetine. She reports that her mood is currently stable and she has no neurovegetative symptoms of depression, though she is mildly anxious about starting her new cancer treatment. She has no other medical conditions and takes no other medications. Vital signs are within normal limits. Physical examination reveals a healing surgical incision on the right breast, with no erythema or discharge. Which of the following is the most appropriate next step in the pharmacologic management of this patient's depression?

  1. A
    Continue paroxetine at the current dosage
  2. B
    Discontinue paroxetine and initiate fluoxetine
  3. C
    Discontinue paroxetine and initiate bupropion
  4. Discontinue paroxetine and initiate venlafaxineAnswer
  5. E
    Continue paroxetine and increase the planned dosage of tamoxifen

Answer

Discontinue paroxetine and initiate venlafaxine
The correct course of action is to discontinue paroxetine and initiate venlafaxine. Tamoxifen is a selective estrogen receptor modulator (SERM) used as adjuvant therapy for hormone receptor-positive breast cancer. It is a prodrug that must be converted by the hepatic cytochrome P450 2D6 (CYP2D6) enzyme into its highly active metabolite, endoxifen. Paroxetine is a strong inhibitor of CYP2D6. Co-administration of paroxetine and tamoxifen prevents the formation of endoxifen, leading to reduced therapeutic efficacy and an increased rate of breast cancer recurrence. Venlafaxine, a serotonin-norepinephrine reuptake inhibitor (SNRI), is a weak or non-inhibitor of CYP2D6. It is considered a safe and highly effective alternative for treating depression (as well as hot flashes) in patients taking tamoxifen.

Step-by-Step Solution

1
Identify the patient's concurrent medications and planned therapies.
The patient is currently taking paroxetine for major depressive disorder and is scheduled to begin tamoxifen for estrogen receptor-positive breast cancer.
Medication reconciliation is the critical first step to avoid adverse drug events when new therapies are introduced.
2
Evaluate potential pharmacokinetic and pharmacodynamic interactions.
Paroxetine is identified as a strong CYP2D6 inhibitor. Tamoxifen is a prodrug that requires activation by CYP2D6 into its active metabolite, endoxifen.
Concomitant use of a strong CYP2D6 inhibitor prevents the activation of tamoxifen, significantly reducing its efficacy and increasing the risk of breast cancer recurrence.
3
Determine the appropriate clinical intervention to manage the interaction.
Select an effective alternative antidepressant that does not strongly inhibit CYP2D6, such as venlafaxine.
Venlafaxine safely maintains psychiatric stability without compromising the efficacy of the patient's critical oncologic therapy.

Key Concept

Management of drug-drug interactions involving CYP2D6 inhibitors and tamoxifen

Alternative Method

While venlafaxine is an excellent choice, selective serotonin reuptake inhibitors (SSRIs) that are weak or non-inhibitors of CYP2D6, such as escitalopram or citalopram, are also safe and viable alternative options for managing depression in patients on tamoxifen.
Estimated Time:1m 0s
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Topics

Pharmacologic Interventions

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